Patent Publication Number: US-11644221-B1

Title: Open cycle thermal management system with a vapor pump device

Description:
CLAIM OF PRIORITY 
     This application claims priority under 35 USC § 119(e) to U.S. Provisional Patent Application Ser. No. 62/813,831, filed on Mar. 5, 2019, and entitled “THERMAL MANAGEMENT SYSTEMS,” the entire contents of which are hereby incorporated by reference. 
    
    
     BACKGROUND 
     This disclosure relates to refrigeration. 
     Refrigeration systems absorb thermal energy from the heat sources operating at temperatures below the temperature of the surrounding environment, and discharge thermal energy into the surrounding environment. 
     Conventional refrigeration systems can include at least a compressor, a heat rejection exchanger (i.e., a condenser), a liquid refrigerant receiver, an expansion device, and a heat absorption exchanger (i.e., an evaporator). Such systems can be used to maintain operating temperature set points for a wide variety of cooled heat sources (loads, processes, equipment, systems) thermally interacting with the evaporator. Closed-circuit refrigeration systems may pump significant amounts of absorbed thermal energy from heat sources into the surrounding environment. In closed-circuit systems, compressors are used to compress vapor from the evaporation and condensers are used to condense the vapor to cool the vapor into a liquid. The combination of condensers and compressors can add significant amount of weight and can consume relatively large amounts of electrical power. In general, the larger the amount of absorbed thermal energy that the system is designed to handle, the heavier the refrigeration system and the larger the amount of power consumed during operation, even when cooling of a heat source occurs over relatively short time periods. 
     SUMMARY 
     According to an aspect, a thermal management system includes an open-circuit refrigeration system having an open-circuit refrigerant fluid flow path, the open-circuit refrigeration system including a receiver configured to store a refrigerant fluid, with the receiver having an outlet. an evaporator having an evaporator inlet and an evaporator outlet, the evaporator configured to receive the refrigerant fluid at the evaporator inlet, extract heat from a heat load that contacts the evaporator and provide refrigerant vapor at the evaporator outlet, and a vapor pump device having a vapor pump inlet that receives the refrigerant vapor and having a vapor pump outlet that outputs compressed refrigerant vapor to an exhaust line coupled to the vapor pump outlet, with the receiver, the evaporator, the vapor pump device, and the exhaust line connected in the open-circuit refrigerant fluid flow path. 
     Embodiments of the thermal management systems may include any one or more of the following features or other features particular to the aspect. 
     The vapor pump device is configurable to control a refrigerant fluid pressure according to a specified evaporation temperature. The open-circuit refrigeration system is configured to control temperature of a heat load in proximity to the evaporator at least in part by operation of the vapor pump device. The pump device is a variable speed pump configured to regulate refrigerant flow through the evaporator. The vapor pump device is a compressor. The vapor pump device is a vacuum pump. The refrigerant fluid is water. The refrigerant fluid is ammonia. During operation refrigerant fluid is discharged from the exhaust line as a vapor, so that the discharged refrigerant fluid is not returned to the receiver. 
     The thermal management system further includes a control device disposed in the open-circuit refrigerant fluid flow path between the outlet of the receiver and the evaporator inlet to control a vapor quality of the refrigerant fluid at the evaporator outlet. The control device is a mechanically controllable expansion device. The thermal management system further includes a sensor device disposed in proximity to the evaporator outlet, which sensor device provides a sensor signal that is a measure of a thermodynamic property of the refrigerant fluid at the evaporator outlet. 
     The control device is an electronically controlled expansion device, that is directly controlled by the sensor signal. The electronically controlled expansion device is indirectly controlled by the sensor signal. The electronically controlled expansion device is configured to receive a control signal that is produced, at least in part, by the controller system processing the sensor signal, and which control signal indirectly controls the electronically controlled expansion device. 
     The thermal management system further includes a controller system that includes a processor device and memory coupled to the processor device, with the processor device executing computer instructions to provide one or more control signals. The thermal management system further includes an electronically controlled expansion device disposed in the open-circuit refrigerant fluid flow path between the outlet of the receiver and the evaporator inlet, a sensor device that measures a thermodynamic property of the refrigerant fluid at the evaporator outlet, and wherein the controller system is configurable by execution of the computer instructions to control one or more of the vapor pump device and the electronically controlled expansion device. 
     The thermal management system further includes a sensor device disposed in proximity to the evaporator outlet to measure a vapor quality of the refrigerant fluid at the evaporator outlet. The open-circuit refrigeration system is configured to operate with a vapor quality at the evaporator outlet substantially equal to the critical vapor quality, where vapor quality is the ratio of mass of vapor to mass of liquid plus mass of vapor and the critical vapor quality is equal to 1. 
     The open-circuit refrigeration system is configured to operate with a vapor quality at the evaporator outlet substantially equal to the critical vapor quality, which is controlled with respect to a vapor quality set point, by operation of the control device. 
     The receiver is a first receiver, with the open-circuit refrigeration system further including a second receiver configured to store a gas, with the second receiver coupled in the open-circuit refrigerant fluid flow path to the first receiver. The thermal management system further includes a control device that is configurable to control a flow of the gas from the second receiver to the first receiver to regulate refrigerant pressure in the first receiver and maintain pressure in the first receiver during operation of the open-circuit refrigeration system. The control device is a pressure regulator. The control device is a first control device, the open-circuit refrigeration system, further includes a second control device that is configurable to receive liquid refrigerant fluid from the first receiver at a first pressure, expand the liquid refrigerant fluid to generate a two-phase liquid/vapor refrigerant fluid mixture at a second pressure, and direct the refrigerant fluid mixture into the evaporator. 
     The second control device includes an expansion valve that is coupled between the outlet of the first receiver and the evaporator inlet, and which expansion valve provides a constant-enthalpy expansion of the liquid refrigerant fluid to generate the refrigerant fluid mixture. The refrigerant fluid comprises ammonia. The gas does not react chemically with the refrigerant fluid. The gas comprises at least one gas selected from the group consisting of nitrogen, argon, xenon, and helium. 
     The thermal management further includes a thermal load in thermal communication with the evaporator that is configured to remove heat from the thermal load. The thermal management wherein the thermal load is a high energy load. 
     According to an aspect a thermal management method includes transporting a refrigerant fluid along a refrigerant fluid flow path that extends from a receiver that stores refrigerant fluid, through an evaporator that has an evaporator inlet and a evaporator outlet, to a vapor pump device, and to an exhaust line, extracting heat from a heat load in contact with the evaporator pumping by the vapor pump device, the refrigerant vapor from the refrigerant fluid flow path into the exhaust line, so that the refrigerant vapor that is pumped into the exhaust line is not returned to the refrigerant fluid flow path. 
     The method includes regulating operation of the vapor pump device to regulate a temperature of the heat load according to a vacuum pressure corresponding to a specified evaporation temperature. The method includes regulating a refrigerant flow through the evaporator by varying a pumping speed of the vapor pump device. The method includes regulating a vapor quality of the refrigerant fluid emerging from the evaporator outlet. The method includes controlling a vapor quality of the refrigerant fluid emerging from the evaporator outlet to be substantially equal to the critical vapor quality, where vapor quality is the ratio of mass of vapor to mass of liquid plus mass of vapor and the critical vapor quality is equal to 1. 
     The vapor quality is controlled by expanding refrigerant fluid from the receiver into a two-phase refrigerant fluid, with an expansion device that is coupled between the outlet of the receiver and the evaporator inlet, with operation of the expansion device controlled according to a control signal. 
     The method includes regulating a temperature of the heat load according to a vacuum pressure corresponding to a specified evaporation temperature. The refrigerant fluid is water or ammonia. 
     The receiver is a first receiver, and the method further includes transporting a gas from a second receiver to the first receiver at least prior to transporting or during transporting of the refrigerant fluid to control refrigerant pressure in the first receiver. The refrigerant fluid is ammonia. The gas does not react chemically with the ammonia. The gas comprises at least one gas selected from the group consisting of nitrogen, argon, xenon, and helium. The method includes controlling a vapor quality of the refrigerant fluid emerging from the evaporator outlet to be substantially equal to the critical vapor quality, where vapor quality is the ratio of mass of vapor to mass of liquid plus mass of vapor and the critical vapor quality is equal to 1. The vapor quality is controlled by expanding refrigerant fluid from the receiver into a two-phase refrigerant fluid, with an expansion device that is coupled between the outlet of the receiver and the evaporator inlet, with operation of the expansion device controlled according to a control signal. 
     Other aspects are also disclosed. 
     One or more of the aspects of the open-circuit refrigeration systems disclosed herein may have one or more of the following advantages, amongst other advantages. 
     For example, relative to closed-circuit systems, the absence of condensers can result in a significant reduction in the overall size and mass of such systems, relative to conventional closed-circuit systems, particularly when the open-circuit refrigeration systems are sized for operation over relatively short time periods. The inclusion of a compressor in the open-circuit system provides additional upstream control of vapor pressure and hence load temperature, but such compressors would generally be smaller and lighter in size and consume less power for a given amount of refrigeration relative to a closed-circuit system because the compressor would not be required to compress vapor back to a liquid phase. 
     The benefit of maintaining the refrigerant fluid within a two-phase (liquid and vapor) region of the refrigerant fluid&#39;s phase diagram, is that the heat extracted from high heat flux loads can be used to drive a constant-temperature liquid to vapor phase transition of the refrigerant fluid, allowing the refrigerant fluid to absorb heat from a high heat flux load without undergoing a significant temperature change. In practical applications, a pressure drop exists during this transition and associated evaporating temperature drop exists as well. Consequently, the temperature of a high heat flux load can be stabilized within a range of temperatures that is relatively small, even though the amount of heat generated by the load and absorbed by the refrigerant fluid is relatively large. In addition, the boiling heat transfer coefficients of a two phase refrigerant fluid may be significantly higher than the heat transfer coefficients of single-phase refrigerant fluid. 
     The details of one or more embodiments are set forth in the accompanying drawings and the description below. Other features and advantages will be apparent from the description, drawings, and claims. 
    
    
     
       DESCRIPTION OF DRAWINGS 
         FIG.  1    is a schematic diagram of an example of a thermal management system that includes an open-circuit refrigeration system with an exhaust compressor or vacuum pump. 
         FIGS.  2 A- 2 F  are schematic diagrams of other examples of a thermal management system that includes alternative open-circuit refrigeration systems with an exhaust compressor or vacuum pump. 
         FIG.  3    is a schematic diagram of another example of a thermal management system that includes an alternative open-circuit refrigeration system with an exhaust compressor or vacuum pump and an ejector. 
         FIG.  4    is a schematic diagram of an ejector. 
         FIGS.  5 A- 5 F  are schematic diagrams of other examples of a thermal management system that includes alternative open-circuit refrigeration systems with an exhaust compressor or vacuum pump and an ejector. 
         FIG.  6    is a schematic diagram of another example of a thermal management system that includes an open-circuit refrigeration system with exhaust compressor or vacuum pump and pump for liquid recirculation, useful for controlling vapor pressure in a refrigerant receiver. 
         FIGS.  6 A- 6 C  are schematic diagrams of examples of a junction and arrangements of the junction in open-circuit refrigeration system configurations of  FIG.  6   . 
         FIGS.  7 A- 7 F  are schematic diagrams of other examples of a thermal management system that includes alternative open-circuit refrigeration systems with exhaust compressor or vacuum and pump. 
         FIGS.  8 ,  8 A  are schematic diagram of another example of a thermal management system that includes alternative open-circuit refrigeration systems that have gas compression for receiver vapor pressure control. 
         FIG.  9    is a schematic diagram of an example of a receiver for refrigerant fluid in a thermal management system. 
         FIG.  10    is a schematic diagram of an example of a receiver for refrigerant fluid in the thermal management system. 
         FIGS.  11 A and  11 B  are schematic diagrams showing side and end views, respectively, of an example of the heat load that includes refrigerant fluid channels. 
         FIGS.  12 ,  12 A- 12 C  are diagrammatical views of liquid separators. 
         FIG.  13    is a schematic diagram of another example of a thermal management system that includes an alternative open-circuit refrigeration system with an exhaust compressor or vacuum pump, ejector and a recuperative heat exchanger. 
         FIG.  13 A  is a schematic diagram of an example of the recuperative heat exchanger useful in  FIG.  13   . 
         FIG.  14    is a schematic diagram of an example of the thermal management system of  FIG.  1    that includes one or more sensors connected to a controller. 
         FIG.  15    is a block diagram of a controller. 
         FIG.  16    is a schematic diagram of an example of a thermal management system that includes a power generation apparatus. 
         FIG.  17    is a schematic diagram of an example of directed energy system that includes a thermal management system. 
     
    
    
     DETAILED DESCRIPTION 
     I. General Introduction 
     Cooling of large loads and high heat flux loads that are also highly temperature sensitive can present a number of challenges. On one hand, such loads generate significant quantities of heat that is extracted during cooling. In conventional closed-cycle refrigeration systems, cooling high heat flux loads typically involves circulating refrigerant fluid at a relatively high mass flow rate. However, closed-cycle system components that are used for refrigerant fluid circulation—include large compressors to compress vapor at a low pressure to vapor at a high pressure and condensers to remove heat from the compressed vapor at the high pressure and convert to a liquid—are typically heavy and consume significant power. As a result, many closed-cycle systems are not well suited for deployment in mobile platforms—such as on small vehicles or in space—where size and weight constraints may make the use of large compressors and condensers impractical. 
     On the other hand, temperature sensitive loads such as electronic components and devices may require temperature regulation within a relatively narrow range of operating temperatures. Maintaining the temperature of such a load to within a small tolerance of a temperature set point can be challenging when a single-phase refrigerant fluid is used for heat extraction, since the refrigerant fluid itself will increase in temperature as heat is absorbed from the load. 
     Directed energy systems that are mounted to mobile vehicles such as trucks or exist in space may present many of the foregoing operating challenges, as such systems may include high heat flux, temperature sensitive components that require precise cooling during operation in a relatively short time. The thermal management systems disclosed herein, while generally applicable to the cooling of a wide variety of heat loads, are particularly well suited for operation with such directed energy systems. 
     In particular, the thermal management systems and methods disclosed herein include a number of features that reduce both overall size and weight relative to conventional refrigeration systems, and still extract excess heat energy from both high heat flux, highly temperature sensitive components and relatively temperature insensitive components, to accurately match temperature set points for the components. At the same time, the disclosed thermal management systems that use a compressor or vacuum pump (when the evaporating pressure is substantially below ambient pressure) would in general require less power than conventional closed circuitry systems for a given amount of refrigeration over a specified period(s) of operation. Whereas certain conventional refrigeration systems used closed-circuit refrigerant flow paths, the systems and methods disclosed herein use open-cycle refrigerant flow paths. Depending upon the nature of the refrigerant fluid, exhaust refrigerant fluid may be incinerated as fuel, chemically treated, and/or simply discharged at the end of the flow path. 
     II. Thermal Management Systems with Open-Circuit Refrigeration Systems with Compressor Exhaust 
     Referring to  FIG.  1   , an example of a thermal management system  10  that includes an open-circuit refrigeration system with compressor-assisted exhaust (OCRSCE)  10   a  is shown. System  10  includes a receiver  12 , an optional valve  16 , a control device  18  (i.e., an fluid expansion device  18  that is optional in some embodiments), an evaporator  32 , a vapor pumping device  19 , e.g., a compressor or a vacuum pump, (compressor/vacuum pump  19 ) and conduits  24   a - 24   f , with conduit  24   f  at or being an exhaust line  38 . The compressor or vacuum pump  19  (compressor/vacuum pump  19 ) can be a single-speed, multi-speed or a variable speed device. A heat load  34  is coupled to the evaporator  32 . OCRSCE  10   a  in some embodiments also includes a controller  17  (see  FIG.  15    for an exemplary embodiment) that produces control signals, e.g.,  17   a - 17   c , based on sensed thermodynamic properties to control operation of the various control devices such as the optional valve  16 , the control device  18 , the compressor/vacuum pump  19 , etc., as needed. That is, controller  17  may receive signals (of sensed thermodynamic properties of the system), and send control signals (as appropriate) to the expansion device  16 , the optional solenoid valve  18 , a motor of the compressor/vacuum pump  19  or a control to change the speed or to shut off or starting the compressor/vacuum pump  19 , etc. 
     As used herein “compressor or vacuum pump” or “compressor/vacuum pump  19 ” generally refers to a compressor or a vacuum pump as alternative devices for use as vapor pumping device  19  (as well as  59  and  119 , in  FIGS.  3 ,  5 A- 8 A , discussed below). A compressor is, in general, a device that increases the pressure of a gas by reducing the gas&#39; volume. The compressor is similar to a vacuum pump as both increase the pressure on a fluid and both can transport the fluid through a pipe, and also reduce the volume of the compressed gas or vapor. Usually the term compressor refers to devices operating at and above ambient pressure and vacuum pumps refer to devices operating below ambient pressure. In general, either a “compressor” or a “vacuum pump” are suitable alternative devices for use as the component  19  or  59  or  119 , taking into consideration specific requirements of a given system  10 . 
     Control device  18  functions as a flow control device. In general, control device  18  can be implemented as any one or more of a variety of different mechanical and/or electronic devices. For example, in some embodiments, control device  18  can be implemented as a fixed orifice, a capillary tube, and/or a mechanical or electronic expansion valve  18   a  ( FIG.  2 A ). In general, fixed orifices and capillary tubes are passive flow restriction elements which do not actively regulate refrigerant fluid flow. 
     Mechanical expansion valves (usually called thermostatic or thermal expansion valves) are typically flow control devices that enthalpically expand a refrigerant fluid from a first pressure to an evaporating pressure, controlling superheat at the evaporator  34  exit. Mechanical expansion valves generally include an orifice, a moving seat that changes the cross-sectional area of the orifice and the refrigerant fluid volume and mass flow rates, a diaphragm moving the seat, and a bulb (features not illustrated) at the evaporator exit. The bulb is charged with a fluid and it hermetically fluidly communicates with a chamber above the diaphragm. The bulb senses the refrigerant fluid temperature at the evaporator exit (or another location) and the pressure of the fluid inside the bulb transfers the pressure in the bulb through the chamber to the diaphragm, and moves the diaphragm and the seat to close or to open the orifice. 
     Typical electrical expansion valves include an orifice, a moving seat, a motor or actuator that changes the position of the seat with respect to the orifice, a controller, and pressure and temperature sensors at the evaporator exit. The controller calculates the superheat for the expanded refrigerant fluid based on pressure and temperature measurements at the evaporator exit. If the superheat is above a set-point value, the seat moves to increase the cross-sectional area and the refrigerant fluid volume and mass flow rates to match the superheat set-point value. If the superheat is below the set-point value the seat moves to decrease the cross-sectional area and the refrigerant fluid flow rates. 
     Examples of suitable commercially available expansion valves that can function as control device  18  include, but are not limited to, thermostatic expansion valves available from the Sporlan Division of Parker Hannifin Corporation (Washington, Mo.) and from Danfoss (Syddanmark, Denmark). 
     Receiver  12  includes an inlet port  12   a , an outlet port  12   b , a pressure relief valve  12   c , and an optional heater  12   d . Receiver  12  is typically implemented as an insulated vessel that stores a refrigerant fluid at relatively high pressure. The use of the compressor/vacuum pump  19  greatly assists in controlled removal or exhaust of vapor from the system  10   a . (See discussion for  FIG.  10    for details on refrigerant receivers.) 
     Evaporator  32  can be implemented in a variety of ways. In general, the evaporator  32  functions as a heat exchanger, providing thermal contact between the refrigerant fluid and the heat load  34 . Typically, the evaporator  32  includes one or more flow channels extending internally between an inlet and an outlet of the evaporator, allowing refrigerant fluid to flow through the evaporator and absorb heat from the heat load  34 . (See discussion for  FIGS.  11 A,  11 B  for details on evaporators). 
     The compressor/vacuum pump  19  generally functions to control the fluid pressure upstream of the evaporator  32 . In general, the compressor/vacuum pump  19  can be implemented using a variety of different pump/compressor technologies. In general, the compressor/vacuum pump  19  is selected based on the refrigerant fluid volume flow rate, the pressure differential across the compressor/vacuum pump  19 , and the pressure and temperature at the compressor or vacuum pump inlet. The compressor/vacuum pump  19  is configured by the controller  17  or otherwise to control a temperature of the heat load by control of the fluid pressure upstream of the evaporator  32 . 
     Refrigerant Fluids 
     A variety of different refrigerant fluids can be used in system  10 . For these open-circuit refrigeration systems, in general emissions regulations and operating environments may limit the types of refrigerant fluids that can be used. For example, in certain embodiments, the refrigerant fluid can be ammonia having very large latent heat; after passing through the cooling circuit, the ammonia refrigerant can be disposed of by incineration, by chemical treatment (i.e., neutralization), and/or by direct venting to the atmosphere. In certain embodiments, the refrigerant fluid can be an ammonia-based mixture that includes ammonia and one or more other substances. For example, mixtures can include one or more additives that facilitate ammonia absorption or ammonia burning. 
     More generally, any fluid can be used as a refrigerant in the open-circuit refrigeration systems disclosed herein, provided that the fluid is suitable for cooling heat load  34  (e.g., the fluid boils at an appropriate temperature) and, in embodiments where the refrigerant fluid is exhausted directly to the environment, regulations and other safety and operating considerations do not inhibit such discharge. 
     In particular, the open-circuit refrigeration system  10  can operate with water as the refrigerant. The receiver  12  may be charged with water at atmospheric pressure. During operation the water is enthalpically expanded into a vacuum pressure corresponding to a desired evaporating temperature, cools heat load  34 , evaporates into a vapor, and the vapor is discharged by the vacuum pump/compressor  19  into ambient environment. 
     While, in the OCRSCE  10   a , the compressor consumes power, the discharge pressure can be lower than the discharge pressure of an equivalent closed-circuit refrigeration system, and, therefore, the power consumed by the compressor/vacuum pump  19  can be less than the power consumed by a compressor of the equivalent closed-circuit refrigerant system. Moreover a power requirement can be reduced if an ejector is added, as will be discussed below (see  FIG.  3   , etc.) 
     Referring now to  FIGS.  2 A- 2 E  some alternative configurations  10   b - 10   g  of the OCRSCE are shown. Items illustrated and referenced, but not mentioned in the discussion below are discussed and referenced in  FIG.  1   . 
     Also discussed below will be a general OCRSCE system configuration that is one of several open-circuit refrigeration configurations that include two receivers (one for a pressurizing gas and the other for refrigerant), but which otherwise parallel OCRSCE configurations  10   a - 10   g.    
     Referring to  FIG.  2 A , OCRSCE  10   b  is shown having the control device  18  of  FIG.  1   , implemented as an electronically controlled expansion device  18   a  (expansion device  18   a ). The expansion device  18   a  is operated with a sensor device  40  that controls the expansion device  18   a  either directly or through the controller  17 . The evaporator  32  operates in two phase (liquid/gas) and superheated region with controlled superheat. The sensor controlled expansion device  18   a  and the sensor  40  provide a mechanism to measure and control superheat. The compressor/vacuum pump  19  in general functions to control refrigerant fluid pressure upstream of the evaporator  32 . The use of the compressor/vacuum pump  19  assists in controlled removal or exhaust of refrigerant vapor from the system  10   b . After passing through compressor/vacuum pump  19 , the refrigerant fluid is discharged as exhaust through conduit  24   f , which functions as an exhaust line  38  for system  10   b . Typically, compressor/vacuum pump  19  adjusts the upstream refrigerant fluid pressure in system  10 . The compressor/vacuum pump  19  is configurable to control the temperature of heat load  34  that can be adjusted (by changing a pumping rate) to selectively change a temperature set point value (i.e., a target temperature) for heat load  34 . 
     The vapor quality of the refrigerant fluid after passing through evaporator  32  can be controlled either directly or indirectly with respect to a vapor quality set point by, e.g., the sensor device and/or controller  17  controlling operation of the expansion valve  18 . The evaporator  32  may be configured to maintain exit vapor quality below the critical vapor quality defined as “1.” Vapor quality is the ratio of mass of vapor to mass of liquid+mass of vapor and is generally kept in a range of approximately 0.5 to almost 1.0; more specifically 0.6 to 0.95; more specifically 0.75 to 0.9; more specifically 0.8 to 0.9 or more specifically about 0.8 to 0.85. “Vapor quality” is defined as mass of vapor/total mass (vapor+liquid). In this sense, vapor quality cannot exceed “1” or be equal to a value less than “0.” 
     Referring now to  FIG.  2 B , OCRSCE  10   c  is shown to include the electronically controlled expansion device  18   a . The expansion device  18   a  is operated with a sensor device  42  that measures superheat of refrigerant exiting from the evaporator  32 . As in systems  10   a  and  10   b  ( FIGS.  1 ,  2 A ), the compressor/vacuum pump  19  functions to control refrigerant fluid pressure upstream of the evaporator  32 , and thus assists in controlled removal or exhaust of refrigerant vapor from the system  10   c  through conduit  24   f  that functions as the exhaust line  38  for system  10   c . The compressor/vacuum pump  19  is configurable to control the temperature of heat load  34  as discussed above for system  10   b . The vapor quality of the refrigerant fluid after passing through evaporator  32  can be controlled either directly or indirectly with respect to a vapor quality set point by, e.g., the sensor device and/or controller  17  controlling operation of the expansion valve  18 . 
     Referring now to  FIG.  2 C , OCRSCE  10   d  includes the electronically controlled expansion device  18   a  operated with the sensor device  40  that controls the expansion device  18  either directly or with the controller  17 , as shown. The evaporator  32  operates in a two phase (liquid/gas) region of refrigerant phase. Also shown in  FIG.  2 C , is a second load  34   a  that operates in a superheated region with controlled superheat. The sensor controlled expansion device  18   a  and sensor  40  provide a mechanism to measure and control superheat at the exit of the second load  34   a . As in systems  10   a  and  10   b  ( FIGS.  1 ,  2 A ), the compressor/vacuum pump  19  functions to control refrigerant fluid pressure upstream of the evaporator  32 , and thus assists in controlled removal or exhaust of refrigerant vapor from the system  10   d  through conduit  24   f  that functions as the exhaust line  38  for system  10   d . The compressor/vacuum pump  19  is configurable to control the temperature of heat load  34  as discussed above for system  10   b.    
     Referring now to  FIG.  2 D , OCRSCE  10   e  includes the electronically controlled expansion device  18   a  operated with the sensor device  40  that controls the expansion device  18  either directly or (by the controller  17 , not shown) and a back pressure regulator  36  disposed in line with an output of the vacuum pump or compressor  19  via conduit  24   f  and the exhaust line  38 . As in systems  10   a  and  10   b  ( FIGS.  1 ,  2 A ), the compressor/vacuum pump  19  functions to control refrigerant fluid pressure upstream of the evaporator  32 , and thus assists in controlled removal or exhaust of refrigerant vapor from the system  10   e  through conduit  24   f  that functions as the exhaust line  38  for system  10   e . The compressor/vacuum pump  19  is configurable to control the temperature of heat load  34  as discussed above for system  10   b . However, in the implementation of  FIG.  2 D , the back pressure regulator  36  also functions to control removal or exhaust of refrigerant vapor from the system  10   e  at the exhaust line  38  and thus can assist with and reduce the needed capacity (concomitant therewith size, weight and power consumption characteristics of the compressor/vacuum pump  19 ) to control the temperature of heat load  34 . 
     Referring now to  FIG.  2 E , OCRSCE  10   f  includes the electronically controlled expansion device  18   a  operated with the sensor device  40  that controls the expansion device  16  either directly or (by the controller  17 ) and the optional back pressure regulator  46  disposed in line with the output of the vacuum pump or compressor  19  and the exhaust line. Also included in  FIG.  2 E  is a junction device  48  disposed between the back pressure regulator  36  and the output of the vacuum pump or compressor  19 . The junction device  48  is located at a compressor/vacuum pump  19  outlet. The junction device  48  couples refrigerant discharge at the compressor/vacuum pump  19  outlet to an inlet to the receiver  12 . Compressor refrigerant discharge is used to control pressure in the receiver  12  so that the pressure remains high enough to extend operation of the OCRSCE  16 . As the amount of liquid refrigerant is consumed refrigerate pressure in the receiver is reduced. In this implementation, the back pressure regulator  36  when applied can maintain a relatively constant pressure in the receiver  12  during entire period of operation of the OCRSCE  10   f.    
     The junction device  48  has an input port coupled to the vacuum pump or compressor  19 , a first output port coupled to the back pressure regulator  46  and a second output port coupled to an input to the receiver  12 . Compressor discharge is used to control pressure in the receiver  12  allowing for the evaporator and back pressure regulator to be fixed control devices. 
     In the embodiments of  FIGS.  1 ,  2 A -E, discussed above, control of superheat is required since most implementations of the compressor/vacuum pump  19 , usually do not allow for or tolerate the presence of liquid at the inlet of the compressor/vacuum pump  19 . 
     Referring now to  FIG.  2 F , this embodiment allows for operation of the evaporator  32  at a vapor quality of “1” or less than “1” at the evaporator exit by the use of a recuperative heat exchanger  50  that evaporates any remaining refrigerant liquid prior to the refrigerant fluid being fed to the inlet of the compressor/vacuum pump  19 . OCRSCE  10   g  includes the electronically controlled expansion device  18   a  operated with the sensor device  40  that controls the expansion device  16  either directly or (by the controller  17 ) and the recuperative heat exchanger  50 . The recuperative heat exchanger  50  is used for transferring heat energy from the refrigerant fluid emerging from evaporator  32  to refrigerant fluid upstream from control device  18 . Inclusion of the recuperative heat exchanger  50  thus can reduce mass flow rate demand, and also allows for operation of the evaporator  32  within threshold of vapor quality, as mentioned. 
     In this implementation of  FIG.  2 F , a back pressure regulator (not shown) is optional, but if applied, the back pressure regulator can maintain a relatively constant pressure in the receiver  12  during entire period of operation of the OCRSCE  10   g.    
     The discussion below regarding vapor quality presumes that the recuperative heat exchanger  50  is configured to generate sufficient superheat. 
     The vapor quality of the refrigerant fluid after passing through evaporator  32  can be controlled either directly or indirectly with respect to a vapor quality set point by, e.g., the controller  17 . The evaporator  32  may be configured to maintain exit vapor quality below the critical vapor quality defined as “1.” Vapor quality is the ratio of mass of vapor to mass of liquid+mass of vapor and is generally kept in a range of approximately 0.5 to almost 1.0; more specifically 0.6 to 0.95; more specifically 0.75 to 0.9; more specifically 0.8 to 0.9 or more specifically about 0.8 to 0.85. “Vapor quality” is defined as mass of vapor/total mass (vapor+liquid). In this sense, vapor quality cannot exceed “1” or be equal to a value less than “0.” 
     In practice, vapor quality may be expressed as “equilibrium thermodynamic quality” that is calculated as follows:
 
