HEAT PUMP WATER HEATER

Abstract

A heat pump water heater employing a thermostatic mixing valve integrated with the water lines carrying water from a water tank to a heat pump and then back to the water tank to achieve one pass technology to heat the water quickly and to achieve rapid recovery time without the use of supplemental heating sources.

Description

BACKGROUND OF THE INVENTION

The present invention relates to water heaters utilizing heat pump technology to raise the temperature of water in a tank to a sufficient level to be useful for domestic hot water purposes.

Water heaters for domestic use are well known in the art. Typically, water contained within a water tank is brought into the bottom of the tank from an external source, such as a municipal water supply or from a well. Such water is quite cold, and requires heating to at least 100° F., and preferably between 120° F. and 140° F., for domestic hot water usage. In the known art, the water may be heated by oil, gas, or coal, as a byproduct of the heating system used to heat the house itself. More popularly, the water is heated by electricity. Electric hot water heaters often use resistance heating elements to directly heat the water in the tank.

Heat pump technology has become a desirable alternative to resistance heating. Heat pumps remove heat energy from the ambient air through a combination of compression and evaporation; this energy may then be used to heat water in the water tank. The heating is indirect; that is, the heat energy generated by the heat pump is captured in a separate refrigerant fluid that is circulated within a closed system. A heat exchanger interfaces between the refrigerant and the water to be heated, allowing heat energy to be transferred from the refrigerant to the water. Heat pumps operate with higher efficiency than electric resistance heaters, and heat pump water heaters are known in the art.

Currently available heat pump water heaters fall into three basic configurations. These are wrap-around, immersion, and pumped. See FIGS. 3A, 3B, and 3C. In the wrap-around version (FIG. 3B), water never leaves the tank, but rather refrigerant is heated by the heat pump and passes through coils which are wrapped externally about the tank. Heat passes from the coils through the wall of the water tank, thereby heating all of the water contained in the tank simultaneously. In the immersion version (FIG. 3C), refrigerant is heated and passes through coils which are immersed within the interior of the water tank, again heating all of the water contained in the tank simultaneously. In both of these versions, “recovery time” (the time it takes freshly added cold water to be brought up to the desired temperature) is delayed, since the heat energy is provided to all of the water in the tank, not just to the newly added water.

In pumped versions (FIG. 3A), water is pumped out of the bottom of the water tank, passes through a heat exchanger in order to be heated by the refrigerant, then is returned to the top of the water tank (from which hot water is drawn for use). This means that only the coldest water in the tank (the water located at the bottom) is being heated. However, the amount of heat energy that can be added to the cold water by the heat pump is fairly low, between 7° F. and 10° F. at a time, so the water being added to the top of the water tank is initially not much warmer than the water contained in the bottom of the tank, and significantly less than the desired temperature. The water therefore must be continuously pumped out of the bottom of the water tank, heated by the heat pump, and returned to the top of the water tank. This process, known as multi-pass, eventually heats all of the water in the tank, but again with a delayed recovery time. For example, the water from the bottom of the tank might be 50° F., the heat pump/heat exchanger may be able to heat it to 60° F. (based on the amount of energy produced by the heat pump and the flow rate of the water as controlled by the flow pump), and the 60° F. water is returned to the top of the tank. However, the desired temperature may be 120° F., so the water entering the tank actually cools the water at the top of the tank slightly. Multiple passes through the system are required, with each pass raising the temperature incrementally, and the hot-on-top/cold-on-bottom temperature stratification of the water within the tank is lost, with the temperature mixing and eventually heating the entirety of the tank to 120° F. This takes a long time and leads to very poor water recovery after use. An alternative configuration of prior art pumped versions is similar to that described above, except that the water heated by the heat pump is returned to the bottom of the water tank, rather than to the top. This is also a multi-pass system. Thus, the heating of the water within the tank occurs from the bottom up, rather than from the top down. As with the previously described configuration, this also results in very slow recovery time.

Residential heat pump water heaters are limited to one of these configurations because the only known mechanisms in the prior art for allowing “one-pass” operation (as described below) are computer controlled sensors and valving (also as described below), which are too costly for small scale residential applications.

