Solar Thermal Electric Energy

The use of solar thermal for the production of electricity has been demonstrated at several places. Solar I is a 10-megawatt plant located near Barstow, California. It has 1818 heliostats with a total area of 71,130 square meters and is one of the largest solar thermal plants in the world. 81 The heliostats track the sun and reflect the solar energy into a central receiver. Here the highly concentrated solar energy boils water, making steam to run the turbines. The power varies as the angle of the sun changes throughout the day. When the sun is low in the sky, the heliostats shade one another. The shading reduces the time the plant can produce energy to about 75% of the daylight hours.

Hellemans, Alexander, "Solar Homes for the Masses", Science, Vol. 285, No. 5428, July 30, 1999, Page 679

"Energy In Transition 1985 - 2010", The National Research Council of the National Academy of Sciences, Washington C. C. 1979

http://www.energy.sandia.gov/sunlab

The Solar Dish Stirling Electric Generating Module at Huntington Beach, California, has 82 curved glass mirrors arranged to form a parabolic collection surface with a total area of 90 square meters. On a clear day, this unit will produce 25-KWe (Kilo Watt electric). There is a similar plant at Santa Rosa substation in Palm Springs, California. It uses an 87 square meter parabolic collector and a Stirling engine to produce electricity. This installation has set records of 30% peak and 20% continuous for the conversion of sunlight to electricity during cloud free daylight hours.

These solar plants are being tested to evaluate their desirability. One area of concern is how the output from the plants can be integrated into the electric power grid. The midday peak of production does not mesh well with the peak demand that occurs late in the afternoon. On some days, clouds prevent any operation. On other days the power drops quickly when clouds drift in front of the collectors and comes back abruptly when the clouds drift on past. These transients cause dramatic thermal expansion and contraction shocks to the system. Accommodating for these shocks produces design challenges and reduces the system lifetime.

These abrupt changes also cause a large drop in the output power. Even when they are only a small part of the total power in the grid, these transients are difficult to handle. If the solar energy makes up a large part of the power input to the grid, transients become impossible to handle. The sporadic nature of solar energy limits its use to situations in which it only supplies a small part of the total energy or to systems that have significant storage capability.

Another concern is keeping the solar collectors clean. Grime and dust deposited on the mirror surface can rapidly reduce the amount of energy reflected. This in turn decreases the output of the plant. The collectors require windshield wipers to perform frequent cleaning; cleaning and/or windshield wiper maintenance causes a significant increase in operating cost.

Solar thermal technology has many problems. These are, cost of facilities, large areas of land required for the hardware, unreliability of sunlight, and potential high continuous maintenance cost for cleaning. It is clear that the cost of energy must increase significantly before solar thermal plants become economical. Even when other fuel costs become high enough that solar is economical, the intermittent energy production of the solar collectors will make their use very inconvenient.

It should be noted that all of the demonstration plants are located in the southwest United States where solar energy is more reliably available than in other parts of the country. These southwest locations are far from the major markets. Plants built close to the major energy markets in the north and east will be more costly and less effective because the weather is less favorable for solar energy collection. In the northern states, during the winter, weeks may pass without any direct sun light.

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