The Race for PV Conversion Efficiency

published: 2011-06-01 9:59 | editor: | category: Knowledge

Research labs and solar manufacturers are all vying for the next generation of high efficiency PV solar devices

There is a race on for the “Holy Grail” of solar energy: Making PVs more efficient.

Towards this goal, scientists and engineers worldwide have are trying to develop a low-cost solar cell, which is highly efficient, affordable and easy to manufacture. The focus of breakthroughs lately has been on CPV and thin-film PV applications.

But PV solar energy continues to suffer from a low efficiency of the conversion process that result in expensive initial investments to manufacture and build such systems. PV efficiency is the key metric for commercial silicon-based solar panels, which can collect only a theoretical maximum of about 30 percent of available light. The current silicon-based solar panels available on the market convert barely 20 percent of available solar energy into an equivalent amount of electricity. Higher efficiencies can lead to a higher rate of return for not only manufactures, but project owners and investors.

In just the past month, several breakthroughs have been announced by government, university and private company researchers who have been focusing on dramatically increasing CPV and thin-film PV efficiency. Some of the approaches researchers are taking are not only radical, but they are producing radical results.

CIGS

Researchers at the Swiss Federal Laboratories for Materials Science and Technology (Empa) announced May 19, 2011, their ability to increase the conversion efficiency of flexible CIGS PV cells - made from copper indium gallium (di)selenide - to world record –breaking 18.7 percent.

The 18.7 percent, which was certified by Germany’s Fraunhofer Institute for Solar Energy Systems in Freiburg, surpasses the same researcher’s previous record in June 2010 of 17.6 percent.

Ayodhya Tiwari, who heads Empa's Laboratory for Thin Film and Photovoltaics, believes these "flexible and lightweight CIGS solar cells with efficiencies comparable to the ‘best-in-class’ will have excellent potential to bring about a paradigm shift and to enable low-cost solar electricity in the near future."

"The new record value for flexible CIGS solar cells of 18.7 percent nearly closes the ‘efficiency gap’ to solar cells based on polycrystalline silicon wafers or CIGS thin film cells on glass,” says Tiwari.

These high-efficiency CIGS PV cells allow for potentially lower manufacturing and processing costs while their light weight and flexibility also means lower shipping and installation costs.

“The continuous improvement in energy conversion efficiencies of flexible CIGS solar cells is no small feat,” says Empa Director, Gian-Luca Bona. “Next, we need to transfer these innovations to industry for large scale production of low-cost solar modules to take off."

Empa researchers have partnered with FLISOM to refine the manufacturing and production process for commercializing the technology.

The 3-D Trend

California-basedSolar3D announced May 8, 2011, that they were moving faster than expected on the development of their 3-dimensional PV technology for increasing conversion efficiency.

 “Our fast progress has pushed us past the primary design and simulation phase onto fabrication research and planning activities. Our team is spending more time in the lab and clean room preparing for the actual building of a prototype, said Jim Nelson, CEO of Solar3D.

Solar3D researchers say they were inspired by techniques used in fiber optic devices. What Solar3D’s 3-dimensional design does is to trap the sun’s photons inside micro-photovoltaic structures where they bounce around until they are eventually converted into electrons.

 “Our effort is focused on completing the prototype so that we can initiate conversations with a fabrication partner in the semiconductor business. We have made substantial preparation to begin those discussions,” says Nelson. “We are getting closer and closer to making our super efficient 3-dimensional silicon solar cell a reality.”

The company’s engineers are now completing the design of a prototype that will allow them to calculate the overall conversion efficiency which they believe will dramatically change the economics of solar energy.

Meanwhile, researchers from the U.S. Department of Energy's Oak Ridge National Laboratory, led by Jun Xu, are also working within the 3-D realm and announced on April 29, 2011 they had developed a 3-D nanocone-based PV cell that can increase the conversion efficiency by almost 80 percent.

"We designed the three-dimensional structure to provide an intrinsic electric field distribution that promotes efficient charge transport and high efficiency in converting energy from sunlight into electricity," Xu said. lead researcher and a member of ORNL's Chemical Sciences Division.

The nanocone-based PV cell developed at Oak Ridge is made up of an “n-type” nanocone made of zinc oxide wrapped with a “p-type” semiconductor made of polycrystalline cadmium telluride. The n-type nanoncone functions as a junction and electron conductor, while the p-type serves as the photon absorber and conductor.

According to Xu, this design substantially overcomes the problem of poor transport of charges - negative electrons and positive holes - generated by solar photons that typically become trapped by defects in bulk materials which degrade the efficiency.

"To solve the entrapment problems that reduce solar cell efficiency, we created a nanocone-based solar cell, invented methods to synthesize these cells and demonstrated improved charge collection efficiency," said Xu.

In addition to the increased efficiency, the manufacturing process is inexpensive and has a lower risk of defects in semiconductors, according to Xu.

CPV

The efficiency of concentrated photovoltaic cells (CPV) has experienced only minor improvements in the range of 0.4 percent annually for the past 4 years. But during the past nine months, San Jose, California-basedSolar Junction has had remarkable success with their 5.5 mm x 5.5 mm CPV cells.

Solar Junction announced another world record on April 14, 2011 by achieving 43.5 percent conversion efficiency, considered a major development for the CPV market since it was achieved on commercial-ready cells.

The cell’s 43.5 percent conversion efficiency was confirmed by the U.S. Department of Energy’s National Renewable Energy Laboratory’s (NREL) Measurement and Characterization Laboratory. The previous CPV conversion record was held by NREL scientists with a 41.6 percent rate, which was done in a laboratory setting.

“There’s no question that we’ve been on a nine-month tear,” said Jim Weldon, CEO of Solar Junction.

According to the company, increases in CPV cell efficiencies are key to improving CPV economics, and with each cell efficiency gain leveraged and multiplied in value by the components that account for the remaining 80 percent of total system costs.

Weldon says “we have a highly extensible technology that is actually delivering a clear and continued path to higher efficiencies in both the short and long term. That bodes well for CPV.”

Solar Junction’s CPV cells incorporate a proprietary adjustable spectrum lattice using multi-junction cells, where each layer is designed to capture different light wavelengths to convert as much sunlight as possible to electricity. The cells are designed to be fitted into current CPV collectors and the company plans on shipping the modules by the end of the year.

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