 X =( h−h ′)/( h″−h ′),
 
where h is specific enthalpy, specific entropy or specific volume, h′ is of saturated liquid and “is of saturated vapor. In this case X can be mathematically below 0 or above 1, unless the calculation process is forced to operate differently. Either approach is acceptable.
 
     During operation of system  10 , cooling can be initiated by a variety of different mechanisms. In some embodiments, for example, system  10  includes a temperature sensor attached to load  34  (as will be discussed subsequently). When the temperature of load  34  exceeds a certain temperature set point (i.e., threshold value), the controller  17  connected to the temperature sensor can initiate cooling of load  34 . Alternatively, in certain embodiments, system  10  operates essentially continuously—provided that the refrigerant fluid pressure within receiver  12  is sufficient—to cool load  34 . As soon as receiver  12  is charged with refrigerant fluid, refrigerant fluid is ready to be directed into evaporator  32  to cool load  34 . In general, cooling is initiated when a user of the system or the heat load issues a cooling demand. 
     System Cooling Operations 
     Upon initiation of a cooling operation, refrigerant fluid from receiver  12  is discharged from outlet  12   b , through optional valve  18  if present, and is transported through conduit  24   a  to control device  18 , which directly or indirectly controls vapor quality (or superheat) at the evaporator outlet. In the following discussion, control device  18  is implemented as an expansion valve. However, it should be understood that more generally, control device  18  can be implemented as any component or device that performs the functional steps described below and provides for vapor quality control (or superheat) at the evaporator outlet. 
     Once inside the expansion valve  18 , the refrigerant fluid undergoes constant enthalpy expansion from an initial pressure p r  (i.e., the receiver pressure) to an evaporation pressure p e  at the outlet of the expansion valve. In general, the evaporation pressure p e  depends on a variety of factors, e.g., the desired temperature set point value (i.e., the target temperature) at which load  34  is to be maintained and the heat input generated by the heat load  34 . Set points are discussed below. 
     The initial pressure p r  in the receiver tends to be in equilibrium with the surrounding temperature and is different for different refrigerants. The pressure p e  in the evaporator depends on the evaporating temperature, which is lower than the heat load temperature and is defined during design of the system. The system is operational as long as the receiver-to-evaporator pressure difference (p r −p e ) is sufficient to drive adequate refrigerant fluid flow through the expansion device  18 . After undergoing constant enthalpy expansion in the expansion device  18  (or valve  18 ), the liquid refrigerant fluid is converted to a mixture of liquid and vapor phases at the temperature of the fluid and at the evaporation pressure p e . The two-phase refrigerant fluid mixture is transported via conduit  24   b  to evaporator  32 . 
     When the two-phase mixture of refrigerant fluid is directed into evaporator  32 , the liquid phase absorbs heat from load  34 , driving a phase transition of the liquid refrigerant fluid into the vapor phase. For the embodiments of  FIGS.  1 ,  2 A- 2 E , the phase transition should be to a complete vapor phase, as it is not desirable to have liquid at inlets to the compressor/vacuum pump  19 . 
     For the embodiment of  FIG.  2 F , the phase transition need not be to a complete vapor phase, that is, the evaporator can operate with a vapor quality of less than 1, as it is permitted to have liquid at the outlet of the evaporator  34 , provided that the recuperative heat exchanger  50  is used to evaporate any remaining refrigerant liquid prior to the refrigerant fluid being fed to the inlet of the compressor/vacuum pump  19 . Because this phase transition occurs at (nominally) constant temperature, the temperature of the refrigerant fluid mixture within evaporator  32  remains unchanged, provided at least some liquid refrigerant fluid remains in evaporator  32  to absorb heat. 
     Further, the constant temperature of the refrigerant fluid mixture within evaporator  32  can be controlled by adjusting the pressure p e  of the refrigerant fluid, since adjustment of p e  changes the boiling temperature of the refrigerant fluid. Thus, by regulating the refrigerant fluid pressure p e  upstream from evaporator  32  (e.g., using compressor/vacuum pump  19 ), the temperature of the refrigerant fluid within evaporator  32  (and, nominally, the temperature of heat load  34 ) can be controlled to match a specific temperature set-point value for heat load  34 , ensuring that load  34  is maintained at, or very near, a target temperature. 
     The pressure drop across the evaporator  32  causes drop of the temperature of the refrigerant mixture (which is the evaporating temperature), but still the evaporator  32  can be configured to maintain the heat load temperature within the set tolerances. 
     In some embodiments, for example, the evaporation pressure of the refrigerant fluid can be adjusted by the compressor/vacuum pump  19  to ensure that the temperature of heat load  34  is maintained to within ±5 degrees C. (e.g., to within ±4 degrees C., to within ±3 degrees C., to within ±2 degrees C., to within ±1 degree C.) of the temperature set point value for load  34 . 
     As discussed above, within evaporator  32 , a portion of the liquid refrigerant in the two-phase refrigerant fluid mixture is converted to refrigerant vapor by undergoing a phase change. As a result, the refrigerant fluid mixture that emerges from evaporator  32  has a higher vapor quality (i.e., the fraction of the vapor phase that exists in refrigerant fluid mixture) than the refrigerant fluid mixture that enters evaporator  32 . 
     As the refrigerant fluid mixture emerges from evaporator  32 , a portion of the refrigerant fluid can optionally be used to cool one or more additional heat loads. Typically, for example, the refrigerant fluid that emerges from evaporator  32  is nearly all in the vapor phase. The refrigerant fluid vapor (or, more precisely, high vapor quality fluid vapor) can be directed into a heat exchanger (not shown) coupled to another heat load, and can absorb heat from the additional heat load during propagation through the heat exchanger. Examples of systems in which the refrigerant fluid emerging from evaporator  32  is used to cool additional heat loads will be discussed in more detail subsequently. 
     The refrigerant fluid emerging from evaporator  32  is transported through conduit  24   e  to compressor/vacuum pump  19 , (or to one side of the recuperative heat exchanger  50  in  FIG.  2 F ) which directly or indirectly controls the upstream pressure, that is, the evaporating pressure p e  in the system  10 . After passing through compressor/vacuum pump  19 , the refrigerant fluid is discharged as exhaust through conduit  24   f , which functions as the exhaust line  38  for the system  10 . Refrigerant fluid discharge can occur directly into the environment surrounding system  10 . Alternatively, in some embodiments, the refrigerant fluid can be further processed; various features and aspects of such processing are discussed in further detail below. 
     It should be noted that the foregoing steps, while discussed sequentially for purposes of clarity, occur simultaneously and continuously during cooling operations. In other words during operation, refrigerant fluid is continuously being discharged from receiver  12 , undergoing continuous expansion in control device  18 , flowing continuously through evaporator  32  and compressor/vacuum pump  19 , being discharged from system  10 , while heat load  34  is being cooled. 
     During operation of system  10 , as refrigerant fluid is drawn from receiver  12  and used to cool heat load  34 , the receiver pressure p r  falls. If the refrigerant fluid pressure p r  in receiver  12  is reduced to a value that is too low, the pressure differential p r -p e  may not be adequate to drive sufficient refrigerant fluid mass flow to provide adequate cooling of heat load  34 . Accordingly, when the refrigerant fluid pressure p r  in receiver  12  is reduced to a value that is sufficiently low, the capacity of system  10  to maintain a particular temperature set point value for load  34  may be compromised. Therefore, the pressure in the receiver or pressure drop across the expansion valve (or any related refrigerant fluid pressure or pressure drop in system  10 ) can be an indicator of the remaining operational time. An appropriate warning signal can be issued (e.g., by the controller  17 ) to indicate that, in a certain period of time, the system may no longer be able to maintain adequate cooling performance; operation of the system can even be halted if the refrigerant fluid pressure in receiver  12  reaches the low-end threshold value. 
     It should be noted that while in  FIG.  1    only a single receiver  12  is shown, in some embodiments, system  10  can include multiple receivers to allow for operation of the system over an extended time period. Each of the multiple receivers can supply refrigerant fluid to the system to extend to total operating time period. Some embodiments may include plurality of evaporators connected in parallel, which may or may not be accompanied by a plurality of expansion valves and plurality of evaporators. 
     III. System Operational Control 
     As discussed in the previous section, by adjusting the pressure p e  of the refrigerant fluid, the temperature at which the liquid refrigerant phase undergoes vaporization within evaporator  32  can be controlled. Thus, in general, the temperature of heat load  34  can be controlled by a device or component of system  10  that regulates the pressure of the refrigerant fluid within evaporator  32 . Typically, compressor/vacuum pump  19  adjusts the upstream refrigerant fluid pressure in system  10 . Accordingly, compressor/vacuum pump  19  is generally configured to control the temperature of heat load  34 , and can be adjusted (by changing a pumping rate) to selectively change a temperature set point value (i.e., a target temperature) for heat load  34 . 
     Other important system operating parameters are the superheat and the vapor quality of the refrigerant fluid emerging from evaporator  32 , as also discussed above. Vapor quality, as mentioned, is a number from 0 to 1, represents the fraction of the refrigerant fluid that is in the vapor phase. 
     In the embodiment in  FIG.  2 F , which can operate in the two phase region, because heat absorbed from load  34  is used to drive a constant-temperature evaporation of liquid refrigerant to form refrigerant vapor in evaporator  32 , it is generally important to ensure that, for a particular volume of refrigerant fluid propagating through evaporator  32 , at least some of the refrigerant fluid remains in liquid form right up to the point at which the exit aperture of evaporator  32  is reached to allow continued heat absorption from load  34  without causing a temperature increase of the refrigerant fluid. However, if the fluid is fully converted to the vapor phase after propagating only partially through evaporator  32 , further heat absorption by the (now vapor-phase) refrigerant fluid within evaporator  32  will lead to a temperature increase of the refrigerant fluid and heat load  34 . On the other hand, liquid-phase refrigerant fluid that emerges from evaporator  32  represents unused heat-absorbing capacity, in that the liquid refrigerant fluid did not absorb sufficient heat from load  34  to undergo a phase change. To ensure that system  10  operates efficiently, the amount of unused heat-absorbing capacity should remain relatively small. 
     In addition, the boiling heat transfer coefficient that characterizes the effectiveness of heat transfer from load  34  to the refrigerant fluid is typically very sensitive to vapor quality. When the vapor quality increases from zero to a certain value, called a critical vapor quality, the heat transfer coefficient increases. In general, the critical vapor quality and heat transfer coefficient values vary widely for different refrigerant fluids, and heat and mass fluxes. For all such refrigerant fluids and operating conditions, the systems and methods disclosed herein control the vapor quality at the outlet of the evaporator such that the vapor quality approaches the threshold of the critical vapor quality. 
     When the vapor quality exceeds the critical vapor quality, the heat transfer coefficient is abruptly reduced to a very low value, causing dry-out within evaporator  32 . In this region of operation, the two-phase mixture behaves as superheated vapor. 
     The embodiments of  FIGS.  1 ,  2 A- 2 E , operate with controlled superheat at the evaporator exit. That is, the embodiments of  FIGS.  1 ,  2 A- 2 E  are configured to operate such that vapor quality is at 1, at the evaporator exit, so that refrigerant liquid is not fed to the inlet of the compressor/vacuum pump  19 . 
     To make maximum use of the heat-absorbing capacity of the two-phase refrigerant fluid mixture, the vapor quality of the refrigerant fluid emerging from evaporator  32  (in the implementation of  FIG.  2 F ) should nominally be nearly equal to the critical vapor quality, but (in the implementations of  FIG.  1 ,  2 A- 2 E ) should be equal to the critical vapor quality, i.e., completely in the vapor phase. 
     Accordingly, to both efficiently use the heat-absorbing capacity of the two-phase refrigerant fluid mixture and also ensure that the temperature of heat load  34  remains approximately constant at the phase transition temperature of the refrigerant fluid in evaporator  32 , the systems and methods of  FIGS.  1 ,  2 A- 2 F  are generally configured to adjust the vapor quality of the refrigerant fluid emerging from evaporator  32  to a value that is less than (of  FIG.  2 F ) or equal to ( FIGS.  1 ,  2 A- 2 E ) the critical vapor quality. 
     Another important operating consideration for system  10  is the mass flow rate of refrigerant fluid within the system. The evaporator can be configured to provide minimal mass flow rate controlling maximal vapor quality, which is the critical vapor quality. By minimizing the mass flow rate of the refrigerant fluid according to the cooling requirements for heat load  34 , system  10  operates efficiently. Each reduction in the mass flow rate of the refrigerant fluid (while maintaining the same temperature set point value for heat load  34 ) means that the charge of refrigerant fluid added to reservoir  12  initially lasts longer, providing further operating time for system  10 . 
     Within evaporator  32 , the vapor quality of a given quantity of refrigerant fluid varies from the evaporator inlet (where vapor quality is lowest) to the evaporator outlet (where vapor quality is highest). Nonetheless, to realize the lowest possible mass flow rate of the refrigerant fluid within the system, the effective vapor quality of the refrigerant fluid within evaporator  32 —even when accounting for variations that occur within evaporator  32 —should match the critical vapor quality as closely as possible according to the considerations mentioned above. 
     In summary, to ensure that the system operates efficiently and the mass flow rate of the refrigerant fluid is relatively low, and at the same time the temperature of heat load  34  is maintained within a relatively small tolerance, system  10  adjusts the vapor quality of the refrigerant fluid emerging from evaporator  32  to a value such that an effective vapor quality within evaporator  32  matches, or nearly matches, the critical vapor quality. 
     In system  10 , control device  18  is generally configured to control the vapor quality of the refrigerant fluid emerging from evaporator  32 . As an example, when control device  18  is implemented as an expansion valve, the expansion valve regulates the mass flow rate of the refrigerant fluid through the valve. In turn, for a given set of operating conditions (e.g., ambient temperature, initial pressure in the receiver, temperature set point value for heat load  34 , heat load  34 ), the vapor quality determines mass flow rate of the refrigerant fluid emerging from evaporator  32 . 
     Control device  18  typically controls the vapor quality of the refrigerant fluid emerging from evaporator  32  in response to information about at least one thermodynamic quantity that is either directly or indirectly related to the vapor quality. Compressor/vacuum pump  19  is configured to control a temperature of the heat load  34  (via upstream refrigerant fluid pressure adjustments) in response to information about at least one thermodynamic quantity that is directly or indirectly related to the temperature of heat load  34 . The one or more thermodynamic quantities upon which adjustment of control device  18  is based may be different from the one or more thermodynamic quantities upon which adjustment of compressor/vacuum pump  19  is based. 
     In general, a wide variety of different measurement and control strategies can be implemented in system  10  to achieve the control objectives discussed above. These strategies are presented below. Generally, control device  18  is connected to a first measurement device and compressor/vacuum pump  19  is connected to a second measurement device. The first and second measurement devices provide information about the thermodynamic quantities upon which adjustments of the first and second control devices are based. The first and second measurement devices can be implemented in many different ways, depending upon the nature of the first and second control devices. In addition, in the implementations of  FIGS.  2 A- 2 E  sensor device  40  can measure thermodynamic quantities that can be used to determine, by the controller  17 , superheat at the outlet of the evaporator  32 , and controller  17 , in turn generates control signals to control operation of the expansion device  18 , and as appropriate the compressor/vacuum pump  19  and back pressure regulator  36 . 
     IV. Thermal Management Systems with Open-Circuit Refrigeration Systems with Compressor Exhaust and Ejector Boost Assist 
     Referring now to  FIG.  3   , an alternative thermal management system  50  includes an open-circuit refrigeration system with compressor-assisted exhaust and ejector boost assist (OCRSCAEE)  50   a  that has a refrigerant fluid flow path  55   a . The addition of an ejector allows recirculation of liquid non-evaporated in the evaporator operating within the threshold of critical vapor quality. 
     Also discussed below will be an OCRSCAEE system configuration that is one of several open-circuit refrigeration with ejector system configurations that include two receivers, but which otherwise parallel OCRSCAEE configurations  50   a - 50   g.    
     In  FIG.  3   , the OCRSCAEE  50   a  includes a compressor or a vacuum pump  59  (functionally similar to the compressor or the vacuum pump  19  of  FIGS.  1 ,  2 A- 2 F ). OCRSCAEE  50   a  is one of several open-circuit refrigeration with ejector system configurations  50   a - 50   g  that will be discussed herein. 
     OCRSCAEE  50   a  includes a receiver  52  that receives and is configured to store refrigerant, an optional solenoid valve  56  and an optional control device  56 , e.g., an expansion device  58 . The liquid refrigerant in the receiver  52  can be maintained in two-phase or (if high pressure is maintained in the receiver by the compressor) in a subcooled state (e.g., as a liquid existing at a temperature below its normal boiling point temperature) even at high ambient and liquid refrigerant temperatures. Both, either or neither of the solenoid control valve  56  and the optional expansion device  58  are used (i.e., or not used) in each of the embodiments of an OCRSCAEE, as will be described in  FIGS.  3 , and  5 A- 5 F . 
     The OCRSCAEE  50   a  also includes an ejector  66  and a liquid separator  68 . The ejector  66  has a primary inlet or high pressure inlet  66   a  that is coupled to the refrigerant receiver  52  (either directly or through the optional solenoid control valve  56  and/or optional control device  58 ). In OCRSCAEE  50   a  outlet  66   c  of the ejector  66  is coupled to an inlet  68   a  of the liquid separator  68 . The ejector  66  also has a secondary inlet or low pressure inlet  66   b . The liquid separator  68  in addition to the inlet  68   a , has a first outlet (vapor side outlet)  68   b  and a second outlet  68   c  (liquid side outlet). The first outlet  68   b  of the liquid separator  68  is coupled to an inlet (not referenced) of the compressor/vacuum pump  59  that has an outlet (not referenced) that feeds an exhaust line  67 . 
     The OCRSCAEE  50   a  also includes an optional expansion device  70  and an evaporator  32  (similar or the same as the evaporator  32  of  FIGS.  1 ,  2 A- 2 F ). The evaporator  32  is coupled to secondary inlet of the ejector  66  and the second outlet  68   c  (liquid side outlet) of the liquid separator  68 . 
     The thermal management system  50  includes the heat load  34  (similar or the same as the load of  FIGS.  1 - 2 F ) that is coupled to OCRSCAEE  50   a  in thermal communication with the evaporator  32 . 
     The evaporator  32  is configured to extract heat from the heat load  34  that is in contact with the evaporator  32 . Conduits  64   a - 64   j  couple the various aforementioned items, as shown. OCRSCAEE  50   a  in some embodiments also includes a controller  17  that produces control signals to control operation of the various ones of devices  56 ,  58 ,  70 , etc. as needed, as well as compressor/vacuum pump  59 . Controller  17  is similar to controller  17  of  FIG.  1   . 
     The OCRSCAEE  50   a  can be viewed as including three circuits. A first circuit  55   a  being the refrigerant flow path  55   a  that includes the receiver  52  and two downstream circuits  55   b  and  55   c  that are downstream from the liquid separator  68 . Downstream circuit  55   b  carries liquid from the liquid separator  68  and includes the expansion device  70  that feeds the evaporator  32 . The downstream circuit  55   c  includes the compressor/vacuum pump  59 , and which exhausts vapor via the exhaust line  67 . Receiver  52  is typically implemented as an insulated vessel that stores refrigerant fluid, at relatively high pressures. The control device  58  is configurable to control a flow of the refrigerant from the receiver  52  to ejector  66 . The control device  58  can be an expansion valve. 
     In some implementations, the OCRSCAEE  50   a  includes a junction  57  between the compressor/vacuum pump  59  and the exhaust line  67 . (Optionally, a back pressure regulator, not shown could also be included as in  FIG.  2 E ). The junction  57  is interposed to divert a portion of compressed gas from the compressor/vacuum pump  59  back to the receiver  52  to maintain high pressure in the receiver  52  and to maintain the liquid refrigerant in the receiver in a sub-cooled state. 
     In some embodiments, refrigerant flow through the OCRSCAEE  50   a  is controlled either solely by the ejector  66  and compressor/vacuum pump  59  or by those components aided by either one or all of the solenoid valve  56  and expansion control device (e.g., expansion valve)  58 , depending on requirements of the application, e.g., ranges of mass flow rates, cooling requirements, receiver capacity, ambient temperatures, heat load, etc. While both solenoid valve  56  and expansion control device  58  are optionally, in some implementation either or both would be used and would function as a flow control device(s) to control refrigerant flow into the primary inlet  66   a  of the ejector  66 . 
     In some embodiments expansion device  58  is integrated with the ejector  66 . In OCRSCAEE  50   a  (as well as the other embodiments discussed below) the expansion control device  58  may be required under some circumstances where there are or can be significant changes in, e.g., an ambient temperature, which might impose additional control requirements on the OCRSCAEE  50   a.    
     In general, the valve  56  is implemented as a solenoid control valve  56  or any one or more of a variety of different mechanical and/or electronic devices. A solenoid valve includes a solenoid that uses an electric current to generate a magnetic field to control a mechanism to regulate an opening in a valve to control fluid flow. The valve  56  is configurable to stop refrigerant flow as an on/off valve. 
     The compressor/vacuum pump  59  at the vapor side outlet  68   b  of the liquid separator  68  generally functions to control the vapor pressure upstream of the compressor/vacuum pump  59  and hence temperature of the heat load  34 . In OCRSCAEE  50   a , the compressor/vacuum pump  59  controls the refrigerant fluid vapor pressure from the liquid separator  68 , and indirectly controls evaporating pressure/temperature and hence heat load temperature. In general, compressor/vacuum pump  59  can be implemented using a variety of different techniques, as discussed above for compressor/vacuum pump  19 . 
     For device  58  a mechanical expansion valve or an electrically controlled expansion valve could be used, as discussed above for expansion control device  18  ( FIG.  1   ). The expansion valve  70  can be an electrically controlled expansion valve, similar to electrically controlled expansion control device  18   a  discussed above. 
     In some embodiments discussed below, the controller  17  can be used with sensors to calculate a value of superheat for the expanded refrigerant fluid based on pressure and temperature measurements at the liquid separator exit. The controller  17  can generate control signals to control electrical expansion valves, as discussed above for device  18   a  ( FIG.  2 A ). 
     Some loads require maintaining thermal contact between heat load  34  and evaporator  32  with the refrigerant being in the two-phase region (of a phase diagram for the refrigerant) and, therefore, the expansion device or valve  70  maintains a proper vapor quality at the evaporator exit. Alternatively, a sensor communicating with the controller  17  may monitor pressure in the refrigerant receiver  52 , as well as a pressure differential across the expansion valve  56 , a pressure drop across the evaporator  32 , a liquid level in the liquid separator  68 , and power input into electrically actuated heat loads, or a combination of the above. 
     Examples of suitable commercially available expansion valves that can function as device  70  include, but are not limited to, thermostatic expansion valves available from the Sporlan Division of Parker Hannifin Corporation (Washington, Mo.) and from Danfoss (Syddanmark, Denmark). 
     In  FIG.  3   , the evaporator  32  has an outlet that is coupled, via conduit  64   j  to the secondary inlet  66   b  (low-pressure inlet) of the ejector  66 . The evaporator  32  has an inlet that is coupled via conduit  64   i  to an outlet of the expansion device  70 . The expansion device  70  and conduit  64   h  and  64   i , thus couple the inlet of the evaporator  32  to the liquid side outlet  68   c  of the liquid separator  68 . In this configuration, the ejector  66  acts as a “pump,” that may assist in reducing pumping demand by the compressor/vacuum pump  59 . The ejector  66  assists in “pumping” a secondary fluid flow, e.g., liquid/vapor from the evaporator  32 , by using energy of the primary refrigerant flow from the refrigerant receiver  52 . The use of the ejector  66  can significantly reduce the size or pumping capacity needed for a given cooling demand and as a result reduce the amount of power consumed by the compressor/vacuum pump  59 . 
     Ejector  66  operation is discussed in  FIG.  4   . The evaporator  32  can be implemented as discussed above and as will be further discussed below. 
     Referring now also to  FIG.  4   , a typical configuration for the ejector  66  is shown. This exemplary ejector  66  includes the primary inlet  66   a , secondary or suction inlet  66   b  and the outlet  66   c . The primary inlet feeds a motive nozzle  66   d , the secondary or suction inlet  66   b  feeds secondary nozzle(s)  66   e  that are coupled to a suction chamber  66   f . A mixing chamber  66   g  of a constant area receives the primary flow of refrigerant and secondary flow of refrigerant and mixes these flows. A diffuser  66   h  diffuses the flow to deliver an expanded flow at the outlet  66   c.    
     In one embodiment, the ejector  66  is passively controlled by built-in flow control. Also, the OCRSCAEE  50   a  may employ the optional flow control device(s)  56 ,  58  upstream of the ejector  66 . 
     Liquid refrigerant from the refrigerant receiver is the primary flow. In the motive nozzle  66   d  potential energy of the primary flow at the inlet  66   a  is converted into kinetic energy reducing the potential energy (the established static pressure) of the primary flow. The secondary flow at the inlet  66   b  from the outlet of the evaporator  32  has a pressure that is higher than an established static pressure in the suction chamber  66   f , and thus the secondary flow is entrained through the suction inlet (secondary inlet  66   b ) and the secondary nozzle(s)  66   f  internal to the ejector  66 . The two streams (primary flow and secondary flow) mix together in the mixing section  66   f . In the diffuser section  66   g , the kinetic energy of the mixed streams is converted into potential energy elevating the pressure of the mixed flow liquid/vapor refrigerant that leaves the ejector outlet  66   c  and is fed to the liquid separator  68 . 
