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PowerPedia:Solar Energy

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Solar Energy covers the various solar innovations that increase collection efficiency and reduce costs. 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. This page is dedicated to exploring the future of solar energy: how technological advancements and future solar energy concepts might provide entirely new solar energy delivery systems at some point in the future.

Contents

Overview

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.

Technology Classifications

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

Direct or Indirect

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

  • Sunlight hits a photovoltaic 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.

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

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. 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.

Power technologies

Energy Tower and solar updraft towers

An Energy Tower is a new concept for producing electrical power for consumer consumption, the brainchild of Professor Dan Zaslavsky. An Energy Tower produces electricity by drawing the energy from the air around it. 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. An Energy Tower is a tall hollow cylinder with a water spray system at the top. The water is pumped up to the top of the tower and then sprayed inside the tower which cools the warm air hovering at the top. The cooled air, being denser than the outside warmer air, falls to the bottom of the cylinder which causes a turbine at the bottom of the cylinder to spin. The turbine is connected to a generator which produces the electricity. The tower should optimally be situated in a hot dry climate, which thus allows for the greatest extraction of energy from the air. The need for large quantities of water may be solved by choosing a location that is not too far from the coast. An alternative approach to this is the Solar updraft tower, but requires a huge diameter (up to 7 or 8 kilometres) agricultural glass house collectors to capture the solar heated air. Even though energy towers use some energy (about 50% of the turbine output) by having to pump water to the top and pressurizing nozzles, their advantage is that they require no such large collection areas, because dry air, if available, is continuously drawn at the top from the surroundings. Energy Towers 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.

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. The solar updraft tower is a type of renewable-energy power plant. Air is heated in a wide greenhouse and the resulting convection causes the air to rise and escape through a tall tower. The moving air drives turbines which produce electricity. There are no solar updraft towers in operation at present. A research prototype operated in Spain in the 1980s, and EnviroMission proposes to construct a full scale power station using this technology in Australia. The generating ability of a solar updraft power plant depends primarily on two factors: the size of the collector area and chimney height.

With a larger collector area, more volume of air is warmed up to flow up the chimney; collector areas as large as 7 km in diameter have been considered. With a larger chimney height, the pressure difference increases the stack effect; chimneys as tall as 1000 m have been considered. Further, a combined increase of the collector area and the chimney height leads to massively larger productivity of the power plant. Heat can be stored inside the collector area greenhouse, to be used to warm the air later on. Water, with its relatively high specific heat capacity, can be filled in tubes placed under the collector increasing the energy storage as needed. Turbines can be installed in a ring around the base of the tower, with a horizontal axis, as planned for the Australian project described below and seen in the diagram above; or (as in the prototype in Spain) a single vertical axis turbine can be installed inside the chimney. Solar towers do not produce CO2 emissions, but do require a large initial construction investment. A solar updraft tower power station would directly impact a significant area of land if it were designed to generate as much electricity as is produced by modern power stations using other technology such as burning coal. However, the land under the collector could be used for farming or other purposes.

Space-Based Solar Collectors

Traditionally, capturing solar energy has been an earth-based technological process. Solar panels or arrays are placed on buildings or on the ground to collect the incoming energy from the sun. There is, however, a novel idea for capturing solar energy from space-based satellites and beaming it down to earth.

The solar energy captured by the satellites would be converted to microwaves and beamed to diode converters on the ground, which in turn would convert the energy into useful electricity. An obvious advantage of space-based solar collectors is the fact that the sun's energy is much stronger outside of the earth's atmosphere, due to the absence of the atmosphere's blocking effect, and tremendous amounts of energy could be delivered to earth for use as electricity. The major disadvantage is cost, as satellite technology is quite expensive, although it is certainly an idea to consider for future use, especially as space travel becomes more common and eventually perhaps satellite technology becomes less costly.

Space Solar Collectors Future

The National Aeronautics and Space Administration (NASA) has studied Space-Based Solar Collectors in the past and is apparently still considering their use in the future (perhaps by 2015). Here's a piece from an article about NASA's Space-Based Solar plans:

Solar panels have been used to help power spacecraft since the earliest days of the space age, and the dream of someday using spacecraft to harness the sun's rays for use on Earth has existed at least as long. NASA studied the concept extensively in the 1970s before rejecting space-based solar power stations as too costly and impractical. But NASA recently dusted off those old plans and is taking a fresh look at the concept.