In existing pumped version configurations for residential use, in order to improve recovery time, an integrated electric resistance heater is used to augment the energy supplied by the heat pump. The resistance heater is located in the top portion of the water tank, and quickly heats the water in the top of the tank. However, in order to be effective, these resistance heaters are typically 4.5 kW drawing power from a 240V outlet. Many residential homes, though, do not have 240V power available to their water heaters, but rather only have 120V power. A resistance heater running off 120V power typically puts out at most only 900 W, which is insufficient for heating a sufficient amount of water quickly enough to result in acceptable recovery times. Thus, while the above-described configuration of currently available “pumped” heat pump water heaters having integrated resistance heating elements overcomes the limitations described above in regard to wrap-around and immersion heat pump water heaters, it is not workable in those residential spaces having only 120V power available.

Larger, industrial heat pump water heaters use sophisticated computer controlled sensors and valving to retain the water within the heat pump/heat exchanger unit, so that rather than being heated only a few degrees at a time, the water is heated to the desired temperature before being returned to the tank. Thus, water from the bottom of the tank at 50° F. enters the heat pump/heat exchanger, which by operation of the control system retains the water within the heat pump/heat exchanger until it heats it to 120° F., then sends the heated water into the top of the water tank. This is considered “one pass” technology, and it ensures that the water at the top of the tank (which is what is first used) is at the desired temperature, rather than being cooled by only partially heated return water as described above, and tank heat stratification is maintained. The cold water at the bottom of the tank is only slowly heated, but the water for immediate use is always at the desired temperature. This results in very quick recovery time.

However, “one pass” systems controlled by computer controlled sensors and valving are too expensive for residential use. The device of the present invention solves this problem by replacing the computer controlled sensors and valving with a mechanical thermostatic mixing valve. A thermostatic mixing valve is relatively inexpensive (when scaled for residential use) and it serves the same purpose as the computer controlled sensors and valving: it allows the cold water entering the heat pump/beat exchanger unit to be retained therein until it achieves the desired final temperature. The remainder of the cycle is as described for the commercial units. The “one pass” commercial units cannot use the thermostatic mixing valve technology, though, because at commercial scale thermostatic mixing valves become prohibitively expensive, and the computer controlled sensors and valving is more cost efficient. Similarly, the residential units cannot use the computer controlled sensors and valving, because at that scale that system is too expensive.

It is thus shown that there is a need to provide a heat pump water heater that can be used in residential homes operating on 120V power which can not only heat water to useable levels but also provide rapid recovery time by using a one-pass system.

It is therefore an object of the present invention to provide for a heat pump water heater that can be used in residential homes operating on 120V power.

It is a further object of the present invention to provide for a heat pump water heater which can heat water to useable levels.

It is a further object of the present invention to provide for a heat pump water heater which can provide rapid recovery time by using a one-pass system.

Other objects of the present invention will be readily apparent from the description that follows.

SUMMARY OF THE INVENTION

The present invention is a pumped version heat pump water heater. As with prior art configurations, water is pumped out of the bottom of the water tank, passes through a heat exchanger in order to be heated by the refrigerant, then is returned to the top of the water tank. However, rather than being a multi-pass process, whereby the water is only heated incrementally by the heat pump and so must be continuously circulated out of and back into the tank, the device of the present invention is a one-pass process, whereby the water to be heated is retained within the heat pump system until it is fully heated to the desired temperature, and only then is the water returned to the top of the water tank. This means that the water at the top of the water tank is immediately available for use, rather than having to be recirculated over and over in and out of the water tank and through the heat pump, or heated by an integrated resistance heater. Because there is no need for a resistance heater, the device does not require 240V power but rather can run on 120V power.

