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The topic of Solar Energy is one that has generated enormous relevance to the today's mainstream society, but it has also generated heated controversy. In an effort to dispel and/ or confirm the underlying myths that surround Solar Energy and Solar Cells the following article expounds the intricacies of this particular renewable energy.

Myths

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Solar Energy Conversion is Efficient

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Even the most ardent supporters of solar energy agree that the technology has not reached a point where it can significantly contribute to world-wide energy generation. While the U.S. Department of Energy [1] states that, “[t]he solar energy resource in a 100-mile-square area of Nevada could supply the United States with all its electricity (about 800 gigawatts) using modestly efficient (10%) commercial PV modules”, it also acknowledges that “…assuming that the proper investments are made now and are sustained, the industry will become significant in the next few decades.” The key phrase to note is that investment must be applied to this field if the industry is to grow. Currently, the technology is well suited for small applications such as outdoor lighting. However, even slightly larger systems such as solar water heaters traditionally have conventional backup systems to provide constant performance.[2] This need for backup systems is caused by both intrinsic engineering difficulties in PV cell technology as well as the characteristic of solar energy density.

Solar Cell Design is Mature

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As HowStuffWorks.com points out , there are currently two large areas of PV technology that need to be addressed.[3] The first is that current PV cells capture only a small fraction of the electromagnetic spectrum. This is due to what’s known as the band gap energy of the PV material, or the amount of energy needed to knock an electron loose from the material and generate energy. The desired band gap energy is what determines the material used in the PV cell. As it turns out, for a single material PV cell, in this material selection process there must be a balance between the amount of energy absorbed from the sun and the resulting strength of the electric field generated in a PV cell. If the band gap energy is too high, the resulting electric field is too weak to be collected. If the band gap energy is too low, not enough of the electromagnetic spectrum is absorbed. Thus, for single material cells, there must be a give-and-take relationship between these two effects. From an engineering perspective, more research must be undertaken to move the material selection process from a balancing act to one of finding the combination that yields both large spectrum energy absorption and strong electric fields. Possible courses of research to find a solution include experimentation with composite PV cells or better energy collection methods.

The second engineering difficulty associated with PV cells is the behavior of silicon. While the properties of the material allow for the photovoltaic effect to occur at junctions between P-type and N-type silicon,[4] the issue remains that silicon is a semiconductor. Unlike metals which allow for the free passage of electrons through the material, silicon has a relatively high internal resistance which leads to losses during conversion. Current solutions to minimize losses involve covering a PV cell with a metallic grid to minimize the distance an electron has to travel through the silicon. More research in materials science could produce a material that is both a conductor as well as a PV material. However, this material does not exist at present, and therein current PV cells still have high series losses that decrease efficiency and lead to lower performance.

Solar Cells Work Well Everywhere

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Another aspect that impacts PV cell energy conversion is the sun itself. The map from the Solar Energy Industries Association shows the solar radiation per month for the continental United States.[5] This association uses this map to argue that solar cells work anywhere because the sun shines everywhere. However, closer analysis of this map illustrates that only a small portion of the southwest enjoys high solar energy density. That high energy density amounts to 7-8 kWh/m2/day. In the Great Lakes Region and the Pacific Northwest, energy density is so low that no usable solar energy can be collected from some areas. To put this problem into perspective, the Energy Information Administration reports that the average home used 10,656 kWh/year in 2001.[6] Assuming the best solar energy density in the country and that average home use of electricity has not significantly increased since 2001, it would take more than 3 square meters of solar panels operating at 100% efficiency to meet this demand. If you were to try and power an average home in the Great Lakes Region (assuming an optimal energy density of 2 kWh/m2/day), the square area of solar panels would skyrocket to more than 14.5 square meters. While this still might seem a reasonable amount, the U.S. Department of Energy suggests that modestly efficient solar cells convert only 10% of the solar energy into electricity. At that conversion rate, you would need 30 square meters of solar cells per house in the southwest (comparable to the average floor space in a one car garage) and 145 square meters of solar cells per house in the Great Lakes (~1800 sq. feet or comparable to the floor space in a small house). Multiply those areas by the number of homes in your community and the space requirement really starts to add up.

