Patent Publication Number: US-2022228513-A1

Title: Waste heat recovery system and control

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to U.S. Provisional Application No. 62/855,234, filed May 31, 2019, entitled “Waste Heat Recovery System and Control,” the contents of which are incorporated herein in their entirety. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates to waste heat recovery (WHR) systems, and in particular to WHR systems used with vehicles. 
     BACKGROUND 
     A WHR system recovers heat energy from a vehicle component or system, such as from an internal combustion engine of the vehicle that would otherwise be lost. The more waste heat energy that is extracted from the component or system by a WHR system, the greater the potential efficiency of the engine. In other words, rather than the extracted heat being lost, the extracted heat energy may be repurposed to, e.g., supplement the power output from the internal combustion engine, thereby increasing the efficiency of the system. Some WHR systems use a Rankine cycle (RC), which is a thermodynamic process in which heat is transferred to a working fluid of the RC circuit. The working fluid is pumped through a heat exchanger where the working fluid is vaporized. The vapor is passed through an expander and then through a condenser, where the vapor is condensed back into liquid. The expander may drive a generator to generate electrical energy. An organic RC (ORC) is an RC in which the working fluid is an organic, high molecular mass fluid with a liquid-vapor phase change at a lower temperature than that of water. Such a fluid allows for heat recovery from lower temperature sources relative to other non-organic RC circuits. 
     SUMMARY 
     In one aspect, a WHR system includes at least two pressure circuits, each pressure circuit including at least one heat exchanger, each of the at least two pressure circuits providing a working fluid to the respective at least one heat exchanger and to an expander having a plurality of inputs corresponding to a number of the at least two circuits, and a controller communicably coupled with the at least two pressure circuits. The controller is configured to receive a first pressure value and a first temperature value from a first sensor positioned at one of an inputs of the expander. The controller is further configured to determine a superheated temperature value from the first pressure value based on at least one characteristic of the working fluid. The controller is also configured to determine whether the first temperature value is greater than the superheated temperature value. The controller is further configured to control, responsive to determining that the first temperature value is greater than the superheated temperature value, at least one of a pump pumping the working fluid or a bypass valve positioned across the pump to increase a flow of the working fluid. 
     In another aspect, a method is provided for operating WHR system that includes at least two pressure circuits, each pressure circuit including at least one heat exchanger, the at least two circuits providing a working fluid to the respective at least one heat exchanger and to an expander having a plurality of inputs corresponding to a number of the at least two circuits. The method includes receiving, at a controller, a first pressure value and a first temperature value from a first sensor positioned at one of an inputs of the expander. The method further includes determining, at the controller, a superheated temperature value from the first pressure value based on at least one characteristic of the working fluid. The method also includes determining, at the controller, whether the first temperature value is greater than the superheated temperature value. The method further includes controlling, by the controller responsive to determining that the first temperature value is greater than the superheated temperature value, at least one of a pump pumping the working fluid or a bypass valve positioned across the pump to increase a flow of the working fluid. 
     In one or more implementations, the method further includes receiving, at the controller, a second temperature value and a second pressure value from a second sensor positioned at an output of a subcooler receiving working fluids from the at least two circuits. The method also includes determining, at the controller, a subcooled temperature value based on the second pressure value and at least one characteristic of the working fluid. The method further includes determining, at the controller, whether the second temperature value is less than the subcooled temperature value. The method also includes controlling, by the controller, responsive to determining that the second temperature value is less than the subcooled temperature value, at least one valve increasing flow of the working fluid to or from a working fluid reservoir to the subcooler. 
     In one or more implementations, the method also includes receiving, at the controller, a third temperature value from a third sensor positioned at an output of at least one heat exchanger. The method further includes determining, at the controller, whether the third temperature value is less than a target temperature value. The method also includes controlling, by the controller responsive to determining that the third temperature value is less than the target temperature value, a bypass valve across the at least one heat exchanger to allow at least a portion of the working fluid to flow from upstream of the at least one heat exchanger to downstream of the at least one heat exchanger. 
     In one or more implementations, the method further includes receiving, at the controller, a third pressure value form a fourth sensor positioned at least one input of the plurality of inputs of the expander. The method also includes determining, at the controller, that the third pressure value is greater than a target pressure value. The method further includes controlling, by the controller responsive to determining that the third pressure value is greater than the target pressure value, a bypass valve across the expander to allow at least a portion of the working fluid from upstream of the at least one input of the plurality of inputs of the expander to downstream of an output of the expander. 
     In one or more implementations, the WHR system includes a transfer valve positioned between a first pressure circuit and a second pressure circuit of the at least two pressure circuits, the first pressure circuit having a working fluid pressure that is less than a working fluid pressure of the second pressure circuit. The method further includes receiving, at the controller, a fourth temperature value from a fourth temperature sensor positioned at an output of a heat exchanger in the first pressure circuit. The method also includes determining, at the controller, that the fourth temperature value is above a first circuit temperature target value. The method additionally includes controlling, by the controller, the transfer valve to transfer working fluid from the second pressure circuit to the first pressure circuit. 
     In one or more implementations, the WHR system includes a common pump providing working fluid to both a first pressure circuit and a second pressure circuit of the at least two pressure circuits, the first pressure circuit having a working fluid pressure that is less than a working fluid pressure of the second pressure circuit, and a transfer valve diverting a portion of the working fluid from the second pressure circuit to the first pressure circuit. The method further includes receiving, at the controller, a fourth temperature value from a fourth temperature sensor positioned at an output of a heat exchanger in the first pressure circuit. The method also includes determining, at the controller, that the fourth temperature value is above a first circuit temperature target value. The method additionally includes controlling, by the controller, the transfer valve to increase transfer of working fluid from the second pressure circuit to the first pressure circuit. 
