From basic considerations such as shown in Figure 2, some general remarks may be given for the planning of a solar assisted air conditioning system:

■ If the thermally driven cooling system runs with a comparatively low COP and a fossil fueled heat source backup is foreseen, a high average solar fraction is required in order to achieve significant primary energy savings. This has to be ensured by a proper design of the system, e. g. a large solar collector area, sufficient storage volumes and other measures in order to maximise the use of solar thermal system.

■ A conventional electrically driven compression chiller may be used as backup system alternatively to a heat backup. In this concept, each unit of cold provided by the solar thermally driven chiller reduces the cold to be delivered by the conventional chiller. This

approach leads to primary energy savings even at low values of solar fraction. The solar system then serves mainly to reduce the electrical energy consumption.

■ If a heat backup system using fuels is applied, any replacement of fossil fuels by fuels from renewable souces such as biomass will increase the fossil fuel conversion factor and thus decreases the primary energy consumption of the thermally driven system.

■ A solar thermally autonomous air conditioning system does not require any other cold source and therefore always works at the limit with a solar fraction of 1.0.

■ The utilisation of the solar collector system should be maximised in any case, e. g., by supplying heat also to other loads such as to the building heating system and for hot water production.

More information on guidelines, design approaches and examples for solar assisted air­conditioning is provided in the following projects:

— International Energy Agency (IEA) Solar Heating and Cooling Programme (Task 25, Solar Assisted Air-Conditioning of Buildings) /3/;

— Solar Air Conditioning in Europe (SACE), funded by the EC /4/;

— CLIMASOL, funded by the EC, actually in process and finished 2005 /5/.