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User control is vital for best performing solar water heating

The way householders use their solar thermal system is critical to getting the best results from solar water heating systems, according to new research. The Energy Saving Trust (EST) is calling on installers to give better advice to users on how to control their solar water heating: in terms of volume of hot water use, timing of back-up heating and hot water use, and temperatures required.

Here Comes the Sun: a field trial of solar water heating systems is the largest independent trial of solar water heating in the UK and Eire. It monitored real-life performance in 88 homes. Sites were monitored over a full calendar year from April 2010.

Overall the trials found that solar water heating systems have the potential to work well in the UK and the Republic of Ireland when installed properly and controlled adequately by the user. However, it identified a huge range of performance, with the best system producing 98 per cent of the household’s hot water, and the worst just 9 per cent. The median across all systems was 39 per cent. Well-installed and properly used systems provided around 60 per cent of a household’s hot water.

Generally householders in the trial were happy with their solar water heating systems: 84 per cent were “satisfied” with their system, and over half were “very satisfied”. This is despite the costs of solar thermal still being high, compared with the savings in energy bills it yields. Typical savings from a well-installed and properly used system in the trial are £55/year when replacing gas and £80/year when replacing electric immersion heating; however, they also vary depending on hot water usage, and how the consumer controls the system. This compares with installation costs of £3,000 to £5,000 for an average system.

Which is better – flat plate or evacuated tube collectors is a perennial question, so it’s good to see the field trial providing an answer. It demonstrated that there was little difference in total solar energy yield between systems that used flat-plate solar collectors and those that used evacuated-tube solar collectors. The median for flat-plate collectors was 1,156kWh per year and the median for evacuated-tubes 1,140kWh per year. Although evacuated tube systems have higher insulation, flat-plate solar collectors generally have a larger working area as a proportion of the collector size. This was supported by findings from the trial.

The performance of pumps run by mains electricity versus pumps run by a small PV panel is also a running argument. The trial found that where mains electricity was used, generally only a small amount of energy was used by the pump in comparison to the total heat energy provided by the system (the median was about 5 per cent and 55kWh per year in total). However, it did identify a few faulty systems where the pump had been running when there was no solar energy, which had led to excessive electricity consumption (c. 180kWh per year).

Heat loss from inadequately insulated cylinders and pipes was another reason that some systems were not achieving the performance they should. This is a problem EST has also observed in boiler trials, and it is calling for it to be addressed through industry standards on insulation across all heating systems.

Overall, however, the field trial found that a well-installed and properly used solar water heating system is likely to provide both savings in energy bills and carbon savings.  The current price barriers to investment in solar hot water systems are expected to come down when the renewable heat incentive is introduced in October 2012. In the meantime, the  renewable heat premium payments offer £300 grants to help with the capital cost of installation (until March 2012).

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