Abstract:
A modular evaporator which can be assembled from a number of standard modules is provided. Depending on the requirements, the modular evaporator can be assembled to meet a wide range of design cooling loads. Additionally, the modular evaporator is capable of generating and holding ice for thermal storage purposes, eliminating the need for external ice storage tanks. Furthermore, the heat transfer and thermal storage fluid for the evaporator can simply be water which considerably simplifies the system, lowers the cost, and increases the efficiency of the heat transfer loop.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application is related to and claims priority from prior provisional application Ser. No. 61/365,443, filed Jul. 19, 2010, entitled Modular Evaporator and Thermal Energy Storage for Chillers, the contents of which are incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to cold thermal energy storage systems as well as using such systems to optimize and reduce energy consumption of a building. In particular, this invention relates to a novel modular evaporator and thermal energy storage system for chillers. 
       BACKGROUND 
       [0003]    Improving the energy efficiency of building comfort systems has become increasingly more important due to rising energy costs, as well as increased awareness and concern over global warming as a result of humanity&#39;s rising consumption of carbon fuels for electrical energy generation, direct burn heating, and domestic hot water appliances. One area where these concerns can be addressed is through improving the efficiency of solar-based HVAC systems by generating ice during hours of sunshine for later use during the night or cloud cover when solar radiation is inadequate. In traditional HVAC system this can also be beneficial through leveling demand by shifting some of the load during peak hours of a day to off-peak times, thereby eliminating the need to build and run expensive and inefficient peak generator turbines (peakers). 
         [0004]    Demand control and increased efficiency is primarily accomplished by shifting the burden of cooling from the hottest time of the day to the nighttime when ambient temperatures, as well as demand, are considerably lower. Refrigeration equipment efficiency increases when the temperature lift requirement decreases. The difference in temperature lift between a hot day and a cool night can often be as high as 50%, thereby resulting in a massive drop in refrigeration equipment lift requirements and a corresponding efficiency increase. The demand for cooling is usually highest during peak hours when outside temperatures and solar radiation are at their highest levels which results in increased electrical consumption. In order to prevent strain on the power grid, utilities are often forced to use gas turbine peak generators for only a few of the hottest hours of the year. The efficiency of these generators is typically 40 to 50% lower than steam turbines which generate most of our electricity. An alternative to peak generation is Thermal Energy Storage (TES) technology. 
         [0005]    While there are different types of thermal storage systems on the market the most common designs are based on cold water or two-phase ice/water storage. In recent years the ice storage systems have increased in popularity due to a considerably higher energy storage density. Currently ice storage systems are commonly used in large buildings and campuses. These systems will generally contain chillers which cool a secondary heat transfer media (such as an ethylene-glycol solution) to below the freezing point of water and circulate it through the heat exchangers of ice storage tanks. 
         [0006]    Ice storage tanks are usually comprised of rectangular or cylindrical water-filled vessels containing heat exchangers. The heat exchangers are primarily made of circular copper or plastic tubing. The cooling solution flows through the heat exchanger thereby freezing the water. Examples of such systems are disclosed in U.S. Pat. No. 4,831,831 to Carter et al. and U.S. Pat. No. 6,247,522 to Kaplan et al. 
         [0007]    These types of systems have several shortcomings. First they occupy a considerable amount of floor space for the chiller and the ice storage tanks Secondly, the solutions used as the heat transfer media are generally expensive, toxic, and have inferior heat transfer properties to water which increases the required pumping energy. And finally, the ratio of ice volume to the full volume of the storage tank is not very high due to the heat exchanger coil occupying a considerable amount of the tank&#39;s volume. 
         [0008]    The process of calculating the growth of freezing water around multiple tubes is complicated and costly thereby making it impractical for commercial markets. The heat exchanger design is usually accomplished through an experimental approach which is expensive, time consuming and rarely produces satisfactory results. Pockets of water can be encapsulated by ice, then, when these pockets freeze, expansion can generate very high pressures which can damage the tubes and/or the shell. This problem is generally solved by restricting the entire tank water volume from freezing solid which in turn further reduces the average ice storage density and increases the size and weight of the tank required for meeting the cooling demand. 
         [0009]    Another example of an approach used for ice-based thermal energy storage systems is disclosed in U.S. Pat. No. 7,124,594 to McRell. The thermal energy storage apparatus is comprised of a tank filled with water and a heat exchanger consisting of a multitude of spiral copper tubing coils connected to upper and lower headers. During the ice generating mode these coils are filled with liquid refrigerant provided by a condensing unit which evaporates and freezes the surrounding water. During cooling mode the liquid refrigerant is pumped into cooling coils inside the air conditioning equipment where it evaporates and is fed into the ice storage tank coils, surrounded by a slurry of ice and water, and is cooled and condensed back into liquid. 