The device of the present invention comprises a heat pump, a heat exchanger, a circulating pump, refrigerant piping, water piping, and a thermostatic mixing valve. The circulating pump and thermostatic mixing valve are integrated with the water piping. The water piping runs from the water tank to the thermostatic mixing valve, then to the circulating pump, then to the heat exchanger, then to a T-junction; one portion of the water piping exiting the T-junction goes back to the thermostatic mixing valve, while the other portion of the water piping exiting the T-junction runs to the water tank. The portion of the water piping running from the thermostatic mixing valve to the heat exchanger and then back to the thermostatic mixing valve is referred to as circulation piping. See FIG. 2.

During operation, the heat pump extracts heat energy from the ambient air and transfers this energy to a refrigerant. The refrigerant circulates through the refrigerant piping, a portion of which is integrated with the heat exchanger. Water from the water tank flows through the water piping by means of the circulating pump. A portion of the circulation piping is integrated with the heat exchanger. Heat energy contained in the refrigerant is transferred to the water in the circulation piping by the heat exchanger. The thermostatic mixing valve controls whether the water in the circulation piping is recirculated through the heat exchanger to acquire additional heat energy or is returned to the water tank. The thermostatic mixing valve is field adjustable and can be set so that the water leaving the device will always be within a couple of degrees of a set point selected by the user. That set point can be selected within the range of 80°-145° F. The device will produce 120° F. usable hot water instantaneously at a rate of between 0.20-0.82 gallons per minute, depending on the beginning temperature of the water. When domestic hot water is called for, the water heated by the heat pump will be immediately available.

Use of the thermostatic mixing valve is the central aspect of the present invention. A thermostatic mixing valve is a valve that blends hot water with cold water. It is typically used to ensure constant, safe shower and bath outlet temperatures, preventing scalding. This technology is well known in the art. In brief, a thermostatic mixing valve comprises a hot water inlet, a cold water inlet, a mixed water outlet, a thermostat element, a stem assembly, a hot valve seat, and a cold valve seat. See FIG. 5. The active part of the thermostatic mixing valve, the thermostat element, expands and contracts in response to changes in the temperatures and relative pressures of the incoming hot and cold water entering through the inlets. It typically has a wax core. As the thermostat element expands and contracts, it moves the stem assembly back and forth to correctly proportion the amount of cold and hot water entering the valve. It does this by partially covering and uncovering the inlets, whereby movement of the stem assembly in one direction simultaneously increases the size of the opening of one water inlet while decreasing the size of the opening of the other water inlet. At the extreme, a water inlet may be completely closed. Using a thermostatic mixing valve in the device of the present invention allows the device to retain water within the circulation piping until its temperature is sufficiently raised by the heat pump to cause the thermostatic mixing valve to open its cold water inlet. As a result of the use of the thermostatic mixing valve, water returned to the water tank is always within the desired temperature range, making it immediately available for domestic hot water consumption.

It is to be understood that the foregoing and following description of the invention is intended to be illustrative and exemplary rather than restrictive of the invention as claimed. These and other aspects, advantages, and features of the invention will become apparent to those skilled in the art after review of the entire specification, accompanying figures, and claims.

DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a schematic drawing of one embodiment of the heat pump water heater of the present invention.

FIG. 2 depicts a water line piping diagram of the device of the present invention.

FIG. 3A depicts a prior art embodiment of a heat pump water heater wherein water from the water tank is pumped through a heat exchanger to obtain heat energy created by a heat pump.

FIG. 3B depicts another prior art embodiment of a heat pump water heater wherein refrigerant heated by a heat pump is circulated through piping surrounding the water tank, thereby heating the water in the water tank through conduction through the walls of the water tank.

FIG. 3C depicts yet another prior art embodiment of a heat pump water heater wherein refrigerant heated by a heat pump is circulated through piping immersed within the water tank, thereby heating the water in the water tank through direct conduction.

FIG. 4 is a graph depicting the temperature of water in the water tank over various periods of time as hot water is used domestically.

FIG. 5 depicts a generic thermostatic mixing valve.

DETAILED DESCRIPTION OF THE INVENTION

During operation, cold water flows out of the bottom 12 of the water tank 10 into inflow piping 140 of the water piping 130 and through the thermostatic mixing valve 180 (which is adjusted to a desired temperature) and then to the heat pump 110 for heating; and heated water flows out of the heat pump 110 through outflow piping 150 of the water piping 130 into the top 14 of the water tank 10, either to replenish the hot water in the tank 10 or for immediate domestic consumption. See FIG. 1.

