Patent Grant
8438864
The present subject matter relates generally to water heaters, and more particularly to a heat pump water heater.
Heat pump water heaters are gaining broader acceptance as a more economic and ecologically-friendly alternative to electric water heaters. These systems utilize a condenser configured in a heat exchange relationship with the water storage tank, for example wrapped around the tank in a series of coils. During operation of the vapor compression heat pump cycle, a refrigerant exits an evaporator as a superheated vapor and/or high quality vapor mixture. Upon exiting the evaporator, the refrigerant enters a compressor where the pressure and temperature increase and the refrigerant becomes a superheated vapor. The superheated vapor from the compressor enters the condenser, wherein the superheated vapor transfers energy to the water within a storage tank and returns to a saturated liquid and/or high quality liquid vapor mixture. Conventional refrigerants are able to reject heat to the water in the storage tank via condensation in the condenser.
Carbon dioxide (CO2) has emerged as a natural, ecologically friendly replacement for CFC and HCFC refrigerants. CO2, however, has a low critical point and thus operates on a transcritical cycle wherein it evaporates in the subcritical region and rejects (transfers) heat at temperatures above the critical point in a gas cooler instead of a condenser. U.S. Pat. No. 7,210,303 describes a transcritical heat pump water heater system.
An impediment to wide scale acceptance of transcritical heat pump systems, including water heater systems, is the perceived lower efficiency of the transcritical CO2 vapor compression cycle as compared to the CFC and HCFC systems. In this regard, improvements are constantly being pursued to increase the thermodynamic efficiency and coefficient of performance (COP) of the transcritical systems. The present invention relates to such improvements.
Aspects and advantages of the invention will be set forth in part in the following description, or may be obvious from the description, or may be learned through practice of the invention.
In a particular embodiment of the invention, a method is provided for increasing efficiency of a transcritical heat pump water heater system. The method involves compressing a refrigerant, such as CO2, to a supercritical point and passing the refrigerant through a gas cooler that is wrapped at least partially around a water storage tank, wherein the refrigerant transfers heat to water stored in the tank. The hot water is discharged from the storage tank from a location proximate to a top of the tank, and cold water is introduced into the storage tank at a location proximate to a bottom of the tank. The refrigerant is directed to flow through the gas cooler from a top point to a lowermost point in a flow direction such that the refrigerant exits the gas cooler proximate to the bottom of the tank at a location of the coldest water within the storage tank. This method utilizes the temperature gradient of the water in the tank to maximize the temperature loss of the refrigerant as it circulates through the gas cooler, thereby increasing the thermodynamic efficiency of the heat pump cycle.
In a particular embodiment, the gas cooler is configured as a coiled heat exchanger wrapped in a series of coils around at least a portion of the storage tank. It is desirable in particular embodiments to maintain a stratified layer of relatively cold water proximate to the bottom of the tank where the refrigerant exit is located for enhanced cooling of the refrigerant. One means for achieving this is to concentrate the coils of the gas cooler closer to the top portion of the tank (above a mid level point). For example, the top portion may have a greater coil density as compared to the bottom portion of the tank, with the goal being to have high heat transfer above the low coil density portion to allow cooling of the coils at the bottom of the tank. In this configuration, the gas cooler transfers most of its heat to the water in the top portion of the tank and a well defined layer of relatively cold water is established in the bottom portion of the tank. In a further embodiment, it may be desired to define a section of the tank below the mid level point that is essentially void of coils of the gas cooler. A high density section of coils of may be provided below the void section of the tank proximate to the refrigerant exit so that the refrigerant is cooled even further prior to exiting the gas cooler.
The thermodynamically efficient method embodiments may result in hot water being generated at a temperature that is above a desired level. It this regard, it may be desired to mix cold water (e.g. with a controllable mixing valve) with the hot water discharged from the storage tank to reduce the temperature of the downstream hot water.
To take even further advantage of the incoming cold water, in a particular embodiment, the cold water is introduced into the storage tank at a cold water inlet that is proximate to the bottom of the tank. In other words, the cold water is not preheated by the hot water in the storage tank (as with a conventional dip tube design in an electric water heater system).
The present invention also encompasses any manner of a transcritical heat pump water heater system having the gas cooler and storage tank configuration as discussed above.
These and other features, aspects and advantages of the present invention will become better understood with reference to the following description and appended claims. The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention.