As now envisioned, each "Sun Tower" might stretch 22 miles in length, with pairs of 100- to 200-meter diameter solar collectors arranged along a backbone. From its perch in geosynchronous orbit, a Sun Tower would beam 1.2 billion watts of power to ground stations via microwave -- enough electricity for 1.2 million homes and 40 percent more than generated by either of the Beaver Valley Nuclear Power Stations. NASA eyes solar energy collectors in space by 2015

  • Solar Power from Space As reported in the EPRI Journal, sun-facing photovoltaic arrays in stationary Earth orbit at an altitude of 22,300 miles would receive eight times as much sunlight as they would at the earth's surface, on average.
  • Power from Space Solar power collected in space and beamed to Earth could be an environmentally friendly solution to our planet's growing energy problems.
  • What ever happened to solar power satellites? At the end of June 2004, a conference about space based solar power generation was held in Granada, Spain. The conference provided progress reports from groups in Europe, the US, and Japan who are working on concepts and plans for building solar power plants in orbit that would beam electricity down for use on Earth.

Nanotechnology Collectors

Solar technology has been stuck for decades, with only minor incremental improvements in silicon-based solar cells, never achieving more than 30% efficiency ratios for converting sunlight to electricity. However, this malaise in solar technology appears to be coming to an end, due to advancements in nanotechnology (building structure on the molecular level).

Recent nanotechnology breakthroughs in regards to solar have promised new solar cell designs capable of capturing a much wider range of solar energy, which would be much more efficient at converting solar energy to electricity (approaching 60% efficiency), more versatile (able to be painted onto just about any surface), and less costly than today's solar technology. These nanotechnology advancements in solar energy technology appear to be finally advancing solar beyond it's initial silicon-based limitations.

Nano Solar Collectors Future

  • Nanotechnology Plus Plastic Electronics: Solar Cells A new generation of solar cells that combines nanotechnology with plastic electronics has been launched with the development of a semiconductor-polymer photovoltaic device by researchers with the U.S. Department of Energy's Lawrence Berkeley National Laboratory (Berkeley Lab) and the University of California at Berkeley (UCB). Such hybrid solar cells will be cheaper and easier to make than their semiconductor counterparts, and could be made in the same nearly infinite variety of shapes as pure polymers.
  • Nanotechnology May Give Plastic Solar Cells A BoostRIT researchers, led by Ryne Raffaelle, professor of physics and microsystems engineering and director of the NanoPower Research Laboratories, hope to develop an improved polymer solar cell using nanomaterial additives. Raffaelle and his team will use a thin polymer film that can be rolled out in sheets. The film will contain nanoscale pieces of semiconductor material and single-walled carbon nanotubes to maximize energy conversion.
  • http://www.news.utoronto.ca/bin6/050110-832.asp

Solar fibers

A photovoltaic device not using silicon is currently in development. [1] 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. [2]

Concentrated Solar Stirling Engine

One unexpected new use for the Stirling Engine, an engine originally designed to deal with a 19th Century problem (the fact that steam engines had a tendency to explode) is to use them in combination with solar technology to generate electricity from the temperature differential created by concentrating hot solar power on a Stirling Engine. Instead of converting sunlight directly into electricity via photovoltaic solar cells, this method utilizes a solar concentrating dish to focus incoming solar energy on a Stirling Engine, which in turn generates electricity.

This novel approach to using solar to create electricity is made possible by the fact that the Stirling Engine is a heat engine that turns solar provided heat directly into energy (in this case electricity). It might not turn out to be the most efficient way to create electricity by solar means, but it is worthy of exploration as it might be useful in unique situations, and further developments in efficiency might make it suitable for large scale implementation in sunny regions of the world. I believe the main advantage of this system is that it would be less expensive to build than large photovoltaic solar cell arrays, as there is no need for expensive silicon in this form of solar energy. It's a matter of using a dish with large mirrors to focus the solar energy (heat) on the Stirling Engine. The following article explains how the Solar/Stirling Engine design works and plans for future deployment of this technology to create utility scale electricity.

