Direct use of solar radiation

Simply because a very large energy flux falls on Britain through the year, it is wrongly assumed by many that the life to which they have become accustomed can be supported with solar energy. Even with the reduced level of energy use we managed to achieve with proper insulation, the average level of radiation occurring during the two months when we need warmth most - December and January - is down to an average over the period of 15 watts per square metre. This is vanishingly small. To make any use of such small radiation requires that correspondingly very large areas be set aside to low temperature collection, which will be redundant during the summer months when average radiation, over the month of June for example, is 225 watts per square metre. Thus, unless you can set aside at least 40 square metres of cheap solar collecting surface, facing due south at an angle of around 70° to the horizontal, it is best to forget energy self-sufficiency from the sun alone.

It is not so silly, however, to use the sun when it does shine hot to provide between 50 and 90% of your hot water, usually during the months of April to September when it is warm enough. 4 square metres of collecting surface plumbed into the hot water system of your house can provide up to 100kw-h/m² per year. At 1p per kw-h this could be worth £24 a year, and at 2p per kwh, which is the price we are likely to be paying soon, it would be worth £48 a year.

For anyone wishing to embark on this course it should be remembered that while solar collectors are very efficient in the lower temperature ranges up to around 10-17°C, this 'efficiency' falls off very quickly when the temperature of the heat transfer fluid inside the collector rises above 35-40°C. In tests carried out on an unglazed solar collector we found that we could achieve an average of about 50% efficiency at 40°C. By putting a clear plastic glazing on top we increased the average efficiency from about 50 to 60% in the higher temperature ranges. It is really not worth spending a great deal on glazing, since the extra energy you collect will take years to pay back the additional cost of the framing and glazing, in terms of hot water.

I doubt whether it is worth spending money on making your own solar collector. The cheapest do-it-yourself collector is a scrap steel radiator stripped down and painted black. Insulation and aluminium foil underneath stops radiation and conduction losses. So far as possible, collectors should face south. The cheapest place to site them is on the south wall of the house. The most expensive place is on the south roof, since you will need the services, or skills, of a roofer and carpenter, in addition to those of a plumber and electrician, unless you have these skills yourself.