Tricks To Taming Solar Power

In 1839, scientists discovered that certain materials could convert sunlight to electrical current. But it took until the middle of the 20th century to understand how that worked, launching the science of photovoltaics (nowadays shortened to PV).

Much of the effort in PV concentrated on miniaturizing solar cells so common today in watches and calculators. Other work involved using sunlight to power satellites and spaceships.

But what about using the sun to make electricity in more ordinary situations?

For decades, solar power enthusiasts promoted the benefits of using sunlight as a source of energy—after all, it’s free. But it costs to capture that energy, convert it to electricity, then distribute it when and where it’s needed.

Making solar power practical faces three major hurdles: efficiency, reliability, and cost.

A measure of a solar cell’s usefulness is its “conversion efficiency.” In other words, the amount of solar energy it can transform into electrical energy. The earliest PV devices could only convert about 1-2 percent of the sunlight falling on them into electric energy. That’s been raised to 7-17 percent. Today’s higher conversion efficiencies mean higher yields of electricity, making solar cells much more practical.

Reliability raises more complex issues. Although solar panels and other PV devices require little maintenance and can function for a long time, the amount of sunlight, the fuel source, can be very unreliable.

In Kentucky, the extremes vary from a little more than nine hours of potential sunlight a day in December to more than 14 hours a day in June. Unpredictable interference from clouds (which can linger for many dreary days in a row) further complicates the situation.

That’s why many PV applications must also include a battery system to store electricity as it’s produced. The batteries can then supply a steady flow of electricity when sunlight is not available. Construction advances have improved battery capacities and made them last longer, but that adds another layer of cost.

Yet another hurdle rises from the nature of electricity itself. PV systems generate “direct current,” or DC electricity, the kind produced by batteries.

But America’s utility grid and consumer appliances and motors use electricity in the form of “alternating current,” or AC. Anyone who wants to operate conventional equipment with electricity from a solar cell must install an intermediate device to convert it from DC to AC. That’s commonly done, but the extra step means less energy and efficiency, and the addition of still more costs.

Although much cheaper than it was just a decade ago, electricity from a solar energy system today still costs about 20 cents per kilowatt-hour—no bargain when compared to the rates for electricity from conventional sources such as fossil fuels, which range from 6 to 13 cents per kilowatt-hour.

Utility-scale PV power plants are still a long way in the future.

John Holt, manager of Generation and Fuel for the National Rural Electric Cooperative Association, says, “Right now, solar power doesn’t make a whole lot of economic sense for normal homes, either. But there are very important niche markets in America for solar power. In states such as Texas and Nebraska, with huge farms and ranches, if water is needed for livestock two miles from the nearest grid, a PV water pump can be a very attractive alternative.” If it would cost $10,000 a mile to run a normal distribution line from the grid to the pump site, then installing an independent PV system costing only several hundred dollars is a much cheaper way to get the electricity.

Practical use of solar electricity today calls for carefully weighing the costs and benefits, then matching the energy source to the proper use.

Holt says, “NRECA encourages the development and use of renewable energy. Solar power will continue to be an important part of the mix as the price of solar-generated electricity continues to come down.”

To find out more about solar power, visit this Web site: www.eere.energy.gov/solar/photovoltaics.html

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