GreenEnergyForEarth.com – Solar & Wind Power Products

Solar energy can be used directly by providing light, heat or electrical power. Electrical power can be provided directly from photovoltaic cells (solar cells) or indirectly from steam produced by concentrating solar power.

Solar power can be used most places but is especially efficient in areas with a lot of sunny days. At lower latitudes and in the summer, more energy is produced because the sun is higher in the sky. According to The Electronic Project at The University of Oregon, on a summer day at 40 degree North latitude, the earth at ground level receives approximately 600 watts per square meter or 4.8 kilowatt hours per square meter. This is equivalent to 0.13 gallons of gasoline. However, the actual amount of power per square that is usable depends on the efficiency of the technology used.

Solar energy can be direct, concentrated (thermal) or photovoltaic (solar cells)

Direct

Windows, skylights, solar tubes and fiber optics are methods of providing natural light to buildings during the day reducing or eliminating the need for electric light. In cool weather, windows and skylights can reduce the need for other sources of heat as the sun helps warm the room. Curtains or other window coverings can be used at night or when the window is not illuminated by the sun to help prevent heat loss. In hot climates windows and skylights can made of materials that allow in light but reduce that amount of heat produced.

Solar tubes are a type of skylight which is round and deep. They concentrate light and are smaller then conventional skylights.

Fiber optics can be used to bring sunlight far into a building.

Concentrated (Thermal)

Thermal technologies use the sun’s heat energy to heat substances (such as water or air) for applications such pool, water or space heating for homes and businesses. The products used for this application are usually called solar thermal collectors and can be mounted on the roof of a building or in some other sunny location. The sun’s heat can also be used to produce electricity on a large utility-scale by converting the sun’s heat energy into mechanical energy.

Flat plates with dark-colored materials can be used to heat water or air. For much higher temperatures, lenses and mirrors are used to concentrate the sun’s energy.

Concentrating solar power systems can be sized for village power (10 kilowatts) or grid-connected applications (hundreds of megawatts). Some systems use thermal storage during cloudy periods or at night to produce electricity outside of the traditional solar daytime window. Others can be combined with natural gas and the resulting hybrid power plants provide high-value, dispatchable power. These attributes, along with high solar-to-electric conversion efficiencies, make concentrating solar power an attractive renewable energy option in the southwest United States and other sunbelt regions worldwide.

Of all of these technologies the solar dish / Stirling engine has the highest energy efficiency. A single solar dish-Stirling engine installed at Sandia National Laboratories National Solar Thermal Test Facility produces as much as 25 kW of electricity, with a conversion efficiency of 30%.

Solar parabolic trough plants have been built with efficiencies of about 20%. Fresnel reflectors have an efficiency that is slightly lower (but this is compensated by the denser packing)
Concentrated solar power can be used for cooking. One solar oven can bake, boil or steam at temperatures up to 400° F. For more information about his product and where to purchase, please go to GLOBAL SUN OVEN® . Some larger solar ovens that adjust their position and curvature to the angle of the sun, can reach a temperature of 1200° F.

One of the challenges is storing solar energy. Heat storage allows a solar thermal plant to produce electricity at night and on overcast days. Heat is transferred to a thermal storage medium in an insulated reservoir during the day, and withdrawn for power generation at night. Thermal storage media include pressurized steam, concrete, a variety of phase change materials, and molten salts such as sodium and potassium nitrate.

Photovoltaic (Solar Cells)

Photovoltaics is a semiconductor-based technology which converts light energy directly into an electric current that can either be used immediately or stored, such as in a battery, for later use. PV panels/modules are very versatile and can be mounted in a variety of sizes and applications; e.g. on the roof or awning of a building, on roadside emergency phones or as very large arrays consisting of multiple panels/modules. Currently they are being integrated into building materials (such as PV shingles, which replace conventional roofing shingles).

Photovoltaics are best known as a method for generating solar power by using solar cells packaged in photovoltaic modules, often electrically connected in multiples as solar photovoltaic arrays to convert energy from the sun into electricity. To explain the photovoltaic solar panel more simply, photons from sunlight knock electrons into a higher state of energy, creating electricity.