     In yet another aspect, a system includes an engine, and a WHR system. The WHR system includes at least two pressure circuits, each pressure circuit including at least one heat exchanger receiving at least one fluid associated with the engine, the at least two pressure circuits providing a working fluid to the respective at least one heat exchanger and to an expander having a plurality of inputs corresponding to a number of the at least two circuits, and a controller communicably coupled with the at least two pressure circuits. The controller is configured to receive a first pressure value and a first temperature value from a first sensor positioned at one of an inputs of the expander. The controller is further configured to determine a superheated temperature value from the first pressure value based on at least one characteristic of the working fluid. The controller is also configured to determine whether the first temperature value is greater than the superheated temperature value. The controller is further configured to control, responsive to determining that the first temperature value is greater than the superheated temperature value, at least one of a pump pumping the working fluid or a bypass valve positioned across the pump to increase a flow of the working fluid. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The skilled artisan will understand that the drawings primarily are for illustrative purposes and are not intended to limit the scope of the subject matter described herein. The drawings are not necessarily to scale; in some instances, various aspects of the subject matter disclosed herein may be shown exaggerated or enlarged in the drawings to facilitate an understanding of different features. In the drawings, like reference characters generally refer to like features (e.g., functionally similar and/or structurally similar elements). 
         FIG. 1  shows a first example WHR system, according to an embodiment of the present disclosure. 
         FIG. 2  shows a second example WHR system, according to an embodiment of the present disclosure. 
         FIG. 3  shows a flow diagram of an example process for maintaining temperatures of the working fluid. 
         FIG. 4  shows a flow diagram of an example process for controlling a temperature of a fluid cooled by a heat exchanger. 
         FIG. 5  shows a flow diagram of an example process for controlling working fluid pressure at inputs of the expander. 
         FIG. 6  shows a flow diagram of a process for controlling transfer of working fluid from a high pressure circuit to a low pressure circuit, according to an embodiment of the present disclosure. 
         FIG. 7  shows a third example WHR system, according to an embodiment of the present disclosure. 
         FIG. 8  shows a fourth example WHR system, according to an embodiment of the present disclosure. 
     
    
    
     The features and advantages of the inventive concepts disclosed herein will become more apparent from the detailed description set forth below when taken in conjunction with the drawings. 
     DETAILED DESCRIPTION 
     Following below are more detailed descriptions of various concepts related to, and embodiments of, inventive WHR systems and methods of operating WHR systems. It should be appreciated that various concepts introduced above and discussed in greater detail below may be implemented in any of numerous ways, as the disclosed concepts are not limited to any particular manner of implementation. Examples of specific implementations and applications are provided primarily for illustrative purposes. 
     WHR systems can recover thermal or other forms of energy in a vehicle that would otherwise be dissipated and lost to the environment, and help convert the energy into usable electrical or mechanical energy. In particular, the WHR systems can absorb heat generated by various components of a vehicle, such as, for example, the engine or the exhaust. The WHR system can use the absorbed heat to impart motion to a heated working fluid, which, in turn, can drive or rotate a driving shaft. The driving shaft can be coupled to a final drive (such as, for example, wheels) or can be coupled to a drive shaft of a motor/generator that can convert the imparted torque into electrical energy. In hybrid vehicles, the WHR system can provide the electrical energy for charging a battery, which, in turn, can provide power to one or more electrical motors that drive the vehicle. 
     Some WHR systems can include more than one working fluid circuit. For example, a WHR system can include a high pressure circuit and a low pressure circuit, where the working fluid in the high pressure circuit is operated at a pressure higher than that of the working fluid in the low pressure circuit. Each circuit may absorb heat from more than one heat sources. For example, the heat sources can include the engine coolant, the engine charge air, the engine exhaust, the engine exhaust gas recirculation (EGR) system, etc. These heat sources provide various degrees of heat energy to the working fluid at various portions of the respective high and low pressure circuits. Moreover, the heat energy provided by these heat sources can vary over time. It is desired for the working fluid in each circuit to be operated within particular ranges of temperature and pressure that result in a more effective heat recovery. With multiple circuits and multiple heat sources the operating temperature or pressure of the working fluid may deviate from the desired ranges of temperature and pressure. This can result in inefficient heat recovery and conversion. This can also result in inefficient operation of the engine, in particular, where the temperature of certain fluids, such as the exhaust in the EGR, is desired to be maintained within strict range of values. 
     The WHR systems discussed herein provide a solution to the problem of maintaining the WHR circuit within the desired operating conditions. In particular, the WHR systems provide a control circuit that senses the temperature and pressure of the working fluid at various portions of the high pressure and low pressure circuits, and controls one or more parameters, such as for example, a flow rate of the working fluid and bypass valves, to maintain the temperature and pressure at these various portions of the circuit to within the desired range of values. This results in substantial increase in the performance of both the engine and the heat recovery system. 
       FIG. 1  shows a first example WHR system  100 . The first WHR system  100  includes a high pressure working fluid circuit  124  and a low pressure working fluid circuit  122 . The high pressure circuit  124  circulates a working fluid that is maintained at a higher temperature and pressure than the working fluid in the low pressure circuit  122 . In some scenarios, the high pressure circuit  124  can be connected to heat exchangers that exchange heat with high temperature heat sources, while the low pressure circuit can be connected to heat exchangers that exchange heat with relatively lower temperature heat sources. The low pressure circuit  122  includes a low pressure pump  118  and a recuperator  108  positioned downstream of the low pressure pump  118 . It should be noted that the terms “upstream” and “downstream” refer to the flow direction of the working fluid in the WHR system  100 . One or more heat exchangers are positioned downstream of the recuperator  108 . For example, a mixed use heat exchanger (MUHE)  106  is positioned downstream of the recuperator  108  and a low temperature heat exchanger (LTHE)  102  is positioned downstream of the MUHE  106 . The low pressure working fluid at the output of the LTHE  102  is provided to a low pressure input of the expander  110 . 