A full and enabling disclosure of the present invention, including the best mode thereof, directed to one of ordinary skill in the art, is set forth in the specification, which makes reference to the appended figures, in which:
Reference now will be made in detail to embodiments of the invention, one or more examples of which are illustrated in the drawings. Each example is provided by way of explanation of the invention, not limitation of the invention. In fact, it will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the scope or spirit of the invention. For instance, features illustrated or described as part of one embodiment can be used with another embodiment to yield a still further embodiment. Thus, it is intended that the present invention covers such modifications and variations as come within the scope of the appended claims and their equivalents.
The water storage tank 102 in the system 100 of
The system 100 may also include supplemental electric heating elements 128 placed near the top and bottom of the water storage tank 102 to heat the water. In general, the heating elements 128 are activated in situations wherein the demand for hot water placed on the system 100 exceeds the heating capability of the heat pump system.
The heated water exits the tank 102 at a hot water exit 124 and flows to the consumer's residential plumbing, or other location where the system 100 is installed. The system 100 may include a temperature sensor 125 positioned to sense the temperature of the water in the upper region of the tank and may also have additional temperature sensors placed at various locations for sensing other temperatures, such as heat pump condenser inlet and outlet temperatures, ambient temperature, etc.
The system 100 may also include a controller 126, equipped with a microprocessor, that determines which of the compressor 130 and/or electric resistance heating elements 128 shall be energized, and for how long, in order to heat the water within the water storage tank 102 to a setpoint temperature. The controller 1126 may receive any manner of temperature readings (e.g., from sensor 125), flow signals, setpoint, and so forth, to implement its control functions.
The gas cooler 122 is configured in a heat exchange relationship with at least a portion of the tank 102, depending on the particular configuration of the gas cooler 122. For example, the gas cooler 122 may be a planar or plate-like heat exchanger that is wrapped at least partially around the tank 102. In the illustrated embodiment, the gas cooler 122 is a coiled loop heat exchanger having a plurality of tube coils 114 wrapped around at least a portion of the tank 102. These coils may be disposed between the shell 130 and the tank 102, as depicted in the figures.
As a aspect of the invention, it has been determined that the thermodynamic efficiency of the transcritical HPWH system 100 is quite sensitive to the temperature of the CO2 refrigerant that exits the gas cooler 112, and that further reduction of the temperature of the refrigerant prior to the expansion device 110 and evaporator 104 can lead to a meaningful increase in the overall efficiency of the system. In this regard, the system 100 is uniquely configured so that the specific heat of the refrigerant, at an appropriate operating pressure and tank temperature, coincides with the location of the upper coils. The refrigerant is directed to flow through the gas cooler 112 from a top point at a refrigerant inlet 116 to a lowermost point at a refrigerant exit 118 in a flow direction such that the refrigerant exits the gas cooler 112 proximate to the tank bottom 132, which generally corresponds to the location of the coldest water within the storage tank 102. This configuration utilizes the temperature gradient of the water in the tank 102 to maximize the temperature loss of the refrigerant as it circulates through the gas cooler 112.
The coils 114 of the gas cooler 114 are wrapped around at least a portion of the storage tank 102 to transfer heat to the water while utilizing the temperature gradient of the water in the tank to maximize the temperature drop (glide) of the refrigerant. Thus, it may be desirable in particular embodiments to maintain a stratified layer of relatively cold water at the tank bottom 132 proximate to the refrigerant exit 118. Referring to
To take even further advantage of the incoming cold water, in particular embodiments as depicted in
Referring to
It is possible that, in certain embodiments, the system 100 may generate hot water that exceeds a desired temperature. In this situation, it may be desired to mix cold water with a controllable mixing valve 138 (
This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they include structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.
| Number | Name | Date | Kind |
|---|---|---|---|
| 2575325 | Ambrose et al. | Nov 1951 | A |
| 3213850 | Heeger | Oct 1965 | A |
| 4257355 | Cook | Mar 1981 | A |
| 4556104 | Engelhardt | Dec 1985 | A |
| 4658803 | Ball et al. | Apr 1987 | A |
| 5946927 | Dieckmann et al. | Sep 1999 | A |
| 7210303 | Zhang et al. | May 2007 | B2 |
| 7310960 | Sienel et al. | Dec 2007 | B2 |
| 20060191276 | Sienel et al. | Aug 2006 | A1 |
| 20100209084 | Nelson et al. | Aug 2010 | A1 |
| Number | Date | Country | |
|---|---|---|---|
| 20120060521 A1 | Mar 2012 | US |