The first practical application of photovoltaics was to power orbiting satellites and other spacecraft, but today the majority of photovoltaic modules are used for grid connected power generation. In this case an inverter is required to convert the DC to AC. There is a smaller market for off grid power for remote dwellings and roadside emergency telephones.

Amount of captured solar energy depends critically on orientation of collector with respect to the angle of the Sun. Under optimum conditions, one can achieve fluxes as high as 1000 Watts per sq. meter.In the Winter, for a location at 40 degrees latitude, the sun is lower in the sky and the average flux received is about 300 Watts per sq. meter.

The first solar cell was built in 1883, by Charles Fritts, who coated the semiconductor selenium with an extremely thin layer of gold to form the junctions. The device was only around 1% efficient.. In 1954 when Bell Laboratories, experimenting with semiconductors, accidentally found that silicon doped with certain impurities was very sensitive to light. This resulted in the production of the first practical solar cells with a sunlight energy conversion efficiency of around 6 percent.

In the United States, the first 17% efficient air mass zero (AM0) single-junction GaAs solar cells were manufactured in production quantities in 1988 by Applied Solar Energy Corporation (ASEC). The “dual junction” cell was accidentally produced in quantity by ASEC in 1989 as a result of the change from GaAs on GaAs substrates to GaAs on Germanium (Ge) substrates. The accidental doping of Ge with the GaAs buffer layer created higher open circuit voltages, demonstrating the potential of using the Ge substrate as another cell. As GaAs single-junction cells topped 19% AM0 production efficiency in 1993, ASEC developed the first dual junction cells for spacecraft use in the United States, with a starting efficiency of approximately 20%. These cells did not utilize the Ge as a second cell, but used another GaAs-based cell with different doping. Eventually GaAs dual junction cells reached production efficiencies of about 22%. Triple Junction solar cells began with AM0 efficiencies of approximately 24% in 2000, 26% in 2002, 28% in 2005, and in 2007 have evolved to a 30% AM0 production efficiency, currently in qualification.

In 2007, two companies in the United States, Emcore Photovoltaics and Spectrolab, produce 95% of the world’s Triple Junction solar cells which have a commercial efficiency of 38%. In 2006 Spectrolab’s cells achieved 40.7% efficiency in lab testing.

Scientists at the U.S. Department of Energy’s National Renewable Energy Laboratory (NREL) have set a world record in solar cell efficiency with a photovoltaic device that converts 40.8 percent of the light that hits it into electricity. This is the highest confirmed efficiency of any photovoltaic device to date.

Solar cell efficiencies vary from 6% for amorphous silicon-based solar cells to 40.7% with multiple-junction research lab cells and 42.8% with multiple dies assembled into a hybrid package.[

Solar cell energy conversion efficiencies for commercially available multicrystalline Si solar cells are around 14-19%. The highest efficiency cells have not always been the most economical — for example a 30% efficient multijunction cell based on exotic materials such as gallium arsenide or indium selenide and produced in low volume might well cost one hundred times as much as an 8% efficient amorphous silicon cell in mass production, while only delivering about four times the electrical power.
However, there is a way to “boost” solar power. By increasing the light intensity, typically photogenerated carriers are increased, resulting in increased efficiency by up to 15%. These so-called “concentrator systems” have only begun to become cost-competitive as a result of the development of high efficiency GaAs cells. The increase in intensity is typically accomplished by using concentrating optics. A typical concentrator system may use a light intensity 6-400 times the sun, and increase the efficiency of a one sun GaAs cell from 31% at AM 1.5 to 35%.

Although the selling price of modules is still too high to compete with grid electricity in most places, significant financial incentives in Japan and then Germany triggered a huge growth in demand, followed quickly by production.

The Forbes magazine article, “Sun Worshippers” indicates that “within 7 years the cost of unsubsidized solar power could be down to 10 per kwh – what the average American consumer pays today for juice …” Solar power electricity generated today represents only 0.03% of the US electricity power. However, silicon and polysilicon are getting cheaper, solar wafers are getting thinner and conversion of solar to electricity is getting more efficient; all of which will continue with greater use. Hopefully, Congress will extend the federal tax credits that are due to expire December 31, 2008.

Solar cells are expected to be capable of producing electricity for twenty to thirty years without a significant decrease in efficiency.