     Similarly, the high pressure circuit  124  includes a high pressure pump  120  and the recuperator  108  positioned downstream of the low pressure pump  118 . The recuperator  108  can include two working fluid channels, one for the high pressure circuit  124  and another for the low pressure circuit  122 . The high pressure circuit  124  further includes the MUHE  106  positioned downstream of the recuperator  108 . Like the recuperator  108 , the MUHE  106  includes a high pressure channel for the high pressure working fluid in the high pressure circuit  124  and a low pressure channel for the low pressure working fluid in the low pressure circuit  122 . The high pressure circuit  124  further includes a high temperature heat exchanger (HTHE) 104 positioned downstream of the MUHE  106  and receives high pressure working fluid from the high pressure channel of the MUHE  106 . The high pressure working fluid at the output of the HTHE  104  is provided to a high pressure input of the expander  110 . 
     The high pressure circuit  124  and the low pressure circuit  122  are combined into a common return circuit  154  at an output of the expander  110 . The working fluid from the output of the expander  110  is provided to a return channel of the recuperator  108 . The return circuit  154  further includes a condenser  114  and a subcooler positioned downstream of the return channel of the recuperator  108 . The working fluid at the output of the subcooler  116  is provided back to the low pressure pump  118  and the high pressure pump  120  positioned downstream of the low pressure pump  118 . 
     In some embodiments, the low pressure pump  118  and the high pressure pump  120  can be fixed flow pumps. In some embodiments, the pumps can be variable flow pumps. The recuperator  108  can be utilized to lower the temperature of the working fluid in the return circuit  154  and increase the temperature of the working fluids in the high pressure circuit  124  and the low pressure circuit  122 . The recuperator  108  receives the working fluid in the return circuit  154  from the expander  110 . This working fluid is typically at high temperature and in or close to a vapor state. The recuperator  108 , by transferring the heat from the working fluid in the return circuit  154  to the working fluid in the low pressure circuit  122  and the high pressure circuit  124  reduces the burden on the condenser  114  for cooling the working fluid. 
     The MUHE  106  can be coupled to low or medium temperature heat sources. In some embodiments, the MUHE  106  can be coupled to heat sources that are at a temperature that is lower than the boiling temperature of the working fluids in the low pressure circuit  122  and the high pressure circuit  124 . As an example, the MUHE  106  can be coupled to a charge air cooler circuit of a turbocharger. The charge air cooler circuit of a vehicle can receive charge air from a compressor of the turbocharger, cool the charge air, and provide the cooled charge air to an intake manifold of the engine. The MUHE  106  can contribute to cooling the charge air by transferring heat from the charge air to the working fluids in the low pressure circuit  122  and the high pressure circuit  124 . In some examples, the MUHE  106  can be coupled to other heat sources, such as the exhaust gas for the EGR system, or any other fluid of the vehicle. In some embodiments, the MUHE  106  can be optional. That is, the output of the recuperator  108  could be directly provided to the LTHE  102  and the HTHE  104 . 
     The LTHE  102  can be coupled to low temperature heat sources, similar to those discussed above in relation to the MUHE  106 . The HTHE  104  can be coupled to high temperature heat sources. As an example, the high temperature heat sources can include those heat sources that have a temperature that is greater than the boiling temperature of the working fluid in the high pressure circuit  124 . In some embodiments, the HTHE  104  can be coupled to heat sources such as the EGR exhaust gas from the EGR system, the exhaust gas directed to the tail pipe of the vehicle, an engine coolant from an engine cooling circuit, or some other high temperature heat source. The HTHE  104  and the LTHE  102  can provide enough heat to change the state of the working fluids in the respective low pressure and high pressure circuits  122  and  124  to the vapor state. 
     The expander  110  receives at least two working fluid inputs—one from the LTHE  102  and another from the HTHE  104 . In one embodiment, the expander  110  can be a dual input expander, with each input fluidly coupled to a respective one of the high pressure circuit  124  and the low pressure circuit  122 . In another embodiment, the expander  110  can include twin expanders, with each expander being fluidly coupled to a respective one of the high pressure circuit  124  and the low pressure circuit  122 . As the substantially vaporized working fluids travel through the expander  110 , the vapor expands and loses pressure, thereby driving a turbine of the expander  110  to generate useful work. In some embodiments, the turbine of the expander  110  is operatively coupled to a generator, which can convert the mechanical energy of the rotating turbine into electrical energy. In some embodiments, the turbine of the expander can be coupled to a crankshaft of the engine, an engine accessory shaft, and/or other components, such as, for example, via a gear or belt drive so as to transfer the mechanical energy from the turbine to those devices. According to various embodiments, the expander  110  can include a piston expander, a screw expander, a scroll expander, a gerotor expander, or other type of expander. In some embodiments, the expander  110  can have variable geometry input nozzles. The variable geometry nozzle can be adjusted to change the flow rate verses the pressure characteristics of the expander. 
     The condenser  114  is positioned along the return circuit  154  downstream of the recuperator  108 . The condenser  114  is structured to receive a high temperature working fluid and transfer heat from the working fluid to the ambient environment, thereby substantially or fully condensing the working fluid back to a liquid state. In some embodiments, the condenser  114  can be at least partially air cooled, and can be positioned off-engine in a vehicle cooling package area structured to receive ram air. 
