Desiccant cooling systems

Desiccant cooling systems are basically open cycle systems, using water as refrigerant in direct contact with air. The thermally driven cooling cycle is a combination of evaporative cooling with air dehumidification by a desiccant, i.e. a hygroscopic material. For this purpose, liquid or solid materials can be employed. The term ‘open’ is used to indicate that the refrigerant is discarded from the system after providing the cooling effect and new refrigerant is supplied in its place in an open-ended loop. Therefore only water is possible as refrigerant with direct contact to the surrounding air. The common technology applied today uses rotating desiccant wheels, equipped either with silica gel or lithium-chloride as sorption material.

Solid Desiccant Cooling

The main components of a solar assisted desiccant cooling system are shown in the figure on the right. The basic process in providing conditioned air may be described as follows.

Warm and humid air enters the slowly rotating desiccant wheel and is dehumidified by adsorption of water (1-2). Since the air is heated up by the adsorption heat, a heat recovery wheel is passed (2-3), resulting in a significant pre-cooling of the supply air stream. Subsequently, the air is humidified and thus further cooled by a controlled humidifier (3-4) according to the set-values of supply air temperature and humidity. The exhaust air stream of the rooms is humidified (6-7) close to the saturation point to exploit the full cooling potential in order to allow an effective heat recovery (7-8). Finally, the sorption wheel has to be regenerated (9-10) by applying heat in a comparatively low temperature range from 50 °C-75 °C and to allow a continuous operation of the dehumidification process.

Solid desiccant systems can also be used to provide heating for periods with low heating demand.
Flat-plate solar thermal collectors are normally applied as heating system in solar assisted desiccant cooling systems. The solar system may consist of collectors using water as fluid and a water storage, which will increase the utilisation of the solar system. This configuration however requires an additional water/air heat exchanger, to connect the solar system to the air system.

Special design of the desiccant cycle is needed in case of extreme outdoor conditions such as e.g. coastal areas of the Mediterranean region. Due to the high humidity of ambient air, a standard configuration of the desiccant cooling cycle is not able to reduce the humidity down to a level that is low enough to employ direct evaporative cooling. More complex designs of the desiccant air handling unit employing for instance another enthalpy wheel or additional air coolers supplied by chilled water can overcome this problem. A novel approach is the dehumidification and simultaneously cooling of the supply air in an air-to-air heat exchanger, in which the supply air is dehumidified through sorptive coatings at the heat exchanger wall, and cooled by the returned air, which was humidified close to saturation in order to lower the return air temperature before entering the heat exchanger. The simultaneously dehumidification and cooling improves the efficiency of the system. As a consequence, the supply air humidification may be avoided in moderate climates. Since the sorption material in the supply side of the heat exchanger will be saturated after some time, a periodic operation with two heat exchangers of which one is regenerated, is required. A pilot project in Germany for testing this new concept is currently in the design phase.

Liquid Desiccant Cooling

A new technology, close to market introduction, are desiccant cooling systems using a liquid water-lithium chloride solution as sorption material. This type of systems shows several advantages like higher air dehumidification at the same driving temperature range of solid desiccant cooling systems, and the possibility of high energy storage by storing the concentrated solution. This technology is a promising option for a further increase in exploitation of solar thermal systems for air conditioning. Currently, a few systems of this type are installed in Germany in pilot and demonstration applications, driven either with solar thermal heat or from other heat sources.

Schematic drawing of a desiccant cooling system (click to enlarge)