Domestic Solar Water Heating
Key FactsThe current grant system in UK is confused and constantly over-subscribed. There is clearly a strong demand, but uncertainty caused by Government policy means the supply seriously lags the demand. The Government grant schemes has at least resulted in a listing for approved energy-saving products (3).
The main market drivers are environmental concerns, rising fossil fuel costs, the "Merton" rule (4) and security of supply.
Where in the World you are directly affects the available solar radiation as this is a function of the angle of the Sun to the Earth. The prevailing climate, especially hours of cloud-free sunlight, modifies this figure. The geographical location also affects the relative amounts of beam and diffuse radiation available. For solar collectors, diffuse radiation is more effective than with photovoltaic cells that work best with direct sunlight.
Within a fairly large range of angle, the inclination of the solar collectors has little impact in the UK. Of more importance is the orientation with the solar collectors facing between south-west and south-east.(5) However, a higher angle (> 53°) is better as summer collection is reduced, but improved in spring and autumn, this spreading the available effective radiation over the year. Collectors set at a higher angle are better washed by rain, and require a little less roof space.
Domestic solar heating is generally not suitable for space heating, as heat is produced at the wrong time of year! Systems should be sized bases on the average daily hot water usage in the summer. The minimises overheating risks, increasing the survival time of anti-freeze, thus allowing longer service intervals. Design engineers should use detailed computational simulations such as T*SOL® (6) and RETScreen(7). Some simple "rules of thumb" are: install 0.75-1m2 tube per person if at optimum orientation and pitch, 35 litres per m2 of collector in addition to other storage. 25 litres storage (cylinder) capacity per person is legal minimum, with 50 litres an ideal.
DIY systems, e.g. (8) are available, but problems such as corrosion and legionella bacteria forming in water heated to below 60°C can occur.
Further Information