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The amazing solar power
February 26, 2008

The Sun supplies an average of 174 watts of power to every square meter of the Earth’s surface. That’s 89 petawatts worldwide – nearly 6,000 times the power used by all human beings. The total amount of fossil fuels used by humans since the start of civilization is equivalent to less than 30 days of sunshine, and the total estimated remaining fossil fuel resources add up to less than 39 days of sunshine. So why are we still driving in gasoline powered cars, living in homes heated by natural gas, and using electricity generated from coal? Why aren’t we making more use of our most abundant, cheapest, and greenest energy source? Because it turns out that transforming that energy into something we can use is rather difficult, and until recently the low efficiency and high cost of solar cells was prohibitive. But that’s all changing, and there are some exciting new developments in photovoltaics.

A solar cell works basically like this:

• Sunlight hits the solar cell
• Photons in the sunlight knock loose some of the electrons within the solar cell
• These “freed” electrons then flow through the solar cell, producing electricity

The problem is that many of the photons are either absorbed by, reflected by, or pass through the solar cell without knocking loose any electrons. The first solar cell, built by Charles Fritts in 1883, was only 1% efficient. This means it was only able to convert 1% of the incident light energy into electricity. Skip ahead a hundred years, and the efficiency of the most advanced (and expensive) solar cells had only increased to 17% - still too low and too costly for anything but satellite applications. But in the past twenty-five years we’ve seen that number increase to over 40% in the laboratory, and mass produced cells are now in the 10%-15% range. The general trend in efficiency is upward, and the general trend in cost is downward.

One focus of recent research has been to develop cheaper ways of producing the silicon necessary to build conventional solar cells. Currently it takes over two years for a solar cell to produce the amount of energy it took to make the silicon it contains. But new methods of silicon synthesis such as electrolysis in molten salt promise to reduce costs, and new methods of machining and thin-film deposition promise to reduce the amount of silicon (and thus the total cost and energy) needed to fabricate solar cells. Nanocrystalline cells use extremely thin layers to create flexible solar cell “sheets.” Organic solar cells use thin films of polymers to create flexible and potentially biodegradable cells.

Other research has focused on increasing efficiency. Research into multijunction gallium arsenide cells has produced the highest efficiencies to date – as high as 40.7%. These cells consist of 20 to 30 thin layers that are able to extract energy from the full spectrum of sunlight. This technology could eventually produce electricity at 8-10 cents per kilowatt-hour – about the cost of electricity today.

These are just a sample of the many exciting developments in photovoltaics. And as the need for energy independence and clean energy production rises, one thing seems clear – the future of solar energy is indisputably bright.

Sean, gearhead at large

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