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 he energy consumed by buildings - primarily for heating, lighting and ventilation - accounts for more than 40% of the EU total, and for 30% of its CO2 emissions. As standards of living rise, growing use of electrical air-conditioning offsets much of the energy saving achieved through more efficient heating technology and building design(1).
Solar energy can fairly easily be employed to reduce the load on conventionally-powered heating systems. Solar cooling requires more complex technologies. But it enjoys one great advantage over solar heating - energy is most readily available just when the demand for cooling is highest. Solar energy can therefore be used immediately, making bulky and costly systems for storage unnecessary.
Competitive Technology
Germany's Fraunhofer Institute of Solar Energy Systems (ISE) has developed an air-conditioning unit which, though more expensive to install than a standard, electrically driven system of the same capacity, will consume only half as much electricity.
"All ventilation systems use electricity to push air through the building," explains Dr Hans- Martin Henning of ISE. "Our design also incorporates a thermal heat source as a back-up. Our objective was to design a system which would be attractive to real customers - in terms both of comfort and of cost. As yet, the solar-driven desiccant cooling technology cannot compete with small domestic window units, but for large non-residential buildings such as hospitals, hotels and offices, we are confident that it is commercially viable."
ISE has already tested its concept in a small-scale application in Germany. Now, in a Thermie project, it is installing a much larger demonstration system in the offices of Atecnic, a Portuguese manufacturer of air- conditioning equipment, in Sintra, near Lisbon. Henning hopes to commission the system, which will serve the whole of the building's 350m2 first floor, in time to monitor its performance throughout the 1999 summer season.
"Atecnic is an ideal end-user," he says. "Air-conditioning is their business, so they understand exactly how the system needs to perform in order to win acceptance in the market place. If the company is happy with its own installation, it will be in an excellent position to recommend the system to its customers."
A second Portuguese partner, the research institute INETI, will be responsible for analysing performance data as the basis for an assessment of the overall energy savings achieved in practice.
Temperature and Humidity
How can the same sunlight which beats against the windows of an office be used to cool the air which passes through it?
In fact, temperature reduction is achieved through evaporative cooling. As air is drawn into the building it is humidified, and its temperature falls. "But where the humidity of the outside air is already high, there is very little potential for evaporative cooling," Henning explains. "Besides, the building's occupants want less humidity, not more. Our system dries the air first. Humidification can then bring its temperature down to a comfortable 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'. Hot water from a solar collector heats the outlet air to around 65°C. As the wheel turns, it passes through the stream of hot air, which drives off the water, desorbing the gel for the next cycle. A second wheel - a rotary heat exchanger - pre-cools the inlet air, pre-heating the outlet air at the same time.
"The wheels create two feedback loops between the inlet and outlet flows, making it very hard to control both temperature and humidity," says Henning. "The biggest technical challenge is to maintain conditions indoors within the desired limits, despite changing inputs from the solar collector. We think we have succeeded, but we will monitor the system closely during the first summer to ensure that it meets the end-user's requirements."
Retrofit
The take-up of all renewable energy technologies has been slowed by the depressed prices of conventional forms of energy, now at their lowest since the 1960s. The financial savings offered by new, energy-efficient technologies are often too small to justify the initial investment - except as part of large capital projects, where only the additional cost needs to be justified.
However, only 2% of Europe's building stock is replaced each year. Technologies which can be installed in existing structures, as Atecnic's system has been, can spread faster. "The retrofit market may turn out to be very important," Henning confirms. "Of course, you need space on the roof for the solar collectors, and a site for the air-conditioning unit, which is larger than a conventional system."
Optimal configuration of the system depends upon local climate. In the warm and humid conditions at Trapani, in Sicily, ISE found that the lowest overall air-conditioning costs can be achieved by using solar energy only for dehumidification, relying on conventional electrically powered air-chilling systems for temperature reduction.
(1)As noted in the first report of the External Advisory Group on cleaner energy systems and economic and efficient energy use, December 1998. The report is available on-line at
http://europa.eu.int/comm/dg12/fp5/eag-energy1.html
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