Low-cost heat pump water heater

Abstract

An improved heat pump water heater of the type having a water tank with an exterior surface and defining a water chamber, a top on the water tank with at least one opening therethrough in communication with the water chamber, and a heat pump of the type having a compressor, the compressor being in fluid communication with an annular condenser assembly via a first refrigerant conduit, the annular condenser assembly being in fluid communication with an expansion device through a second refrigerant conduit, the expansion device being in fluid communication with an evaporator through a third refrigerant conduit, the evaporator being in fluid communication with the compressor through a fourth refrigerant conduit and control means therefore, the improvement comprising disposing the annular condenser assembly through the opening in the top of the water tank and into the water chamber, the water tank opening further comprising a geometric twisted sleeve positioned to form a helical insertion pattern in the annular condenser assembly, the annular condenser assembly further comprising an elongate outer tube having a closed bottom end and an open and opposed upper end in flow communication with the first refrigerant conduit, an elongate return tube disposed within the outer tube and having an open bottom end and a closed and opposed top end in flow communication with the second refrigerant conduit, an elongate capillary tube disposed within the return tube and having an open bottom end and an open and opposed top end in flow communication with the second refrigerant conduit, and an air gap disposed between the capillary tube and the return tube.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Patent Application 60/664,776 filed Mar. 24, 2005, and is herein incorporated by reference. This application is also related to U.S. Pat. No. 6,233,958, issued May 22, 2001, herein incorporated by reference in its entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

This invention was made with United States Government support under Contract No. DE-AC05-00OR22725 between the United States Department of Energy and U. T. Battelle, LLC. The United States Government has certain rights in this invention.

FIELD OF THE INVENTION

This invention is in the general field of water heaters and specifically teaches a low-cost embodiment for a heat pump water heater.

BACKGROUND OF THE INVENTION

U.S. Pat. No. 5,946,927 teaches a “drop-in” heat pump water heater (HPWH), and ECR International produces a HPWH based around this patent. U.S. Pat. No. 6,233,958 teaches a HPWH with bayonet condenser. Residential U.S.-made HPWHs are manufactured and sold by AERS (Atlanta, Ga.) and Nyle Special Products (Bangon, Me.); however the small market that was present is drying up due to high costs based on old technologies. HPWHs are also manufactured and sold in Asia and Australia at low volumes due to high first costs.

After much work on the HPWH, the U.S. Department of Energy (DOE) has concluded that high costs are the principal market barrier for the HPWH, and DOE has established a goal of $500 for a 50-gallon residential HPWH. A preferred way of attaining this goal is taught in this invention.

There are basically three types of residential HPWHs. One is the desuperheater which is connected to the 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. One problem with a desuperheater HPWH is that the house space conditioning load is usually not in sync with the water heating load. That is, when hot water is needed, the house might not need cooling or heating, the desuperheater is therefore inactive, and water heating is provided by inefficient, backup electric resistance water heaters. For this reason, field data show that desuperheaters provide only 20-30% of all hot water needs. A second type of HPWH is a dedicated stand alone unit that pumps water from the water tank and heats it using a small, dedicated heat pump. This type of HPWH is bulky, requires a water pump, and its costs tend to be high (typically $1000, or more). The third type is a HPWH mounted on the top of the water tank to form a single, integrated unit. Integrated HPWHs heat water in a number of ways: some use small pumps to circulate water between an external condenser and the tank itself while others heat the exterior of the tank letting the warm tank transfer heat to the water that is contained inside. Finally, HPWHs that use an immersed condenser are taught; however, the immersed condenser requires a large hole be present on the top of the water tank through which the condenser assembly can be installed. A problem with all of these approaches is a market one: the costs are simply too high and as a result, the market for the HPWH is vanishing. Most of the HPWHs mentioned require a tank that is special in some way and therefore expensive. The wraparound condenser design (probably the best recent attempt at a residential HPWH) begins its manufacture with a bare metal tank, wrapping a heat exchanger coil around the tank, affixing temperature sensors to the tank, insulating the wrapped tank, shrouding it with a metal cover, and finally installing a small heat pump on the top. The major water heater manufacturers, who dominate the water heating distribution chain, have shown no interest in the wraparound design nor in any other HPWH for that matter. They are tank manufacturers, and moving into any field beyond that is simply too involved and expensive. Other attempts at a residential HPWH have faired even worse. Today, none of the major water heater manufacturers who produce conventional electric water heaters by the millions are interested in the current HPWH design. Yet, if the market for a HPWH is to materialize, the major tank manufacturers must be players. Needed then is a simple, low-cost HPWH design based on (1) use of a conventional insulated electric water heater—everywhere available; (2) a pre-charged HPWH design—one that does not require the services of refrigeration trades for installation; (3) HPWH controls that work with the existing controls for the conventional electric water heater and simply plug together; and (4) an overall design approach in which the HPWH and electric resistance tank can be “married” in the factory with little additional labor and no additional manufacturing processes. Only in this way, using the invention herein, will the economics for the heat pump water heater be favorable enough to support a reasonable market.

