Concentrating Solar Power (CSP) Systems

Concentrating solar power (CSP) systems are not for the backyard enthusiast. These are utility-scale, renewable-energy power plants located in arid desserts regions that can produce hundreds of megawatts of electricity. CSP plants produce electricity by using mirrors to focus sunlight on a central point to heat water or another heat-carrying fluid. The high-temperature fluid then creates steam to drive a turbine and generator to produce electricity. There are three different types of concentrating solar power systems and are classified by how they collect the sun's energy.

Power Tower Systems

These large installations use large, flat mirrors to reflect sunlight at a collector located at the top of a tower. Numerous mirrors encircle the tower and each is constantly adjusted to focus the sunlight towards the central receiver. The large amount of solar energy is captured by a heat-carrying fluid which is then used to produce high-pressure steam to turn a turbine and generator.

In the first systems, water and steam were used to convert the solar energy into electricity. It was successful but electricity production would stop when clouds obscured the sun or after the sun set. Advanced designs use molten nitrate salt which can hold more heat than water & steam. The substance is continuously cycled through the solar beam to collect more and more heat and then stored for later use or passed through in a heat exchanger to create steam that drives the turbine. This method allows for continuous electricity generation during cloudy periods and into the evening.

Linear CSP Systems

These systems use either parabolic or flat mirrors to reflect and focus the sunlight onto a receiver tube strategically placed above the reflecting surface. The fluid in the receiver tube is heated by the concentrated sunlight which then produces steam to drive the turbine and generator. Some systems use molten nitrate salt to collect the solar energy for later use.

These systems do not need a tall, centrally located tower as in the Power Tower system. They do take up large amounts of land as numerous mirrors are needed to generate enough electricity to make the installation worth while. The angle of the mirrors is constantly adjusted to ensure the heat-collecting tube is constantly exposed to the concentrated solar beam.

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Dish/Engine CSP Systems

These stand-alone systems consist of a large parabolic dish, similar to the older satellite TV dishes, but have a reflective surface that concentrates the solar energy to a focal point. A heat collector and engine is mounted at this focal point to create electricity. The entire unit is constantly adjusted to directly face the sun.

As a stand alone system, these units produce less than 25 kW of electricity. The heated fluid is used to move a piston a Stirling engine which in turn can drive a crankshaft of a generator. No turbine is necessary. These units are too small to heat molten nitrate salt and therefore only produce electricity when the sun is shining.

Advantages of CSP Systems

• Solar farms can be placed in hot, dry inhospitable locations where human and wildlife is sparse.

• The entire system uses established technology that is readily available (mirrors, tubes and electrical generators).

• These units produce electricity during the day when air-condition loading is high. These clean-running, daytime power plants help supplement other primary electrical generation sources.

Disadvantages of CSP Systems

• Expensive transmission lines need to be installed in remote desert locations to get the useful energy to market. Since electrical production may not be continuous, it takes a longer time for CSP systems to pay for themselves.

• These units are most efficient during the daylight hours and cannot be relied upon as primary electrical sources. Even the systems that use heat storage techniques may stop producing electricity if it's overcast for a number of days.

• Small dish/engine systems cannot be economically used in a backyard to power a house. It is currently too expensive to build a system with a mirror alignment system and a Stirling engine.

Concentrating solar power plants have been around since the 1980's. They proved that large installations can be efficient and can pay for themselves if an economy of scale is achieved. Advances in system design have made these power plants more efficient at producing electricity but since they only collect solar energy when the sun is shining, they are not as cheap to operate as conventional hydro, nuclear or coal-fired power plants.