Heat pumps have enormous potential for saving energy, particularly in industrial processes. They are the only heat recovery systems which enable the temperature of waste heat to be raised to more useful levels. Although the principle of the heat pump has been known since the middle of the nineteenth century, there was little incentive to develop them in a time of cheap and abundant energy.
Recent research and development has indicated that heat pump performance is likely to improve over the coming years. Improvements in component design and in use of waste heat sources will raise heat pump performance. With regard to technical aspects, the many years of experience that have brought about important findings for the planning and design of heat pump systems can be used. Moreover, new ideas and equipment appearing in the last decade have simplified the construction of the heat pump heating and cooling systems.
Heat pumps look and operate very much like air conditioners (only for forced-air systems) with the notable exception that they provide both heating and cooling. While heat pumps and air conditioners require the use of some different components, they both operate on the same basic principles.
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.
Almost all heat pumps currently in operation are based either on a vapor compression, or on an absorption cycle. Theoretically, heat pumping can be achieved by many more thermodynamic cycles and processes, including Stirling and Vuilleumier cycles, singlephase cycles (e.g. with air, CO2 or noble gases), solid-vapor sorption systems, hybrid systems (notably combining the vapor compression and absorption cycle), thermoelectric cycle and electromagnetic and acoustic processes. Some of these are entering the market or have reached technical maturity, and are expected to become significant in the future.
A heat pump is essentially a heat engine operating in reverse and can be defined as a device that moves heat from a region of low temperature to a region of higher temperature. The residential air-to-air heat pump, the type most commonly in use, extracts heat from low temperature outside air and delivers this heat indoors. To accomplish this and in accordance with the second law of thermodynamics work is done on the working fluid (i.e. a refrigerant) of the heat pump.
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. Electricallydriven 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.
For large-scale applications, heat pumps using a combustion furnace for supplemental heat and/or temperature peaking have become popular due to:
• their applicability to the retrofit market as add-on units to existing oil or gas furnaces and boilers, and
• the improved performance of the combined system compared with electric-resistance heat-supplemented heat pumps.
In this regard, heat pumps operating with supplementary heat are often said to be operating in a bivalent mode. A heat pump operating with electric resistance heating or without other back-up is said to be operating in a monovalent mode. With the exception of certain control components designed to regulate compressor and furnace operation, essentially standard heat pump components are used. The system is operated in the heat pump mode down to a predetermined temperature called the balance point and the furnace is switched on when supplementary heat is required or, in the case of air distribution systems, during heat pump defrosting. Some systems switch the compressor off completely below the balance point while others allow parallel heat pump and furnace operation down to –10°C for an air source heat pump. The heat pump technology is of special interest in colder climates where the traditional means of heating existing buildings is gas or oil and a requirement for some air-conditioning as an add-on arises. The systems can also be used for heating alone in conjunction with conventional furnaces. Even in the coldest climates there are a sufficient number of heating days above the balance point of an existing heat pump to make this combination worthy of consideration.