     The subcooler  116  is positioned in the return circuit  154  downstream of the condenser  114 . The subcooler  116  is structured to ensure that the working fluid is in a subcooled liquid state before being provided to the low pressure and high pressure pumps  118  and  120 . Ensuring that the working fluid is in the liquid state reduces the risk of cavitation in the pumps, thereby improving the performance and the reliability of the pumps. 
     The receiver  112  is coupled to the condenser  114  and the subcooler  116  and can serve as a reservoir for the working fluid. Flow in and out of the receiver  112  can be used to control the flow rate of the working fluid through the subcooler  116  and fluid inventory in the condenser  114 . A first receiver valve  148  can be positioned downstream of the receiver  112  and upstream of the subcooler  116 . The first receiver valve  148  can be open during normal operation to increase liquid subcooling entering the pump or closed to decrease it. A second receiver valve  146  is positioned between a point that is downstream of the receiver  112  and upstream of the first receiver valve  148 , and a point that is downstream of the low pressure pump  118 . The second receiver valve  146  can be closed during normal operation, but can be opened to decrease the amount of subcooling offered by the subcooler  116 . 
     The WHR system  100  further includes a control system including a controller  170 , several sensors, and several actuators. For example, the sensors can include temperature and pressure sensors, and the actuators can include valves. The sensors and the valves can be communicably coupled to the controller  170 . In particular, the sensors can provide the controller  170  with values of the measured parameters, and the valves can receive actuating signals from the controller to actuate the valves. As shown in  FIG. 1 , the WHR system  100  includes subcooler sensors  158 , MUHE sensors  126  and  128 , LTHE sensors  132 , HTHE sensors  130 , and expander input sensors  134  and  136 . In some embodiment, all of the components processing working fluid in the WHR circuit can include sensors at their respective inputs and outputs. The actuators can include a low pressure pump bypass valve  144  that is positioned across the low pressure pump  118 , a high pressure pump bypass valve  150  that is positioned across the high pressure pump  120 , the low pressure pump  118  and the high pressure pump  120  (in particular to control their flow rate). The actuators also include a MUHE bypass valve  142  positioned across the MUHE  106 . The MUHE bypass valve  142  can include two valves, one valve each for the low pressure channel and the high pressure channel. The actuators further include the LTHE bypass valve  138  positioned across the LTHE  102 , and the HTHE bypass valve  140  positioned across the HTHE  104 . The actuators also include the expander bypass valve  156  positioned across the expander  110 . The expander bypass valve  156  can comprise two valves—one for bypassing the high pressure input and another for bypassing the low pressure input to the expander  110 . The actuators further include the first receiver valve  148  and the second receiver valve  146 . The actuator also can include a transfer valve  152  that can transfer working fluid between the high pressure circuit  124  and the low pressure circuit  122 . Details of the operation of the controller  170  are discussed further below. 
       FIG. 2  shows a second example WHR system  200 . The second example WHR system  200  is similar to the first example WHR system  100  discussed in relation to  FIG. 1  in that both the first and the second example WHR systems  100  and  200  include high pressure and low pressure working fluid circuits. In the second WHR system  200 , however, only one pump is used. For example, only the high pressure pump  120  is used to pump the working fluid. This means that only one channel of both the recuperator  108  and the MUHE  106  is utilized for heat exchange. At the output of the MUHE  106 , the working fluid is split into a high pressure circuit  224  and a low pressure circuit  222 . The pressure in the low pressure circuit  222  is maintained by a flow regulation valve  204 , which can be controlled by the controller  170 . In some embodiments, the flow regulation valve  204  can be positioned downstream of the recuperator  108  when the MUHE  106  is not present in the system, or to maintain the dual channel MUHE  106  discussed above in relation to  FIG. 1 . The second example WHR system  200  can be more reliable compared to the first WHR system  100  due to the reduced number of components. 
     With the large number of heat sources providing heat to the working fluid, and temperature requirements of some of the heat sources (e.g., the charge air), the desirable temperature and pressure of the working fluid can be maintained despite dynamic changes in the input heat and temperature requirement of the heat sources. In particular, the controller  170  can be configured to maintain the working fluid at the desired temperature at various locations in the system. For example, the controller  170  can be configured to maintain the working fluid at a superheated state before it enters the expander  110 , and to maintain the working fluid at a liquid or subcooled state when it exits the subcooler  116 . The controller  170  can also control the temperature and pressure of the working fluid based on the temperature requirements of the heat sources, such as the charge air temperature requirement. The controller  170  can be configured to manage the fluid inventory within the system. The controller  170  can further be configured to maintain the operation of the WHR system. Each of these control scenarios is discussed in detail below. 
       FIG. 3  shows a flow diagram of an example process  300  for maintaining temperatures of the working fluid. It is desirable to maintain the pressure and the temperature of the working fluid in both the low pressure circuit  122  and the high pressure circuit  124 , in particular at the input of the expander  110 , within ranges that correspond to the superheated state of the working fluid. Maintaining the working fluid in the superheated state at the input of the expander  110  can increase the efficiency of the WHR system, increase the power output of the expander  110 , and also reduce the risk of condensation within the expander—thereby reducing the risk of damage to the expander  110  due to liquid droplets. In addition, maintaining the temperature of the working fluid at subcooled temperatures at the output of the subcooler  116  can increase the amount of cooling provided to the engine components, thereby increasing engine performance. The process  300  includes receiving temperature T and pressure P input at an input of the expander  110  ( 302 ). Referring to  FIG. 1 , the temperature T and the pressure P can be measured by the expander input sensors  134  and  136  at the inputs of the expander  110 . Focusing on the high pressure circuit  124 , the controller can receive the temperature T and pressure P values from the expander sensor  134  (although the controller  170  can execute the process  300  to similarly maintain the temperature in the low pressure circuit  122  as well). The controller  170  can receive actual values of the T and P from the expander sensor  134 , or receive values that are representative of the values of the T and P. In such instances, the controller  170  can calculate the actual values of the temperature T and pressure P based on the representative values based on a look up table or a formula. 
