A Fresnel lens is a type of lens invented by French physicist Augustin-Jean Fresnel. Originally developed for lighthouses, the design enables the construction of lenses of large aperture and short focal length without the weight and volume of material which would be required in conventional lens design. Compared to earlier lenses, the Fresnel lens is much thinner, thus passing more light and allowing lighthouses to be visible over much longer distances.
Since plastic Fresnel lenses can be made larger than glass lenses, as well as being much cheaper and lighter, they are used to concentrate sunlight for heating in solar cookers, solar forges, and solar collectors to heat water for domestic use.
New applications have appeared in solar energy, where Fresnel lenses are used to concentrate sunlight (with a ratio of almost 500) onto solar cells. Thus the active solar cell surface can be reduced to a fraction compared to conventional solar modules. This offers a considerable cost-saving potential by low material consumption, and it is possible to use high-quality and expensive solar cells, which achieve a very high efficiency under concentration due to thermodynamic effects.
GreenPowerScience featured large Fresnel Lenses in their Youtube broadcast. They combined a medium sized Stirling Engine with a Fresnel Lens creating mechanical power directly from the sun. Fresnel reflectors are used in concentrating solar power (CSP) plants to concentrate solar energy from the sun. Amonix uses a Fresnel lens for a photovoltaic system that concentrates the solar energy over 500 times onto a small solar cell. There is also a development effort to create a 50% efficient photovoltaic system using a "Rainbow Concentrator"
Concentrating Solar Power (CSP) systems use lenses or mirrors and tracking systems to focus a large area of sunlight into a small beam. The concentrated light is then used as a heat source for a conventional power plant. A wide range of concentrating technologies exists; the most developed are the solar trough, parabolic dish and solar power tower. These methods vary in the way they track the Sun and focus light. In all these systems a working fluid is heated by the concentrated sunlight, and is then used for power generation or energy storage.
Source: Wikipedia (All text is available under the terms of the GNU Free Documentation License)
For More Information: Solar Concentrators & Fresnel Lenses