Solar energy

The Sun provides 1,366 watts/meter² at the distance of the Earth's orbit, but less at ground level.

US annual average solar energy received by a latitude tilt photovoltaic cell.

The term solar power is used to describe a number of methods of harnessing energy from the light of the Sun. It has been used in many traditional technologies for centuries and has come into widespread use where other power supplies are absent, such as in remote locations and in space. Its use is spreading as the environmental costs and limited supply of other power sources such as fossil fuels are realized.

Energy from the Sun

Theoretical annual mean insolation, at the top of Earth's atmosphere (top) and at the surface on a horizontal square meter .

Global solar energy resources. The colors in the map show the actual local solar energy, averaged through the years of 1991-1993. The scale is in watts per square meter.
The land area required to supply the current global primary energy demand by solar energy using available technology is represented by the dark disks.

The rate at which solar radiation reaches a unit of area in space in the region of the Earth's orbit is 1,366 W/m², as measured upon a surface normal (at a right angle) to the Sun. This number is referred to as the solar constant. The atmosphere reflects 6% and absorbs 16% of incoming radiation resulting in a peak power at sea level of 1,020 W/m² . Average cloud cover reduces incoming radiation by 20% through reflection and 16% through absorption. The image on the right shows the average solar power available on the surface in W/m² calculated from satellite cloud data averaged over three years from 1991 to 1993 (24 hours a day). For example, in North America the average power of the solar radiation lies somewhere between 125 and 375 W/m², between 3 and 9 kWh/m²/day.

It should be noted that this is the maximum available power, and not the power delivered by solar power technology. For example, photovoltaic panels currently have an efficiency of ca. 15% and, hence, a solar panel delivers 19 to 56 W/m² or 0.45-1.35 kWh/m²/day (annual day and night average). The dark disks in the image on the right are an example for the land areas that, if covered with solar panels, would produce slightly more energy in the form of electricity than the total primary energy supply in 2003. That is, solar cells with an assumed 8% efficiency installed in these areas would deliver a bit more energy in the form of electricity than what is currently available from oil, gas, hydropower, nuclear power, etc. combined.

It should also be noted that a recent concern is that of Global dimming, an effect of pollution that is allowing less and less sunlight to reach the Earth's surface. It is intricately linked with pollution particles and Global warming, and is mostly of concern for issues of Global climate change, but is also of concern to proponents of Solar Power due to the existing and potential future decreases in available Solar Energy. The order of magnitude is about 10% less solar energy available at sea level, mostly due to more intense cloud reflections back into outer space. That is, the clouds are whiter and brighter because the pollution dust serves as a vapor-liquid phase change initiation site and generates clouds where otherwise there would be a moisture filled but otherwise clear sky.

After passing through the Earth's atmosphere, most of the sun's energy is in the form of visible and Infrared radiations. Plants use solar energy to create chemical energy through photosynthesis. Humans regularly use this energy burning wood or fossil fuels, or when simply eating the plants.

Types of technologies

Most solar energy used today is harnessed as heat or electricity.

Solar design in architecture

Main articles: Passive solar and Active solar

Solar design can be used to achieve comfortable temperature and light levels with little or no additional energy. This can be through passive solar, where maximising the entrance of sunlight in cold conditions and reducing it in hot weather; and active solar, using additional devices such as pumps and fans to direct warm and cool air or fluid.

Solar heating systems

Main article: Solar hot water

Solar hot water systems use sunlight to heat water. These systems may be used to heat domestic hot water or for space heating but are most commonly used to heat pools. These systems are basically composed of solar thermal collectors and a storage tank. The three basic classifications of solar water heaters are:

• Active systems which use pumps to circulate water or a heat transfer fluid.
• Passive systems which circulate water or a heat transfer fluid by natural circulation. These are also called thermosiphon systems.
• Batch systems using a tank directly heated by sunlight.

A Trombe wall is a thermal mass that is heated by sunlight during the day and radiates stored heat during the evening.

