In an open sorption storage system air is transporting water vapor and heat in and out of the packed bed of solid adsorbents (see figure 7) or a reactor where the air is in contact with a liquid desiccant. In Desorption mode a hot air stream enters the packed bed or the reactor, desorbs the water from the adsorbent or the salt solution and exits the bed cooler and saturated. In adsorption mode the previously humidified, cool air enters the desorbed packed bed or the concentrated solution. The adsorbent or the solution (or absorbent) adsorbs (or absorbs) the water vapor and releases the heat of sorption. The air exits warm and dry. In case of a solid adsorption it can be very hot. In case of a liquid absorption the dehumidification of the air is the main purpose.

The desorption energy QDes is the energy input to the thermochemical storage system, whereas the heat of adsorption energy QAds can be used for heating. The heat of condensation QCond can be additionally used, if it is available on a usable temperature level, which is depending on the inlet air conditions. The energy for evaporation QEvap has to be available at a low temperature level, which can not be used otherwise (right scheme of figure 7). The desiccant cooling process is based on the dehumidification of the air during the adsorption mode only.

Thermal energy storage is achieved by separating the desorption step (charging mode) from the adsorption step (discharging mode). After desorption the adsorbent can theoretically stay in this desorbed state, being referred to as charged in the following, without any thermal losses until the adsorption or absorptions process is activated [5], [6].