Renewable Energy - Energy for the Future

The Sun is a massive nuclear furnace radiating energy into space. One thousandth of a millionth of the Sun's output of around 400 000 000 000 000 000 000 000 000 watts is intercepted by the Earth. About 30 per cent of this energy is reflected back into space. The rest is absorbed by the atmosphere, land and oceans, or in the evaporation, convection and precipitation of water.

The amount of solar energy absorbed by the Earth and its atmosphere in just one year is equivalent to one hundred times the energy stored in the world's proven reserves of fossil fuels. If we could capture just one ten thousandth of this energy, using solar collectors, specially designed buildings, wind and water turbines, wave energy converters, trees and other fuel crops, we could supply more useful energy in a year than we currently get from burning coal, oil and gas. Unlike fossil fuels, renewable energy cannot be exhausted and does not produce polluting emissions that contribute to serious environmental threats, such as climate change and acid rain.

Solar power

Solar collectors are widely used for domestic hot water in a number of countries. The highest concentration is in Japan, with over 1.5 million in Tokyo alone; there is also a very active market in Australia. Solar energy provides 3 per cent of primary energy use in Israel, where two thirds of homes have solar water heaters.

Despite its northern latitude, the UK receives roughly half as much solar energy as some of the sunniest parts of the world. In theory, there is sufficient solar energy in the UK to provide all the space and water heating of well-designed dwellings. In practice, seasonal swings in availability make year-round solar heating impractical. The fact that about half of the incoming solar energy in the UK is in the form of scattered, diffuse light which cannot be focused using mirrors and lenses rules out building large-scale thermal solar power stations like those now working successfully in California, but there are other things we can do.

Solar Architecture (passive solar)

Buildings with large south-facing and small north-facing windows and high levels of thermal insulation can trap the Sun's energy and store it as heat within the structure. This 'free heat' is then distributed around the building by the processes of conduction, convection and thermal radiation, keeping the inside comfortable when the outside temperature falls. It is more economic to incorporate passive solar design at the construction stage. Over the past decade, hundreds of thousands of buildings incorporating these so-called 'passive solar' features have been erected (in the UK the highest concentration is in Milton Keynes). Integrating passive solar design with energy efficiency measures can reduce energy consumption in new housing for both heating and

lighting. Heating bills can be cut by more than half for little extra cost. For existing buildings, passive solar features, including conservatories and glazed courtyards can be fitted during major rehabilitation work.

Solar Water Heating (active solar)

Solar heating panels fitted to the outside of buildings capture the Sun's heat and heat water for domestic use. A matt black metal surface covered by glass and insulated absorbs and contains the sun's energy. This heat is transferred to water flowing through pipes attached to the metal plate and this hot water is then used to heat a normal cylinder in a domestic hot water system, or to heat swimming pools or other low-temperature loads. Solar water heating is usually installed in conjunction with a conventional heating system as in the UK it is unable to provide all hot water necessary for a household's use. Over 40,000 panels have been installed in the UK to date, but for many costs are prohibitively high. There is, however, considerable potential for cost reduction through design improvement and mass production of advanced systems. In future it may be possible to provide year-round solar heating for small groups of dwellings by coupling up banks of solar collectors toa sunken interseasonal heat store. Solar energy could also make an input into large district heating schemes.

Photovoltaics (solar cells)

Photovoltaics (PVs) convert sunlight into electricity. They are made from silicon, the second most abundant element in the Earth' crust (although it should be noted that some exotic and sometimes toxic materials are used in manufacture). The cells are fitted in panels to the exterior of buildings and are clean, safe and silent in operation. Although at the moment they are an expensive method of generating electricity for the grid, they compete very successfully with conventional systems for a wide range of applications in remote locations, and are being used more and more in the developing world. For example, solar cells provide power for refrigeration and water pumping in hot desert regions, and for telecommunication equipment on mountain tops.

The cost of photovoltaic modules has fallen dramatically in recent years - by a factor of ten between 1975 and 1990, and one Japanese/American company plans to market a new type of module which is predicted to halve the (1993) cost of photovoltaic electricity. Increased funding for research and demonstration projects will reduce costs further. At the moment, the best option for reducing overall generation costs seems to rest in integrating PV modules into building design. In fact, it has been shown that PVs are already economic if they substitute for polished stone (such as marble) in building facades. The Oxford Solar House was designed to use as little energy as possible and to investigate the potential of building integrated photovoltaics to contribute cost-effectively to domestic electricity supply. The building costs (around £800/m2) were comparable with typical architect designed homes in the UK, and over the first year the house was a net exporter of electricity to the grid. Further, new developments include the manufacture of photovoltaic roofing 'shingles' to replace conventional tiles.

Contacts:

The UK Solar Energy Society - c/o School of Engineering, Oxford Brookes University
Gypsy Lane Campus, Headington
Oxford, OX3 0BP, United Kingdom
Tel: (+44) 1865 484 367
Fax: (+44) 1865 484 263

IT Power - for PVs projects in the developing world

Building Research Establishment - for passive solar design