Solar cooking

Main article: Solar cooker

Pictured: Solar Cooker

A solar box cooker traps the Sun's power in an insulated box; such boxes 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

Main articles: Daylighting and Light tube

The interior of a building can be lit during daylight hours using light tubes.

For instance, fiber optic light pipes can be connected to a parabolic collector mounted on the roof. The manufacturer claims this gives a more natural interior light and can be used to reduce the energy demands of electric lighting.

Photovoltaics

Main article: Photovoltaics

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 sunlight. Until recently, their use has been limited due to high manufacturing costs. One cost effective use has been in very low-power devices such as calculators with LCDs. Another use has been in remote applications such as roadside emergency telephones, remote sensing, cathodic protection of pipe lines, and limited "off grid" home power applications. A third use has been in powering orbiting satellites and other spacecraft.

Total peak power of installed PV is around 5,300 MW as of the end of 2005. This is only one part of solar-generated electric power. For solar reflector plants see below.

Declining manufacturing costs (dropping at 3 to 5% a year in recent years) are expanding the range of cost-effective uses. The average lowest retail cost of a large photovoltaic array 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 fibers

A photovoltaic device not using silicon is currently in development. [2] The device consists of a "solar tape," containing titanium dioxide (TiO2) in the form of a tape or fiber that could be combined with clothing or building materials. The material has inferior efficiency to conventional photovoltaics (5% for an initial commercial version to "near 12%" in the lab as of 2004, versus 15-30% for conventional cells). Its advantages are its low manufacturing cost, low weight, flexibility, function in artificial light, and resulting versatility. [3]

Concentrating Photovoltaic (CPV) systems

Despite major progress made over the last decade the use of solar panels remains relatively expensive compared to conventional electricity generation. One promising way to reduce cost even further is by using concentrating photovoltaic systems. The idea is to concentrate sunlight by lenses or mirrors onto a small panel of high-efficiency solar cells. That way expensive solar panels are replaced by cheap plastic or glass, thus dramatically reducing the cost per watt. In addition, the amount of solar energy harvested per m² is increased, thus reducing the area needed for generating solar power.

High-efficiency cells have been developed for special applications such as satellites and space exploration which require high-performance. GaAs multijunction devices are the most efficient solar cells to date, reaching as high as 39% efficiency. They are also some of the most expensive cells per unit area (up to US$40/cm²).

In Concentrating Photovoltaic systems solar energy is concentrated several hundred times, which increases the solar energy conversion efficiency and reduces the semiconductor area needed per watt of power output. This may be beneficial as an application for multi-junction solar cells, as the high costs and technical challenges of generating large area multi-junction photovoltaics are prohibitive relative to current silicon PV technologies.

Since concentrating photovoltaics requires solar tracking the approach is most suited for large utility scale applications. Different approaches are being evaluated for that purpose, in particular Fresnel lenses, parabolic trough concentration systems, and solar dishes.

For examples of concentrating photovoltaic systems suited for rooftop installation on commercial buildings, see the "Sunflower", and the "SunCube" for domestic applications.

Solar thermal electric power plants

Solar Two, a concentrating solar power plant (an example of solar thermal energy).

Main article: Solar thermal energy

Solar thermal energy can be used to heat a fluid to high temperatures and use it to produce electric power.

Solar updraft tower

Main article: Solar updraft tower

A Solar updraft tower is a relatively low tech solar thermal power plant where air passes under a very large agricultural glass house (between 2 and 8 km 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.

Energy Tower

An Energy tower is an alternative proposal for the Solar updraft tower. The "Energy Tower" is driven by spraying water at the top of the tower; evaporation of water causes a downdraft by cooling the air thereby increasing its density, driving windturbines at the bottom of the tower. It requires a hot arid climate and large quantities of water (seawater may be used for this purpose) but it does not require the large glass house of the Solar updraft tower.

