• Number 332  |
• March 7, 2011

Super-efficient cells key to low-cost solar power

Thinking big while focusing on small, a solar company and a national energy lab combined talents to develop a solar power concentrator that generates electricity at prices competitive with natural gas.

The Amonix 7700 Concentrated Photovoltaic (CPV) Solar Power Generator, developed by Amonix and DOE’s National Renewable Energy Laboratory, is the size of an IMAX screen but costs much less than comparable generators, partly because of the efficiency of its small solar cells. It delivers more "energy per acre" than anything yet available in the solar energy world.  The public-private partnership won a 2010 R&D 100 award.

The 7700 uses acrylic Fresnel lenses to concentrate sunlight up to 500 times its usual intensity and direct it onto 7,560 tiny, highly efficient multi-junction PV cells.

The cells, originally developed by NREL scientists, can convert 41.6 percent of the sunlight that shines on them into usable electricity in a laboratory setting, a world record. The multi-junction cells on the Amonix 7700 are achieving 31 percent efficiency at the module level and 27 percent at the system level in the field, the highest ever achieved for an operating CPV concentrator.

A six-inch square silicon wafer in traditional photovoltaic (PV) panels produces about 2.5 watts of electricity. That same-sized wafer, cut into hundreds of square-centimeter cells in the Amonix 7700, each teamed with a Fresnel lens, produces more than 1,500 watts. It reduces the required area for cells 500 times.

The key breakthrough that lifted the 7700 to a 50 percent greater power output than previous generations of Amonix generators was the substitution of the multi-junction cells made of gallium indium arsenide and gallium phosphide for the more common silicon cells.

A conundrum was how to use the highly efficient cells without breaking the bank. Researchers solved that problem by teaming an inexpensive Fresnel lens — at less than $2 a pop — with each of the 7,560 high efficiency solar cells that make up one 53-kilowatt 7700 system. The 500-power amplification of the Fresnel lens allowed the solar cells to be tiny — thus a small fraction of the cost of bigger cells — while still packing record-setting efficiency.

Solar energy has found a niche on rooftops, especially of green-minded homeowners. But if it is to play a major role in the broader electricity market, it needs to come in at or below the costs of electricity generated from coal, which is projected to cost from 6 cents to 15 cents per kilowatt-hour in four years. The 7700's cost per kilowatt-hour is expected to be well within those price ranges as production and sales continue to grow.

The concentrator also is kinder to the environment than most large systems, using no water in its operation. Propped up two feet above the land, it doesn't hinder the movement of wildlife. "You simply can't put enough solar systems on rooftops to achieve the scale and capacity necessary to generate electricity in the quantities required by utilities and by society," said Amonix's founder and chief technical officer, Vahan Garboushian. "This is a technology that can meet the terawatt (trillions of watts) needs of the world for clean electricity."

[Bill Scanlon, 303.275.4051,
william.scanlon@nrel.gov]