Abstract:
An improved non-stratified heat pump water heater wherein the condenser assembly of the heat pump has at least one non-vertical section and is inserted into the water tank through an existing opening in the top of the tank. The condenser assembly has either a tube-in-a-tube style or U-tube style elongated heat exchanger construction. As the refrigerant condenses along the interior surface of the condenser assembly, the heat from the refrigerant is transferred to the water. Two embodiments of the invention include a water heater having a separate sacrificial anode rod and a water heater wherein the condenser assembly acts not only as a heat exchanger but also as the sacrificial anode in the water tank by being constructed of a metal which is more likely to corrode than the metal of the tank.

Description:
CROSS REFERENCES TO RELATED APPLICATIONS  
       [0001]    This application is a continuation-in-part of U.S. Ser. No. 09/396,061, filed Sep. 15, 1999, to be issued as U.S. Pat. No. 6,233,958, entitled “Heat Pump Water Heater and Method of Making Same”, which is herein incorporated by reference in its entirety. 
     
    
     STATEMENT REGARDING FEDERAL SPONSORSHIP  
       [0002] This invention was made with Government support under contract no. DE-AC05-00OR22725 to UT-Battelle, LLC, awarded by the United States Department of Energy. The Government has certain rights in the invention. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0003]    This invention relates generally to the combination of a heat pump and a water heater and, more specifically, to the construction of a heat pump water heater condenser assembly, having at least one non-vertical section, inserted into the water heater tank through an existing opening in the top of the water tank. The non-vertical section on the condenser assembly promotes heat transfer to the water from the condenser assembly and produces an essentially non-stratified water temperature profile in the water heater tank. Without the non-vertical condenser section, a top-to-bottom temperature gradient of approximately 30 degree F occurs and causes a stratified water temperature profile inside the water heater tank.  
           [0004]    Heat pump water heaters (HPWH) are an energy-efficient way to heat water with electricity, typically providing the same amount of hot water at one-half to one-third the energy used in electric resistance water heaters. A HPWH works by transferring heat, not by creating heat. Through a reverse application of the standard vapor compression refrigeration cycle, a heat pump water heater uses an electrically driven compressor to remove heat energy from a low-temperature heat source (ambient room air) and move it to a higher-temperature heat sink, the water stored in the hot-water tank. The energy supplied to heat the water is primarily electrical energy needed to operate the compressor. The energy supplied to heat the water comes from both the heat transferred from the ambient air and the energy used to operate the compressor in the system. Because less energy is needed to move heat than to create heat, the effective efficiency of the heat pump water heater system, defined as the ratio of hot water energy output to energy input to the water heater, is greater than 100%. The effective efficiency is called the Coefficient of Performance (COP).  
           [0005]    A typical residential HPWH operates by extracting heat from a moderate-temperature source (such as room air), and moving it to a higher-temperature heat sink, the residence hot 0  water supply. This heated water is then stored in a hot-water storage tank for later use. The physics and operation of the HPWH is identical to the vapor compression refrigeration/heat pump cycle used for space conditioning heat pumps, air conditioners, and refrigerators. FIG. 2 shows the components used in a vapor compression refrigeration/heat pump cycle: compressor, condenser, evaporator, and expansion device. The flow of refrigerant between components in this closed cycle is also illustrated.  
           [0006]    In the compressor, refrigerant vapor is compressed, thereby raising its temperature and pressure. This vapor then moves to the condenser. In the condenser, heat flows from the hot refrigerant to water surrounding the condenser. As heat leaves the refrigerant, the refrigerant condenses to a high-pressure, liquid state. The heat removed from the refrigerant as it changes to a liquid is transferred to the water.  
           [0007]    The high pressure, liquid refrigerant leaves the condenser at a temperature slightly above the temperature of the water surrounding the condenser. The liquid passes to an expansion device, where it is rapidly depressurized, and some of the liquid refrigerant flashes back into vapor. The vaporization of a portion of the refrigerant causes the remaining refrigerant to cool rapidly, and the refrigerant leaves the expansion device as a low-temperature mixture of fluid and vapor. This cold mixture then enters the evaporator, where it absorbs heat from air blown over the evaporator coils. The liquid portion of the refrigerant evaporates, and the vapor then moves back to the low-pressure side of the compressor at a temperature slightly below the temperature of the heat source.  
