To those not acquainted with solar air-conditioning (SAC), the technologies might seem to be a contradiction in terms. In all SAC solar heat is used to drive a cooling process. Generally, SAC can be distinguished into:

Closed cycle systems
Closed cycle systems are equipped with thermally driven chillers, which provide chilled water that is either used in air handling units to supply cooled and dehumidified air or that is distributed via a chilled network to decentralized room installations such as fan coils or chilled ceilings. Available thermally driven chillers on the market are absorption chillers, which are most common, and adsorption chillers, offered currently by few manufacturers only. A component, necessary in all chilled water systems, is a heat rejection system.

Open cycle systems
Open cycle systems allow complete air-conditioning by supplying cooled and dehumidified air. The “refrigerant” is always water, which is brought into direct contact with the atmosphere. The most common open systems are desiccant cooling systems with a rotating dehumidification wheel and a solid sorbent.

Among the thermally driven cooling systems, closed cycle systems have currently a central position. According to a survey in 2003, approx. 71 % of the installations were equipped with chillers (63 % using absorption technology and 8 % using adsorption chillers). Besides, 29 % of the installations are desiccant cooling systems.By the way, solar cooling systems have a fast growing market with several new actors. New interesting developments and concepts are appearing in the different technologies.In a near future, the barriers to overcome are the organisation of the sector, the cost reductions and the adaptation of the machines and the main components to technical specifications of solar cooling systems.

Overview of the most common solar assisted air conditioning technologies

Type of refrigerant cycle

Closed Refrigerant flows
in closed cycle

Open Refrigerant is in
direct contact with air


Chilled water

Dehumidification of air and evaporative cooling

Phase of sorbent





Typical material duo

Water - silica gel

Water - water/lithium bromide, ammonia- water

Water - silica gel / Water - lithium chloride

Water - calcium chloride, Water - lithium chloride

Available technology

Adsorption chiller

Absorption chiller

Desiccant cooling

Close to market introduction

Typical cooling capacity

5.5 - 500 kW

4.5 kW - 5 MW

20 - 350 kW (per module)


Typical coefficient of performance *

0.5 - 0.7

0.6 - 0.75
(single effect)

0.5 - > 1

> 1

Typical driving temperature

65 - 90 °C

80 - 110 °C

50 - 95 °C

50 - 70 °C

Solar collectors

tubes, flat plate

tubes, flat plate

Flat plate, air collectors

Flat plate, air collectors

* Coefficient of Performance (COP) = chilling capacity / driving heat

Desiccant cooling system assisted by solar energy from air collectors and PV moduls. Pompeu Fabra Library (Mataró, Spain) | Source: AIGUASOL



Capacity 105 kWc, 448 m˛ selective flat plate collectors (Crete, Greece) | Source: CRES