Solar pond

A solar pond is a relatively low-tech, low cost approach to harvesting solar energy. The principle is to fill a pond with 3 layers of water:

1. A top layer with a low salt content
2. An intermediate insulating layer with a salt gradient, which sets up a density gradient that prevents heat exchange by natural convection in the water.
3. A bottom layer has with a high salt content which reaches a temperature approaching 90 degrees Celsius.

The different densities in the layers due to their salt content prevent convection currents developing which would normally transfer the heat to the surface and then to the air above. The heat trapped in the salty bottom layer can be used for different purposes, such as heating of buildings, industrial processes, or generating electricity. There is one in use at Bhuj, Gujarat, India and another at the University of Texas El Paso .

Solar chemical

Solar chemical refers to a number of possible processes that harness solar energy by absorbing sunlight in a chemical reaction in a way similar to photosynthesis in plants but without using living organisms. No practical process has yet emerged.
A promising approach is to use focused sunlight to provide the energy needed to split water into its constituent hydrogen and oxygen in the presence of a metallic catalyst such as zinc.

While metals, such as zinc, have been shown to drive photoelectrolysis of water, more research has focused on semiconductors. Further research has examined transition metal compounds, in particular titania, titanates, niobates, tantalates. Unfortunately, these materials exhibit very low efficiencies, because they require ultraviolet light to drive the photoelectrolysis of water. Current materials also require an electrical voltage bias for the hydrogen and oxygen gas to evolve from the surface, another disadvantage. Current research is focusing on the development of materials capable of the same water splitting reaction using lower energy visible light.

It is also possible to use solar energy to drive industrial chemical processes without a requirement for fossil fuel.

Biofuels

Main article: Biofuel

The oil in plant seeds, in chemical terms, very closely resembles that of petroleum. Many, since the invention of the Diesel engine, have been using this form of captured solar energy as a fuel comparable to petrodiesel - for functional use in any diesel engine or generator and known as Biodiesel. A 1998 joint study by the U.S. Department of Energy (DOE) and the U.S. Department of Agriculture (USDA) traced many of the various costs involved in the production of biodiesel and found that overall, it yields 3.2 units of fuel product energy for every unit of fossil fuel energy consumed. Other Biofuels include ethanol, wood for stoves, ovens and furnaces, and methane gas produced from biofuels through chemical processes.

Classifications of solar power technology

Solar power technologies can be classified in a number of ways.

Direct or Indirect

A photovoltaic cell produces electricity directly from solar energy

Direct solar power involves a single transformation of sunlight which results in a useable form of energy.

• 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 may be 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 is collected using focusing mirrors and transmitted via optical fibers into a building's interior to supplement lighting.
• Sunlight strikes a light mill and causes the vanes to rotate as mechanical energy, little practical application has yet been found for this effect.

File:Itaipu2.jpg

Hydroelectric power stations produce indirect solar power. The Itaipu Dam, Brazil / Paraguay

Indirect solar power involves multiple transformations of sunlight which result in a useable form of energy.

• Vegetation uses photosynthesis to convert solar energy to chemical energy incorporated in biomass. Biomass may be burned directly to produce heat and electricity or processed into methane (natural gas), hydrogen and other biofuels.
• Hydroelectric dams and wind turbines are powered by solar energy through its interaction with the Earth's atmosphere and the resulting weather phenomena.
• Ocean thermal energy production uses the thermal gradients present across ocean depths to generate power. These temperature differences are due to the energy of the sun.
• Fossil fuels are ultimately derived from solar energy captured by vegetation in the geological past.

Passive or Active

Passive solar systems use non-mechanical techniques of capturing, converting and distributing sunlight into useable forms of energy such as heating, lighting or ventillation. These techniques include selecting materials with favorable thermal properties, designing spaces that naturally circulate air and referencing the position of a building to the sun.

• Passive solar water heaters use a thermosiphon to circulate fluid.
• A Trombe wall circulates air by natural circulation and acts as a thermal mass which absorbs heat during the day and radiates heat at night.
• Clerestory windows, light shelves, skylights and light tubes can be used among other daylighting techniques to illuminate a building's interior.
• Passive solar water distillers may use capillary action to pump water.