         [0010]    These systems are complicated and expensive. Also the density of ice storage is relatively low due to the fact that some of the water must remain unfrozen to ensure proper water circulation at the beginning of the cooling mode and to prevent coil damage due to the high pressures generated by the expansion of freezing ice. 
         [0011]    U.S. Pat. No. 6,079,481 to Lowenstein et al. discloses a thermal energy storage system where the heat exchanger assembly is made of substantially flat profile boards disposed in a rectangular tank filled with water. A cooling medium with a low freezing temperature flows from a chiller through the boards and freezes the water, and then this solution flows through the load and back through the boards thawing the ice. While this design is potentially capable of increasing thermal energy storage density, it still requires separate spaces for the chiller and the thermal storage unit and requires a heat transfer medium with a freezing temperature below that of water. 
         [0012]    Medium capacity chillers usually have direct expansion, tubes-in-shell evaporators. The refrigerant flows through the tubes and the water (or another heat transfer medium) circulates through the shell. Refrigerant is injected in the tubes and evaporates to cool the water. Each evaporator is designed for a specific load, so a chiller manufacturer must carry multiple models of evaporators with a wide range of capacities. Another shortcoming of tubes-in-shell evaporators is the necessity to prevent the heat transfer fluid from freezing on the tubes which would lead to reduction of their heat transfer properties and even their damage. 
       SUMMARY OF THE INVENTION 
       [0013]    According to preferred embodiments of the present invention, a modular evaporator which can be assembled from a number of standard modules is provided. Depending on the requirements, the modular evaporator can be assembled to meet a wide range of design cooling loads. Additionally, the modular evaporator is capable of generating and holding ice for thermal storage purposes, eliminating the need for external ice storage tanks. Furthermore, the heat transfer and thermal storage fluid for the evaporator can simply be water which considerably simplifies the system, lowers the cost, and increases the efficiency of the heat transfer loop. 
         [0014]    According to a preferred embodiment the modular evaporator and thermal energy storage apparatus comprises of a number of rectangular modules and two end plates. Cold plates are located between adjacent modules. Modules contain manifolds for distribution of water and refrigerant. All modules and end plates are compressed together forming a water tight vessel that is filled with water. Liquid refrigerant is distributed through the refrigerant manifolds and headers and is injected into the cold plates where it evaporates thereby cooling the cold plates and is then removed from the suction headers and manifolds by the chiller compressor. Water can be pumped into the modular evaporator through water supply manifolds and headers to jet-generating nozzles. In cooling mode, the jets thaw the ice and/or transfer heat to the cold plates. 
         [0015]    Advantages of certain embodiments may include more compact thermal energy storage systems, simplification and reduction in the cost of production of chillers and thermal energy systems, and a considerable increase in the energy efficiency and comfort level of the conditioned environment. 
         [0016]    Other advantages will be readily apparent to one skilled in the art from the following figures, descriptions, and claims. Moreover, while specific advantages were enumerated above, various embodiments can include some additional advantages while others may be absent. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]      FIG. 1  is a simplified exploded view of a modular evaporator and thermal energy storage apparatus, according to a preferred embodiment of the present invention; 
           [0018]      FIG. 2  is a view of the assembled modular evaporator and thermal energy storage apparatus; 
           [0019]      FIG. 3  is a view of the assembled modular evaporator and thermal energy storage apparatus with a separated end plate; 
           [0020]      FIG. 4  is a view of a single module and cold plate of the modular evaporator and thermal energy storage apparatus; 
           [0021]      FIG. 5  is a horizontal cross section view of the module showing the water turbulence generated by the jets from staggered nozzles; 
           [0022]      FIG. 6  is a view of a single module and cold plate of a modular evaporator and thermal energy storage apparatus, according to an alternate preferred embodiment of the present invention; 
           [0023]      FIG. 7A  is a view of a cold plate comprised of a number of brazed together micro multiport extrusions, headers, and manifolds; 
           [0024]      FIG. 7B  is a view of a number of brazed cold plates assembled together with gaskets; 
           [0025]      FIG. 8A  is a cross section view of a cold plate comprising of a flat sheet of metal and a rolled profile before brazing or bonding; 
           [0026]      FIG. 8B  is a cross section view of a brazed cold plate comprising of a flat sheet of metal and a rolled profile; 
           [0027]      FIG. 9  is a vertical cross section view of the module (with a second module attached); 
           [0028]      FIG. 10  is a block diagram of the chiller and thermal energy storage system containing a single modular evaporator, according to a preferred embodiment of the present invention; 
           [0029]      FIG. 11  is a graph illustrating the process of ice growth in a system of  FIG. 10 ; 
           [0030]      FIG. 12  is a block diagram of the chiller and thermal energy storage system containing multiple modular evaporators; and 
           [0031]      FIG. 13  is a graph illustrating the process of ice growth in a system of  FIG. 12 . 
       
