The Renewable Heat Incentive is likely to create a much bigger market for solar thermal systems, and it's important to optimise the design to ensure that all of the potential benefits are realised.
The long-awaited Renewable Heat Incentive (RHI) has now been announced and the first phase, for non-residential buildings, will be introduced in July of this year. The Department of Energy and Climate Change (DECC) estimates that the RHI will generate a seven-fold increase in the uptake of qualifying technologies by 2020.

Clearly a significant proportion of this projected increase will be from the wider use of solar thermal systems - a variable-output heating technology that requires careful design if it is to deliver maximum benefits. For example, solar irradiation levels in the UK fluctuate widely, from 100 Wh/ sq m of collection area on a cloudy day up to 1,000 Wh/sq m on a sunny day.

Given this potential ten-fold variation in solar irradiation levels it is essential that any solar thermal system in the UK has 100% back-up from one or more other heat sources. The challenge, therefore, is to ensure that each of the heat sources used provides optimum efficiency under all of the conditions that may be experienced.

Achieving this comes down to careful system design that reflects a clear understanding of anticipated heat loads through the year. This, in turn, will depend on the nature of the project.

Low operating temperatures

For example, a project with a swimming pool provides a large heat sink with relatively low operating temperatures (typically 26-30°C) that result in high collector efficiencies. The fact that most indoor pools have a large roof area for mounting the solar collectors also makes such projects ideal for solar thermal heating.

However, the majority of projects do not have a swimming pool and the majority of solar thermal systems in non-residential projects are likely to be used to heat or pre-heat domestic hot water (DHW). In fact, given the wide variation in demand for DHW in larger, non-residential projects it is usually more cost-effective to size the solar thermal system to pre-heat DHW. Additional heat sources can then be used to bring it up to the required temperature.

In this scenario, the solar energy will typically be stored in a thermal storage vessel, using a heat exchanger to pre-heat the cold feed. This enables the solar energy to be stored at higher temperatures without risk of scalding. It also has the additional advantage that it minimises the storage volumes of pre-heated potable water where regular pasteurisation forms part of the Legionella control regime.
In the case of non-potable systems, thermal storage vessels can be connected in series, using diverting valves to circulate the water and enhance stratification within the vessels. This arrangement will facilitate the integration of biomass or other boilers in support of the system.

Where low grade heating systems such as underfloor heating are in use, maximum advantage may be gained from increasing the angle of inclination of the solar collectors. Although this will reduce their efficiency in the summer it will increase efficiency when the sun is lower in the sky during other seasons, so that the solar thermal system makes a contribution for more of the year. Indeed, in summer the system may meet the demand of the underfloor heating and still have capacity to contribute to DHW.

In all such cases the choice of auxiliary heating is clearly important and may include other low carbon technologies such as biomass boilers or heat pumps - or conventional gas or oil fired boilers - or any combination of these. Heat pumps, of course, may also vary in their heat output through the year if they are solely dependent on ambient temperatures rather than using heat recovered from other processes. For that reason, additional heat sources will still be required when heat pumps are used.

So it is vital to ensure that the system is highly flexible with a control strategy that leverages maximum performance from the low carbon heat sources and only uses conventional heat sources when necessary. In the case of DHW, a system that uses stainless steel buffers fed by minimum storage, high output calorifiers or plate heat exchangers will provide a flexible solution. Ideally, the cold mains water will be pre-heated using solar thermal or heat pump systems. And, of course, an anti-Legionella regime will need to be in place.

The RHI will provide a much faster payback for projects using solar thermal, and this will completely alter the return on investment (ROI) calculations that are generally made when considering the use of low carbon heat sources. Therefore, as the RHI fuels an increase in the use of such technologies, building services engineers have a key role to play in optimising performance and ensuring that maximum ROI is achieved through the most appropriate use and control of low carbon heat sources.

• Kevin Stones is technical director of Hoval