WATER HEATER WITH MULTIPLE CONDENSERS

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001]This application claims priority to U.S. Provisional Ser. No. 63/380,648, which was filed on Oct. 24, 2022, the entire contents of which is hereby incorporated by reference.

BACKGROUND

[0002]The present invention relates to water heaters, and more particularly to commercial or residential hot water heaters, and even more particularly to heat pump hot water heaters.

[0003]Heat pump hot water heaters operate by transferring heat from a generally low temperature heat source (e.g. ambient air, the ground, surface water) to water that is to be heated, by way of a refrigeration cycle. Heat is absorbed to a liquid or mostly liquid refrigerant flow on the low-pressure side of the refrigerant cycle, thereby vaporizing the refrigerant, after which the now vapor refrigerant is compressed to a high pressure and high temperature vapor state. The absorbed heat (along with heat occurring due to efficiency losses in the compressor) is then transferred to a flow or store of water in order to produce hot water. In some cases, the heated water is stored in a tank to be used on demand.

[0004]In some heat pump water heater systems (sometimes referred to as integrated or unitary systems) the water tank and refrigeration cycle components are integrated into a single unit. In such systems, the heat transfer from the refrigerant to the water can be accomplished by way of a condenser that is disposed against an outer surface of the water tank. As the hot refrigerant travels through the condenser channels, heat is transferred through the channel walls and the tank wall to the water within the tank, thereby heating the water and cooling and condensing the refrigerant. The heated water near the tank wall will then rise to the top of the tank by buoyancy, to be replaced by cooler water within the tank.

[0005]Since only a small portion of the water within the tank is in contact with the heated tank wall surface, the heating performance of such a system may be limited. To increase the performance, the condenser may be increased in size so as to cover the whole height of the tank, but this may cause the water temperature at the top of the tank to undesirably increase to a temperature that exceeds the desired water delivery temperature, a phenomenon referred to as “stacking”. The tank size itself may be increased in order to provide additional heating surface area, but this is often undesirable as the available space to locate the water heater system is often limited.

[0006]A need therefore exists for an integrated heat pump water heater design that addresses these and other issues known in the art.

BRIEF DESCRIPTION OF THE DRAWINGS

[0007]FIG. 1 is a schematic illustration of an exemplary heat pump water heater system as known in the art.

[0008]FIG. 2 is a schematic illustration of a heat pump water heater system using two condensers in a first configuration.

[0009]FIG. 3 is a schematic illustration of the heat pump water heater system using two condensers in a second configuration.

[0010]FIG. 4 is a schematic illustration of the heat pump water heater system using two condensers in a third configuration.

[0011]FIG. 5 is a Pressure-Enthalpy diagram depicting a refrigerant cycle of the heat pump water heater system.

DETAILED DESCRIPTION

[0012]Before any embodiments of the invention are explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the following drawings. The invention is capable of other embodiments and of being practiced or of being carried out in various ways. While three different heat pump water systems are shown, these systems are combinable to yield other configurations that are not necessarily explicitly disclosed herein. Also, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting.

[0013]FIG. 1 illustrates a heat pump water heater system 1 known in the art. The system 1 includes a tank 5, a compressor 10, an expansion device 60, and an evaporator 15. The tank includes a tank inlet 25 that is fluidly connected to a tank outlet 30 by way of a tank volume 35. The tank inlet 25 delivers cold water into the tank volume 35 through a port 26 near the bottom of the tank 5. Once the water has been heated to a desired temperature, the heated water is drawn out through the tank outlet 30 for use. A condenser 20 is disposed against a wall 40 of the tank 5, such that the condenser 20, by way of the wall 40, is able to transfer heat into the water within the volume 35 of the tank 5. In some cases, the condenser is a micro-channel style heat exchanger that includes multiple channels arranged in a series-parallel arrangement, each channel encircling the tank volume 35, as shown in FIG. 1. In other cases, the condenser is a single coil that is spirally wrapped around the tank wall.