Sandia, Stirling to build solar dish engine power plant Goal is to deploy solar dish farms with 20,000 units producing energy

ALBUQUERQUE, N.M. — The National Nuclear Security Administration’s Sandia National Laboratories is joining forces with Stirling Energy Systems, Inc. (SES) of Phoenix to build and test six new solar dish-engine systems for electricity generation that will provide enough grid-ready solar electricity to power more than 40 homes. Solar/Stirling Engine Story Continues
Sandia, Stirling Energy Systems set new world record for solar-to-grid conversion efficiency. 31.25 percent efficiency rate topples 1984 record. Solar efficiency record set Feb. 12, 2008

Photovoltaics

Solar cells, also referred to as photovoltaic cells (or just photovoltaics), 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.

Quantum Dot Solar Cell Materials

Quantum Dot Materials Can Reduce Heat, Boost Electrical Output Monday, May 23, 2005

Golden, Colo. — Researchers at the U.S. Department of Energy's National Renewable Energy Laboratory (NREL) have shown that nanotechnology may greatly increase the amount of electricity produced by solar cells. In a paper published in a May issue of the American Chemical Society's Nano Letters journal, an NREL team found that tiny "nanocrystals," also known as "quantum dots," produce as many as three electrons from one high energy photon of sunlight. When today's photovoltaic solar cells absorb a photon of sunlight, the energy gets converted to at most one electron, and the rest is lost as heat. "We have shown that solar cells based on quantum dots theoretically could convert more than 65 percent of the sun's energy into electricity, approximately doubling the efficiency of solar cells," Nozik said. The best cells today convert about 33 percent of the sun's energy into electricity. Quantum Dot Materials Story Continues

Full Spectrum Solar Cell

Researchers in the Materials Sciences Division (MSD) of Lawrence Berkeley National Laboratory, working with crystal-growing teams at Cornell University and Japan's Ritsumeikan University, have discovered a newly established low band gap for indium nitride, which means that the indium gallium nitride system of alloys (In1-xGaxN) can cover the full solar spectrum, which could dramatically increase solar conversion efficiencies. The serendipitous discovery means that a single system of alloys incorporating indium, gallium, and nitrogen can convert virtually the full spectrum of sunlight -- from the near infrared to the far ultraviolet -- to electrical current, at solar conversion efficiencies that are estimated to approach an astounding 70% (compared to the best solar conversion efficiencies currently possible that are approximately 30%).

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. [citation needed]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.

Total Spectrum Solar Consentrator

Solar thermal electric power plants

Total Spectrum Solar Consentrator is a concept where you first consentrate the suns rays using parabolic mirrors or a fresnel lense, and then you spread them out in a spectum using a prism, wich enables you to place optimal photo-electric materials for each frequency along the spectrum. Solar thermal energy can be used to heat a fluid to high temperatures and use it to produce electric power. Solar thermal energy refers to the idea of harnessing solar power for practical applications from solar heating to electrical power generation. Solar thermal collectors, such as solar hot water panels, are commonly used to generate solar hot water for domestic and light industrial applications. Solar thermal energy is used in architecture and building design to control heating and ventilation in both active solar and passive solar designs. This article is devoted primarily to solar power generation facilities, that is, solar power plants that generate electricity by converting solar energy to heat, to drive a thermal power plant. The article on photovoltaics reviews solar power generation by means of solar electric panels.

Concentrated solar power (CSP) plants

In concentrating collectors, the area intercepting the solar radiation is greater, sometimes hundreds of times greater, than the absorber area. Where temperatures below about 200 °F are sufficient, such as for space heating, flat-plate collectors of the nonconcentrating type are generally used. These hold temperatures "in stagnation" at between 150 and 220 degrees Celsius.

Power tower designs

Power towers (also know as 'central tower' power plants or 'heliostat' power plants (power towers) use an array of flat, moveable mirrors (called heliostats) 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. Neither of these are currently used for active energy generation. In South Africa, a solar power plant is planned with 4000 to 5000 heliostat mirrors, each having an area of 140 m²

Dish designs

A dish system uses a large, reflective, parabolic dish (similar in shape to satellite television dish). It focuses all the sunlight that strikes the dish up onto to a single point above the dish, where a thermal collector is used to capture the heat and transform it into a useful form. Dish systems, like power towers, can achieve much higher temperatures due to the higher concentration of light which they receive. 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.

Fresnel designs

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). These systems claim to offer lower overall costs because they permit the heat-absorbing element to be shared between several mirrors. The mirrors can therefore be smaller and do not require complex pivoting couplings for the fluid flowing from the absorber. The design can also permit mirrors to be placed closer together, allowing for a more efficient use of land area.