    
    
     DETAILED DESCRIPTION 
       [0032]    A preferred embodiment of a modular evaporator  100  incorporating the principles of the present invention is depicted in  FIG. 1 . As shown, and as will be described in greater detail, the modular evaporator  100  comprises several modules  101  held between respective end plates  102 ,  103 . As depicted, direct expansion cold plates  104  are located between adjacent modules  101  in such a way that they are capable of being compressed by the modules  101 . The cold plates  104  are sealed by gaskets. 
         [0033]    An assembled modular evaporator  100  is depicted in  FIG. 2 . The modules  101  and end plates  102 ,  103  are compressed and fastened together, preferably, by rods  205 , forming a water tight vessel. The assembled modular evaporator  100  includes water supply sockets  201  and water return sockets  203 , liquid refrigerant sockets  204 , and suction sockets  202 .  FIG. 3  shows the modular evaporator with the end plate  102  detached. 
         [0034]      FIG. 4  depicts a single module  101  in more detail. It is to be understood that the sockets  201 ,  202 ,  203 , and  204  are extensions of the manifolds formed by holes  402 ,  411 ,  406 , and  407  in the module  101 , respectively. The horizontal cross section A-A of the module  101  with an additional cold plate associated with adjacent module is represented by  FIG. 5 . The vertical headers  502  are connected to the supply manifolds  402 . Dual sets of nozzles  403  and  404  are located on the vertical bars of the module frame  414  with angles  506 , preferably opposite from one another respective to the horizontal axis of the vertical plane of the module such that the nozzles are in fluidic communication with the headers  502 . Vertical slots  405  are connected to the horizontal water return manifolds  406  and function as the drains of the modular evaporator (slots or holes may be used for this purpose). The cold plates  104  can be formed of flat multiport aluminum extrusions  501  assembled side by side, as shown. The width of the cold plates  104  is selected to provide the distance  505  between the ends of the nozzles  403 ,  404  and the vertical edges of the cold plates  104  approximately equal to the half of the distance between two adjacent cold plates  104 . The rate of ice grow in the ice generating mode in the horizontal direction parallel to the plate is considerably slower than in the perpendicular direction. The selection of the distance  505  assures that ice doesn&#39;t reach the nozzles, and that water in the area adjacent to the nozzles never freezes when the water in the spaces between the adjacent cold plates is frozen solid and the process of the ice formation is stopped. 
         [0035]    A method of determining this event is provided in U.S. Pat. No. 7,832,217 to Reich, which is herein incorporated by reference in its entirety. 
         [0036]    It is to be understood that gasket  410  prevents water leakage from the modular evaporator when all the modules are compressed together by the rods  205 . Preferably, the periphery of each module  101  is covered with thermal insulation  401 . 
         [0037]    The vertical cross section B-B of the two modules  101  side-by-side is depicted in  FIG. 9 . Voids  408  and  903  on the bottom of the module and  412  and  901  on the top of the module form liquid refrigerant headers and suction headers when adjacent modules are compressed together. The gaskets  409  and  413  prevent the refrigerant from escaping the hermetic refrigerant system. The headers are connected with liquid refrigerant manifolds  407  and suction manifolds  411 . Although two liquid and two suction manifolds are depicted in the figures, it should be understood that the design may be implemented with any number of manifolds. The cold plate  104  protrudes into the headers to prevent accidental obstruction of the fine ports of the extrusions  501  by debris. The cold plate  104  is bonded to the frame of the module by bonding compound  902  both on liquid and suction sides. Alternatively, refrigerant leak prevention can be accomplished by completely surrounding the cold plates by gaskets on both the liquid and suction header sides. 
