Since its early times, man has seen the great potential in the sun as an enormous source of power and energy. And over the course of history, man has also devised means to tap into this great energy source in the heavens. Indeed, the sun is an ideal power giver: the solar energy is ever-flowing, inexhaustible and free from creating pollution.
Every day, the sun showers Earth with several thousand times as much energy as we use. Even the small amount that strikes our roof is many times higher than all the energy that comes in through power lines. With the sun straight overhead, a single acre of land receives some four thousand horsepower, equivalent to about a large railroad locomotive. In less than three days, the solar energy reaching Earth more than matches the estimated total of all the fossil fuels on Earth!
MilestonesThe term photovoltaic has been in used in English since 1849 and it comes from the Greek word “phos”, meaning “light” and “voltaic”, meaning electrical. This last term is derived directly from the name of the Italian physicist Volta, after whom the volt, the unit of electrical potential, is named.
The harnessing of solar energy is not new. In fact, the development of solar energy dates back more than 100 years, to the middle of the industrial revolution. Several pioneering solar power plants were constructed to produce steam from the heat of the sun, which was used to drive the machinery of the time. At the same time, Henri Becquerel discovered the photovoltaic effect, in 1839.
However, the first solar cell wasn't built until 1883, when Charles Fritts coated the semiconductor selenium with an extremely thin layer of gold, to form the junctions. But the device was only mildly efficient, so the modern solar cell was patented a lot later, in 1946, by Russel Ohl.
The modern age of solar technology arrived in 1954, when Bell Laboratories 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%.
The first spacecraft to use solar panels was the US satellite Vanguard 1, launched in March 1958, with solar cells made by Hoffman Electronics. This milestone created interest in producing and launching a geostationary communications satellite, in which solar energy would provide a viable power supply. This was a crucial development which stimulated funding from several governments into research for improved solar cells.
The first highly effective GaAs (Gallium arsenide) heterostructure solar cells were created by Zhores Alferov and his team of USSR researchers, in the 1970. But, the abilities of companies to manufacture the GaAs solar cell was limited, until the 1980s, by the fact that Metal Organic Chemical Vapor Deposition (MOCVD or OMCVD) production equipment wasn't developed up to that date.
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). As GaAs single-junction cells topped 19% AM0 production efficiency in 1993, ASEC developed the first dual junction cells for spacecraft used in the United States, with a starting efficiency of approximately 20%. 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 from the United States, Emcore Photovoltaics and Spectrolab, have produced 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. And even this has been topped...
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.
Types of solar cells
Solar cells are classified into three generations, which indicates the order of which each became prominent. Thus, the first generation cells consist of large-area, high quality and single junction devices. They involve high energy and labor inputs, which make them rather unpractical today, because of their rather elevated costs. Single junction silicone devices are approaching the theoretical limiting efficiency of 33% and achieve cost parity with fossil fuel energy generation after a payback period of 5-7 years.
Second generation materials have been developed to address energy requirements and production costs of solar cells. It is commonly accepted that as manufacturing techniques evolve production costs will be dominated by constituent materials.
For the second generation solar cells, alternative techniques have been used, such as vapor deposition and electroplating, as well as the most successful second generation materials, such as cadmium telluride (CdTe), copper indium gallium selenide, amorphous silicon and micromorphous silicon. These materials are applied in a thin film to a supporting substrate such as glass or ceramics, reducing material mass and therefore costs.
Third generation technologies aim to enhance poor electrical performance of second generation thin film technologies, while maintaining very low production costs. These solar cells are aimed at reaching an even higher efficiency levels than the second generation cells (they are targeting conversion efficiencies of 30-60%) and there are already a few approaches to achieving these high efficiencies, such as: multijunction photovoltaic cell, modifying incident spectrum (concentration), use of excess thermal generation to enhance voltages or carrier collection.
Advantages of photovoltaic power
Photovoltaic solar power is one of the most promising renewable energy sources in the world. Compared to nonrenewable sources, such as coal, gas, oil and nuclear, the advantages are more than clear: it's totally non-polluting, has no moving parts to break down and doesn't require too much maintenance.
Also, it doesn't need a large scale installation to operate, unlike conventional power generation stations. Rooftop power can be added as more homes or businesses are added to a community, thereby allowing power generation to keep in step with growing needs without having to overbuild generation capacity as is often the case with conventional large scale power systems.
Photovoltaic solar power is even more advantageous than other renewable energy resources, such as wind, water, or even solar thermal power, because, unlike them, it doesn't need turbines and generators, not to mention the fact that photovoltaic sells can last for decades.
Uses of solar power
Until solar power came along, people who wanted to live in remote areas frequently had to pay large fees to have a power cable run to their house. Now, a remote home can be virtually self-sufficient with solar power. Even in areas where power lines are nearby, solar may be a viable alternative to being connected to a power company.
An average home has more than enough roof area to produce enough solar electricity to supply all of its power needs. With an inverter, which converts direct current (DC) power from the solar cells to alternating current (AC), which is what most home appliances run on, a solar home can look and operate very much like a home that is connected to a power line.
For recreational vehicles, solar power provides the freedom to go to more remote locations, without relying on a plug-in power source or a noisy electric generator. Systems for RVs can be small, for charging batteries only or large enough to power the entire vehicle for a period of time. Similarly, boats can use solar power for many of their power needs, rather than a generator or engine.
These are probably the most obvious uses of the solar panels. However, some of the most important applications of solar energy are nearly invisible. Thus, telecommunications, oil companies, and highway safety equipment all rely on solar power for dependable, constant power, far from any power lines (solar power is used for many lighted highway signs, eliminating the need for diesel generators and many communication repeaters in remote areas use solar power).
Siemens Solar alone has shipped over 130 megawatts of modules since its inception, and the use of solar power is projected to grow at 10-15% per year from now until the year 2010.
Given this growth, solar power will be a much larger part of our lives in 2010 than it is today. Homes could incorporate solar power at the time that they are built, dramatically reducing both the cost of buying solar power and the cost of utility bills. New communications technology may make living in remote areas a practical reality given the availability of solar power. Mobile uses will undoubtedly increase. And industrial applications will continue to enjoy the versatility of solar power.