[0014]The refrigerant, after heating water in the tank volume 35, travels from the condenser 20 to the evaporator 15 by way of a return line 55. The return line 55 includes the expansion device. By way of example, the expansion device 60 can take the form of a thermal expansion valve, an electronically controlled expansion valve, a fixed orifice, etc. Upstream of the expansion device 60, the refrigerant in the return line 55 is in a high-pressure liquid state, having been condensed in the condenser 20. As the refrigerant passes through the expansion device 60, it transitions to a low-pressure two-phase (liquid and vapor) state. This transition occurs essentially adiabatically (i.e. with virtually no gain or loss of heat, and no mechanical work performed). The return line 55 connects to the evaporator 15. Because the expansion in the expansion device 60 is adiabatic, the two-phase refrigerant exits the expansion device 60 and enters the evaporator 15 at a substantially lower temperature than that at which it entered the return line 55.

[0015]In the evaporator 15, the refrigerant absorbs heat from a heat source (for example, ambient air or a water source). Due to the low boiling temperature of the low-pressure refrigerant, a substantial quantity of latent heat can be delivered to the refrigerant from a relatively low temperature heat source. The refrigerant is vaporized in the evaporator 15, and exits as a low-pressure, slightly superheated vapor. The evaporator 15 is connected to the compressor 10 by way of a first line 45. Within the compressor 10, the refrigerant is compressed to a high-pressure, superheated vapor state. The compressor 10 is connected to the condenser 20 by a second line 50. As a result, to heat water within the tank volume 35, the refrigerant travels through a loop circuit from the condenser 20, into the evaporator 15, into the compressor 10, and back into the condenser 20.

[0016]The thermodynamic cycle of the refrigerant as it passes through the loop circuit can be understood with reference to FIG. 5. FIG. 5 shows a pressure-enthalpy diagram for an exemplary refrigerant (R134a), with a liquid/vapor phase boundary envelope 250. Also shown are three isotherm lines, corresponding to 50° F., 140° F., and 180° F. As can be seen from the horizontal nature of the isotherm lines within the dome of the envelope 250, adding/removing heat (enthalpy) to/from the refrigerant when it is within that dome occurs at a constant temperature.

[0017]With continued reference to FIG. 5, the refrigerant cycle is shown by the quadrilateral loop extending between points 200, 210, 220, and 230. The points 200 and 210 represent the refrigerant into and out of, respectively, the compressor 10. The refrigerant enters the compressor at a low-pressure and low-temperature vapor state, and is compressed to a substantially higher pressure and substantially higher temperature vapor state. In the condenser 20, between points 210 and 220, the vapor refrigerant gives up its heat to the water and is condensed to a subcooled liquid state. The expansion device 60 is between the points 220 and 230, wherein the high-pressure liquid refrigerant is expanded, at a constant enthalpy, to a low-pressure two-phase state with a low boiling point. The cycle is completed with the evaporator 15, wherein heat is added to the refrigerant at a relatively low temperature in order to produce the superheated vapor state at 200.

[0018]Water will tend to sit within a bottom of the tank volume 35 as a result of gravity. Further, due to buoyancy effects, hotter water in the tank volume 35 will tend to rise toward the top of the tank volume 35 and cooler water will tend to sink toward the bottom of the tank volume 35. Because of these principles, the tank outlet 30 is positioned at the top of the tank 5, where the hottest water is located, and the tank inlet 25 is positioned at the bottom of the tank 5, so as to not cool the already-heated water located in other portions of the tank volume 35 (alternatively, the inlet can be located at the top of the tank and an internal dip tube can be used to deliver the cold water to the bottom of the tank). The refrigerant is at its highest temperature when within the second line 50 exiting the compressor 10. As a result, the second line 50 connects to condenser 20 on the tank 5 at a location near the top of the tank to ensure that the water near the tank outlet 30 is at the target temperature during a water draw.

[0019]FIG. 2 illustrates an embodiment of a heat pump water heater system 1 that provides advantages over the embodiment shown in FIG. 1. The heat pump water heater system 1 in FIG. 2 includes a first condenser 65 positioned along the second line 50. In some embodiments, the first condenser 65 is a brazed plate heat exchanger and/or a double wall plate heat exchanger. This type of heat exchanger provides an advantage in that it can efficiently heat a liquid in a relatively small dimensional area. In other embodiments, the first condenser 65 is an annular coiled heat exchanger. As a result of being placed along the second line 50, refrigerant at its hottest temperature at the exit of the compressor 10 passes through the condenser 65 on its way to the condenser 20. The condenser 65 includes a condenser inlet 70. When there is a draw of water from the tank 5, the condenser inlet 70 receives new, cold water that has not yet passed though the tank 5. This new, cold water passes into the condenser inlet 70, and through the condenser 65 on its way to the tank inlet 25. Heat exchange occurs in the condenser 65 from the heated refrigerant to the new, cold water, resulting in a pre-heating of the new, cold water before the water is introduced into the tank volume 35. This “pre-heated” water is then heated further in the tank 5 by the second condenser 20 as the heated refrigerant passes from the first condenser 65 and into the second condenser 20 that heats water within the tank volume 35.

[0020]The condenser inlet 70 includes a T-junction 85 to connect a mid-tank line 75 to the condenser 65. The mid-tank line 75 connects to the tank volume 35 at a point between the tank inlet 25 and the tank outlet 30 (i.e. between the top and bottom of the tank), and is intended to pull water from the tank volume 35 that may be partially heated, but is not yet heated to the desired, set temperature for the water to reach before the water is pulled for use through the tank outlet 30. By way of example, the mid-tank line 75 may connect through a port 76 at a middle third of the tank 5 along the vertical orientation, while the tank inlet port 26 is located in a bottom third of the tank 5 along the vertical orientation. The mid-tank line 75 includes a check valve 80 to prevent cold incoming water from directly entering the tank volume 35, and a pump 90 is positioned between the condenser 65 and the tank inlet 25 to pull water through the condenser 65 and into the tank inlet 25. During a first mode operation of the heat pump water heater system 1 in FIG. 2, the pump 90 does not operate, and supply water pressure moves new, cold water through the second condenser and into the tank inlet 25. In this configuration, no water moves through the mid-tank line 75 into the condenser 65. This mode of operation functions to pre-heat the water during a water draw, as described above.

[0021]In a second mode of operation where there is no water draw, the pump 90 operates to pull water through the first condenser 65 and into the tank inlet 25. In this mode of operation, water within the tank volume 35 is drawn into the condenser 65 through the mid-tank line 75, and is heated by refrigerant flowing through the condenser 65. No or little new, cold water is drawn into the condenser inlet 70 in this mode of operation. In a third mode of operation, the pump 90 operates during a water draw, and incoming cold water and water from the tank are combined in the T-junction 85 and directed through the condenser 65.

[0022]In addition to preheating the water at the bottom of the tank volume 35, the embodiment of FIG. 2 can also reduce undesirable temperature stacking in the tank 5. Stacking occurs when water at the very top of the tank volume 35 is at a substantially higher temperature than the desired setpoint. Such a phenomenon can result in the delivery of undesirably hot water during the initial period of a water draw. Referring back to FIG. 5, it can be seen that the temperature of the refrigerant exiting the compressor 10 at the point 210 (180° F.) is substantially higher than the desired water temperature setpoint, which is ideally slightly below the condensing temperature (for example, 140° F.). The point 215 on the thermodynamic cycle represents the state at which the refrigerant exits the first condenser 65 and enters the second condenser 20. Having rejected some of its heat to the water flow in the condenser 65, the refrigerant has been de-superheated and enters the condenser 20 at the top end of the tank 5 at or near its condensing temperature. In this way, overheating of the water at the top of the tank volume 35 is avoided.

