Solar thermal power plants are electricity generation plants that utilize energy from the Sun to heat a fluid to a high temperature. This fluid then transfers its heat to water, which then becomes superheated steam. This steam is then used to turn turbines in a power plant, and this mechanical energy is converted into electricity by a generator. This type of generation is essentially the same as electricity generation that uses fossil fuels, but instead heats steam using sunlight instead of combustion of fossil fuels. These systems use solar collectors to concentrate the Sun's rays on one point to achieve appropriately high temperatures.
There are two types of systems to collect solar radiation and store it: passive systems and active systems. Solar thermal power plants are considered active systems. These plants are designed to operate using only solar energy, but most plants can use fossil fuel combustion to supplement output when needed.
Despite the fact that there are several different types of solar thermal power plants, they are all the same in that they utilize mirrors to reflect and concentrate sunlight on a point. At this point the solar energy is collected and converted to heat energy, which creates steam and runs a generator. This creates electricity.
These troughs, also known as line focus collectors, are composed of a long, parabolic shaped reflector that concentrates incident sunlight on a pipe that runs down the trough. The collectors sometimes utilize a single-axis Solar tracking system to track the Sun across the sky as it moves from east to west to ensure that there is always maximum solar energy incident on the mirrors. The receiver pipe in the center can reach temperatures upward of 400°C as the trough focuses Sun at 30-100 times its normal intensity.
These troughs are lined up in rows on a solar field. A heat transfer fluid is heated as it is run through the pipes in the parabolic trough. This fluid then returns to heat exchangers at a central location where the heat is transferred to water, generating high-pressure superheated steam. This steam then moves a turbine to power a generator and produce electricity. The heat transfer fluid is then cooled and run back through the solar field.
These are large parabolic dishes that use motors to track the Sun. This ensures that they always receive the highest possible amount of incoming solar radiation that they then concentrate at the focal point of the dish. These dishes can concentrate sunlight much better than parabolic troughs and the fluid run through them can reach temperatures upwards of 750°C.
In these systems, a Stirling engine coverts heat to mechanical energy by compressing working fluid when cold and allowing the heated fluid to expand outward in a piston or move through a turbine. A generator then converts this mechanical energy to electricity.
Solar power towers are large towers that act as a central receiver for solar energy. They stand in the middle of a large array of mirrors that all concentrate sunlight on a point in the tower. These large number of flat, sun tracking mirrors are known as heliostats. In the tower, there is a mounted heat exchanger where the heat exchange fluid is warmed. The heat concentrated to this point can be 1500 times as intense as incident sunlight. The hot fluid is then used to create steam to run a turbine and generator, producing electricity. One drawback with these towers is they must be very large to be economical.
Because these systems can generate steam of such high temperatures, the conversion of heat energy to electricity is more efficient. As well, these plants get around the issue of being unable to efficiently store electricity by being able to store heat instead. The storage of heat is more efficient and cost-effective than storing electricity.
Additionally, these plants can produce dispatchable baseload energy, which is important as it means these plants produce a reliable amount of energy and can be turned on or up at will, meeting the energy demands of society. In addition to this, solar thermal power plants represent a type of electricity generation technology that is cleaner than generating electricity by using fossil fuels. Thus, these are some of the cleanest options for generating electricity. Despite this, there are still associated environmental effects of these plants as a full life cycle analysis can show all associated carbon dioxide emissions involved in the building of these plants. However, emissions are still much lower than those associated with fossil fuel plants.
Some of the drawbacks include the large amount of land necessary for these plants to operate efficiently. As well, the water demand of these plants can also be seen as an issue, as the production of enough steam requires large volumes of water. A final potential impact of the use of large focusing mirrors is the harmful effect these plants have on birds. Birds that fly in the way of the focused rays of Sun can be incinerated. Some reports of bird deaths at power plants such as these amount the deaths to about one bird every two minutes.
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