(written by: Willy Yanto Wijaya)
What is heat pump?
A device that transfers heat from one location (lower temperature) to another location (higher temperature) by using input work.
Since heat naturally flows from higher temperature to lower temperature, input work is required to have a reverse flow of heat.
Description of heat pump
Heat pump generally consists of a condenser, evaporator, compressor, expansion valve, and refrigerant.
A condenser is a part of heat pump where condensation (vapour -> liquid) occurs, and therefore heat is released (located inside the room to be heated). Evaporator is located outside, where evaporation (liquid -> vapour) process is expected to extract the heat from the cold environment. Compressor and expansion valve are parts of a heat pump to adjust the pressure and temperature of the circulating fluid (refrigerant). In fact, it is by exploiting the properties of the refrigerant that we can enable the process of heat transfer from colder environment to the warmer environment.
Types of heat pumps
Basically heat pump can be classified into 2 types:
- Compression Heat Pump
- Absorption Heat Pump
Compression heat pump relies on mechanical work (electricity) to power the compressor to adjust the refrigerant properties. Meanwhile, in absorption heat pump, a compressor is replaced by a set of more complex absorption system. The energy input for the absorption heat pump is usually in the form of heat.
Absorption heat pump is much more complex and expensive compared to compression heat pump, however if there are abundant sources of cheap thermal energy, absorption heat pump can be an attractive option despite its initial high cost investment.
Development of heat pump technologies
Technologies for the compression heat pumps have been commercially established, with a wide range of applications, in particular for the purpose of residential heating. After the discovery of CFC (Freon) in 1930 by Thomas Midgley, this refrigerant soon became a popular choice for applications in heat pumps and refrigerators due to its favourable thermodynamic properties. However, since its adverse effect on the ozone layer was discovered in the 1980s, it has been mostly replaced with natural refrigerants such as ammonia, carbon dioxide, or non-halogenated hydrocarbons.
In fact, in a heat pump system, many aspects are kept being researched for improvements. This includes researches upon the heat exchange and transfer phenomena, properties of refrigerants, compression and expansion processes, phase changes, and flow dynamics. For the case of absorption heat pump, a wider range of issues is even needed to be addressed. For example, if the source of the heat input energy is geothermal energy, understanding of the characteristics of this energy source is certainly required.
Besides these two most common types of heat pump, some new types of refrigeration systems have also been developed, such as: magnetic refrigeration, thermoelectric, and thermo acoustic refrigeration systems.
Fig. Typical Schematic of Absorption Heat Pump
(source: WY Wijaya, S Kawasaki, H Watanabe, K Okazaki, “Evaluation of combined absorption heat pump – methanol steam reforming system: Feasibility criterion as a measure of system performance”, Energy Convers Manage. 52 (4), 1974-1982 (2011))