[0023]FIG. 3 illustrates another configuration of a heat pump water heater system 1, and differs from the embodiment shown in FIG. 2 in that the condenser inlet 70 connects to a T-junction 95. The T-junction 95 also connects to the condenser 65 and to the tank inlet 25. The T-junction allows new, cold water to be drawn into the tank inlet 25, or to be drawn through the second condenser 65, through the mid-tank line 75, and into the tank volume 35. A pump 100 is positioned along the mid-tank line 75. When there is a draw of hot water from the tank 5 and the pump 100 is not in operation, new, cold water is drawn into the tank inlet 25 directly. In a first mode of operation, the pump 100 operates while there is a draw of hot water. In that first mode of operation, in the event that the flow rate of the pump 100 is less than the rate of the hot water draw, a portion of the new, cold water (which has a flow rate equal to that of the hot water draw) is drawn into the tank inlet 25 and another portion is drawn through the condenser 65. This mode of operation pre-heats some of the new, cold water entering the tank volume 35 via the condenser 65, helps to prevent stacking, and increases the overall efficiency of the system 1. The pump 100 can alternatively be operated at a higher flow rate, such that the flow rate of water through the pump (and, consequently, through the condenser 65) exceeds that of the hot water draw. In such an operation, a portion of the water is drawn from the tank volume 35 by way of the tank inlet 25 and is combined with the incoming cold water at the T-junction 95.

[0024]In a second mode of operation where there is no draw of hot water, the pump 100 is activated, and operates to draw water from the tank volume 35 through the tank inlet 25, and pump this water through the condenser 65. After being heated in the condenser 65, the water will enter the tank volume 35 via the mid-tank line 75. In this mode, no water is drawn through the condenser inlet 70, which is connected to the water supply, for example a municipal water supply. Operation in the second mode provides additional heating of water taken from the bottom of the tank volume 35, and introduces this water into the middle of the tank volume 35 via the mid-tank line 75, which provides pre-heating to the coldest water in the tank volume 35 (i.e., water drawn from the bottom of the tank 5 through the tank inlet 25). As a result, additional heating is provided to water in the coldest part of the tank 5, but without affecting water towards the very top of the tank volume 35, where water should remain at or near the target temperature for immediate use.

[0025]Both the compressor 10 and the pump 90/100 are communicatively coupled to a controller 150. The controller 150 is configured to activate the compressor 10 in response to a signal that indicates a need to heat water. The water tank 5 can be provided with one or more temperature sensors to monitor the temperature of the water in the tank. In the embodiment of FIGS. 2 and 3, four temperature sensors 140a-d are arranged on the tank wall 40 at various locations along the vertical orientation of the tank. Specifically, the temperature sensor 140a monitors the temperature of the water at the top of the tank volume 35, the temperature sensor 140d monitors the temperature of the water at the bottom of the tank volume 35, the temperature sensor 140b monitors the temperature of the water at an upper middle location of the tank volume 35, and the temperature sensor 140c monitors the temperature of the water at a lower middle location of the tank volume 35. It should, however, be understood that in some embodiments that there may be more or fewer temperature sensors.

[0026]In one non-limiting example, the controller 150 is configured to determine a need to heat water by receiving the temperature signals from the upper middle sensor 140b and the lower middle sensor 140c, and comparing the average of those two temperatures to the desired hot water temperature setpoint. When the average temperature is less than the setpoint by a predetermined amount (for example, 8 degrees Fahrenheit) the controller determines that heating of the water is needed. Such a condition occurs, for example, when one or more draws of hot water from the tank outlet has caused the bottom of the tank volume 35 to fill with cold water up to about the level of the lower middle temperature sensor 140c. The controller 150 can additionally be configured to turn off the compressor 10 when another criteria is reached, for example when the average temperature is closer to or at the setpoint.