     In the context of open-circuit refrigeration systems, the use of the ejector  66  allows for recirculation of liquid refrigerant captured by the liquid separator  68  to increase the efficiency of the system  50 . That is, by allowing for some recirculation of refrigerant, but without the need for a compressor or a condenser, as in a closed cycle refrigeration system, this recirculation reduces the required amount of refrigerant needed for a given amount of cooling over a given period of operation. The ejector  66  also can reduce the power requirements of the compressor/vacuum pump  59 . 
     The system including the evaporator  32  may be configured to maintain exit vapor quality below the critical vapor quality defined as “1”, as discussed above. 
     Referring back to  FIG.  3   , the OCRSCAEE  50   a  operates as follows. The refrigerant from the receiver  52  (primary flow) is fed to the primary inlet of the ejector  66  and expands at a constant entropy in the ejector  66  (in ideal case; in reality the nozzle is characterized by the isentropic efficiency of the ejector) and turns into a two-phase (gas/liquid) state. The refrigerant in the two-phase state from the ejector  66  enters the liquid separator  68 , with liquid exiting the liquid separator at outlet  68   c  (liquid side outlet) and vapor exiting the separator  68  at outlet  68   b  the (vapor side outlet) depending on separation efficiency. The liquid stream exiting at outlet  68   c  enters and is expanded in the expansion device  70  into a liquid/vapor stream that enters the evaporator  32 . The expansion device  70  is configured to maintain suitable vapor quality at the evaporator exit (or a superheat if this is acceptable to operate the heat load) and related recirculation rate. 
     The evaporator  32  provides cooling duty and discharges the refrigerant in a two-phase state or a superheated state into the secondary inlet  66   b  of the ejector  66 . The ejector  66  entrains the refrigerant flow exiting the evaporator  32  and mixes it in the mixing section of the ejector with the primary flow from the receiver  54 . Vapor exits from the vapor side outlet  68   b  of the liquid separator  68  is drawn to the exhaust line  67  by the compressor/vacuum pump  59  and is exhausted by the exhaust line  67 . The compressor/vacuum pump  59  regulates the pressure upstream of the compressor/vacuum pump  59  so as to maintain upstream refrigerant fluid pressure in OCRSCAEE  50   a  and thus indirectly control temperature of a heat load  34 . 
       FIGS.  5 A- 5 G  discussed below are alterative open-circuit refrigeration systems with ejector booster assist and compressor (OCRSCAEE) configurations. 
     Referring now to  FIG.  5 A , the system  50  includes an alternative open-circuit refrigeration system (OCRSCAEE)  50   b . OCRSCAEE  50   b  includes the receiver  52 , optionally, the solenoid control valve  56  and/or optionally the expansion control device  58 , as discussed above. OCRSCAEE  50   b  also includes the ejector  66  having the primary inlet  66   a  that is coupled to receiver  52  directly (or through the control device  58  and solenoid control valve  56 , if used) via conduit  64   a ,  64   b , and the ejector has the outlet  66   c.    
     In OCRSCAEE  50   b , the evaporator  32  inlet is coupled to the outlet  66   c  of the ejector  66 , via conduit  64   c , and the evaporator outlet is coupled to the inlet  68   a  of the liquid separator  68 , via conduit  64   k . The heat load  34  is coupled to the evaporator  32 . The evaporator  32  is configured to extract heat from heat load  34  that is in contact with the evaporator  32 . In OCRSCAEE  50   b , the expansion device  70  is coupled between the liquid outlet  68   c  of the liquid separator  68  and the suction or secondary inlet  66   b  of the ejector  66 . 
     The second outlet (vapor side outlet) of the liquid separator  68  is coupled to compressor/vacuum pump  59  and is exhausted by the exhaust line  67 . The compressor/vacuum pump  59  regulates the pressure upstream of the compressor/vacuum pump  59  so as to maintain upstream refrigerant fluid pressure in OCRSCAEE  50   a  and indirectly control temperature of the heat load  34 . Conduits  64   a - 64   f ,  64   h ,  64   i  and  64   k  couple the various aforementioned items as shown. With OCRSCAEE  50   b , the recirculation rate is equal to the vapor quality at the evaporator exit. The expansion device  70  is optional but when used, is a fixed orifice device. The control valve  58  or other control device that is built in the motive nozzle of the ejector provides active control of the thermodynamic parameters of refrigerant state at the evaporator exit. 
     The OCRSCAEE  50   b  operates as follows. The liquid refrigerant from the receiver  52  is fed to the ejector  66  and expands at a constant entropy in the ejector  66  (in an ideal case; in reality the nozzle is characterized by the ejector isentropic efficiency), and turns into a two-phase (gas/liquid) state. The refrigerant in the two-phase state enters the evaporator  32  that provides cooling duty and discharges the refrigerant in a two-phase state at an exit vapor quality (fraction of vapor to liquid) below a unit vapor quality (“1”). The discharged refrigerant is fed to the inlet of the liquid separator  68 , where the liquid separator  68  separates the discharge refrigerant with only or substantially only liquid exiting the liquid separator at outlet  68   c  (liquid side outlet) and only or substantially only vapor exiting the separator  68  at outlet  68   b  the (vapor side outlet). The vapor side may contain some liquid droplets since the liquid separator  68  has a separation efficiency below a “unit” separation. The liquid stream exiting at outlet  68   c  enters and is expanded in the optional expansion device  70 , if used, into a liquid/vapor stream that enters the suction or secondary inlet of the ejector  66 . The ejector  66  entrains the refrigerant flow exiting the expansion valve by the refrigerant from the receiver  54 . 
     In OCRSCAEE  50   b , by placing the evaporator  32  between the outlet of the ejector  66  and the inlet of the liquid separator  68 , OCRSCAEE  50   b  avoids the necessity of having liquid refrigerant pass through the liquid separator  29  during the initial charging of the evaporator  32  with the liquid refrigerant, in contrast with the OCRSCAEE  50   a  ( FIG.  3   ). The ejector “pumping” capacity of liquid is higher than the pumping capacity of vapor, therefore the recirculation rate may be increased and the requirements for the precise control of the vapor quality at the evaporator exit, ease. At the same time liquid trapped in the liquid separator  68  may be wasted after the OCRSCAEE shuts down. 
     The OCRSCAEE  50   b  can also be viewed as including three circuits, the first circuit  55   a  being the refrigerant flow path as in  FIG.  3    and two circuits  55   b ′ and  55   c . Circuit  55   b ′ however is upstream from the liquid separator  68  and carries vapor/liquid from the evaporator  32  to the inlet to the liquid separator  68 . The downstream circuit  55   c  exhausts vapor from liquid separator  68  via the compressor/vacuum pump  59  to the exhaust line  67 . 
     When a fixed orifice device is not used, the expansion valve  58  can be an electrically controlled expansion valve. Typical electrical expansion valves include an orifice, a moving seat, a motor or actuator that changes the position of the seat with respect to the orifice, the controller  17  (see  FIG.  15   ), and sensors. The sensors may monitor vapor quality at the evaporator exit, pressure in the refrigerant receiver  52 , pressure differential across the expansion valve  58 , pressure drop across the evaporator  32 , liquid level in the liquid separator  68 , power input into electrically actuated heat loads or a combination of the above. 
     Examples of suitable commercially available expansion valves that can function as device  70  include, but are not limited to, thermostatic expansion valves available from the Sporlan Division of Parker Hannifin Corporation (Washington, Mo.) and from Danfoss (Syddanmark, Denmark). Also, the expansion valve  58  can be integrated into the motive nozzle of the ejector. 
     Referring now to  FIG.  5 B , the system  10  includes another alternative open-circuit refrigeration system (OCRSCAEE)  50   c . OCRSCAEE  50   c  includes the receiver  52  and the optional expansion valve  58  and optional solenoid control valve  56 , coupled to inlet  66   a  of the ejector  66 , and the liquid separator  68 . The OCRSCAEE  50   c  includes the expansion device  70  coupled to the liquid side outlet  68   c  of the liquid separator  68 . The second outlet  68   b  (vapor side outlet) of the liquid separator  68  is coupled, via conduit  64   e ,  64   f  and the compressor/vacuum pump  59  to the exhaust line  67 . 
     The OCRSCAEE  50   c  also includes a first evaporator  32   a . A heat load  34   a  is coupled to the evaporator  32   a . The evaporator  32   a  is configured to extract heat from heat load  34   a  that is in contact with the evaporator  32   a . The evaporator  32   a  is coupled to the outlet  66   c  of the ejector  66  and the inlet  68   a  of the liquid separator  68 . The OCRSCAEE  50   c  also includes a second evaporator  32   b  having an inlet coupled to the outlet of the expansion device  70 , and the second evaporator  32   b  has an outlet coupled to the suction inlet  66   b  of the ejector  66 . A heat load  34   b  is coupled to the evaporator  32   b . The evaporator  32   b  is configured to extract heat from heat load  34   b  that is in contact with the evaporator  32   b . Conduits  64   a - 64   f ,  64   h - 64   k  couple the various aforementioned items, as shown. 
     The cooling capacity of the OCRSCAEE  50   a  is sensitive to recirculation rate; also, this configuration can operate with loads that allow for operation in superheat regions. OCRSCAEE  50   a  reduces compression ratio requirements for the compressor  59  and reduces compressor power and compressor size and weight requirements relative to a conventional closed-circuit refrigeration system. The OCRSCAEE  50   b  system is not sensitive to recirculation rate, which may be beneficial when the heat loads may significantly reduce recirculation rate. An operating advantage of the OCRSCAEE  50   c  is that by placing evaporators  32   a ,  32   b  at both the outlet  66   c  and the secondary inlet  66   b  of the ejector  66 , it is possible to run the evaporators  32   a ,  32   b  combining the features of the configurations mentioned above. 
     The OCRSCAEE  50   c  can also be viewed as including three circuits. The first circuit  55   a  being the refrigerant flow path as in  FIG.  1    and two circuits  55   b ″ and  55   c . Circuit  55   b ″ being upstream and downstream from the liquid separator  68 , carrying liquid from the liquid outlet of the liquid separator  68  and carrying vapor/liquid from the evaporator  32  into the inlet of the liquid separator  68 . The downstream circuit  55   c  exhausts vapor via the compressor/vacuum pump  59  to the exhaust line  67 . 
     Referring now to  FIG.  5 C , the system  10  can include another alternative open-circuit refrigeration system (OCRSCAEE)  50   d . OCRSCAEE  50   d  includes the receiver  12 , (optional valve  58  and optional solenoid control valve  56 ), ejector  66 , and liquid separator  68 , as discussed above. The OCRSCAEE  50   d  includes the expansion device  70  coupled to the liquid side outlet  68   c  of the liquid separator  68 . 
     The OCRSCAEE  50   d  also includes a single evaporator  32   c  that is attached downstream from and upstream of the ejector  66 . A first heat load  34   a  is coupled to the evaporator  32   c . The evaporator  32   c  is configured to extract heat from the first load  34   a  that is in contact with the evaporator  32   c . A second heat load  34   b  is also coupled to the evaporator  32   c . The evaporator  32   c  is configured to extract heat from the second load  34   a  that is in contact with the evaporator  32   c . The evaporator  32   c  has a first inlet that is coupled, via conduit  64   d , to the outlet  66   c  of the ejector  66  and also has a first outlet that is coupled, via conduit  64   k , to the inlet  68   a  of the liquid separator  68 . The evaporator  32   c  has a second inlet that is coupled, via conduit  64   i , to the outlet of the expansion device  70  and has a second outlet that is coupled, via conduit  64   j , to the suction inlet  66   b  of the ejector  66 . The second outlet  68   b  (vapor side outlet) of the liquid separator  68  is coupled, via conduit  64   e ,  64   l , and the compressor/vacuum pump  59  to the exhaust line  67 . Conduits  64   a - 64   f ,  64   h - 64   l  couple the various aforementioned items, as shown. 
     In this embodiment, the single evaporator  32   c  is attached downstream from and upstream of the ejector  66  and requires a single evaporator in comparison with the configuration of  FIG.  4    having the two evaporators  32   a ,  32   b  ( FIG.  4   ). The OCRSCAEE  50   d  can also be viewed as including the three circuits  55   a ,  55   b ″ and  55   c  as described in  FIG.  5 B . 
     Referring now to  FIG.  5 D , the system  10  includes an alternative open-circuit refrigeration system (OCRSCAEE)  50   e . OCRSCAEE  50   e  includes the receiver  52 , (optional valve  58  and optional solenoid control valve  56 ), ejector  66 , liquid separator  68 , and the evaporators  32   a ,  32   b , as discussed in  FIG.  5 B . The evaporators  32   a ,  32   b  have the first heat load  34   a  and the second heat load coupled to the evaporators  32   a ,  32   b  respectively, with the evaporators  32   a ,  32   b  configured to extract heat from the loads  34   a ,  34   b  in contact with the evaporators. 
     In this embodiment, the OCRSCAEE  50   e  also includes an expansion device  70   a . The expansion device  70   a  is a sensor controlled expansion device, such as an electrically controlled expansion valve. The evaporators  32   a ,  32   b  operate in two phase (liquid/gas) and superheated region with controlled superheat. The controllable expansion device  70   a  is coupled to the liquid side outlet  68   c  of the separator  68  and the evaporator  32  having a control port that is fed from a sensor  40 . The sensor controlled expansion device  70   a  and sensor  40  provide a mechanism to measure and control superheat at the evaporator exit. The second outlet  68   b  (vapor side outlet) of the liquid separator  68  is coupled via the compressor/vacuum pump  59  to the exhaust line  67 . The OCRSCAEE  50   e  can also be viewed as including the three circuits  55   a ,  55   b ″ and  55   c  as described in  FIG.  5 B . Conduits  64   a - 64   f ,  64   h - 64   k  couple the various aforementioned items, as shown. 
     In  FIG.  5 D , the vapor quality of the refrigerant fluid after passing through evaporator  32   a  can be controlled either directly or indirectly with respect to a vapor quality set point by the controller  17 . In some embodiments, as shown in  FIGS.  5 D , the system  10  includes a sensor  40  that provides a measurement of superheat, and indirectly, vapor quality. For example, in  FIG.  5 D , sensor  40  is a combination of temperature and pressure sensors that measure the refrigerant fluid superheat downstream from the heat load, and transmits the measurements to the controller (not shown). The controller adjusts the expansion valve device  70  based on the measured superheat relative to a superheat set point value. By doing so, controller indirectly adjusts the vapor quality of the refrigerant fluid emerging from evaporator  32 . 
     Referring now to  FIG.  5 E , the system  10  includes an alternative open-circuit refrigeration system (OCRSCAEE)  50   f . OCRSCAEE  50   f  includes the receiver  52 , optional expansion device  58  and optional solenoid control valve  56 , ejector  66 , liquid separator  68 , expansion device  70  and the evaporators  32   a ,  32   b , as discussed in  FIG.  5 B , as well as, a second expansion device  71  and a third evaporator  39  coupled to a third load  41 . The evaporators  32   a ,  32   b  have the first heat load  34   a  and the second heat load  34   b  coupled to the evaporators  32   a ,  32   b  respectively, with the evaporators  32   a ,  32   b  configured to extract heat from the loads  34   a ,  34   b  in contact with the evaporators. The evaporator  39  is configured to extract heat from the load  41  in contact with the evaporator  39 . An inlet of the evaporator  39  is coupled to an outlet of the expansion device  71 , that is coupled to a port of a junction device  75 . The other ports of the junction device  75  are coupled to the outlet of expansion valve  70  and an inlet to the evaporator  32   b.    
     The second outlet  68   b  (vapor side outlet) of the liquid separator  68  is coupled via the compressor/vacuum pump  59  and a back pressure regulator  53  to the exhaust line  67  that may include an and junction  75   a  and optional flow control device that when included splits exhausted vapor into an inlet to the receiver  52  and out the exhaust line. (If the junction  75   a  and optional flow control device are not included, the exhaust is feed into the receiver inlet.) An outlet of the evaporator is coupled to exhaust line  67   a , and a sensor  40   a  that measures a thermodynamic property of the refrigerant flow and generates a signal (either directly or via the controller  17 ) to control expansion device  71  to control superheat. 
     However, rather than the liquid separator vapor outlet being coupled to exhaust refrigerant vapor, as discussed above ( FIGS.  3 ,  5 A- 5 D ), the liquid separator vapor outlet  28   b  can be coupled via junction  75   a  to the inlet to the refrigerant receiver  52  and the exhaust line  67 , such that discharged vapor from the compressor  59  feeds the receiver  52  (in this instance the receiver containing a refrigerant that is under pressure, e.g., ammonia) and also some or none of the vapor can be exhausted via the exhaust line  67 . In this embodiment there need not be any exhaust (from the compressor circuit, e.g., compressor and liquid separator vapor side) and obviates the need for a nitrogen (gas) receiver (discussed below). Pressure can be regulated by the use of the optional flow control device that will regulate the amount of vapor that is exhausted at line  67 . The compressor is configured to maintain high pressure in the gas refrigerant (ammonia) receiver  52 . The (OCRSCAEE)  50   f  will be configured to maintain vapor quality at the evaporator exit and amount of liquid in the liquid separator sufficient to operate the second and third evaporators. 
     The compressor/vacuum pump  59  and the back pressure regulator  53  control vapor pressure upstream of the compressor/vacuum pump  59 . Conduits  64   a - 64   f ,  64   i - 64   m  couple the various aforementioned items, as shown with the conduit  64   m  coupling the evaporator  39  outlet to the second exhaust line  67   a . Controller  17  can be included to control operation of, e.g., devices  56 ,  58 ,  59 ,  70  and  71 , etc., for instance. 
     The evaporators  32   a ,  32   b  operate in two phase (liquid/gas) and the third evaporator  33  operates in superheated region with controlled superheat. OCRSCAEE  50   f  includes the controllable expansion device  71  that has an inlet attached to the outlet of expansion valve  70  and has an outlet attached to the evaporator  39 . The expansion valve  71  has a control port that is fed from a sensor  40   a . The sensor  40   a  controls the expansion valve  71  and provides a mechanism to measure and control superheat. The OCRSCAEE  50   f  can also be viewed as including the three circuits  55   a ,  55   b ″ and  55   c  as described in  FIG.  5 B . 
     In  FIGS.  5 D,  5 E , the vapor quality of the refrigerant fluid after passing through evaporator  32  can be controlled either directly or indirectly with respect to a vapor quality set point by the controller  17 . In some embodiments, as shown in  FIGS.  5 D,  5 E , the system  10  includes a sensor  40  or  40   a  that provides a measurement of superheat, and indirectly, vapor quality. For example, in  FIG.  5 D , sensor  40  is a combination of temperature and pressure sensors that measure the refrigerant fluid superheat downstream from the heat load, and transmits the measurements to the controller (not shown). The controller adjusts the expansion valve device  70  based on the measured superheat relative to a superheat set point value. By doing so, controller indirectly adjusts the vapor quality of the refrigerant fluid emerging from evaporator  32 . 
     Referring now to  FIG.  5 F , the system  10  includes another alternative open-circuit refrigeration system (OCRSCAEE)  50   g . OCRSCAEE  50   g  includes the receiver  52 , (optional expansion valve  58  and optional solenoid control valve  56 ), ejector  66 , liquid separator  68 , the expansion device  70  and the evaporators  32   a ,  32   b ,  33 , and heat load  34   a ,  34   b  and  33   a , as discussed in  FIG.  5 E , (but without the expansion valve  71  of  FIG.  5 E ). In this embodiment the OCRSCAEE  50   g  includes the third evaporator  39  that shares the same expansion valve, i.e., expansion valve  70 , as the evaporators  32   a ,  32   b  via junction  75 . The evaporators  32   a ,  32   b  operate in two phase (liquid/gas) and evaporator  39  operates in superheated region with controlled superheat. 
     The second outlet  68   b  (vapor side outlet) of the liquid separator  68  is coupled via the compressor/vacuum pump  59  and a back pressure regulator  53  to the exhaust line  67  that may include an and junction  75   a  and optional flow control device that when included splits exhausted vapor into an inlet to the receiver  52  and out the exhaust line. (If the junction  75   a  and optional flow control device are not included, the exhaust is feed into the receiver inlet.) 
     However, rather than the liquid separator vapor outlet being coupled to exhaust refrigerant vapor, as discussed above ( FIGS.  3 ,  5 A- 5 D ), the liquid separator vapor outlet  28   b  can be coupled via junction  75   a  to the inlet to the refrigerant receiver  52  and the exhaust line  67 , such that discharged vapor from the compressor  59  feeds the receiver  52  (in this instance the receiver containing a refrigerant that is under pressure, e.g., ammonia) and also some or none of the vapor can be exhausted via the exhaust line  67 . In this embodiment there need not be any exhaust (from the compressor circuit, e.g., compressor and liquid separator vapor side) and obviates the need for a nitrogen (gas) receiver (discussed below). The compressor/vacuum pump  59  is configured to maintain high pressure in the gas refrigerant (ammonia) receiver  52 . The (OCRSCAEE)  50   g  will be configured to maintain vapor quality at the evaporator exit and amount of liquid in the liquid separator sufficient to operate the second and third evaporators. 
     The compressor/vacuum pump  59  and the back pressure regulator  53  control vapor pressure upstream of the compressor/vacuum pump  59 . In OCRSCAEP  50   g , the compressor/vacuum pump  59  is a control device that controls the vapor pressure from the liquid separator  68  and indirectly controls evaporating pressure/temperature. The back pressure regulator  53  regulates fluid pressure upstream from the regulator, i.e., regulates the pressure at the outlet pump at the inlet of the regulator  53  according to a set pressure point value to provide an additional degree of control of exhaust pressure at the exhaust line  67 . 
     Conduits  64   a - 64   f ,  64   h - 64   l  couple the various aforementioned items, as shown. Additional conduits (not referenced) couple the evaporator  33  to a second exhaust line  67   a  and second back pressure regulator. The OCRSCAEE  50   g  can also be viewed as including the three circuits  55   a ,  55   b ″ and  55   c , as described in  FIG.  5   . Controller  17  can be included to control operation of, e.g., devices  56 ,  58 ,  59 ,  53 ,  59  and  70 , for instance. 
     V. Thermal Management Systems with Open-Circuit Refrigeration Systems with Compressor Exhaust and Compressor Boost Assist 
     Referring now to  FIG.  6   , an alternative thermal management system  110  includes an open-circuit refrigeration system with compressor-assisted exhaust and pump boost assist (OCRSCAEP)  110   a  that has a refrigerant fluid flow path  115   a . The addition of a pump allows recirculation of non-evaporated refrigerant liquid in the evaporator  32  operating within the threshold of critical vapor quality. 
     In  FIG.  6   , embodiment  110   a  of the OCRSCAEP is one of several open-circuit refrigeration systems with vacuum pump/compressor and pump  110   a - 110   f  system configurations that will be discussed herein and includes a compressor or a vacuum pump  59  (functionally similar to the compressor or the vacuum pump arrangements  19  and  59  of  FIGS.  1 ,  2 A- 2 F,  3  and  5 A- 5     f ). OCRSCAEP  110   a  is one of several open-circuit refrigeration vacuum assist with pump system configurations  110   a - 110   g  that will be discussed herein. 
     OCRSCAEP  110   a  includes a receiver  112  that is configured to store liquid refrigerant, i.e., subcooled liquid refrigerant. OCRSCAEP  110   a  also includes an optional first control device, e.g., an optional solenoid control valve  116 , and an optional control device, e. g., an expansion valve  118 . OCRSCAEP  110   a  includes a junction device  136  that has first and second ports configured as inlets, and a third port configured as an outlet. A first one of the inlets of the junction device  136  is coupled to an outlet of the receiver  112  and the second one of the inlets of the junction device  136  is coupled to an outlet of a pump  100 . An inlet of the optional solenoid control valve  116  (if used) is coupled to the outlet of the receiver  112 . Otherwise, the outlet of receiver is coupled directly to the one of the inlets of the junction device  136  and the outlet of the junction device  136  feeds an inlet of the second control device, e. g., the expansion valve  118  (if used) or, if nether solenoid control valve  116  nor the expansion valve  118  is used, the outlet of the junction device  136  is coupled to an inlet of an evaporator  32 . When the expansion valve  118  is used, the outlet of the junction device  136  is coupled to the inlet of the expansion valve  118  and the outlet of the expansion device is coupled to the inlet of the evaporator  32 , as shown. 
     OCRSCAEP  110   a  in some embodiments also includes a controller  17  that produces control signals to control operation of the various ones of valves  116 ,  118 , etc., pump  100 , and compressor/vacuum pump  59 , as needed. Controller  17  is described in  FIG.  15   . 
     Referring momentarily to  FIGS.  6 A- 6 D  that show details of the junction device  136  and alternative locations for the junction device  136 . 
       FIG.  6 A  shows a diagrammatical view of the junction device  136  having at least three ports any of which could be inlets or outlets. Generally, in the configurations below two of the ports would be inlets and one would be an outlet and refrigerant flows from the two ports acting as inlets would be combined and exit the outlet. 
       FIG.  6 B  shows the location for the junction device  136 , as set out in  FIG.  6   , having one of the inlets and the outlet interposed between solenoid valve  116  and expansion valve  118 , and having its other inlet coupled to the outlet of the evaporator  32 . 
       FIG.  6 C  shows another alternative location for the junction device  136  having one of the inlets and the outlet interposed between the outlet of the expansion valve  118  and the evaporator  32  ( FIG.  2   ) or liquid separator  28  ( FIG.  3   ) and having its other inlet coupled to the outlet of the evaporator  32 . 
     Any of the configurations that will be discussed below in  FIGS.  7 A to  7 E  can have the junction device  136  placed in the various locations shown in  FIG.  6 B or  6 C . If both of the optional solenoid control valve  116  and optional expansion valve  118  are not included, then all of the locations for the junction device  136  are, in essence, the same provided that there are no other intervening functional devices between the outlet of the receiver  112  and the inlet (that is in the refrigerant flow path  115   a ) of the junction device  136 . 