Sol Solution Has developed a photovoltaic system using a linear Fresnel design. The system takes advantage of chromatic aberration to create a 'Rainbow Concentrator' to separate and concentrate the solar spectrum. This allows higher efficiencies for solar cells that are optimized for a specific range of wavelengths.
You can find their link Here.

Conversion rates

Of these technologies the solar dish/stirling has the highest energy efficiency (the current record is a conversion efficiency of 30% of solar energy). A single solar dish-Stirling engine installed at Sandia National Laboratories’ National Solar Thermal Test Facility produces as much as 25 kW of electricity, while its footprint is a hundred times smaller than the spain solar updraft tower. Solar trough plants have been built with efficiencies of about 20%. The Concentrated Solar Power (CSP) Plant using the parabolic trough principle called the SEGS system, in California in the United States, produces 330 MW, and it is currently the largest solar thermal energy system in operation. Furthermore, Southern California Edison announced an agreement to purchase solar powered Stirling engines from Stirling Energy Systems over a twenty year period and in quantities (20,000 units) sufficient to generate 500 megawatts of electricity. Stirling Energy Systems announced another agreement with San Diego Gas & Electric to provide between 300 and 900 megawatts of electricity.

The gross conversion efficiencies (taking into account that the solar dishes or troughs occupy only a fraction of the total area of the power plant) are determined by net generating capacity over the solar energy that falls on the total area of the solar plant. The 500-megawatt (MW) SCE/SES plant would extract about 2.75% of the solar power (1 kW/m²; see Solar power for a discussion) that impinges on its 4,500-acres (18.2 km²).[11] For the 50MW AndaSol Power Plant that is being built in Spain (total area of 1,300×1,500 m = 1.95 km²) gross conversion efficiency comes out at 2.6%.

Biofuels

The oil in plant seeds, in chemical terms, very closely resembles that of petroleum and is reffered as "Biofuel". 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. [24] Other Biofuels include ethanol, wood for stoves, ovens and furnaces, and methane gas produced from biofuels through chemical processes.

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 cooking

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.

Directory:Solar Cooking

Solar lighting

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. Solar lighting is the passive solar practice of placing windows, or other transparent media, and reflective surfaces so that, during the day, natural sunlight provides effective internal illumination. In general passive solar technique, buildings are designed such as to account for local climate, in particular the luminance of the sky.

Light tubes or light pipes are used for transporting or distributing natural or artificial light. In their application to daylighting, they are also called solar tubes, solar pipes, daylight pipes, or solar light pipes. Generally speaking, a light pipe or light tube may refer to:

  • a tube or pipe for transport of light to another location, minimizing the loss of light;
  • a transparent tube or pipe for distribution of light over its length, either for equidistribution along the entire length (see also sulfur lamp) or for controlled light leakage. Both have the purpose of lighting, for example in Architecture.

In cooler parts of northern countries with largely overcast sky, a house will be designed with minimal windows on the north side but more and larger windows on the south side. This is because in the Northern Hemisphere, above the Tropic of Cancer, there is no direct sunlight on the north wall of a house from the autumnal equinox to the spring equinox, north-side windows are ineffective at daylighting. South-side windows receive at least some direct sunlight on any sunny day of the year, so they are effective at Solar lighting areas of the house adjacent to the windows. One disadvantage of relying on conventional window space for Solar lighting is that, especially during mid-winter, it tends to be highly directional light that casts deep shadows.

Another important element in creating Solar lighting is the use of clerestory windows. These are high, vertically-placed windows oriented to the sun to admit sunlight for daylighting: towards the south in the Northern Hemisphere, and towards the north in the Southern Hemisphere. In the case of a passive solar house, these may provide a direct light path to north-side (in the northern hemisphere; south-side in the southern) rooms that otherwise would not be illuminated. Often, clerestory windows also shine onto interior wall surfaces painted white or another light color. These walls are placed so as to reflect indirect light to interior areas where it is needed. This method has the advantage of reducing the directionality of light to make it softer and more diffuse, reducing shadows.

