Heat flows naturally from a higher to a lower temperature. Heat pumps, however, are able to force the heat flow in the other direction, using a relatively small amount of high quality drive energy (electricity, fuel, or high-temperature waste heat). Thus heat pumps can transfer heat from natural heat sources in the surroundings, such as the air, ground or water, or from man-made heat sources such as industrial or domestic waste, to a building or an industrial application. Heat pumps can also be used for cooling. Heat is then transferred in the opposite direction, from the application that is cooled, to surroundings at a higher temperature. Sometimes the excess heat from cooling is used to meet a simultaneous heat demand.
In order to transport heat from a heat source to a heat sink, external energy is needed to drive the heat pump. Theoretically, the total heat delivered by the heat pump is equal to the heat extracted from the heat source, plus the amount of drive energy supplied. Electrically-driven heat pumps for heating buildings typically supply 100 kWh of heat with just 20-40 kWh of electricity. Many industrial heat pumps can achieve even higher performance, and supply the same amount of heat with only 3-10 kWh of electricity.
Because heat pumps consume less primary energy than conventional heating systems, they are an important technology for reducing gas emissions that harm the environment, such as carbon dioxide (CO2), sulphur dioxide (SO2) and nitrogen oxides (NOx). However, the overall environmental impact of electric heat pumps depends very much on how the electricity is produced. Heat pumps driven by electricity from, for instance, hydropower or renewable energy reduce emissions more significantly than if the electricity is generated by coal, oil or gas-fired power plants.
The two main heat pump types
Almost all heat pumps currently in operation are either based on a vapour compression, or on an absorption cycle. These two principles will be briefly discussed in the following two sections.
Theoretically, heat pumping can be achieved by many more thermodynamic cycles and processes. These include Stirling and Vuilleumier cycles, single-phase cycles (e.g. with air, CO2 or noble gases), solid-vapour sorption systems, hybrid systems (notably combining the vapour compression and absorption cycle) and electromagnetic and acoustic processes. Some of these are entering the market or have reached technical maturity, and could become significant in the future.
The great majority of heat pumps work onthe principle of the vapour compression cycle. The main components in such a heat pump system are the compressor, the expansion valve and two heat exchangers referred to as evaporator and condenser. The components are connected to form a closed circuit, as shown in Figure 1. A volatile liquid, known as the working fluid or refrigerant, circulates through the four components.
||Figure 1: Closed cycle, electric motor-driven vapour compression heat pump|
||Figure 2: Closed cycle, engine-driven vapour compression heat pump.|
||Figure 3: Absorption heat pump |
In the evaporator the temperature of the liquid working fluid is kept lower than the temperature of the heat source, causing heat to flow from the heat source to the liquid, and the working fluid evaporates. Vapour from the evaporator is compressed to a higher pressure and temperature. The hot vapour then enters the condenser, where it condenses and gives off useful heat. Finally, the high-pressure working fluid is expanded to the evaporator pressure and temperature in the expansion valve. The working fluid is returned to its original state and once again enters the evaporator. The compressor is usually driven by an electric motor and sometimes by a combustion engine.
- An electric motor drives the compressor (see Fig ure 1) with very low energy losses. The overall energy efficiency of the heat pump strongly depends on the efficiency by which the electricity is generated. This is discussed in the section on Heat pump performance.
- When the compressor is driven by a gas or diesel engine (see Figure 2), heat from the cooling water and exhaust gas is used in addition to the condenser heat.
- Industrial vapour compression type heat pumps often use the process fluid itself as working fluid in an open cycle. These heat pumps are generally referred to as mechanical vapour recompressors, or MVRs; refer to the section on Heat pumps in industry.
Absorption heat pumps are thermally driven, which means that heat rather than mechanical energy is supplied to drive the cycle. Absorption heat pumps for space conditioning are often gas-fired, while industrial installations are usually driven by high-pressure steam or waste heat.
Absorption systems utilise the ability of liquids or salts to absorb the vapour of the working fluid. The most common working pairs for absorption systems are:
- water (working fluid) and lithium bromide (absorbent); and
- ammonia (working fluid) and water (absorbent).
In absorption systems, compression of the working fluid is achieved thermally in a solution circuit which consists of an absorber, a solution pump, a generator and an expansion valve as shown in Figure 3. Low-pressure vapour from the evaporator is absorbed in the absorbent. This process generates heat. The solution is pumped to high pressure and then enters the generator, where the working fluid is boiled off with an external heat supply at a high temperature. The working fluid (vapour) is condensed in the condenser while the absorbent is returned to the absorber via the expansion valve.
Heat is extracted from the heat source in the evaporator. Useful heat is given off at medium temperature in the condenser and in the absorber. In the generator high-temperature heat is supplied to run the process. A small amount of electricity may be needed to operate the solution pump. For heat transformers, which through the same absorption processes can upgrade waste heat without requiring an external heat source, refer to the section on Heat pumps in industry.