Concentrating Solar _____________________________________________________________________________________________

Concentrated sunlight has been used to perform useful tasks from the time of ancient China. A legend claims Archimedes used polished shields to concentrate sunlight on the invading Roman fleet and repel them from Syracuse in 212 BC. Leonardo Da Vinci conceived using large scale solar concentrators to weld copper in the 15th century. In 1866, Auguste Mouchout successfully powered a steam engine with sunlight, the first known example of a concentrating solar-powered mechanical device. Over the following 50 years, inventors such as John Ericsson, and Frank Shuman developed solar-powered devices for irrigation, refrigeration and locomotion. The progeny of these early developments are the concentrating solar thermal power plants of today.[64]

Concentrating Solar Thermal (CST) systems use lenses or mirrors and tracking systems to focus a large area of sunlight into a small beam. This is then used to generate electricity. Moreover, the high temperatures produced by CST systems can be used to provide process heat and steam for a variety of secondary commercial applications (cogeneration). However, CST technologies require direct insolation to function and are of limited use in locations with significant cloud cover. The main methods for producing a concentrated beam are the solar trough, solar power tower and parabolic dish; the solar bowl is more rarely used. Each concentration method is capable of producing high temperatures and high efficiencies, but they vary in the way they track the sun and focus light.

A solar trough consists of a linear parabolic reflector that concentrates light onto a receiver positioned along the reflector's focal line. The reflector is made to follow the sun during the daylight hours by tracking along a single axis. A working fluid (oil, water) flows through the receiver and is heated to 500 °C before transferring its heat to a distillation or power generation system.[65] Trough systems are the most developed CST technology. The Solar Electric Generating System (SEGS) plants in California and Plataforma Solar de Almería's SSPS-DCS plant in Spain are representatives of this technology.[65]

A parabolic dish or dish/engine system consists of a stand-alone parabolic reflector that concentrates light onto a receiver positioned at the reflector's focal point. The reflector tracks the sun along two axes. A working fluid (hydrogen, helium, air or water) flows through the receiver where it is heated to 1000 °C before transferring its heat to a Stirling engine for power generation. Parabolic dish systems display the highest solar-to-electric efficiency among CST technologies and their modular nature offers scalability. The Stirling Energy Systems (SES) and Science Applications International Corporation (SAIC) dishes at UNLV and the Big Dish in Canberra, Australia, are representatives of this technology.

The PS10 solar power tower near Seville concentrates sunlight from a field of heliostats on a central tower.A solar power tower consists of an array of flat reflectors (heliostats) that concentrate light on a central receiver atop a tower. Focusing is critical and the reflectors track the sun through the day and the year on two axes. A working fluid (air, water, molten salt) flows through the receiver where it is heated up to 1500 °C before transferring its heat to a power generation or energy storage system. Power towers are less advanced than trough systems but they offer higher efficiency and better energy storage capability. The Solar Two in Daggett, California and the Planta Solar 10 (PS10) in Sanlucar la Mayor, Spain are representatives of this technology.

A solar bowl consists of a fixed parabolic reflector that concentrates light onto a receiver which tracks the focus of light as the sun moves across the sky. One has been constructed in Marseilles, France and another in Auroville, India.

Concentrating Photovoltaic (CPV) systems convert concentrated light into electricity by PV rather than heat engines. They also use tracking systems, mirrors, and lenses to achieve high concentration ratios and are able to reach efficiencies above 40%. [66] A solar power station planned for Victoria, Australia will use heliostat concentrating PV technology similar to the power tower concept