Solar energy

Solar power is tapped directly from the sun; which has an intensity on a clear day of 1,000 W / m² on the ground with the sun directly overhead.

Many ways of tapping solar power have been developed. Solar heating helps heat buildings and provide hot water. Just a few south-facing windows with summer-shading can substantially reduce heating bills in the winter, without overheating a building in the summer.

Invented by Horace de Saussure, a Swiss naturalist experimenting as early as 1767, a solar box cooker traps the sun's power in an insulated box; for cooking, pasteurization and fruit canning.

Solar cells convert sunlight to electrity using the photoelectric effect. Solar energy can be used to heat fluids to drive turbines in electic generators. Plants use photosynthesis to convert solar energy to chemical, which can later be burned as fuel to generate electricity.

Electricity generation by solar power has several difficulties, as it is diffuse and intermittent. First, solar input is interrupted by night and by cloud cover, which means that solar electric generation inevitably has a low capacity factor, typically less than 15%. Also, there is a low intensity of incoming radiation, and converting this to high grade electricity is still relatively inefficient (12 - 16 percent), though this has been the subject of much research over several decades.

One method of solar utilization utilises sunlight acting on photovoltaic (PV) cells to produce electricity. This has application on satellites, earthbound signalling and communication equipment, such as remote area telecommunications equipment, off-grid installations such as remote homes and traffic signals, and low power applications like calculators and garden lighting. Sales of solar PV modules are increasing strongly as their efficiency increases and price decreases.

For a stand-alone system some means must be employed to store the collected energy during hours of darkness or cloud - either as electricity in batteries, or in some other form such as hydrogen (produced by electrolysis of water) or superconductors. In either case, an extra stage of energy conversion is involved with consequent energy losses, thus lowering overall net efficiency, and greatly increasing capital costs.

Several experimental PV power plants mostly of 300 - 500 kW capacity are connected to electricity grids in Europe and USA. Japan has 150 MWe installed. A large solar PV plant is planned for Crete. Research continues into ways to make the actual solar collecting cells less expensive and more efficient. Other major research is investigating economic ways to store the energy which is collected from the sun's rays during the day.

A solar thermal power plant has a system of mirrors to concentrate the sunlight on to an absorber, the energy then being used to drive turbines. The concentrator is usually a parabolic mirror trough oriented north-south, which tracks the sun's path through the day. The absorber is located at the focal point and converts the solar radiation to heat (about 400oC) which is transferred into a fluid such as synthetic oil or sulfur. The fluid drives a conventional turbine and generator. Several such installations in modules of 80 MW are now operating. Each module requires about 50 hectares of land and needs very precise engineering and control. These plants are supplemented by a gas-fired boiler which generates about a quarter of the overall power output and keeps them warm overnight. Over 350 MWe capacity worldwide has supplied about 80% of the total solar electricity to the mid 1990s.

An important role of solar energy in the future will be that of direct heating. Much of our energy need is for heat below 60 degrees C - eg. in hot water systems. A lot more, particularly in industry, is for heat in the range 60 - 110 degrees C. Together these may account for a significant proportion of primary energy use in industrialised nations. The first need can readily be supplied by solar power much of the time in some places, and the second application commercially is probably not far off. Such uses will diminish to some extent both the demand for electricity and the consumption of fossil fuels, particularly if coupled with energy conservation measures such as insulation.

With adequate insulation, heat pumps utilising the conventional refrigeration cycle can be used to warm and cool buildings, with very little energy input other than from the sun in some climates.

"Solar not nuclear" is a catch-cry of both antinuclear environmental groups and many technological optimists, particularly as advances in direct solar heating continue to be made.