Scientists have known for a long time that the sun's beams carry an enormous amount of energy down to earth in the forms of light and heat. However, it's been a challenge to tap this energy and efficiently convert it into usable electricity. Photovoltaic (PV) panels, for instance, can convert the sun's light into electricity to power a household or building to some extent, but remain comparatively expensive or inefficient.
Some believe that solar thermal technology, however, is the Holy Grail for cheap, clean, renewable energy. Whereas fossil fuels may lead to global climate change and are becoming more expensive, solar thermal energy is clean and might soon become competitively priced. By converting the sun's heat into usable electricity, this technology could help wean countries off fossil fuels.
Solar thermal power doesn't convert solar energy directly to electricity like PV panels do. Instead it does so indirectly -- often using heat to turn water to steam. And, much like in fossil-fueled power plants, this steam can turn a turbine that powers an electric generator.
A few different systems can collect solar heat:
- Solar tower: When surrounded by heliostats, which are special mirrors that track the sun, a solar tower can collect the sun's energy. The central receiver at the top of the tower collects and stores heat in special liquid salts that are transported from the tower to a steam generator.
- Parabolic troughs: This kind of system uses the shape of parabola to intensify heat and collect it from the sun no matter what time of day. Tubes filled with a special fluid run through this system. As the special fluid travels by, it warms up and essentially "collects" the heat. The fluid then carries its heat to the water, which converts to steam. What's special about the fluid is that it is an oil that can reach temperatures of 750 degrees Fahrenheit (400 degrees Celsius) [source: Biello]. Molten salts can store extra heat for use when the sun isn't available.
- Fresnal reflector: Instead of heating fluid, this mechanism heats water directly. It can do this efficiently by increasing the atmospheric pressure. This steam can then go on to power a turbine.
- Solar dish: These move with the sun to collect heat all day. Using mirrors, they reflect sunlight onto a focal point. They then convert this heat to mechanical power by heating a compressed fluid. Then, expanding that hot compressed fluid can operate a piston and create useful energy.
The California Mojave Desert is home to power plants that use parabolic troughs as well as solar towers. Various countries, such as Spain, Morocco and Israel, are building or planning to build solar thermal power plants as well [source: Wald].
How can plants like these help countries that don't get much sun? Some say it's quite feasible to install a system of transmission lines that would carry high-voltage direct current long distances. This way, countries rich with sun could erect solar thermal power plants and send this power to places that don't get as much sun.
As the technology develops, some believe it will soon become cheap enough to compete with fossil fuels, especially with the help of high taxes on carbon emissions. Specifically, this might work if the price of solar thermal power comes down to about 10 cents per kilowatt-hour [source: Biello].
Browse the resources on the next page to learn more about renewable energy.