Solar Energy Storage Design is Mature, Efficient, and Environmentally Friendly

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It is true that the sun shines most everywhere on the planet, and thus it is intuitive to think that solar power is a great alternative energy source since it works wherever the sun shines. Unfortunately, when the sun doesn’t shine (due to clouds, weather, solar eclipse, etc) no energy is produced from photovoltaic cells. Therein, a means of storing excess solar energy when the sun is out for those rainy days is crucial to the technology. Because of this, it is also intuitive to think that the problem of energy storage would have been already solved. Unfortunately, while energy storage solutions do exist, they are either inefficient, dangerous to the environment, or both.

Storage Method 1: Water

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Two main energy storage solutions exist for solar energy today. The more environmentally friendly of the two involves the conversion of solar energy into heat stored in tanks of water that can then be distributed throughout a home as either hot water or heat.[7] It is also conceivable that this hot water could be converted back into electricity later. However, such systems require a great deal of insulation because heat loss could drastically decrease the efficiency of the system. Besides the loss from heat escaping from the system, energy is lost each time it is converted from one form to another. A conversion from solar to electricity to heat and potentially back to electricity is very inefficient. Due in part to these inefficiencies as well as the desire to store a usable quantity of energy, heat storage units are also large, taking up valuable floor space in homes.

Storage Method 2: Batteries

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Traditional battery storage solves many of the problems of heat storage but also introduces new issues. Batteries are smaller than heat tanks and can hold a sizable amount of electricity per unit volume.[8] The solar energy is also only converted once (from solar to DC electricity) rather than through an intermediate step. However, as even teenagers know,[9] batteries pose a serious risk to the environment once their usable life has expired. Decomposing batteries in land fills around the world introduce toxic chemicals into the environment. Through groundwater supplies, these chemicals can be re-introduced into society in the form of drinking water.

In either case, the solutions for energy storage that currently exist are not sufficient for solar power to reach its full potential, and the idea that mature design solutions do exist today is simply a myth. Low performance is currently balanced against environmental ramifications in the selection of energy storage systems, but both must be addressed and improved through additional research if a permanent solution is to be found. Even leading researchers from MIT note that, “The major hurdle to overcome [in solar energy] is developing a cost-effective method of storage” [10] and “…that advances in chemistry such as the development of suitable catalysts for water-splitting are crucial for solar energy to reach its full potential.” Such advances are on the horizon, but it will take time, energy and money to achieve.

Solar Energy Transmission Can Use Existing Power Infrastructure

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Since parts of the United States have a disproportionately large amount of solar radiation, an idea would be to collect and convert solar energy where it is most abundant and then distribute it via an electric grid to other parts of the country. Even if a solar energy system was decentralized, moving the electricity from the solar array in the lot down the street to your house requires some type of distribution grid. However, there is a major engineering issue with such a plan. Currently, electricity is distributed across your town and the country using alternating current, or AC. Solar cells, on the other hand, produce direct current, or DC electricity. While all of the appliances in your homes use DC electricity (it is converted from AC by the devices), unfortunately solar energy cannot be distributed using the energy grid in existence today.

The solution proposed in the January 2008 issue of Scientific American is the construction of a brand-new, DC power distribution backbone across the entirety of the country. Such a system would deliver high voltage DC electricity from solar cells and other renewable energy sources to homes across America. The problem is that, while DC lines have been tested and proven, no one has every installed them on a continental scale. As the authors of the article admits, “No major technical advances [in DC power distribution] seem to be needed, but more experience would help refine operations.”

Additionally, although the method of transportation could be made more efficient, the amount of power transportation would be significantly larger under a system where electricity is generated relatively close to major consumption points. Large-scale power transportation using HVDC lines would entail significant losses. Also, because it is difficult to create multi-terminal HVDC systems, lines would carry large amount of electricity to major converter stations, which would then use existing infrastructure to distribute power locally using AC.[11][12] This means that rather than replacing current inefficiencies in AC power transmission with long distance HVDC transmission, a new system would likely be merely adding greater inefficiency to the transmission of power. This means that a much greater amount of electricity would need to be generated under the new system than is currently generated in order to compensate for increased power losses.

Then, there is the problem of funding. The authors estimate that $420 would need to be spent between 2011 and 2050 to cover the cost of the new infrastructure as well as make renewable energy competitive. Even after the expenditure, solar energy could account for only 69% of electricity production and 35% of the United States’ total energy needs.[13] Not only can solar power not use existing infrastructure, but it would take 1/25th of the national debt [14] during the 2005 fiscal year to make it competitive with existing energy technologies.

