Solar power describes a number of methods of harnessing energy from the light of the sun. It has been present in many traditional building methods for centuries but has become of increasing interest in developed countries as other power sources such as fossil fuels become more scarce and expensive both in financial and environmental terms. It is often employed where other supplies of power are absent such as in remote locations and in space.
As the earth orbits the sun it receives 1,410 W / m2 as measured upon a surface kept normal (at a right angle) to the sun. Of this, approximately 19% of the energy is absorbed by the atmosphere, while clouds on average reflect a further 35% of the total energy.
After passing through the Earth's atmosphere, most of the sun's energy is in the form of visible and ultraviolet light. Plants use solar energy to create chemical energy through photosynthesis. We use this energy when we burn wood or fossil fuels.
Classifications of solar power
Solar power can be classified as direct or indirect.
Direct solar power involves only one transformation into a usable form.
- Sunlight hits a photovoltaic cell creating electricity. (Photovoltaics are classified as direct although the electricity is usually converted to another form of energy such as light or mechanical energy before becoming useful.)
- Sunlight hits a dark surface and the surface warms when the light is converted to heat by interacting with matter. The heat is used to heat a room or water.
Indirect solar power involves more than one transformation to reach a usable form.
- Systems to close insulating shutters or move shades.
- Systems to close insulating shutters or move shades.
Solar power can also be classified as passive or active.
Passive solar systems are systems that do not involve the input of any other forms of energy apart from the incoming sunlight.
Active solar This usually refers to system which use additional mechanisms such as circulation pumps, air blowers or automatic systems which aim collectors at the sun.
Types of solar power applications
Most solar energy used today is harnessed as heat or electricity.
Solar design in architecture
Solar design is the use of architectural features to replace the use of grid electricity and fossil fuels with the use of solar energy and decrease the energy needed in a home or building with insulation and efficient lighting and appliances.
- Architectural features used in solar design:
- South-facing (for the Northern Hemisphere) or north-facing (for the Southern Hemisphere) windows with insulated glazing that has high ultraviolet transmitance.
- Thermal masses. Which are any masses like walls or roofs that absorb and hold the suns heat. Materials with high specific heat like stone, concrete, adobe or water work best. see Trombe walls.
- Insulating shutter s for windows to be closed at night and on overcast days. These trap solar heat in the building.
- Fixed awnings positioned to create shade in the summer and exposure to the sun in the winter.
- Movable awnings to be repositioned seasonally.
- A well insulated and sealed building envelope.
- Exhaust fans in high humidity areas.
- Passive or active warm air solar panels. Pass air over black surfaces fixed behind a glass pane. The air is heated by the sun and flows into the building
- Active solar panels using water or antifreeze solutions. These get hot in the sun and the hot liquid is used to heat the building or in a solar hot water system
- Passive solar panels for preheating potable water.
- Photovoltaic systems to provide electricity.
- Solar Tower Turbine Cluster Village.
Solar hot water
Solar hot water systems are quite common in some countries where a small flat panel collector is mounted on the roof and able to meet most of a household's hot water needs. Cheaper flat panel collectors are also often used to heat swimming pools, thereby extending their swimming seasons.
A solar box cooker traps the sun's power in an insulated box; these have been successfully used for cooking, pasteurization and fruit canning. Solar cooking is helping many developing countries, both reducing the demands for local firewood and maintaining a cleaner environment for the cooks. The first known record of a western solar oven is attributed to Horace de Saussure , a Swiss naturalist experimenting as early as 1767.
Solar cells (also referred to as photovoltaic cells) are devices or banks of devices that use the photoelectric effect of semiconductors to generate electricity directly from the sunlight. As their manufacturing costs have remained high during the twentieth century, their use has been limited to very low power devices such as calculators with LCD displays or to generate electricity for isolated locations which could afford the technology. The most important use to date has been to power orbiting satellites and other spacecraft. As manufacturing costs decreased in the last decade of the twentieth century, solar power has become cost effective for many remote low power applications such as roadside emergency telephones, remote sensing, and limited "off grid" home power applications.
Solar power plants
Solar power plants generally use reflectors to concentrate sunlight into a heat absorber.
- Heliostatmirror power plants use an array of flat moveable mirrors to focus the sun's rays upon a collector tower. A vast amount of energy is transported from the tower and stored by using a high temperature liquid like sodium. This in turn is used to heat water for use in stream turbines.
- A parabolic trough power plant is another type of solar thermal collector. It consists of a series of troughs rather like rainwater guttering with a hollow tube running its length. Sunlight is reflected by the mirror and concentrated on the tube. Heat transfer fluid runs through the tube to absorb heat from the concentrated sunlight and is used to power a steam turbine.
- A Parabolic reflector power plant is rather like a large satellite dish but with the inside surface made of mirror material. They focus all the suns energy to a single point and can achieve very high temperatures. They are often used directly with a stirling engine or steam engine to obtain power directly.
- A Solar Tower uses air, which passes under a very large agricultural glass house (between 2-30 kilometres in diameter), is heated by the sun and channeled upwards towards a convection tower like those used in ordinary power stations where it rises naturally, thereby driving wind turbines, which generate electricity.
There have been experiments to harness energy by absorbing sunlight in a chemical reaction in a way similar to photosynthesis without using living organisms but no practical process has yet emerged.
Deployment of Solar Power
Deployment of solar power depends largely upon local conditions and requirements. For example, while certain European or U.S. states could benefit from a public hot water utility, such systems would be both impractical and counter-productive in countries like Australia or states like New Mexico. As all industrialised nations share a need for electricity, it is clear that solar power will increasingly be used to supply a cheap, reliable electricity supply.
Many other types of power generation are indirectly solar-powered. Plants use photosynthesis to convert solar energy to chemical energy, which can later be burned as fuel to generate electricity; oil and coal originated as plants. Hydroelectric dams and wind turbines are indirectly powered by the sun.
In some areas of the U.S., solar electric systems are already competitive with utility systems. As of 2002, there is a list of technical conditions: There must be many sunny days. The systems must sell power to the grid, avoiding battery costs. The solar systems must be inexpensively mass-purchased, which usually means they must be installed at the time of construction. Finally, the region must have high power prices. For example, Southern California has about 260 sunny days a year, making it an excellent venue. It yields about 9%/yr returns of investment when systems are installed at $9/watt (not cheap, but feasible), and utility prices are at $0.095 per kilowatt-hour (the current base rate). On grid solar power can be especially feasible when combined with time-of-use net metering, since the time maximum production is largely coincident with the time of highest pricing.
For a stand-alone system, some means must be employed to store the collected energy for use during hours of darkness or cloud cover - either as electrochemically in batteries, or in some other form such as hydrogen (produced by electrolysis of water), flywheels in vacuum, or superconductors. Storage always has an extra stage of energy conversion, with consequent energy losses, greatly increasing capital costs.
Several experimental photovoltaic (PV) power plants of 300 - 500 kW capacity are connected to electricity grids in Europe and the U.S. Japan has 150 MWe installed. A large solar PV plant is planned for the island of Crete. Research continues into ways to make the actual solar collecting cells less expensive and more efficient. Other major research is investigating economic ways to store the energy which is collected from the sun's rays during the day.
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