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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 the environmental costs and limited supply of other power sources such as fossil fuels are realized. It is already in widespread use where other supplies of power are absent such as in remote locations and in space.

As the Earth orbits the Sun, it receives approximately 1,400 W / m² of energy, as measured upon a surface kept normal (at a right angle) to the Sun (this number is referred to as the solar constant). Of the energy received, roughly 19% is absorbed by the atmosphere, while clouds on average reflect a further 35% of the total energy. The generally accepted standard is 1020 watts per square meter at sea level.

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 or when we consume the plants as a source of food.

Classifications of solar power

A wide range of solar power technologies exist. These can be classified in a number of different ways.

Method of energy transformation

Solar energy can be tranformed for use elswhere or utilised directly.

Direct solar power involves only one transformation into a usable form. For example:

Sunlight hits a photovoltaic cell (also called a photoelectric cell) creating electricity.
Sunlight hits the dark absorber surface of a solar thermal collector and the surface warms. The heat energy is carried away by a fluid circuit.
Sunlight strikes a solar sail on a space craft and is converted directly into a force on the sail which causes motion of the craft.
Sunlight strikes a light mill and causes the vanes to rotate, although little practical application has yet been found for this effect.
Sunlight is focused on an externally mounted fibre optic cable which conducts sunlight into building interiors to supplement lighting.

Indirect solar power involves more than one transformation to reach a usable form. Many other types of power generation are indirectly solar-powered. Some of these are so indirect that they are often excluded from discussion of solar power:

Vegetation use photosynthesis to convert solar energy to chemical energy, which can later be burned as fuel to generate electricity, see biofuel.
Energy obtained from oil, coal and peat originated as solar energy captured by vegetation in the remote geological past and fossilised. Hence the term Fossil fuel. Though strictly solar power, the great time delay between the input of the solar energy and its recovery means these are not normally classified as such.
Hydroelectric dams and wind turbines are indirectly powered by solar energy through its interaction with the Earth's atmosphere and the resulting weather phenomena.
Energy obtained from methane (natural gas) may be derived from solar energy either as a biofuel or fossil fuel , but some methane derives from the primeval gas cloud which formed the Solar system and is therefore not solar in origin.
Ocean thermal energy production uses the thermal gradients that are present across ocean depths to generate power. These temperature differences are ultimately due to the energy of the sun.

Complexity of mechanism

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, although (in the case of solar heat through windows) there may be draperies or panels used to reduce nighttime heat losses and thermostatically or manually operated vents (but not fans) to prevent overheating. Passive solar water heaters, for instance, use a thermosiphon and have no pumps. The thermosiphon only operates when hot, to reduce nighttime heat loss. Other space heating systems use a thermal diode to similar effect. Passive solar water distillers may rely upon capillary action to pump water.

Active solar systems use additional mechanisms such as circulation pumps, air blowers or tracking systems which aim collectors at the sun. These mechanisms are typically powered by electricity and may have additional electronic or computerized automatic controls.

Focus type

Effective use of solar radiation often requires the radiation (light) to be focussed to give a higher intensity beam. Consequently, another scheme for classifying solar power systems is

Point focus. A parabolic dish or a series of heliostats are used to concentrate light at a point (the focus). At the focus you might place high-concentration photovoltaic cells (solar cells) or a thermal energy 'receiver'. Solar One was an example of the latter.
Line focus. A parabolic trough or a series of long narrow mirrors are used to concentrate light along a line. The SEGS systems in California are an example of this type of system.
Non-focussing systems include solar domestic hot water systems and most photovoltaic cells. These systems have the advantage that they can make use of diffuse solar radiation (which can not be focussed). However, if high temperatures are required, this type of system is usually not suitable, because of the lower radiation intensity.

Types of solar power technologies

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 transmittance.
Thermal masses -- any masses such as walls or roofs that absorb and hold the sun's heat. Materials with high specific heat like stone, concrete, adobe or water work best. See Trombe walls.
Insulating shutters 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 thermal solar panels using a heat transfer fluid (water or antifreeze solution). These are heated by the sun and the heat is carried away by circulation of the fluid for domestic hot water or building heating or other uses.
Passive thermal solar panels for preheating domestic hot water.
Photovoltaic systems to provide electricity.
Solar chimneys for cooling.
Planting deciduous trees near the windows. The leaves will give shade in summer but fall in winter to let the sunlight enter the building.

