Adsorption Chillers

Adsorption chillers use solid sorption materials instead of liquid solutions. Market available systems use water as refrigerant and silica gel as sorbent; but recently, an alternative to silicagel is zeolith for some manufacturers. So the two technologies now available are: Silicagel/H2O and Zeolith/H2O.

The machines consist of two sorbent compartments (see figure) one evaporator and one condenser. While the sorbent in the first compartment is regenerated using hot water from the external heat source, e.g. the solar collector, the sorbent in the second compartment adsorbs the water vapour entering from the evaporator. Compartment 2 has to be cooled in order to enable a continuous adsorption. Due to the low pressure conditions in the evaporator, the refrigerant in the evaporator is transferred into the gas phase by taking up the evaporation heat from the chilled water loop and thereby producing the useful "cold". If the sorption material in the adsorption compartment is saturated with water vapour to a certain degree, the chambers are switched over in their function.

To date, only a few Asian and European manufacturers produce adsorption chillers. The two historical actors are Japanese, but now a German manufacturer has entered the market.
Under typical operation conditions with a driving temperature of 80 C, the systems achieve a coefficient of performance (COP) of about 0.6, but operation is possible even with temperatures of approx. 60 C. The capacity of the chillers ranges from 5.5 kW to 500 kW chilling power.

The simple mechanical construction of adsorption chillers and their expected robustness is an advantage. There is no danger of crystallisation and thus no limitation in temperatures. There is no internal solution pump and electricity consumtion is reduced to a minimum. A disadvantage is the comparatively large volume and weight. Furthermore, due to the small number of produced items, the price of adsorption chillers is currently still high. A large potential for improvements is expected in the construction of the heat exchangers in the adsorber compartments, which would reduce volume and weight considerably in future generations of adsorption chillers.

Schematic drawing of an adsorption chiller (click to enlarge)