     Returning to  FIG.  6   , the OCRSCAEP  110   a  also includes an evaporator  32  that has an inlet coupled to an outlet of the expansion valve  118 . The evaporator  32  also has an outlet coupled to an inlet  128   a  of a liquid separator  128 . The liquid separator  128  in addition to the inlet  128   a , has a first outlet (vapor side outlet)  128   b  and a second outlet  128   c  (liquid side outlet). The first outlet  128   b  of the liquid separator  128  is coupled to an inlet (not referenced) of third control device, such as a compressor or vacuum pump  119  that controls a vapor pressure in the evaporator  32 . The compressor or vacuum pump  119  has an outlet (not referenced) that feeds an exhaust line  27 . The second outlet of the liquid separator  128  is coupled to an inlet of a pump  100 . An output of the pump  100  is coupled to the second input of the junction device  136 . In the liquid separator  128  only or substantially only liquid exits the liquid separator at outlet  128   c  (liquid side outlet) and only or substantially only vapor exits the separator  128  at outlet  128   b  the (vapor side outlet). Conduits  124   a - 124   k  couple the various aforementioned items, as shown. 
     The OCRSCAEP  110   a  can be viewed as including three circuits. A first circuit  115   a  being the refrigerant flow path  115   a  that includes the receiver  112  and two downstream circuits  115   b  and  115   c  that are downstream from the liquid separator  128 . Downstream circuit  115   b  carries liquid from the liquid separator  128  via the pump  100 , which liquid is pumped back into the evaporator  32  indirectly via the junction device  136  and the downstream circuit  115   c  that includes the back pressure regulator  29 , which exhausts vapor via the exhaust line  27 . Receiver  112  is typically implemented as an insulated vessel. 
     The compressor/vacuum pump  119  at the vapor side outlet  128   b  of the liquid separator  128  generally functions to control the vapor pressure upstream of the compressor or vacuum pump  119 . In OCRSCAEP  110   a , compressor or vacuum pump  119  is a control device that controls the vapor pressure from the liquid separator  128  and indirectly controls evaporating pressure/temperature. Various types of pumps can be used for compressor/vacuum pump  119 . The compressor or vacuum pump  119  handles vapor and regulates fluid pressure upstream from the regulator, i.e., regulates the pressure at the inlet to the regulator  29  according to a set pressure point value. 
     Various types of pumps can be used for pump  100 . Exemplary types include gear, centrifugal, rotary vane, types. When choosing a pump, the pump should be capable to withstand the expected fluid flows, including criteria such as temperature ranges for the fluids, and materials of the pump should be compatible with the properties of the fluid. A subcooled refrigerant can be provided at the pump  100  outlet to avoid cavitation. To do that a certain liquid level in the liquid separator  128  may provide hydrostatic pressure corresponding to that sub-cooling. 
     The evaporator  32  can be implemented in a variety of ways as discussed above and as will be further discussed below. 
     In  FIG.  6   , the evaporator  32  is coupled to the inlet  128   a  of the liquid separator  128  and to an outlet of the expansion device  116 . The liquid refrigerant from the refrigerant receiver  112  mixes with an amount of pumped refrigerant from the pump  100 , and expands at a constant enthalpy in the expansion device  116 . The expansion device  116  turns the liquid into a two-phase mixture. The two-phase mixture stream enters the evaporator  32 . The evaporator  32  absorbs heat from the heat load. A liquid/vapor refrigerant flow from the evaporator enters the liquid separator  128 . The liquid stream exiting the liquid separator  128  is pumped by the pump  100  back into the expansion device  118  via the junction device  136 . In this configuration, the pump  100  indirectly pumps a secondary refrigerant fluid flow, e.g., a recirculation liquid refrigerant flow from the evaporator  32 , via the liquid separator  128 , back via the expansion device  118  into the evaporator  32 . 
     If the junction  136  is upstream of the expansion device  118 , in some cases the pump  100  may return a portion of the liquid refrigerant from the liquid separator  128 , effectively back to the receiver  14  (via the junction device  136 ) so long as the remaining liquid column in the liquid separator remains sufficiently high to permit substantially cavitation-free operation of the pump  100 . 
     The evaporator  32  may be configured to maintain exit vapor quality below the so called “critical vapor quality” defined as “1”, as discussed above. Any vapor that may be included in the refrigerant stream will be discharged at the vapor phase outlet of the liquid separator  128 . Refrigerant vapor exits from the vapor side outlet  128   b  of the liquid separator  128  and is exhausted by the exhaust line  97 . The compressor or vacuum pump  119 , regulates the pressure upstream of the evaporator  32 , so as to maintain upstream refrigerant fluid pressure in the OCRSCAEP  110   a.    
     As mentioned above, the OCRSCAEP  110   a  of  FIG.  6   , is one of several alternative system architectures that have a liquid separator  128 , compressor/vacuum pump  119  and liquid pump  100  as part of the OCRSCAEP cooling system. 
     Referring now to  FIG.  7 A , an alternative open-circuit refrigeration system with pump (OCRSCAEP)  110   b  configuration is shown. OCRSCAEP  110   b  includes receiver  112 , (optional solenoid control valve  116 , optional expansion valve  118 ), evaporator  32 , liquid separator  128 , pump  100 , and compressor/vacuum pump  119  coupled to the exhaust line  97 , as discussed above. OCRSCAEP  110   b  also includes the junction device  136 . The junction device  136  has one port as an inlet coupled to the outlet of the pump  100 , and a second port as an outlet coupled to the inlet to the evaporator, but in OCRSCAEP  110   b , the junction device  136  has the third port functioning as a second inlet coupled to the output of the expansion valve  118 . Conduits  124   a - 124   i  couple the various aforementioned items as shown. 
     In OCRSCAEP  110   b , the pumped liquid from the pump  100  is fed directly into the inlet to the evaporator  32  along with the primary refrigerant flow from the expansion valve  118 . These liquid refrigerant steams from the refrigerant receiver and the pump are mixed downstream from the expansion valve  118 . 
     The heat load  34  is coupled to the evaporator  32 . The evaporator  32  is configured to extract heat from heat load  34  that is in contact with the evaporator  32  and to control the vapor quality at the outlet of the evaporator. 
     The OCRSCAEP  110   b  can also be viewed as including three circuits. The first circuit  115   a  being the refrigerant flow path and the two circuits  115   b  and  115   c  as in  FIG.  1   . 
     The OCRSCAEP  110   b  operates as follows. The liquid refrigerant from the receiver  112  is fed to the expansion valve  118  and expands at a constant enthalpy in the expansion valve  118  turning into a two-phase (gas/liquid) mixture. This two-phase liquid/vapor refrigerant stream and the pumped liquid refrigerant stream from the pump  100 , enters the evaporator  32  that provides cooling duty, and discharges the refrigerant in a two-phase state at a relatively high exit vapor quality (fraction of vapor to liquid, as discussed above). The discharged refrigerant is fed to the inlet of the liquid separator  128 , where the liquid separator  128  separates the discharge refrigerant with only or substantially only liquid exiting the liquid separator at outlet  128   c  (liquid side outlet) and only or substantially only vapor exiting the separator  128  at outlet  128   b  the (vapor side outlet). The liquid stream exiting at outlet  128   c  enters and is pumped by the pump  100  into the second inlet of the junction. Vapor from the vapor side of the liquid separator  128  is fed to the compressor or vacuum pump  119  to the exhaust line. 
     OCRSCAEP  110   b  provides an operational advantage over the embodiment of OCRSCAEP  110   a  ( FIG.  6   ) since the pump  100  can operate across a reduced pressure differential (pressure difference between inlet and outlet of the pump  100 ). In the context of open-circuit refrigeration systems, the use of the pump  100  allows for some recirculation of liquid refrigerant from the liquid separator  128  to enable operation at reduced vapor quality at the evaporator  32  outlet, that also avoids discharging remaining liquid out of the system at less than the separation efficiency of the liquid separator  128  allows. That is, by allowing for some recirculation of liquid phase refrigerant, but without the need for a compressor and condenser, as in a closed cycle refrigeration system, this recirculation reduces the required amount of refrigerant needed for a given amount of cooling over a given period of operation relative to open-circuit systems without recirculation, while also potentially reducing, size, power and weight characteristics relative to closed-circuit systems. 
     The configuration above reduces the vapor quality at the evaporator  32  inlet and thus may improve refrigerant distribution (of the two phase mixture) in the evaporator  32 . 
     During start-up both OCRSCAEP  110   a  and OCRSCAEP  110   b  ( FIGS.  6 ,  7 A ) need to charge the evaporator  32  with liquid refrigerant. However, in both OCRSCAEP  110   a  and OCRSCAEP  110   b , by placing the evaporator  32  between the outlet of the expansion device and the inlet of the liquid separator, these configurations avoid the necessity of having liquid refrigerant first pass through the liquid separator  128  during the initial charging of the evaporator  32  with the liquid refrigerant, in contrast with the OCRSCAEP  110   c  ( FIG.  7 B ). At the same time, liquid refrigerant that is trapped in the liquid separator  128  may be wasted after the OCRSCAEP  110   b  shuts down. 
     Referring now to  FIG.  7 B , the system  10  includes another alternative open-circuit refrigeration system with pump (OCRSCAEP)  110   c . OCRSCAEP  110   c  includes the receiver  112 , the compressor or vacuum pump  119  and the second receiver  14  as discussed for  FIG.  6   . OCRSCAEP  110   c  also includes solenoid control valve  116 , expansion valve,  118 , liquid separator  128 , pump  100 , and compressor/vacuum pump  119 , coupled to the exhaust line  97 , as discussed above. 
     OCRSCAEP  110   c  also includes the junction device  136  and evaporator  32 . The junction device  136  has one port as an inlet coupled to the outlet of the expansion valve  118 , a second port as an outlet coupled to the inlet  128   a  of the liquid separator  128  and has a third port as a second inlet coupled to the evaporator  32 . Outlets  128   b  and  128   c  of the liquid separator are coupled as discussed above. OCRSCAEP  110   c  has the inlet to the evaporator  32  coupled to the output of the pump  100  and has the outlet coupled to the second inlet of the junction device  136 . A heat load  34  is coupled to the evaporator  32 . The evaporator  32  is configured to extract heat from heat load  34  that is in contact with the evaporator  32 . Conduits  24   a - 24   m  couple the various aforementioned items as shown. 
     Vapor quality downstream from the expansion valve  118  is higher than the vapor quality downstream from the pump  100 . An operating advantage of the OCRSCAEP  110   c  is that by placing the evaporator  32  downstream from the pump  100  better refrigerant distribution is provided with this component configuration since liquid refrigerant enters the evaporator  32  rather than a liquid/vapor stream. 
     The OCRSCAEP  110   c  can also be viewed as including three circuits, the first circuit  115   a  being the refrigerant flow path and the other two being the circuits  115   b  and  115   c , as in  FIG.  1   . 
     Evaporators of the configurations  110   a - 110   b  ( FIGS.  6  and  7 A ) operate below a vapor quality of 1. These architectures are not very sensitive to the pumping flow capacity and do not need a precise flow control, i.e., a constant speed pump configured to meet highest load requirements can be employed for pump  100 . 
     The evaporator  32  of the configuration in  FIG.  7 B  may allow a superheat. The configuration of  FIG.  7 B  may be sensitive to the pumping flow capacity. If the evaporator of  FIG.  7 B  is configured to strictly maintain vapor quality at the evaporator exit, vapor quality control may be provided by a variable speed pump (not shown) controlled by the controller  17  ( FIG.  15   ) acting on a value of vapor quality that is sensed downstream from the evaporator  32 . If the evaporator  32  of  FIG.  7 B , is configured to operate in the range extended into the superheated region and the pump  100 , the superheat control may be provided by a variable speed pump and a controller acting on pressure and temperatures sensed downstream from the evaporator. 
     Referring now to  FIG.  7 C , the system  10  can include another alternative open-circuit refrigeration system with pump (OCRSCAEP)  110   d . OCRSCAEP  110   d  includes the receiver  112 , optional solenoid control valve  116 , optional expansion valve  118 , pump  100 , liquid separator  128 , and compressor or vacuum pump  119 , coupled to the exhaust line  97 , as discussed above. OCRSCAEP  110   d  also includes the junction device  136 , a first evaporator  32   a  and a second evaporator  32   b  (or can be a single evaporator as in  FIG.  5 C ). The junction device  136  has a first port as an inlet coupled to the outlet of the expansion valve  118 . The junction device  136  has a second port as an outlet coupled to an inlet of the first evaporator  32   a , with the first evaporator  32   a  having an outlet coupled to the inlet of the liquid separator  128 , and the junction device  136  has a third port as a second inlet coupled to an outlet of the evaporator  32   b  with the evaporator  32   b  having an inlet that is coupled to the outlet of the pump  100 . A heat load  34   a  is coupled to the evaporator  32   a  and a heat load  34   b  is coupled to the evaporator  32   b . The evaporators  32   a ,  32   b  are configured to extract heat from the respective loads  34   a ,  34   b  that are in contact with the corresponding evaporators  32   a ,  32   b . Conduits  24   a - 24   k  couple the various aforementioned items as shown. 
     An operating advantage of the OCRSCAEP  110   d  is that by placing evaporators  32   a ,  32   b  at both the outlet and the second inlet of the junction device  136 , it is possible to combine loads which require operation in two-phase region and which allows operation with a superheat. 
     The OCRSCAEP  110   d  can also be viewed as including three circuits, the first circuit  115   a  being the refrigerant flow path as in  FIG.  1    and two circuits  115   b ″ and  115   c . Circuit  115   b ″ being upstream and downstream from the liquid separator  128 , carries liquid from the liquid outlet of the liquid separator  128  and carrying vapor/liquid from the evaporator  32   a  into the inlet of the liquid separator  128 . The downstream circuit  115   c  exhausts vapor via the back pressure regulator  29  to the exhaust line  27 . 
     Referring now to  FIG.  7 D , the system  10  can include another alternative open-circuit refrigeration system with pump (OCRSCAEP)  110   e . OCRSCAEP  110   e  includes the receiver  112 , the optional expansion valve  118 , and optional solenoid control valve  116 , pump  100 , liquid separator  128 , and compressor or vacuum pump  119 , coupled to the exhaust line  97 , as discussed above. 
     The OCRSCAEP  110   e  also includes a single evaporator  32   c  that is attached downstream from and upstream of the junction device  136 . A first heat load  34   a  is coupled to the evaporator  32   c . The evaporator  32   c  is configured to extract heat from the first load  34   a  that is in contact with the evaporator  32   c . A second heat load  34   b  is also coupled to the evaporator  32   c . The evaporator  32   c  is configured to extract heat from the second load  34   a  that is in contact with the evaporator  32   c . The evaporator  32   c  has a first inlet that is coupled to the outlet  136   c  of the junction device  136  and a first outlet that is coupled to the inlet  128   a  of the liquid separator  128 . The evaporator  32   c  has a second inlet that is coupled to the outlet of the pump  100  and has a second outlet that is coupled to the inlet  136   b  of the junction device  136 . The second outlet  128   b  (liquid side outlet) of the liquid separator  128  is coupled via the compressor or vacuum pump  119  to the exhaust line  97 . Conduits  124   a - 124   i  couple the various aforementioned items, as shown. 
     In this embodiment, the single evaporator  32   c  is attached downstream from and upstream of the junction  136  and requires a single evaporator in comparison with the configuration of  FIG.  7 C  having the two evaporators  32   a ,  32   b . The OCRSCAEP  110   e  can also be viewed as including the three circuits  115   a ,  115   b ″ and  115   c  as described in  FIG.  4   . 
     Referring now to  FIG.  7 E , the system  10  includes an alternative open-circuit refrigeration system with pump (OCRSCAEP)  110   f . OCRSCAEP  110   f  includes the receiver  112 , optional expansion valve  118 , optional solenoid control valve  116 , pump  100 , liquid separator  128 , and compressor or vacuum pump  119  coupled to the exhaust line  97 . However, rather than the outlet of the compressor being coupled to the exhaust line  97  to exhaust refrigerant vapor, as discussed above, the outlet of the compressor is coupled via (optional) junction  75   a  to an inlet to the refrigerant receiver  112  and to an optional flow control device, such that discharge from the compressor/vacuum pump  119  feeds the receiver  112  (in this instance the receiver containing a refrigerant that is under pressure, e.g., ammonia) and can be optionally exhausted. In this embodiment there need not be any exhaust (from the compressor circuit, e.g., compressor and liquid separator vapor side) that may obviate a need for a nitrogen (gas) receiver (discussed below). The compressor or vacuum pump  119  is configured to maintain high pressure in the gas refrigerant (ammonia) receiver  112 . The (OCRSCAEP)  110   f  will be configured to maintain vapor quality at the evaporator exit and amount of liquid in the liquid separator sufficient to operate the second evaporator. The OCRSCAEP  110   f  also includes the evaporators  32   a ,  32   b  (or can be a single evaporator as in  FIG.  7 D ). 
     In this embodiment, the OCRSCAEP  110   f  also has the liquid separator  128  configured to have a second outlet  128   d  (such a function could be provided with another junction device). The second outlet  128   d  diverts a portion of the liquid exiting the liquid separator  128  into a third evaporator  32   c  that is in thermal contact with a heat load  35  and which extracts heat from the heat load  35  and exhausts vapor from a second vapor exhaust line  97   a.    
     An operating advantage of the OCRSCAEP  110   f  is that by placing evaporators  32   a ,  32   b  at both the outlet and the second inlet of the junction device  136 , it is possible to run the evaporators  32   a ,  32   b  with changing refrigerant rates through the junction device  136  to change at different temperatures or change recirculating rates. By using the evaporators  32   a ,  32   b , the configuration reduces vapor quality at the outlet of the evaporator  32   b  and thus increases circulation rate, as the pump  100  would be ‘pumping’ less vapor and more liquid. That is, with OCRSCAEP  110   d  the evaporator  32   b  is downstream from the pump  100  and better refrigerant distribution could be provided with this component configuration since liquid refrigerant enters the evaporator  32   b  rather than a liquid/vapor stream as could be for the evaporator  32   a.    
     In addition, some heat loads that may be cooled by an evaporator in the superheated phase region, at the same time do not need to actively control superheat. The open-circuit refrigeration system  110   f  employs the additional evaporator circuit  33 , with an evaporator cooling heat loads in two-phase and superheated regions. The exhaust lines may or may not be combined. The third evaporator  33  can be fed a portion of the liquid refrigerant and operate in superheated region without the need for active superheat control. 
     The OCRSCAEP  110   f  can also be viewed as including the three circuits  115   a ,  115   b ″ as described above, a third circuit  115   c ′ (modified “exhaust” having the feedback to the receiver  112 ) and a fourth circuit  15   d  being the evaporator  33  and exhaust line  27   a . Controller  17  can be included to control operation of, e.g., devices  116 ,  118 ,  100 , and  119 , for instance. 
     Referring now to  FIG.  7 F , the system  10  includes an alternative open-circuit refrigeration system with pump (OCRSCAEP)  110   g . OCRSCAEP  110   g  includes the refrigerant receiver  112 , optional expansion valve  118  and solenoid control valve  116 , the pump  100 , the liquid separator  128 , and the compressor/vacuum pump  119  coupled to the exhaust line  97 . As with  FIG.  7 E , However, rather than the outlet of the compressor being coupled to the exhaust line  97  to exhaust refrigerant vapor, as discussed above, the outlet of the compressor is coupled via (optional) junction  75   a  to an inlet to the refrigerant receiver  112  and to an optional flow control device, such that discharge from the compressor/vacuum pump  119  feeds the receiver  112  (in this instance the receiver containing a refrigerant that is under pressure, e.g., ammonia) and can be optionally exhausted However, rather than the outlet of the compressor being coupled to the exhaust line  97  to exhaust refrigerant vapor, as discussed above, the outlet of the compressor is coupled via (optional) junction  75   a  to an inlet to the refrigerant receiver  112  and to an optional flow control device, such that discharge from the compressor/vacuum pump  119  feeds the receiver  112  (in this instance the receiver containing a refrigerant that is under pressure, e.g., ammonia) and can be optionally exhausted. In this embodiment there need not be any exhaust (from the compressor circuit, e.g., compressor and liquid separator vapor side  128   b ) and thus may obviate or reduce the need for a nitrogen (gas) receiver (discussed below). The OCRSCAEP  110   g  also includes the evaporators  32   a ,  32   b  (or can be a single evaporator as in  FIG.  7 D ). The compressor  119  is configured to maintain high pressure in the gas refrigerant (ammonia) receiver  112 . The (OCRSCAEP)  110   g  can be configured to maintain vapor quality at the evaporator exit and amount of liquid in the liquid separator  128  sufficient to operate the second evaporator  32   b.    
     In this embodiment, the OCRSCAEP  110   g  also has the liquid separator  128  configured to have the second outlet  128   d  (such a function could be provided with another junction device). The second outlet  128   d  diverts a portion of the liquid exiting the liquid separator  128  into the third evaporator  32   c  that is in thermal contact with the heat load  35 , and which extracts heat from the heat load  35 , and exhausts vapor from a second vapor exhaust line  97   a.    
     OCRSCAEP  110   g  also includes a second expansion device  147  having an inlet coupled to the second outlet of the liquid separator  128  and having an outlet coupled to the inlet to the third evaporator  32   c . OCRSCAEP  110   g  also includes a sensor device  145 . The expansion valve  147  has a control port that is fed from the sensor  145  or controller  17 , which control the expansion valve  147  and provide a mechanism to measure and control superheat. The sensor  145  is disposed approximate to the outlet of the evaporator  32   c  and provides a measurement of superheat, and indirectly, vapor quality. For example, sensor  145  is a combination of temperature and pressure sensors that measure the refrigerant fluid superheat downstream from the heat load, and transmits the measurements to the controller  17 . The controller  17  produces control signals to adjust the expansion valve device  147  based on the measured superheat relative to a superheat set point value in response to signal(s) from the sensor  145 . By doing so, controller indirectly adjusts the vapor quality of the refrigerant fluid emerging from evaporator  33 . Conduits  24   a - 24   m  couple the various aforementioned items, as shown. The evaporators  32   a ,  32   b  operate in two phase (liquid/gas) and the third evaporator  32   c  operates in superheated region with controlled superheat. 
     The OCRSCAEP  110   g  can also be viewed as including the three circuits  115  and  115   b ″ and as described in  FIG.  4   , the third circuit  115   c ′ (modified “exhaust” having the feedback to the receiver  112 ) and a fourth circuit  15   d  being the evaporator  33  and exhaust line  27   a . Controller  17  can be included to control operation of, e.g., devices  116 ,  118 ,  100 ,  29   a ,  119  and  147 , for instance. 
     VI. Thermal Management Systems with Open-Circuit Refrigeration Systems with Compressor Exhaust and Pump Boost Assist with Gas Induced Compression of Refrigerant 
     The system  10  can use a different family of open-circuit refrigeration systems. For example, each of the Open-circuit Refrigeration Systems with Compressor Exhaust (OCRSCE) configurations  10   a - 10   g  or the Open-circuit Refrigeration Systems with Compressor Exhaust and Ejector Boost Assist (OCRSCAEE) configurations  50   a - 50   g  or the Open-circuit Refrigeration Systems with Compressor Exhaust and Pump Boost Assist (OCRSCAEP) configurations  110   a - 110   g  can be further configured with a second receiver  14 . 
     Referring now to  FIG.  8   , using OCRSCE  10   a  ( FIG.  1   ) as exemplary, an OCRSCE  10   a ′ is shown with the features of OCRSCE  10   a  ( FIG.  1   ) and with a second receiver  14  that stores a gas, and which is coupled, via conduit  24   k  to a control device  13  configurable to control a flow of gas from the second receiver  14 , via conduit  24   l  to the first receiver  12  regulate pressure in and control refrigerant flow from the first receiver  12 . This embodiment uses the compressor  19  (as in  FIGS.  1 ,  2 A- 2 F ) that can maintain a required level of refrigerant pressure in the receiver  12 , however, use of the compressor  19  does not preclude using the receiver  14  for also maintaining a required level of refrigerant pressure in the receiver  12 . 
     The control device  13  can be a pressure regulator that regulates a pressure at an outlet of the pressure regulator  13 . Pressure regulator  13  generally functions to control the gas pressure from gas receiver  14  that is upstream of the refrigerant receiver  12 . Transporting a gas from the gas receiver  14  into the refrigerant receiver  12  through pressure regulator  13 , either prior to or during transporting of the refrigerant fluid from the refrigerant receiver  12 , functions to control the pressure in the refrigerant receiver  12  and the refrigerant fluid pressure upstream from the evaporator  32  ( FIG.  1   ) especially when the optional valves  16  and  18  are not used. 
     Pressure regulator  13  would be used at the outlet of the receiver  14  to regulate the pressure in the refrigerant receiver  12 . For example, the pressure regulator  13  could start in a closed position, and as refrigerant pressure in the receiver  12  drops the pressure regulator  13  can be controlled to start opening to allow gas from the receiver  14  to flow into the receiver  14  to substantially maintain a desired pressure in the receiver  12  and thus provide a certain sub-cooling of the refrigerant in the receiver  12 , and a certain refrigerant mass flow rate through the expansion device  16 , and evaporator  32 , and, as a result, a desired cooling capacity for one or more heat loads  34 . 
     In general, pressure regulator  13  can be implemented using a variety of different mechanical and electronic devices. Typically, for example, pressure regulator  13  can be implemented as a flow regulation device that will match an output pressure to a desired output pressure setting value. In general, a wide range of different mechanical and electrical/electronic devices can be used as pressure regulator  13 . Typically, a mechanical pressure regulator includes a restricting element, a loading element, and a measuring element. The restricting element is a valve that can provide a variable restriction to the flow. The loading element, e.g., a weight, a spring, a piston actuator, etc., applies a needed force to the restricting element. The measuring element functions to determine when the inlet flow is equal to the outlet flow. 
     Considerations when the receiver  12  stores a refrigerant such as ammonia are discussed below. 
     Referring now to  FIG.  8 A , while the second receiver  14  that stores the gas to regulate pressure in the first receiver  12  was described for the embodiment of  FIG.  1   , each of the embodiments described in  FIGS.  2 - 3 ,  5 A- 7 D ) can use the gas receiver  14  to maintain pressure in the refrigerant receiver  12  ( FIGS.  2 A- 2 F ) or  52  ( FIGS.  3 ,  5 A- 5 D ) or  112  ( FIGS.  6 ,  7 A- 7 E ). The embodiments discussed in  FIGS.  5 E- 5 F and  7 E- 7 F  generally would not need the second receiver  14 , as the exhaust from the compressor or vacuum pump ( 59  or  119 ) is used to maintain vapor pressure in the respective receivers  52 ,  112 . 
