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Let the Sun Shine

By Benjamin Kahane

There are two major types of solar power technologies: photovoltaic and solar thermal. Photovoltaic solar power utilizes the photoelectric effect. A semiconductor material absorbs light and the photons in the light beam are routed through the semiconductor and harnessed in the form of direct current electricity. The semiconductor cells are electrically tied together in what is commonly called a solar module. These modules can be used to power a direct current source, such as a battery bank or a water pump, or more commonly the power can be converted into alternating current power to be used in a home or fed into the electric grid. Solar thermal power harnesses the heat derived from the sun’s rays. A common use of solar thermal power is to mount a solar thermal collector on the roof to use the hot water running through the collector to heat the water for the house. This is widespread in Israel. However, with respect to electricity generation, the sun’s rays also can be used to heat a fluid, converting water into steam to turn an electricity generating turbine. The United States has about 500 megawatts of operational solar thermal power, most of which comes from the largest single project, a 354 megawatt plant in California’s Mojave Desert. Photovoltaic power is much more widespread, mostly because it is a much more scalable technology. The total U.S. grid connected photovoltaic capacity in 2010 was 2,152.5 megawatts — more than four times the total solar thermal electric power installed — and installed photovoltaic capacity is growing at an exponential rate. Currently, solar power represents a very small portion of the total energy demands of the United States — less than half of 1 percent of the country’s energy usage — but advances in solar cell manufacturing processes and competition in the market are allowing the American solar energy sector to grow rapidly. However, solar power is not without its negative impacts to the environment. Although solar energy creates no pollution during its operation, the manufacturing and installation of solar panels certainly does. Still, it’s estimated that solar water heating reduces the units of heat per unit of fossil fuel energy by a factor of two compared to heating with natural gas, and by a factor of at least eight when compared to electric water heating. There are have been many studies done to try and tabulate the amount of fossil fuels burned in creating a solar farm to quantify how much greener solar is than conventional fuel electricity generation plants. The problem is that there are many factors to consider and most of the studies show different findings; the bottom line, however, is that the longer the plant is in operation, the smaller its carbon footprint gets. Also, every study shows that a solar power plant is an easy decision if your main concern is greenhouse gas emissions, even if they do not agree on the total tabulated emissions from the solar plant. There is also public concern about some of the materials used for certain photovoltaic modules. Some modules contain potentially hazardous materials such as arsenic and cadmium. Although the materials are safe for human contact while encapsulated in a photovoltaic module, the danger lies near the end of the module’s useful lifespan if it is not disposed of properly. Technological progress has been made in both the photovoltaic and solar thermal industries, but the majority of growth in the world has been with photovoltaics. The introduction of thin-film solar wafers promises much cheaper semiconductor material with only slightly reduced efficiency. Many wafer manufacturers have been analyzing the production processes to find ways to produce the same cells at lower costs. There also have been leaps in recorded efficiencies of solar cells. For example, Solar Junction, a Silicon Valley start-up, has recorded a peak efficiency of 43.5 percent in their proprietary solar cell, a world record. The company uses multi-junction technology to take advantage of many different band-gaps of light striking the solar cell. By both increasing efficiencies and reducing production cost, the case to build a solar installation will become stronger and stronger. ________________________________________________________________________________________ Benjamin Kahane is a utility scale project engineer at SunEdison, where he designs photovoltaic solar energy systems. He has provided engineering support for the development of more than 100 megawatts of ground-mounted photovoltaic projects across North America. Kahane previously worked as a project engineer developing photovoltaic installations at Conergy. He earned his master’s degree in sustainable energy engineering at the University of Maryland, College Park.

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