SOLAR THERMAL SYSTEMS

Solar energy can also be used to make electricity.

Some solar power plants, like the one in the picture to the right in California's Mojave Desert, use a highly curved mirror called a parabolic trough to focus the sunlight on a pipe running down a central point above the curve of the mirror. The mirror focuses the sunlight to strike the pipe, and it gets so hot that it can boil water into steam. That steam can then be used to turn a turbine to make electricity. In California's Mojave desert, there are huge rows of solar mirrors arranged in what's called "solar thermal power plants" that use this idea to make electricity for more than 350,000 homes. The problem with solar energy is that it works only when the sun is shining. So, on cloudy days and at night, the power plants can't create energy. Some solar plants, are a "hybrid" technology. During the daytime they use the sun. At night and on cloudy days they burn natural gas to boil the water so they can continue to make electricity. Another form of solar power plants to make electricity is called a Central Tower Power Plant, like the one to the right - the Solar Two Project. Sunlight is reflected off 1,800 mirrors circling the tall tower. The mirrors are called heliostats and move and turn to face the sun all day long.The light is reflected back to the top of the tower in the center of the circle where a fluid is turned very hot by the sun's rays. That fluid can be used to boil water to make steam to turn a turbine and a generator. This experimental power plant is called Solar II. It was re-built in California's desert using newer technologies than when it was first built in the early 1980s. Solar II will use the sunlight to change heat into mechanical energy in the turbine. The power plant will make enough electricity to power about 10,000 homes. Scientists say larger central tower power plants can make electricity for 100,000 to 200,000 homes.

 Solar energy technologies are poised for significant growth in the 21st century. More and more architects and contractors are recognizing the value of passive solar and learning how to effectively incorporate it into building designs. Solar hot water systems can compete economically with conventional systems in some areas. And as the cost of solar PV continues to decline, these systems will penetrate increasingly larger markets. In fact, the solar PV industry aims to provide half of all new U.S. electricity generation by 2025.

Aggressive financial incentives in Germany and Japan have made these countries global leaders in solar deployment for years. But the United States is catching up thanks particularly to strong state-level policy support. The rolling blackouts and soaring energy prices experienced by California in 2000 and 2001 have motivated its leaders to create new incentives for solar and other renewable energy technologies. In January 2006, the California Public Utility Commission approved the California Solar Initiative, which dedicates $3.2 billion over 11 years to develop 3,000 megawatts of new solar electricity, equal to placing PV systems on a million rooftops.

In the 1890s solar water heaters were being used all over the United States. They proved to be a big improvement over wood and coal-burning stoves. Artificial gas made from coal was available too to heat water, but it cost 10 times the price we pay for natural gas today. And electricity was even more expensive if you even had any in your town! Many homes used solar water heaters. In 1897, 30 percent of the homes in Pasadena, just east of Los Angeles, were equipped with solar water heaters. As mechanical improvements were made, solar systems were used in Arizona, Florida and many other sunny parts of the United States. The picture shown here is a solar water heater installed on the front roof of a house in Pomona Valley, California, in 1911 (the panels are circled above the four windows). By 1920, ten of thousands of solar water heaters had been sold. By then, however, large deposits of oil and natural gas were discovered in the western United States. As these low cost fuels became available, solar water systems began to be replaced with heaters burning fossil fuels. Today, solar water heaters are making a comeback. There are more than half a million of them in California alone! They heat water for use inside homes and businesses. They also heat swimming pools like in the picture. Panels on the roof of a building, like this one on the right, contain water pipes. When the sun hits the panels and the pipes, the sunlight warms them. That warmed water can then be used in a swimming pool.

Other states are following suit. Arizona, Colorado, New Jersey, and Pennsylvania have specific requirements for solar energy as part of their renewable electricity standards. Many more states offer rebates, production incentives, and tax incentives, as well as loan and grant programs. Even the federal government is offering a 30 percent tax credit (up to $2,000) for the purchase and installation of residential PV systems and solar water heaters.

As the solar industry continues to expand, there will be occasional bumps in the road. For example, demand for manufacturing-quality silicon from the solar energy and semiconductor industries has led to shortages that have temporarily driven up PV costs. In addition, some utilities continue to put up roadblocks for grid-connected PV systems. But these problems will be overcome, and solar energy will play an increasingly integral role in ending our national dependence on fossil fuels, combating the threat of global warming, and securing a future based on clean and sustainable energy.

By using mirrors and lenses to concentrate the rays of the sun, solar thermal systems can produce very high temperatures—as high as 3,000 degrees Celsius. This intense heat can be used in industrial applications or to produce electricity.

Solar concentrators come in three main designs: parabolic troughs, parabolic dishes, and central receivers. The most common is parabolic troughs—long, curved mirrors that concentrate sunlight on a liquid inside a tube that runs parallel to the mirror. The liquid, at about 300 degrees Celsius, runs to a central collector, where it produces steam that drives an electric turbine.

Parabolic dish concentrators are similar to trough concentrators, but focus the sunlight on a single point. Dishes can produce much higher temperatures, and so, in principle, should produce electricity more efficiently. But because they are more complicated, they have not succeeded outside of demonstration projects.

A more promising variation uses a stirling engine to produce power. Unlike a car's internal combustion engine, in which gasoline exploding inside the engine produces heat that causes the air inside the engine to expand and push out on the pistons, a stirling engine produces heat by way of mirrors that reflect sunlight on the outside of the engine. These dish-stirling generators produce about 30 kilowatts of power, and can be used to replace diesel generators in remote locations.

The third type of concentrator system is a central receiver. One such plant in California features a "power tower" design in which a 17-acre field of mirrors concentrates sunlight on the top of an 80-meter tower. The intense heat boils water, producing steam that drives a 10-megawatt generator at the base of the tower. The first version of this facility, Solar One, operated from 1982 to 1988 but had a number of problems. Reconfigured as Solar Two during the early to mid-1990s, the facility is successfully demonstrating the ability to collect and store solar energy efficiently. Solar Two's success has opened the door for further development of this technology.

To date, the parabolic trough has had the greatest commercial success of the three solar concentrator designs, in large part due to the nine Solar Electric Generating Stations (SEGS) built in California's Mojave Desert from 1985 to 1991. Ranging from 14 to 80 megawatts and with a total capacity of 354 megawatts, each of these plants is still operating effectively.

As a result of state and federal policies and incentives, more commercial-scale solar concentrator projects are under development. Modified versions of the SEGS plants are being constructed in Arizona (one megawatt) and Nevada (65 megawatts). In addition, Stirling Energy Systems received approval from the California Public Utility Commission in October 2005 to build a 500-megawatt facility (with the option to add 350 megawatts) in the Mojave Desert using the parabolic dish design. Beginning in January 2009, the plant will supply power to Southern California Edison under a 20-year contract that will help the utility meet its requirements under the state's renewable electricity standard.