The Energy Saving Trust (EST) has recently published the results of the most comprehensive study ever of the performance of solar water heating. Eighty eight homes, 12 months of monitoring and millions of data points later the results are out and the report brilliantly highlights both the strengths and weaknesses of this technology.
Customers were clearly delighted with their solar heating systems, with 84% of households being “satisfied” and 50% “very satisfied”.
Those systems that were categorised as “well installed and properly used” achieved excellent results with the report showing a typical energy saving of 1,500 kWh/year, corresponding to the solar system providing around 60% of the household’s hot water energy.
The report also found many systems where users had timed their back-up heating in a way that was detrimental to the energy saving.
Most solar systems in the study used a twin coil cylinder, where the solar system heats the whole volume of the cylinder from the base, with the back-up heater (most often a boiler) heating only a portion at the top of the cylinder. Read a fuller explanation of how a twin coil cylinder works here.
If the household controls their back-up heater well, it is timed to be on for a short time and switch off before the main period of hot water use starts. As hot water is taken from the top of the cylinder, cold water enters at the bottom. The cylinder has plenty of cold water in it for the solar system to work on the next day.
By contrast, if the back-up heater runs during or after hot water use, then the cold water let into the cylinder is quickly heated up. By the time the solar system starts to work, the only cold water available is at the very bottom part of the cylinder where the back-up heater couldn’t reach. Once this zone reaches its maximum temperature, there is nowhere left for the solar panel to put its heat; it switches off and solar energy that could have been collected goes to waste.
An example showing poor use of back-up heating is shown in the image above. The main water use is in the evening and the following morning (as before). The back-up heater is timed to coincide with morning water use, and runs after water use is finished, ensuring that most of the cylinder is already hot as the day begins and severely limiting the amount of solar energy that can be collected.
One approach is to try and educate the householder about how to get the most from their solar heating system. Better information from their solar installers might have lifted more of the households in the study into the higher performing category. After the systems in the study were installed, a new installer standard, the Microgeneration Certification Scheme (MCS), came into force. It requires installers to leave printed materials and durable stickers about the place to explain how to get the best performance out of the solar system.
The problem with this approach is that as we move from solar heating being a fringe hobby to a mainstream technology, more and more diverse people will end up in houses with solar heating. Unlike the sample in the EST trial, these people may not have bought it for themselves – it may have come with a new house, been fitted by a social landlord or a previous owner. They may not be bothered about optimising their solar saving, or able to program the boiler timer.
Fortunately there is a technical fix that reduces the sensitivity of solar water heating systems to the user settings of heating systems.
If the solar heating system provides a larger volume of water that the back-up heater cannot heat, then there is more likely to be cold water available for the solar system to heat when there is sunlight available. This can be achieved by selecting a twin coil cylinder with an adequate solar dedicated volume, or having a separate solar cylinder that pre-heats water before it reaches the main cylinder.According to the Domestic Heating Compliance Guide, the solar system should provide at least 25 litres of solar dedicated volume per square metre of solar panel aperture area, but few do.
Specifiers of solar heating systems who are installing solar heating for others (for example social landlords or property developers) should consider carefully before choosing solar heating systems that rely upon achieving user-behaviours to achieve results. Particular care should be taken with systems which retro-fit onto existing cylinders without providing any solar dedicated volume. Although these seem attractive because there is no need to replace the existing cylinder, they are extremely sensitive to the timing of back-up heating.
Instead, consider systems that are much less sensitive to user behaviour – specify hot water storage with an adequate solar dedicated volume.