         [0038]    An alternative preferred design of the module  101  is depicted in  FIG. 6 . This design has both liquid and suction refrigerant manifolds on the top side of the frame of the module. The cold plate has a lower header made of a tube  604  with a slot running lengthwise across the tube. The tube has plugs  607  on both sides. The tube  604  is brazed to the bottoms of the multiport extrusions  609  and  610  and serves as the bottom distribution header. The top header  608  has two slots  602  to accommodate dividers which are installed in the process of assembly and divide the header in three parts. The sections located at the ends of the header are connected to the liquid manifolds  611  and serve as liquid refrigerant headers. The central section of the header is connected to the suction manifold  601  and serves as a suction header. The multiport extrusions  610  located on the sides of the cold plate have fluidic communication with the liquid refrigerant header. The direction of flow of the refrigerant in these extrusions is shown by the arrows  605 . The central extrusions  609  are in fluidic communication with the suction part of the header  608 . The direction of flow of the refrigerant in these extrusions is shown by the arrows  606 . The gaps  603  between the adjacent extrusions connected to the liquid and suction parts of the header  608  are made wide enough to accommodate the dividers between the liquid and suction parts of the header  608 . 
         [0039]    The refrigerant when injected in the liquid sections of the header  608  flows through the ports of the extrusions  610  down to the bottom header  604  which provide fluidic communication among all the extrusions of the cold plate. Then the refrigerant flows through the ports of the extrusions  609  to the suction section of the header  608 . It is possible to reverse the liquid and suction manifolds. The design can be also implemented with any numbers of liquid and suction headers (for example, one liquid and two suction). 
         [0040]    An alternative preferred design of the cold plates is shown in the  FIG. 7A . In this case, the cold plate comprises a number of multiport extrusions  501 , liquid header tube  702 , suction header tube  703 , liquid manifold sections  704 , and suction manifold sections  705 . Slots run along the length of the header tubes and the extrusions are inserted in these slots. The manifold sections  705  and  706  have both male and female connectors  704  and  707 . The whole assembly is brazed together. When this cold plate is inserted inside the module frames  101  and modules are compressed together with gaskets at connectors  704  and  707  a refrigerant tight assembly is formed. This assembly  710  without frames is depicted in  FIG. 7B . 
         [0041]    Another alternative preferred design of the cold plate is presented in  FIG. 8A . The plate is comprised of two parts, a rolled sheet of metal with multiple channels  802 , and a flat sheet of metal  801 . These two sheets  801 ,  802  are brazed together forming a multiport heat exchanger  800  depicted in  FIG. 8B . In lieu of rolled channel, corrugated sheet metal can be bonded between two flat metal plates. 
         [0042]    The connection of the modular evaporator in the refrigerant and water loops is shown in  FIG. 10 . The compressor  1001  compresses the dry low pressure cool refrigerant coming through the suction line from the modular evaporator  100  converting it into hot high pressure gas. This gas enters the condenser  1002  and condenses there into liquid. Although an air cooled condenser is shown in  FIG. 10  it can also be water cooled. The liquid refrigerant enters the receiver  1003  and accumulates there. The liquid refrigerant goes from the receiver  1003  through the filter-drier  1013  to the expansion valve  1004  which rations the liquid into the modular evaporator  100  and reduces its pressure partly flashing it into gas. The low pressure liquid and gas mixture flows through the liquid manifolds  407 , the liquid headers formed by voids  408  and  903  and into the ports of the cold plates  104 . There the liquid refrigerant evaporates cooling the cold plates. The controller  1005  receives signals from the pressure sensor  1009  and temperature sensor  1010  in the suction line, calculates the superheat, and modulates the expansion valve  1004  to maintain the superheat at the set point. This control strategy assures that the maximum volume in the internal space of the cold plates has liquid refrigerant present, and at the same time, only a negligible quantity of liquid refrigerant leaves the cold plates. An almost dry, low pressure refrigerant vapor travels from the cold plates to the suction header formed by voids  412  and  901  of adjacent plates, through suction manifolds  411  and back to the suction line of the compressor  1001 . 
         [0043]    The water loop of the system can be arranged in several configurations. In a preferred embodiment, it comprises of a main circulating pump  1007  which circulates water through the modular evaporator  100  and the main loop  114 . Local pumps  1012  circulate water through loads  1008 . 
         [0044]    The system of  FIG. 10  can function in the following distinctive modes: chiller mode, ice generation mode, ice harvesting mode, and hybrid mode. 