Skylights are often used for daylighting, but they have half the insulating value of a similarly constructed windows because warm air rises to the ceiling where the skylight is located. In addition, the amount of light skylights deliver peaks in the around midday, when the additional light and heat it provides is least needed. Poorly constructed skylights may have leak problems and single-paned ones may weep with condensation. Still, skylights have a useful place in daylighting, particularly in large rooms or interior rooms with no windows to the outside. Using skylights with at least two panes of glass and a heat reflecting coating will increase their energy efficiency. Another type of device used are light tubes, also called solar tubes, placed into a roof and admitting light to a focused area of the interior. These somewhat resemble recessed light fixtures in the ceiling. They do not allow as much heat transfer as skylights because they have less exposed surface area. It is also easier to retrofit light tubes into existing buildings, especially those with deep roof constructions.

Deployment of solar power to energy grids

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. 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.

North America

In some areas of the United States, solar electric systems are already competitive with utility systems. As of 2005, there is a list of technical conditions that factor into the economic feasibility of going solar: the amount of sunlight that the area receives; the purchase cost of the system; the ability of the system owner to sell power back to the electric grid; and most important, the competing power prices from the local utility. For example, a photovoltaic system installed in Boston, Massachusetts, produces 25% less electricity than it would in Albuquerque, New Mexico, but yields roughly the same savings on utility bills since electricity costs more in Boston.

In addition to these considerations, many states and regions offer substantial incentives to improve the economics for potential consumers. Congress recently adopted the first federal tax breaks for residential solar since 1985 -- temporary credits available for systems installed in 2006 or 2007. Homeowners can claim one federal credit of up to $2,000 to cover 30% of a photovoltaic system's cost and another 30% credit of up to $2,000 for a solar thermal system. Fifteen states also offer tax breaks for solar, and two dozen states offer direct consumer rebates. Solar One is a pilot solar-thermal project in the Mojave Desert near Barstow, California. It uses heliostats, and molten salts storage technology, to achieve longer periods of power generation. Solar Two, also near Barstow, has now built and elaborated on the success of Solar One. It was an R&D project in Barstow, California, financed by the US federal Department of Energy. Solar Two used liquid salts as a storage medium in order to continue to provide energy for much of the time when sunlight is not available. Its success has led to the larger Solar Tres project in Spain.

On August 11, 2005, Southern California Edison announced an agreement to purchase solar powered Stirling engines from Stirling Energy Systems over a twenty year period and in quantities (20,000 units) sufficient to generate 500 megawatts of electricity.In some areas of the United States, solar electric systems are already competitive with utility systems. As of 2005, there is a list of technical conditions that factor into the economic feasibility of going solar: the amount of sunlight that the area receives; the purchase cost of the system; the ability of the system owner to sell power back to the electric grid; and most important, the competing power prices from the local utility. For example, a photovoltaic system installed in Boston, Massachusetts, produces 25% less electricity than it would in Albuquerque, New Mexico, but yields roughly the same savings on utility bills since electricity costs more in Boston.

In addition to these considerations, many states and regions offer substantial incentives to improve the economics for potential consumers. Congress recently adopted the first federal tax breaks for residential solar since 1985 -- temporary credits available for systems installed in 2006 or 2007. Homeowners can claim one federal credit of up to $2,000 to cover 30% of a photovoltaic system's cost and another 30% credit of up to $2,000 for a solar thermal system. Fifteen states also offer tax breaks for solar, and two dozen states offer direct consumer rebates. Solar One is a pilot solar-thermal project in the Mojave Desert near Barstow, California. It uses heliostats, and molten salts storage technology, to achieve longer periods of power generation. Solar Two, also near Barstow, has now built and elaborated on the success of Solar One. It was an R&D project in Barstow, California, financed by the US federal Department of Energy. Solar Two used liquid salts as a storage medium in order to continue to provide energy for much of the time when sunlight is not available. Its success has led to the larger Solar Tres project in Spain.

On August 11, 2005, Southern California Edison announced an agreement to purchase solar powered Stirling engines from Stirling Energy Systems over a twenty year period and in quantities (20,000 units) sufficient to generate 500 megawatts of electricity. These systems — to be installed on a 4,500 acre (18 km²) solar farm — will use mirrors to direct and concentrate sunlight onto the engines which will 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 electricity.

