The basic functional principle behind cold production is the compression of the refrigerant which will evaporate at low temperatures and pressure while absorbing heat. The refrigerant will condense again at a higher temperature and pressure level and release the heat it previously absorbed again. When in a liquid state, the refrigerant can then be evaporated again.
Apart from absorption chillers using the paired workings materials lithium bromide / water and water / ammonium, highly porous solids (adsorbents), e. g. silica gel or zeolite, are also suitable for thermal cooling. Characteristic of these materials is that they absorb water vapour or other vapours and adhere to the surface of pores. They are, therefore, used, among other things, as drying agents. The refrigerant commonly used is water.
The compression of the refrigerant in adsorption chillers is achieved using heat generated by the cyclic adsorption and desorption of water vapour on the adsorbent.
In order for this process to progress as efficiently as possible, foreign gases must be excluded to the greatest possible extent. This requires that the enclosure be hermetically sealed and evacuated.
The cold production process is composed of the following steps:
Step 1: Desorption – drying the adsorbent
The adsorbent is dried by adding heat. In the process, water vapour is released (desorbed) and flows into the condenser where it is liquefied. This process releases heat which must be dissipated into the environment using a suitable re-cooling unit. When the adsorbent is sufficiently dry, the heat supply is stopped, and the top check valve will close.
Step 2: Adsorption – adhesion of water vapour to the adsorbent
Once the adsorption has cooled down, the reverse reaction will take place, and the liquid condensate will evaporate. The bottom check valve opening to the condenser opens, and the dried adsorbent will absorb water vapour.
Cold water is generated inside the condenser and used for air-conditioning. Heat is generated during the adsorption process and must be dissipated as well.
Step 3: Return of the condensate
In a final step, the condensate is returned to the condenser, completing the cycle.
The generation of continuous cooling requires the anti-cyclic operation of two adsorbers, i. e. one adsorber desorbs while the other one adsorbs and generates cold.