         [0045]    In the chiller mode the compressor  1001  and water pumps  1007  and  1012  are on. The refrigerant&#39;s suction pressure is kept at a point corresponding to a temperature above the freezing point of water by modulating the expansion valve. The water pump  1007  injects warm water from the loads  1008  into the evaporator  100 , flows through the manifolds  402 , vertical headers  502  and into the nozzles  403  and  404 . The nozzles generate water jets directed at the surfaces of the cold plates which facilitate the heat transfer from the water to the cold plates causing the liquid refrigerant to evaporate. The cooled water leaves the modular evaporator through drain slots  405  and return manifolds  406  and is injected in the main water loop  1014 . The pumps  1012  extract the required quantity of cold water from the main loop  1014  to feed the loads  1008 . The warm water from the load is injected back into the main loop  1014 . 
         [0046]    The water supply temperature  1011  is measured by the controller  1005 . When the supply water temperature  1011  drops to the set point (which is above the freezing temperature of water) the controller  1005  turns the compressor off. When the supply water temperature rises to the set point plus a dead band the compressor is turned back on. A large volume of water in the modular evaporator minimizes cycling of the compressor. The other way of controlling the supply water temperature is by modulating the output capacity of the compressor. 
         [0047]    The preferred embodiment of the module depicted in  FIG. 6  has two columns of nozzles  403  and  404  on each vertical bar of the module frame. The nozzles on each column are staggered both adjacently and on the opposing sides of the frame. This staggering facilitates intensive turbulence in the water space between the two adjacent cold plates  104 . The turbulence is illustrated in  FIG. 5  by arrows  503  and  504 . This turbulence in turn facilitates an increase in the rate of heat transfer between the water and the cold plates. The angle  506  between the nozzle axis and the module plane is selected to maximize the jet flow on the surface of the cold plate and, at the same time, minimizing leakage of the jet water into the adjacent space. 
         [0048]    In ice generating mode the compressor  1001  is on and the pumps  1007  and  1012  are off. The ice grows on both sides of the cold plates  104 . Ice is a relatively good thermal insulator by comparison with water in the presence of convection. Therefore the heat transfer rate from the freezing water to the refrigerant drops during the process of ice growth. As a result of this process the suction pressure also drops as shown in graph  1101  on  FIG. 11 . When the pressure drops to the set point  1102 , the controller  1005  starts opening the hot gas bypass valve  1006  thereby injecting hot gas into the modular evaporator  100  and maintaining the suction pressure at a constant set point. Alternatively, instead of using this hot gas bypass technique, compressor capacity modulation can be used. 
         [0049]    Another way of controlling the suction pressure is having multiple modular evaporators connected in parallel as shown in  FIG. 12 . Each evaporator has its own modulating expansion valve with shutdown capability  1201 . The graph of the process of ice growth is shown in  FIG. 13 . The process starts with ice growth in the first evaporator. When suction pressure reaches the set point  1301  the second evaporator is turned on, and so on. The process continues until the last evaporator is turned on by opening the corresponding valve  1201  and the suction pressure is dropped to the set point  1301 . When the pressure drops to the set point  1102 , the controller  1005  starts opening the hot gas bypass valve  1006 , thereby injecting hot gas into the modular evaporators  100  and maintaining the suction pressure at a constant set point. The process of ice growth continues until the desired amount of ice is accumulated or the thickness of the ice on each side of the cold plates is equal of the half the distance between two adjacent plates. 
         [0050]    One of the major advantages of the flat cold plate heat exchanger is the predictability of the process of ice growth. The outside surface of the ice slab is approximately parallel to the plate. When the water freezes it expands and squeezes out excess water between the ice slabs to the sides preventing excessive pressure build up. The method of calculating the ice thickness is disclosed in the U.S. Pat. No. 7,832,217 to Reich. Using measurements of the refrigerant in the suction line from the pressure sensor  1009  and the temperature sensor  1010 , the controller  1005  calculates an integral starting from the moment when ice accumulation begins (refrigerant saturation temperature Tr drops below freezing point of water): 
         [0000]        I ( t )=∫ Tr ( T )* dT  
 