Solar heating systems

Solar heating systems are generally composed of solar thermal collectors, a fluid system to move the heat from the collector to its point of usage, and usually a reservoir to stock the heat for subsequent use. The systems may be used to heat domestic hot water, to heat a swimming pool, to provide heat for a heating circuit (usually radiators or floor heating coils). The heat can also be used for industrial applications or as an energy input to other uses (such as cooling equipment).

In many climates, a solar heating system can provide a very high percentage of domestic hot water energy. In many northern European countries, combined systems (hot water and space heating) are used to provide 15 to 25% of home heating energy.

Residential solar thermal installations can be subdivided in two kind of systems: compact and pumped systems. Both include typically an auxiliary energy source (electric heating element or connection to a gas or fuel oil central heating system) that is activated when the water in the tank falls below a minimum temperature setting(i. e. 50 ºC), so hot water is available always, even in rainy days.

Compact systems

Consist of a tank for the heated water, a few panels and pipes. Based on the thermo siphon principle, the water flows upwards when heated in the panel. When this water enters the tank (placed in the upper part) it expels some cold water from inside, so there is no need for pumps. A typical system for a 4 members home in a sunny region consists of a 300 liters tank and 2 panels (2 square meters each).

"Direct" compact systems are not suitable for cold climates, because at nighttime the remaining water in the panels can freeze and damage them. Besides, the tank is placed together with the panels, generally outside the house (even if the can be hidden beneath the tiles). Some compact systems have a “primary circuit”. This primary circuit includes the collectors and the external part of the tank. A graphical explanation of the thermosyphon principle can be found at this site Instead of water, some non-toxic antifreezing liquid is used. When this liquid is heated up, it flows to the external part of the tank, transferring the heat to the water placed inside. However, direct systems are slightly cheaper and more efficient.

A compact system can save up to 4.5 tonnes per annum of gas emissions. So, in order to achieve the aims of the Kyoto Protocol, several countries are offering subsidies to the end user. Some systems can work for up to 25 years with minimum maintenance. These kinds of systems can be redeemed in 6 years, and they achieve a positive balance of energy (energy used to build them minus energy they save) of 1.5 years. Most part of the year, when the electric heating element is not working, these systems don't use any external source for power (as water flows due to thermosyphon principle).

Usually flat solar thermal collectors are used, but a few compact systems with vacuum tubes can be found.

Pumped systems

They are commonly used in bigger installations (hotels, gyms, and so forth) and the main difference is that the storage tank is placed inside the building, and thus require a controller that measures when the water is hotter in the panels than in the tank, and a pump for transferring water between the two. Most controllers also activate the pump when the outside temperature gets close to 0º C, in order to prevent the water from freezing and thus damaging the panels.

These systems can be controlled remotely, by means of the data logger and a modem-connection.

The most commonly used panel is the flat panel, but sometimes cheaper ones, like polypropylene panels (for swimming pools), or higher-performing ones like vacuum tubes are used.

Solar heating thermal collectors

There are three main kind of solar thermal collectors in common use:

Formed Plastic Collectors (such as polypropelene, EPDM or PET plastics). These consist of tubes or formed panels through which water is circulated and heated by the sun's radiation. Used for extending the swiming season in swiming pools. In some countries heating a open-air swiming pool with non-renewable energy sources is not allowed, and then these cheap systems offer a good solution. This panel is not suitable for year round uses like providing hot water for home use, mainly due to its lack of insulation which reduces its effectiveness greatly when the ambient air temperature is lower than than temperature of the fluid being heated.

Flat Collector. It consists of a thin absorber sheet (usually copper, to which a selective coating is applied) backed by a grid or coil of fluid tubing and placed in an insulated casing with (usually) a glass cover. Fluid is circulated through the tubing to remove the heat from the absorber and transport it to an insulated water tank, to a heat exchanger, or to some other device for using the heated fluid. Flat-plate collectors for solar water heating had a popularity in Florida and Southern California in the 1920s. There was a resurgence of interest in them in North America in the 1970s. With various improvements, the collectors of this basic design have frequently been used in "off-grid" home situations (or in other sorts of buildings), but now they are present in all most every city in the world. Naturally, like a lot of solar-heating strategies that have been available until recently, conventional flat-plate solar collectors were originally developed for use in sunny, warm climates. Benefits from this kind of collector are considerably diminished when colder or cloudy days present unfavorable conditions.