Active solar systems use mechanical components such as pumps and fans to process sunlight into useable forms of energy.

Concentrating or Non-concentrating

Point focus parabolic dish with Stirling engine and its solar tracker at Plataforma Solar de Almería (PSA) in Spain.

Concentrating solar power systems use lenses or mirrors and tracking systems to focus sunlight into a high intensity beam capable of producing high temperatures and conversion efficiencies. Concentrating solar power systems are sub-classified by focus and tracking type.

• Line focus
  • A solar trough consists of an elongated parabolic reflector aligned on a north-south axis that uses single-axis tracking to follow the sun from east to west and concentrate light along a line formed at the parabola's focus. The SEGS systems in California are an example of this type of system.
• Point focus
  • A power tower consists of an array of flat reflectors that use dual-axis tracking to follow the sun and concentrate light at a single point on the tower where a thermal receiver is located.
  • A parabolic dish or dish/engine system consists of a stand-alone unit that uses dual-axis tracking to follow the sun and focuses light at a single point where photovoltaic cells or a thermal receiver is located.

The scientific solar furnace at Odeillo, French Cerdagne

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

Non-concentrating photovoltaic and solar thermal systems do not concentrate sunlight. Non-concentrating solar thermal systems absorb sunlight directly to a working fluid or thermal mass. These systems have the ability unlike concentrating solar power systems of effectively utilizing diffuse solar radiation; however, the maximum temperatures attainable are below 200 °C and conversion efficiencies are low.

• Solar water heating use dark solar thermal collectors to absorb sunlight and heat water or a working fluid.
• A Trombe wall consists of a thermal mass and air channel which are both heated by sunlight. The heated air circulates by natural circulation and the thermal mass absorbs heat which is radiated in the evening.

Advantages and disadvantages of Solar power

Advantages

• Solar power is pollution free during use. Production end wastes and emissions are manageable using existing pollution controls. Decommisioning end recycling technologies are under development.

• Facilities can operate with very little maintenance or intervention after initial setup.

• Solar power is becoming more and more economical as costs associated with production decreases, the technology becomes more effective in energy conversion, and the costs of other energy source alternatives increase.

• In situations where connection to the electricity grid is difficult, costly, or impossible (such as island communities, areas not served by a power grid, illuminated roadside signs, and ocean-going vessels) harvesting solar power is often an economically competitive alternative to energy from traditional sources.

• When grid connected, solar electric generation can displace the highest cost electricity during times of peak demand (in most climatic regions), can reduce grid loading, and can eliminate the need for local battery power for use in times of darkness and high local demand; such application is encouraged by net metering. Time-of-use net metering can be highly favorable to small photovoltaic systems.

• Grid connected solar electricity can be used locally thus minimizing transmission/distribution losses (approximately 7.2%).

Disadvantages

• Limited potential: total terrestial power incidence is $1.5\times 10^{17}{\hbox{ Watt}}$ which gives a theoretical maximum to the power gained from earth-based solar and wind energy.

• Intermittency: It is not available at night and is reduced when there is cloud cover, decreasing the reliability of peak output performance or requiring a means of energy storage.

• For power grids to stay functional at all times,
  • energy storage facilities, such as Pumped-storage hydroelectric facilities, are needed to 'gapfill' low points in solar generation
  • other renewable energy sources (i.e., wind, geothermal, tidal, wave, ocean power, etc) would need to be active, or
  • backup conventional powerplants would be needed. There is an energy cost to keep coal-burning power plants 'hot', which includes the burning of coal to keep boilers at temperature. Natural gas power plants can quickly come up to full load without requiring significant standby idling . Without changes in the energy supply and control system (such as a shift to using current hydropower as nighttime/backup across wider regions or the incorporation of more renewable power), few coal power plants could be displaced, according to critics.