BRIEF DESCRIPTION OF THE INVENTION

An improved heat pump water heater of the type having a water tank with an exterior surface and defining a water chamber, a top on the water tank with at least one opening therethrough in communication with the water chamber, and a heat pump of the type having a compressor, the compressor being in fluid communication with an annular condenser assembly via a first refrigerant conduit, the annular condenser assembly being in fluid communication with an expansion device through a second refrigerant conduit, the expansion device being in fluid communication with an evaporator through a third refrigerant conduit, the evaporator being in fluid communication with the compressor through a fourth refrigerant conduit and control means therefore, the improvement comprising disposing the annular condenser assembly through the opening in the top of the water tank and into the water chamber, the water tank opening further comprising a geometric twisted sleeve positioned to form a helical insertion pattern in the annular condenser assembly, the annular condenser assembly further comprising an elongate outer tube having a closed bottom end and an open and opposed upper end in flow communication with the first refrigerant conduit, an elongate return tube disposed within the outer tube and having an open bottom end and a closed and opposed top end in flow communication with the second refrigerant conduit, an elongate capillary tube disposed within the return tube and having an open bottom end and an open and opposed top end in flow communication with the second refrigerant conduit, and an air gap disposed between the capillary tube and the return tube.

The invention comprises a low-cost type HPWH with the heat pump unit mounted on top of a conventional hot water storage tank. Low-cost connotes turning an inexpensive, conventional resistance-type electric water heater into an HPWH with no changes to the tank or to the existing controls.

The invention addresses the need for a low-cost HPWH by using a conventional, insulated electric water heater and controls as presently produced and found throughout the United States.

The invention teaches a method for installing a condenser with a large surface area through the small threaded opening at the top of the water heater.

The invention comprises a simple method (geometric twisted sleeve) for arranging the internal condenser by adjusting the number of coils, pitch, radius and location within the tank to affect an optimal, efficient design.

The invention comprises a linear condenser design that avoids loss of refrigerant subcooling as the refrigerant travels upward along its path out of the storage tank.

The invention comprises a linear condenser design that incorporates the expansion device into the condenser itself forming into a single element the two functions of an immersed condenser and an expansion device.

The invention comprises a HPWH control system that uses to the greatest extent all of the conventional controls of the baseline resistance water heater, and applies them in a unique way as part of a HPWH control system that can heat water using resistance heaters or using the heat pump.

The invention can be applied to a new water heater by a manufacturer, retrofitted to an existing water heater, and applied to a gas water heater resulting in a dual-fuel, efficient water heater for both residential and commercial buildings.