           [0008]    This continuing cycle results in movement of heat from the ambient air to the higher-temperature residential hot-water supply. In residential HPWHs, the heat source is typically air from inside the residence, although with proper duct design, the air could come from inside the residence, from outdoors, or can be set manually to come from either depending on climate conditions.  
           [0009]    Electrical energy is required to operate both the compressor in the HPWH and a fan that continually blows air across the evaporator coils when the unit is operating. Depending on the system design, a water pump may also be needed to circulate water between the condenser and the storage tank. The compressor, however, is the major electrical load in an HPWH. Most of the energy consumed by the compressor is used to compress and subsequently heat the refrigerant vapor, with only a small fraction of energy lost as heat from the shell of the compressor. Since the total energy to the hot water comes from the energy transferred from the heat source, as well as virtually all the energy that is used by the compressor, the net amount of heat energy transferred to the hot water is considerably higher than the net input of electrical energy by the compressor. In residential HPWHs, the heat energy supplied to the water is typically between two and three times the amount of electrical energy required to operate the HPWH.  
           [0010]    By contrast, electrical energy in a standard electric water heater is converted directly to heat in an electrically resistive element. Since the conversion efficiency from electrical energy to heat energy is 100% and the element is completely immersed in the water, the amount of heat energy supplied to the water in a standard electric water heater is equal to the electrical energy supplied to the elements. By providing more hot water per unit of electricity consumed, the HPWH saves energy and money.  
           [0011]    Residential HPWH units are wired with electrical resistance backup for heating water during period when the HPWH will not operate satisfactorily. Backup electric resistance heat may prove necessary if the heat pump unit fails, or if the temperature of the heat source is too low for the HPWH to operate effectively. Some designs also allow the use of backup resistance heat if the hot-water load is significantly above the heat pump capacity.  
           [0012]    There are basically two types of HPWHs currently available on the market. One is the desuperheater, which is connected to a heat pump system that is used for house cooling and heating. The desuperheater takes part of the heat from the compressor discharge gas and uses it for domestic water heating. The problem with a desuperheater HPWH is that the house load might not match the water heating load. In other words, when hot water is needed, the house might not need cooling or heating, and this results in inefficient use of the heat pump system.  
           [0013]    Another type of HPWH is a dedicated stand-alone unit. It pumps water from the water or storage tank, heats it in the HPWH using a heat pump and then circulates it back to the storage tank. While an advantage of the stand-alone is that storage tanks or HPWH units can be replaced separately as they wear out, this type of HPWH is bulky and requires a water pump to pump water from the tank to and from the condenser. The cost for such a HPWH tends to be high. An HPWH produced by Crispaire Corp. of Norcross, Ga. (Model R106K3) is mounted on the water tank.  
           [0014]    There is a third type of HPWH, which is not on the market yet, but has been designed and developed. In this new design, the condenser coils are wrapped around (over half of the exterior of the tank wall with the balance of the refrigeration system (including controls, expansion device, compressor, fan and evaporator assembly) being mounted on top of the tank. Thus, the water is heated by heating the tank walls with the obvious disadvantage being that the condenser is not in direct contact with the water so as to have the most efficient heat transfer occur. The system is designed to be a single package, including the modified tank. This type of HPWH requires special manufacturing to wrap the copper coil around the tank wall. Also, contact resistance between wall and the coil must be minimized to insure proper system operation. Again, in case the tank must be replaced, the condenser coil will have to be cut, which involves taking refrigerant out of the heat pump first. Then, a new tank, with the coil wrapped around its wall, will have to be connected to the compressor-evaporator assembly, and then evacuation and refrigerant charging. Full replacement of the entire system will likely be the best option, and first or replacement costs could be high. Enviromaster International (EMI), with support from the Department of Energy&#39;s ENERGY STAR Program through Oak Ridge National Laboratory, is developing this type of HPWH aimed at the large electric water heater replacement market.  
