Heat pump water heater

Abstract

A water tank is provided. The water tank has a cylindrical tank wall and an inner volume to contain water. The water heater also includes a refrigerant subsystem including a condenser. The condenser has at least one refrigerant coil winding around the cylindrical tank wall. The at least one refrigerant coil has an outer surface partially disposed against the cylindrical tank wall to transfer heat between refrigerant flowing through the at least one refrigerant coil and water contained within the water tank. The water heater also includes a water circulation loop including at least one water coil arranged externally to the water tank. The at least one water coil has an outer surface partially disposed against the outer surface of the at least one refrigerant coil to transfer heat between refrigerant flowing through the at least one refrigerant coil and water circulating through the at least one water coil.

Description

HEAT PUMP WATER HEATERCROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to United States provisional patent application no. 63 / 486,783, filed on February 24. 2023, the entire contents of which are hereby incorporated by reference.BACKGROUND

[0002] The present disclosure relates to water heaters, and more particularly to heat pump water heaters. A heat pump water heater typically heats water by exchanging between a heated and compressed refrigerant and the water. The heated water is then stored in a tank, from which it may be drawn for use when hot water is demanded. Some known types of heat pump water heaters operate to heat the water while it is resident within the tank, by way of a condenser in the form of refrigerant tubing that is wrapped around the metal storage tank. Thermal energy is transferred from the refrigerant through the tank wall and into the water as the refrigerant passes through this wrap around condenser.

[0003] One known deficiency of present heat pump water heaters is that the wrap around condenser surface area is limited by the tank surface area. This can limit heat transfer if a high heating capacity is needed. Also, due to the water side heat transfer on the tank surface inside the tank being limited by natural convention (i.e. by buoyancy-driven movement of the heated water away from the tank wall), the water side heat transfer coefficient is very low compared with the heat transfer coefficient inside the surrounding refrigerant tubes, which results in the tube area not being fully utilized for heat transfer. To have more heating capacity, more heat transfer surface area is needed, typically requiring longer refrigerant tubes, which is costly and complicated, and may be limited by the available tank surface area.

[0004] Accordingly, a need exists for a heat pump water heater that can provide a higher heating capacity by effectively increasing the wrap around condenser surface area. Further, a need exists for a heat pump water heater that can provide a more efficient heat transfer from the refrigerant tubes to the tank water, and limit losses due to natural convection.SUMMARY

[0005] A heat pump water heater according to the present disclosure provides advantageous solutions to these and other know n problems in the art. A w ater tank is provided. The water tank has a cylindrical tank wall and an inner volume to contain water. The water heater also includes a refrigerant subsystem including a condenser. The condenser has at least one refrigerant coil winding around the cylindrical tank wall. The at least one refrigerant coil has an outer surface partially disposed against the cylindrical tank wall to transfer heat between refrigerant flow ing through the at least one refrigerant coil and water contained within the water tank. The water heater also includes a water circulation loop including at least one water coil arranged externally to the water tank. The at least one water coil has an outer surface partially disposed against the outer surface of the at least one refrigerant coil to transfer heat between refrigerant flowing through the at least one refrigerant coil and w ater circulating through the at least one water coil.

[0006] In some embodiments, the water circulation loop circulates water from a lower portion of the w ater tank inner volume to an upper portion of the w ater tank inner volume by way of the water coil.

[0007] In some embodiments, the water circulation loop includes a pump having a pump inlet and a pump outlet. In some embodiments, the pump inlet is in fluid communication with a low er portion of the w ater tank inner volume to draw w ater therefrom when the pump is operational. In some embodiments, the pump outlet is in fluid communication with an inlet end of the at least one water coil.

[0008] In some embodiments, the water tank includes a connection spud arranged at a top end of the water tank. In some embodiments, the connection spud is in fluid communication with an outlet end of the at least one water coil to direct heated water from the water circulation loop into an upper portion of the water tank inner volume w hen the pump is operational.

[0009] In some embodiments, the water circulation loop includes a dip tube extending through the connection spud to the lower portion of the water tank inner volume in order to place the pump inlet in fluid communication with the lower portion of the water tank inner volume.

[0010] In some embodiments, the at least one refrigerant coil includes a first refrigerant coil and a second refrigerant coil. In some embodiments, the first and second refrigerant coils provide hydraulically parallel flow paths for the refrigerant.

