With the growing impetus for power companies to install renewable energy, solar is the fastest growing of the alternative energy forms. The global solar photovoltaic market reached 6,000 megawatts of installed capacity and generated $37 billion of revenues last year. That sounds like a big number, but solar accounts for a mere 0.08% (less than a tenth of one percent) of U.S. energy production. While the figure is slightly higher in some areas, it is around that level globally.
Germany has quickly emerged as a leader in photovoltaics. That may be surprising, in that Germany is hardly the sunniest place in the world, receiving just 1500 hours of direct sunshine a year (equivalent to about 150 ten hour days of direct sun).
The solar industry in Germany arose from a government policy adopted in 2000 that sought to obtain a quarter of the country’s energy needs from renewable sources by 2020. Achieving that goal involves guaranteed minimum prices paid to green energy producers. The higher costs are shared among the power utilities, the government, and consumers through a system known as feed-in tariffs.
While the system involves higher energy prices in the near term, the bold policy has catapulted the country to the forefront of the solar industry, creating tens of thousands of jobs, with the potential for further substantial growth. A side benefit is cleaner air, as the country has been able to reduce the amount of coal it burns to produce electricity.
German solar feed-in-tariffs are currently the equivalent of $0.53 per kilowatt- hour (kWh), declining at 5-10% per annum under a 20 year program. The high assured price provides a reliable revenue stream for suppliers, leading to a great deal of research and development that has spawned an industry.
Ontario, Canada recently proposed similar tariffs up to US$0.64 per kWh, which would make that Canadian province the most favourable location worldwide for solar systems (below 100kW). To put those feed-in tariffs into perspective, the electricity rates are roughly $0.15-0.25 per kWh in Germany and $0.06-0.13 in Ontario.
While limitless amounts of solar energy are available for the taking, the challenge continues to be installing systems to harness that energy on a cost effective basis. The world-wide economic slowdown has made it even more difficult to pry subsidies from either consumers or governments.
Technology enhancements are within reach that could make solar cell systems compete head-to-head with conventional energy. After all, solar systems are virtually free to operate once they are in place. Improvements in capital costs could spark a massive change in the way the world gets its energy.
Solar Technology Is Advancing In Two Directions
Technology advances are aimed at two aspects. One is to find ways to manufacture cells less expensively. The second is to get more electricity out of the sunlight falling on the solar cells.
Traditional solar cells are comprised of silicon and crystal, which render the photovoltaic cells heavy, fragile and dependent on a supporting frame. Breakthrough technologies using copper, indium, gallium and selenium (CIGS) have allowed cells to become thinner, cheaper and more durable. Those cells are so thin that they can be mounted on flexible materials, like cloth.
One pioneering company, PowerFilm, developed solar-powered field shelters for the U.S. Army. As seen in the photo below, the photovoltaic cells are mounted directly onto the fabric roof of the shelter. Such cells provide exceptional convenience for a remote field operation, but the technology is not applicable to mass production of electricity: efficiencies are in the 6% region and costs are far too high to be competitive with conventional power sources.
Another innovative company, Nanosolar, has developed a proprietary ink system that allows thin-film panels to be manufactured on a machine that is similar to a printing press. Nanosolar's cells have efficiencies as high as 14.6%.
The company is selling their product for under a dollar per watt of capacity, at the leading edge of the present cost structure. While important in small applications, that technology has limitations in the context of major power plants.
Getting More Watts Per Cell
The focus of research is shifting to boosting efficiencies. That shift arises because the cost of the solar material is only a portion of the total installed cost of a solar power system. As solar plants increase in scale, the amount of real estate taken up by the solar arrays is becoming enormous, extending to square miles. The most important factor is the total amount of electricity that can be extracted from the system.
Most silicon photovoltaic cells are about 15% efficient. In other words, 85% of the energy that hits a cell is lost due to reflection or ineffective absorption. The sidebar summarizes the research that is underway to improve on the efficiencies.
The challenge of increasing the efficiency of solar cells represents perhaps the most important element in the field of alternative energy. A combination of lower costs and higher efficiencies would catapult solar technology to the forefront of alternative energy. Both of those objectives are within reach.
Solar has potential to become an extremely important part of long term energy planning, but on its own cannot provide a comprehensive solution. Due to its intermittent nature, it currently requires a mix of alternative energy sources that can provide electricity in times of reduced output (clouds and overnight).
Solar technology will inevitably take a prominent role in the global energy mix. Investors in this field have the potential for huge returns as more companies develop and implement solar technology.
Light can be converted directly to electricity in a process called photovoltaics. When light hits the surface of certain materials, electrons are knocked loose from the atoms. When two complementary photo-sensitive semiconductor materials are used together, the freed electrons can be induced to flow from one material to the other, thereby creating an electric current.
In essence, one semiconductor layer has an abundance of unattached electrons and is referred to as the n-Type semiconductor. The other layer has an excess demand for electrons and is designated the p-Type semiconductor.