         BRIEF SUMMARY OF THE INVENTION  
         [0015]    The above disadvantages of the prior art are overcome by the present invention wherein the unit of the HPWH is an integral part of the water tank by mounting the heat pump unit on top of the water tank and inserting a heat pump water heater condenser assembly having at least one non-vertical section into the water tank through an existing opening, such as the hole in the top cover for the anode rod. The condenser assembly is of a tube-in-a-tube design and is bent so that a non-vertical section of the assembly is extended near the bottom of the tank in an essentially horizontal position. Any non-vertical component of the condenser assembly acts to de-stratify the water tank and essentially any geometry having a sufficient non-vertical section can be constructed to insert in top openings of the water heater tank thereby de-stratifying the tank.  
           [0016]    The condenser assembly has either a tube-in-a-tube style or U-tube style elongated heat exchanger construction. As the refrigerant condenses along the interior surface of the condenser assembly, the heat from the refrigerant is transferred to the water. Two embodiments of the invention include 1) a water heater having a separate sacrificial anode rod and 2) a water heater wherein the condenser assembly acts not only as a heat exchanger but also as the sacrificial anode in the water tank by being constructed of a metal which is more likely to corrode than the metal of the tank.  
           [0017]    When a straight, or “U” tube, condenser assembly without a non-vertical section is inserted into a water tank from the top, the water heating process produced a stratified water temperature profile as shown in FIG. 7. The water temperature stratification causes at least two problems. One is that when the water temperature at the top of the tank reaches the heat pump water heater thermostat setpoint, around 135 degrees F (“hot”), water at the bottom of the tank remains cold and limits the availability of “hot” water to the user. Another problem is that once the water temperature stratifies, the effectiveness of natural convective heat transfer is reduced, and heat pump performance is decreased. Natural convective forces are used to properly mix the water thereby creating a non-stratified, or homogeneous, temperature profile in the tank. With proper water mixing in the tank, the condenser assembly will contact and transfer heat to the homogeneous tank of water thereby maintaining heat pump capacity and performance through proper refrigerant condensing temperatures. Proper condensing temperatures in the vapor-compression cycle of the heat pump will keep the compressor from overheating and maintain adequate refrigerant flow in the system. This results in longer compressor life and more efficient operation of the heat pump cycle.  
           [0018]    Specifically, the present invention is a non-stratified heat pump water heater of the type having a water tank with an exterior surface and being formed of a first metal and defining a water chamber, a top on the water tank with at least one opening there through and a heat pump of the type having a compressor being in fluid communication with a condenser assembly via a first refrigerant conduit, the condenser assembly being in fluid communication with an expansion valve through a second refrigerant conduit, the expansion valve being in fluid communication with an evaporator through a third refrigerant conduit and the evaporator being in fluid communication with the compressor through a fourth refrigerant conduit. In the first embodiment of the present invention, the improvement comprises the condenser assembly being formed of an outer body and having a closed bottom and an opposed upper end which is in flow communication with the first refrigerant conduit and an inner body disposed within the outer body and having an open bottom and an opposed top which is in flow communication with the second refrigerant conduit. In this manner, superheated vapor from the compressor enters at the top of the outer body from the first refrigerant conduit and condenses along the length of the outer body with heat from the refrigerant in the first refrigerant conduit being transferred through the outer body to the water in the water tank. The condensed refrigerant then travels up through the inner body into the second refrigerant line.  
           [0019]    The first embodiment can be used, for instance, where there are two anode rod holes. One of those openings can be used for the condenser assembly where both the outer and inner bodies are made from conventional materials, such as copper.  
           [0020]    In the second embodiment of the present invention, the condenser assembly of the present invention replaces the existing anode rod in the water tank and the assembly is disposed within the tank through the existing anode hole in the tank top. In the second embodiment, the tank wall is constructed of a first metal and the improvement comprises having the condenser assembly being formed of an outer body constructed of a second metal capable of corroding at a rate greater than the rate of corrosion of the first metal and having a closed bottom, an opposed upper end which is in flow communication with the first refrigerant conduit and an inner body disposed within the outer body and having an open bottom and an opposed top which is in flow communication with the second refrigerant conduit. Thus, the outer body functions both as an anode as well as a heat exchanger with the refrigerant flowing through the condenser assembly in the same manner as in the first embodiment.  
