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Shedding light on heat from the sun

Edward Blake Thomas exposes some of the myths within the solar thermal sector
With the cost of fossil fuels continuing to rise rapidly and with business-operating margins becoming ever tighter, consultants and contractors are now giving serious consideration to the specification and installation of solar thermal systems to meet domestic hot water demand.

That said, it's still necessary to de-bunk a number of the myths around solar thermal technology, so that the carbon and energy saving benefits can be fully realised.

Myth: Collectors only work in hot countries
Many still think that solar thermal technology is not suitable for the UK climate, where rain and cloud, rather than year-round sunshine still prevail.

In the northern hemisphere, the optimum performance of a solar thermal collector will occur when the unit faces due south. However, thanks to the efficiency of modern collectors, installations facing south east, south west, or those which are even orientated westward can now provide a meaningful energy input.

Both conventional and façade type solar thermal collectors can supply up to 60 per cent of a building's annual hot water demand. Solar irradiation in a UK winter is about 30 per cent of its summertime peak. Even so, with a suitably sized array, hot water can still be generated. It is not unusual for water to be heated to 60 deg C in October and November, for example, helping reduce carbon emissions.

Myth: All collectors look and perform in the same way
The design and manufacture of the collector is critical in determining its efficiency, and should be one of, if not the critical factor in selection and specification.
For example, the design and welding method used to connect copper absorber pipes and manifolds to the back of the absorbers will have an impact on efficiency of the heat transfer process. The type of coating used on the collector will also affect its performance. The construction also impacts on efficiency over time when corrosion can dramatically reduce performance.

Evacuated tube type collectors also vary in efficiency. These collectors typically comprise high-efficiency, double-walled evacuated glass tubes, each containing a heat absorbing plate. The vacuum between the tube's twin walls helps reduce both convection and conduction heat losses from the absorber, to improve the collector's efficiency, in cold, ambient conditions such as the UK.

This efficiency is further enhanced by a high-efficiency selective coating applied to the outer wall of the inner glass tube, and in some cases the absorber, which helps increase the amount of heat absorbed by the unit. The result is a solar thermal collector which can absorb over 92 per cent of the sun's energy.

Myth: Solar thermal systems are maintenance-free
Performance is not completely maintenance-free. Regular checks are advised to ensure that solar thermal collectors continue to perform to the specific design criteria and deliver to the highest efficiency. Speak to your supplier to determine the frequency of regular checks and for further information.

Myth: Solar cannot heat swimming pools
Solar thermal collectors are a great heating solution for hotel swimming pools, health clubs and leisure centre applications. A well-designed system can contribute significantly to heating pools and spas in the UK, helping to reach temperatures of over 30 deg C in the summer. This type of application is where the biggest savings can be made as the annual cost of heating pools is very high.

Myth: Solar thermal collectors are expensive to install
Solar thermal collectors are a viable, mass-market heating solution for commercial heating projects and should not be a seen as a niche product, or as expensive, but necessary lipservice to a client's renewable obligations.

Solar thermal technology is a cost-effective, renewable solution, capable of generating abundant supplies of hot water in many different UK commercial HVAC applications, including health spa and leisure centre projects. But be aware that not all solar thermal collectors are the same.

// The author is Camel Solar product manager at AET //
13 May 2013

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