US20150159886A1

US20150159886A1 - Heat pump water heater and method

Info

Publication number: US20150159886A1
Application number: US14/102,561
Authority: US
Inventors: John Dennis Bush; Ronald Edward Domitrovic; K. R. Amarnath
Original assignee: Electric Power Research Institute Inc
Current assignee: Electric Power Research Institute Inc
Priority date: 2013-12-11
Filing date: 2013-12-11
Publication date: 2015-06-11

Abstract

A heat pump water heater and method is disclosed. The heat pump water heater includes a stratified water tank configured to hold water of different temperatures therein and a heat exchange unit. The heat exchange unit includes a pump configured to remove water from a bottom of the stratified tank, a heat pump having a condenser configured to receive the water from the pump and supply heat to the water flowing through the condenser such that the condenser heats the water to a pre-determined temperature, and a resistance heater configured to receive the heated water from the condenser. The resistance heater supplies additional heat to the water if the pre-determined temperature of the water exiting the condenser does not equal or exceed a pre-determined set-point. The heated water from the resistance heater is supplied to a top of the stratified tank for use by a residence or utility.

Description

BACKGROUND OF THE INVENTION

This application relates to a heat pump water heater and method and, more particularly, to a heat pump water heater adapted to increase efficiency and, thereby control electric load.
In pursuit of energy efficiency, new technologies are sought to provide cost effective energy savings compared to legacy technologies. One such technology is Heat Pump Water Heaters (HPWHs). A residential electric water heater draws approximately 4.5 kW (15 kBTU/hr) when running, and for a family of four, consumes nearly 4.8 MWh (16.4 MMBTU) annually. Water heating results in nearly 11% of the residential carbon dioxide budget and over 11% of the typical utility bill.
In 2015, new federal regulations are expected to require that electric water heaters larger than 55 gallons have energy factors close to 2.0 which will essentially eliminate residential electric resistance water heaters and, thereby push HPWHs to the forefront. The one exception being considered is for electric resistance water heaters that provide “utility services”. The exception is being considered in part due to the inability of heat pumps to provide the same services.
Accordingly, there are problems to address as a result of the pursuit of energy efficiency and the inability of heat pumps to provide certain services. The first is the ability for water heaters to provide grid services and the second is to provide those services in an energy efficient manner. Current electric resistance water heaters are capable of providing grid services, such as providing demand response, frequency regulation, and other services. However, electric resistance elements are highly inefficient compared with heat pumps. Heat pump water heaters as they currently are made can provide less of the same services, or provide those services only when the heat pump is not running, thereby eliminating the efficiency gains. Also, today's heat pumps are not as efficient as they could be.

BRIEF SUMMARY OF THE INVENTION

These and other shortcomings of the prior art are addressed by the present invention, which provides a heat pump water heater adapted to increase efficiency and provide grid services such as Demand Response.
According to one aspect of the invention, a heat pump water heater configured to provide grid related services such as Demand Response includes a stratified water tank configured to hold water of different temperatures therein and a heat exchange unit. The heat exchange unit includes a pump configured to remove water from a bottom of the stratified tank, a heat pump having a condenser configured to receive the water from the pump and supply heat to the water flowing through the condenser such that the condenser heats the water to a pre-determined temperature, and a resistance heater configured to receive the heated water from the condenser. The resistance heater supplies additional heat to the water if the pre-determined temperature of the water exiting the condenser does not equal or exceed a pre-determined set-point. The heated water from the resistance heater is supplied to a top of the stratified tank for use by a residence or utility.
According to another aspect of the invention, a method of providing hot water to a residence or utility includes the steps of providing a heat pump water heater, having a stratified water tank, and a heat exchange unit having a heat pump and an in-line resistance heater. The method further includes the steps of pulling water from a bottom of the stratified water tank and supplying it to the heat pump, using the heat pump to heat the water to a higher temperature, using the in-line resistance heater to provide additional heat to the water exiting the heat pump, and storing the heated water in a top of the stratified water tank for use by a residence or utility.

