Solar Power

Solar power (energy/unit time) uses electromagnetic radiation emitted from the sun.   Solar power, a renewable energy source, has been used in many traditional technologies for centuries, and is in widespread use where other power supplies are absent, such as in remote locations and in space.   Solar power is currently used in a number of applications: ^{Wiki n.p.}

• Heat (hot water, building heat, cooking)
• Electricity generation (photovoltaics, heat engines)
• Transportation (solar car)
• Desalination of seawater
• Photosynthesis of plants

Solar radiation reaches the Earth's upper atmosphere at a rate of 1366 watts per square meter (W/m2).   While traveling through the atmosphere, 6% of the incoming solar radiation is reflected and 16% is absorbed resulting in a peak irradiance at the equator of 1,020 W/m².   Average atmospheric conditions (clouds, dust, pollutants) further reduce insolation by 20% through reflection and 3% through absorption.   Atmospheric conditions not only reduce the amount of radiation reaching the Earth's surface, but also affect the quality of radiation by diffusing incoming light and altering its spectrum.   For example, in North America the average radiation 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).   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 kWh/m²/day.   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. ^{Wiki n.p.}

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 to see and keep warm.   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 time period of 1961–90, mostly from increased reflection from clouds back into outer space. ^{Wiki n.p.}

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.   Solar water heaters on rooftops use sunlight to heat water.   Solar hot water systems were used extensively in the United States up to the 1920s until replaced by relatively cheap and more reliable conventional heating fuels.   The economic advantage of conventional heating fuels has varied over time resulting in periodic interest in solar hot water.   However, solar hot water and heating technologies have yet to show the sustained momentum they lost in the 1920s.   Nevertheless, the recent spikes and erratic availability of conventional fuels has resulted in a renewed interest in solar heating technologies.   Solar water heating is particularly appropriate for low temperature applications (100-150F).   This advantage has been successfully applied to heating swimming pools where solar water heating can economically increase pool use.   Solar water heating is also used in stand alone or hybrid domestic water heating systems.   Solar water heating systems are composed of solar thermal collectors, a storage tank and a circulation loop. ^{Wiki n.p.}

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, which can be 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 community.   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. ^{Wiki n.p.}

Daylighting is a passive solar method of using 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.   Although difficult to quantify, the use of natural light also offers physiological and psychological benefits compared to conventional lighting.   Daylighting features include building orientation, window orientation, exterior shading, sawtooth roofs, clerestory windows, light shelves, skylights and light tubes.   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.   Hybrid solar lighting (HSL) is an active solar method of using natural light to provide illumination.   Hybrid solar lighting systems collect sunlight using focusing mirrors that track the sun.   The collected light is transmitted via optical fibers into a building's interior to supplement conventional lighting.   Daylight saving time (DST) can be seen as a method of using solar energy by matching available sunlight to the hours of the day in which it is most useful. ^{Wiki n.p.}

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.   To take advantage of the incoming electromagnetic radiation from the sun, solar panels can be attached to each house or building.   The panels should be mounted perpendicular to the arc of the sun to maximize usefulness.   The easiest way to use this electricity is by connecting the solar panels to a grid tie inverter.   However, these solar panels may also be used to charge batteries or other energy storage device.   Solar panels produce more power during summer months because they receive more sunlight.   The cost payback time may take over 10 years depending on the cost of grid electricity and tax rebates.   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 - 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%.   Between 2000 and 2004, the increase in worldwide solar energy capacity was an annualized 60%. ^{Wiki n.p.}

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 in an electrical power plant.   The solar heat coming from the sun is reflected off the mirrors and is concentrated on the top of the tower where it will heat water or oil to boiling point.   After the water or oil is heated it will be transferred to the power plant where it will make steam to turn a turbine to generate electricity.   Alternately, 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.   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. ^{Wiki n.p.}

A parabolic solar collector can concentrate the sun's rays on the heating element of a Stirling engine.   The entire unit acts as a solar tracker.   Solar energy converted to heat in a concentrating dish or trough parabolic collector can be used to drive a Stirling engine, a type of hot air 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.   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. ^{Wiki n.p.}

A solar updraft tower, also known as a solar chimney, 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, and 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.   An solar downdraft 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, but does not require the large glass house of the solar updraft tower.   A solar pond is a pool of water that 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: ^{Wiki n.p.}

• 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. ^{Wiki n.p.}

Solar chemical is any process that harnesses solar energy by absorbing sunlight and using it to drive an endothermic or photoelectrochemical chemical reaction.   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.   Another approach is to use focused sunlight to provide the energy needed to split water via photoelectrolysis into its c onstituent hydrogen and oxygen in the presence of a metallic catalyst such as zinc.   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.   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. ^{Wiki n.p.}

Advantages: The 89 petawatts of sunlight reaching the earth's surface is plentiful compared to the 15 terawatts of average power consumed by humans.   Additionally, solar electric generation has the highest power density (global mean of 170 W/m2) among renewable energies.   Solar power is pollution free during use.   Production end wastes and emissions are manageable using existing pollution controls.   Facilities can operate with little maintenance or intervention after initial setup.   Solar electric generation is economically competitive where grid connection or fuel transport is difficult, costly or impossible.   Examples include satellites, island communities, remote locations and ocean vessels.   When grid-connected, solar electric generation can displace the highest cost electricity during times of peak demand, can reduce grid loading, and can eliminate the need for local battery power for use in times of darkness and high local demand.   Grid-connected solar electricity can be used locally thus minimizing transmission/distribution losses, which are approximately 7.2%.   Once the initial capital cost of building a solar power plant has been spent, operating costs are low compared to existing power technologies. ^{Wiki n.p.}

Disadvantages: Solar electricity can currently be more expensive than electricity generated by other sources.   Solar heat and electricity are not available at night and may be unavailable due to weather conditions; therefore, a storage or complementary power system is required for most applications.   Solar energy has limited power density: Average daily radiation in the contiguous U.S. is 3-9 kW·h/m2 usable by 7-19.7% efficient solar panels.   Solar cells produce DC which must be converted to AC when used in currently existing distribution grids.   This incurs an energy loss of 4-12%. ^{Wiki n.p.}

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: ^{Wiki n.p.}

• Using traditional batteries
• Thermal mass
• Pumped-storage hydroelectricity
• Flow batteries
• Molten salt
• Cryogenic liquid air or nitrogen
• Compressed air in cylinders and in caverns
• Flywheel energy storage
• Hydrogen produced by electrolysis
• Hydraulic accumulator
• Superconducting magnetic energy storages
• Vegetable oil economy

Storage always has an extra stage of energy conversion, with consequent energy losses, increasing the total 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 dependent 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, solar capacity parallels energy demand because much of the electricity is for removing heat produced by too much solar energy, i.e., air conditioners.   This is less true in the winter.   Wind power complements solar power since it can produce energy when there is no sunlight. ^{Wiki n.p.}

Development of a practical solar powered car has been an engineering goal for 20 years.   The center of this development is the World Solar Challenge, a biannual solar powered car race over 3021 km (1877mi) 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 (42 mph).   By the 2005 race this had increased to an average speed of greater than 100 km/h (62 mph), even though the cars were faced with the 110 km/h (68 mph) South Australia speed limit. ^{Wiki n.p.}

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