. Pre-selection of technologies

In the first planning phase of a solar assisted air conditioning system, the type of the building to be equipped with the air-conditioning system and the use of the building has to be closer analysed. High attention should be paid to architectural and technical measures which reduce the cooling load, e. g. by implementation of external shadings, night ventilation in combination with the thermal inertia of the building and by decreasing the internal loads as far as possible.

For further investigations, an annually load file should be prepared, containing at least hourly values of cooling and heating loads as well as humidification and dehumidification requirements. The load structure of a considered building area depends beside the physical properties of the building and the thermal and radiative gains on the usage of the building,

i. e. the frequency of occupation, occupation density and on the additional technical equipment in the rooms. The determination of a representative time series of loads is necessary since the correlation between solar radiation power and heating/cooling loads determines the energy demand from auxiliary sources as well as the utilisation of the solar thermal sub-system. Including these effects into the annual energy balance allows to assess primary energy savings, to choose appropriate sizes of the core components and thus to obtain reasonable economic figures of the planned installation. Several load calculation methods, e. g., according to the German standard VDI 2078 /1/, and building simulation programs like TRNSYS, ESP, etc., may be applied for this purpose.

A simplified decision scheme is presented in Figure 1 in order to support the pre-selection of air-conditioning technologies, which can be used in combination with solar thermal systems.


Figure 1: Simplified decision scheme of thermally driven air-conditioning systems. It is assumed that both, indoor temperature and humidity are to be controlled. The starting point is the assessment of the cooling loads and of the required air exchange rates. If the installation of a centralized air handling unit is feasible, the following basic decision is, whether or not the hygienic air change rate is sufficient to cover the cooling loads (latent + sensible). This will be typically the case in rooms with high ventilation rates, as required e. g. in lecture rooms. Next, the tightness of the building shell has to be considered for the decision, whether a supply/return air system makes sense or not. Depending on these building and load oriented tasks, the decision on the distribution medium is made: either a pure chilled water system, a pure air system or combined solutions are possible. Finally, the technology (third column) has to be selected. In case a chilled water supply is necessary, the lowest required temperature level of the chilled water is determined by the question

Distribution medium

whether air dehumidification is realised by cooling the air below the dew point (conventional technique) or whether air dehumidification is realised by a desiccant process. In the latter case, the temperature of the chilled water may be higher since it has to cover sensible loads only. In extreme climates with high ambient air humidities, special configurations of the desiccant cooling cycle are required, when this technology will be employed.

Short cuts: DEC = Desiccant Cooling system, AHU = Air Handling Unit.

The scheme presented in Figure 1 can help to approach to the most appropriate technical

solution but does not cover the following questions:

— Necessity of a backup system for the cold/heat production or to allow solar autonomous operation of the solar assisted air-conditioning system;

— Type of the thermally driven chiller, e. g., absorption chiller or adsorption chiller;

— Flexibility in comfort conditions, e. g., to allow certain deviations from the desired air states;

— Economical issues;

— Availability of water for humidifiers in an air-handling unit or for cooling towers;

— Comfort habits for room installations: fan coils have lowest investment cost, but allow dehumidification only when connected to a drainage system; chilled ceilings and other gravity cooling systems require high investment cost, but provide high comfort.

Additional constraints may arise from an architectural / planning point of view or from

economic considerations:

— possibility to install the required air ducts in existing buildings

— question how the desired technical solution fits to the technical infrastructure of an exis­ting building (e. g., if a cold water distribution system already exists this might lead to such a system because of economical considerations even if fig. 1 leads to a pure air system)

— available area to install the required solar collector field

— available space for technical equipment (e. g. thermally driven chillers, buffer storage, desiccant air handling unit)