UAB "Contentus" - Solar energy - Photovoltaic

CSP - Concentrated Solar Power

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 or is concentrated onto photovoltaic surfaces.
Concentrating solar power systems are divided into concentrating solar thermal (CST) and concentrating photovoltaics (CPV).
Concentrating solar thermal (CST) is used to produce renewable heat or electricity (generally, in the latter case, through steam). The concentrated light is then used as a heat source for a conventional power plant (solar thermoelectricity).
A wide range of concentrating technologies exist, including the parabolic trough, Concentrating Linear Fresnel Reflector, Solar chimney and solar power tower. Each concentration method is capable of producing high temperatures and correspondingly high thermodynamic efficiencies, but they vary in the way that they track the Sun and focus light. Due to new innovations in the technology, concentrating solar thermal is being more and more cost-effective.
In 2007 completed the 14 MW power station in Clark County, Nevada and the 20 MW site in Beneixama, Spain are characteristic of the trend toward larger solar power stations in the US and Europe. As an intermittent power source, solar power requires a backup supply, which can partially be complemented with wind or biogas power. Local backup usually can be with batteries or thermal storage systems, while utilities normally use pumped-hydro storage.

Solar energy is not available at night, during cloudy weather and energy storage is an important issue because modern energy systems usually assume continuous availability of energy.
Thermal mass systems can store solar energy in the form of heat at domestically useful temperatures for daily or seasonal durations. Thermal storage systems generally use readily available materials with high specific heat capacities such as water, earth and stone. Well-designed systems can lower peak demand, shift time-of-use to off-peak hours and reduce overall heating and cooling requirements.
Phase change materials * such as paraffin wax, fatty acids and salt hydrates are another thermal storage media. These materials are inexpensive, readily available, and can deliver domestically useful temperatures (approximately 64 °C).
Solar energy can be stored at high temperatures using molten salts. Salts are an effective storage medium because they are low-cost, have a high specific heat capacity and can deliver heat at temperatures compatible with conventional power systems. This method of energy storage, allowing it to store in storage tanks with an annual storage efficiency of about 99%.
*A phase change material (PCM) is a substance with a high heat of fusion which, melting and solidifying at a certain temperature, is capable of storing and releasing large amounts of energy. Heat is absorbed or released when the material changes from solid to liquid and vice versa; thus, PCMs are classified as latent heat storage (LHS) units.

The 11 megawatt PS10 near Seville in Spain solar power tower produces electricity from the sun using 624 large movable mirrors called heliostats.

Concentrated Photovoltaics (CPV) is the technology, which holds the greatest promise in meeting the energy challenges facing the world. A concentrating photovoltaic (CPV) system converts light energy into electrical energy in the same way that conventional photovoltaic technology does. The difference in the technologies consists in the addition of an optical system that focuses a large area of sunlight onto each cell.
Concentrating photovoltaic’s (CPV) systems employ sunlight concentrated onto photovoltaic surfaces for the purpose of more efficient electrical power production. Solar concentrators of all varieties may be used, and often these where mounted on a solar tracker in order to keep the focal point upon the cell as the Sun moves across the sky.CPV systems can be as telescopes, trained on the suns position and feeding the concentrated light to the cell.

In case of ratio magnification, CPV system designs used different varies very widely, but three classes of systems have developed:

Low concentration, where the magnification ratio is less than 10X, uses conventional silicon solar cells and simple concentrations (for ex. mirrors);
Medium concentration, between 10X and 150X, systems additionally require active solar tracking and cooling;
High concentration, where the ratio lies above 150X, but is usually less than 1000X. Systems employ concentrating optics consisting of dish reflectors or Fresnel lenses that concentrate sunlight to intensities of 200 suns or more. It require to use expensive Multi-junction solar cells to prevent thermal destruction and to manage temperature related performance losses. R&D into multi-junction photovoltaics usually are sponsored by governments and the astronautics industry.

Concentrating photovoltaic technology offers solar cell efficiencies greater than 40% and reduction in costs of cells relative to optics. There are no moving parts, no intervening heat transfer surface, no thermal mass, as well as CPV operation is near-ambient temperature.