(A). During the initial “priming” of the heat pump 110, the cold water inlet 182 of the thermostatic mixing valve 180 is closed off to the inflow piping 140 of the water piping 130. Water already in the circulation piping 160 is pumped by the circulating pump 190 through the circulation piping 160 through the heat exchanger 170, where it receives heat energy produced by the heat pump 110 and delivered through the refrigerant piping 120 to the heat exchanger 170. The partially heated water then passes through the T-junction 162 and continues through the circulation piping 160 back to the thermostatic mixing valve 180. (Water does not flow out of the circulation piping 160 through the outflow piping 150 of the water piping 130 during this step because the outflow piping 150 is connected to the top 14 of the pressurized water tank 10. Since the cold water inlet 182 of the thermostatic mixing valve 180 is closed to the inflow piping 140, water is not flowing out of the water tank 10 and the water tank 10 remains fully pressurized, preventing more water from entering the tank 10 via the outflow piping 150. Alternatively, if there is call for hot water, the tank 10 remains pressurized by the corresponding inflow of water into the water tank 10 from the external water supply, again preventing water from flowing through the outflow piping 150.) If the water within the circulation piping 160 has not yet reached a sufficiently warm temperature, the cold water inlet 182 of the thermostatic mixing valve 180 remains closed and the water repeats this circuit through the circulation piping 160 as described above, with each repetition adding more heat energy to the water and thus eventually raising the temperature of the water in the circulation piping 160 to the desired temperature.

(B). Once the water in the circulation piping 160 reaches the desired temperature, the cold water inlet 182 of the thermostatic mixing valve 180 begins to open, letting cold water from the bottom 12 of the water tank 10 into the circulation piping 160 through the inflow piping 140. Because water from the water tank 10 is now flowing into the circulation piping 160, a corresponding amount of heated water in the circulation piping 160 flows out of the circulation piping 160 through the outflow piping 150 into the top 14 of the water tank 10; this water is already at the desired temperature. The thermostatic mixing valve 180 controls the amount of the flow of the cold water into the circulation piping 160. Water already in the circulation piping 160 that had been heated to the desired temperature during the “priming” phase but which did not flow back to the water tank 10 through the outflow piping 150 remains in the circulation piping 160 and enters the hot water inlet 184 of the thermostatic mixing valve 180, to be mixed with the cold water and then returned to the heat exchanger 170 through the mixed water outlet 186 of the thermostatic mixing valve 180. This combined hot and cold water flowing through the circulation piping 160 is much closer to the desired temperature, and the heat exchanger 170 can easily bring it up to the desired temperature. Cold water entering the circulation piping 160 through the inflow piping 140 during this phase is therefore heated to the desired temperature in fewer passes through the heat exchanger 170, even in as few as a single pass. If too much cold water enters through the inflow piping 140, or if the heat exchanger 170 does not bring the water to the desired temperature in a single pass, the thermostatic mixing valve 180 closes off access through its cold water inlet 182 (or restricts entry of water to some degree) and the process reverts back to operation as described in phase (A), above.

(C). Once the water in the tank 10 (measured by a temperature probe located at a distance close to the bottom 12 of the tank 10) achieves the desired temperature (meaning, the entire tank 10 is filled with water at the desired temperature), the device turns off. As hot water is used from the tank 10, additional cold water enters the tank 10 at the bottom 12; the temperature probe detects the resulting drop in temperature at the bottom 12 of the tank 10 and restarts the device; the thermostatic mixing valve 180 controls whether the restart needs to go through phase (A) or whether it can immediately go into phase (B).