         [0000]    where Tr is changing with time t. The thickness of the ice on one side of a cold plate is calculated using the following formula: 
         [0000]    
       
         
           
             x 
             = 
             
               
                 
                   2 
                   * 
                   I 
                   * 
                   K 
                   * 
                   
                     Ui 
                     / 
                     pi 
                   
                 
                 ci 
               
             
           
         
       
     
         [0000]    where Ui is the thermal conductance of ice, ρi is the density of ice, Ci is the latent heat of ice, and K is a correction coefficient associated with the design parameters of the heat exchanger (experimentally derived). When the thickness of the ice reaches the desired value or the half distance between adjacent cold plates (whichever is greater) the process of ice growth is stopped by turning off the compressor. 
         [0051]    In the ice harvesting mode the compressor is turned off and the water pumps are turned on. The warm water coming from the loads  1008  are fed to the nozzles  403  and  404  which generate warm water jets and thaw the ice. Cold water is supplied to the loads  1008  by pumps  1007  and  1012 . 
         [0052]    In hybrid mode the compressor  1001 , as well as the water pumps  1007  and  1012 , are on. The temperature of the cold plates  104  are allowed to drop below the freezing point of the water. Ice grows on the surfaces of the cold plates. Simultaneously warm water jets generated by the nozzles  403  and  404  thaw the ice. When the heat load drops, the quantity of ice increases. Conversely, when the load increases, the quantity of accumulated ice decreases. As a result the sum of the latent heat of the thawed ice and the refrigeration cycle match the cooling load. This mode allows for a reduction in the installed capacity of the whole refrigeration system. In other words, a smaller compressor and condensing unit could be utilized. It should be understood that instead of water other heat transfer liquids can be used, as an example, a solution of ethylene glycol in water. 
         [0053]    While this invention has been described in conjunction with the various exemplary embodiments outlined above, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art. Accordingly, the exemplary embodiments of the invention, as set forth above, are intended to be illustrative, not limiting. Various changes may be made without departing from the spirit and scope of the invention.