Solar energy

Solar power is a method of generating energy (usually electricity) from the light of the sun. It has become of increasing interest as other power sources such as finite fossil fuels become more scarce and expensive, in both financial and environmental terms. 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 reflect 35% of the total energy on average.

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. There have been experiments to create fuel by absorbing sunlight in a chemical reaction in a way similar to photosynthesis without using living organisms.

Most solar energy used today is converted into heat or electricity.

Types of solar power

Methods of solar energy have been classified using the terms direct, indirect, passive and active.

Direct solar energy involves only one transformation into a usable form.
Examples:

• Sunlight hits a photovoltaic cell creating electricity. (Photovoltaics are classified as direct despite the fact that 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 energy involves more than one transformation to reach a usable form.
Example:

• Systems to close insulating shutters or move shades.
  

Passive solar systems are considered direct systems although sometimes they involve convective flow which technically is a conversion of heat into mechanical energy.

Active solar energy refers to systems that use electrical, mechanical or chemical mechanisms to increase the effectiveness of the collection system. Indirect collection systems are almost always active systems.

Solar design is the use of architectural features to replace the use of 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:

• Solar Tower Turbine Cluster Village.
• South-facing (for the Northern Hemisphere) or north-facing (for the Southern Hemisphere) windows with insulated glazing that has high ultraviolet transmitance.
• Thermal masses.
• Insulating shutters for windows to be closed at night and on overcast days.
• 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.
• Passive or active Trombe walls.
• Active solar panels using water or antifreeze solutions.
• Passive solar panels for preheating potable water.
• Photovoltaic systems to provide electricity.
• Windmills to provide electricity.

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.

Solar cooking is helping in 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. 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 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 generally use reflectors to concentrate sunlight into a heat absorber.

• Heliostat mirror power plants focus the sun's rays upon a collector tower. The vast amount of energy is generally transported from the tower and stored by use of a high temperature fluid. Liquid sodium is often used as the transport and storage fluid. The energy is then extracted as needed by such means as heating water for use in stream turbines.
• Trough concentrators have been used successfully in the State of California (in the U.S.) to generate 350MW of power in the past two decades. The parabolic troughs can increase the amount of solar radiation striking the tubes up to 30 or 60 times, where synthetic oil is heated to 390°C. The oil is then pumped into a generating station and used to power a steam turbine.
• Parabolic reflectors are most often used with a stirling engine or similar device at its focus. As the single parabolic reflector achieves a greater focusing accuracy than any larger bank of mirrors can achieve, the focus is used to achieve a higher temperature which in turn allows a very efficient conversion of heat into mechanical power to drive a electrical generator. Parabolic reflectors can also be used to generate steam to power turbines to generate electricity.

Applying 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.

See also

Main Renewable resource, Renewable energy, Sustainable design

Solar: Solar box cooker, Solar thermal energy, Sun, Solar power satellite, Current solar income

Energy crisis: 1973 energy crisis, 1979 energy crisis

Electricity: Electricity generation, Electricity retailing, Energy storage, Green electricity, Direct current, Photoelectric effect, Power station, Power supply, Microwave power transmission, Solar cell, Power plant, Solar Tower

Lists: List of conservation topics, List of physics topics

People: Leonardo da Vinci, Charles Eames, Charles Kettering, Menachem Mendel Schneerson

Other: Autonomous building, Solar-Club/CERN-Geneva-Switzerland, Electric vehicle, Lightvessel, Mass driver, Clock of the Long Now, Tidal power, Cumulonimbus Smart 1, Science in the United States, Slope Point, Back to the land, Architectural engineering, Ecology, Geomorphology, List of conservation topics, Nine Nations of North America