Concentrated Solar Power





Concentrated solar power is technology that utilizes reflectors to focus the sunlight hitting a large area onto a smaller area. Primarily this is done to drive heat engines but concentrators also exist that focus sunlight on photovoltaic panels. Concentrated solar power (CSP) systems are primarily commercial power plant type installations although some small scale systems have been created by hobbyists. One of the earliest examples of CSP is the story of Archimedes mirror, this technology was supposedly used as a weapon to ignite ships but it is doubted by historians. First confirmed use of CSP was in 1866 by Auguste Mouchout who used a parabolic trough system to power a steam engine.

Solar Stirling Engine

Solar Stirling engines make use of a parabolic dish that reflects light onto a receiver mounted at the focal point of the dish. This receiver is typically a cylinder that contains a thermal working fluid such as water. When the receiver is heated by the focused sunlight the working fluid contained within is also heated causing it to expand. Expansion of the working fluid drives a piston that simultaneously drives a flywheel and also presses the fluid away from the heated side of the receiver. Removed from the heat the fluid begins to cool and contracts driving the piston the other way and exposing the fluid to the heat again where the cycle continues.

Animation of a beta type Stirling engine.

Animation of a beta type Stirling engine. Image by Van Helsing. Copyright: GNU FDL

Common configurations include a dual piston and single piston setup both of which drive a flywheel which is connected to a generator. Typically solar Stirling engines will have a tracking system that properly positions the parabolic reflector in relation to the Sun known as heliostats.

Solar Towers

Solar towers utilize an array of Sun tracking reflectors to concentrate light onto a single receiver mounted on a tower. Within this receiver is a working fluid (commonly molten salt) that can reach as high as 1,800 degrees Fahrenheit. Typically this working fluid will be run through pipes in contact with another pipe containing water. Heat transfers from the working fluid to the water causing it to vaporize and become steam which is then used to power steam turbines generating electricity.

Sierra SunTower 5 MW facility in California

Sierra SunTower 5 MW facility in California. Image by Bardot. Copyright: GNU FDL

Solar tower CSP systems are very efficient and offer better energy storage than other solar power technology sometimes being able to generate electricity 24 hours a day. Torresol’s 19.9 MW concentrating solar power plant became the first to accomplish this goal in July of 2011.

Concentrated Photovoltaics

Concentrated photovoltaics (CPV) use reflectors or lenses to concentrate sunlight from an area larger than a photovoltaic cell onto the cell in order to increase electrical generation. This method allows for less expensive photovoltaic energy to be produced by increasing the efficiency of a given solar cell. Unfortunately solar cell efficiency also decreases as temperature rises so it becomes a challenge of controlling temperature to ensure a net gain in efficiency. Another problem is that reflectors require heliostats to track the Sun in order to maintain focus on the solar cell which can consume more power and increase cost of the system eliminating gains from concentration technology. In spite of these drawbacks CPV technology can be advantageous in some situations. Below is a video showcasing very small high efficiency solar cells in combination with reflectors to produce cheaper high efficiency solar panels.

Parabolic Troughs

Parabolic trough CSP technology can be thought of as a distributed version of solar power towers. Instead of concentrating light onto a tower linear parabolic reflectors (imagine a hollow half circle) focus light onto a pipe that is suspended along the focal point of the reflectors. Inside this pipe is a working fluid (commonly molten salt) that is heated by the concentrated sunlight to temperatures reaching 680 degrees Fahrenheit.

Parabolic trough CSP at Kramer Junction.

Parabolic trough CSP at Kramer Junction.

Many rows of parabolic troughs are commonly used and the working fluid travels through the pipes back to a central location. There it passes next to pipes containing water which is heated and vaporizes creating steam to power steam turbines for the generation of electricity.

Fresnel Reflectors

Fresnel reflectors operate similarly to parabolic troughs but use multiple rows of reflectors to concentrate light onto a single pipe system. Fresnel reflectors are cheaper to produce than parabolic troughs but commonly aren’t as efficient. While this is a newer CSP technology it is quickly becoming popular.

Nevada's Solar One Fresnel reflectors.

Nevada's Solar One Fresnel reflectors. Image by Ausra. Copyright: GNU FDL

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