There are many different materials used in photovoltaics. The most common material is silicon, the same material that forms the basis of virtually all electronic devices (hence “Silicon Valley” as the nickname for the heart of electronic research and development in California). To explain the principal, we will discuss one variation of silicon-based materials.
In pure crystalline silicon, four electrons from each silicon atom bond with adjacent atoms. The n-layer is created by adding phosphorous into the silicon crystal lattice (“doping”). Phosphorous has five available electrons (or “valence” electrons), only four of which become bonded into the crystal. One of the electrons can be induced to move.
The p-layer is created by doping the silicon crystal with boron, which has three valence electrons. The silicon atom is looking to share four atoms, and therefore the boron-doped crystal is able to accept extra electrons.
Sunlight, acting on the surface of the n-layer, frees electrons which flow from the n-layer toward the p-layer. This is exactly the same concept as in a battery, where electrons flow through the electrolyte from one pole to the other.
As in the battery, an external circuit allows the flow of electrons to be harnessed as an electric current.
More about Silicon
Silicon is one of the most common elements in the earth’s crust: it is the prevalent element in most rocks. Window glass is silicon dioxide. While abundant, silicon for use in electronics and photovoltaic cells is expensive, as it must be of a high purity. Very thin layers of silicon material must be created, with regular crystal structures.
Doping to create the n-layer and the p-layer is complex, as the doping material must be injected into the crystal lattice without upsetting the structure. The most common method of doping at present is to coat a layer of silicon material with the doping material and then heat the surface and maintain a high temperature while the doping atoms work into the crystal lattice.
Concentrating photovoltaic systems use mirrors to direct additional sunlight onto the photovoltaic surfaces, thereby increasing power generation. That approach works only with direct sunlight, as the diffused light coming through clouds cannot be concentrated.
Further, to be effective, the mirrors require tracking mechanisms to follow the sun. The effectiveness of concentrating photovoltaic systems is therefore limited to areas with abundant direct light and the cost involved in installing the mirrors and the tracking mechanisms offsets much of the gains in efficiency. With the cost of photovoltaic cells coming down, it is becoming more effective to simply install photocells in place of the concentrating mirrors.
Research now is directed to increasing the amount of electricity produced from the light reaching the solar cells. One of the limits to efficiency in a photo cell is that a particular semiconductor material reacts only to a limited range of light color, or wavelength. That is, a high proportion of a particular color range can be converted, but the other colors do not impact the solar cell.
A broader range of wavelengths can be captured with tandem cells. Those cells utilize multiple n-layer/p-layer pairs, with each pair tuned to a different color range. However, the multi-layer approach works only to the extent that light can pass through the upper layers to reach the lower levels.
Transparency of the layers is accomplished by using ultra-thin layers: The top layer is able to extract energy from the light, with the unused wavelengths passing through to the lower levels. Tandem cells are in use, confirming the applicability of the concept. The challenge is that with present technology, obtaining suitably thin layers of appropriate quality is prohibitively expensive for large scale applications. NASA has done a great deal of work with tandem cells, as their high efficiency makes them useful for high-performance applications such as satellites.
Solar thermal: Generating electricity from the heat of the sun.
Concentrating the sun’s rays with mirrors can produce extremely high temperatures which can be used to produce electricity.
One type of solar thermal system uses u-shaped mirrors that focus sunlight onto a pipe that contains a fluid capable of withstanding extremely high temperatures. The heated fluid is used to boil water, which produces electricity in a conventional steam turbine generator.
The parabolic shape of the trough is designed to maximize the amount of sunlight concentrated onto the pipe as the sun moves through the sky. However, with the sun moving through two dimensions over the course of the day and the year, there is no shape that works for all sun positions. As a result, efficiencies are less than for systems that use flat mirrors that actively track the sun.
Solar power towers use flat mirrors that move on two axes to provide accurate tracking of the sun, maintaining a reflection directly onto the target. A large field of heliostats follow the sun to focus sunlight onto the collector tower.
It is important to note that only direct sunlight can be reflected. The diffused light of a cloudy day cannot be effectively reflected.
The concentrated energy of several hundred heliostats produces temperatures of 500°C to as much as 1000°C or more. That intense heat is used directly or indirectly to boil water. The resulting steam power drives a turbine to produce electricity.
Clearly, solar thermal only works in areas with abundant sunny weather and where there is vast amount of real estate available at low cost. Installations are expensive, as each of the hundreds of mirrors requires sophisticated equipment to track the sun. Aside from the limitations of converting solar radiation to heat, the systems are restricted by the inherent inefficiencies of the steam turbine and generator systems.
With those limitations, the economics of solar thermal systems using present technology do not support large scale development unless there are subsidies or other incentives. It does not appear that there is a great deal of scope to further improve efficiencies or bring down installation costs.
Solar thermal is a demonstrated technology that provides a near term alternative as power utilities strive to meet renewable targets imposed by legislation.
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