     The process  300  further includes determining a desired superheated temperature Tsh based on the pressure P at the input of the expander  110  ( 304 ). The superheated temperature Tsh of the working fluid can vary based on the pressure the working fluid is operating under. In some embodiments, the controller  170  can use a look up table that is representative of the characteristic saturation curve of the working fluid. The look up table can provide the value of Tsh based on the measured value of P. In some embodiments, the controller  170  can use a formula to determine the superheated temperature Tsh. In some embodiments, the Tsh can be a range of values instead of a single temperature value. Under some operating conditions, it can be advantageous to maintain the working fluid above the normal Tsh target, as maintaining the temperature above the normal Tsh target can increase the efficiency of the WHR system  100  and increase the power generated by the expander  110 . Under some other operating conditions, it may be beneficial to reduce the temperature of the working fluid at the input of the expander to increase the capacity of heat extraction that can be provided to the various heat exchangers. This, in turn, can increase the efficiency and power output of the engine. 
     The process  300  includes determining whether the measured temperature T at the input of the expander  110  is greater than the target superheated temperature Tsh ( 306 ). As mentioned above, in some embodiments, Tsh target can represent a range of values. The controller  170  can determine whether the measured temperature T is greater than the range of values represented by Tsh target. The process  300  includes, in response to the measured temperature T being greater than the superheated temperature Tsh target, controlling the high pressure pump  120  and/or the high pressure pump bypass valve  150  to increase the flow rate of the high pressure pump  120  ( 308 ). For example, if the high pressure pump  120  is a variable flow rate pump, the controller  170  can control the pump itself to increase the flow rate, and therefore, the pressure of the high pressure circuit  124 . In embodiments, where the high pressure pump  120  is a fixed flow rate pump, the controller  170  can close the high pressure pump bypass valve  150  to feed a smaller portion of the fluid downstream of the high pressure pump  120  back upstream of the high pressure pump  120 , thereby increasing the effective flow rate, and therefore the pressure, of the high pressure circuit  124 . By increasing the pressure, the temperature T of the inlet of the expander  110  can be reduced to the desired Tsh target value or range of values. 
     The process  300  further includes, in response to the temperature T at the input of the expander  110  not being greater than the superheated temperature Tsh target, controlling the pump or the bypass valve to decrease the flow ( 310 ). In instances where the temperature T is undesirably below the superheated temperature Tsh, the controller  170  can decrease the effective flow of the high pressure circuit  124 . For example, the controller  170  can directly control the high pressure pump  120  if the pump is a variable flow pump, and/or can open, if closed, the high pressure pump bypass valve  150  to decrease the flow rate and the pressure in the high pressure circuit  124 . Thus, the controller  170  can control the pumps or the bypass valves across the pumps to control the flow rate, and therefore, the pressure. The controller  170  can therefore match the pressure of the working fluid to the temperature of the heat exchangers (indicated by the temperature of the working fluid at the input of the expander) to maintain the working fluid in the target superheated state. 
     The process  300  includes maintaining the temperature of the working fluid at the output of the subcooler  116 . The process  300  includes receiving temperature T and pressure P values from the output of the subcooler ( 312 ). In particular, the controller  170  can receive the temperature T and pressure P values from the subcooler sensors  158  positioned at the output of the subcooler  116 . Subcooling temperatures can refer to the temperature of the working fluid below its saturation temperature. Each working fluid, based on its saturation curve, can have a corresponding saturation temperature corresponding to a current pressure of the working fluid. Subcooling refers to reducing the temperature of the working fluid below the saturation temperature. The degree of subcooling can refer to the amount by which the temperature of the working fluid is below the saturation temperature. Increasing the amount or degree of subcooling can result in lower working fluid temperature, which can be utilized to further cool the fluids of the engine or the vehicle in the heat exchangers, thereby increasing the efficiency or the power output of the engine. On the other hand, lower subcooling allows for reduced condenser pressure and an increase in the expander  110  power. In some embodiments, the ideal subcooling temperature Tsc can be determined by the operator based on the tradeoff between increased engine power and increased expander power. 
     The process  300  includes determining the Tsc based on the P measured at the output of the subcooler  116  ( 314 ). As mentioned above, the degree of subcooling can be based on the saturation temperature of the working fluid. The saturation temperature of the working fluid may vary based on the operating pressure of the working fluid. The controller  170  can determine, based on a look up table or a mathematical formula representative of the characteristic curve of the working fluid, the saturation temperature, and therefore the Tsc based on the desired degree of subcooling. As an example, the controller  170  can determine Tsc=Tsat−x, where Tsat is the saturation temperature of the working fluid and ‘x’ is amount of desired subcooling in degrees. 
     The process  300  includes determining whether the temperature T at the output of the subcooler is greater than the desired subcooling temperature Tsc ( 316 ). In some embodiments, the Tsc can represent a range of values, and the controller  170  can determine whether the measured value of T is less than the range of values of Tsc. The process  300  includes, in response to a determination that the sensed temperature is less than the desired sub cooling temperature Tsc, increasing a flow of working fluid from the receiver  112  to the subcooler  116  ( 318 ), which increases subcooling. In particular, the controller  170  can open the first receiver valve  148  and close the second receiver valve  146 , allowing flow of working fluid from the receiver  112  to the subcooler. This increase flow of working fluid from the receiver  112  to the subcooler  116  increases the amount of subcooling at the output of the subcooler  116 . 