     For each of the configurations, the refrigerant flow is controlled either solely by the compressor/vacuum pump  29 ,  59  or  119  or the combination of the expansion device  18 ,  58 ,  118 , and the compressor/vacuum pump  29 ,  59  or  119  or those components in combination with the other optional components. The control strategies of those controls depends on requirements of the application, e.g., ranges of mass flow rates, cooling requirements, receiver capacity, ambient temperatures, heat load, etc. 
     When ambient temperature is very low and, as a result, pressure in the receiver  12  is low and possibly insufficient to drive refrigerant fluid flow through any of the open-circuit refrigeration systems discussed above, the gas from the gas receiver  14  is used to compress the liquid refrigerant in the receiver  12 . The gas pressure supplied by the gas receiver  14  compresses liquid refrigerant in the receiver  12  and maintains the liquid refrigerant in a sub-cooled state even at high ambient and liquid refrigerant temperatures. 
     Referring to  FIG.  9   , in some embodiments, refrigerant receiver  12  is positioned inside gas receiver  14 , as shown. An example of a gas receiver  14  with an internal refrigerant receiver  12  has an exit port  160  that allows gas to exit gas receiver into the refrigerant receiver  12 . and a check valve  150  positioned at the inlet of refrigerant receiver  12  ensure that gas flows into the refrigerant receiver  12  but refrigerant does not flow backward into gas receiver  14  from refrigerant receiver  12 . Refrigerant fluid leaves refrigerant receiver  12  through outlet  152 . Refrigerant receiver  12  is charged with refrigerant fluid through inlet  154 , while gas receiver  14  is charged with gas through charging port  156 . An optional pressure sensor  158  can be positioned in charging port  156 . 
     In certain embodiments, a combined refrigerant and gas receiver (i.e., a single receiver, not shown) is charged with both refrigerant fluid and gas. Because the refrigerant fluid is entirely in a liquid phase, the refrigerant fluid rests on the bottom of the refrigerant receiver  12 , while the gas occupies the portion of the internal volume above the liquid refrigerant fluid. During operation, the refrigerant fluid leaves through an outlet at the bottom of receiver, while the gas remains in receiver. In some embodiments the receiver is configured in such a way and initial gas charge is such that the gas pressure in the receiver will be always high enough to maintain the remaining refrigerant at the bottom in sub-cooled state. Otherwise, which is also an option, liquid refrigerant will evaporate; the gas will mix with evaporated vapor. The total pressure above the liquid phase will be equal to the sum of partial nitrogen pressure and partial vapor refrigerant pressure. 
     A variety of different gases can be introduced into gas receiver  14  to maintain the pressure in refrigerant receiver  12 . In general, gases that are used are inert (or relatively inert) with respect to the refrigerant fluid. As an example, when a refrigerant fluid such as ammonia is used, suitable gases that can be introduced into gas receiver  14  include, but are not limited to, one or more of nitrogen, argon, xenon, and helium. 
     It should be appreciated that while any of the open-circuit systems discussed herein are shown with a single gas receiver  14  and single refrigerant receiver  12  more generally such systems can include any number of gas receivers and refrigerant receivers (and associated components, e.g., control devices, check valves, ports, and sensors) discussed above. 
     Refrigerants and Considerations for Choosing Configurations 
     As discussed above any two-phase refrigerant (liquid/gas phases) can be used. Of particular interest are water and ammonia. 
     Water&#39;s latent heat is significantly higher than the latent heat of ammonia. Therefore, water as the refrigerant provides an open-circuit refrigeration system (generally OCRS) having a significantly lower mass flow rate demand than, e.g., ammonia and thus the OCRS would require less water to complete a given task than that of ammonia. The density of water density is significantly higher than the density of liquid ammonia, and thus the receiver  12  could be configured to either occupy a much smaller volume or store significantly more refrigerant than a receiver storing ammonia, for a given task. 
     When any of the open-circuit refrigeration configurations use water as the refrigerant it is more likely that the use of the second (gas receiver  14 ) would not be necessary for most applications. When the receiver  12  uses water, the receiver  12  may be charged with the water at atmospheric pressure. During operation, the water is enthalpically expanded into a vacuum pressure corresponding to a desired evaporating temperature (e.g., 20° C.), cools the load, evaporates, and vapor is discharged by the vacuum pump or compressor  19  into ambient environment. If water applied to cool evaporating temperatures at 100° C. and higher the OCRS operating pressures are above atmospheric pressures. 
     Ammonia on the other hand under standard conditions of pressure and temperature is in a liquid or two-phase state. Thus, the receiver  12  typically will store ammonia at a saturated pressure corresponding to the surrounding temperature. Nitrogen (or other gases) under standard conditions of pressure and temperature is stored in a gaseous state. The gas receiver  14  serves to help elevate pressure in the ammonia receiver above saturation pressure and to maintain ammonia in a subcooled liquid state. The pressure in the receiver  12  storing ammonia will change during operation when there is no gas receiver  14  controlling the pressure, which complicates the control function of the expansion control device (expansion valve)  18  that will thus receive the refrigerant flow at a reducing pressure. The use of the gas receiver  14  can stabilize pressure in the receiver  12  during operation, by adjusting the expansion control device  18  (e.g., automatically or by controller  17 ) based on a measurement of the evaporation pressure (pe) of the refrigerant fluid and/or a measurement of the evaporation temperature of the refrigerant fluid. With expansion control device  18  adjusted in this manner, the compressor or vacuum pump  119  can be adjusted (e.g., automatically or by controller  17 ) based on measurements of one or more of the following system parameter values: the pressure drop across expansion control device  18 , the pressure drop across evaporator  32 , the refrigerant fluid pressure in receiver  12 , the vapor quality of the refrigerant fluid emerging from evaporator  32  (or at another location in the system), the superheat value of the refrigerant fluid, and the temperature of heat load  34 . 
     In certain embodiments, expansion control device  18  is adjusted (e.g., automatically or by controller  17 ) based on a measurement of the temperature of heat load  34 . With expansion control device  18  adjusted in this manner, compressor vacuum pump  19  can be adjusted (e.g., automatically or by controller  17 ) based on measurements of one or more of the following system parameter values: the pressure drop across expansion control device  18 , the pressure drop across evaporator  32 , the refrigerant fluid pressure in receiver  12 , the vapor quality of the refrigerant fluid emerging from evaporator  32  (or at another location in the system), the superheat value of the refrigerant fluid, and the evaporation pressure (pe) and/or evaporation temperature of the refrigerant fluid. 
     In some embodiments, controller  17  controls the compressor/vacuum pump  19  based on a measurement of the evaporation pressure p e  of the refrigerant fluid downstream from expansion control device  18  (e.g., measured by sensor) and/or a measurement of the evaporation temperature of the refrigerant fluid (e.g., measured by sensor). With compressor vacuum/pump  19  adjusted based on this measurement, controller  17  can adjust expansion control device  18  based on measurements of one or more of the following system parameter values: the pressure drop (p r -p e ) across expansion control device  18 , the pressure drop across evaporator  32 , the refrigerant fluid pressure in receiver  12  (p r ), the vapor quality of the refrigerant fluid emerging from evaporator  32  (or at another location in the system), the superheat value of the refrigerant fluid in the system, and the temperature of heat load  34 . 
     In certain embodiments, controller  17  adjusts compressor/vacuum pump  19  based on a measurement of the temperature of heat load  34  (e.g., measured by a sensor). Controller  17  can also adjust expansion control device  18  based on measurements of one or more of the following system parameter values: the pressure drop (p r -p e ) across first control device  18 , the pressure drop across evaporator  32 , the refrigerant fluid pressure in receiver  12  (p r ), the vapor quality of the refrigerant fluid emerging from evaporator  32  (or at another location in the system), the superheat value of the refrigerant fluid in the system, the evaporation pressure (p e ) of the refrigerant fluid, and the evaporation temperature of the refrigerant fluid. 
     To adjust either control device  13  or compressor/vacuum pump  19  based on a particular value of a measured system parameter value, controller  17  compares the measured value to a set point value (or threshold value) for the system parameter, as will be discussed below. 
     A variety of different refrigerant fluids can be used in any of the OCRSCAEP configurations. For open-circuit refrigeration systems in general, emissions regulations and operating environments may limit the types of refrigerant fluids that can be used. For example, in certain embodiments, the refrigerant fluid can be ammonia having very large latent heat; after passing through the cooling circuit, vaporized ammonia that is captured at the vapor port of the liquid separator can be disposed of by incineration, by chemical treatment (i.e., neutralization), and/or by direct venting to the atmosphere. Any liquid captured in the liquid separator is recycled back into the OCRSCAEP (either directly or indirectly). 
     Since liquid refrigerant temperature is sensitive to ambient temperature, the density of liquid refrigerant changes even though the pressure in the receiver  12  remains the same. Also, the liquid refrigerant temperature impacts the vapor quality at the evaporator inlet. Therefore, the refrigerant mass and volume flow rates change and the control devices  13  and  18  can be used. 
       FIG.  10    shows a schematic diagram of an example of receiver  12  (or receiver  14 ). Receiver  12  includes an inlet port  12   a , an outlet port  12   b , a pressure relief valve  12   c , and a heater  12   d . To charge receiver  12 , refrigerant fluid is typically introduced into receiver  12  via inlet port  12   a , and this can be done, for example, at service locations. Operating in the field the refrigerant exits receiver  12  through outlet port  12   b  that is connected to conduit  24   a  ( FIG.  1   ). In case of emergency, if the fluid pressure within receiver  12  exceeds a pressure limit value, pressure relief valve  12   c  opens to allow a portion of the refrigerant fluid to escape through valve  12   c  to reduce the fluid pressure within receiver  12 . For receiver  14  gas would be introduced. 
     When ambient temperature is very low and, as a result, pressure in the receiver  12  is low and insufficient to drive refrigerant fluid flow through the system, the gas from the gas receiver  14  is used to compress liquid refrigerant in the receiver  12 . The gas pressure supplied by the gas receiver  114  compresses the liquid refrigerant in the receiver  12  and maintains the liquid refrigerant in a sub-cooled state even at high ambient and liquid refrigerant temperatures. 
     A heater  12   d  can be used in those embodiments that do not include the gas receiver  14  to control vapor pressure of the liquid refrigerant in the receiver  12 . The heater  12  is connected via a control line to a controller ( FIG.  13   ). Heater  12   d , which can be implemented as a resistive heating element (e.g., a strip heater) or any of a wide variety of different types of heating elements, can be activated by controller to heat the refrigerant fluid within receiver  12 . Receiver  12  can also include insulation (not shown in  FIG.  2   ) applied around the receiver to reduce thermal losses. 
     In general, receiver  12  can have a variety of different shapes. In some embodiments, for example, the receiver is cylindrical. Examples of other possible shapes include, but are not limited to, rectangular prismatic, cubic, and conical. In certain embodiments, receiver  12  can be oriented such that outlet port  12   b  is positioned at the bottom of the receiver. In this manner, the liquid portion of the refrigerant fluid within receiver  12  is discharged first through outlet port  12   b , prior to discharge of refrigerant vapor. In certain embodiments, the refrigerant fluid can be an ammonia-based mixture that includes ammonia and one or more other substances. For example, mixtures can include one or more additives that facilitate ammonia absorption or ammonia burning. 
     More generally, any fluid can be used as a refrigerant in the open-circuit refrigeration systems disclosed herein, provided that the fluid is suitable for cooling heat load  34   a  (e.g., the fluid boils at an appropriate temperature) and, in embodiments where the refrigerant fluid is exhausted directly to the environment, regulations and other safety and operating considerations do not inhibit such discharge. 
     Evaporator 
     A variety of different evaporator types can be used in system  10 . In general, any cold plate may function as the evaporator  32  of the open-circuit refrigeration systems disclosed herein. Evaporator  32  can accommodate any number of and types of refrigerant fluid channels (including mini/micro-channel tubes), blocks of printed circuit heat exchanging structures, or more generally, any heat exchanging structures that are used to transport single-phase or two-phase fluids. The evaporator  32  and/or components thereof, such as fluid transport channels, can be attached to the heat load mechanically, or can be welded, brazed, or bonded to the heat load in any manner. 
     In some embodiments, evaporator  32  (or certain components thereof) can be fabricated as part of the heat load  34  or otherwise integrated into heat load  34 .  FIGS.  11 A and  11 B  show side and end views, respectively, of a heat load  34  with one or more integrated refrigerant fluid channels  302 . The portion of head lead  34  with the refrigerant fluid channel(s)  302  effectively functions as the evaporator  32  for the system. 
     In general, evaporator  32  functions as a heat exchanger, providing thermal contact between the refrigerant fluid and heat load  34  that is coupled to the any of the open-circuit refrigeration systems  10   a - 10   g  or  50   a - 50   f  or  110   a - 110   f  discussed above. Typically, evaporator  32  includes one or more flow channels extending internally between an inlet and an outlet of the evaporator, allowing refrigerant fluid to flow through the evaporator and absorb heat from heat load  34 . A variety of different evaporators can be used. In general, any cold plate may function as the evaporator of the open-circuit refrigeration systems disclosed herein. Evaporator  32  can accommodate any number and type of refrigerant fluid channels (including mini/micro-channel tubes), blocks of printed circuit heat exchanging structures, or more generally, any heat exchanging structures that are used to transport single-phase or two-phase fluids. The evaporator  32  and/or components thereof, such as fluid transport channels, can be attached to the heat load mechanically, or can be welded, brazed, or bonded to the heat load in any manner. In some embodiments, evaporator  32  (or certain components thereof) can be fabricated as part of heat load  34  or otherwise integrated into the heat load  34 . 
     The evaporator  32  can be implemented as plurality of evaporators connected in parallel and/or in series. 
       FIGS.  11 A and  11 B  show side and end views, respectively, of a heat load  34  on a thermally conductive body  62   a  with one or more integrated refrigerant fluid channels  62   b . The body  62   a  supporting the heat load  34 , which has the refrigerant fluid channel(s)  62   b  effectively functions as the evaporator  32  for the system. The thermally conductive body  62   a  can be configured as a cold plate or as a heat exchanging element (such as a mini-channel heat exchanger). Alternatively, the heat loads  34  can be attached to both sides of the thermally conductive body. 
     During operation of system  10 , cooling can be initiated by a variety of different mechanisms. In some embodiments, for example, system  10  includes a temperature sensor attached to load  34 . When the temperature of load  34  exceeds a certain temperature set point (i.e., threshold value), the controller  17  ( FIG.  15   ) connected to the temperature sensor can initiate cooling of load  34 . Alternatively, in certain embodiments, system  10  operates essentially continuously—provided that the refrigerant fluid pressure within receiver  12  is sufficient—to cool load  34 . As soon as receiver  12  is charged with refrigerant fluid, refrigerant fluid is ready to be directed into evaporator  32  to cool load  34 . In general, cooling is initiated when a user of the system  10  or the heat load  34  issues a cooling demand. 
     Upon initiation of a cooling operation, refrigerant fluid from receiver  12  is discharged from the outlet of the receiver  12  and transported through the conduit  24   a , through optional valve  16 , if present, and is transported through conduit  24   b  to expansion control device  18 , if present, which directly or indirectly controls vapor quality at the evaporator outlet. 
     In the following discussion, pertaining to  FIG.  1   , expansion control device  18  and solenoid valve  16  are not present and thus refrigerant fluid from receiver  12  enters the evaporator via conduit directly. The refrigerant fluid cools heat load  34  by having liquid refrigerant convert into a vapor with the vapor being exhausted from the exhaust line through action of the compressor/vacuum pump  19 . The initial temperature in the receiver  12  tends to be in equilibrium with the surrounding temperature, and the initial temperature establishes an initial pressure that is different for different refrigerants. The pressure in the evaporator  32  depends on the evaporating temperature, which is lower than the heat load temperature, and is defined during design of the system  10 . 
     In the following discussion, pertaining to  FIG.  3   , the control device  116  and the valve  118  are not present and thus refrigerant fluid from receiver  12  enters the evaporator  32  via conduit directly into the primary inlet of the ejector  66 . Once inside the ejector  66 , the refrigerant fluid undergoes constant enthalpy expansion from an initial pressure p r  (i.e., the receiver pressure) to an evaporation pressure p e  at the outlet of the ejector  66 . 
     In general, the evaporation pressure p e  depends on a variety of factors, most notably the desired temperature set point value (i.e., the target temperature) at which load  34  is to be maintained and the heat input generated by the heat load. The pressure in the evaporator  32  depends on the evaporating temperature, which is lower than the heat load temperature, and is defined during design of the system, as well as subsequent recirculation of refrigerant from the expansion valve  70 , which is entrained by the primary flow. The system  10  is operational as long the receiver-to-evaporator pressure difference is sufficient to drive adequate refrigerant fluid flow through the ejector  66 . 
     With any of the embodiments, the initial temperature in the receiver  12  (or  52 ,  112 ) tends to be in equilibrium with the surrounding temperature, and the initial temperature establishes an initial pressure that is different for different refrigerants. 
     Returning to  FIG.  5 A  and presuming use of the gas receiver  14  (not actually shown in  FIG.  5 A ), for operation with gas receiver  14 , at some point the gas receiver  14  feeds gas via pressure regulator and conduits (not shown) into the refrigerant receiver  12  (or  52 ,  112  for the other embodiments). The gas flow can occur at activation of the OCRSCAEP  50   b  or can occur at some point after activation of the OCRSCAEP  50   b . Similar operational factors apply for OCRSCAEP  50   a  and OCRSCAEP&#39;s  50   c - 50   g.    
     After undergoing expansion in the ejector  66 , the liquid refrigerant fluid is converted to a mixture of liquid and vapor phases at the temperature of the fluid and evaporation pressure p e . The refrigerant fluid in the two-phase state is transported via conduit  24   f  to the liquid separator  68 . Liquid from the liquid separator is fed to the expansion valve  70  is converted to a mixture of liquid and vapor phases at the temperature of the fluid and evaporation pressure p e . 
     When the refrigerant fluid in the two-phase state is directed into evaporator  32 , the liquid phase absorbs heat from load  34 , driving a phase transition of the liquid refrigerant fluid into the vapor phase. Because this phase transition occurs at (nominally) constant temperature, the temperature of the refrigerant fluid two-phase state within evaporator  32  remains unchanged, provided at least some liquid refrigerant fluid remains in evaporator  32  to absorb heat. 
     Further, the constant temperature of the refrigerant fluid in the two-phase state within evaporator  32  can be controlled by adjusting the pressure p e  of the refrigerant fluid, since adjustment of p e  changes the boiling temperature of the refrigerant fluid. Thus, by regulating the refrigerant fluid pressure p e  upstream from evaporator  32  (e.g., using pressure regulator  13 ), the temperature of the refrigerant fluid within evaporator  32  (and, nominally, the temperature of heat load  34 ) can be controlled to match a specific temperature set-point value for load  34 , ensuring that load  34  is maintained at, or very near, a target temperature. The pressure drop across the evaporator  32  causes a drop of the temperature of the refrigerant mixture (which is the evaporating temperature), but still the evaporator  32  can be configured to maintain the heat load temperature within the set tolerances. 
     In some embodiments, for example, the evaporation pressure of the refrigerant fluid can be adjusted by the compressor/vacuum pump  19  (or  59 ,  119 ) to ensure that the temperature of heat load  34  is maintained to within ±5 degrees C. (e.g., to within ±4 degrees C., to within ±3 degrees C., to within ±2 degrees C., to within ±1 degree C.) of the temperature set point value for load  34 . 
     As discussed above for OCRSCAEE  50   a , within evaporator  32 , a portion of the liquid refrigerant is converted to refrigerant vapor by undergoing a phase change. As a result, the refrigerant fluid two-phase state that emerges from evaporator  32  has a higher vapor quality (i.e., the fraction of the vapor phase that exists in refrigerant fluid mixture) than the refrigerant fluid two-phase state that enters evaporator  32 . As the refrigerant fluid two-phase state emerges from evaporator  32 , the refrigerant fluid is directed into the secondary (low pressure) inlet of the ejector  66  and is entrained by the primary flow (from receiver  12 ) fed to the inlet  66   a  of the ejector  66 . 
     The refrigerant fluid emerging from evaporator  32  is transported through conduit  24   j  to the compressor/vacuum pump  19  (or  59 ,  119 ), which directly or indirectly controls the upstream pressure, that is, the evaporating pressure p e  in the system. After passing through the compressor/vacuum pump  19  (or  59 ,  119 ), the refrigerant fluid is discharged as exhaust through conduit, which functions as an exhaust line for system  10 . Refrigerant fluid discharge can occur directly into the environment surrounding system  10 . Alternatively, in some embodiments, the refrigerant fluid can be further processed; various features and aspects of such processing are discussed in further detail below. 
     It should be noted that the foregoing, while discussed sequentially for purposes of clarity, occurs simultaneously and continuously during cooling operations. In other words, gas from receiver  14  is continuously being discharged, as needed, into the receiver  12  and the refrigerant fluid is continuously being discharged from receiver  12 , undergoing continuous expansion in ejector  66 , continuously being separated into liquid and vapor phases in liquid separator  68 , vapor is exhausted through the compressor/vacuum pump  19  (or  59 ,  119 ), while liquid is flowing through expansion valve  70  into evaporator  32  and from evaporator  32  into the low pressure inlet of the ejector  66 , which flow is entrained by the primary flow. Refrigerant flows continuously through evaporator  32  while heat load  34  is being cooled. 
     During operation of system  10 , as refrigerant fluid is drawn from receiver  12  and used to cool heat load  34 , the receiver pressure p r  falls. However, this pressure can be maintained by gas from gas receiver  14  (for embodiments  50   a - 50   g ). With either embodiments  50   a - 50   g  or  11   a , if the refrigerant fluid pressure p r  in receiver  12  is reduced to a value that is too low, the pressure differential p r -p e  may not be adequate to drive sufficient refrigerant fluid mass flow to provide adequate cooling of heat load  34 . Accordingly, when the refrigerant fluid pressure p r  in receiver  12  is reduced to a value that is sufficiently low, the capacity of system  10  to maintain a particular temperature set point value for load  34  may be compromised. Therefore, the pressure in the receiver or pressure drop across the expansion valve  70  (or any related refrigerant fluid pressure or pressure drop in system  10 ) can be an indicator of the remaining operational time. An appropriate warning signal can be issued (e.g., by the controller) to indicate that in certain periods of time, the system may no longer be able to maintain adequate cooling performance; operation of the system can even be halted if the refrigerant fluid pressure in receiver  12  reaches the low-end threshold value. 
     The refrigerant fluid that emerges from the vapor side  68   b  of the liquid separator  68  is all or nearly all in the vapor phase. As in OCRSCAEE  50   f ,  50   g , the refrigerant fluid vapor (at a saturated or very high vapor quality fluid vapor, e.g., about 0.95 or higher) can be directed into a heat exchanger coupled to another heat load, and can absorb heat from the additional heat load during propagation through the heat exchanger to cool additional heat loads as discussed in more detail subsequently. 
     It should be noted that while in the figures only a single receiver  12  is shown, in some embodiments, system  10  can include multiple receivers  12  to allow for operation of the system  10  over an extended time period. Each of the multiple receivers  12  can supply refrigerant fluid to the system  10  to extend to total operating time period. Some embodiments may include plurality of evaporators connected in parallel, which may or may not be accompanied by plurality of expansion valves and plurality of evaporators. 
     VII. System Operational Control 
     As discussed in the previous section, by adjusting the pressure p e  of the refrigerant fluid, the temperature at which the liquid refrigerant phase undergoes vaporization within evaporator  32  can be controlled. Thus, in general, the temperature of heat load  34  can be controlled by a device or component of system  10  that regulates the pressure of the refrigerant fluid within evaporator  32 . Typically, compressor or vacuum pump  19  (or  59 ,  119 ) adjusts the upstream refrigerant fluid pressure in system  10 . Accordingly, compressor or vacuum pump  19  is generally configured to control the temperature of heat load  34 , and can be adjusted to selectively change a temperature set point value (i.e., a target temperature) for heat load  34 . 
     Another system operating parameter is the vapor quality of the refrigerant fluid emerging from evaporator  32 . Vapor quality is a number from 0 to 1 and represents the fraction of the refrigerant fluid that is in the vapor phase. Because heat absorbed from load  34  is used to drive a constant-temperature evaporation of liquid refrigerant to form refrigerant vapor in evaporator  32 , it is generally important to ensure that, for a particular volume of refrigerant fluid propagating through evaporator  32 , at least some of the refrigerant fluid remains in liquid form right up to the point at which the refrigerant exits the evaporator  32  to allow continued heat absorption from heat load  34  without causing a temperature increase of the refrigerant fluid. If the fluid is fully converted to the vapor phase after propagating only partially through evaporator  32 , further heat absorption by the (now vapor-phase or two-phase with vapor quality above the critical one driving the evaporation process in the dry-out) refrigerant fluid within evaporator  32  will lead to a temperature increase of the refrigerant fluid and heat load  34 . 
