Solar thermal electricity

In areas where there is substantial sunshine, solar energy can be used to generate electricity in a number of ways. One method which has been successfully demonstrated is the solar chimney.

Designed primarily for desert locations it consists of a tall column surrounded by a glass solar collector. In effect it is a chimney surrounded by a huge solar collector or greenhouse. The air is heated by the circular greenhouse and drawn through the chimney which acts as a thermal accelerator. Within the chimney are one or more vertical axis turbines. A prototype has been built in Manzanares, Spain, with a 195 metre tower served by a greenhouse collector 240 metres in diameter. The collector warms the air by around 17°C creating an updraught of 12 metres per second giving an output of 50 kilowatts (Figure 4.1).

The project has demonstrated the viability of the principle and plans are being drawn up for a giant version in Mildura, Australia. The economics suggest that the tower would produce about 650 gigawatt hours per year or enough to serve a population of 70 000. The tower will be 1000 metres high with a solar collector of glass and plastic 7 kilometres across. The updraught would be about 15 metres per second or 54 km/hr and will drive 32 turbines at the base. The outer areas of the collector, where the temperature would be near the ambient level, would be used to grow food. The plant would operate over night by using daytime heat to warm underground water pipes connected to an insulated chamber, returning heat to the surface of the collector during the night. The scheme would carry low maintenance costs and would have a life expectancy of 100 years. Construction of the tower will consume an estimated 700 000 m3 of high strength concrete. A lookout gallery at the top of the tower promises to be a not-to-be-missed tourist attraction (see New Scientist, 31 July 2004, pp. 42-45).

The Almeria region of Spain is the sunniest location in Europe, achieving about 3000 hours of sun a year. This is why the area has been chosen to demonstrate another technology for producing electricity called the SolAir project. In essence it produces superheated steam to drive a turbine.

The idea has been made possible by the development of ceramics that can tolerate high temperatures. At ground level 300 large mirrors or heliostats each 70 m2 track the passage of the sun and focus its rays on a silicon carbide ceramic heat absorber. The surface of the absorber reaches 1000°C. Air blown through its honeycomb structure reaches 680°C. The hot air travels down the absorber tower to a heat exchanger where it generates steam to drive a conventional turbine. The system produces up to 1 megawatt of electricity. The ceramic is also able to store heat to compensate for cloudy conditions.

According to the Spanish Ministry of Science: 'In five to ten years' time there should be several plants across Europe, each 15 to 20 times larger than the demonstration plant and together generating hundreds

Figure 4.1

Solar chimney generator

Figure 4.1

Solar chimney generator of megawatts' (New Scientist,'Power of the midday sun', 10 April 2004). At current prices it is expected to produce electricity at one third the price of photovoltaics. Plans are already in place to locate these plants along the Algerian coast to export electricity to Europe. Egypt is also warming to the possibility of this new export opportunity.

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