Solar Thermal Absorption Cooling Systems

Introduction to S T A C S

The demand for domestic air conditioning is growing in Wales. Based on current climate change scenarios Wales is expected to experience a rise in average annual daily temperature of between 2°C and 4°C by 2080 (UKCIP). Therefore the demand for domestic air conditioning systems is likely to continue to rise over this century.

All air conditioning systems require an energy input to function. Most standard vapour compression chillers are motor or engine driven, deriving their energy inputs mainly from fossil fuel sources. In order to mitigate the potential contribution to climate change of increased numbers of air conditioning units in Wales it is necessary to seek alternative energy sources to provide the energy input required for cooling.

Absorption chillers have been used on a large scale for industrial scale cooling since the 1950’s. Generally they are run using low-grade waste heat from other industrial processes to provide chilled water for space cooling. A number of different refrigerant cycles exist, the most common being lithium / bromide and ammonia / water. Most chillers fall into 2 categories: single effect or double effect, with double effect chillers being the more efficient while requiring a higher grade heat input. Triple effect chillers are currently under development.

Solar thermal collectors can provide renewable heat sources of over 100°C in summer. Most domestic systems comprise either a flat plate collector or a series of evacuated tubes connected to a fluid circulation loop. Evacuated tube systems (also known as vacuum tubes) produce steam that rises in the tube and warms a carrier fluid via a heat exchanger. Evacuated tubes offer the advantage over flat plate collectors that they work efficiently with high absorber temperatures and with lower solar radiation. They are also more efficient at capturing the available solar energy.

Solar thermal systems can be installed on domestic dwellings where orientation and shading conditions are favourable (for example roof mounted). The heat produced by such systems is usually used to provide domestic hot water but could also be used to drive a small absorption chiller. The potential benefit of such a system would be that peak cooling demand in summer would exactly coincide with peak solar radiation available. Hot water would still be available throughout the year (as with a standard solar thermal system) and the solar thermal could further be used to provide space heating during the winter months if surplus heat were available. Such a system could potentially produce a significantly lower carbon emission than a standard air conditioning system. With rising fuel costs such systems could help to alleviate fuel poverty particularly in social housing.

Currently no domestic scale solar thermal absorption chiller systems exist in Wales. This project will involve the design, installation and performance monitoring of such a system. Further to this the implications for building design will be assessed both in terms of new build and retrofit through a review of Welsh housing stock to determine the suitability of the system for use in existing dwellings. Aesthetic and planning considerations will also be examined and computer modelling will be employed with real data from the test system to assess performance for a range of dwelling types.

This work commenced in August 2006 and is due for completion in June 2008


Solar Thermal Collector

Solar Thermal Collector

Domestic Scale Absorption Chiller

Domestic Scale Absorption Chiller

WSA Roof Schematic

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