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Concentrated Solar Power (CSP)

CSP refers to any technique, usually mirrors or lens, used to concentrate large areas of solar energy onto a smaller area. The solar energy, which is converted to heat, is used to drive a heat engine and thus produce electricity. In fact, some of these systems produce steam to drive turbines, just as a traditional coal-fired or nuclear power plant would. CSP units can be small-scale for residential use or large-scale for supply power to a grid.

History of Concentrated Solar Power

While there is no direct proof that it actually occurred, legend holds that Archimedes used a “burning glass” to concentrate sunlight on invading Roman ships and set them afire. Archaeological evidence is non-existent, but the Greek army did demonstrate that technique to much success in 1973.

Beginning in 1866, concentrated solar power as a way powering steam engines became popular. The first recorded use was a parabolic trough used by Auguste Moucout to produce steam. This invention was quickly followed by solar concentrators for refrigeration and irrigation. Modern CSP design began in 1968 when an Italia professor built the first CSP plant using a solar tower.

Types of Concentrated Solar Power

There are four major categories of CSP, at least a few of which were mentioned above. Almost all commercially viable forms of CSP use reflective mirrors, with the exception of the Fresnel lens. There are a number of applications in which CSP technology is combined with photovoltaic cells to increase the efficiency of PV systems. PV-CSP combinations rely on lens for concentrating solar energy. Many PV panels have reflective backings to ensure that sunlight that passes through the panel is reflected back into it and used.

One important consideration of CSP is incident angle or the angle at which light hits the apparatus. This is critical to obtaining maximum efficiency. To ensure the proper angle of irradiance is maintained, most CSP systems have sophisticated tracking capabilities. Tracking systems make CSP more efficient, but also more prone to failure.

One final term to be familiar with in regard to CSP is “working fluid.” This term simply replies to the liquid or gas that is being heated and used to do the work of the system in generating electricity. Water is not a popular choice in CSP. Molten salt and oil are the most commonly used working fluids.

Parabolic Trough

A parabolic trough can be thought of as one third of a tube. The mirrored inner surface is used to reflect light and also to concentrate it on a focal point. Most of these systems follow the sun and track on a single axis in order to heat the working fluid, which is usually molten salt, during the daylight hours. Temperatures between 150 and 350 degrees Celsius can be achieved with trough systems.

In comparison to other CSP, the major advantage of parabolic troughs is the fact that troughs are the most thoroughly developed CSP technology, with large scale applications throughout the world including California, Nevada, Spain, and Israel. The major disadvantage of trough systems is their lower overall efficiency. Even though 60 to 80% of sunlight is collected to heat the working fluid, inefficiencies in the overall transfer result in only 15% of the sunlight being used to generate electricity.

Dish

Dish systems are stand-alone systems often used in residential and small-scale applications. They consist of a single parabolic dish, much like a satellite dish, that reflects sunlight (heat energy) onto a heat engine. So, there are two primary components to a dish system, the dish and the heat engine.

The dish is reflective and tracks the sun on two axes (horizontal and vertical). Temperatures achieved range from 250 to 700 degrees Celsius. These high temperatures are the primary advantage if dish systems, making them exceptionally efficient. The solar to electricity conversion of dish systems is approximately 30%.

The second component, the heat engine, is usually of the Stirling design but can also be a simple steam engine. In order to work properly, this engine must be mounted to the focal point of the dish, meaning a rigid, strong support structure is necessary. This is a drawback in terms of cost and complexity. The other major drawback of these systems is their complexity as a result of the need to track on two axes to maintain efficiency.

Power Tower (Heliostat)

Power tower designs are for large-scale, industrial production of electricity that is supplied to the grid. These systems consist of a field of reflective mirrors that concentrate their energy on a single location several hundred feet above the ground on a tower. There are currently several heliostats under design and a working plant in Spain.

The major advantage of a heliostat is that temperatures as high as 1000 degrees Celsius can be reached, making these systems very efficient. Also, because of their large scale and high temperature operation, thermal energy can be stored with efficiencies as high as 98% for use during dark hours.

There are several disadvantages to these systems. First, each mirror must have dual tracking capabilities. Because the tower itself does not move, the mirrors must be aligned not only to collect sunlight, but also to appropriately reflect it onto the tower. The second drawback is that large arrays of mirrors are needed that take up vast tracts of land.

Fresnel Lenses

Fresnel lenses are actually reflectors that have a very shallow curve or none at all (meaning they are flat). Several rows of Fresnel reflectors concentrate sunlight onto a single focal point (as opposed to one focal point per mirror in dish and parabolic systems). The major advantage of using a Fresnel system is that they are simple and less expensive. They require single axis tracking and flat mirrors are inexpensive to manufacture. The downside to Fresnel systems is that rows of reflectors can block or shade other reflectors, lowering efficiency.

Efficiencies and Major Hurdles

CSP ranges in efficiency from 15% to a high of over 31%, making CSP at least as efficient as photovoltaic systems. When this is combined with their decreased cost and easy scalability, there are many reasons to consider that CSP will become more widespread for large-scale power generation then PV systems will.

A minor hurdle to CSP is the need to keep the mirrors and reflectors clean. Water can be used in most applications, but this can be a problem in arid settings where CSP location would otherwise be ideal. Many of these systems are designed to use as little water as possible through the application of electrostatic repulsion of dirt and wind removal of debris. No system can operate independent of water, however.

The largest hurdle to overcome with CSP is storage of heat energy for use during times when the sun is not available. Additionally, CSP plants can only be located in relatively sunny locations, meaning electricity must be transported great distances to residential locations. This means the electrical grid in many locations must be upgraded to provide efficient transport of electricity. The cost of upgrading the grid should be considered as part of the cost of building a large-scale CSP plant.
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