Solar power

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.


Classifications of solar power

Direct or indirect

Solar power can be classified as direct or indirect.

Direct solar power

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Solar Cell

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

  • Sunlight hits a photovoltaic cell (also called photoelectric cell) creating electricity.
  • 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.
  • Sunlight strikes a Solar sail on a space craft and is converted directly into a force on the sail which caused motion of the craft.

Indirect solar power

Indirect solar power involves more than one transformation to reach a usable form.

  • Systems to close insulating shutters or move shades.

Many other types of power generation are indirectly solar-powered.

Passive or active

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.

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 is an example of this type of system.
  • 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 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 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 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 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 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 the swimming season.

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). 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.

A newer design, "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.

Residential solar thermal energy can be subdivided in two kind of systems: compact and pumped systems:

Compact systems

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Compact system with primary circuit

Consist on 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 in the tank (placed in the upper part) it expulses 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).

These 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. Instead of water, some non-toxic antifreezing it's placed. When this liquid is heated up, it flows to the external part of the tank, transferring the heat to the water placed inside.

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Primary circuit liquid comes from the panels and flows between red and blue layers and back to the panels, heating the water to consume placed inside the blue layer

These systems 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 en user. Some systems can work for up to 25 years with a minimum maintenance. These kind of systems can be redeemed in 6 years, and they have a positive balance of energy (energy used to build them minus energy they save) of 1.5 years. Most of the year, when the electric heating element is not working, these systems don't use any external source for power.

Pumped systems

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Pumped system

They are commonly used in bigger installations (hotels, gyms..) and the main difference is that the storage tank is placed inside the building, and so there is need for a pump and a controller that measures when the water is hotter in the panels than in the tank so the pump must start working. Most controllers activate also the pump when the outside temperature gets close to 0? C, so the movement 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.

Both compact and pumped systems include typically a electric element that is activated when the water in the tank gets lower than certain temperature (i. e. 45 ?C), so hot water is available even in rainy days.

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

Solar keymark is a european quality certificate

Photovoltaic cells

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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 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 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.
  • 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[1] (,1282,65936,00.html) 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.

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.

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: -

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 effectively uses the power grid as a battery.

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.

North America

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 [necessary for solar power to be economic?]: 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 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.

Europe & Japan

Several experimental photovoltaic (PV) power plants of 300 - 500 kW capacity are connected to electricity grids in Europe and the U.S. As of 2004, Japan had 1200 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.

A large parabolic reflector solar furnace is located in the Pyrenees at Odeillo, France. It is used for various research purposes. [2] (

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