Air Source Heat Pump Water Heaters
The basic operation of a heat pump water heater (HPWH) can be readily understood by examining it as a black box, without regard to the inner workings. Using this simplified approach, Figure 1 illustrates the three energy flows involved with a typical HPWH. The HPWH consumes electric energy and it removes heat from the heat source, producing a cooling effect. The energy gained is then delivered by the HPWH as heating output. For an air-source HPWH, the heat source is usually warm, humid interior air. Water-source heat pumps usually rely on a chilled water loop or a cooling tower loop as a heat source.
HPWHs use a small amount of electricity to upgrade the temperature of a large amount of heat and deliver it to meet a thermal load. The water heating efficiency of a heat pump water heater is always greater than 100%, and usually substantially greater. In addition to the water heating output, HPWHs often provide a useful cooling and dehumidification effect with no additional energy input.
The electric energy input results in two useful effects: cooling and heating. The heating output (electrical input + heat removed from the heat source) is applied toward a water heating load. The cooling output is often used to cool and dehumidify the interior of a building. Since HPWHs have efficiencies greater than 100%, water heating efficiency for a HPWH is described by the coefficient of performance or COP, instead of using the term "efficiency." The water heating COP is the ratio of the useful water heating output to the electric energy input.
Under typical conditions, an air-source HPWH delivers about 10,000 BTUH of water heating for every kilowatt of electric power it uses. It typically achieves a maximum temperature of about 130-150°F depending on the refrigerant used. While heating water, the HPWH also provides a cooling effect of about 6700 BTUH per kilowatt. Like a conventional air conditioner, typically about 75% of the cooling output is sensible cooling and 25% is latent cooling or dehumidification at standard rating conditions.
The fundamental principles of operation for a HPWH are the same as those of a room air conditioner, a refrigerator, or an air-to-air heat pump. The basic functional components of a heat pump water heater are the evaporator, compressor, condenser, and expansion device, as shown in Figure 2.
Heat is transferred by the flow of refrigerant (currently HCFC-22 or HFC-134a), taking advantage of the large amount of heat absorbed and released when the refrigerant evaporates and condenses. The flow of refrigerant is caused by the pressure differential created by the compressor. The compressor and condenser operate at higher pressure; that portion of the refrigeration system is called the high side. The portion containing the evaporator and the expansion device is called the low side. The compressor pulls refrigerant from the evaporator on the low side and discharges it to the condenser on the high side, much like a pump lifting water uphill. The expansion device resists the flow of refrigerant back to the low side, maintaining the pressure differential.
The Refrigeration Cycle
The refrigeration cycle is best understood by following a portion of the refrigerant around the cycle. The processes that occur in the major components are described in the following paragraphs:
Drawn by the compressor, refrigerant gas (vapor) leaves the evaporator at low pressure and low temperature and flows through the suction line to the compressor. As the compressor compresses the vapor to a higher pressure, its temperature rises (in the same manner as a bicycle pump becomes warm when pumping up a tire).
Refrigerant leaves the compressor as a high-temperature gas at high pressure.
The compressor pushes hot, high-pressure refrigerant through the discharge line to the condenser. The condenser is simply a heat exchanger that removes heat from the hot gas and releases it to a heat sink (for HPWHs, the water being heated). The removal of heat from the hot gas causes it to condense to a liquid.
Refrigerant leaves the condenser as an intermediate-temperature liquid at high pressure.
Pressure Drop & Expansion
Liquid refrigerant flows from the condenser through the liquid line to the expansion device. By acting as a flow restrictor, the expansion device maintains high pressure on the condenser side and low pressure on the evaporator side. In larger commercial heat pump water heaters, the expansion device is an expansion valve. In smaller systems, it may be a capillary tube.
As the liquid moves through the expansion device, its pressure is suddenly lowered. The pressure drop causes some of the liquid refrigerant to flash (evaporate very quickly) into vapor. The evaporation of a portion of the liquid cools the remaining liquid, in the same way evaporation cools your skin when you step out of the shower.
Refrigerant leaves the expansion device as a low-temperature mixture of gas and liquid at low pressure
The cold, low-pressure mixture of liquid and gas refrigerant then flows to the evaporator. The evaporator is another heat exchanger that allows heat to move from a heat source (the air inside a building for most air-source HPWHs) to the refrigerant. As the liquid refrigerant evaporates to a gas, the evaporator removes heat from the heat source. The evaporator in an air-source HPWH provides a cooling and dehumidification effect for the building interior as the evaporator removes heat from the air. Dehumidification takes place only when the evaporator surface temperature is below the air's dewpoint temperature, allowing moisture to condense.
