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Solar energy is the most important source of energy available to the earth and its inhabitants. Without it there would be no life. It is the energy source that drives the photosynthesis reaction. As such, it is responsible for all the biomass on the surface of the earth and is the origin of fossil fuels, the products of photosynthesis millions of years ago and now buried beneath the earth's surface. Solar energy creates the world's winds, it evaporates the water which is responsible for rain; waves and ocean thermal power are both a result of insolation. In fact, apart from nuclear energy, geothermal energy and tidal power, the sun is responsible for all the forms of energy which are exploited by man.

All these different sources of energy, each derived from the sun, can be used to generate electricity. However solar energy can also be used directly to generate electricity. This can be achieved most simply by exploiting the heat contained in the sun's radiation, but electricity can also be generated directly from light using an electronic device called a solar cell. Both methods are valuable renewable sources of electricity.

The solar energy resource

The energy radiated by the sun is around 7% ultraviolet light, 47% visible light and 46% infrared light. Its energy content at the distance of the earth from the sun is around 1.4 kW/m2. Each year around 1500 million TWh of solar energy reaches the earth.

Not all this energy reaches the surface of the earth. Much of the shorter wavelength ultraviolet radiation is absorbed in the atmosphere. Water vapour and carbon dioxide absorb longer wavelength energy while dust particles scatter more radiation, dispersing some of it back into space. Clouds also reflect light back into space.

When all these factors are taken into account, around 47% of the energy, 700 million TWh actually reaches the surface. This is 14,000 times the amount of energy, 50,000 TWh, used by mankind each year. Much of this solar energy strikes the world's oceans and is inaccessible. Even so, with reasonably efficient energy conversion systems, less than 1% of the world's land area would provide sufficient energy to meet global electricity demand, around 15,000 TWh.

Let us put this into a more practical perspective. A group of solar thermal power plants were built in California in the late 1980s and early 1990s. The design of these plants was based on an estimated solar energy input of 2725kWh/m2/year. This is equivalent to 22.75 GWh for each hectare each year. On this basis, assuming a conversion efficiency of 10%,! 10 million ha, or 100,000 km2 (316 km X 316 km), could generate enough energy to supply the entire USA.

This may appear to be a large area but the demand is not onerous. Such an area could be found quite easily, particularly if desert areas were exploited. In fact solar electricity generation takes up less land than most hydropower projects where these include reservoirs. Indeed the land requirements of some hydro schemes can be as much as 50 times a typical solar project yielding the same output.2

But in spite of its enormous potential, global solar electricity generating capacity is tiny. According to European Union estimates, there was probably less than 800 MW of installed capacity in 1995 (including all types of solar generation technologies). Between 1995 and the end of 2003, gross world production of solar cells was around 2600 MW. With little other additional solar capacity, total global solar generating capacity may have been 3400 MW at the beginning of 2004.

Sites for solar power generation

In principle solar power can be generated anywhere on the earth but some regions are better than others. Places where the sun shines frequently and regularly are preferable to regions where cloud cover is common. The brighter the sunlight, the greater the output and the more advantageous the economics of the generating plant. Many of the world's developing countries, where demand for electricity is growing rapidly, offer good conditions for solar electricity generation.

Solar generating stations do not take up enormous amounts of land but they do require many times the space of a similarly sized fossil fuel power plant. But solar power does not necessarily require large contiguous areas of land in order to generate electricity. Solar panels can be made in small modular units which can be incorporated into buildings so that power generation can share space used for other purposes.

Distributed generation of this type has many advantages. In California, and elsewhere, there is a major daytime grid demand peak resulting from the use of air-conditioning systems. As the air-conditioning systems are used to combat heat generated by the sun, distributed solar electricity generation matches this demand perfectly. Recent experience has shown that domestic solar panels virtually eliminate this additional demand from the houses to which they are fitted.3

Solar technology

There are two ways of turning the energy contained in sunlight into electricity. The first, called solar thermal generation, involves using the sun simply as a source of heat. This heat is captured, concentrated and used to drive a heat engine. The heat engine may be a conventional steam turbine, in which case the heat will be used to generate steam, but it could also be a gas turbine or a sterling engine.

The second way of capturing solar energy and converting it into electricity involves use of the photovoltaic or solar cell. The solar cell is a solid-state device like a transistor or microchip. It uses the physical characteristics of a semiconductor such as silicon to turn the sunlight directly into electricity.

The simplicity of the solar cell makes it an extremely attractive method of generating electricity. However the manufacture of the silicon required for solar cells is energy intensive. The solar thermal plant, although more complex is currently cheaper and uses more conventional power station technology.

Whatever its type, a solar power plant has a major weakness. It can only generate electricity when the sun is shining. During the night there is no sunlight and so no electricity. In order to circumvent this problem, a solar power station must either have some form of conventional fuel back-up, or it must incorporate energy storage. Solar cells are frequently coupled with rechargeable batteries in order to provide continuous power in remote locations. Solar thermal power plants can also be designed with heat storage systems which allow them to supply power in the absence of the sun.

Solar thermal power generation

The sun is a source of high-quality heat which can easily be exploited for power generation. This was recognised as early as 1907 when the first patent for a solar collector was granted in Germany to Dr W. Maier. The development of modern solar thermal power technology began in the 1970s and was finally proved in the late 1980s with a series of commercial solar thermal power plants in California.

In spite of the success of these plants no further commercial plants have been built anywhere in the world. Research has continued, however, and interest accelerated at the beginning of the twenty-first century following renewed support from government and international agencies. It seems likely that several new projects will be built before the end of the first decade of the new century.

Modern solar thermal research has concentrated on three different approaches to converting solar energy into electricity. All require sunlight to be collected and concentrated to provide a high-energy source. The first uses a parabolic trough-shaped mirror to focus the energy contained in sunlight onto an energy collector at the focus of the parabola. These parabolic trough solar units can be deployed in massive arrays to provide a large generating capacity.

The second approach, called a solar tower, employs a solar energy collector mounted atop a large tower. A field of mirrors is used to direct sunlight onto the collector where the concentrated heat is used in a power generation system. Both this and the parabolic trough system can be used to build utility-sized power plants. The third system, usually called the solar dish, comprises a parabolic dish with a solar heat engine mounted at its focus. Dishes are usually only 10-50 kW in capacity but can achieve high-energy conversion efficiency. Fields of dishes are needed to produce a high-capacity power plant.

There is also a novel technique being explored in Australia called a solar chimney. This involves building a massive greenhouse, in the centre of which is an extremely tall chimney. The chimney sucks hot air from the greenhouse, creating a massive updraft. Fans or turbines placed inside the chimney can capture energy from this updraft to generate electricity. It is estimated that 40 km2 of greenhouse and a chimney 1000 m high will be needed to generate 200 kW.

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