           [0021]    Thus, the second embodiment is preferably utilized if there is only one anode rod hole on the tank. In the construction of the condenser assembly of the second embodiment, the second metal for the outer body of the condenser assembly is selected from the group consisting of aluminum, magnesium or zinc and the inner body is formed of copper.  
           [0022]    Both embodiments of the present invention avoid the need for an additional water pump as the connection HPWH&#39;s or wrapping the condenser around the water tank. Because the immersed heat exchanger is in direct contact with the water, the heating efficiency of the present invention will be high.  
           [0023]    The present invention will save space, labor, and cost to manufacture. Most important, the invention can be added on to an existing water tank without any modification of the water tank so that it will be easier for water heater tank manufacturers to accept this type of HPWH and incorporate it into existing product lines. 
       
    
    
     BRIEF DESCRIPTION OF THE FIGURES OF DRAWINGS  
       [0024]    [0024]FIG. 1 is a vertical cross-sectioned schematic view of a conventional electric water heater.  
         [0025]    [0025]FIG. 2 is a schematic view of the major components of a conventional heat pump.  
         [0026]    [0026]FIG. 3 is a perspective schematic view of the exterior of the heat pump water heater of the present invention with the heat exchanger exploded away from the hot water tank.  
         [0027]    [0027]FIG. 4 is a schematic view of the major operational components of the heat pump water heater of the present invention.  
         [0028]    [0028]FIG. 5 is a vertical cross-sectional schematic view of the condenser assembly of the present invention.  
         [0029]    [0029]FIG. 6 a  is a 1-circuit tube-in-tube style condenser assembly with a non-vertical section near the bottom of the tank.  
         [0030]    [0030]FIG. 6 b  is a parallel 2-circuit U-tube style condenser assembly with two non-vertical sections near the bottom of the tank.  
         [0031]    [0031]FIG. 7 is a graph of water temperature vs. time for a vertical tube-in-tube condenser assembly style.  
         [0032]    [0032]FIG. 8 is a graph of water temperature vs. time for a non-vertical 2-circuit tube-in-tube condenser assembly style.  
         [0033]    [0033]FIG. 9 is a graph of water temperature vs. time for a non-vertical 4-circuit U-tube condenser assembly style. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0034]    The present invention is more particularly described in the following examples that are intended as illustrative only since numerous modifications and variations therein will be apparent to those skilled in the art. As used in the specification and in the claims, “a” can mean one or more, depending upon the context in which it is used. The preferred embodiment is now described, in which like numbers indicate like parts throughout the figures.  
         [0035]    [0035]FIG. 1 is a schematic view of a conventional electric water heater  10  comprising the following elements: an outer metal case  12  with a heavy inner steel tank  14  that holds the hot water. Typically, the tank  14  holds 40 to 60 gallons. The steel tank  14  normally has a bonded glass liner  16  to keep rust out of the water. Insulation  18  surrounds the tank  14 . A drain valve  20  to drain the tank  14  extends through the metal case  12  adjacent the bottom of the water heater  10 . A dip tube  22  to let cold water into the tank  14  and a pipe  24  to let hot water out of the tank  14  extend vertically through the top or cover  26  of the tank  14 . Heating elements  28  to heat the water extend into the interior of the tank  14 . A thermostat  30  to control the temperature of the water inside the tank  14  is disposed on the outside of the case  12 .  
         [0036]    A sacrificial anode rod  32  downwardly extends from the cover  26  into the water within the tank  14 . Anodes of metals such as aluminum, magnesium, or zinc are sometimes installed in water heaters and other tanks to control corrosion of the tank. The introduction of the anode creates a galvanic cell in which the magnesium or zinc will go into solution (be corroded) more quickly than the metal of tank  14  thereby imparting a cathodic (negative) charge to the tank metal(s) and preventing tank corrosion. This corroding of the anode metal is called “the sacrifice of the anode.” 