Individual Electricity Generation Can Replace Large Scale Production

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While large-scale production of solar power would be most efficient in certain portions of the world, transportation of electricity can generate costly losses. These losses could be mitigated by decentralized production and even personal electricity generation using household solar panels. The common household solar electric systems include grid-tied AC systems and stand-alone off-grid systems with energy storage that produce both AC and DC electricity. Grid-tied systems necessitate the ability to tie into the local electricity grid. Inverters that meet these requirements must not emit noise, which can interfere with the reception of equipment, must switch off in the case of a grid failure, and must retain acceptable levels of harmonic distortion. These inverters can be costly, but allow the generator to use electricity generated during the day and sell power to the grid that is unused, then buy electricity from the grid at night without the requirement of personal energy storage.[15]

The second common solar electric system involves an off-grid system with personal storage capabilities. This method avoids the necessity of an expensive inverter. Batteries are commonly used to store electricity, but the electrochemical conversion of electricity is only 75% efficient, and a regulator is usually necessary to prevent damage to the batteries while charging. This allows the electricity to be stored as DC power. The electricity could then be used in DC applications directly or converted into AC power. Conversion to AC requires an inverter, which will introduce greater inefficiency into the system, but also allows the use of common household appliances, which are more readily found to accept AC power rather than DC.[16]

The best site for solar panel installation is usually a south-facing roof. The systems are usually easy to remove and reinstall to allow for roof maintenance, but the initial installation may require additional bracing, as panels usually weigh 2.5 pounds per square foot. An initial structural assessment determines if the roof will need additional support to bear solar panels.[17]

The largest barrier currently facing consumers who want to implement personal generation systems has traditionally been the large initial cost of installing solar panels. The initial cost is being reduced as technology advances, however. In 2006, the average cost to install a residential-sized system was between $6.50 and $7.50 per Watt.[18] Nanosolar, a company based in California, claims that a new photovoltaic “thin film” technology will allow production of solar panels for only $0.99 per Watt.[19] Also, some firms are beginning to sell contract-based solar solutions. SunEdison installs and manages solar arrays for businesses. Customers pay for the solar energy used over a 20 year period at rates no higher than grid rates, and SunEdison owns all the panels and covers the costs.[20]

Danger to the Ecosystem

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Solar cells are a great source of energy, but only if placed in the right location. Many power plants using photovoltaic cells have been built in isolated, hot areas. Although this is a cheap and efficient location, the previous occupants of the land are commonly ignored. Solar power plants are often quite large and built in the desert, putting one of the most sensitive ecosystems at risk of permanent damage. Many plants and animals in the desert are highly immobile and very sensitive to sudden changes and destruction of their environment which is commonly seen as a useless piece of land. Large solar cell power plants require 600,000 square meters or more, and can only be placed in areas which are currently uninhabited and previously were home to many rare and potentially endangered species.[21] For example, the U.S. Air Force recently covered 140 acres of the Mojave Desert with photovoltaic cells. The Mojave Desert is currently home to about 2000 plant species, 45 different mammals, and 100 types of birds. 500 of these plant species only exist in the Mojave and could be lost forever.[22]

Solar cells can be quite useful however if properly placed, especially on previously unused surfaces such as the tops of buildings. This too can be problematic however since it requires the removal of trees that block exposure to the sun. Trees of this size have been around for decades if not longer and are now being cut down in some areas to increase roof sun exposure. Several lawsuits have already erupted in California amongst neighbors as the battle between preserving nature and using green energy continues.

Further Potential Environmental Impacts

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Although solar power has very low environmental impact as compared to fossil fuels, there are some potential environmental impacts associated with solar power that should not be completely ignored.

One potential environmental impact of solar power is related to obtaining and processing the materials required for solar power generation. Current PV cells use large quantities of silicon. Although silicon is a very common element, it must be mined (usually in the form of silicon dioxide) and processed. Silicon is produced from silicon dioxide by stripping the oxygen atoms from silicon dioxide through the combustion of wood, charcoal, or coal in an arc furnace. This reaction produces pure silicon and carbon monoxide. In addition to silicon PV cells, thin films of cadmium telluride, copper-indium selenide, gallium arsenide, and dye-sensitized titanium dioxide can be used to produce PV cells. These thin film cells use less light absorbing material than silicon cells, but the materials themselves are less common and some are toxic. In comparison to uses in other industries and similar environmental impacts of other power technologies, these difficulties are very minor.