[0011] In some embodiments, the first refrigerant coil is offset from the second refrigerant coil along an axial direction of the cylindrical tank wall to define a constant gap dimension between the first and second refrigerant coil in the axial direction. In some embodiments, the water coil includes a tube with an outer diameter that is greater than the constant gap dimension.

[0012] In some embodiments, refrigerant flows through the at least one refrigerant coil from an upper end of the at least one refrigerant coil to a lower end of the at least one refrigerant coil. In some embodiments, water circulates through the at least one water coil from a lower end of the at least one w ater coil to an upper end of the at least one water coil.

[0013] In some embodiments, the cylindrical tank wall includes an upper portion that is not in contact with the at least one refrigerant coil. In some embodiments, the upper portion is at least the top third of the cylindrical tank wall.

[0014] In some embodiments, the at least one water coil includes a first water coil and a second water coil. In some embodiments, the first and second water coils provide hydraulically parallel flow paths for the water.

[0015] A method of operating a heat pump water heater includes, according to some embodiments: monitoring at least one temperature of a volume of water within a water tank; comparing the at least one temperature to at least one temperature setpoint to determine whether heating of the volume of water is needed; in response to determining that heating of the volume of water is needed, operating a compressor to flow heated refrigerant through a condenser; operating a pump to circulate a flow of water outside of the water tank from a first location in the volume of water to a second location in the volume of water; and simultaneously transferring heat from the refrigerant flowing through the condenser to both water within the water tank and to the flow of water circulating outside of the water tank.

[0016] In some embodiments, the method of operating a heat pump water heater further includes: monitoring a temperature of the flow of water after having transferred heat to the flow7of water from the refrigerant; comparing the temperature of the flow of water to a temperaturethreshold; and adjusting a flow rate of the flow of water in order to drive said temperature of the flow of water to be closer to the temperature threshold.

[0017] Other aspects of the invention will become apparent by consideration of the detailed description and accompanying drawings.BRIEF DESCRIPTION OF THE DRAWINGS

[0018] FIG. 1 is a front view of a heat pump w ater heater according to a first construction.

[0019] FIG. 2 is a section view of a portion of the heat pump water heater of FIG. 1.

[0020] FIG. 3 is an enlarged section view of the portion of the heat pump w ater heater shown in FIG. 2.

[0021] FIG. 4 is a detail view of a portion of a water circulation loop of the heat pump water heater of FIG. 1.

[0022] FIG. 5 is a schematic view of a refrigerant subsystem for the heat pump water heater of FIG. 1.

[0023] FIG. 6A is a section view of a portion of the heat pump water heater of FIG. 1 with a w ater return tube according to a first construction.

[0024] FIG. 6B is a section view of a portion of the heat pump water heater of FIG. 1 with a water return tube according to a second construction.

[0025] FIG. 7 is a flow diagram of a method of operating a heat pump w ater heater in order to heat water.

[0026] FIG. 8 is a section view of a portion of the heat pump water heater of FIG. 1 illustrating another example of a condenser.

[0027] FIG. 9 is a section view of a portion of the heat pump water heater of FIG. 1 illustrating yet another example of a condenser.DETAILED DESCRIPTION

[0028] 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.

[0029] 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. The modifier “about" used in connection with a quantity is inclusive of the stated value and has the meaning dictated by the context (for example, it includes at least the degree of error associated with the measurement of the particular quantity). The modifier “about” should also be considered as disclosing the range defined by the absolute values of the two endpoints. For example, the expression “from about 2 to about 4” also discloses the range “from 2 to 4”. The term “about” may refer to plus or minus 10% of the indicated number. For example, “about 10%” may indicate a range of 9% to 11%, and “about 1%” may mean from 0.9- 1.1. Other meanings of “about” may be apparent from the context, such as rounding off, so, for example “about 1” may also mean from 0.5 to 1.4.

[0030] FIG. 1 illustrates a front view of a heat pump water heater, which can be generally referred to as a water heater 5. This disclosure is not intended to limit the embodiments shown and described herein to heat pump water heaters, as other fluid heaters or fluid chillers are contemplated. The water heater 5 includes a tank 10 having at least one wall 15. The wall 15 shown herein is generally cylindrical, although other tank wall shapes are contemplated such as circular or oblong. The w all 15 can be steel. The wall 15 extends in an axial direction 20 generally aligned with a centerline of the cylindrical tank 10. The tank 10 is capped at the top by a top wall 25. In some embodiments, the top wall 25 is domed. The tank 10 holds a fluid, such as water, so that the fluid can be heated for residential or commercial use.

