With collector and tank
Solar energy can be used to heat your home as well. One way is to use a large area of collector and a large hot water storage tank. The rule of thumb is to allow 50-60 litres of thermal storage per square metre of collector area, although some people might suggest up to 1401itres/m2. For example, a tank containing 3000 litres or 3m3 of water (i.e. a tank 1m x 1m x 3m) will hold 87kWh if it starts at 50°C and cools to 25°C. This assumes that you have a heating system that can use water at low temperatures such as underfloor heating with pipes buried in a concrete floor.
This calculation is based on the specific heat of water which is about 1.16Wh/1 deg C. This means that one litre of water cooled through 1 deg C will give out 1.16Wh of stored heat. The 87kWh stored in the 3m3 of water, which will weigh 3 tonnes, would heat a very well insulated house for about 21 days. Such a storage tank would need a collector area of 30-60m2 depending on the efficiency. The cost of tanks, which must have a very thick layer of insulation round them to keep the heat in and must not be liable to corrode or leak, is only one factor which makes this type of heating system very expensive.
A much simpler way to take advantage of the sun is to use your windows as solar collectors. A vertical window facing south will receive about 700kWh per square metre per year, or just over 300kWh during the winter heating system (1 October to 30 April). If the window is double-glazed about 70 per cent of the solar radiation reaches the inside of your house, providing about 200kWh. The same window will lose about 150kWh over the same period if it is not protected. But if it is covered with insulated shutters for sixteen hours a day, the heat lost will drop to about 75kWh, giving a useful gain to the building of 125kWh per square metre over the heating season. Providing the building is thermally heavy, with externally applied insulation on heavy masonry walls, a concrete floor and brick internal partitions, this solar heat gain will be stored in the structure and given out slowly into the interior.
This sort of passive heating system, which requires no mechanical parts and uses the house itself as the collector and storage system, can reduce the need for conventional fuels. However it is not easy to control and requires everyone’s cooperation. For this reason cheap, reliable and elegantly simple passive systems have found little support from architects and engineers building experimental projects; they prefer to install the maximum number of pumps, thermistors, electronic controls and motorised valves to give their schemes technological sophistication. One argument is that people do not want to be involved in operating solar collectors and the complex systems are supposed to be as unobtrusive in their operation as gas central heating.
Another source of passive solar heating is the familiar conservatory or lean-to south facing greenhouse. A conservatory can reduce the heating load of a building in several ways. First the air in the conservatory becomes warm when the sun shines because of the so called ‘greenhouse effect’. The wall against which the conservatory is built will then lose less heat because the temperature difference across its two sides will be less than that between the inside of the house and the outside. In the spring and autumn the conservatory may well become hotter than the inside of the house and there will then be a flow of heat through the wall to the inside of the building. Any heat flow can be improved by putting vents into the house near the top of the conservatory where the hottest air will collect. If the vents have insulated covers they can be opened when the air in the conservatory is warmer than that in the house, thus providing a source of solar-heated air. Finally, the conservatory, even at night when there is no sunshine, will trap a layer of still air against the house. The effect of wind increases the heat loss from the surface of a building so the trapped layer of air will have an insulating effect.
It is not easy to evaluate the contribution made by a conservatory as much will depend on variable factors such as the size of the conservatory, the construction and insulation of the house to which it is attached, the type of glazing and how airtight the conservatory is, as well as the number of hours of sunshine. Estimates of energy contributions range from 10 to 60 per cent of a building’s total space heating demand. The good thing about conservatories is that they are not just solar heaters. You can grow figs, grapes, peaches and apricots as well as tomatoes, aubergines and melons in a conservatory and you can use it as an extension to the house on sunny winter days or wet spring ones. You cannot do any of these with a solar panel.