[0027]The controller can further be configured to operate the pump 90/100 whenever the compressor 10 is in operation. With particular reference to the embodiment of FIG. 3, with such a configuration, the condenser 65 will always be operating to de-superheat the refrigerant prior to it entering the condenser 20, so that temperature stacking in the water tank 5 is avoided. Cold water (either new water from condenser inlet 70, cold tank water from tank inlet 25, or a combination) is heated by the refrigerant in the condenser 65, and is delivered into the tank volume 35 via the mid-tank line 75. At the point in time at which the compressor 10 and pump 100 are first turned on, the water within the lower portion of the tank volume 35 (e.g. approximately below the lower middle temperature sensor 140c) will be cold water. As the compressor 10 operates, the water flowing through the condenser 65 will be heated at a first rate. The refrigerant flowing through condenser 20 will provide little to no heat to the water within the tank volume 35 until it reaches the location along the vertical orientation of the tank 5 where the water is cold (i.e. below the setpoint), and the water within the tank volume 35 below that point will be heated by the refrigerant in the condenser 35 at a second rate. Referring back to the exemplary P-h diagram of FIG. 5, the first heating rate is equal to the change in enthalpy from point 210 to point 215, multiplied by the refrigerant mass flow rate; the second heating rate is equal to the change in enthalpy from point 215 to point 220, multiplied by the refrigerant mass flow rate.

[0028]After the hot water draw from the tank outlet 30 stops, the compressor 10 and pump 100 can continue to operate until the turn-off criteria is met. While the system continues to operate with no draw, the water within the bottom portion of the tank will continue to be heated by both the condenser 65 and the portion of the condenser 20 that is in contact with a portion of the tank containing colder water. The location of the mid-tank line 75 can advantageously be selected so as to avoid premature turn-off of the system. By way of example, the mid-tank line 75 can connect to the tank via a port 76 that is located between the upper-middle temperature sensor 140b and the lower-middle temperature sensor 140c.

[0029]In some embodiments, the pump 100 is a variable speed pump. The controller 150 is configured to operate the pump 100 at a desired speed, depending on the circumstances within the tank. By way of example, the controller 150 can adjust the speed of the pump 100 in response to feedback from a flow sensor (not shown) located along the water line 25 so as to maintain some minimum flow rate out from the tank inlet 26 during a water draw. In this manner, the water flowing to the tank volume 35 through the port 76 will always meet or exceed the rate of draw from the tank volume 35 through the tank outlet 30 while the compressor 10 and pump 100 are operational, thereby ensuring that cold water in the bottom of the tank will not rise above the location of the port 76. As another example, the controller 150 can adjust the speed of the pump 100 in response to calculating a rate of change of the average temperatures measured by the sensors 140b and 140c. The pump 100 can be operated at a first, initial rate when the compressor 10 begins operation, and the controller 150 can then monitor the rate of change of that average temperature. A negative rate of change indicates that level of colder water in the bottom of the tank is rising closer to the sensor 140b, and in response the controller 150 can increase the speed of the pump 100 to deliver more heated water to the midpoint of the tank.

[0030]In other embodiments, the controller 150 can be configured to operate the water pump only when there is no draw of water through the water outlet 30. By way of example, in the embodiment of FIG. 2, the pump 90 may be left idle when there is a water draw even if the compressor 10 is operating to circulate refrigerant through the refrigerant circuit, since all of the cold replacement water is directed through the condenser 65 to the water inlet line 25. When the water draw stops, it may be desirable to continue heating water in the tank volume 35. In order to do so, the controller 150 can engage the pump 90 to circulate water from the tank volume 35 through the condenser, so that additional heating of the water in the bottom portion of the tank volume 35 without causing temperature stratification at the top of the tank volume 35 can continue to be achieved. A flow sensor can be arranged along the outlet line 30 or the water inlet line 70 and can be communicatively coupled to the controller 150, so that the controller 150 can detect when there is no draw and can activate the pump 90.