BRIEF DESCRIPTION OF THE INVENTION

The subject matter that is regarded as the invention may be best understood by reference to the following description taken in conjunction with the accompanying drawing figures in which:

FIG. 1 shows a prior art heat pump water heater;

FIG. 2 shows a prior art heat pump water heater with external condensers; and

FIG. 3 shows a heat pump water heater according to an embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

Referring to the drawings, a typical prior art heat pump water heater (HPWH) is shown in FIG. 1. The key elements of a typical HPWH are (1) a heat pump with a condensing coil that is integral to the tank (typically wrapped around the outside of the tank, occasionally immersed as shown) and (2) one or two electric resistance elements used for backup heat which are immersed in the tank and separate from all heat pump components.
Manufacturers have also developed devices with external condensers, as shown in FIG. 2. An advantage of such a configuration is that the condensing coil always has low-temperature water entering because of stratification, which is good for efficiency. A wrap-around or immersed condenser, shown in FIG. 1, heats the tank in bulk, and efficiency decreases as the average tank temperature increases.
In general, heat pump water heaters use electricity to move heat from one place to another instead of generating heat directly. Therefore, they can be two to three times more energy efficient than conventional electric resistance water heaters. Instead of heating stored water directly with a conventional electric element or with a burner, as in the case of a gas unit, a heat pump water heater transfers available heat from the ambient air, intensifies the heat and transfers the heat into the water, a far more cost and energy-efficient process.
Referring now to FIG. 3, a heat pump water heater in accordance with an embodiment of the invention is illustrated and shown generally at reference numeral 10. The heater 10 includes a stratified water tank 11 configured to hold both cold water coming in from a utility and hot water supplied by a heat exchange unit 12. The heat exchange unit 12 includes a heat pump 13, a resistance heater 14, and a pump 16. The heat pump 13 includes an expansion valve 17, evaporator 18, fan 19, compressor 20, and condenser 21.
As shown, water from a utility is supplied to the tank 11. Pump 16 draws the water from the bottom of the tank 11 so that the water can be heated by the condenser 21. The pump 16 may be a variable speed pump or may consist of a non-variable speed pump with a regulating valve to vary the flow-rate of the water. The control for the flow-rate is based on the outlet water temperature.
In order for condenser 21 to provide heat to the water flowing therethrough, a refrigerant flowing through the heat pump 13 is subjected to a vapor-compression cycle. More particularly, starting with the evaporator 18, a low-pressure refrigerant enters the evaporator 18 where the refrigerant absorbs heat from its surroundings and boils into a gaseous state. The refrigerant is then compressed by the compressor 20 into a hot and highly pressurized vapor. The refrigerant is then cooled by the condenser 21 by transferring heat into the cold water from the tank 11 flowing through the condenser 21. In transferring heat from the refrigerant to the water, the refrigerant becomes a high-pressure, moderate temperature liquid. The refrigerant then flows through the expansion valve 17 where the refrigerant becomes a low-pressure liquid. The cycle then starts again.
The water flowing through the condenser 21 is heated to a set-point or lower. The water exits the condenser 21 and flows into an in-line resistance heater 14. The in-line resistance heater 14 makes sure that the water is heated to the set-point. In other words, if the condenser 21 heats the water to a temperature below the set-point, then the in-line resistance heater 14 provides the additional heat to bring the temperature of the water to the set-point.
In addition to providing additional heat to bring the water up to the set-point, the resistance heater 14 may be used to provide:

- Higher-power operation as a “Demand Response: Load Up” signal response.
- Ancillary services by modulating power to the element via pulse width modulation or other control.
- Potential elevated-temperature storage in advance of “Demand Response: Load Down” events or for load shifting, with the heat pump providing heat as high as is efficient and the resistance element providing additional temperature boost.
- Additional heating capacity as needed, either for fast recovery or to boost capacity due to low ambient temperatures.