• Locations at high latitudes or with frequent substantial cloud cover offer reduced potential for solar power use.

• It can only realistically be used to power transport vehicles by converting light energy into another form of energy (e.g. battery stored electricity or by electrolysing water to produce hydrogen) suitable for transport, incurring an energy penalty similar to coal or nuclear electricity generation. While the burning of gasoline in an internal combustion engine is only about 20%-25% efficient , depending on driving mode, the use of battery electric technology can match or exceed that efficiency when various external factors are included, such as the loss of energy in the production of gasoline and the energy cost of battery manufacture and recycling.

• Solar cells produce DC which must be converted to AC when used in currently existing distribution grids. This incurs an energy penalty of 5-10%.

Energy storage

Main article: 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
• Cryogenic liquid air or nitrogen
• Compressed air in a cylinder
• Flywheel energy storage
• Hydrogen produced by electrolysis of water and then available for pollution free combustion
• Hydraulic accumulator
• Pumped-storage hydroelectricity
• Molten salt
• Superconducting magnetic energy storages

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. However, since the grid always has a positive outflow, the result is exactly the same.

Electric power costs are highly dependant on the consumption per time of day, since plants must be built for peak power (not average power). Expensive gas-fired "peaking generators" must be used when base capacity is insufficient. Fortunately for solar, solar capacity parallels energy demand -since much of the electricity is for removing heat produced by too much solar energy (air conditioners)! This is less true in the winter. Wind power complements solar power since it can produce energy when there is no sunlight.

Deployment of solar power to energy grids

See main article Deployment of solar power to energy grids

Deployment of solar power depends largely upon local conditions and requirements. All industrialised nations share a need for electricity and it is clear that solar power will increasingly be used as an option for electricity supply.

Deployment of Solar power in transport

The solar powered car The Nuna 3 built by the Dutch Nuna team

Development of a practical solar powered car has been an engineering goal for twenty years. The center of this development is the World Solar Challenge, a biannual solar powered car race over 3021 km through central Australia from Darwin to Adelaide. The race's stated objective is to promote research into solar-powered cars. Teams from universities and enterprises participate. In 1987 when it was founded the winner's average speed was 67 km/h. By the 2005 race this had increased to a record average speed of 103 km/h.

See also

1973 energy crisis Absorption (electromagnetic radiation) Energy crisis Energy storage Microgeneration European Union Climate Change Programme Future energy development Green electricity Global dimming List of conservation topics Photoelectric effect

Renewable energy Solar air conditioning Solar car Solar ponds Solar Power Satellite Solar power tower Solar updraft tower Sustainable design Thermosolar Timeline of solar energy Trans-Mediterranean Renewable Energy Cooperation (TREC)

Corporate ownership of solar technology

In 1979 Ray Reece described how US corporations prevented the growth of the solar industry in his book The Sun Betrayed: Report on the Corporate Seizure of U.S. Solar Energy Development which provides "This is a disturbing history of the collusion between federal and corporate energy executives to control the development of solar energy."

External links

• Solar Energy International. Renewable Energy Education for a Sustainable Future
• Solar Power Energy Stocks Come to Light as Oil and Gas Prices Soar
• Solar Energy Industries Association is the national trade association for the US solar energy industry and has information on current commercial technologies and market developments.
• Direct solar U.S. Department of Energy: Energy Efficiency and Renewable Energy - Thermal water splitting
• U.S. Department of Energy: Energy Efficiency and Renewable Energy Solar History Timeline
• National Renewable Energy Laboratory: Concentrating Solar Power (CSP)
• ESTIF - European Solar Thermal Industry organization (statistics, market situation)
• Library of public domain images of operational Solar Power projects from the World-wide Information System for Renewable Energy (WIRE).
• Solar Facts - Information about all aspects of solar
• Solar Power for Everyone - DIY Solar power resources.
• Solar Power Wireless Mesh DIY Wireless Mesh Solar Experiment