What has been described is a one pass system. The device is designed to quickly heat the water from the bottom 12 of the tank 10 while leaving the hot water at the top 14 of the tank 10 ready for immediate use. Due to heat stratification, the cold water entering the tank 10 will not be accessible for domestic hot water use. Moreover, if the outflow piping 150 of the device is piped to the hot-outlet of the water tank 10, the device will provide usable hot water instantly any time the heat pump water heater 100 is running. Given any amount of time to recover, the water tank 10 will be at least partially full of usable hot water. In typical conditions, 30-minutes after a complete draw-down of the tank 10, the device will have reheated fifteen gallons of water to 120° F., which is enough for a shower, while a prior art 120V heat pump water heater with integrated resistance heating will have a tank full of luke-warm water.

Integrated heat pump water heaters, when operating in heat pump only mode, are only able to add minimally heated water to the water tank. In order to get any water in the water tank up to a usable temperature (such as 105° F. or 125° F.), these systems have to heat the entire contents of the water tank. So, after a series of demanding hot water uses, such as multiple showers or a bath, there is no usable hot water available in the water tank for up to several hours. Lab performance testing supports this. A 240V water heater addresses this problem by placing a 4.5 kW resistance electric heating element in the upper ⅓-¼ of the water tank. This allows the water heater to heat just the top fifteen gallons of water (as opposed to forty-five or more) and this smaller volume can be raised to a hot, usable temperature in a shorter amount of time. However, a 120V water heater does not have this option. At that voltage, any resistance elements will be less than 900 W. That is insufficient power to raise the temperature of fifteen gallons of water quickly. Because the device of the present invention operates in one pass mode, it is able to effectively heat water contained in the tank 10 from the top 14 of the tank 10 down (by adding sufficiently heated water to the top 14 of the tank 10), which means hot water, at a usable temperature, is available more quickly than with an integrated heat pump water heater approach using only a 120V resistance heating element.

While the preferred embodiments of the present invention have been described, modifications can be made and other embodiments may be devised without departing from the spirit of the invention.

Example of Use

A prototype of the device of the present invention was installed in a real world environment to test its efficacy. The device was installed in the basement of a single family home with two adults and two children, ages three and six. The average temperature in the basement in February has been 47° F. Pex pipe and sharkbite fittings connect the device to a 40 gallon capacity electric water heater. The two-pole, 240V circuit breaker was switched off to the water heater, thereby disabling the resistance heating element and making the device of the present invention the sole source of heat. The device was plugged into the a 120V outlet on a 15 A breaker. The device draws up to 900 W momentarily during startup and averages about 700 W.

FIG. 4 displays a screen capture of the two primary temperature sensors on the water side of the device installed in the single family home from 6:00 PM to 3:00 AM on February 8th and 9th, 2022. At the time this graph was produced, the device was set to produce 120° F. minimum hot water. In the graph, one line 200 is the temperature of the water leaving the device. The other line 210 is the temperature of the water located low in the water tank, just 12″ or so from the bottom. This low position is what determines when the device needs to operate. A lower temperature probe location means the device will activate and begin heating water after only 5-10 gallons of hot water is consumed. This ensures that the water tank is topped off and ready for a large demand at any time.

Just after 7 pm a bath was prepared for the three year old, and the six year old got in the shower in a separate bathroom. As cold water entered the tank from the municipal water supply, the temperature probe detected the cold water coming in to the bottom of the tank, which triggered the device to begin making hot water. The device responded by producing 120° F. water which was then sent to the hot water pipe, just outside the tank, and instantly consumed by the bath and shower. The contribution from the device was but a small fraction of the total flow going to the fixtures, with the majority of the hot water coming from the tank. Both shower and bath were as long as desired with no fluctuation in temperature. By 9 pm, the water in the lower portion of the tank was beginning to heat up, indicating that the tank was becoming heat soaked, just two hours after the heavy use. The sharp drop in temperature just after 9 pm occurred when an adult showered. Again, the temperature began recovering quickly, and within two hours the water in the bottom of the tank was already approaching 110° F. By just after 2 am, the water in the bottom of the tank reached its set point of 125° F. and the device tuned off.

