Heat Pumps

From Thermal-FluidsPedia

As explained in the second law, the thermal energy of any substance vanishes only at absolute zero temperature (-273°C). This means even cool outdoor winter air has some thermal energy. The heat pump is a device that utilizes this thermal energy by bringing it into the house and heating its interior. Of course, by removing this heat, we make the outside temperature even cooler! In order to move this energy inside, a heat pump circulates a fluid called refrigerant which absorbs heat from outdoor air and releases it inside. Heat pumps can be used for cooling as well. This process is the reverse of the heating process; it removes the heat from the space to be cooled and dumps it into the already warm outdoor environment.

The operation of a heat pump as a heating device is shown in Figure 1a. The refrigerant flows inside a closed loop. At point 1, the refrigerant vapor enters a compressor where it is compressed to a temperature warmer than the indoor air. The compressor needs some energy, usually in the form of electricity, in order to function. At point 2, the heated refrigerant enters an indoor heat exchanger where it gives off its thermal energy and, with the aid of a fan, uniformly heats the room. In the process, some of the vapor is condensed back to liquid (point 3) inside the heat exchanger (thus the name condenser). The liquid is then expanded through a valve or capillary tube at point 4, where it rapidly expands and cools. The mixture then enters a heat exchanger, where the heat from outside air causes it to vaporize (thus the name evaporator). An outside fan facilitates this heat transfer. The refrigerant vapor leaves the evaporator at point 1 and the cycle repeats.

Figure 1: Heat Pumps a) heating cycle; b) cooling cycle

During summer, when cooling instead of heating is desired, the direction of flow is reversed (See Figure 1b) and the heat exchangers reverse functions. In this mode, the outside heat exchanger is a condenser where heat is extracted, whereas the inside heat exchanger acts as an evaporator, removing heat from the space where cooling is desired.

The performance of heat pumps is defined in terms of the amount of heating or cooling that is achieved per amount of work energy (electricity) that is needed to run the compressor. This is called the coefficient of performance (COP). The COP for practical heat pumps is around 4-5, meaning heat pumps can provide four to five times more heating or cooling than electricity consumed. In comparison, electric heaters have efficiencies close to 100%, oil and gas heaters have efficiencies of 50% to 90%, and wood stoves’ heating efficiencies range from 20% to 60%. The COP decreases markedly once outside temperatures fall below around −5 or −10 °C.

In the United States, it is customary to specify the performance of a heat pump by two numbers, the COP as defined above for heating, and the Energy Efficiency Ratio (EER) for cooling. The EER is expressed in Btu/Wh, and is the ratio of cooling capacity given in Btu/h to required electrical power in watts. ( a ) For large commercial units, chillers are rated in kilowatts per ton. Because each ton of refrigeration is equal to 12,000 Btu, we have