[0031] The top wall 25 includes a tank inlet 30 and a tank outlet 35. The tank inlet 30 supplies water to the tank 10 from a water source, and the tank outlet 35 delivers heated water from the tank 10. For example, the tank outlet 35 in some examples is connected to the hot water line of a residence.

[0032] The water heater 5 shown in FIG. 1 further includes a refrigerant subsystem 40, which is used to heat water within the tank 10. As best shown in FIG. 5, the refrigerant subsystem 40 includes an expansion valve 45, an evaporator 50, a compressor 55, and a condenser 60. The evaporator 50 boils the refrigerant at low pressure, then the compressor 55 pressurizes the refrigerant vapor created by the evaporator 50 and directs the refrigerant to the condenser 60. The expansion valve 45 removes pressure from the liquid refrigerant to allow expansion or change of state from a liquid to a vapor in the evaporator 50. In some embodiments the expansion valve 45, the evaporator 50, and the compressor 55 are integrated into the top of the water heater 5, and in other embodiments can be housed within a separate structure remote from the water heater 5.

[0033] The heated refrigerant travels through the condenser 60, and back into the evaporator 50 and the compressor 55 to complete the refrigerant loop. Referring back to Fig 1, the condenser 60 is in the form of one or more coils that are wrapped around the wall 15, which is cylindrical in this example. In the embodiment shown in FIG. 1. two refrigerant coils 60a, 60b are shown that provide the heated refrigerant hydraulically in parallel around the cylindrical wall 15. In other examples, more refrigerant coils can be provided. Including at least tw o refrigerant coils 60a, 60b can increase the heat transfer rate in the condenser 60 and limit pressure drop of the refrigerant along the refrigerant coils 60a, 60b. In some other embodiments, a single refrigerant coil with increased flow area can be used to the same effect.

[0034] The water heater 5 further includes a water circulation loop 65. The water circulation loop 65 draws water from within the tank 10 and passes the water through a water coil 70. The water coil 70 passes the water drawn into the w ater circulation loop 65 past the first and second refrigerant coils 60a, 60b to heat the water within the water coil 70. The coils 60a, 60b, and 70 can manufactured from aluminum, copper, or stainless steel, or a combination thereof. While only a single water coil 70 is shown, in some embodiments multiple water coils 70 can be provided, and in some embodiments, the multiple water coils each have a smaller diameter than an equivalent single w ater coil.

[0035] While the interaction between the water coil 70 and the first and second refrigerant coils 60a, 60b is better shown in Figs. 2 and 3. the general flow of the water and refrigerant throughout the water heater 5 is shown with arrows in FIG. 1. Due to the effects of convection during heating of the water within the tank 10, water will tend to rise from a lower portion of the tank 75 to an upper portion of the tank 80 as the water is heated. As a result, the refrigerantcoils 60a, 60b wrap around the cylindrical wall 15 starting near the upper portion of the tank 80, and are wrapped around the tank 10 to extend gradually down toward the lower portion of the tank 75. Since the refrigerant entering the condenser 60 is at a superheated temperature that is higher than the condensing temperature and the refrigerant exiting the condenser 60 is at a subcooled temperature that is lower than the condensing temperature, this arrangement ensures that refrigerant in the refrigerant coils 60a, 60b is hot enough to heat water in the tank 10 toward the upper portion of the tank 80 (where the water in the tank 10 is hottest). The refrigerant coils 60a, 60b shown in FIG. 1 do not start wrapping around the tank 10 at the very top of the cylindrical wall 15 (i.e., near the top wall 25) so that the heated w ater in the upper portion of the tank 80 is not overheated to a temperature that may be above a target output temperature of the water heater 5. In some embodiments, the top one-third of the cylindrical wall 15 is not contacted by the coils 60a, 60b, 70 such that the top w all 25 is spaced apart from the upper end of the refrigerant coil 105 and the upper end of the water coil 110 by at least one third of the length of the wall 15 along the axial direction 20. This reduces the risk of overheating the water near the upper portion of the tank 80, which can be referred to as stacking when the water at the upper portion of the tank 80 becomes heated far above a target value. However, in other embodiments, the refrigerant coils 60a, 60b extend along the entire length of the tank 10 along the axial direction 20.

