Concentrated solar power (CSP) concentrates energy from the sun’s rays to heat a thermal receiver suitable to hold high temperatures. Unlike a photovoltaic system where sunlight is converted to electricity, the thermal receiver converts sunlight to heat where it will be transported to a steam generator to convert it to electricity.
Concentrated solar power has been in operation for some time and is a proven solar energy technology. It is not as marketable and simple as PV modules but the government of United States and Spain supported the technology to respond to the global crisis. Concentrated solar power is mostly used by large power grid companies as it can collect and generate more electricity than PV panels.
There are four current concentrated solar power technologies categorized in their ability to focus the sun’s rays and receive the sun’s energy: parabolic troughs, parabolic dishes, linear Fresnel collectors, and towers or CRS.
Parabolic troughs
Parabolic troughs consist of two parallel lines of mirror or reflector curved in a single direction to focus the sun’s energy to a fluid carrying receiver placed in the heart of the curved trough mirror. The sun’s energy heat the fluid inside the tube and the generated heat energy is used to generate electricity using a steam engine or generator. The tubes or absorber collectors are generally made of stainless steel and coated with a selective coating. Both the reflector and absorber collector move with the sun as it crosses the sky. This ensures that the sun is continually focused on the receiver pipes. Parallel lines of parabolic troughs are called collector fields.
Parabolic trough plants are hybrid. When there is insufficient sunlight, the plants burn natural gas, coal or fuel to meet load requirements.
Parabolic dishes
Parabolic dish plants use dish mirrors to concentrate sunlight and focus it to a thermal receiver. Unlike parabolic troughs, parabolic dish is a standalone unit, which is composed of a collector, thermal receiver, and an engine. The entire unit tracks the sun all throughout the day. The engine eliminates the need for heat transfer fluid and cooling water. Instead, the engine or generator is air-cooled.
Parabolic dish uses dual axis collectors, allowing it to capture maximum amount of sunlight during the day. Compared to other concentrated solar power technologies, parabolic dishes offer the highest solar energy to electricity conversion as it can achieve extremely high temperatures and high efficiencies.
One of the drawbacks of small parabolic dishes is it does not work well with thermal storage. The converted electricity must be fed immediately to solar grids. However, very large parabolic dishes are found to be more compatible with thermal storage and natural gas back-up. Manufacturers as well as promoters see huge parabolic dishes to compete with larger solar thermal plants in the future.
Linear Fresnel Reflectors
This concentrated solar power technology is much like parabolic troughs as it uses a single axis collector and a fluid-carrying receiver. However, linear Fresnel collectors are made of long rows of ground mounted mirrors or reflectors to concentrate the sun’s rays. The thermal receiver is elevated and fixed atop the reflectors.
Although the design has lower efficiency than parabolic troughs, it requires less land and is also more inexpensive than troughs. The current design of LFRs made possible direct steam generation by allowing water to be fed directly on the thermal receiver and be boiled at about 50 bars of atmospheric pressure. The saturated steam produced is used to power a steam cycle. LFRs produce lower optical efficiency compared to troughs and are low in compatibility with thermal storage.
Power Towers
Also known as central receiver systems, power towers employ thousands of field tracking reflectors, heliostats, to collect sun’s radiation. The reflectors are mounted atop a fixed tower. Sunlight is absorbed by molten salt or pressurized water, working the fluid flowing through the receiver and serving as thermal storage as well. Power towers surpassed the operating temperature of parabolic troughs and linear Fresnel reflectors but not parabolic dishes. Power tower design offers more flexibility as designers can choose a wide array of heliostats, thermal storage, power blocks, and transfer fluids. This concentrated solar power technology has more potential for lower operating costs than line-focus technologies like parabolic troughs and LFRs.
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