     The process  300  includes controlling the receiver valves to decrease the flow of working fluid from the receiver  112  to the subcooler  116  ( 320 ). In particular, the controller  170  can close the first receiver valve  148  and open the second receiver valve  146  to reduce the effective flow of the working fluid from the receiver  112  to the subcooler  116 , thereby reducing the amount of subcooling. The process can repeatedly monitor both the superheating and the subcooling of the working fluid at various locations in the working fluid circuit. 
       FIG. 4  shows a flow diagram of an example process  400  for controlling a temperature of a fluid cooled by a heat exchanger. In particular, the controller  170  can execute the process to control the temperature of engine fluids cooled by the one or more heat exchangers in the high pressure circuit  124  and the low pressure circuit  122 . As an example, the controller  170  can execute the process  400  to control the temperature of the charge air cooled by the MUHE  106  shown in  FIG. 1 . However, it should be noted that the controller  170  can execute similar processes in association with the other heat exchangers in the WHR system  100 , such as, for example, the LTHE  102  and the HTHE  104 . The process  400  includes determining a temperature at an output of the heat exchanger ( 402 ). In some embodiments, sensors  128  (or  126 ) at the working fluid output of the MUHE  106  can be used to indicate the temperature of the fluid cooled by the working fluid. In some embodiments, the WHR system  100  can include temperature and/or pressure sensors at the heat source output of the MUHE  106 . The process  400  further includes determining whether the temperature of the heat source is below a target temperature ( 404 ). In some embodiments, the charge air, which is provide to the input manifold of the engine, may have to be provided within certain temperature ranges to be effective. The operator can select the value or a range of values for target temperature Ttarget. 
     The process  400  includes opening a bypass valve if the temperature of the heat source is less than the target temperature ( 406 ). Having the temperature of the heat source below the target temperature Ttarget or a target temperature range, can indicate that the heat source is being over cooled. To reduce the amount of cooling provided to the heat source fluid, the controller  170  can activate the MUHE bypass valve  142  to bypass at least a portion of the working fluid from the input of the MUHE  106  to the output of the MUHE  106 . By bypassing some of the working fluid, the amount of cooling of the heat source fluid will be reduced, thereby potentially raising the temperature of the heat source fluid. The process  400  also includes closing a bypass valve if the temperature of the heat source is greater than the target temperature Ttarget or a range of target temperatures ( 408 ). Having the temperature greater than the target temperature can indicate that the heat source fluid is not getting enough cooling. Thus, the controller  170  can close, if open, the MUHE bypass valve  142 . In some embodiments, the MUHE bypass valve  142  can be a variable flow valve. In some such embodiments, the controller  170  can control the valve to regulate the amount of working fluid that is bypassed to attain the desired target temperature. The controller  170  can similarly bypass the working fluid in relation to both the low pressure channel and the high pressure channel of the heat exchangers. In some embodiments, the WHR system  100  can include at least one bypass valve for each channel through the heat exchanger. In some embodiments, the controller  170  can control the bypass valve associated with the high temperature channel first to control the temperature of the heat source fluid. As the high temperature channel carries high pressure working fluid, it offers a relatively higher level of heat exchange compared to the working fluid in the low pressure channel. In some embodiments, for an even faster response, the controller  170  can activate the bypass valve of both the high pressure and low pressure channels. 
       FIG. 5  shows a flow diagram of an example process  500  for controlling working fluid pressure at inputs of the expander. In some embodiment, the pressure at the expander  110  inputs may increase beyond a target pressure range or value. The target pressure range or value can be representative of a design pressure value of the WHR system  100 , which is preferably not to be exceeded. The controller can control the expander bypass valve  156  to control the input pressure. The process  500  includes determining a pressure P at an input of the expander  110  ( 502 ). The controller  170  can receive the pressure measured by the sensors  136  and  134  positioned at the inputs of the expander  110 . The process further includes determining whether the measured value of the pressure is greater than the target pressure value Ptarget ( 504 ). If the pressure is greater than the target pressure value, the controller  170  can activate the expander bypass valve  156  to bypass a portion of the working fluid at the input of the expander  110  to the output of the expander  110 . The controller  170  can monitor the pressure at both the high pressure and the low pressure inputs of the expander  110  and independently open the bypass valves associated with the high and low pressure inputs ( 506 ). The process  500  includes closing the bypass valve if the pressure at the input of the expander  110  is not greater than the target pressure ( 508 ). This can indicate that the pressure has stabilized and the controller  170  can therefore close the expander bypass valve  156 . 
       FIG. 6  shows a flow diagram of a process  600  for controlling transfer of working fluid from a high pressure circuit to a low pressure circuit. In some embodiments, the low pressure circuit  122  may become over-utilized and may not be able to sustain the desired cooling of the engine fluids. In particular, assume that the temperatures of the heat source fluids at the outputs of the MUHE  106  or the LTHE  102  is greater than the desired temperatures. This may occur when the working fluid in the low pressure circuit  122  does not have the capacity to absorb the heat provided by the heat source fluids. In some such instances, the controller  170  can transfer a portion of the working fluid in the high pressure circuit  124  into the low pressure circuit  122 . The process includes determining a temperature of a heat source fluid at an output of a heat exchanger in the low pressure circuit ( 602 ). The controller  170  can receive temperature T values from one or more heat exchangers, namely the MUHE  106  and the LTHE  102  in the low pressure circuit  122 . Temperature sensors at the output of channels of the heat exchangers receiving the heat source fluids can provide the controller  170  with the temperature values. The process  600  include determining that the temperature T is above a target temperature Ttarget value or a range of values ( 604 ). The temperature being above the target values can indicate that the working fluid is incapable of absorbing sufficient heat from the heat source fluid. The process  600  includes opening a transfer valve if the temperature is greater than the target temperature ( 606 ). The controller  170  can open the transfer valve  152  that is positioned between the low pressure circuit  122  and the high pressure circuit  124  at the output of the MUHE  106  to transfer working fluid from the high pressure circuit  124  to the low pressure circuit  122 . It should be noted that the transfer valve can be positioned anywhere in the WHR system  100  between the low pressure circuit  122  and the high pressure circuit  124 . By transferring the working fluid from the high pressure circuit  124  to the low pressure circuit  122  the capacity of the low pressure circuit  122  can be increased. The process  600  further includes closing the transfer valve if the temperature is below the target value or range of values ( 608 ). The temperature being below the target temperature can indicate that the working fluid in the low pressure circuit is capable of absorbing sufficient heat from the heat source fluids to maintain the temperature below the target temperature. The controller can control the transfer valve  152  to cease the transfer of the working fluid from the high pressure circuit  124  to the low pressure circuit. 