A second potential environmental impact of solar power is based on the use of large solar farms to produce power. These farms, which cover large land areas, may disrupt the local environment or ecosystem of the area in which they are placed, especially if constructed on the scale necessary to provide a significant amount of world (or national) energy demand. In an era in which a major environmental concern is the rapid expansion of urban and suburban areas and agricultural/cultivated areas and the shrinkage of wilderness and natural areas, the use of vast tracts of land as solar farms could do serious damage to unique local ecosystems. Desert ecosystems in particular are ideal for solar power installation but are relatively fragile and might easily be destroyed by the construction of large solar farms. In addition to the ethical issues involved, current United States law regarding preservation of threatened and endangered species might prevent the construction of such solar farms under statutes designed to protect habitats of said species.

Impact on Other Industries

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Although silicon is very common (making up about a quarter of the earth’s crust) and is easy to produce from sand, the purification of that product is difficult and expensive. Although such refineries have long existed to produce silicon for electronics, the increasing demand for purified silicon as PV cell production grows has caused silicon prices to rapidly increase. Between August 2004 and August 2006, silicon prices more than doubled. As demand for electronics and PV cells continues to rise, so will the cost of silicon, possibly leading to increases in prices for all manner of electronic devices.

Solar Cells are not durable

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There are a wide variety of solar cells that are currently in use today. Each have their own respective durability and need to be taken into consideration when evaluating whether or not a particular type of solar cell is right for a given location. An example of this is the use of thin-film technology, which has the advantage of being mass-produced for a relatively low cost.[23] With this thin film though, the general attitude is that they are far less durable than their single crystal rivals. However, according to The International Society for Optical Engineering, SPIE, they are creating thin film solar cells that are displaying excellent durability.[24]

One important frontier of solar energy is outer space. For low earth orbit, a solar array is able to exhibit a high durability of only losing a few percent efficiency per year.[25] However, when a satellite enters a region of higher exposer to radiation, such as high earth orbit, then efficiency can be cut in half in the just one year. The Space Environmental Durability Branch of the NASA Glenn Research Center has been putting a significant amount of effort into determine the best materials and solar cells for the harsh conditions of space.[26]

Truths

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Diminishing cost barriers to solar energy

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A common argument against conversion to solar energy production is prohibitive up-front cost. While it is still the case that the average electrical power derived from oil or coal plants costs less than solar energy, this trend is steadily moving toward a reversal. The change is being effected from both sides of the issue. Oil prices are increasing while economies of scale and advanced research in solar cell technology are driving the cost of solar systems down.

In 2005, based on average photovoltaic cell efficiencies of 6.5% - 13.5% and a cost of around $350 per square meter of cell, solar-generated electricity cost on the order of $.2/kWh. Projections for cost in 2011 predict a decrease to $.12/kWh. When compared to the average cost of commercially produced (coal or oil) electricity, $.06/kWh, it is clear that on simple “bottom line” analysis, fossil fuels still have the edge.

However, according to earth-policy.org, “between 1976 and 2000, each doubling of cumulative production resulted in a price drop of 20%.” [27] As more environment-conscious consumers show interest in solar energy, the prices will continue to decrease.

In addition to the economies of scale helping to reduce solar panel costs, research is being conducted around the world to develop higher efficiency and lower cost cells. Multi-junction PV cells utilizing an array of materials designed to absorb a wide spectrum of light have been built with efficiencies of up to 40%. The [Defense Advanced Research Projects Agency] (DARPA) has sponsored a program to produce cells of at least 50% efficiency by 2010. Though these multi-junction devices are expensive, their high efficiency would make them suitable for certain applications. On the other end of the spectrum are cheaper technologies like thin film cells and dye-sensitized cells. These cells trade high efficiency for low cost. Thin film technologies require very little material to work and thus do not need large, expensive crystals. Dye-sensitized cells function well in low-light conditions when compared with their [silicon] counterparts, and could prove to be a viable option for household generation. Both of these technologies are in their infancy, and as more studies are conducted, the cost to produce them on a large scale will decrease and their overall efficiency will increase accordingly.

