Solar power in applications

"Alternative Energy"

Solar power (also known as solar energy) is the technology of obtaining usable energy from the light of the sun. Solar energy 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. Solar energy is energy deployed from the sun by nuclear reactions inside its core and outer levels.

Solar energy is currently used in a number of applications:
Heat (hot water, building heat, cooking)
Electricity generation (photovoltaics, heat engines)
Desalination of seawater.

Contents
1 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.

Map of global solar energy resources. The colours show the average available solar energy on the surface during 1991 to 1993. For comparison, the dark disks represent the land area required to supply the primary energy demand in the year 2010 using currently available technology (i.e. with a conversion efficiency of 8%).


Solar radiation reaches the Earth's upper atmosphere at a rate of 1366 watts per square meter (W/m2).[1] The first map shows how the solar energy varies in different latitudes.
While traveling through the atmosphere 6% of the incoming solar radiation (insolation) is reflected and 16% is absorbed resulting in a peak irradiance at the equator of 1,020 W/m².[2] Average atmospheric conditions (clouds, dust, pollutants) further reduce insolation by 20% through reflection and 3% through absorption.[3] Atmospheric conditions not only reduce the quantity of insolation reaching the Earth's surface but also affect the quality of insolation by diffusing incoming light and altering its spectrum.


The second map shows the average global irradiance calculated from satellite data collected from 1991 to 1993. For example, in North America the average insolation at ground level over an entire year (including nights and periods of cloudy weather) lies between 125 and 375 W/m² (3 to 9 kWh/m²/day).[4] This represents the available power, and not the delivered power. At present, photovoltaic panels typically convert about 15% of incident sunlight into electricity; therefore, a solar panel in the contiguous United States on average delivers 19 to 56 W/m² or 0.45 - 1.35 (kW·h/m²)/day.[5]


The dark disks in the third map on the right are an example of the land areas that, if covered with 8% efficient solar panels, would produce slightly more energy in the form of electricity than the total world primary energy supply in 2003.[6] While average insolation and power offer insight into solar power's potential on a regional scale, locally relevant conditions are of primary importance to the potential of a specific site.
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.


A recent concern is global dimming, an effect of pollution that is allowing less sunlight to reach the Earth's surface. It is intricately linked with pollution particles and global warming, and it is mostly of concern for issues of global climate change, but is also of concern to proponents of solar power because of the existing and potential future decreases in available solar energy. The order of magnitude is about 4% less solar energy available at sea level over the timeframe of 1961–90, mostly from increased reflection from clouds back into outer space.[7]


1.1 Types of technologies


Many technologies have been developed to make use of solar radiation. Some of these technologies make direct use of the solar energy (e.g. to provide light, heat, etc.), while others produce electricity.



1.2 Solar design in architecture



Main article: Passive solar building design
Solar design in architecture involves the use of appropriate solar technologies to maintain a building’s environment at a comfortable temperature through the sun's daily and annual cycles. It may do this by storing solar energy as heat in the walls of a building, which then acts to heat the building at night. Another approach is to keep the interior cool during a hot day by designing in natural convection through the building’s interior.



1.3 Solar heating systems


Solar heating systems
Main articles: Solar hot water and Solar combisystem

Solar water heaters on a rooftop in Jerusalem, Israel
Solar hot water systems use sunlight to heat water. They may be used to heat domestic hot water, for space heating or to heat swimming pools. These systems are composed of solar thermal collectors, a storage tank and a circulation loop.[8] The three basic classifications of solar water heaters are:
Batch systems which consist of a tank that is directly heated by sunlight. These are the oldest and simplest solar water heater designs, however; the exposed tank can be vulnerable to cooldown.[9]


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.


A Trombe wall is a passive solar heating and ventilation system consisting of an air channel sandwiched between a window and a sun-facing wall. Sunlight heats the air space during the day causing natural circulation through vents at the top and bottom of the wall and storing heat in the thermal mass. During the evening the Trombe wall radiates stored heat.[10]
A transpired collector is an active solar heating and ventilation system consisting of a perforated sun-facing wall which acts as a solar thermal collector. The collector pre-heats air as it is drawn into the building's ventilation system through the perforations. These systems are inexpensive and commercial models have achieved efficiencies above 70%. Most systems pay for themselves within 4-8 years.[11]

1.4 Solar cooking


Solar Cookers use sunshine as a source of heat for cooking as an alternative to fire.
A solar box cooker traps the sun's energy 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 breathing environment for the cooks.
The first known western solar oven is attributed to Horace de Saussure in 1767, which impressed Sir John Herschel enough to build one for cooking meals on his astronomical expedition to the Cape of Good Hope in Africa in 1830.[12] Today, there are many different designs in use around the world.[13]



1.5 Solar lighting


Main articles: Daylighting and Light tube
Solar lighting or daylighting is the use of natural light to provide illumination. Daylighting directly offsets energy use in electric lighting systems and indirectly offsets energy use through a reduction in cooling load.[14] Although difficult to quantify, the use of natural light also offers physiological and psychological benefits.


Daylighting features include building orientation, window orientation, exterior shading, sawtooth roofs, clerestory windows, light shelves, Hybrid Solar Lighting[15], skylights and light tubes.[16] These features may be incorporated in existing structures but are most effective when integrated in a solar design package which accounts for factors such as glare, heat gain, heat loss and time-of-use. Architectural trends increasingly recognize daylighting as a cornerstone of sustainable design.