[0036] Referring now to the water coil 70, water within the water coil 70 is drawn from near the bottom of the tank 10 (as will be described in more detail with specific reference to FIGs. 4 and 6A-B), w here the w ater is coldest. Water flows through the water coil from a w ater coil inlet end 85 to a water coil outlet end 90 in a flow- direction reversed from the flow direction of the refrigerant in the condenser 60. More specifically, water in the water coil 70 flows from the low er portion of the tank 75, and the water coil 70 wraps around the cylindrical wall 15 to gradually move the water from the low er portion of the tank 75 to the upper portion of the tank 80. Because w ater in the water coil 70 is drawn from near the bottom of the tank 10, heat exchange can still occur between the refrigerant and the relatively cool water even after the refrigerant has discharged most of its heat energy (i.e., when the refrigerant is at the lower end of the refrigerant coil 95). Thus, by aligning the coolest water with the coolest refrigerant along the coil lengths (i.e., from a lower end of the refrigerant coil 95 and a lower end of the water coil 100 all the w ay to an upper end of the refrigerant coil 105 and an upper end of the water coil 110) and by flowing the refrigerant in an opposite direction relative to the flow of the water, heat exchange from the refrigerant to the water can be maximized over the coil 60a, 60b,70 lengths. In some alternate embodiments, however, the pressure drop through the water coil 70 (and, consequently, the power that must be consumed in order to move the water therethrough) can be reduced by not extending the water coil 70 to the very lowermost and / or uppermost turns of the condenser coil 60, since the most heat transfer effective portion of the condenser will be in that central region where the refrigerant is in a two-phase state.

[0037] Thus, additional heat is transferred to water within the water coil 70. This heat would have been lost in a conventional system (since this heat from the condenser coils could radiate away from the tank 10 in the absence of the water coil 70) or would have remained within the refrigerant as it is returned to the evaporator 50. While not shown in FIG. 1, the tank 10 can include an insulation layer surrounding the tank 10, and the refrigerant coils 60a, 60b as well as the water coil 70 can both pass within the insulation layer. In some embodiments, the insulation layer is a foam, such as (for example) a bl own polyurethane foam. Through the dual heating functionality , the water heater 5 is able to obtain a much higher heat transfer rate compared to a conventional heat pump water heater having the same tank size. Further, because the coils 60a, 60b, and 70 are within the foam, there is still minimal standby loss of the design shown in FIG. 1 compared to a conventional design.

[0038] The water heater 5 shown in FIG. 1 thus addresses disadvantages of other known w ater heaters by providing dual heating of the water by the refrigerant coils 60a, 60b, i.e., by heating of water within the tank 10 and simultaneously heating of water within the water coil 70. In some embodiments, there is a 10-20 degree Fahrenheit temperature rise of the water that has passed through the water coil 70. The design can thus increase heating capacity of the water heater 5 without increasing the number of refrigerant coils 60a. 60b. In some embodiments, the refrigerant flow' rate may have to increase compared to conventional heat pump w ater heaters to keep up with the additional heating demands created by the w ater coil 70.

[0039] FIG. 2 illustrates a section view of a portion of the w ater heater 5. The tank wall 15 separates the w ater 115 within the tank volume from the refrigerant coils 60a, 60b, which abut the wall 15. The refrigerant coils 60a, 60b are spaced apart along the axial direction 20 by a gap dimension 120. The gap dimension 120 remains substantially constant along the entire path of the refrigerant coils 60a, 60b from the upper end of the refrigerant coils 105 to the lower end of the refrigerant coils 95 as the refrigerant coils 60a, 60b spiral down the axial direction 20 of the w all 15, as also shown in FIG. 1. Referring again to FIG. 2, the w ater coil 70 ispositioned in contact with both refrigerant coils 60a, 60b such that the water coil 70 rests at least partially within the space defining the gap dimension 120 between the first and second refrigerant coils 60a, 60b. Thermal paste can be applied at the contact points between the refrigerant coils 60a, 60b and the wall 15 and can be applied at the contact points between the refrigerant coils 60a, 60b and the water coil 70. The thermal paste can reduce thermal resistance between the structures. The water coil 70 is illustrated as a tube that has an outer diameter 125. As shown, the water coil 70 outer diameter 125 is greater than the gap dimension 120. In FIG. 2 the refrigerant coils 60, 60b have the same diameter, although other relative dimensions of the coils 60a, 60b, and 70 are contemplated.