One clear application is to residential electric water heating. The annual operating costs for a conventional residential water heater are about $350. The HPWH would cut these costs in half. Applied to a gas water heater, the HPWH technology would provide a “dual-fuel” capability. And finally, since the HPWH has an air-source evaporator, the HPWH provides some cooling and dehumidification to the space where is the HPWH is located.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic of a HPWH with an internal condenser coil.

FIG. 2 is a diagram showing the condenser coil inserted and formed through a sleeve having an expansion device.

FIG. 3 is a graph showing the field test data from a previous HPWH with an immersed condenser.

FIG. 4 is a schematic of a HPWH control system.

FIG. 5 is a schematic of a HPWH with the condenser wrapped around the tank wall.

DETAILED DESCRIPTION OF THE INVENTION

One of the biggest challenges to be met is finding a way to install a large surface area condenser into the conventional tank through one of the small pipe fittings (¾″ FPT) that is already on the top of all water heaters. This is somewhat akin to building a ship in a bottle with a small neck. The invention overcomes this problem by installing a short, geometric twisted sleeve through the small diameter top fitting in the top of the tank, and then pushing a long condenser assembly through the open sleeve. As the linear condenser is pushed through the sleeve, it forms itself into a helix inside the tank. The radius, pitch and location of the helix inside the tank is pre-determined by the geometry of the sleeve. Calculations show that a 5/16-in diameter condenser can be pushed through a ¾-inch, sleeved hole in a tank, and that a 50-foot long condenser section, 5/16-in diameter, will form a helical condenser inside the tank providing more than 4 sq. ft. of condenser surface inside of a conventional water tank that is only 3.5 feet long. The sleeve, having served its purpose, remains in the tank. A major advantage of this design is that a small manufactured package consisting of an air-source heat pump and linear condenser assembly can be simply mated to a conventional, insulated hot water tank to make an HPWH.

A second constraint is loss of performance when a linear condenser, designed as a assembly, is used to heat water in a tank. Conventional immersed condenser art takes the form of a probe or “U-tube” design in which the condenser's inlet and exit are at the same location; usually at the top of the tank where the water is the hottest. This means that as superheated refrigerant from the compressor enters the condenser and passes downward towards the end of the condenser at the bottom of the tank, it condenses and becomes subcooled. The refrigerant then returns from the bottom of the condenser, passing Upwards and out of the condenser at the top of the tank. The problem is that the hot water at the top of the tank heats the refrigerant so that it loses its subcooling that it attained at the bottom of the tank. As the refrigerant, having lost its subcooling continues to the expansion device, refrigerant metering and control are lost, the operation of the cycle becomes unstable and the performance and efficiency of the system drop. To overcome this problem, the condenser in the invention has a unique feature that ensures that the refrigerant returning from the bottom of the tank is not reheated by the hot water at the top of the tank. This is done by using an annular condenser design that allows the refrigerant to return from the bottom of the tank (from the end of the condenser assembly) through a small, return tube that is insulated from the outer annulus. The design is such that an air space is formed in the return tube, and the air space effectively insulates the cooler refrigerant returning from the bottom of the tank from the hot condensing refrigerant passing downwards through the annulus of the assembly. The invention takes the condenser design one step further. Since the small, central return tube of the condenser provides no heating function, it is used as the expansion device in the cycle, that is, it serves as a capillary tube to meter low temperature, low pressure refrigerant directly into the evaporator of the cycle. Therefore, the linear condenser assembly is a condenser on it's outside where it is always adjacent to the water to be heated, and is an expansion device along the central tube. This simplifies the cycle, reduces its costs and improving reliability as compared to HPWHs that use active thermostatic expansion valves.