         [0037]    [0037]FIG. 2 schematically depicts the major components of a heat pump  34  which comprises a compressor  36  being in fluid communication with a condenser  38  through a first refrigerant line  40 , the condenser  38  being in fluid communication with an expansion device  42  through a second refrigerant line  44 , the expansion device  42  being in fluid communication with an evaporator  46  through a third refrigerant line  48  and the evaporator  46  being in fluid communication with the compressor  36  through a fourth refrigerant line  50 . An electric fan  52  is associated with the evaporator  46 . The conventional controls means for the heat pump  34  are not shown.  
         [0038]    Drawn by the compressor  36 , refrigerant gas (vapor) leaves the evaporator  46  at low pressure and low temperature and flows through the fourth refrigerant suction line  50  to the compressor  36 . As the compressor  36  compresses the vapor to a higher pressure, its temperature rises so that the refrigerant leaves the compressor  36  as a high-temperature gas at high pressure. The compressor  36  pushes the hot, high-pressure refrigerant vapor through the first refrigerant or discharge line  40  to the condenser  38 . The condenser  38  is simply a heat exchanger that removes heat from the hot gas and releases it to a heat sink that, for heat pump water heaters, is the water heater  10 . The removal of heat from the hot gas causes it to condense to a liquid with the condenser heat being used to heat the water.  
         [0039]    Refrigerant leaves the condenser  38  as an intermediate-temperature liquid at high pressure through the second refrigerant or liquid line  44  to the expansion device  42 . By acting as a flow restrictor, the expansion device  42  maintains high pressure on the condenser side and low pressure on the evaporator side. In larger commercial heat pump water heaters, the expansion device  44  is an expansion valve. In smaller systems, it may be a capillary tube.  
         [0040]    As the liquid moves through the expansion device  42 , its pressure is suddenly lowered. The pressure drop causes some of the liquid refrigerant to flash (evaporate very quickly) into vapor. The evaporation of a portion of the liquid cools the remaining liquid so that the refrigerant leaves the expansion device  42  as a low-temperature mixture of gas and liquid at low pressure which then flows through the third refrigerant line  48  to the evaporator  46 . The evaporator  46  is another heat exchanger that allows heat to move from a heat source (the air inside a building for most air-source HPWHs) to the refrigerant. As the liquid refrigerant evaporates to a gas, the evaporator  46  removes heat from the heat source. In an air-source HPWH, the evaporator  42  provides a cooling and dehumidification effect to the building interior as it removes heat from the air. The refrigerant leaves the evaporator  46  through the fourth refrigerant line  50  as a low-temperature gas at low pressure and enters the compressor  36  completing the cycle.  
         [0041]    In combining a heat pump  34  with a water heater  10 , to produce a heat pump water heater  100 , as shown schematically in FIG. 3, an energy-efficient system is created to heat the water so as to provide the same amount of hot water at possibly one-half to one-third the energy used in an electric resistance water heater  10 . Considerably more energy is transferred to the water in the tank than is used to operate the heat pump.  
         [0042]    The construction of the HPWH  100  includes placing some of the components of FIG. 4, the compressor  136 , the evaporator  146  (along with a fan  147 ), the expansion device  142 , the control means (not shown) and associated refrigerant conduits  140 ,  144 ,  148  and  150 , within a circular housing  160  that fits on top  126  of the water heater  110  of FIG. 3.  
         [0043]    Any suitable means can be employed to secure the connector  162  to the opening in the top  126 . The bottom surface  164  can have a neck portion (not shown) with threads thereon which are complimentary in shape to the threaded openings in the top  126 . With the first embodiment of HPWH  100 , the condenser assembly  138 , as shown in FIG. 5, includes a union-type connector  162  with a bottom surface  164  which is used to fasten the assembly  138  to the tank top  126  through one of the existing ¾″ threaded openings in the top  126 . Extending into the interior of the connector  162  is the first conduit line  140  which exits from compressor  136 .  
