A low temperature waste heat flow can be upgraded to useful high temperature heat with the use of a heat pump. Among the different types of heat pumps that have been developed, the mechanical heat pump is the most widely used. Its operating principle is based on compression and expansion of a working fluid, or so called 'refrigerant'. A heat pump has four main components: evaporator, compressor, condenser and expansion device. The refrigerant is the working fluid that passes through all these components. In the evaporator heat is extracted from a waste heat source. In the condenser this heat is delivered to the consumer at a higher temperature level. Electric energy is required to drive the compressor and this energy is added to the heat that is available in the condenser. The efficiency of the heat pump is denoted by its COP (coefficient of performance), defined as the ratio of total heat delivered by the heat pump to the amount of electricity needed to drive the heat pump.
The thermodynamic cycle
The operating principle of a heat pump is based on the physical property that the boiling point of a fluid increases with pressure. By lowering pressure, a medium can be evaporated at low temperatures while an increase of pressure will lead to a high boiling point. The graph at the left side shows this principle. The black line shows the relation between pressure and boiling point of, in this case, Ammonia. At low pressure and temperature Ammonia is evaporated in the evaporator. The energy needed for this is provided by a waste-heat flow. The compressor increases the pressure of the Ammonia vapour. The vapour is then condensed at high pressure and temperature inside the condenser. During the condensation of Ammonia, heat is released: a useful source of energy. The liquid Ammonia is transported to the expansion device that lowers pressure. The low temperature, low pressure Ammonia flows to the evaporator: the starting point of yet another cycle. Read more
The figure above shows the heat pump cycle for the refrigerant Ammonia. For large scale industrial applications, Ammonia is the most suitable refrigerant for heat pumps that deliver heat up to a temperature of 90 °C. Ammonia is considered to be one of the most efficient refrigerants. However its use comes with certain safety measures. Therefore Ammonia is mainly used for large industrial installations. The choice of refrigerant for a certain application is determined by the temperature range of its thermodynamic cycle and the size of the installation needed. Refrigerants can be divided into two groups: natural refrigerants (Butane, Ammonia, CO2) and synthetic refrigerants (R134A, R407C, R410A). For commercial applications synthetic refrigerants are favoured over natural ones. A disadvantage of synthetic refrigerants is their strong contribution to the greenhouse effect in case of leakage. The negative impact of synthetic refrigerants is, for example, 1300-2100 times higher compared to CO2. Read more
Different types of heat pumps
The mechanical heat pump is the heat pump that is most conventionally applied in the industry. However, several other types of heat pumps are available.
Mechanical heat pump: The mechanical heat pump is the most commonly used and commercially feasible heat pump. It's principle of operation: The pressure of a refrigerant is increased with a compressor causing a rise in boiling temperature. There are two varieties of mechanical heat pumps: a system with direct expansion (DX system) and a system in which a vessel is used to separate gaseous and fluid refrigerant. Read more
Gas engine heat pump: The gas engine heat pump consists of a mechanical heat pump and a gas engine. Different to a conventional heat pump, where the compressor is driven with an electric motor, the compressor of a gas engine heat pump is driven by a gas engine. The heat from the motor cooling and flue gases of the gas engine can be utilized. Therefore the heat pump capacity can be lower.
Absorption heat pump: The principle of operation of an absorption heat pump is based on evaporation of a refrigerant and its absorption into an absorbing medium. Well known combinations of refrigerant and absorbing medium are Lithium-Bromide and Water and Ammonia and Water. Driving force in this type of heat pump is thermal energy. Absorption heat pumps can be very useful when both heating and cooling are necessary. Read more
Adsorption heat pump: Although based on the same principles as the absorption heat pump, the adsorption heat pump uses a solid instead of a fluid as absorption medium. Read more
Transcritical CO2 heat pump: Above 31°C CO2 falls in the trans-critical range; meaning that no distinction can be made between fluid or gaseous phase. Due to this phenomenon, heat can be released at a range of temperatures instead of a fixed temperature. Read more
Hybrid heat pump: A hybrid heat pump is the combination of a mechanical and an absorption heat pump. The principle of operation is based on the fact that absorption of Ammonia in Water takes place at much higher temperatures as compared to condensation of Ammonia at constant pressure. Read more
Thermoacoustic heat pump: