Solar Power Systems
The Solar Resource
Solar technologies use the sun's energy and light to provide heat, light, hot water, electricity, and even cooling, for homes, businesses, and industry. Similarly, the solar resource for generating power from concentrating solar power systems is also varied and plentiful. For instance, enough electric power for the entire country could be generated by covering about 9 percent of the land in Nevada, or a plot of land of about 100 miles square, with parabolic trough solar systems. The amount of power generated by a concentrating solar power plant depends on the amount of direct sunlight. Like concentrating photovoltaic concentrators, these technologies use only direct-beam sunlight, rather than diffuse solar radiation.
The southwestern United States potentially offers the best development opportunity for concentrating solar power technologies in the world. Concentrating solar power plants produce electric power by converting the sun's energy into high-temperature heat using various mirror configurations. The heat is then channeled through a conventional generator. The plants consist of two parts: one that collects solar energy and converts it to heat, and another that converts heat energy to electricity. There are three kinds of concentrating solar power systems; troughs, dish/engines, and power towers, which are classified by how they collect the solar energy.
Trough Systems
The sun's energy is concentrated by parabolic trough-shaped reflectors onto a receiver pipe running along the inside of the curved surface. This energy heats oil flowing through the pipe, and the heat energy is then used to generate electricity in a conventional steam generator.
A collector field comprises many troughs in parallel rows aligned on a north-south axis. This enables tracking the sun from east to west during the day to ensure that the sun is continuously focused on the receiver pipes. Individual trough systems currently can generate about 80 megawatts of electricity.
Advanced developments in this technology continues, with direct steam generation in the receiver tubes promising lower generating costs in the next decade. Hybrid systems - solar used with combined cycles are also in use.
Dish/Engine Systems
A Dish/Engine System is an electric generator that uses sunlight instead of gas or coal to produce electricity. The major parts of a system are the solar concentrator and the power conversion unit.
THE DISH, which is more specifically referred to as a concentrator, is the primary solar component of the system. It collects solar energy coming directly from the sun and focuses it on a small area. The resultant solar beam has all of the power of the sunlight hitting the dish but is concentrated in a small area so that it can be more efficiently utilized. Glass mirrors reflect approximately 92% of the sunlight that hits them, are relatively inexpensive, can be cleaned, and last a long time in the outdoor environment, making them an excellent choice for the reflective surface of a solar concentrator. The dish structure must track the sun continuously to reflect the beam on the thermal receiver.
THE POWER CONVERSION UNIT includes the thermal receiver and the engine/generator. The thermal receiver is the interface between the dish and the engine/generator. It absorbs the concentrated beam of solar energy, converts it to heat, and transfers the heat to the engine/generator. A thermal receiver can be a bank of tubes with a cooling fluid, usually hydrogen or helium, which is the heat transfer medium and also the working fluid for the conversion engine. Alternate thermal receivers are heat pipes wherein the boiling and condensing of an intermediate fluid is used to transfer the heat to the engine. The engine/generator system is the subsystem that takes the heat from the thermal receiver and uses it to produce electricity.
Power Tower Systems
A power tower converts sunshine into clean electricity for the world's electricity grids. The technology utilizes many large, sun-tracking mirrors (heliostats) to focus sunlight on a receiver at the top of a tower. A heat transfer fluid heated in the receiver is used to generate steam, which in turn, is used in a conventional turbine-generator to produce electricity.
With thermal storage, power towers can operate at an annual capacity factor of 65%, which means they can potentially operate for 65% of the year without the need for a back-up fuel source. Without energy storage, solar technologies like this are limited to annual capacity factors near 25%. The power tower's ability to operate for extended periods of time on stored solar energy separates it from other renewable energy technologies.
One key competitive advantage of concentrating solar energy systems is their close resemblance to most of the power plants operated by the nation's power industry. Concentrating solar power technologies utilize many of the same technologies and equipment used by conventional central station power plants, simply substituting the concentrated power of the sun for the combustion of fossil fuels to provide the energy for conversion into electricity. This evolutionary aspect as distinguished from revolutionary or disruptive results in easy integration into today's central station based electric utility grid. It also makes concentrating solar power technologies the most cost-effective solar option for the production of large-scale electricity generation.