Evacuated tube collectors are made of a series of modular tubes, mounted parallel, whose number can be added to or reduced as hot-water-delivery needs change. This type of collector consists of rows of parallel transparent glass tubes, each of which contains an absorber tube (in place of the absorber plate to which metal tubes are attached in a flat-plate collector). The tubes are covered with a special light-modulating coating. In an evacuated-tube collector, sunlight passing through an outer glass tube heats the absorber tube contained within it, and in doing so the heat is transferred to a liquid flowing through the tube. The heated liquid circulates through a heat exchanger and gives off its heat to water that is stored in a storage tank (which itself may be kept warm partially by sunlight). Evacuated-tube collectors heat to higher temperatures. Even in some northern climates, this sort of system may capture excess heat which can also be used to supply room heat in winter. However they are more expensive and fragile than flat panels.

Solar thermal cooling

There are some new applications of thermal hot water, like air cooling, currently under development. The absorber machine works basically as a fridge; it uses hot water to compress a gas that once expanded will produce an endothermic reaction, cooling the air. The main problem right now is that the absorber machine works with liquid at 90ºC, a pretty high temperature to be reached with pumped solar pannels with no auxiliary power supply. Some commercial systems are expected to be relased soon.

The same pumped solar thermal installation can be used for producing hot water the whole year, cooling in summertime and partially heating the building in wintertime.

ESTIF European Solar Thermal Industry organization. Statistics, market situation...

Solar keymark is a European quality certificate

Photovoltaic cells

The solar panels (photovoltaic arrays) on this small yacht at sea can charge the 12 V batteries at up to 9 Amps in full, direct sunlight

Solar cells (also referred to as photovoltaic cells) are devices or banks of devices that use the photovoltaic effect of semiconductors to generate electricity directly from the sunlight. Because of high manufacturing costs, their use has been limited until recently. One cost-effective use has been in very low-power devices such as calculators with LCDs. Another has been remote applications such as roadside emergency telephones, remote sensing, cathodic protection of pipe lines, and limited "off grid" home power applications. A third has been to power orbiting satellites and other spacecraft.

However, the continual decline of manufacturing costs (dropping at 3% to 5% a year in recent years) is expanding the range of cost-effective uses. The average retail cost of a large solar panel declined from $7.50 to $4 per watt between 1990 and 2005. With many jurisdictions now giving tax and rebate incentives, solar electric power can now pay for itself in five to ten years in many places. "Grid-connected" systems - that is, systems with no battery that connect to the utility grid through a special inverter - now make up the largest part of the market. In 2004 the worldwide production of solar cells increased by 60%. 2005 is expected to see large growth again, but shortages of refined silicon have been hampering production worldwide since late 2004.

Solar thermal electric power plants

The two main types of solar thermal power plants are Solar Chimneys and Concentrating Solar Power (CSP) plants.

Concentrating solar power (CSP) plants

Solar thermal power plants generally use reflectors to concentrate sunlight into a heat absorber. Such powerplants are known as Concentrating Solar Power (CSP) plants.

Heliostat mirror power plants (power towers) use an array of flat, moveable mirrors to focus the sun's rays upon a collector tower (the target). The high energy at this point of concentrated sunlight is transferred to a substance that can store the heat for later use. The more recent heat transfer material that has been successfully demonstrated is liquid sodium. Sodium is a metal with a high heat capacity, allowing that energy to be stored and drawn off throughout the evening. That energy can, in turn, be used to boil water for use in steam turbines. Water had originally been used as a heat transfer medium in earlier power tower versions (where the resultant steam was used to power a turbine). This system did not allow for power generation during the evening. Examples of heliostat based power plants are the 10 MWe Solar One, Solar Two and the 15 MW Solar Tres plants. In South Africa a solar power plant is planned with 4000 to 5000 heliostat mirrors, each having an area of 140 m².

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, oil in the Luz systems, 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. It focuses all the sun's energy to a single point and can achieve very high temperatures. Typically the dish is coupled with a Stirling engine in a Dish-Stirling System, but also sometimes a steam engine is used. These create rotational kinetic energy that can be converted to electricity using an electric generator. Planned 850 megawatt Solar Stirling Condenser array [1] [2].

A linear Fresnel reflector power plant uses a series of carefully angled plane mirrors to focus light onto a linear absorber. Recent prototypes of these types of systems have been built in Australia (CLFR) and Belgium (SolarMundo).

Solar chimney

A solar chimney is a solar thermal power plant where air passes under a very large agricultural glass house (between 2 and 30 kilometres in diameter), is heated by the sun and channeled upwards towards a convection tower. It then rises naturally and is used to drive turbines, which generate electricity.

Solar chemical

There have been experiments[3] 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.

A promising approach is to use focussed sunlight to provide the energy needed to split water into its constituent hydrogen and oxygen in the presence of metalic zinc. [4]

Solar cooking

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 western solar oven is attributed to Horace de Saussure.

Solar lighting

The interior of a building can be lit during daylight hours using fibre optic light pipes connected to a parabolic collector mounted on the roof. The manufacturer claim this gives a more natural interior light and can be used to reduce the energy demands of electric lighting. [5]

Energy storage

See main article at Grid energy storage
For a stand-alone system, some means must be employed to store the collected energy for use during hours of darkness or cloud cover. The following list includes both mature and immature techniques: -

Electrochemically in batteries,
Hydrogen produced by electrolysis of water and then available for pollution free combustion (see direct solar thermal water splitting),
Compressed air in a cylinder,
Pumped-storage hydroelectricity
Flywheel energy storage,
Molten salt
Superconducting magnetic energy storages.
Cryogenic liquid air or nitrogen

Storage always has an extra stage of energy conversion, with consequent energy losses, greatly increasing capital costs. One way around this is to export excess power to the power grid, drawing it back when needed. This appears to use the power grid as a battery but in fact is relying on conventional energy production through the grid during the night.

Deployment of solar power

Deployment of solar power depends largely upon local conditions and requirements. But 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.

Several experimental photovoltaic (PV) power plants of 300 - 600 kW capacity are connected to electricity grids in Europe and the U.S. Other major research is investigating economic ways to store the energy which is collected from the sun's rays during the day.

North America

A laundromat in California powered by solar panels on the roof.

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 for economic feasibility: 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 building construction. Finally, the region must have high power prices. For example, Southern California has about 260 sunny days a year, making it the best possible venue. Even there, it only yields about 4%/yr returns on investment when systems are installed at $9/watt (not cheap, but feasible), utility prices are at $0.095 per kilowatt-hour (the current base rate), and neglecting all maintenance costs. On-grid solar power can be especially feasible when combined with time-of-use net metering, since the time of maximum production is largely coincident with the time of highest pricing.

On August 11, 2005, Southern California Edison announced an agreement to purchase solar powered Stirling engines from Stirling Energy Systems [6] over a twenty year period and in quantity (20,000 units) sufficient to generate 500 megawatts of electricity. These systems - to be installed on a 4,500 acre solar farm - will use mirrors to direct and concentrate sunlight onto the engines which will in turn drive generators. Less than a month later, Stirling Energy Systems announced another agreement with San Diego Gas & Electric to provide between 300 and 900 megawatts of electricy.

Japan

As of 2004, Japan had 1200 MWe installed.

Europe

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. A large parabolic reflector solar furnace is located in the Pyrenees at Odeillo, France. It is used for various research purposes. [7]. Another site is the Loser in Austria.

Israel

The Israeli government recently (2005) announced an international contract for building a 100MW solar power plant to supply the electricity needs of more than 200,000 Israelis living in southern Israel. The plan may eventually allow the creation of a gigantic 500MW power plant making Israel a leader in solar power production. [8]

India

In terms of overall installed PV capacity, India comes fourth after Japan, the US and Germany (Indian Ministry of Non-conventional Energy Sources 2002). Government support and subsidies have been a major influence in its progress.^[9]

References

^ Solar energy heats up India is Rapidly Developing Solar Energy via Photovoltaic & Thermal Systems

External links

The International Energy Agency runs many large multinational solar power projects under the SolarPACES umbrella. They are Task I.1: Central Generation Systems, Task I.2: Distributed Generation Systems and Task I.3: CSP Market Development.
International Conference on Renewable Energy and Power Quality (ICREPQ´04) (PDF file)
Home Power Magazine
National Renewable Energy Laboratory: Concentrating Solar Power (CSP)
PowerPedia Solar Energy Directory Focuses on futuristic solar energy delivery systems
Library of public domain images of operational Solar Power projects from the World-wide Information System for Renewable Energy (WIRE).
Solar Panel Information provides general information about the practical and ethical applications of solar technology.
Residential Solar Power Systems - Photo Gallery
U.S. Department of Energy: Energy Efficiency and Renewable Energy Solar History Timeline
Solar Garden Lights Applications of solar energy for households.
Solar Power Related News
Solar Energy Magazine
Maintaining the largest solar power plant in the world - An article