Demystifying the solar myths

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 DHW (domestic hot water) demand in commercial HVAC applications.

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.  

Suitable for all

First of all, 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 sleek, 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% of a building’s annual hot water demand. Solar irradiation in a UK winter is about 30% 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°C in October and November, for example, helping reduce carbon emissions.

Another interesting point to note is that in the Middle East and southern Europe, solar thermal systems have even been linked to absorption chillers to provide chilled water by using heat from the sun. There are applications that suit this hybrid approach in the UK too. It represents a perfect synergy of technologies, which proves that contrary to popular belief, solar can be used for both heating and cooling.

Design and manufacture

Performance efficiency is another area of concern for those of us active in the solar thermal sector. The myth persists that pretty well all types of collector look the same and behave at the same levels of efficiency. Not true.

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 the 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% of the sun’s energy, enabling it to work effectively even during the coldest winter months. Tests show such units can achieve temperatures well above boiling point and some units have stagnation temperatures of around 250°C.

The industry accepted mark of quality is the Solar Keymark. At present, the Solar Keymark is only a voluntary third party certification mark for solar thermal products, demonstrating that the products conform to the relevant European standards and fulfils additional requirements. As a result, some Solar Keymark certified collectors still vary hugely with regards to performance and specification. At present, the industry is void of a single definitive test to demonstrate long term performance and efficiency over a period of time. 

The problem for specifiers is that the number of poorly manufactured and less expensive products on the market (which can stop performing after as little as three to four years due to internal corrosion) has increased within the last few years. The result is that differentiating a long term solution from a short term fix is harder than ever. As a guide, quality collectors should still perform to the same level of efficiency after 15 to 20 years or more.

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.

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°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.

Cost of installation

In the past, when the market was in its infancy and the technology untried, solar thermal collectors suffered from something of a reputation issue. The product technology lifecycle for solar thermal is maturing. With that maturity and substantial improvements in manufacturing technology, the cost of high quality collectors is reducing year-on-year. As a result, 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 lip service to the renewable obligations of a client.  

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. Be critical of the panel’s design and manufacture. Bear in mind that although collectors don’t require maintenance, regular checks are advised to ensure you continue to achieve the highest efficiency. Do your research and make sure that the right solar thermal collector is specified for the project.