     In some embodiments, the expander  110  can provide variable flow into the expander  110  for the same entry pressure. In some embodiments, the expander  110  can include variable geometry nozzles or have a variable speed or variable displacement, to allow for variable flow. In some embodiments, the controller  170  can control the expander  110  to allow increased flow through the expander  110  without increasing the pressure and the saturation temperature of the working fluid. This can allow the expander to generate high power without affecting the pressure and temperature of the working fluid. Furthermore, this can allow the adjustment of the flow v. pressure curve of the working fluid in the high pressure circuit  124  independently of that of the working fluid in the low pressure circuit  122 . This can provide increased cooling of the heat source fluids while increasing the power generated by the expander  110 . 
       FIG. 7  shows a third example WHR system  700 . The third example WHR system  700  is similar to the second example WHR system  200  discussed in relation to  FIG. 2 . However, the third example WHR system  700  additionally includes a first flow adjustment valve  770  positioned between the high pressure circuit  224  and the low pressure circuit  222 . In particular, an input of the first flow adjustment valve  770  receives working fluid from a position in the high pressure circuit  224  that is downstream of the HTHE  104  and upstream of the expander  110 . The output of the first flow adjustment valve  770  selectively provides working fluid at a position in the low pressure circuit  222  that is downstream of the LTHE  102  and upstream of the expander  110 . In some instances, the controller  107  can open the first flow adjustment valve  770  to provide excess heat energy in the high pressure circuit  224  to the low pressure circuit  222 . The excess heat energy in the working fluid may be imparted by a high temperature source, such as, for example, a tail pipe exhaust of a vehicle. The expander  110  can have a flow capacity over which the expander  110  may not be able to convert the heat energy in the working fluid into useful work. Specifically, if the flow rate of the working fluid entering the expander  110  is greater than a threshold value that corresponds to a flow capacity of the expander  110 , the additional heat energy in the working fluid may not be converted by the expander  110  into useful work. The excess heat instead may result in the temperature of the working fluid in the return circuit  154  to be high, thereby increasing the burden on the condenser  114  to cool the working fluid. In such instances, where there is excess heat energy in the working fluid in one circuit, the excess heat energy can be transferred to the other circuit. 
     In some embodiments, the excess heat energy in the working fluid in the high pressure circuit  124  can be transferred to the working fluid in the low pressure circuit  122 . In particular, excess heat and flow of the working fluid in the high pressure circuit  224  can be transferred to the low pressure circuit  222  if the low pressure circuit portion of the expander  110  has unused flow capacity. As shown in  FIG. 7 , the first flow adjustment valve  770  is positioned between the high pressure circuit  224  and the low pressure circuit  222  upstream of the expander  110 . The excess heat energy that would otherwise be wasted in the high pressure circuit  224 , can now be utilized in the low pressure circuit  222  because of the available capacity in the low pressure circuit portion of the expander  110 . The controller  170  can receive temperature of the working fluid at the high pressure circuit  224  from the expander input sensors  134 , and can receive a flow rate in the low pressure circuit  222  from the expander input sensor  136 . If the temperature of the working fluid in the high pressure circuit  224  at the input of the expander  110  is greater than a threshold value, and the flow rate of the working fluid in the low pressure circuit  222  at the input of the expander  110  is below a threshold value, the controller  170  can open the first flow adjustment valve  770  to transfer a portion of the high temperature working fluid from the high pressure circuit  224  to the low pressure circuit  222  upstream of the expander  110 . The controller  170  can monitor the temperature received from the expander input sensor  134  in the high pressure circuit  224 , and the flow rate of the working fluid in the low pressure circuit at the input of the expander  110 . If the temperature decreases below a threshold value that indicates no excess heat energy and/or the flow rate in the low pressure circuit  222  is equal to or greater than a flow capacity of the expander  110  in the low pressure circuit  222 , the controller  170  can close the first flow adjustment valve  770 . 
       FIG. 8  shows a fourth example WHR system  800 . The fourth example WHR system  800  is similar to the second example WHR system  200  discussed in relation to  FIG. 2 . However, the fourth example WHR system  800  additionally includes a second flow adjustment valve  880  positioned between the high pressure circuit  224  and the low pressure circuit  222 . In particular, the input of the second flow adjustment valve  880  receives working fluid from a position in the high pressure circuit  224  that is downstream of the HTHE  104  and upstream of the high pressure circuit input of the expander  110 . The output of the second flow adjustment valve  880  selectively provides working fluid to a position in the low pressure circuit that is upstream of the LTHE  102 . The fourth example WHR system  800  can be used to provide heat back to heat sources coupled with the LTHE  102 . 