     On the other hand, liquid-phase refrigerant fluid that emerges from evaporator  32  represents unused heat-absorbing capacity, in that the liquid refrigerant fluid did not absorb sufficient heat from load  34  to undergo a phase change. To ensure that system  10  operates efficiently, the amount of unused heat-absorbing capacity should remain relatively small, and should be defined by the critical vapor quality. 
     In addition, the boiling heat transfer coefficient that characterizes the effectiveness of heat transfer from load  34  to the refrigerant fluid is typically very sensitive to vapor quality. Vapor quality is a thermodynamic property which is a ratio of mass of vapor to total mass of vapor+liquid. As mentioned above, the “critical vapor quality” is a vapor quality=1. When the vapor quality increases from zero towards the critical vapor quality, the heat transfer coefficient increases. However, when the vapor quality reaches the “critical vapor quality,” the heat transfer coefficient is abruptly reduced to a very low value, causing dry out within evaporator  32 . In this region of operation, the two-phase mixture behaves as superheated vapor. 
     In general, the critical vapor quality and heat transfer coefficient values vary widely for different refrigerant fluids, and heat and mass fluxes. For all such refrigerant fluids and operating conditions, the systems and methods disclosed herein control the vapor quality at the outlet of the evaporator such that the vapor quality approaches the threshold of the critical vapor quality. 
     To make maximum use of the heat-absorbing capacity of the two-phase refrigerant fluid state, the vapor quality of the refrigerant fluid emerging from evaporator  32  should nominally be equal to the critical vapor quality. Accordingly, to both efficiently use the heat-absorbing capacity of the two-phase refrigerant fluid and also ensure that the temperature of heat load  34  remains approximately constant at the phase transition temperature of the refrigerant fluid in evaporator  32 , the systems and methods disclosed herein are generally configured to adjust the vapor quality of the refrigerant fluid emerging from evaporator  32  to a value that is less than or almost equal to the critical vapor quality. 
     Another operating consideration for system  10  is the mass flow rate of refrigerant fluid within the system. In open-circuit systems with recirculation of non-evaporated liquid, the mass flow rate is minimized as long as the system discharges at the highest possible vapor quality, which discharge is defined by liquid separator efficiency. Evaporator  32  can be configured to provide minimal mass flow rate controlling maximal vapor quality, which is the critical vapor quality. By minimizing the mass flow rate of the refrigerant fluid according to the cooling requirements for heat load  34 , system  10  operates efficiently. Each reduction in the mass flow rate of the refrigerant fluid (while maintaining the same temperature set point value for heat load  34 ) means that the charge of refrigerant fluid added to receiver  12  initially lasts longer, providing further operating time for system  10 . 
     Within evaporator  32 , the vapor quality of a given quantity of refrigerant fluid varies from the evaporator inlet (where vapor quality is lowest) to the evaporator outlet (where vapor quality is highest). Nonetheless, to realize the lowest possible mass flow rate of the refrigerant fluid within the system, the effective vapor quality of the refrigerant fluid within evaporator  32 —even when accounting for variations that occur within evaporator  32 —should match the critical vapor quality as closely as possible. 
     In summary, to ensure that the system operates efficiently and the mass flow rate of the refrigerant fluid is relatively low, and at the same time the temperature of heat load  34  is maintained within a relatively small tolerance, system  10  adjusts the vapor quality of the refrigerant fluid emerging from evaporator  32  to a value such that an effective vapor quality within evaporator  32  matches, or nearly matches, the critical vapor quality. 
     System  10  is generally configured to control the heat load  34  temperature. In some embodiments that use the control device  70 , that device  70  can control the vapor quality of the refrigerant fluid emerging from evaporator  32  in response to information about at least one thermodynamic quantity that is either directly or indirectly related to the vapor quality. Control device  29  typically adjusts the temperature of heat load  34  (via upstream refrigerant fluid pressure adjustments) in response to information about at least one thermodynamic quantity that is directly or indirectly related to the temperature of heat load  34 . The one or more thermodynamic quantities upon which adjustment of control device  70  is based are different from the one or more thermodynamic quantities upon which adjustment of compressor vacuum pump  19  is based. 
     The evaporator  32  can be configured to maintain exit vapor quality below the critical vapor quality. That is, for a given set of requirements, e.g., mass flow rate of refrigerant, ambient operating conditions, set point temperature, heat load, desired vapor quality exiting the evaporator, etc., the physical configuration of the evaporate  32  is determined such that the desired vapor quality would be achieved or substantially achieved. This would entail determining a suitable size, e.g., length, width, shape and materials, of the evaporator given the expected operating conditions. Conventional thermodynamic principles can be used to design such an evaporator for a specific set of requirements. In such an instance where the evaporator  32  is configured to maintain exit vapor quality this could eliminate the need for another control device, e.g., at the input to the evaporator  32 . 
     In general, a wide variety of different measurement and control strategies can be implemented in system  10  to achieve the control objectives discussed above. Generally, the control devices can be controlled by measuring a thermodynamic quantity upon which signals are produced to control and adjust the respective devices. The measurements can be implemented in various different ways, depending upon the nature of the devices and the design of the system. As an example, embodiments can optionally include mechanical devices that are controlled by electrical signals, e.g., solenoid controlled valves, regulators, etc. The signals can be produced by sensors and fed to the devices or can be processed by controllers to produce signals to control the devices. The devices can be purely mechanically controlled as well. 
     It should generally be understood that various control strategies, control devices, and measurement devices can be implemented in a variety of combinations in the systems disclosed herein. Thus, for example, any of the control devices can be implemented as mechanically-controlled devices. In addition, systems with mixed control in which one of the devices is a mechanically controlled device and others are electronically-adjustable devices can also be implemented, along with systems in which all of the control devices are electronically-adjustable devices that are controlled in response to signals measured by one or more sensors and or by sensor signals processed by controller (e.g., dedicated or general processor) circuits. In some embodiments, the systems disclosed herein can include sensors and/or measurement devices that measure various system properties and operating parameters, and transmit electrical signals corresponding to the measured information. 
       FIG.  12    depicts an configuration for the liquid separator  128 , (implemented as a coalescing liquid separator or a flash drum for example), which has ports  128   a - 128   c  coupled to conduits (not referenced). Other conventional details such as membranes or meshes, etc. are not shown. Similar considerations can be used for the liquid separator  68  ( FIGS.  3 ,  5 A- 5 F ). 
       FIGS.  12 A- 12 C  depict alternative configurations of the liquid separator  128  (implemented as a flash drum for example), which has ports  128   a - 128   c  coupled to conduits (not referenced), especially useful for the open-circuit refrigeration system with pump (OCRSCAEP) configurations. 
     In  FIG.  12 A , the pump  100  is located distal from the liquid separator port  128   c . This configuration potentially presents the possibility of cavitation. To minimize the possibility of cavitation one of the configurations of  FIG.  12 B or  12 C  can be used. 
     In  FIG.  12 B , the pump  100  is located distal from the liquid separator port  128   c , but the height at which the inlet is located is higher than that of  FIG.  12 A . This would result in an increase in liquid pressure at the outlet  128   c  of the liquid separator  128  and concomitant therewith an increase in liquid pressure at the inlet of the pump  100 . Increasing the pressure at the inlet to the pump should minimize possibility of cavitation. 
     Another strategy is presented in  FIG.  12 C , where the pump  100  is located proximate to or indeed, as shown, inside of the liquid separator port  128   c . In addition although not shown the height at which the inlet is located can be adjusted to that of  FIG.  12 B , rather than the height of  FIG.  12 A  as shown in  FIG.  12 C . This would result in an increase in liquid pressure at the inlet of the pump  100  further minimizing the possibility of cavitation. 
     Another alternative strategy that can be used for any of the configurations depicted involves the use of a sensor  129   a  that produces a signal that is a measure of the height of a column of liquid in the liquid separator. The signal is sent to the controller  17  that will be used to start the pump  100 , once a sufficient height of liquid is contained by the liquid separator  128 . 
     Another alternative strategy that can be used for any of the configurations depicted involves the use of a recuperative heat exchanger. The recuperative heat exchanger is an evaporator, which brings into thermal contact two refrigerant streams. In the above systems, a first of the streams is the liquid stream leaving the liquid separator. A second stream is the liquid refrigerant expanded to a pressure lower than the evaporator pressure in the evaporator  32  and evaporating the related evaporating temperature lower than the liquid temperature at the liquid separator exit. Thus, the liquid from the liquid separator exit is subcooled, rejecting thermal energy to the second side of the recuperative heat exchanger. The second side absorbs the rejected thermal energy due to evaporating and superheating of the second refrigerant stream. 
     Referring now to  FIG.  13   , the system  10  includes another alternative open-circuit refrigeration system with ejector configuration  50   b ′″ that is similar to the open-circuit refrigeration system (OCRSCAEE)  50   b  of  FIG.  2   , including the receiver  52 , but also including the gas receiver  14  and the pressure regulator  13 , the solenoid control valve  58 , expansion valve  56 , evaporator  32 , liquid separator  68 , ejector  66  and compressor/vacuum pump  59  coupled to the exhaust line  27 , as discussed above in  FIG.  3   . Conduits  64   a - 64   k  couple the various aforementioned items, as shown. Compressor/vacuum pump  59  is coupled between a recuperative heat exchanger  79  and a back pressure regulator  29 . 
     The OCRSCAEP  50   b ′″ also includes the recuperative heat exchanger  79  having two fluid paths. A first fluid path is between a first inlet and first outlet of the recuperative heat exchanger  79 . The first fluid path has the first inlet of recuperative heat exchanger  79  coupled to the outlet of the receiver  12  and the first outlet of the recuperative heat exchanger  79  coupled to the inlet of the valve  58 . A second fluid path is between a second inlet and second outlet of the recuperative heat exchanger  79 . The second fluid path has the second inlet of recuperative heat exchanger  79  coupled to the vapor side outlet of the liquid separator  68  and the second outlet of the recuperative heat exchanger  79  is coupled to the inlet of the back pressure regulator  29 . 
     In this configuration, the receiver  52  is integrated with the recuperative heat exchanger  100 . The recuperative heat exchanger  79  provides thermal contact between the liquid refrigerant leaving the receiver  52  and the refrigerant vapor from the liquid separator  68 . The use of the recuperative heat exchanger  79 , at the outlet of the receiver  52  may further reduce liquid refrigerant mass flow rate demand from the receiver  52  by re-using the enthalpy of the exhaust vapor to precool the refrigerant liquid entering the evaporator that reduces the enthalpy of the refrigerant entering the evaporator and thus reduces mass flow rate demand and provides a relative increase in energy efficiency of the system  10 . 
     The OCRSCAEP  50   b ′″ with the recuperative heat exchanger  79  can be used with at least the embodiments  50   a ,  50   c - 50   d  or  11   a  (and corresponding analogs). 
     Referring now to  FIG.  13 A , one embodiment of the recuperative heat exchanger  79  is a helical-coil type heat exchanger that includes a shell  102  and a helical coil  104  that is inside the shell  102 . The refrigerant liquid stream from the receiver  12  flows though the shell  102  while the vapor stream from the vapor side of the liquid separator flows through the coil  104 . The coil  104  can be made of different heat exchanger elements: conventional tubes, mini-channel tubes, cold plate type tubes, etc. The shape of the coil channels can be different as well. Heat from the vapor is transferred from the vapor to the liquid. 
       FIG.  14    shows the thermal management system  10  of  FIG.  3    (representative of any of the variations, e.g.,  FIGS.  1 ,  2 A- 2 F,  5 A- 5 F,  6 - 7 F , discussed above) but also including the gas receiver  14  and pressure regulator  13  ( FIGS.  8  and  8 A , discussed above, both not shown in  FIG.  3   ) with a number of different sensors generally  73  each of which is optional. Various combinations of the sensors  73  shown can be used to measure thermodynamic properties of the system  10  that are used to adjust the control devices or pumps discussed above and which signals are processed by the controller  17 . In  FIG.  14   , connections (not shown) are provided between each of the sensors and controller  17  (wired or wireless). In many embodiments, system includes only certain combinations of the sensors shown (e.g., one, two, three, or four of the sensors) to provide suitable control signals for the first and/or second control device. 
       FIG.  15    shows the controller  17  that includes a processor  17   a , memory  17   b , storage  17   c , and I/O interfaces  17   d , all of which are connected/coupled together via a bus  17   e . Any two of the optional devices, as pressure sensors upstream and downstream from a control device, can be configured to measure information about a pressure differential p r -p e  across the respective control device and to transmit electronic signals corresponding to the measured pressure from which a pressure difference information can be generated by the controller  17 . Other sensors such as flow sensors and temperature sensors can be used as well. In certain embodiments, sensors can be replaced by a single pressure differential sensor, a first end of which is connected adjacent to an inlet and a second end of which is connected adjacent to an outlet of a device to which differential pressure is to be measured, such as the evaporator. The pressure differential sensor measures and transmits information about the refrigerant fluid pressure drop across the device, e.g., the evaporator  32 . 
     Temperatures sensor can be positioned adjacent to an inlet or an outlet of e.g., the evaporator  32  or between the inlet and the outlet. Such as temperature sensor measures temperature information for the refrigerant fluid within evaporator  32  (which represents the evaporating temperature) and transmits an electronic signal corresponding to the measured information. A temperature sensor can be attached to heat load  34 , which measures temperature information for the load and transmits an electronic signal corresponding to the measured information. An optional temperature sensor can be adjacent to the outlet of evaporator  32  that measures and transmits information about the temperature of the refrigerant fluid as it emerges from evaporator  32 . 
     In certain embodiments, the systems disclosed herein are configured to determine superheat information for the refrigerant fluid based on temperature and pressure information for the refrigerant fluid measured by any of the sensors disclosed herein. The superheat of the refrigerant vapor refers to the difference between the temperature of the refrigerant fluid vapor at a measurement point in the system and the saturated vapor temperature of the refrigerant fluid defined by the refrigerant pressure at the measurement point in the system. 
     To determine the superheat associated with the refrigerant fluid, the system controller  17  (as described) receives information about the refrigerant fluid vapor pressure after emerging from a heat exchanger downstream from evaporator  32 , and uses calibration information, a lookup table, a mathematical relationship, or other information to determine the saturated vapor temperature for the refrigerant fluid from the pressure information. The controller  17  also receives information about the actual temperature of the refrigerant fluid, and then calculates the superheat associated with the refrigerant fluid as the difference between the actual temperature of the refrigerant fluid and the saturated vapor temperature for the refrigerant fluid. 
     The foregoing temperature sensors can be implemented in a variety of ways in system  10 . As one example, thermocouples and thermistors can function as temperature sensors in system  10 . Examples of suitable commercially available temperature sensors for use in system  10  include, but are not limited to, the  88000  series thermocouple surface probes (available from OMEGA Engineering Inc., Norwalk, Conn.). 
     System  10  can include a vapor quality sensor that measures vapor quality of the refrigerant fluid emerging from evaporator  32 . Typically, such a sensor is implemented as a capacitive sensor that measures a difference in capacitance between the liquid and vapor phases of the refrigerant fluid. The capacitance information can be used to directly determine the vapor quality of the refrigerant fluid (e.g., by system controller  17 ). Alternatively, sensor can determine the vapor quality directly based on the differential capacitance measurements and transmit an electronic signal that includes information about the refrigerant fluid vapor quality. Examples of commercially available vapor quality sensors that can be used in system  10  include, but are not limited to, HBX sensors (available from HB Products, Hasselager, Denmark). 
     The systems disclosed herein can include a system controller  17  that receives measurement signals from one or more system sensors and transmits control signals to the control devices to adjust the refrigerant fluid vapor quality and the heat load temperature. 
     It should generally understood that the systems disclosed herein can include a variety of combinations of the various sensors described above, and controller  17  can receive measurement information periodically or aperiodically from any of the various sensors. Moreover, it should be understood any of the sensors described can operate autonomously, measuring information and transmitting the information to controller  17  (or directly to the first and/or second control device) or, alternatively, any of the sensors described above can measure information when activated by controller  17  via a suitable control signal, and measure and transmit information to controller  17  in response to the activating control signal. 
     To adjust a control device on a particular value of a measured system parameter value, controller  17  compares the measured value to a set point value (or threshold value) for the system parameter. Certain set point values represent a maximum allowable value of a system parameter, and if the measured value is equal to the set point value (or differs from the set point value by 10% or less (e.g., 5% or less, 3% or less, 1% or less) of the set point value), controller  17  adjusts a respective control device to modify the operating state of the system  10 . Certain set point values represent a minimum allowable value of a system parameter, and if the measured value is equal to the set point value (or differs from the set point value by 10% or less (e.g., 5% or less, 3% or less, 1% or less) of the set point value), controller  17  adjusts the respective control device to modify the operating state of the system  10 , and increase the system parameter value. The controller  17  executes algorithms that use the measured sensor value(s) to provide signals that cause the various control devices to adjust refrigerant flow rates, etc. 
     Some set point values represent “target” values of system parameters. For such system parameters, if the measured parameter value differs from the set point value by 1% or more (e.g., 3% or more, 5% or more, 10% or more, 20% or more), controller  17  adjusts the respective control device to adjust the operating state of the system, so that the system parameter value more closely matches the set point value. 
     Optional pressure sensors are configured to measure information about the pressure differential p r -p e  across a control device and to transmit an electronic signal corresponding to the measured pressure difference information. Two sensors can effectively measure p r , p e . In certain embodiments two sensors can be replaced by a single pressure differential sensor. Where a pressure differential sensor is used, a first end of the sensor is connected upstream of a first control device and a second end of the sensor is connected downstream from first control device. 
     System also includes optional pressure sensors positioned at the inlet and outlet, respectively, of evaporator  32 . A sensor measures and transmits information about the refrigerant fluid pressure upstream from evaporator  32 , and a sensor measure and transmit information about the refrigerant fluid pressure downstream from evaporator  32 . This information can be used (e.g., by a system controller) to calculate the refrigerant fluid pressure drop across evaporator  32 . As above, in certain embodiments, sensors can be replaced by a single pressure differential sensor to measure and transmit the refrigerant fluid pressure drop across evaporator  32 . 
     To measure the evaporating pressure (p e ) a sensor can be optionally positioned between the inlet and outlet of evaporator  32 , i.e., internal to evaporator  32 . In such a configuration, the sensor can provide a direct a direct measurement of the evaporating pressure. 
     To measure refrigerant fluid pressure at other locations within system, sensor can also optionally be positioned, for example, in-line along conduit. Alternatively, sensor can be positioned at or near an inlet of compressor/vacuum pump  19 . Pressure sensors at each of these locations can be used to provide information about the refrigerant fluid pressure downstream from evaporator  32 , or the pressure drop across evaporator  32 . 
     System includes an optional temperature sensor which can be positioned adjacent to an inlet or an outlet of evaporator  32 , or between the inlet and the outlet to measure temperature information for the refrigerant fluid within evaporator  32  (which represents the evaporating temperature) and transmits an electronic signal corresponding to the measured information. System can also include an optional temperature sensor attached to heat load  34 , which measures temperature information for the load and transmits an electronic signal corresponding to the measured information. 
     System includes an optional temperature sensor adjacent to the outlet of evaporator  32  that measures and transmits information about the temperature of the refrigerant fluid as it emerges from evaporator  32 . 
     In certain embodiments, the systems disclosed herein are configured to determine superheat information for the refrigerant fluid based on temperature and pressure information for the refrigerant fluid measured by any of the sensors disclosed herein. The superheat of the refrigerant vapor refers to the difference between the temperature of the refrigerant fluid vapor at a measurement point in the system and the saturated vapor temperature of the refrigerant fluid defined by the refrigerant pressure at the measurement point in the system. 
     It should be appreciated that, in the foregoing discussion, any one or various combinations of two sensors discussed in connection with system can correspond to the first measurement device connected to control device  18 , and any one or various combination of two sensors can correspond to the second measurement device connected to compressor/vacuum pump  19 . In general, as discussed previously, the first measurement device provides information corresponding to a first thermodynamic quantity to the first control device, and the second measurement device provides information corresponding to a second thermodynamic quantity to the second control device, where the first and second thermodynamic quantities are different, and therefore allow the first and second control device to independently control two different system properties (e.g., the vapor quality of the refrigerant fluid and the heat load temperature, respectively). 
     In some embodiments, one or more of the sensors shown in system are connected directly to control device  18  and/or to compressor/vacuum pump  19 . The first and second control device can be configured to adaptively respond directly to the transmitted signals from the sensors, thereby providing for automatic adjustment of the system&#39;s operating parameters. In certain embodiments, the first and/or second control device can include processing hardware and/or software components that receive transmitted signals from the sensors, optionally perform computational operations, and activate elements of the first and/or second control device to adjust the control device in response to the sensor signals. 
     In addition, controller  17  is optionally connected to control device  18  and compressor/vacuum pump  19 . In embodiments where either control device  18  or compressor/vacuum pump  19  (or both) is/are implemented as a device controllable via an electrical control signal, controller  17  is configured to transmit suitable control signals to the first and/or second control device to adjust the configuration of these components. In particular, controller  17  is optionally configured to adjust control device  18  to control the vapor quality of the refrigerant fluid in system and optionally configured to adjust compressor/vacuum pump  19  to control the temperature of heat load  34 . 
     During operation of system, controller  17  typically receives measurement signals from one or more sensors. The measurements can be received periodically (e.g., at consistent, recurring intervals) or irregularly, depending upon the nature of the measurements and the manner in which the measurement information is used by controller  17 . In some embodiments, certain measurements are performed by controller  17  after particular conditions—such as a measured parameter value exceeding or falling below an associated set point value—are reached. 
     It should generally be understood that the systems disclosed herein can include a variety of combinations of the various sensors described above, and controller  17  can receive measurement information periodically or aperiodically from any of the various sensors. Moreover, it should be understood any of the sensors described can operate autonomously, measuring information and transmitting the information to controller  17  (or directly to the first and/or second control device), or alternatively, any of the sensors described above can measure information when activated by controller  17  via a suitable control signal, and measure and transmit information to controller  17  in response to the activating control signal. 
     By way of example, Table 1 summarizes various examples of combinations of types of information (e.g., system properties and thermodynamic quantities) that can be measured by the sensors of system and transmitted to controller  17 , to allow controller  17  to generate and transmit suitable control signals to control device  18  and/or compressor/vacuum pump  19 . The types of information shown in Table 1 can generally be measured using any suitable device (including combination of one or more of the sensors discussed herein) to provide measurement information to controller  17 . 
     