Decentralization

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The nature of solar energy harvesting allows for production of solar power to be decentralized, especially in locations which receive large amounts of direct sunlight. However, since all that is needed for a solar cell to run is incoming solar power, a solar cell can be run in any location where the sun shines regardless of ambient temperature which allows for decentralization in any climate. In traditional forms of power production, electrical power generation plants are fairly decentralized, but have not reached the point of being located in single-family homes. Solar cells have been especially useful in single-family home water heating systems due to stratification within the water storage system (this is a good place for the picture of the house to be referenced).[28]

Benefits of decentralization include the fact that it allows for power to be generated and more easily accessible in remote places where power lines might not be located. Also, since solar power can be generated on a home-by-home basis, much of the costs of running an entire generation plant can be cut out of the process, making the production costs of solar energy cheaper than traditional forms of energy. Transportation of solar power in a fully decentralized system becomes nonexistent, which increases the efficiency of the system since power can be used exactly where it is generated. Due to these benefits in decentralization of solar power generation, the National Renewable Energy Laboratory plans to incorporate individual solar power generators in its plan to create zero-energy homes by 2020.[29]

Solar Energy Safety

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Another advantage of solar energy is the relative safety of this method of energy conversion. Because the energy source is solar radiation, which inundates the Earth regardless of whether its energy is harnessed, there is no separate fuel that may be unsafe to store or dangerous to convert into usable energy. Harnessing solar energy also does not lead to the release of any harmful waste products. As a comparison, generating electricity in a nuclear power plant potentially exposes workers and nearby residents to radiation in the event of an accident, and creates radioactive waste. Generating electricity using the fossil fuel coal involves acquiring the coal under hazardous mining conditions that can lead to injuries or death due to mine collapses/explosions or the effects of black-lung disease. The emissions from coal-powered power plants are also dangerous. Coal ash is primarily composed of oxides of silicon, aluminum, iron, calcium, magnesium, titanium, sodium, potassium, arsenic, mercury, and sulfur plus small quantities of uranium and thorium.[30] It is estimated that around 24,000 people die in the United States annually as the result of airborn chemicals, up to 60% of which may be traced back to coal power plants, depending on the chemical.[31] Using photovoltaic cells that directly convert solar radiation into electricity or other methods that generate usable energy from solar power (such as solar heating or harnessing wind energy), a renewable natural resource can be turned into useful power without using potentially dangerous methods or creating harmful waste products.

Indierct solar energy

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While direct solar energy conversion accounts for only 4 terrawatt hours of electricity per year, the total energy accessible from the sun is much greater and from a multitude of sources besides direct solar conversion including wind, ocean thermal, and biomass.[32] Wind power provides 82 TW/h yr, and biomass 227 TW/h yr.[33] The energy stored in wind originally comes from temperature differences created by sun shine on land and water causing the warmer air over land to rise.[34] This energy can then be converted to energy using turbines, which may provide 5 megawatts each.[35] From a certain point of view, all oil, food, and biomass all come from sunlight on plant matter, however, only the biomass provides electricity in a renewable way. The bio fuels are used to power vehicles such as the E85 enabled cars and trucks or it can be burned to produce electricity directly.

Energy Harvest systems

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When considering the direct conversion from sunlight to electricity, various methods are used to concentrate the sunlight into temperature gradients, or incite chemical reactions to produce a potential. The method used in the most variety of situations is the photovoltaic as it can be used on applications as small as an individual light, to house rooftops, to power generating plants with 1866 cells produced by 2006.[36][37][38] For power plants, the most common method is solar through using a parabolic reflector to direct 30-60 times as much sunlight at a tube carrying an energy transfer fluid.[39] Alternatively, sunlight can be focused at a large tower as in the Solar Project or the PS10 solar power tower.[40] Alternative methods are also being attempted to further lower the costs including cheaper mirrors by using inflatable balloons similar to Mylar party balloons, or the use of liquid salt as a heat transfer fluid.[41]

Renewability

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One of the main benefits to solar energy is its renewability. The life span of the sun, which has approximately 5-6 billion years left, essentially makes the sun an endless source of energy. Of the energy produced by the sun, the earth receives approximately 1366 watts per square meter, or 174 petawatts (PW) in entirety, at its upper atmosphere.[42] Upon entering the Earth’s atmosphere, though, much of the solar radiation is lost via absorption, reflection and other various factors. Once the Earth’s surface is reached, the amount of insulation available for average atmospheric conditions has been reduced to 51% of the initial value.[43] Though this is a large reduction, 89 PW is enough to provide a significant source of energy. In 2004, the worldwide energy consumption was .447 zettajoules (ZJ) and is projected to increase to .539 ZJ by 2010.[44] Over one year, the Earth receives approximately 2800 ZJ of solar energy or 22 gigajoules (GJ) per square meter.[45] Assuming the use of solar cells working at 10% efficiency, roughly 245,000 square meters (0.48% of the Earth’s surface) would need to be covered in order to match the world’s energy consumption.[46] While this estimation assumes the region is receiving constant sunlight, it shows the viability of using the sun as a major source of energy.