     As mentioned above, the LTHE  102  can be coupled with low or medium temperature heat sources, such as, for example, an engine coolant. The low temperature heat sources can have temperature that is lower than the boiling temperature of the working fluid. In some instances, such as where the coolant is the heat source, a low temperature of the coolant, such as during engine startup, can result in a cold start of the engine, which, in turn, can increase the proportion of harmful emissions in the exhaust gases. Therefore it is advantageous to have the coolant over a certain threshold value to enable the engine to more quickly warm up and therefore quickly reduce the proportion of harmful emissions in the exhaust gas. The controller  170  can control the second flow adjustment valve such that high temperature working fluid from the high pressure circuit  224  is directed into the LTHE  102  causing the heat from the working fluid to be transferred to the coolant) flowing through the LTHE  102 . As a result, the temperature of the coolant can rise, thereby warming up the engine more quickly than if the coolant were to be allowed to heat up under normal engine operations. 
     The controller  170  can receive temperature of the coolant from a temperature sensor  882  positioned at the outlet of the heat source channel of the LTHE  102 . If the temperature of the coolant is below a threshold value, the controller  170  can open the second flow adjustment valve  880  to allow a portion of the working fluid output by the HTHE  104  to flow into the input of the LTHE  102 . The controller  170  can monitor the coolant temperature reading received from the temperature sensor  882 , and if the temperature of the coolant is equal to or above a second threshold value (can be equal to the first threshold value), the controller  170  can close the second flow adjustment valve  880 . The second threshold value can be indicative of a desired coolant temperature. In this manner, by transferring heat energy from the WHR working fluid to one or more heat sources, the operation of the overall system can be improved. While  FIG. 8  show the transfer of working fluid into the LTHE  102 , the working fluid may also or alternatively be provided to the MUHE  106 . 
     The first and second flow adjustment valves  770  and  880  can be variable flow valves, i.e., the controller  170  can adjust the magnitude of flow rate of the working fluid through the valves. In such instances, the controller  170  can gradually increase the magnitude of flow through the valves, thereby gradually increasing the amount of working fluid flowing from the high pressure circuit  224  to the low pressure circuit  222 . 
     In some embodiments, the WHR systems discussed herein may not include a recuperator. In some embodiments, the WHR system discussed herein may include more than two circuits. That is, the WHR system may include circuits in addition to the low pressure circuit  122  and the high pressure circuit  124 . In some embodiments, the heat exchangers can be arranged in parallel instead of in series as shown in  FIGS. 1 and 2 . In some embodiments, the low pressure pump  118  and the high pressure pump  120  may be arranged in parallel instead of in series as shown in  FIG. 1 . The working fluid can include a number of different fluids such as, for example, R1233zd(E), R245fa, other refrigerants, ethanol, toluene, water, and other fluids or blends of working fluids. 
     For the purpose of this disclosure, the term “coupled” means the joining of two members directly or indirectly to one another. Such joining may be stationary or moveable in nature. Such joining may be achieved with the two members or the two members and any additional intermediate members being integrally formed as a single unitary body with one another or with the two members or the two members and any additional intermediate members being attached to one another. Such joining may be permanent in nature or may be removable or releasable in nature. 
     It should be noted that the orientation of various elements may differ according to other exemplary embodiments, and that such variations are intended to be encompassed by the present disclosure. It is recognized that features of the disclosed embodiments can be incorporated into other disclosed embodiments. 
     It is important to note that the constructions and arrangements of apparatuses or the components thereof as shown in the various exemplary embodiments are illustrative only. Although only a few embodiments have been described in detail in this disclosure, those skilled in the art who review this disclosure will readily appreciate that many modifications are possible (e.g., variations in sizes, dimensions, structures, shapes and proportions of the various elements, values of parameters, mounting arrangements, use of materials, colors, orientations, etc.) without materially departing from the novel teachings and advantages of the subject matter disclosed. For example, elements shown as integrally formed may be constructed of multiple parts or elements, the position of elements may be reversed or otherwise varied, and the nature or number of discrete elements or positions may be altered or varied. The order or sequence of any process or method steps may be varied or re-sequenced according to alternative embodiments. Other substitutions, modifications, changes and omissions may also be made in the design, operating conditions and arrangement of the various exemplary embodiments without departing from the scope of the present disclosure. 
     While various inventive embodiments have been described and illustrated herein, those of ordinary skill in the art will readily envision a variety of other mechanisms and/or structures for performing the function and/or obtaining the results and/or one or more of the advantages described herein, and each of such variations and/or modifications is deemed to be within the scope of the inventive embodiments described herein. More generally, those skilled in the art will readily appreciate that, unless otherwise noted, any parameters, dimensions, materials, and configurations described herein are meant to be exemplary and that the actual parameters, dimensions, materials, and/or configurations will depend upon the specific application or applications for which the inventive teachings is/are used. Those skilled in the art will recognize, or be able to ascertain using no more than routine experimentation, many equivalents to the specific inventive embodiments described herein. It is, therefore, to be understood that the foregoing embodiments are presented by way of example only and that, within the scope of the appended claims and equivalents thereto, inventive embodiments may be practiced otherwise than as specifically described and claimed. Inventive embodiments of the present disclosure are directed to each individual feature, system, article, material, kit, and/or method described herein. In addition, any combination of two or more such features, systems, articles, materials, kits, and/or methods, if such features, systems, articles, materials, kits, and/or methods are not mutually inconsistent, is included within the inventive scope of the present disclosure. 
     Also, the technology described herein may be embodied as a method, of which at least one example has been provided. The acts performed as part of the method may be ordered in any suitable way unless otherwise specifically noted. Accordingly, embodiments may be constructed in which acts are performed in an order different than illustrated, which may include performing some acts simultaneously, even though shown as sequential acts in illustrative embodiments. 
     The claims should not be read as limited to the described order or elements unless stated to that effect. It should be understood that various changes in form and detail may be made by one of ordinary skill in the art without departing from the spirit and scope of the appended claims. All embodiments that come within the spirit and scope of the following claims and equivalents thereto are claimed.