       
         
           
               
               
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                   
                   
                 Measurement Information Used to Adjust First  
               
               
                   
                   
                 Control Device 
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                   
                 FCM 
                 Evap 
                   
                   
                   
                   
                   
                   
               
               
                   
                 Press 
                 Press 
                 Rec 
                   
                   
                 Evap 
                 Evap 
                 HL 
               
               
                   
                 Drop 
                 Drop 
                 Pres 
                 VQ 
                 SH 
                 VQ 
                 P/T 
                 Temp 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
            
               
                 Measure- 
                 FCM 
                   
                   
                   
                   
                   
                   
                 x 
                 x 
               
               
                 ment 
                 Press 
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 Infor- 
                 Drop 
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 mation 
                 Evap 
                   
                   
                   
                   
                   
                   
                 x 
                 x 
               
               
                 Used to 
                 Press 
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 Adjust 
                 Drop 
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 Second 
                 Rec 
                   
                   
                   
                   
                   
                   
                 x 
                 x 
               
               
                 Control 
                 Press 
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 Device 
                 VQ 
                   
                   
                   
                   
                   
                   
                 x 
                 x 
               
               
                   
                 SH 
                   
                   
                   
                   
                   
                   
                 x 
                 x 
               
               
                   
                 Evap 
                   
                   
                   
                   
                   
                   
                 x 
                 x 
               
               
                   
                 VQ 
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                   
                 Evap 
                 x 
                 x 
                 x 
                 x 
                 x 
                 x 
                   
                 x 
               
               
                   
                 P/T 
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                   
                 HL 
                 x 
                 x 
                 x 
                 x 
                 x 
                 x 
                 x 
                   
               
               
                   
                 Temp 
               
               
                   
               
               
                 FCM Press Drop = refrigerant fluid pressure drop across first control device 
               
               
                 Evap Press Drop = refrigerant fluid pressure drop across evaporator 
               
               
                 Rec Press = refrigerant fluid pressure in receiver 
               
               
                 VQ = vapor quality of refrigerant fluid 
               
               
                 SH = superheat of refrigerant fluid 
               
               
                 Evap VQ = vapor quality of refrigerant fluid at evaporator outlet 
               
               
                 Evap P/T = evaporation pressure or temperature 
               
               
                 HL Temp = heat load temperature 
               
            
           
         
       