Availability

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As mentioned previously, an abundance of sunlight hits the Earth’s surface. Any region that receives sunlight has the potential to harvest solar energy, though some not as well as others. The amount of insulation hitting the Earth varies with latitude, allowing more solar radiation near the equator and less towards the poles.[47] This makes solar energy less viable in certain regions. Generally, these regions of lower solar radiation are remote, making the use of solar cells a strong candidate as a source of energy due to the expense associated with transporting other sources such as petroleum. Another concern with the application of solar energy is that solar radiation received at a certain location is not constant. At night, the energy generation of a solar cell stops and cloud cover or other atmospheric conditions can reduce the amount of insulation allowed through to the Earth’s surface. This effect can be limited by choosing site locations with regularly clear skies. Another solution is to absorb sunlight before it enters the Earth’s atmosphere, avoiding the inconsistency of weather. Night limits the amount of time a solar cell can generate energy reducing its possible output. Energy can be stored via batteries and other methods, though, allowing for solar energy consumption to continue despite the sun not being present.

See also

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References

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  1. ^ U.S. Department of Energy: : Energy Efficiency and Renewable Energy. “Learning About PV: The Myths of Solar Electricity.” [Online] 5 January 2006. http://www1.eere.energy.gov/solar/myths.html
  2. ^ The Light Party. “Four Myths About Solar Power.” [Online] 1996. http://www.lightparty.com/Energy/SolarPower.html
  3. ^ Aldous, Scott. “How Solar Cells Work.” [Online] 17 March 2008. http://science.howstuffworks.com/solar-cell4.htm
  4. ^ Wright, Chuck. “The Solar Spring PV Panel.” [Online] 17 March 2008. http://www.chuck-wright.com/SolarSprintPV/SolarSprintPV.html
  5. ^ Solar Energy Industries Association. “Solar Energy Myths and Facts.” [Online] 17 March 2008. http://www.seia.org/mythsandfacts.php
  6. ^ Energy Information Administration. “End-Use Consumption of Electricity 2001.” [Online] 10 May 2007. http://www.eia.doe.gov/emeu/recs/recs2001/enduse2001/enduse2001.html
  7. ^ “Multi-Drum Heat Storage Unit.” [Online] http://www.jc-solarhomes.com/drum.htm
  8. ^ Solarbuzz. “Types of Solar Electric Systems.” [Online] 17 March 2008. http://www.solarbuzz.com/Consumer/SolarSystem.htm
  9. ^ L, Rachel. “Comparing the Electrical Output of “AA” Batteries at 21o and -15o Celsius.” [Online] 2001. http://www.selah.k12.wa.us/SOAR/SciProj2001/RachelL.html
  10. ^ Thomson, Elizabeth A. “Chemists shed light on solar energy storage.” [Online] 7 December 2006. http://web.mit.edu/newsoffice/2006/solar-nocera.html
  11. ^ Wikipedia. “High-Voltage Direct Current.” [Online] 23 March 2008. http://en.wikipedia.org/wiki/HVDC
  12. ^ Fthenakis, Vasilis, Ken Zweibel and James Mason. “A Solar Grand Plan.” [Online] January 2008. http://www.sciam.com/article.cfm?id=a-solar-grand-plan
  13. ^ Fthenakis, Vasilis, Ken Zweibel and James Mason. “A Solar Grand Plan.” [Online] January 2008. http://www.sciam.com/article.cfm?id=a-solar-grand-plan
  14. ^ GPO Access. “Budget of the United States Government: Historical Tables Fiscal Year 2005.” [Online] 17 March 2008. http://www.gpoaccess.gov/usbudget/fy05/hist.html