[0040] FIG. 3 illustrates a more detailed view of a portion of FIG. 2 to illustrate several features more clearly. For example, the refrigerant coils 60a, and 60b can be substantially circular in cross section, but as shown in Figs. 2 and 3 the cross section can also take the form of a “D-shape’', where the flat part of the “D’’ abuts the wall 15. This serves to increase contact between the refrigerant coil 60a and wall 15, and thus increase heat transfer between the two structures. The general heat transfer direction is illustrated by the arrows in FIG. 3. The ‘"D” shape can be formed into the tubes 60a, 60b prior to the tubes being wTapped around the tank wall 15. Alternatively, the tubes 60a, 60b can be formed as round tubes, and the flat portion of the tubes can result from tension applied during the wrapping of the tubes around the tank wall 15.

[0041] Further, while the water coil 70 can be substantially circular in cross section, the cross section can, as shown in FIG. 3, include a concave portion 130 that abuts the refrigerant coil 60a. A similar concave portion can be formed in the water coil 70 to abut the refrigerant coil 60b. Such concave portions can be formed by, for example, maintaining a sufficient tension on the water coil 70 as it is wrapped around the refrigerant coils 60a and 60b during manufacture. The concave portion 130 increases the surface area in contact between the water coil 70 and the refrigerant coil 60a, and thus serves to increase heat transfer between the two structures. In some embodiments, an extruded aluminum sleeve can be provided between the coils 60a, 60b, 70 to increase the contact area, and thus increase heat exchange, between the coils 60a, 60b, 70.

[0042] FIG. 4 illustrates a portion of the water circulation loop 65 in more detail. More specifically, FIG. 4 illustrates the area of a connection spud 135 that connects through the top wall 25 of the tank 10. A pump 170, shown outside of the tank 10 in FIG. 4, draw s water fromwithin the tank 10 via a dip tube 140 connected to an inlet 145 of the pump 170. The pump 170 can be a variable speed pump. The dip tube 140 extends down near the lower portion of the tank 75 (not shown in FIG. 4) to draw the least heated (i.e. , the coolest) water in the tank 10. The pump 170 pumps the water through a pump outlet 150 that is connected to the inlet end 85 of the water coil 70. The water then passes through the water coil 70 and is heated by the first and second refrigerant coils 60a, 60b of the condenser 60. before returning to an outlet end 90 of the water coil 70. The outlet end 90 is connected to a side port inlet 155 of the spud 135, which is in fluid communication with a water return tube 160. The water return tube 160 passes through the connection spud 135 to deliver the heated water back into the tank 10. As shown in FIG. 4, the dip tube 140 and the water return tube 160 can pass through the single connection spud 135, as the dip tube 140 extends within the water return tube 160. In other embodiments, two connection spuds can be provided, one connecting the water coil inlet 85 to the tank 10 and the other connecting the water coil outlet 90 to the tank 10. In still other embodiments, the outlet end 90 of the water coil 70 is not connected to the top wall 25 at all, and is instead connected directly to the tank 10 such that no water return tube 160 is required. In still other embodiments, water coil inlet end 85 passes through the wall 15 at the lower portion of the tank 75, such that relatively colder water from the bottom of the tank 10 is passed into the water coil 70 without the need for the dip tube 140.

[0043] Figs. 6A and 6B illustrate two alternative embodiments of the water return tube 160. The water return tube 160 in FIG. 6A extends further down into the interior volume 165 of the tank 10 than in the embodiment show n in FIG. 6B. Dashed boxes 175 denote the extent along the axial direction 20 of the wall 15 that the water coil 70 and the refrigerant coils 60a, 60b extend along the tank 10. Notably, the water return tube 160 deposits the heated water from the water coil 70 to a position along the axial direction 20 where the water coil 70 and the refrigerant coils 60a, 60b discontinue their spiral around the w all 15. If the w ater return tube 160 deposited the heated water significantly further toward the top w all 25 than where the coils 60a, 60b. 70 end, then the water coil 70 could have a cooling effect on the water in the top of the tank 10, which is the hottest w ater in the tank 10, and has preferably been heated enough to be ready for use. This can decrease the efficiency of the w ater heater 5. This effect is negated in the embodiment shown in FIG. 6B, as the refrigerant coils 60a, 60b extend substantially the entire length of the extent of the w all 15 in the axial direction 20, and thus any cooling effect is compensated for by heat from the refrigerant coils 60a, 60b. In some embodiments, the water coil 70 does not wrap around all the w ay to the bottom of the tank 10, such that the water coil70 does not extend down to the subcooled region of the tank 10. In any case, the tube length and number of wraps around the tank 10 of the water coil 70 does not need to match that of the refrigerant coils 60a, 60b.