Claims

1. A heat pump water heater to be used with a pressurized water tank, said heat pump water heater comprising a heat pump, refrigerant piping, water piping, a heat exchanger, a thermostatic mixing valve, and a circulating pump, wherein the heat pump is configured to transfer heat energy to refrigerant circulated within the refrigerant piping,the water piping is comprised of inflow piping, outflow piping, and circulation piping, wherein the circulation piping is interposed between the inflow piping and the outflow piping, the inflow piping is connected to a lower portion of the water tank and configured to allow water to pass out of the water tank and into the circulation piping, and the outflow piping is connected to an upper portion of the water tank and configured to allow water to pass out of the circulation piping and into the water tank,a portion of the refrigerant piping and a portion of the circulation piping are integrated with the heat exchanger, such that heat energy from the refrigerant contained within the refrigerant piping is transferred to water contained within the circulation piping via the heat exchanger,the thermostatic mixing valve is located at a junction of the inflow piping and the circulation piping, with the inflow piping connected to a cold water inlet of the thermostatic mixing valve, one end of the circulation piping connected to a hot water inlet of the thermostatic mixing valve, and another end of the circulation piping connected to a mixed water outlet of the thermostatic mixing valve,the circulating pump is located inline of the circulation piping between the mixed water outlet of the thermostatic mixing valve and the heat exchanger and is configured to circulate water contained within the water piping, andthe outflow piping is connected to the circulation piping by a T-junction located at a location between the heat exchanger and the hot water inlet of the thermostatic mixing valve;whereby the thermostatic mixing valve is configured to allow a variable quantity of water, including no water, to flow from the water tank to the heat exchanger, and to allow a variable quantity of water within the circulation piping to flow to the heat exchanger.

2. The heat pump water heater of claim 1 wherein the thermostatic mixing valve is adjustable to accommodate a desired temperature of the water contained within the circulation piping.

3. The heat pump water heater of claim 1 wherein the thermostatic mixing valve has a closed state and an opened state, whereby when the thermostatic mixing valve is in the closed state no water enters the circulation piping through the cold water inlet of the thermostatic mixing valve, and when the thermostatic mixing valve is in the opened state water enters the circulation piping through the cold water inlet of the thermostatic mixing valve.

4. The heat pump water heater of claim 3 wherein the heat pump water heater has a priming cycle, whereby during the priming cycle the thermostatic mixing valve is in the closed state, the heat pump is activated, and the circulating pump is activated, such that the heat pump causes heat energy to be transferred to the refrigerant in the refrigerant piping, and the circulating pump causes water located within the circulation piping to circulate within the circulation piping and to flow through the heat exchanger to absorb heat energy from the refrigerant.

5. The heat pump water heater of claim 3 wherein the heat pump water heater has an operating cycle, whereby during the operating cycle the thermostatic mixing valve is in the opened state, the heat pump is activated, and the circulating pump is activated, such that water flows from the water tank through the inflow piping into the circulation piping, the heat pump causes heat energy to be transferred to the refrigerant in the refrigerant piping, the circulating pump causes water located within the circulation piping to circulate within the circulation piping and to flow through the heat exchanger to absorb heat energy from the refrigerant, and water flows into the water tank through the outflow piping.

6. The heat pump water heater of claim 1 wherein the heat pump water heater further comprises a temperature probe located within the water tank proximate to a bottom of the water tank.

7. The heat pump water heater of claim 6 wherein the thermostatic mixing valve has an opened state, whereby when the thermostatic mixing valve is in the opened state water enters the circulation piping through the cold water inlet of the thermostatic mixing valve;the heat pump water heater has an operating cycle, whereby during the operating cycle the thermostatic mixing valve is in the opened state, the heat pump is activated, and the circulating pump is activated, such that water flows from the water tank through the inflow piping into the circulation piping, the heat pump causes heat energy to be transferred to the refrigerant in the refrigerant piping, the circulating pump causes water located within the circulation piping to circulate within the circulation piping and to flow through the heat exchanger to absorb heat energy from the refrigerant, and water flows into the water tank through the outflow piping; andthe temperature probe determines the temperature of the water located in the water tank at the bottom of the water tank, and activates the operating cycle of the heat pump water heater when said temperature of said water is below a desired temperature, and deactivates the operating cycle of the heat pump water heater when said temperature of said water reaches the desired temperature.