The third issue facing a low-cost HPWH that is designed around the use of a conventional, insulated, electric storage water heater with heating elements, is a low-cost method for controlling the operation of the HPWH. HPWH controls need to respond to tank temperatures and to compressor operating conditions. If the bottom of the tank becomes cool due to a hot water draw, the HPWH needs to turn on and operate until the bottom of the tank reaches a setpoint. If the top of the tank becomes cool due to an extended hot water draw, the upper heating element in the tank needs to turn on. And if the compressor finds itself operating outside of an acceptable envelope of discharge or suction temperatures, controls need to change the HPWH's operation, and if acceptable compressor operating conditions cannot be attained, controls must turn off the compressor and return the system to a conventional control strategy in which the lower tank heating element supplemented by the upper element provides the heating. The invention retains and uses the upper and lower heating elements and thermostats that are present in a conventional electric water heater as well as two temperature sensors on the suction and discharge sides of the compressor and an ambient temperature sensor to perform the control functions that are needed. The invention avoids any need for additional sensors, controls and wiring to be applied to the conventional water heater to turn it into a HPWH. Therefore, this invention could be manufactured by a company skilled in vapor compression refrigeration fabrication, shipped to a water heater company (with no skills in vapor compression equipment), connected together with very little labor, and shipped by the water heater company as a new product into their existing distribution network. Therefore, it will be easier for water heater tank manufacturers to accept this type of HPWH and incorporate it into existing product lines.

FIG. 1 is a simple schematic of the HPWH with its internal annular condenser coil 1. The HPWH portion 3 at the top absorbs heat from the surrounding air and, together with the heat generated by compressor operation, heats the water in the tank 2 via the annular condenser coil 1. The compressor, evaporator, fan, linear condenser, expansion device and controls are packaged as a single assembly. The annular condenser coil 1 can be made from copper tubing, and it is possible that for corrosion protection, the exterior of the condenser assembly could be coated with the same material as used in the sacrificial anode of the water heater. 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 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.”

FIG. 2 shows a geometric twisted condenser sleeve 21 that is installed first in the conventional insulated tank 22. A double walled, annular condenser assembly 23 with the return tube 30 along the central axis of the annular condenser assembly 23 is then be inserted into the tank 22 through the sleeve 21. Due to the shape of the sleeve 21, the inserted annular condenser assembly 23 takes a helical shape as it is pushed into the tank. The pitch, diameter and position of the helix inside the tank is determined by the geometry of the sleeve. Experience has shown that locating an immersed condenser in the bottom ⅓ to ½ of the tank works best, heating the cooler water near the bottom of the tank. A large condenser area can be produced by a helix of the largest diameter and smallest pitch subject to limitations of the tank itself. Experience will tell whether it is necessary to remove temporarily all obstructions on the inside of the tank (e.g. the elements, dip tube, anode rods) during insertion of the condenser.

Details of the annular condenser assembly 23 are shown in FIG. 2. The condenser assembly is double-walled for safety since the water in the tank is considered potable. The exterior of the return tube 30, and the inside of the outer tube 24 of the double-wall configuration ensures that a path exists to allow any refrigerant leaks to exit the condenser and tank without contacting the potable water. Experience has shown that good thermal contact between the outer tube 24 and return tube 30 can be provided by hydraulically expanding the return tube 30 against the outer tube 24 during manufacture. This process also leaves a path for the refrigerant 26 to escape in the event of a leak. FIG. 2 also illustrates condenser detail that shows the refrigerant capillary tube 29 along the center of the return tube 30 with a reducer 28 mounted to the end of the capillary tube 29. By making the capillary tube 29 small, two objectives are reached: (1) an air gap 25 is formed next to the return tube 30, and this provides a high degree of thermal insulation between the annular condensing portion and the refrigerant return portion of the assembly, and (2) by proper design, the capillary tube 29 serves wholly or partially as the expansion device so that as the refrigerant 26 exits the assembly, it is at the low temperature and low pressure needed for the evaporator.

FIG. 3 shows the field test data from a previous HPWH with an immersed condenser. These data show that the HPWH is twice as efficient as a conventional electric resistance heating water heater, with a coefficient of performance (COP) energy factor of around 1.7 to 1.8. It is expected that the invention will perform even better due to the greater heat transfer surface from the use of longer tubes.