[0031]FIG. 4 illustrates another embodiment of a heat pump water system 1, and may be combined with the other embodiments disclosed herein. The embodiment shown in FIG. 4 is constructed such that in certain conditions, no pump 90, 100 is required. The tank 5 includes a tank inlet 25 and a tank outlet 30, which are both positioned at the top of the tank 5. The tank inlet 25 is connected to a dip tube 125 that extends down into the tank volume 35 to deliver new, cold water to the bottom of the tank volume 35. A connection line 105 fluidly connects the tank inlet 25 to the tank outlet 30 outside of the tank volume 35. The condenser 65 is positioned along the tank inlet 25 in between the connection line 105 and the tank 5. The condenser 65 has a condenser inlet 115 into which heated refrigerant flows. The heated refrigerant passes through the condenser 65 to heat water within the tank inlet 25. The heated refrigerant then passes through a condenser outlet 120 and into the condenser 20 to heat the water already within the tank 5. Then, the refrigerant passes through a condenser outlet 130 to flow back through a refrigerant loop to be re-heated. In some embodiments, the tank heat exchanger 20 is a coil that spirals along the tank wall 40 from a top of the tank 5 to the bottom of the tank 5, which can be referred to as a tube wrap condenser. The tube wrap condenser could be a microchannel condenser, a single tube with a larger diameter, or multiple, smaller diameter tubes arranged in parallel.

[0032]Referring back to the condenser 65 positioned along the tank inlet 25, the water within the tank inlet 25 that is heated by the condenser 65 is either new, cold water flowing into the tank inlet 25 to replenish the tank 5 or is water already within the tank volume 35 that has been thermosiphoned out from the tank volume 35 and into the tank inlet 25. To heat water already within the tank volume 35 when there is no water draw, a valve 110 in the connection line 105 is opened, and hot refrigerant is passed through the condenser 65. The valve 110 can, for example, be controlled by a controller that is configured to detect both a need to heat water and a lack of water draw, as described previously with reference to other embodiments. Because the water in the tank inlet 25 can now pass from the tank inlet 25, directly to the tank outlet 30 and back into the tank volume 35 though the tank outlet, the heating of the water in the tank inlet 25 by the condenser 65 causes a thermosiphon to pull relatively cooler water in the bottom of the tank volume 35 up through the dip tube 125 and into the tank inlet 25. Then, as this water is heated, the heated water passes though the connection line 105, into the tank outlet 30, and finally back into the tank volume 35. As a result, cooler water within the tank volume 35 is heated by the condenser 65 without the use of a pump, and prevents stacking within the tank 5. In at least some embodiments, the valve 110 can also function as a tempering or mixing valve to reduce the temperature of water removed from the water outlet 30 to a targeted setpoint temperature by blending a portion of cold inlet water with hotter water removed from the tank volume 35.

[0033]The heat pump water heater systems 1 disclosed in FIGS. 2-4 and the accompanying description provide distinct advantages over known heat exchanger systems, such as increasing the overall heating capacity of the water heater system without greatly increasing the size of the system, which helps with packaging and installation. Other advantages include the fact that no complicated control system is required for any of the heat pump water heater systems 1 disclosed herein, instead, the same controller can be used as is used with a more traditional system such as that shown in FIG. 1. Further, because water will not be fully heated within the second condenser 65 (water is not fully heated until heated via the wraparound heat exchanger that heats water within the tank volume 35), scale buildup within the second condenser 65 will be limited. Double wall and brazed plate heat exchangers are known to break as a result of scale buildup, so incomplete heating of the water within the second condenser 65 provides an advantage in that the scale buildup is limited.

[0034]Although the invention has been described in detail with reference to certain preferred embodiments, variations and modifications exist within the scope and spirit of one or more independent aspects of the invention as described.