Solar Thermal – Sustainable Technology Worth Reconsidering

Bill Sinclair, technical director, Adveco

Faced with an increasing expectation to become more actively sustainable, as well as the need to mitigate rising energy costs, now is a good time for commercial organisations to reconsider the integration of a solar thermal system as part of their premises. Not only a proven and extremely reliable technology, for the past 15 years solar thermal has offered a clear path to reducing CO₂ emissions.

Ten years ago, it was very difficult to argue in favour of solar thermal because the numbers really did not stack up against the price of gas. The capital costs of installation and maintenance versus the operational savings meant many early projects failed to recoup their investment, even with the support of RHI, despite a typically cited ten-year payback.

Flash forward to today, and we are in very different situation. Commercial organisations in response to the need to reduce CO₂ are moving water heating away from gas to direct electric which is perceived to be cleaner. But what they are finding is that shift comes with a new financial burden as electricity at 11p/kWh is substantially more expensive than gas. If you can offset that cost, then the numbers now start to look favourable for adopting solar thermal. A ten-year return on investment becomes very achievable, before even factoring in supplementary support from the Government’s Renewable Heat Incentive (RHI) scheme. But be aware that project applications for RHI close in March 2021, and support for solar thermal under the replacement Clean Heat Grant is not currently planned. Despite that, the undisputed carbon and cost savings means we are seeing a definite upswing in demand for new systems.

Solar thermal systems are ideal for organisation that use and rely on large amounts of domestic hot water (DHW)r, but it is important to understand that a solar thermal system will not fully replace an existing water heating system and will not provide space heating. The actual percentage of water heating demand covered by solar thermal will depend on the site and energy consumption habits – around 30% is typical for commercial applications.

As with any technology, issues can arise if a commercial system is poorly designed and/or badly maintained.

A correctly designed and sized system will consider the daily usage and peak demands. Its aim is to serve all peaks from storage, with the size of the peak determining the size of pre-heat. The recovery time for peaks is what ultimately determines number of solar collectors a building requires. The design process also sizes usage with available space. A south-facing and unobstructed roof with an inclination of 30° from the horizontal is optimal, though by no means essential as modern solar collectors can be installed in a variety of permutations. Unsurprisingly, solar thermal collectors do suffer if the building is significantly shaded, in which case a commercial air source heat pump may be a preferred option to produce low carbon heat. Length of pipe run is also important, if collectors are located a considerable distance from plant the system’s thermal losses can be detrimental to efficient operation.

Perhaps the largest negative for early adoption of solar thermal was the possibility of overheating causing pressure blow-off, or worse stagnation of Solar Fluid. Without overheat protection, a poorly maintained system could see fluid ‘cooking’ to the consistency of molasses. This could happen within weeks of installation, blocking collectors and pipework and causing permanent damage.

The response has been to deploy systems that offer drain back, which, as the name implies drains the Solar Fluid from the collector to a reservoir when not in use. Flat plate collectors with an integrated drain back module offer a more cost-effective (as there is no requirement for large solar storage) and more efficient (as there is no call to dump unused heat) approach. The technology has proven itself in the field, with some sites requiring fluid changes after eight or nine years rather than the expected three, and some sites have required no fluid change at all in the past decade.

Quality design work enables calculation of the Solar Fraction, the total annual heat demand for hot water compared to total available from solar inputs. Obviously, solar thermal systems are most productive during the summer months, but during winter a back-up heat system is also required. Designers have two options – gas fired with solar preheat, or electric with heat pump and solar – to balance cost and environmental impact. Solar has always been used as a preheat with coldest water possible to maximise the efficiency and output: this gives maximum free heat with no carbon emissions. But there is a good case now for using solar thermal with heat pumps and electric if set up as a mid-heating system which can lower both carbon and cost.

No single technology currently provides the ‘magic bullet’ of sustainability for commercial-scale projects. Delivered in conjunction with other technologies – high efficiency gas or electric boilers and ASHP – Solar thermal, when correctly sized, commissioned and protected from overheat, is a proven and practical technology. It can help future-proof a hot water system whilst making substantial savings in operational costs (with or without RHI) while dramatically reducing emissions all year round.