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Fresnel Reflectors

Fresnel reflectors are somewhat like parabolic troughs except that Fresnel systems use flat mirrors instead of curved mirrors. Additionally, rather than each mirror reflecting onto its own tube containing working fluid, many mirrors in a Fresnel system reflect sunlight onto the same tube.

Fresnel Reflector

The basic premise behind a Fresnel system is that flat mirrors have a greater surface area than do curved mirrors. By combining multiple flat mirrors around a single focal point, the concentration of sunlight should be great enough to heat a working fluid enough to do work.

Fresnel systems are based on the principle of the Fresnel lens, which was invented by the French physicist Augustin-Jean Fresnel. The Fresnel lens was designed to have a large aperture and short focal length using a minimal of material. The larger the aperture of a lens, the more light it can collect. The shorter the focal length of a lens, the greater will be its ability to concentrate light. Thus, a Fresnel lens can collect and focus a great deal of light using less building material than traditional designs. The same principle can be applied to mirrors and reflective surfaces, resulting in the Fresnel reflector. Clearly, a reduction in materials needed to build a Fresnel lens means the lens are cheaper to build and thus the overall system will cost less. Parabolic reflectors are difficult to build and very expensive as a result.

Like other CSP systems, Fresnel systems utilize a tracking system to maintain optimal solar alignment. A single axis system similar to that in parabolic troughs is used in Fresnel systems as well. Solar tracking in Fresnel systems is more complicated than in other CSP systems because multiple mirrors must move independently of one another to each maintain focus on the receiver tube. In Compact Linear Fresnel Receivers, the complexity is overcome by providing two receiver tubes and thus two focal points. This becomes critical when mirrors overlap one another and could potentially create shade that would reduce effectiveness.

There is a Fresnel solar plant, known as PE 1, operating in Germany. The plant was built by Novatec Biosol in 2009 and covers 4.4 acres or 18,000 square meters. For the area, the plant can produce 1.4 MW of electricity. A 30 MW systems is being designed by Novatec Biosol for application in Spain. There is also a 5 MW operational Fresnel plant in California.

Advantages of Fresnel Systems

As pointed out, one of the primary advantages of a Fresnel system is the reduced cost of implementation compared to other CSP technology. Fresnel installations cost about 2/3 of what parabolic trough installations cost.

Like other CSP systems mentioned, Fresnel systems can be hybridized with other technology to provide continuous power even during times of low solar irradiance. The advantage to such a system is that it is self-sufficient.

Drawbacks of Fresnel Systems

The primary drawback is shading and area use. Because the mirrors all focus on a single point, parts of mirrors can be blocked depending on the incident angle of radiation. There are two solutions to this problem. The first is to spread the mirrors out. This is a low cost solution, but it also means more land is required or fewer mirrors can be placed in a given area. The net result is to decrease efficiency of the system overall.

The second solution is that employed by Compact Linear Fresnel Reflectors, which use multiple receiver tubes to offset shading. Mirrors that would be overly shaded when focused on one receive tube can be focused on another instead. The downside to this implementation is the increased complexity and thus increased cost of having multiple receivers.

Though Fresnel mirrors are less expensive that parabolic troughs, their ability to concentrate sunlight not as great. Whereas parabolic mirrors have a lower threshold of 30X concentration and a higher limit of 80X concentration, Fresnel mirrors are only capable of approximately 30X concentration at best. This is offset by the fact that multiple mirrors are focused on a single receiver tube. Similar temperatures can be reached by both systems and similar efficiencies achieved. With the advent of nanotechnology that has reduced the cost of producing parabolic mirrors in recent years, the competition between Fresnel and parabolic trough will ultimately come down to a balance between cost and simplicity. Like any CSP system, grid efficiency is a major barrier to implementation on a large scale as these facilities must often be located far from the places where the power will be consumed.

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