[0044] FIGs. 6A and 6B also illustrate components that can be used for the implementation of a method of operating the heat pump water heater 5. A controller 180 can be connected to a temperature sensor 185 located within the interior volume 165 of the tank 10, or closely coupled to the thermally conductive wall 15 of the tank 10. The temperature sensor 185 thus measures the temperature of the water 115 within the tank 10. It should be understood that, although only a single temperature sensor 185 is depicted, the tank 10 can be equipped with multiple temperature sensors 185, and such multiple temperature sensors 185 can be arranged at varying locations along the tank 10. By way of example, the tank 10 can be provided with two or more temperature sensors 185 that are at varying locations along the axial direction 20, such that a first one of the temperature sensors 185 is arranged closer to the lower end 75 of that tank 10 than a second one of the temperature sensors 185. The controller 180 can also be connected to a temperature sensor 190 positioned within or on the water coil 70, and in some embodiments within or near to the outlet end 90 of the water coil 70.

[0045] FIG. 7 illustrates a method 700 of operating the water heater 5. The method 700 can be implemented by the controller 180 and includes monitoring a temperature of a volume of water within the water tank 10 with the temperature sensor 185 (block 702). Then, the controller 180 compares the measured temperature Trank to a temperature threshold Tumi to determine whether heating of the volume of water is needed (block 704). By way of example, the temperature threshold Tumi may be equal to, or slightly below, the setpoint temperature of the tank 10, i.e. the temperature at which hot water is desired to be drawn from the tank 10. In response to determining that heating of the volume of water is needed, i.e. that the tank temperature is below the temperature threshold, the controller 180 starts the compressor 55 and the pump 170 (block 706). The compressor 55 then operates to flow heated refrigerant through the condenser 60 (block 708). Simultaneously, the pump 170 operates to circulate a flow of water outside of the water tank 10 via the water coil 70 (block 710). Heat is then simultaneously transferred from the refrigerant flowing through the condenser 60 to both water within the water tank 10 (block 712) and to the flow of water circulating outside of the tank 10 in the water coil 70 (block 714). The flow of water in the water coil 70 is then returned back to the tank 10 at an increased temperature (block 716).

[0046] At block 718, the temperature of the water within the tank is again measured. This measurement can be taken using the same temperature sensor 185 as was used to measure the tank temperature in block 702, or can be taken using another temperature sensor 185. By way of example, the temperature sensor 185 that is used in block 702 may be arranged at a lower location along the axial direction 20 than the temperature sensor 185 that is used in block 718. The controller 180 then compares the temperature Trank measured in block 718 to another temperature threshold TLM to determine whether the volume of water in the tank 10 has been sufficiently heated (block 720). By way of example, the temperature threshold TLM may be equal to, slightly above, or slightly below the temperature threshold Tumi. In response to determining that the water in the tank 10 has been sufficiently heated, i.e. that the tank temperature is above the temperature threshold, the controller 180 stops the compressor 55 and the pump 170 (block 722) and the heating cycle is ended.

[0047] As depicted in block 724, the refrigerant, after having rejected heat to the water in the tank 10 and the water in the coil 70, is directed through the expansion device 45 and the evaporator 50 of the refrigerant subsystem 40. If the compressor 55 is still operating (block 726), then the refrigerant continues to be compressed and directed through the condenser 60 in order to further heat the water. If, on the other hand, the compressor 55 is no longer operating (e.g. by having been stopped by the controller 180 in block 722). then the refrigerant is no longer circulated through the refrigerant subsystem 40.