The compressor 20 may be of variable or fixed capacity. Depending on whether the compressor 20 is of variable or fixed capacity there will be two or three control points: (1) the water flow rate, (2) the electric element heat capacity (or lack thereof), and (3) the heat pump heat capacity (or lack thereof). These three variables are controlled in concert to provide the desired heating output and power response. The size of the electric resistance element is a design variable and there may be advantages to smaller or larger elements for different types of utility usage.
There are several advantages to a heat pump water heater utilizing an in-line resistance heater, including:

- Incorporating a heat pump into grid-tied devices, rather than electric resistance alone, is far more efficient and will result in lower operating costs.
- Having an electric resistance element incorporated into a heat pump water heater allows fast-response frequency modulation and other benefits which may not be feasible with a heat pump-only system.
- External condenser heat pump water heaters with a high degree of tank stratification have a higher annual efficiency than wrap-around condensers.
- Operating the heat pump and electric resistance element together allows the heat pump to heat to a lower temperature, which is good for heat pump efficiency.
- Tank stratification can allow some cool water to be kept at the bottom of the tank “on reserve” to be heated during “Demand Response” events. This cannot be done with an unstratified tank, where the only option would be to heat above set-point (inefficient for heat pumps, requires additional safety hardware).
- Using wrap-around condensers and tank-immersed electric resistance elements to provide a similar service generally means turning off the heat pump when the elements are active, to keep current down.

The foregoing has described a heat pump water heater and method for controlling electric load. While specific embodiments of the present invention have been described, it will be apparent to those skilled in the art that various modifications thereto can be made without departing from the spirit and scope of the invention. Accordingly, the foregoing description of the preferred embodiment of the invention and the best mode for practicing the invention are provided for the purpose of illustration only and not for the purpose of limitation.

Claims

We claim:

1. A heat pump water heater configured to provide grid related services such as Demand Response, comprising:

(a) a stratified water tank configured to hold water of different temperatures therein; and

(b) a heat exchange unit, comprising:

(i) a pump configured to remove water from a bottom of the stratified tank;

(ii) a heat pump having a condenser configured to receive the water from the pump and supply heat to the water flowing through the condenser such that the condenser heats the water to a pre-determined temperature; and

(iii) a resistance heater configured to receive the heated water from the condenser and supply additional heat to the water if the pre-determined temperature of the water exiting the condenser does not equal or exceed a pre-determined set-point, wherein the heated water from the resistance heater is supplied to a top of the stratified tank for use by a residence or utility.

2. The heat pump water heater according to claim 1, wherein the heat pump further includes:

(a) an evaporator configured to provide heat to a low-pressure, low-temperature refrigerant flowing therethrough;

(b) a compressor configured to receive the refrigerant from the evaporator and compress the refrigerant into a high-temperature, high-pressure refrigerant, wherein the high-temperature, high-pressure refrigerant is supplied to the condenser for heating of the water flowing therethrough; and

(c) an expansion valve configured to receive the refrigerant from the condenser in a high-pressure, moderate temperature state and expand it into a low-pressure, low-temperature state for use in the evaporator.

3. The heat pump water heater according to claim 2, wherein the compressor is of a variable capacity design.

4. The heat pump water heater according to claim 1, wherein the resistance heater is configured to provide elevated temperature storage in advance of Demand Response events or for load shifting.

5. The heat pump water heater according to claim 1, wherein the resistance heater is configured to provide additional heating capacity as needed for fast recovery and to boost capacity due to low ambient temperatures.

6. The heat pump water heater according to claim 1, wherein the resistance heater provides the additional heat to the water so that the heat pump can operate at maximum efficiency.

7. A method of providing hot water to a residence or utility, comprising the steps of:

(a) providing a heat pump water heater, having:

(i) a stratified water tank; and

(ii) a heat exchange unit having a heat pump and an in-line resistance heater;

(b) pulling water from a bottom of the stratified water tank and supplying it to the heat pump;

(c) using the heat pump to heat the water to a higher temperature;

(d) using the in-line resistance heater to provide additional heat to the water exiting the heat pump; and

(e) storing the heated water in a top of the stratified water tank for use by a residence or utility.