Daylight saving time (DST) can be seen as a method of utilising solar energy by matching available sunlight to the hours of the day in which it is most useful. DST energy savings have been estimated to reduce total electricity use in California by 0.5% (3400 MWh) and peak electricity use by 3% (1000 MW).[17] However, there is some question whether these estimates are valid. In 2000 when parts of Australia began DST in late winter, overall electricity consumption did not decrease, but the peak load increased.[18]


1.6 Photovoltaics


The solar panels (photovoltaic arrays) on this small yacht at sea can charge the 12 V batteries at up to 9 A 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 because of 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 spacecraft.


Total peak power of installed PV is around 1,700 MW as of the end of 2005.[19] This is only one part of solar-generated electric power.


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.[20] 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 - those systems that use an inverter to connect to the utility grid instead of relying on batteries - now make up the largest part of the market.
In 2003, worldwide production of solar cells increased by 32%.[21] Between 2000 and 2004, the increase in worldwide solar energy capacity was an annualized 60%.[22] 2005 was expected to see large growth again, but shortages of refined silicon have been hampering production worldwide since late 2004.[23] Analysts have predicted similar supply problems for 2006 and 2007.[24]


1.7 Solar thermal electric power plants




Solar Two, a concentrating solar power tower (an example of solar thermal energy applied to electrical power production).
Main article: Solar thermal energy
Solar thermal energy can be focused on a heat exchanger, and converted in a heat engine to produce electric power or applied to other industrial processes.


1.7.1 Power towers


Power towers use an array of flat, movable 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 working fluid for conversion to electrical energy in a heat engine, or in some instances, stored for nighttime usage, in order to provide a more continuous output.





1.7.2 Parabolic troughs

A long row of parabolic mirrors concentrates sunlight on a tube filled with a heat transfer fluid (usually oil). As with the power tower, this heated oil is used to power a conventional steam turbine, or stored for nighttime use. The largest operating solar power plant, as of 2007, is one of the SEGS parabolic trough systems in the Mojave Desert in California, USA (see Solar power plants in the Mojave Desert).


1.7.3 Concentrating collector with steam engine

Solar energy converted to heat in a concentrating collector can be used to boil water into steam (as is done in nuclear and coal power plants) to drive a steam engine or steam turbine. The concentrating collector can be a trough collector, parabolic collector, or power tower.


1.7.4 Concentrating collector with Stirling engine

Solar energy converted to heat in a concentrating (dish or trough parabolic) collector can be used to drive a Stirling engine, a type of heat engine which uses a sealed working gas (i.e. a closed cycle) and does not require a water supply.
Until recently, a solar Stirling system held the record for converting solar energy into electricity (30% at 1,000 watts per square meter).[25] Such concentrating systems produce little or no power in overcast conditions and incorporate a solar tracker to point the device directly at the sun. That record has been broken by a so-called concentrator solar cell produced by Boeing-Spectrolab which claims a conversion efficiency of 40.7 percent.[26]


1.7.5 Solar updraft tower

A solar updraft tower (also known as a solar chimney, but this term is avoided by many proponents due to its association with fossil fuels) 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.


1.7.6 Energy tower



An energy tower is an alternative proposal to the solar updraft tower. It 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 wind turbines at the bottom of the tower. It requires a hot arid climate and large quantities of water (seawater may be used) but does not require the large glass house of the solar updraft tower.


1.8 Solar pond



A solar pond is simply a pool of water which collects and stores solar energy. It contains layers of salt solutions with increasing concentration (and therefore density) to a certain depth, below which the solution has a uniform high salt concentration. It is a relatively low-tech, low-cost approach to harvesting solar energy. The principle is to fill a pond with 3 layers of water:
A top layer with a low salt content.
An intermediate insulating layer with a salt gradient, which sets up a density gradient that prevents heat exchange by natural convection in the water.
A bottom layer with a high salt content which reaches a temperature approaching 90 degrees Celsius.
The layers have different densities due to their different salt content, and this prevents the development of convection currents which would otherwise transfer the heat to the surface and then to the air above. The heat trapped in the salty bottom layer can be used for heating of buildings, industrial processes, generating electricity or other purposes. One such system is in use at Bhuj, Gujarat, India[27] and another at the University of Texas El Paso.[28]


1.9 Solar chemical


Solar chemical is any process that harnesses solar energy by absorbing sunlight and using it to drive an endothermic or photoelectrochemical chemical reaction. Prototypes, but no large-scale systems, have been constructed.
One approach has been to use conventional solar thermal collectors to drive chemical dissociation reactions. Ammonia can be separated into nitrogen and hydrogen at high temperature and with the aid of a catalyst, stored indefinitely, then recombined later to release the heat stored. A prototype system was constructed at the Australian National University[29].
Another approach is to use focused sunlight to provide the energy needed to split water via photoelectrolysis into its constituent hydrogen and oxygen in the presence of a metallic catalyst such as zinc.[30]. Other research in this area has focused on semiconductors, and on the use of examined transition metal compounds, in particular titanium, niobium and tantalum oxides [31]. 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.
Solar thermal energy also has the potential to be used directly to drive chemical processes that require significant amounts of process heat, including at high temperatures that can be otherwise quite hard to attain[32].

From wikipedia

Alternative energy