Solar thermal power systems use mirrors and optical devices to redirect, focus, and concentrate the Sun's rays onto a receiver, where heat is generated. The first application of this technique was recorded in 212 bc, when Archimedes is said to have used mirrors to burn Roman ships attacking Syracuse. Today, the thermal energy can than be used to produce steam-driven generators for electricity production, or to initiate chemical reactions. The concentration of sunlight may be achieved with different technologies, including parabolic troughs, solar power towers and parabolic dishes differing in the way in which they collect solar radiation. Parabolic trough collector systems are commercially available and are the least expensive solar thermal technology. They use single-axis sun-tracking parabolic mirrors to focus sunlight onto a glass tube which runs in the trough and contains a heat-transfer fluid that reaches temperatures up to 400 °C and, via an heat exchanger, generates steam to drive an electric generator. Nine plants, ranging from 10 to 80 MW have been installed in the Southern California Mojave Desert since 1991, providing a total capacity of 354 MW of peak electricity to the grid . Other projects are under way in several countries such as Spain, which has tested at the Plataforma Solar de Almeria, the possibility of using superheated steam directly as the heat-transfer fluid (Fig. 8.11).
One commercially less mature technology than parabolic trough is that of solar towers, which use many large mirrors called heliostats that track the path of the Sun throughout the day and focus the rays on the solar receiver used to heat a
working fluid, typically molten salt, to generate steam and produce electricity. Because solar energy is concentrated on a point rather than a line in the case of parabolic trough, the temperature generated is higher; in a range from 500 to 1500 °C. The Solar One and Solar Two projects, which were also operated in the Mojave Desert until 1999, demonstrated the technology at the 10-MW pilot-scale.
Smaller solar towers were also field tested in different countries, including Italy, France, Spain, and Japan, not only for electricity generation but also other applications requiring high temperatures (such as methane production and material testing) . Solar dishes, which resemble satellite dishes, use parabolic mirrors that track the Sun in two axes, concentrate the sunlight at their focal point, and generate temperatures similar to those of solar towers. In the most common approach, the concentrated solar energy is used to heat the hydrogen or helium working fluid of a Stirling engine . The generated mechanical energy is then converted to electrical energy. On average, the dishes are between 8 and 10 m in diameter, though some can be much larger, like the "Big Dish" in Australia with a surface area of 400 m2. Existing units are small, producing between 10 and 25 kW. Like PV set-ups, they can be installed in large groups to serve utility needs, or in smaller numbers for decentralized energy generation. Stirling Energy System recently announced its plan to construct the largest solar installation in the world, based on solar dishes, in the Mojave Desert in California. When completed in 2010, the facility will be composed of 20 000 25 -kW solar dishes with a production capacity of 500 MW, comparable to a typical fossil fuel power plant [57, 58]. In addition, a 300-MW plant containing 12 000 solar dishes will also be constructed to provide electricity to the city of San Diego, California. Of all solar technologies, solar dish/engine systems have demonstrated the highest solar to electric conversion efficiency at 29%, compared to around 20% for other solar thermal technologies . With receiver temperatures in excess of 1000 °C for solar tower or solar dish technology, processes to generate hydrogen thermochemically could also be exploited and are under investigation. The production cost of electricity with a parabolic trough, the most mature technology, is still high at $0.10-0.15 per kWh for the best power plants . In order to bring solar thermal power generation to the market, the key cost reduction issue is currently being studied through research and technological development in numerous projects around the world. Solar thermal power plants are currently the most likely to become the first large scale solar electricity producers.
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Do we really want the one thing that gives us its resources unconditionally to suffer even more than it is suffering now? Nature, is a part of our being from the earliest human days. We respect Nature and it gives us its bounty, but in the recent past greedy money hungry corporations have made us all so destructive, so wasteful.