In order to be low cost and energy efficient, the HPWH controls must be as simple as possible. FIG. 4 shows the controls of the HPWH and how they take advantage of existing electric water heater controls. The existing electric water heater controls consist of an upper element 45 and its thermostat 42, and a lower element 44 and its thermostat 43. As shown in FIG. 4, the lower element thermostat 43 is slaved to the upper element thermostat 42 so that if the upper portion of the tank is cool, the upper thermostat 42 activates the upper element 45 and deactivates the lower element 44 irrespective of the lower thermostat 43 position. This means that only one heating element can be active at any one time. The power for a typical water heating element is 4500 W with a voltage input of 240 VAC. Based in this, it can be shown that the resistance of this typical heating element is about 12.8 ohms (small). From FIG. 4, it can be seen that the control module 48 provided as part of the HPWH system is powered by the 240 VAC; other inputs to the control module 48 are from temperature sensors that measure the evaporator temperature 40, the compressor discharge temperature 50, the ambient temperature 51 and safetys 41 such as a condensate switch to turn the compressor off in the event of an imminent evaporator condensate pan overflow.

Conventional (resistance heating) operation: If any one of the control conditions needed for operation of the HPWH is not satisfied, no voltage is applied between terminals A and B of relay 49, and the contacts remain closed. In this case, the HPWH behaves as a conventional resistance water heater with the upper and lower thermostats controlling the heat input to the water tank by the elements.

HPWH operation: If all conditions for HPWH operation (e.g. compressor temperatures and ambient temperatures) are within a nominal range, the control module 48 applies a control voltage (e.g. 24 VAC) between terminals A and B. This activates relay 49 thereby opening its contact, and the HPWH is capable of operating. By design, the control module 48 has high input impedance between terminals A and C. Therefore, with either thermostat 42 or 43 “made”, the series connection between terminals A-C of the control module 48 and either heating element forms a voltage divider network. Assume, for example, that the input impedance between module terminals A-C is 130 ohms. Then with the lower thermostat 43 “made”, the voltage between terminals A-C is 218 VAC. With the lower thermostat satisfied (open), the A-C voltage rises to 240 VAC. The control module 48 uses this difference (or change) to operate the HPWH in response to the lower thermostat. By installing upper and lower elements of two different power inputs (e.g. a 3000-W lower element and a 4500-W upper element), the A-C voltage would have three levels to energize three heat sources. If the A-C impedance were 130 ohms as before, a 240 VAC signal between terminals A and C would indicate that both thermostats are satisfied (the compressor and fans can be turned off); a 210 VAC signal between terminals A and C would indicate that the lower thermostat is not satisfied (but the upper one is satisfied) suggesting that the tank is calling for heating at the bottom; and a 218 VAC signal between terminals A and C would indicate that the upper thermostat is not satisfied, but the lower one is satisfied. The ability to use the existing thermostats and the resistance elements to discriminate how the tank is to be heated is part of the uniqueness of this invention. The values chosen for the examples above are arbitrary and serve only to illustrate the idea.

Finally, it will be understood that the preferred embodiment has been disclosed by way of example, and that other modifications may occur to those skilled in the art without departing from the scope and spirit of the appended claims.

Claims

1. An improved heat pump water heater of the type having a water tank with an exterior surface and defining a water chamber, a top on the water tank with at least one opening therethrough in communication with the water chamber, and a heat pump of the type having a compressor, the compressor being in fluid communication with an annular condenser assembly via a first refrigerant conduit, the annular condenser assembly being in fluid communication with an expansion device through a second refrigerant conduit, the expansion device being in fluid communication with an evaporator through a third refrigerant conduit, the evaporator being in fluid communication with the compressor through a fourth refrigerant conduit and control means therefore, the improvement comprising:

disposing the annular condenser assembly through the opening in the top of the water tank and into the water chamber, the water tank opening further comprising a geometric twisted sleeve positioned to form a helical insertion pattern in the annular condenser assembly, the annular condenser assembly further comprising: an elongate outer tube having a closed bottom end and an open and opposed upper end in flow communication with the first refrigerant conduit, an elongate return tube disposed within the outer tube and having an open bottom end and a closed and opposed top end in flow communication with the second refrigerant conduit, an elongate capillary tube disposed within the return tube and having an open bottom end and an open and opposed top end in flow communication with the second refrigerant conduit, and an air gap disposed between the capillary tube and the return tube.