         [0044]    Vertically depending from the bottom surface  164  of the connector  162  is a tube-in-a-tube cylindrical assembly  166  having a non-vertical section  167  formed of an outer body  168  having a closed bottom  170  which define an inner refrigerant chamber  172  that is in fluid communication with the first refrigerant conduit  140  through the opposed upper end  174  of the outer body  168 . Co-axially disposed within the refrigerant chamber  172  is a hollow inner body  176  having an open bottom  178  that is disposed above the bottom  170  and an opposed top  180  which is in flow communication with the second refrigerant conduit  144 .  
         [0045]    The superheated vapor from the first refrigerant conduit  140  enters the connector  162  into the upper end  174  of the outer body  168  and condenses downwardly along the inner wall of the outer body  168 . The heat thereby released is transferred to the water in the tank  114  through the wall of the outer body  168 . The non-vertical section  167  of the cylindrical assembly  166  promotes heat transfer to the water tank  114  and prevents the vertical temperature profile inside tank  114  from stratifying. The condenser refrigerant collects within the refrigerant chamber  172  and flows up the inner body  176  through bottom  178  and into the second refrigerant conduit  144  through top  180 .  
         [0046]    Because the tank water is potable water, appropriate codes usually require that a heat exchanger, such as the outer body  168 , be double-walled. Doucette Industries, Inc. and similar manufacturers provide vented double-wall heat exchangers specifically designed for water heating purposes. The surface area of the outer body  168  strongly affects the overall heat transfer coefficient with the higher surface enhancement, giving the better heat transfer.  
         [0047]    In the second embodiment of the present invention, the overall construction of the condenser assembly is similar to condenser assembly  166  except that the outer body  168 , in addition to acting as a heat exchanger, will also function as the sacrificial anode in the water tank  114 . That is accomplished by forming the outer body  168  of a second metal that is capable of corroding at a rate greater than the rate of corrosion of the first metal of the water tank  114 . The second metal can be selected from the group consisting of aluminum, magnesium or zinc. The inner body  176  can be constructed of copper. The operation of the condenser assembly of the second embodiment is identical to that of the condenser assembly  168  of the first embodiment.  
         [0048]    The length of the condenser assembly  166  for both the first and second embodiments can vary up to approximately the height of the water tank  114 . The outer and inner bodies  168 ,  176  can be of any conventional shapes. The non-vertical section  167  of condenser assembly  166  can be any geometric shape.  
         [0049]    The superheated refrigerant is fed into the interior of the outer body, which has an appropriately shaped outer heat exchange surface, for thermal transfer to the body of the water within the water chamber. The refrigerant then passes through the bottom of an inner tube or body to be directed in an opposite direction out of the condenser assembly to the expansion device through the second refrigerant conduit.  
         [0050]    Other embodiments of the present invention are shown in FIGS. 6 a  and  6   b . In FIG. 6 a , a single tube-in-tube circuit is improved by bending a non-vertical portion of the condenser assembly near the bottom of the tank to destratify the water temperature profile inside the tank. Hot refrigerant gas from the heat pump vapor compression cycle enters the hot water tank  63  inside the inner tube  61  of the condenser assembly and changes into a liquid phase by transferring heat to the surrounding water. Condensed refrigerant liquid leaves the hot water tank  63  through the outer tube  62  and proceeds through the heat pump cycle. FIG. 6 b  further enhances the homogeneous temperature profile in the tank by using two parallel circuits in the U-tube condenser assembly. Hot water temperatures are measured by thermocouples labeled TC-1 through TC-7, which are equally spaced approximately 6 inches apart from the top of the tank to the bottom. Temperature profiles are graphed in FIGS. 7, 8, and  9  and show that the stratified profile in FIG. 7, as measured on a condenser without the non-vertical condenser portion, is improved from approximately 30 degrees F temperature difference from top-to-bottom to essentially 0 degrees F temperature difference in FIGS. 8 and 9 by adding the non-vertical portion to the condenser assembly. FIG. 8 is for a vertical 2-circuit tube-in-tube condenser assembly style. FIG. 9 is for a 4-circuit U-tube condenser assembly style. The non-vertical portion of each condenser assembly style of this invention is an essential feature for the dramatic improvement in water temperature profile and showed unexpected test results for destratifying the vertical temperature profile of the water tank.