Patent Application
20150159886
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.
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.
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:
Referring to the drawings, a typical prior art heat pump water heater (HPWH) is shown in
Manufacturers have also developed devices with external condensers, as shown in
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
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:
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:
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.
