Chilling tale of solar energy
The growing use of electrically powered air-conditioning is undermining efforts to reduce Europe's greenhouse gas emissions. An alternative technology, powered by the sun itself, is currently being demonstrated in a Portuguese office block. Background Buildings account for more than 40% of total energy consumption in the European Union, and for 30% of CO2 emissions. As standards of living rise, the use of conventional air-conditioning is spreading, offsetting much of the energy saving achieved through more efficient heating technology and building design. Solar energy can fairly easily be used to reduce the load on conventionally powered heating systems. Active solar cooling requires more complex technologies, but enjoys one great advantage - energy is most readily available just when the demand for cooling is highest. Solar heat can therefore be used immediately, making it unnecessary to incorporate bulky and costly energy storage systems into the design. Germany's Fraunhofer Institute of Solar Energy Systems (ISE) has developed an air-conditioning unit based on evaporative cooling. Description, impact and results The temperature of air drawn into the building is lowered by humidifying it. By drying the air first, the potential for evaporative cooling is increased so that its temperature can be reduced to 18-19°C without creating unacceptable levels of humidity. Water vapour is adsorbed from the inlet air by silica gel, packed into a dehumidification `wheel'. A solar collector heats the outlet air to around 65°C. As the wheel turns, it passes through the stream of hot air, which dries the gel for the next cycle. A second wheel - a rotary heat exchanger - pre-cools the inlet air and pre-heats the outlet air. As yet, the solar-driven desiccant cooling technology cannot compete with small domestic units, but for large non-residential buildings such as hospitals, hotels and offices, the partners believe it is commercially viable. It will cost a little more to install than a standard, electrically driven system of the same capacity, but will consume only half as much conventional power. Working partnerships ISE has already tested its concept in a small-scale application in Germany. Now, within the Thermie demonstration project, "Solar Desiccant Cooling System for an Office Building in Portugal", it has helped to install a full-scale system in the offices of Portuguese air-conditioning equipment manufacturer Atecnic, in Sintra. The system, which will serve the whole of the building's 350 m2 first floor, has been commissioned in time for the 1999 summer season. Its performance will be analysed by the Portuguese research institute INETI throughout the summer, as the basis for an assessment of the overall energy savings achieved in practice. Involving an air-conditioning specialist as the demonstration project's end-user will speed the optimisation of the system, and its subsequent take-up by the market. Atecnic understands exactly how the system needs to perform in order to win acceptance by its customers, and will be well-placed to recommend it to them.