ADSORPTION REFRIGERATION CYCLE.
Adsorption involves the distribution of molecules between two phases, one of which is a solid and the other is either a liquid or a gas. Adsorption is a well-known and applied technology in terms of water treatment, purification of liquids and gas cleaning processes. However, it was only in 1990s that researchers started to investigate the potential use of adsorption as refrigeration cycle.
The working principle is based on two main phases: desorption and adsorption. Initially, the system is at low pressure and temperature, the adsorbent contained in the adsorber is saturated of refrigerant and it is not able to further adsorb. In order to regenerate it, a desorption phase is initiated: the adsorbent is heated up (process a→b) by using the heat production by the gas boiler or similar, driving the refrigerant out of the adsorbent and raising the system pressure. The desorbed refrigerant condenses as a liquid in the condenser, producing useful heat due to the released heat of condensation at temperature about 60 °C (e.g. domestic hot water, space heating). Subsequent phase is the adsorption one. In this phase, the adsorber is cooled back to ambient temperature and connected to the evaporator, this causes the refrigerant adsorption. Depending on the working mode, during adsorption phase two different useful outputs can be obtained: during heating season, useful heat is produced, since adsorption is a highly exothermic process, during cooling season, useful cooling energy is produced thanks to endothermic evaporation process on the evaporator side.
Adsorption cooling advantages over absorption in the following:
– The simplicity of the equipment used, since there is no requirement for the use of a rectifier
– Allows the use of heat sources with very low temperatures, while absorption systems require heat sources with at least 70 °C
– Compared to the conventional vapour compression, adsorption systems enable the utilization of waste heat or solar energy,
have lower operation costs, have no moving parts and no vibrations.
Main drawback of adsorption technology is the limited adsorption capacity of the adsorbents resulting in low COP and low specific cooling power (SCP).