2. An improved heat pump water heater as claimed in claim 1 wherein the first refrigerant conduit, the expansion device, and the second refrigerant conduit are disposed inside the water tank; the evaporator, the third refrigerant conduit, the compressor and the fourth refrigerant conduit and the control means are disposed on the water tank.

3. An improved heat pump water heater as claimed in claim 1 and further comprising a housing disposed on the top of the water tank and containing therein the compressor and evaporator.

4. An improved heat pump water heater as claimed in claim 1 wherein the annular condenser assembly is constructed of copper.

5. An improved heat pump water heater as claimed in claim 1 wherein the annular condenser assembly is constructed of a first metal and a second metal which is capable of corroding at a rate greater than the rate of corrosion of the first metal.

6. An improved heat pump water heater as claimed in claim 5 wherein the second metal is selected from the group consisting of aluminum, magnesium or zinc.

7. An improved heat pump water heater as claimed in claim 1 wherein the annular condenser assembly is a double walled helical device.

8. An improved heat pump water heater as claimed in claim 1 wherein the coefficient of performance is in the range of approximately 1.7 to 1.8.

9. An improved heat pump water heater as claimed in claim 1 wherein the control means further comprises a control module having a voltage divider network to energize heating sources.

10. An improved heat pump water heater as claimed in claim 9 having three heating sources.

11. A method of constructing an improved heat pump water heater of the type having a water tank formed of a first metal and defining a water chamber, a top on the water tank with at least one opening therethrough for an anode rod to be disposed within the water chamber, and a heat pump of the type having a compressor, the compressor being in fluid communication with an annular condenser assembly via a first refrigerant conduit, the annular condenser assembly being in fluid communication with an expansion device through a second refrigerant conduit, the expansion device being in fluid communication with an evaporator through a third refrigerant conduit, the evaporator being in fluid communication with the compressor through a fourth refrigerant conduit and control means therefore, the comprising the steps of:

a. removing the anode rod from the water tank; and

b. inserting the annular condenser assembly through the opening in the top of the water tank and into the water chamber, the water tank opening further comprising a geometric twisted sleeve positioned to form a helical insertion pattern in the annular condenser assembly, the annular condenser assembly further comprising an elongate outer tube having a closed bottom end and an open and opposed upper end in flow communication with the first refrigerant conduit, an elongate return tube disposed within the outer tube and having an open bottom end and a closed and opposed top end in flow communication with the second refrigerant conduit, an elongate capillary tube disposed within the return tube and having an open bottom end and an open and opposed top end in flow communication with the second refrigerant conduit, and an air gap disposed between the capillary tube and the return tube.

12. A method as claimed in claim 11 wherein the annular condenser assembly is constructed of copper.

13. A method as claimed in claim 11 wherein the annular condenser assembly is constructed of a first metal and a second metal which is capable of corroding at a rate greater than the rate of corrosion of the first metal.

14. A method as claimed in claim 13 wherein the second metal is selected from the group consisting of aluminum, magnesium or zinc.

15. A method as claimed in claim 11 wherein the annular condenser assembly is a double walled helical device.

16. A method as claimed in claim 11 wherein the coefficient of performance is in the range of approximately 1.7 to 1.8.

17. A method as claimed in claim 11 wherein the control means further comprises a control module having a voltage divider network to energize multiple heating sources.

18. A method as claimed in claim 17 further comprising three heating sources.