  15. ^ “Types of Solar Electric Systems.” [Online] 23 March 2008. http://www.solarbuzz.com/Consumer/SolarSystem.htm
  16. ^ “Types of Solar Electric Systems.” [Online] 23 March 2008. http://www.solarbuzz.com/Consumer/SolarSystem.htm
  17. ^ Palo Alto Hardware. “Photovoltaic FAQ.” [Online] 23 March 2008. http://www.paloaltohardware.com/code/faq.html
  18. ^ “Solar Photovoltaic Panels.” [Online] 23 March 2008. http://www.solarpowerfor.us/solar-photovoltaic-panels.html
  19. ^ Vidal, John. "Solar energy 'revolution' brings green power closer." [Online] 29 December 2007. http://www.guardian.co.uk/environment/2007/dec/29/solarpower.renewableenergy
  20. ^ Mandelbaum, Robb. “MIT Conference is Bullish on Solar Power.” [Online] April 2007. http://spectrum.ieee.org/apr07/5036
  21. ^ Welch, William. “Air Force Embraces Solar Power”. USA Today. April 17, 2007
  22. ^ http://www.redrockcanyonlv.org/Mojave_Desert.htm
  23. ^ http://www.thesolarguide.com/solar-energy-systems/types-of-solar-cells.aspx
  24. ^ http://spie.org/x8639.xml?highlight=x2358
  25. ^ http://inventors.about.com/od/sstartinventions/a/Solar_Panels.htm
  26. ^ http://www.grc.nasa.gov/WWW/epbranch/other/solarar.htm
  27. ^ http://www.earth-policy.org/Indicators/indicator12.htm
  28. ^ http://ecow.engr.wisc.edu/cgi-bin/getbig/ne/602/mikecorrad/solarenergy.pdf
  29. ^ http://www.nrel.gov/buildings/zero_energy.html
  30. ^ http://www.ornl.gov/info/ornlreview/rev26-34/text/colmain.html
  31. ^ http://www.abc4.com/content/specials/as_seen_on/story.aspx?content_id=258d00a8-f1a6-4996-9dc1-7ba85d1925be
  32. ^ http://www.iea.org/textbase/papers/2006/renewable_factsheet. p12
  33. ^ http://www.iea.org/textbase/papers/2006/renewable_factsheet. p12
  34. ^ http://www.eia.doe.gov/kids/energyfacts/sources/renewable/wind.html
  35. ^ http://www.eia.doe.gov/kids/energyfacts/sources/renewable/wind.html
  36. ^ http://en.wikipedia.org/wiki/Photovoltaics
  37. ^ http://www.iea-pvps.org/pvpower/download/pvpower26.pdf
  38. ^ http://www.iea-pvps.org/countries
  39. ^ http://www.eng.fsu.edu/~shih/succeed-2000/roadmap/solar%20power%20plant.htm
  40. ^ http://en.wikipedia.org/wiki/Solar_Two http://en.wikipedia.org/wiki/Solar_energy
  41. ^ http://coolearthsolar.com/technology, http://en.wikipedia.org/wiki/Solar_Two
  42. ^ The solar constant of the earth is 1366 W/ m2. The Earth has a cross sectional area of 1.274x1014 m2. Multiplying 1366 W/m2 by 1.274x1014 m2 gives 1.740x1017 W.
  43. ^ “NASA”, 24 April 2007, < http://www.nasa.gov/centers/goddard/images/content/172159main_polar_radiation_budget_large.gif> (23 March 2008)
  44. ^ “Energy Information Administration”, May 2007, < http://www.eia.doe.gov/oiaf/ieo/world.html> (23 March 2008)
  45. ^ Multiplying 8.874x1016 W by 3.1536x107 s gives 2.7985x1024 J. Fifty-one percent of 1366 W/m2 is 696.66 W/m2. Multiplying 696.66W/m2 by 3.1536x107 s gives 2.20x1010 J.
  46. ^ At 10% efficiency, 2.20x109 J would be converted to usable energy by the solar cells per square meter. Dividing the 2010 worldwide energy consumption of .539x1021 J by 2.20x109 J gives 2.45x1011 m2 or 245,000 km2. The Earth has a surface area of 510,007,200 km2. Dividing 245,000 km2 by 510,007,200 km2 gives a ratio of 0.0048.
  47. ^ “National Renewable Energy Laboratory,” n.d., http://www.nrel.gov/gis/solar.html (23 March 2008)
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Category:Solar energy

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