Solar efficiency records complicate PV market

WORLDWIDE--Scientists in the U.S. and Europe both recently announced records for photovoltaic (PV) solar-cell efficiencies, adding more mile markers on the road to low-cost solar energy.

WORLDWIDE--Scientists in the U.S. and Europe both recently announced records for photovoltaic (PV) solar-cell efficiencies, adding more mile markers on the road to low-cost solar energy (see Researchers at the Fraunhofer Institute for Solar Energy Systems (Freiburg, Germany) reported a 39.7% efficiency for a triple-junction solar cell, exceeding their own European record of 37.6% set in July 2008. A group at the U.S. Department of Energy’s National Renewable Energy Laboratory (NREL) led by senior scientist John Geisz announced a world record of 40.8% solar-cell efficiency for an inverted metamorphic triple-junction PV device in August.

Both records are derived from triple-junction solar-cell designs, using III-V materials such as germanium, gallium arsenide, and indium phosphide; they also involve concentrator systems to boost incident light intensity to hundreds or thousands of times that of the sun. These record efficiency solar-cell designs are not yet commercially available, but similar III-V structures with concentrator designs are not only available, they can compete in cost per kW with silicon photovoltaic technology at large PV power plants in sunny locations. Spectrolab (Sylmar, CA), Emcore (Albuquerque, NM), and others offer these designs as the most economical choice in terms of watts per kg for space applications.

However new record efficiencies keep popping up, in hopes of improving their economic prospects. In 2007, Spectrolab achieved the previous record of 40.7% efficiency. Geisz admits that statistically the 0.1% difference is minor, but, he adds, “The previous record was achieved by improving an older design, while our new record is the result of a new approach, with plenty of room for improvement.” In July 2007, the University of Delaware reported a multi-junction solar-cell efficiency of 42.8%, independently confirmed by the U.S. government-run PV Cell and Module Performance group at NREL. If that’s the case, how come 40.8% is a new record? According to Keith Emery, device performance manager at the NREL testing lab, the difference is that the Delaware efficiency number is partly theoretical--the sum of several individual tests of three different solar cells over six months, assuming ideal optics. “Conceptually, numerous modules could currently beat the Geisz group’s record efficiency,” says Emery. “But the big deal is actually assembling them for testing.”

The numbers game

Another detail of interest is the issue of lifetime. According to Emery, 90% to 95% of the solar energy produced or power sold is silicon based, and they are expected to last at least 20 years. But of all the new types of solar cells being developed, there are no guarantees yet about their stability. “Silicon wafers can sit out for months [and still produce the same efficiency],” says Emery, “but the quantum-dot and organic cell efficiencies are very sensitive to air and water. They have to be tested in a glove box. They can never see air.”

Another factor affecting PV efficiency numbers is a quietly encroaching change to the measurement standards for reference spectral irradiance data. Currently, the reference spectrum used to measure solar-cell output is not consistent internationally. The U.S., Germany, and Japan informally use the same calibration standards for evaluating concentrator cells. But a revision of the standard per the International Electrical Commission adopted in May 2008 will artificially bump the total reported PV power sold or delivered up by 0.8%.

The standards change will have an impact of less than 1% in the peak-watt rating for silicon modules, and will allow an orderly international switch to a new reference spectrum. However, according to Solarbuzz LLC (San Francisco), the global PV industry had revenues of over $17 billion in 2007 and market installations reached over 2.8 gigawatts in 2007 and grew 60% over 2006--so even a 1% change is significant.

--Valerie C. Coffey

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