     
     For example, in some embodiments, control device  18  is adjusted (e.g., automatically or by controller  17 ) based on a measurement of the evaporation pressure (p e ) of the refrigerant fluid and/or a measurement of the evaporation temperature of the refrigerant fluid. With control device  18  adjusted in this manner, compressor/vacuum pump  19  can be adjusted (e.g., automatically or by controller  17 ) based on measurements of one or more of the following system parameter values: the pressure drop across control device  18 , the pressure drop across evaporator  32 , the refrigerant fluid pressure in receiver  12 , the vapor quality of the refrigerant fluid emerging from evaporator  32  (or at another location in the system), the superheat value of the refrigerant fluid, and the temperature of heat load  34 . 
     In certain embodiments, control device  18  is adjusted (e.g., automatically or by controller  17 ) based on a measurement of the temperature of heat load  34 . With control device  18  adjusted in this manner, compressor/vacuum pump  19  can be adjusted (e.g., automatically or by controller  17 ) based on measurements of one or more of the following system parameter values: the pressure drop across control device  18 , the pressure drop across evaporator  32 , the refrigerant fluid pressure in receiver  12 , the vapor quality of the refrigerant fluid emerging from evaporator  32  (or at another location in the system), the superheat value of the refrigerant fluid, and the evaporation pressure (p e ) and/or evaporation temperature of the refrigerant fluid. 
     In some embodiments, controller  17  adjusts the speed of the compressor/vacuum pump  19  based on a measurement of the evaporation pressure p e  of the refrigerant fluid downstream from control device  18  and/or a measurement of the evaporation temperature of the refrigerant fluid. With compressor/vacuum pump  19  adjusted based on this measurement, controller  17  can adjust control device  18  based on measurements of one or more of the following system parameter values: the pressure drop (p r -p e ) across control device  18 , the pressure drop across evaporator  32 , the refrigerant fluid pressure in receiver  12  ( p   r ), the vapor quality of the refrigerant fluid emerging from evaporator  32  (or at another location in the system), the superheat value of the refrigerant fluid in the system, and the temperature of heat load  34 . 
     In certain embodiments, controller  17  adjusts the speed of the compressor/vacuum pump  19  based on a measurement of the temperature of heat load  34  (e.g., measured by sensor  124 ). Controller  17  can also adjust control device  18  based on measurements of one or more of the following system parameter values: the pressure drop (p r -p e ) across control device  18 , the pressure drop across evaporator  32 , the refrigerant fluid pressure in receiver  12  (p r ), the vapor quality of the refrigerant fluid emerging from evaporator  32  (or at another location in the system), the superheat value of the refrigerant fluid in the system, the evaporation pressure (p e ) of the refrigerant fluid, and the evaporation temperature of the refrigerant fluid. 
     To adjust either control device  18  ( FIGS.  1 ,  2 A- 2 E ) or control device  68  ( FIGS.  3 ,  5 A- 5 D,  8   ) or control device  116  ( FIGS.  6 ,  7 A- 7 E ) or compressor vacuum pump  19  (or  59 ,  119 ) based on a particular value of a measured system parameter value, controller  17  compares the measured value to a set point value (or threshold value) for the system parameter. Consider  FIG.  1   , certain set point values represent a maximum allowable value of a system parameter, and if the measured value is equal to the set point value (or differs from the set point value by 10% or less (e.g., 5% or less, 3% or less, 1% or less) of the set point value), controller  17  adjusts control device  18  and/or compressor vacuum pump  19  to adjust the operating state of the system, and reduce the system parameter value. 
     Certain set point values represent a minimum allowable value of a system parameter, and if the measured value is equal to the set point value (or differs from the set point value by 10% or less (e.g., 5% or less, 3% or less, 1% or less) of the set point value), controller  17  adjusts control device  18  and/or compressor vacuum pump  19  to adjust the operating state of the system, and increase the system parameter value. 
     Some set point values represent “target” values of system parameters. For such system parameters, if the measured parameter value differs from the set point value by 1% or more (e.g., 3% or more, 5% or more, 10% or more, 20% or more), controller  17  adjusts control device  18  and/or compressor vacuum pump  19  to adjust the operating state of the system, so that the system parameter value more closely matches the set point value. 
     Measured parameter values are assessed in relative terms based on set point values (i.e., as a percentage of set point values). Alternatively, in some embodiments, measured parameter values can be accessed in absolute terms. For example, if a measured system parameter value differs from a set point value by more than a certain amount (e.g., by 1 degree C. or more, 2 degrees C. or more, 3 degrees C. or more, 4 degrees C. or more, 5 degrees C. or more), then controller  17  adjusts control device  18  and/or compressor vacuum pump  19  to adjust the operating state of the system, so that the measured system parameter value more closely matches the set point value. 
     In the foregoing examples, measured parameter values are assessed in relative terms based on set point values (i.e., as a percentage of set point values). Alternatively, in some embodiments, measured parameter values can be in absolute terms. For example, if a measured system parameter value differs from a set point value by more than a certain amount (e.g., by 1 degree C. or more, 2 degrees C. or more, 3 degrees C. or more, 4 degrees C. or more, 5 degrees C. or more), then controller  17  adjusts control device  18  and/or compressor/vacuum pump  19  to adjust the operating state of the system, so that the measured system parameter value more closely matches the set point value. 
     In certain embodiments, refrigerant fluid emerging from evaporator  32  can be used to cool one or more additional heat loads. In addition, systems can include a second heat load connected to a heat exchanger. A variety of mechanical connections can be used to attach second heat load to heat exchanger, including (but not limited to) brazing, clamping, welding, and any of the other connection types discussed herein. 
     Heat exchanger includes one or more flow channels through which high vapor quality refrigerant fluid flows after leaving evaporator  32 . During operation, as the refrigerant fluid vapor phases through the flow channels, it absorbs heat energy from second heat load, cooling second heat load. Typically, second heat load is not as sensitive as heat load  34  to fluctuations in temperature. Accordingly, while second heat load is generally not cooled as precisely relative to a particular temperature set point value as heat load  34 , the refrigerant fluid vapor provides cooling that adequately matches the temperature constraints for second heat load. 
     In general the systems disclosed herein can include more than one (e.g., two or more, three or more, four or more, five or more, or even more) heat loads in addition to heat loads depicted. Each of the additional heat loads can have an associated heat exchanger; in some embodiments, multiple additional heat loads are connected to a single heat exchanger, and in certain embodiments, each additional heat load has its own heat exchanger. Moreover, each of the additional heat loads can be cooled by the superheated refrigerant fluid vapor after a heat exchanger attached to the second load or cooled by the high vapor quality fluid stream that emerges from evaporator  32 . 
     Although evaporator  32  and heat exchanger are implemented as separate components, in certain embodiments, these components can be integrated to form a single heat exchanger, with heat load and second heat load both connected to the single heat exchanger. The refrigerant fluid vapor that is discharged from the evaporator portion of the single heat exchanger is used to cool second heat load, which is connected to a second portion of the single heat exchanger. 
     The vapor quality of the refrigerant fluid after passing through evaporator  32  can be controlled either directly or indirectly with respect to a vapor quality set point by controller  17 . In some embodiments, the system includes a vapor quality sensor that provides a direct measurement of vapor quality, which is transmitted to controller  17 . Controller  17  adjusts control device, e.g.,  19 ,  46 ,  70 ,  59 ,  53 ,  119 , depending on configuration to control the vapor quality relative to the vapor quality set point value. 
     In certain embodiments, the system includes a sensor that measures superheat and indirectly, vapor quality. For example, a combination of temperature and pressure sensors measure the refrigerant fluid superheat downstream from a second heat load, and transmit the measurements to controller  17 . Controller  17  adjusts control device according to the configuration based on the measured superheat relative to a superheat set point value. By doing so, controller  17  indirectly adjusts the vapor quality of the refrigerant fluid emerging from evaporator  32 . 
     In some embodiments, controller  17  can adjust compressor/vacuum pump  19  based on measurements of the superheat value of the refrigerant fluid vapor that are performed downstream from a second heat load that is cooled by the superheated refrigerant fluid vapor. 
     As the two refrigerant fluid streams flow in opposite directions within recuperative heat exchanger, heat is transferred from the refrigerant fluid emerging from evaporator  32  to the refrigerant fluid entering control device  18 . Heat transfer between the refrigerant fluid streams can have a number of advantages. For example, recuperative heat transfer can increase the refrigeration effect in evaporator  32 , thereby reducing the refrigerant mass transfer rate implemented to handle the heat load presented by heat load  34 . Further, by reducing the refrigerant mass transfer rate through evaporator  32 , the amount of refrigerant used to provide cooling duty in a given period of time is reduced. As a result, for a given initial quantity of refrigerant fluid introduced into receiver  12 , the operational time over which the system can operate before an additional refrigerant fluid charge is needed can be extended. Alternatively, for the system to effectively cool heat load  34  for a given period of time, a smaller initial charge of refrigerant fluid into receiver  12  can be used. 
     Because the liquid and vapor phases of the two-phase mixture of refrigerant fluid generated following expansion of the refrigerant fluid in control device  18  can be used for different cooling applications, in some embodiments, the system can include a phase separator to separate the liquid and vapor phases into separate refrigerant streams that follow different flow paths within the system. 
     Further, eliminating (or nearly eliminating) the refrigerant vapor from the refrigerant fluid stream entering evaporator  32  can help to reduce the cross-section of the evaporator and improve film boiling in the refrigerant channels. In film boiling, the liquid phase (in the form of a film) is physically separated from the walls of the refrigerant channels by a layer of refrigerant vapor, leading to poor thermal contact and heat transfer between the refrigerant liquid and the refrigerant channels. Reducing film boiling improves the efficiency of heat transfer and the cooling performance of evaporator  32 . 
     In addition, by eliminating (or nearly eliminating) the refrigerant vapor from the refrigerant fluid stream entering evaporator  32 , distribution of the liquid refrigerant within the channels of evaporator  32  can be made easier. In certain embodiments, vapor present in the refrigerant channels of evaporator  32  can oppose the flow of liquid refrigerant into the channels. Diverting the vapor phase of the refrigerant fluid before the fluid enters evaporator  32  can help to reduce this difficulty. 
     In addition to phase separator, or as an alternative to phase separator, in some embodiments the systems disclosed herein can include a phase separator downstream from evaporator  32 . Such a configuration can be used when the refrigerant fluid emerging from evaporator is not entirely in the vapor phase, and still includes liquid refrigerant fluid. 
     VIII. Additional Features of Thermal Management Systems 
     The foregoing examples of thermal management systems illustrate a number of features that can be included in any of the systems within the scope of this disclosure. In addition, a variety of other features can be present in such systems. 
     In certain embodiments, refrigerant fluid that is discharged from evaporator  32  and passes through conduit and compressor/vacuum pump  19  can be directly discharged as exhaust from conduit without further treatment. Direct discharge provides a convenient and straightforward method for handling spent refrigerant, and has the added advantage that over time, the overall weight of the system is reduced due to the loss of refrigerant fluid. For systems that are mounted to small vehicles or are otherwise mobile, this reduction in weight can be important. 
     In some embodiments, however, refrigerant fluid vapor can be further processed before it is discharged. Further processing may be desirable depending upon the nature of the refrigerant fluid that is used, as direct discharge of unprocessed refrigerant fluid vapor may be hazardous to humans and/or may be deleterious to mechanical and/or electronic devices in the vicinity of the system. For example, the unprocessed refrigerant fluid vapor may be flammable or toxic, or may corrode metallic device components. In situations such as these, additional processing of the refrigerant fluid vapor may be desirable. 
     In general, refrigerant processing apparatus can be implemented in various ways. In some embodiments, refrigerant processing apparatus is a chemical scrubber or water-based scrubber. Within apparatus, the refrigerant fluid is exposed to one or more chemical agents that treat the refrigerant fluid vapor to reduce its deleterious properties. For example, where the refrigerant fluid vapor is basic (e.g., ammonia) or acidic, the refrigerant fluid vapor can be exposed to one or more chemical agents that neutralize the vapor and yield a less basic or acidic product that can be collected for disposal or discharged from apparatus. 
     As another example, where the refrigerant fluid vapor is highly chemically reactive, the refrigerant fluid vapor can be exposed to one or more chemical agents that oxidize, reduce, or otherwise react with the refrigerant fluid vapor to yield a less reactive product that can be collected for disposal or discharged from apparatus. 
     In certain embodiments, refrigerant processing apparatus  802  can be implemented as an adsorptive sink for the refrigerant fluid. Apparatus  802  can include, for example, an adsorbent material bed that binds particles of the refrigerant fluid vapor, trapping the refrigerant fluid within apparatus and preventing discharge. The adsorptive process can sequester the refrigerant fluid particles within the adsorbent material bed, which can then be removed from apparatus and sent for disposal. 
     In some embodiments, where the refrigerant fluid is flammable, refrigerant processing apparatus can be implemented as an incinerator. Incoming refrigerant fluid vapor can be mixed with oxygen or another oxidizing agent and ignited to combust the refrigerant fluid. The combustion products can be discharged from the incinerator or collected (e.g., via an adsorbent material bed) for later disposal. 
     As an alternative, refrigerant processing apparatus can also be implemented as a combustor of an engine or another mechanical power-generating device. Refrigerant fluid vapor from conduit can be mixed with oxygen, for example, and combusted in a piston-based engine or turbine to perform mechanical work, such as providing drive power for a vehicle or driving a generator to produce electricity. In certain embodiments, the generated electricity can be used to provide electrical operating power for one or more devices, including heat load  34 . For example, heat load  34  can include one or more electronic devices that are powered, at least in part, by electrical energy generated from combustion of refrigerant fluid vapor in refrigerant processing apparatus. 
     The thermal management systems disclosed herein can optionally include a phase separator upstream from the refrigerant processing apparatus or a phase separator also downstream from the compressor/vacuum pump  19 . 
     Particularly during start-up of the systems disclosed herein, liquid refrigerant may be present in conduits because the systems generally begin operation before heat load  34  and/or heat load are activated. Accordingly, phase separator functions in a manner similar to phase separators to separate liquid refrigerant fluid from refrigerant vapor. The separated liquid refrigerant fluid can be re-directed to another portion of the system, or retained within phase separator until it is converted to refrigerant vapor. By using phase separator, liquid refrigerant fluid can be prevented from entering refrigerant processing apparatus. 
     IX. Integration with Power Systems 
     In some embodiments, the refrigeration systems disclosed herein can be combined with power systems to form integrated power and thermal systems, in which certain components of the integrated systems are responsible for providing refrigeration functions and certain components of the integrated systems are responsible for generating operating power. 
       FIG.  16    shows an integrated power and thermal management system  10  that includes many features similar to those discussed above (e.g., see  FIG.  1   ). In addition, system  10  includes an engine  140  with an inlet that receives the stream of waste refrigerant fluid that enters conduit after passing through compressor/vacuum pump  19 . Engine  140  can combust the waste refrigerant fluid directly, or alternatively, can mix the waste refrigerant fluid with one or more additives (such as oxidizers) before combustion. Where ammonia is used as the refrigerant fluid in system  10 , suitable engine configurations for both direct ammonia combustion as fuel, and combustion of ammonia mixed with other additives, can be implemented. In general, combustion of ammonia improves the efficiency of power generation by the engine. 
     The energy released from combustion of the refrigerant fluid can be used by engine  140  to generate electrical power, e.g., by using the energy to drive a generator. The electrical power can be delivered via electrical connection to heat load  34  to provide operating power for the load. For example, in certain embodiments, heat load  34  includes one or more electrical circuits and/or electronic devices, and engine  140  provides operating power to the circuits/devices via combustion of refrigerant fluid. Byproducts  142  of the combustion process can be discharged from engine  140  via exhaust conduit, as shown in  FIG.  13   . 
     Various types of engines and power-generating devices can be implemented as engine  140  in system  10 . In some embodiments, for example, engine  140  is a conventional four cycle piston-based engine, and the waste refrigerant fluid is introduced into a combustor of the engine. In certain embodiments, engine  140  is a gas turbine engine, and the waste refrigerant fluid is introduced via the engine inlet to the afterburner of the gas turbine engine. In other embodiments, is introduced into the afterburner of a gas turbine engine or after the afterburner to be incinerated by a spent exhaust stream from the engine. 
     As discussed above, in some embodiments, system  10  can include phase separator (not shown) positioned upstream from engine  140  and either downstream or upstream from compressor/vacuum pump  19 . Phase separator functions to prevent liquid refrigerant fluid from entering engine  140 , which may reduce the efficiency of electrical power generation by engine  140 . 
     X. Start-Up and Temporary Operation 
     In certain embodiments, the thermal management systems disclosed herein operate differently at, and immediately following, system start-up, compared to the manner in which the systems operate after an extended running period. Upon start-up, refrigerant fluid in receiver  12  may be relatively cold, and therefore the receiver pressure (p r ) may be lower than a typical receiver pressure during extended operation of the system. However, if receiver pressure p r  is too low, the system may be unable to maintain a sufficient mass flow rate of refrigerant fluid through evaporator  32  to adequately cool heat load  34 . 
     As discussed, receiver  12  can optionally include a heater  12   d  (especially when the system does not include the gas receiver  14 ). Heater  12   d  can generally be implemented as any of a variety of different conventional heaters, including resistive heaters. In addition, heater  12   d  can correspond to a device or apparatus that transfers some of the enthalpy of the exhaust from engine  140  into receiver  12 , or a device or apparatus that transfers enthalpy from any other heat source into receiver  12 . 
     During cold start-up, controller  17  activates heater  12   d  to evaporate portion of the refrigerant fluid in receiver  12  and raise the vapor pressure and pressure p r  This allows the system to deliver refrigerant fluid into evaporator  32  at a sufficient mass flow rate. As the refrigerant fluid in receiver  12  warms up, heater  12   d  can be deactivated by controller  17 . By heating refrigerant fluid within receiver  12  at start-up, the system can begin to cool heat load  34  after a relatively short warm-up period. 
     Controller  17  can also activate heater  12   d  to re-heat refrigerant fluid in receiver  12  between cooling cycles. Thus, for example, when the system runs periodically to provide intermittent cooling of heat load  34 , controller  17  can activate heater  12   d  when the system is not running to ensure that when system operation resumes, the receiver pressure p r  in receiver  12  is sufficient to deliver refrigerant fluid to evaporator  32  at the desired mass flow rate almost immediately. During the system operation the heater typically provides heat input at a reduced rate to maintain an acceptable refrigerant fluid pressure receiver  12 . Insulation around receiver  12  can help to reduce heating demands. 
     For most applications it is envisioned that the gas receiver  14 , storing nitrogen and/or the compressor can be used to elevate pressure in the refrigerant receiver  12  to start the system, would be more preferred options to the heater  12   d.    
     XI. Integration with Directed Energy Systems 
     The thermal management systems and methods disclosed herein can be implemented as part of (or in conjunction with) directed energy systems such as high energy laser systems. Due to their nature, directed energy systems typically present a number of cooling challenges, including certain heat loads for which temperatures are maintained during operation within a relatively narrow range. 
       FIG.  17    shows one example of a directed energy system, specifically, a high energy laser system  150 . System  150  includes a bank of one or more laser diodes  152  and an amplifier  154  connected to a power source  156 . During operation, laser diodes  152  generate an output radiation beam  158  that is amplified by amplifier  154 , and directed as output beam  160  onto a target. Generation of high energy output beams can result in the production of significant quantities of heat. Certain laser diodes, however, are relatively temperature sensitive, and the operating temperature of such diodes is regulated within a relatively narrow range of temperatures to ensure efficient operation and avoid thermal damage. Amplifiers are also temperature-sensitively, although typically less sensitive than diodes. 
     To regulate the temperatures of various components of directed energy systems such as diodes  152  and amplifier  154 , such systems can include components and features of the thermal management systems disclosed herein. In  FIG.  17   , evaporator  32  is coupled to diodes  152 , while heat exchanger  79  is coupled to amplifier  154 . The other components of the thermal management systems disclosed herein are not shown for clarity. However, it should be understood that any of the features and components discussed above can optionally be included in directed energy systems. Diodes  152 , due to their temperature-sensitive nature, effectively function as heat load  34  in system  150 , while amplifier  154  functions as heat load  35 . 
     System  150  is one example of a directed energy system that can include various features and components of the thermal management systems and methods described herein. However, it should be appreciated that the thermal management systems and methods are general in nature, and can be applied to cool a variety of different heat loads under a wide range of operating conditions. 
     XII. Hardware and Software Implementations 
     Controller  17  can generally be implemented as any one of a variety of different electrical or electronic computing or processing devices, and can perform any combination of the various steps discussed above to control various components of the disclosed thermal management systems. 
     Controller  17  can generally, and optionally, include any one or more of a processor (or multiple processors), a memory, a storage device, and input/output device. Some or all of these components can be interconnected using a system bus. The processor is capable of processing instructions for execution. In some embodiments, the processor is a single-threaded processor. In certain embodiments, the processor is a multi-threaded processor. Typically, the processor is capable of processing instructions stored in the memory or on the storage device to display graphical information for a user interface on the input/output device, and to execute the various monitoring and control functions discussed above. Suitable processors for the systems disclosed herein include both general and special purpose microprocessors, and the sole processor or one of multiple processors of any kind of computer or computing device. 
     The memory stores information within the system, and can be a computer-readable medium, such as a volatile or non-volatile memory. The storage device can be capable of providing mass storage for the controller  17 . In general, the storage device can include any non-transitory tangible media configured to store computer readable instructions. For example, the storage device can include a computer-readable medium and associated components, including: magnetic disks, such as internal hard disks and removable disks; magneto-optical disks; and optical disks. Storage devices suitable for tangibly embodying computer program instructions and data include all forms of non-volatile memory including by way of example, semiconductor memory devices, such as EPROM, EEPROM, and flash memory devices; magnetic disks such as internal hard disks and removable disks; magneto-optical disks; and CD-ROM and DVD-ROM disks. Processors and memory units of the systems disclosed herein can be supplemented by, or incorporated in, ASICs (application-specific integrated circuits). 
     The input/output device provides input/output operations for controller  17 , and can include a keyboard and/or pointing device. In some embodiments, the input/output device includes a display unit for displaying graphical user interfaces and system related information. 
     The features described herein, including components for performing various measurement, monitoring, control, and communication functions, can be implemented in digital electronic circuitry, or in computer hardware, firmware, or in combinations of them. Methods steps can be implemented in a computer program product tangibly embodied in an information carrier, e.g., in a machine-readable storage device, for execution by a programmable processor (e.g., of controller  17 ), and features can be performed by a programmable processor executing such a program of instructions to perform any of the steps and functions described above. Computer programs suitable for execution by one or more system processors include a set of instructions that can be used directly or indirectly, to cause a processor or other computing device executing the instructions to perform certain activities, including the various steps discussed above. 
     Computer programs suitable for use with the systems and methods disclosed herein can be written in any form of programming language, including compiled or interpreted languages, and can be deployed in any form, including as stand-alone programs or as modules, components, subroutines, or other units suitable for use in a computing environment. 
     In addition to one or more processors and/or computing components implemented as part of controller  17 , the systems disclosed herein can include additional processors and/or computing components within any of the control device (e.g., control device  18  and/or compressor/vacuum pump  19 ) and any of the sensors discussed above. Processors and/or computing components of the control devices and sensors, and software programs and instructions that are executed by such processors and/or computing components, can generally have any of the features discussed above in connection with controller  17 . 
     Other Embodiments 
     A number of embodiments have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the disclosure. Accordingly, other embodiments are within the scope of the following claims.