[0048] Although not shown in FIG. 7, the method of operating the heat pump w ater heater 5 can also include monitoring (e.g. by the controller 180) a temperature of the flow of water near the outlet end 90 of the water coil 70 via a temperature sensor 190 such that the temperature is read after heat is transferred from the refrigerant coils 60 to the water in the water coil 70. The controller 180 may compare the temperature of the flow of water to a temperature threshold and adjust a flow rate of the flow of water, via operation of the pump 170, in order to drive the temperature of the flow of water at the outlet end 90 of the water coil 70 to be closer to the temperature threshold. In some embodiments, the pump 170 is a variable speed pump so that the flow rate can be adjusted by the controller 180 to match heating needs of the water heater 5. In other embodiments, the pump 170 is a fixed speed pump. In some embodiments, flow sensors can be used instead of or in conjunction with temperature sensors 185, 190.

[0049] In some embodiments, the pump 170 is controlled such that the pump 170 is not operating during a defrost cycle. In some embodiments, the pump 170 is controlled such that the pump 170 is operating at a relatively higher speed just before the tank 10 is fully recovered (i.e., at a maximum hot water capacity) so the tank 10 can store more hot heated water. In some embodiments, the controller 180 determines whether the pump 170 can be turned off during lower capacity heating of the tank 10. In some embodiments, the controller 180 can control the pump 170 such that, when the controller 180 determines the user is about to run out of hot water, the pump 170 is operated at a relatively higher speed to deliver more hot water to the tank 10 even if the water temperature is not up to the target water temperature in the tank 10. In some embodiments, if the heat pump cannot operate for some reason (such as due to low ambient temperature), the pump 170 can be used to fill the tank 10 completely with hot water via heating from a heating element (e.g., an electric heating element) in the tank 10 or along the water coil 70. In some embodiments, heat traps in the dip tube 140 are used to minimize the water flow inside the dip tube 140 during standby e.g., when there is no water draw through the tank outlet 35. In some embodiments, the dip tube 140 is configured to minimize water mixing in the tank 10 that results from flow through the water coil 70.

[0050] FIG. 8 illustrates a cross-sectional view of an alternative embodiment of the coils. Refrigerant coil 195, instead of being two separate tubes as shown in FIGs, 2 and 3, is instead an elongated body having fluid passages 200 therein. In the illustrated embodiment, four fluid passages 200 are shown that are arranged along the axial direction 20. The water coil 205 is also a different shape compared to the water coil 70 shown in Figs. 2 and 3. The water coil 205 is an elongate “D"’ shape with the flat portion of the “D” abutting a substantially planar portion of the refrigerant coil 195. Operation of the water heater 5 using the refrigerant coil 195 and water coil 205 is substantially the same as that described in other embodiments, the heated refrigerant within the refrigerant coil 195 heats water 115 in the tank 10 through the wall 15 and simultaneously heats water within the w ater coil 205. In still other embodiments, the refrigerant coil 195 can include micro channels, macro channels, or be a simple rounded tube (i.e., more rounded than the “D” shaped tube shown FIG. 3).

[0051] FIG. 9 illustrates a cross-sectional view of yet another alternative embodiment of the coils. In the embodiment of FIG. 9, the refrigerant coil 210 is again an elongated body, but with a single internal channel for the refrigerant to flow through. The refrigerant coil 210 can. for example, be formed from a round tube that is flattened to an oval or obround shape prior tobeing wrapped around the tank wall 15. In place of a single water tube for the water coil 70, multiple smaller tubes arranged to allow the water to flow in parallel therethrough are used. In the exemplary embodiment of FIG. 9, two such water tubes, indicated as 215a and 215b, are used, although it should be understood that alternate embodiments can use more than two such tubes arranged in parallel. The water tubes 215a, 215b can have a generally circular crosssection, and can have a slightly flattened or D-shape in order to improve the thermal contact between the water tubes 215a. 215b and the generally flat surface of the refrigerant coil 210. In order to control the desired close coupling between the water tubes 215a, b and the refrigerant coil 210, ajoining material 220 can be used to mechanically join the tubes 215a and 215b to one another. Such joining can be done, for example, immediately prior to wrapping the water tubes around the refrigerant coil. As one non-limiting example, the tubes 215a. 215b can be formed of a copper alloy and the joining material 220 can be a solder material such as (for example) a copper-tin solder.

[0052] 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.

