For householders who are concerned about their carbon footprint and want to reduce their consumption of energy from fossil fuels, solar power can be an attractive alternative.
The two most common ways of using solar power in a domestic context are to provide electricity supplies and to heat water.
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Solar Cell Technology
Solar cells convert sunlight into electricity. Each cell consists of a thin layer of photosensitive material, normally silicon-based, and it can vary in size; most solar cells measure between around one centimetre and ten centimetres. The cells are assembled into photovoltaic panels or modules; again, the size of the module varies tremendously from one application to another.
A solar powered calculator probably has a panel of three or four small cells. A solar panel designed to be attached to a car battery to keep it topped up might contain 30 cells. For a domestic installation, hundreds of modules can be linked together into arrays that consist of thousands of photovoltaic cells (PVs). One PV normally produces between one and two watts, and in theory there is virtually no limit to the number of PVs that can be linked together in a solar panel; however, cost is an issue because to date, the manufacturing costs of the photovoltaic coating have been high.
How PVs Work
Sunlight reaches us by radiation. Radiation travels in electromagnetic waves consisting of photons, and when the energy waves reach an object they can either pass through it, or be reflected, or be absorbed, depending on the material. PVs are made out of a semi-conducting material. When they absorb photons of sunlight, the energy of the photons dislodges electrons from their atoms. The PV’s special coating attracts free electrons to the surface, and it is this movement of electrons that produces the electrical current.
PVs can only produce electricity when sunlight reaches them. To prolong the installation’s exposure to the sun’s rays, the PV arrays are usually mounted facing south. To further maximise performance, more sophisticated installations can include a tracking device that tilts the panels to follow the sun, and it is also possible to use lenses to concentrate sunlight onto the cells. Solar cells are still an emerging technology; the latest solar cells are able to convert a higher proportion of available sunlight into electricity than earlier versions, and efficiency is expected to increase in the future. It is also likely that new manufacturing methods will bring down costs.
Active Solar Thermal Heating
Electricity produced by solar panels can be used to power electrical appliances, and any surplus can be stored in batteries or sold to the National Grid. Photovoltaic installations are clean and quiet, but they take many years to repay the cost of investment. In many cases it is equally or more cost-effective to install a solar thermal heating system to provide domestic hot water.
These systems use solar panels, or collectors, which are installed on the roof. Flat plate systems consist of an absorber plate in a transparent casing. The plate can be as simple as a sheet of metal, painted black, so in terms of cost, this is massively cheaper than a photovoltaic panel installation. Water is circulated through a system of tubes that are in contact with the plate. The tubes are often made of copper, which is a good conductor, and the heat passes from the plate, through the tube, to the water.
A more sophisticated method of heat collection is the evacuated tube system, which utilises a series glass tubes containing absorber plates. No matter what means is employed to collect the heat, it is then transferred to water, and the hot water is stored in an insulated tank until needed. In favourable conditions, a solar water heating system can provide up to half of a household’s hot water needs over the year. There are many possible variations in design, and because the basic principle of using the thermal energy of sunlight is so simple, it is relatively easy for a DIY enthusiast to build an effective solar thermal heating system at a modest cost.
Environmental Benefits of Active Solar Installations
Photovoltaic panels and solar heating systems can be fitted to existing properties, or can be incorporated into new builds. Their productivity is dependent on the quantity of sunshine received, and in evaluating their environmental benefits the amount and type of energy used in manufacturing the equipment must be taken into account. However, solar power is fully renewable and completely non-polluting, so an effective active solar installation used as an alternative to non-renewable energy sources will certainly help combat climate change.