Refrigerant leaves the evaporator as a low-temperature gas at low pressure, completing the cycle. The cycle is continuous while the machine is in operation, with refrigerant continuously moving through each part of the system.
Most vapor compression refrigeration devices are dedicated to achieving only a single effect. Refrigerators and air conditioners remove heat from food storage compartments and building interiors. In winter, space conditioning heat pumps deliver heat to the interior of a building. The energy flow on the other side of the cycle is incidental. Heat pump water heaters achieve higher efficiency by accomplishing two useful functions simultaneously. They cool the building interior while heating water. See Figure 3. Unlike conventional devices, there is no "waste" of output.
Coefficient of Performance: The Multiplier Effect
Coefficient of Performance (COP) is simply a measure of efficiency or the amount of useful output achieved for a given input. For example, an air-to-air heat pump might operate at a COP of three under favorable heating season conditions. This means that it delivers three units of heat to the building interior for each unit of energy consumed as electric energy.
HPWHs and other refrigeration devices are able to move more energy than they consume by taking advantage of the large amount of heat absorbed and released when the refrigerant evaporates and condenses. Air conditioners, refrigerators, freezers, and air-to-air heat pumps all operate similarly, with COPs normally about 1.7 to 3.2. However, they obtain a useful benefit only on one end of the heat transfer process.
In the example of Figure 4, the refrigeration device provides two units of cooling and consumes one unit of electric energy. The COP for the cooling process is 2.0. If the device is used for heating, the COP is 3.0.
Figure 5 illustrates the heat and energy flows typical of a commercial heat pump water heater. Notice how the HPWH uses the electric energy input to create two useful energy flows. It achieves the same cooling effect as in the previous example while also making use of the heating effect. An energy "investment" in one unit of electric energy yields energy "dividends" of two units of cooling and three units of heating.
Using more specific figures, a typical HPWH operating at normal conditions delivers about 10,000 BTUH of water heating for every kilowatt of electric power input (the equivalent of 3413 BTUH). About 15 gallons of water can be heated per hour through a temperature change of 80°F. The coefficient of performance for water heating is 10,000 / 3413 or about 2.9. In addition, about 6600 BTUH of cooling and dehumidification capacity is delivered for the same one-kilowatt input. The total water heating output is approximately equal to the sum of the electric power input and the cooling capacity.
Rule of thumb: For each kilowatt of electric power input, a typical HPWH operating at normal conditions delivers about 10,000 BTUH of water heating and 6600 BTUH of cooling and dehumidification.
These rule-of-thumb values agree closely with the actual specifications for most specific HPWHs applied in typical conditions. As an example, one commercially available HPWH model uses five thousand Watts (17,100 BTUH) of electric power to deliver 50,000 BTUH of water heating and 31,500 BTUH of cooling. A total of 81,500 BTUH of useful heat flow is provided.
Heat pumps are not a universal solution to water heating and space cooling energy cost reduction. In the right application, they perform exceptionally well; in the wrong application, the results may be disappointing. Potential applications should be evaluated to select sites that offer the best performance and high run time to achieve good return on investment. Poor or marginal applications will only lead to unfulfilled expectations and dissatisfied customers. After a good application has been selected, the appropriate HPWH system must be selected to address the customer's specific priorities for water heating cost savings and space cooling.
High HPWH run time is the key to successful heat pump water heater applications. High run time is achieved by applying a properly sized HPWH in facilities with significant water heating loads.
- Where there are large hot water consumption.
- Laundries and restaurants are common applications for HPWHs.
- Any facility which uses hot water and has a simultaneous need for additional space cooling and dehumidification.
- Where natural gas, liquid propane gas, oil, or electric resistance heat are expensive.
- Facilities where overheating and high humidity are serious problems.
- Residences with a source of waste heat, including basements, attics, and laundry rooms.
HPWHs can sometimes be justified in applications where the cooling output is not valued if the water heating savings are adequate.
Applications To Avoid
- Where HPWH run time will not be adequate to provide enough operating savings to justify the initial cost of the HPWH.
Although it has tangible value, the cooling output from a HPWH may not cause a savings in utility bills. Air conditioning savings are certainly not possible in buildings where there is no air conditioning. Even where air conditioning is installed, it may be undersized so that the additional cooling capacity provided by the HPWH improves comfort conditions rather than reducing the run time of the conventional air conditioner. Air conditioners are frequently not present or undersized in many HPWH applications, including commercial kitchens and laundries.
The relative efficiencies of heat pump water heaters and conventional systems are shown in Figure 1.