The world's largest solar power plant is located in the Mojave Desert. Solel, an Israeli company, operates the plant, which consists of 1000 acres (4 km²) of solar reflectors. This plant produces 90% of the world's commercially produced solar power (excluding photovoltaics). On January 12, 2006, the California Public Utilities Commission approved the California Solar Incentive Program, a comprehensive $2.8 billion program that provides incentives toward solar development over 11 years. These systems — to be installed on a 4,500 acre (18 km²) solar farm — will use mirrors to direct and concentrate sunlight onto the engines which will 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 electricity. The world's largest solar power plant is located in the Mojave Desert. Solel, an Israeli company, operates the plant, which consists of 1000 acres (4 km²) of solar reflectors. This plant produces 90% of the world's commercially produced solar power (excluding photovoltaics). On January 12, 2006, the California Public Utilities Commission approved the California Solar Incentive Program, a comprehensive $2.8 billion program that provides incentives toward solar development over 11 years.

Other continents

Africa is home to the over 9 million km² Sahara desert, whose overall capacity — assuming 50 MW/km² day/night/cloud average with 15% efficient photovoltaic panels — is over 450 TW, or over 4,000,000 terawatt-hours per year. The current global energy consumption by humans, including all oil, natural gas, coal, nuclear, and hydroelectric, is pegged at about 13 TW.

The largest solar power station in Australia is the 400kWp array at Singleton, New South Wales. Other significant solar arrays include the 220 kWp array on the Anangu Pitjantjatjara Lands in South Australia, the 200kWp array at Queen Victoria Market in Melbourne and the 160kWp array at Kogarah Town Square in Sydney. A building-integrated photo voltaic (BIPV) installation of 60kW in Brisbane (at the Hall-Chadwick building) has an uninterruptible power supply (UPS) which gives around 10-15 minutes worth of emergency power in the event of the loss of electricity supply. Any power not used by the UPS is connected to the grid and goes towards reducing the building's overall power bills. Numerous smaller arrays have been established, mainly in remote areas where solar power is cost-competitive with diesel power.

As of 2004, Japan had 1200 MWe installed. Japan currently consumes about half of worldwide production of solar modules, mostly for grid connected residential applications. In terms of overall installed PV capacity, India comes fourth after Japan, Germany, and the United States (Indian Ministry of Non-conventional Energy Sources 2002). Government support and subsidies have been major influences in its progress. India's very long-term solar potential may be unparalleled in the world because it is one of the few places with an ideal combination of both high solar power reception and a large consumer base in the same place. India's theoretical solar potential is about 5000 TW·h per year (i.e. 600 GW), far more than its current total consumption. In 2005, the Israeli government announced an international contract for building a 100 MW 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 500 MW power plant, making Israel a leader in solar power production.

The 10 megawatt Bavaria Solarpark in Germany is the world's largest solar electric system, covering 25 hectares (62 acres) with 57,600 photovoltaic panels. 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. Another site is the Loser in Austria. The Plataforma Solar de Almería (PSA) in Spain, part of the Center for Energy, Environment and Technological Research (CIEMAT), is the largest center for research, development, and testing of concentrating solar technologies in Europe. In the United Kingdom, the second tallest building in Manchester, the CIS Tower, was clad in photovoltaic panels at a cost of £5.5 million and started feeding electricity to the national grid on November 2005. On April 27, 2006, GE Energy Financial Services, PowerLight Corporation and Catavento Lda announced that they will build the world’s largest solar photovoltaic power project. The 11-megawatt solar power plant, comprising 52,000 photovoltaic modules, will be built at a single site in Serpa, Portugal, 200 kilometers (124 miles) southeast of Lisbon in one of Europe’s sunniest areas.

Solar power Pros and Cons

Pros

  • The total terrestrial solar power incidence is 175,000 TW (terrawatts) out of which 125,000 TW reaches the surface (the rest being reflected by clouds and absorbed by the atmosphere) which compares to about 13 TW total energy consumption by humans (oil, gas, nuclear, hydro), and 100 TW total photosynthetic activity (basically supporting all life/food on Earth, less than 0.1% of available radiation energy captured ) - meaning that solar energy could cover many times humanity's energy needs.
  • 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%).

Cons

  • Limited areal power density: For electrical generation with photovoltaics, the average irradiation power density is approximately 1 kW/m2 usable by 8-15% efficient solar panels.
  • 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, the addition of substantial amounts of solar generated electricity would require one or more of the following;
    • 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:

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.

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Solar energy pros and cons

See Also

- PowerPedia main index
- PESWiki home page

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