As solar thermal power technology is such a simple process, it is truly astounding it has taken so long to develop. The method is based on practices and principles that have been around for ages. Yet, only at the turn of the millennium has it found significance for a brilliant future.
Solar thermal power concentrates solar energy by the use of mirrors and magnifiers to turn water to steam. The steam drives a turbine which in turn rotates a generator to create electricity—the same method used by conventional fossil fuel and nuclear power plants. The only difference is in the way steam is created. Simple! But there is more—the crowning glory is energy storage so that when the sun goes down electricity can still be produced.
The most frequently encountered and most efficient types of solar energy concentrators are the parabolic trough and the power tower. In solar thermal power, there is also occasional mention of the less efficient Fresnel reflector and a gaseous dish/engine (or Stirling engine) that is unable to store energy, and hybrids and thermal storage units are evolving technologies—expanding into the future.
The Parabolic Trough
A parabolic trough captures the sun’s energy using large parabolic-shaped reflectors to focus the sun’s energy onto a series of collection tubes. A parabola is like a cylinder that has been cut lengthwise. These energy collectors make up a collection field that consists of large numbers of collection tubes with associated reflectors that are aligned parallel with a north-south orientation and in a row. With a single-axis sun tracking system, the parabola can be rotated east and west to track the sun during the day.
Receiver tubes contain water that is heated by the sun which is focused on the collection tubes. The water is converted to steam, and the steam exits the other side and is conveyed to the turbine for production of electricity. Conversion of water directly to steam is the easiest, cheapest process, but it does not allow for “energy storage.” Wherein storage may be desirable, a special heat-transfer fluid is heated and conveyed to a heat exchanger so water can be converted to steam for use in generating electricity.
Currently, the largest “stand-alone” solar thermal power plant, parabolic trough type, is Solar One in Nevada with a capacity of 64 MW. The power plant covers 400 acres of land. It has 18,240 receivers and generates up to 134 million kWh of electricity per year. This is enough to power about 15,000 U.S. households annually. Allegedly, there is another parabolic trough power plant being built that has an anticipated capacity of 250 MW. Sounds good? Well, it gets better!
Not only is the parabolic trough the best solar thermal and photovoltaic electricity producer, but capacity can be enhanced by a system of what is called “power parks.” Power parks are several stand-alone power plants in an expanded area. Parabolic trough power parks, or collectives, are constrained only by land availability and transmission line capacity. At this time, the largest collective power park in the world is in the Mohave Desert. It has a capacity of 354 MW. If the output is similar, calculated on the basis of power supplied by Solar One in Nevada, the collective in the Mohave Desert would supply about 58,593 homes—approximately 6 percent the projected homes of Los Angeles.*
At first flush, the most promising form solar thermal electrical power and solar thermal power did appear good. But, based on the numbers, just to supply the City of Los Angeles with sufficient power for all households, 17 of the Mohave Desert collectives would be necessary. This is a monumental task, and the power plants would require a tremendous dedication of land. Still, they would require reliable fossil fuel or nuclear power plants for sustainability.
The Power Tower
A power tower focuses the sun’s energy to receiving unit at the top of tower through the use of large, flat, sun-tracking mirror assemblies, known as heliostats. The appearance from a distance is that of a large plate with a tower in the center. The heat from all the mirrors is focused on a central tower which contains water or a heat-transfer fluid. The water is converted to steam, and the heat-transfer fluid goes to a heat exchange unit to convert water to steam. Once, again, the steam generates electricity the old fashion way—by conventional steam run generators.
The world’s largest power tower came on-line in April 2009 and had a power capacity of 20 MW. This capacity is considerably less than that of power towers, and there are no power parks. Although they are alleged to have high solar-to-electrical conversion efficiencies, the tread in new construction in Spain is to install almost all parabolic trough systems. In the United States, the trend swings more toward parabolic trough systems than the power tower plants, but there are a few Fresnel reflectors and Stirling engines.
The Fresnel Reflector
The Fresnel reflector is a linear collector, similar to the parabolic trough. Flat, or slightly curved mirrors are mounted on trackers on the ground, below the collection tube, reflecting solar energy “up” to the collectors. A mall parabolic mirror is sometimes added on top of the tubes to double down on the capturing the sun’s energy.
The largest one, to date, is in Bakersfield, California with a capacity of 5 MW. This is hardly worth writing home about!
The Stirling Engine
The Stirling engine does not rely on conventional steam power. This type of solar thermal power is more direct. It involves a dish-shaped mirror to concentrate solar energy onto a central power conversion unit. The dish is mounted on a structure that tracks the sun continuously throughout the day, and the power conversion unit contains heat-transfer fluid, or gas, and an engine. When the fluid, or gas, heats up it expands. Pressure is created by the expanding gas which drives a piston, crankshaft, and drive assembly much like an automotive engine without the ignition spark. The engine pistons move and produce power in the form of electricity.
This process typically produces 3 to 25 kilowatts per unit. To compare this with the parabolic trough and power towers is like comparing a mosquito to an elephant. There is no comparison.
Heat Storage and Hybrids
Once the site of a power plant has been effectively located, the greatest challenge facing widespread use of solar energy is that the sun is not always available. The sun rises and sets. It goes into hiding at night. It is obscured by environmental conditions. Yet, we require electricity without conditions. Blackouts are not acceptable. A challenge deserves a solution.
One effective solution that is available only to solar thermal power plants, not to photovoltaic power, is heat storage. In most commonly used solar thermal power plants, oil and molten salt heat-transfer fluid can retain heat when stored properly and when created in excess. In other words, create more than needed, and store the rest. Anticipate solar down time and oversize accordingly. This works well in theory, in a perfect world, maybe sometimes in a not so perfect world. Occasionally, all worse case scenarios will and do break down, and it is during these circumstances that reliable fossil fuel and nuclear power plants must support renewable electricity.
Solar thermal power plants can be and are being designed as “hybrids.” The hybrid facilities are easily supplemented by fossil fuel steam generating facilities. Hybrid plants are being integrated into new designs, and renewable energy plants are being integrated with those already existing.
The dynamic duo—solar and wind—have been also been considered complimentary hybrids. Two reliable resources do not a reliable make!
* Population of Los Angeles in 2008 was at about 4,000,000, and occupied homes in 2000 were 1,275, 412 (U.S. Census Bureau). Projection: 4 people per home in 2008 for a conservative guess of 1,000,000 homes in the City of Los Angeles. The 2008 estimated population of California was 36,756,666 (U.S. Census Bureau).
 Solar Energy Technologies Program. U.S. Department of Energy, Energy Efficiency and Renewable Energy. Updated 11/18/2009. Viewed on 12/4/2009 at http://www1.eere.energy.gov/solar/csp.html