Squeezing More Energy from the Sun

July 01, 2010
July/August 2010
A version of this article appears in the July/August 2010 issue of Home Energy Magazine.
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Scientists are figuring out how to squeeze more electricity out of PV panels. PV panels take the sun’s energy and convert it into electricity. The sun’s energy is abundant, but in order for solar electricity to compete with cheaper forms of electricity, such as that produced from highly polluting coal-fired power plants, PV panels have to be more efficient. And they need to be cheaper. Higher efficiency gets you more power for the same area, which saves on costs. But higher efficiency has generally meant more expensive PV modules.


A First Solar PV array produces electricity from the rooftop of a home in San Rafael, California. (Image credit: First Solar)

The Energy Economy of PV

The efficiency of PV panels is measured by the percentage of sun energy falling on the panel that is converted to electricity. According to the National Renewable Energy Laboratory (NREL), commercial PV panels today range in conversion efficiency from about 7% to 17%. Compare that to the efficiency of a typical fossil fuel power plant that has a conversion efficiency (based on fuel input) of about 30%. That means that about 30% of the energy in coal or natural gas gets converted to electricity. The rest is wasted as heat as the fuel is burned to create steam, which is used to turn a turbine, and it is dissipated in the cooling tower, where the steam condenses back to water.

There is some loss of energy, called transmission losses, as the electricity goes from the power plant to your house. With PV cells, the transmission losses are close to zero, since the electricity doesn’t have far to travel. On the other hand, the inverter required to convert the direct current (DC) from the cells to alternating current (AC) to connect to the grid does cause some loss.

Typical PV cells are made from crystalline silicon or other semiconductor materials. One advance in the technology is the creation of thin-film PV cells made of the same, or similar, materials. These cells use less material than crystalline silicon cells, and so are cheaper to make. In addition, they can be used in a greater variety of applications, sometimes meeting more than one use; some are made to act like roofing shingles, producing electricity as well as providing overhead protection against the weather. But thin-film PV is generally less efficient at converting sunlight to electricity, compared to the more-expensive crystalline silicon PV cells. Manufacturer First Solar recently announced thin-film technology that costs $1 per watt to manufacture, compared to traditional crystalline PV technology that costs approximately $3/W.

Another advance in the technology is layering materials that convert different parts of the spectrum of light into electricity. NREL has made some cells that are more than 30% efficient, but they’re very expensive—maybe suitable for use in the space program, where size and weight are critical.

You can use mirrors and a system that tracks the sun to concentrate energy onto PV cells. This typically reduces the efficiency of the cell, since PV cells tend to lose efficiency as they heat up, but less cell area is needed. With concentrating PV systems, some of the electricity that is produced needs to be siphoned off to run a cooling system that keeps the PV cells from overheating.

Since PV systems generate direct current, one way to increase the efficiency of the solar-electric system on your roof is to connect the panels to devices that run on DC. Most roof systems use an inverter to convert the electricity from DC to AC, and there are significant conversion losses in that process.

More and more devices actually do use DC. Most consumer electronics first convert AC to DC in order to operate. Any appliance that uses an inverter drive (high-end washing machines, air conditioners, and furnace fans) converts AC to DC and back to AC to drive the motor at varying speeds. And any fluorescent with an electronic ballast converts AC to DC and back to high-frequency AC. Either AC or DC can power any resistance heating or incandescent lighting device. All LED lighting operates on DC, so the power supply needs to convert AC to DC.

Bending the Cost Curve

Scientists at Lawrence Berkeley National Laboratory (LBNL) are trying to develop materials that are abundant and also suitable for affordable large-scale use in PV cells. The scientists are currently doing research with bismuth ferrite, a ceramic material that is both ferroelectric and ferromagnetic, and which has some interesting photovoltaic properties that operate on the nanoscale. The material could possibly be used in PV applications, resulting in cells that are much more efficient than what is commercially available today but are less expensive.

A 2008 study by the University of California at Berkeley and LBNL found that unconventional, nonsilicon solar-cell material, such as iron pyrite (FeS2) and zinc phosphide (Zn3P2), will produce the same amount of energy at less than 10% efficiency over its lifetime as crystalline silicon PV cells with efficiencies greater than 20%, while using 75% less material.

The most efficient system isn’t necessarily the best from an economic standpoint. PV power will become a bigger market player in the energy production marketplace when manufacturing costs for reliable, long-lasting solar cells fall well below $1/W. What we might find is that the most inexpensively produced PV panel produced from the fewest and cheapest materials will finally become the clean-power source that can slow the growth in, and then begin to supplant, the use of fossil fuels.

Jim Gunshinan is Home Energy’s editor.

 

For more information:

Learn more about solar research at NREL.
Visit First Solar.
Get more info on research at LBNL.
Get a copy of the 2008 study by LBNL and the University of California at Berkeley on nontraditional PV cell materials research and potential.

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