Sun is the main source of energy for our planet earth. Sun's radiations, falling on the earth, can be captured directly for the use in the modern technologies which at present make use of the fossil fuels responsible for the increased emissions of carbon dioxide. Methods of capturing solar energy are as follows:
Active heating and cooling: e.g. a) water heater converts solar radiation into heat, which can be used directly or stored, b) night sky radiation cooling involves cooling of water in shallow trays exposed to night sky.
Passive solar energy design involving orientation, shape and fabric of a building's allows the solar radiations to enter directly in the indoor areas of the building, reducing the need for artificial lighting and heating.
Active heating and cooling (Solar thermal energy)
Active solar water heating uses flat plate solar energy collectors, usually on the roof of a building, to collect the sun's heat via water tubes attached to the flat plate and a water storage tank. The heat is primarily used for heating water in domestic applications, industrial facilities and commercial buildings and also has the growing market for solar swimming pool water heating. In solar thermal energy systems, when the solar energy is not available (in the night) or availability is low (under cloudy or dusty conditions), hot water can be stored for a limited period of time in well-insulated water tanks.
Air may also be heated directly by sun's heat by circulating it through the metal channels or ducts attached to the flat plate collector.
Night sky radiation cooling effect may also be used in various cooling requirements. Under clear night sky conditions when practically there is no solar radiation, the earth surface continues to emit infrared radiation to the sky and is cooled to temperature much lower than surrounding air temperature. Water in a shallow trough having large surface areas when exposed to clear night sky may be cooled to about 15oC below air temperature.
An overview of the status of the solar thermal technology developments in the European Union countries is given on the European commissions website and that for India is available on the website.
Solar photovoltaic energy
Solar photovoltaic (PV) process involves direct conversion of energy of sun's light into electricity using a semiconductor material, also called photoelectric effect. Energy of light radiations falling on the semiconductor material such as silicon is absorbed, and the electrons in the semiconductor are excited and broken free from their atoms. The free electrons flow through the semiconductor material producing electricity.
Numerous solar PV cell technologies, including polycrystalline, mono-crystalline and thin-film have been developed. Small solar PV cells (1 or 2 watts) are arranged in modular panels which are mounted on a building roof or walls, and directly feed electricity into the heating or cooling equipment inside the building. In the latest PV technology, cells integrated into the roof tiles themselves are also available. PV technology has already found numerous uses including small electronic devices, military equipment and space research.
Solar PV cells are also used in both stand-alone and grid-connected systems. As the solar energy is only available during the day and also varies in output due to dusty conditions or cloud cover, PV systems use batteries or other forms of electricity storage to store the electricity for periods when the solar energy availability is low but the demand is high.
An overview of the status of the photovoltaic technology developments in the European Union countries is given on the European commission's website , and that for India is available on the Indian Indian Ministry of New and Renewable Energy (MNRE) website.
Passive solar design
Passive solar techniques make use of solar energy by means of building designs that balance energy requirements, without the use of additional mechanical equipment. Such techniques include use of south facing windows in the northern hemisphere buildings to allow solar energy for heating the space, insulation of walls, roof and floors to minimise the heat loss, and natural ventilation. Building materials having large thermal mass are used to store solar energy for use when there is no sun shine.
Natural solar light can be allowed to reach inner space of a building using open plan floors or specially developed light pipes, to help reduce energy demand. Such natural light conditions also provide better quality light, better work efficiency and better health.
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