Claims

CLAIMSWhat is claimed is:

1. A heat pump water heater comprising: a water tank having a cylindrical tank wall and an inner volume to contain water; a refrigerant subsystem including a condenser, the condenser comprising at least one refrigerant coil winding around the cylindrical tank wall, the at least one refrigerant coil having an outer surface partially disposed against the cylindrical tank wall in order to transfer heat between refrigerant flowing through the at least one refrigerant coil and water contained within the water tank; and a water circulation loop including at least one water coil arranged externally to the water tank, the at least one water coil having an outer surface partially disposed against the outer surface of the at least one refrigerant coil to transfer heat between refrigerant flowing through the at least one refrigerant coil and w ater circulating through the at least one water coil.

2. The heat pump water heater of claim 1, wherein the water circulation loop is configured to circulate water from a low er portion of the w ater tank inner volume to an upper portion of the water tank inner volume by way of the at least one w ater coil.

3. The heat pump w ater heater of claim 1, wherein the w ater circulation loop includes a pump having a pump inlet and a pump outlet, the pump inlet being in fluid communication with a lower portion of the water tank inner volume to draw water therefrom when the pump is operational and the pump outlet being in fluid communication with an inlet end of the at least one water coil.

4. The heat pump water heater of claim 3, wherein the water tank includes a connection spud arranged at a top end of the w ater tank and in fluid communication with an outlet end of the at least one w ater coil to direct heated water from the water circulation loop into an upper portion of the water tank inner volume when the pump is operational.

5. The heat pump water heater of claim 4, wherein the water circulation loop includes a dip tube extending through the connection spud to the lower portion of the water tank inner volume in order to place the pump inlet in fluid communication with the lower portion of the water tank inner volume.

6. The heat pump water heater of claim 1, wherein the at least one refrigerant coil comprises a first refrigerant coil and a second refrigerant coil, the first and second refrigerant coils providing hydraulically parallel flow paths for the refrigerant.

7. The heat pump water heater of claim 6, wherein: the first refrigerant coil is offset from the second refrigerant coil along an axial direction of the cylindrical tank wall to define a constant gap dimension between the first and second refrigerant coil in that axial direction; and the at least one water coil comprises a tube with an outer diameter that is greater than the constant gap dimension.

8. The heat pump water heater of claim 1, wherein refrigerant flows through the at least one refrigerant coil from an upper end of the at least one refrigerant coil to a lower end of the at least one refrigerant coil, and wherein water circulates through the at least one water coil from a lower end of the at least one water coil to an upper end of the at least one water coil.

9. The heat pump water heater of claim 1, wherein the cylindrical tank wall includes an upper portion that is not in contact with the at least one refrigerant coil, said upper portion being at least the top third of the cylindrical tank wall.

10. The heat pump water heater of claim 1, wherein the at least one water coil comprises a first water coil and a second water coil, the first and second water coils providing hydraulically parallel flow paths for the water.

11. The heat pump water heater of claim 10, wherein the first water coil is joined to the second water coil.

12. The heat pump water heater of claim 1, wherein the at least one refrigerant coil includes a first planar surface disposed against the cylindrical tank wall and a second planar surface disposed against the at least one water coil.

13. The heat pump water heater of claim 1, further comprising an insulation layer surrounding the water tank, wherein the at least on refrigerant coil and the at least one water coil are at least partially arranged within the insulation layer.

14. A method of operating a heat pump water heater, comprising: monitoring at least one temperature of a volume of water within a water tank; comparing the at least one temperature to at least one temperature setpoint to determine whether heating of the volume of water is needed; in response to determining that heating of the volume of water is needed, operating a compressor to flow heated refrigerant through a condenser; operating a pump to circulate a flow of water outside of the water tank from a first location in the volume of water to a second location in the volume of water; and simultaneously transferring heat from the refrigerant flowing through the condenser to both water within the water tank and to the flow of water circulating outside of the water tank.

15. The method of claim 14, further comprising: monitoring a temperature of the flow of water after having transferred heat to the flow of water from the refrigerant; comparing said temperature of the flow of water to a temperature threshold; and adjusting a flow rate of the flow of water in order to drive said temperature of the flow of water to be closer to the temperature threshold.

16. The method of claim 14, wherein the water tank has a vertically arranged central axis and wherein the first location in the volume of water is lower, along that vertically arranged central axis, than the second location in the volume of water.

17. The method of claim 16, wherein heat is transferred from the refrigerant flowing through the condenser to water within the water tank through a portion of a cylindrical wall of the water tank and wherein the second location in the volume of water is higher, along the vertically arranged central axis, than said portion of the cylindrical wall of the water tank.

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