SOLAR ENERGY

Solar radiation is an important source of energy, but its distribution on the planet is far from being homogeneous, as it depends on factor such as latitude, seasons, local climate and the time of day.

Solar energy may become unavailable during long periods, some of them, foreseeable, like the night, but others, very difficult to predict, like cloudiness. On these periods, solar systems need a back-up. The main weakness of solar energy systems is our present technological inability to store energy for those periods.

Solar radiation may be utilized in two different ways; in a direct way, to generate heat, or as a means to generate electricity.

Solar collectors are used, in a small scale, to provide heating and hot water to homes and other minor uses. The working temperature stays around 80C. The collectors are useful only in areas not densely populated.

There exist two main technologies for the generation of electricity through solar radiation:  the concentration systems and the photovoltaic systems.

The solar radiation concentration systems use mirrors to heat up a fluid, which then goes through a heat exchanger, generating steam to propel an electricity-generating turbine. The working temperatures can be close to 3000 C, and the efficiency can reach 80%. There are two different solar radiation concentration technologies: the disperse collectors system and the central concentration system.

The disperse collectors technology relies on a net of individual mirrors and pipes leading to the heat exchanger. In the central concentration system, a field of mirrors concentrates solar radiation on a single central tower, heating up a fluid, usually molten salt, which then goes through the heat exchanger.

Photovoltaic systems have the distinct property of converting solar radiation directly into electric energy. They are based on semiconductors technology. The solar cells are made of crystals. Silicium crystals reach efficiencies of around 23%, while some compounds of gallium and arsenic exceed 30%, but at a greater cost. Usually, economy prevails over efficiency and the less expensive, 15% efficiency crystals are chosen for most systems.

Photovoltaic cells provide energy to space vehicles.