Growing public demand for solar electric energy ensures that it will become a significant part of our clean energy needs in the near future.

Generating electricity from the sun’s power has always been clean energy’s Holy Grail, and researchers at the National Renewable Energy Laboratory (NREL) in Golden, Colorado, produced in August 2008 an experimental new photovoltaic (PV) cell that set a record 40.8 percent conversion rate from light to power. The current commercial norm for silicon solar PV arrays is around 12 to 15 percent conversion.

The inverted metamorphic triple-junction solar cell is cutting-edge and offers a glimpse of what might be on the horizon. The design that produced the 40.8 percent cell has room for improvement, and could approach 45 percent with further development.

The new cell uses a germanium wafer as the bottom junction and compositions of gallium indium phosphide and gallium indium arsenide to split the solar spectrum into three equal parts that are absorbed by each of the cell’s three junctions for higher potential efficiencies. The cell is grown on a gallium arsenide wafer, then flipped over and the wafer removed. By removing the wafer, the cell is a natural candidate for space or terrestrial arrays using inexpensive lenses or mirrors to concentrate sunlight onto the surface, with the whole array rotating to follow the sun. The inexpensive light-gathering optics reduces the cell cost to a fraction of the system cost, giving “concentrating photovoltaics” the potential for very inexpensive electricity.

Meanwhile, NREL scientists are researching nanotechnology where the theoretical potential for light-to-power conversion efficiency is huge. They are also collaborating with industry on the potential to produce at commercial scale thin-film copper indium gallium diselenide PV devices that could be rolled out in vast quantities. Such material could be used to clad a building cheaply and not only provide its own electricity, but export surplus to the power grid.

“Efficiencies are lower, but you would also have somewhat lower costs to offset the lower efficiencies,” said NREL Solar Program Manager Roland Hulstrom.

Already production of PV power has roughly doubled every year this century. Recent estimates put PV peak electricity output around 12.4 gigawatts a year. Most installations have been in Germany, Japan, the United States and Spain, which collectively account for about 90 percent of the global total. In some projections, PV could produce enough electricity to meet the needs of 14 percent of the world’s population by 2030.

Interest in the technology has been boosted in recent years as the threat of climate change becomes more evident. However, the technology is not without its challenges. PV cells produce direct current—fine for satellite usage where power-to-weight ratios are critical. But for most terrestrial uses, it must be converted to alternating current, which involves a slight but important loss of power.

Another challenge is that solar, by definition, works best in intense sunlight, which is limited in some countries and climates. Its power, like most renewable energy, must be used immediately or stored. Generally, this is not an issue in the United States where, on a hot summer day when generation from solar PV is peaking, demand for electricity for air conditioning is also peaking. Storage is another issue, and one on which NREL is working.

Consequently, Hulstrom sees a future where solar PV and a sister technology, concentrated solar-thermal electric, could produce significant amounts of the electricity in the United States. In addition, low-carbon solar PV could play a major role in electrolyzing water into its constituent atoms hydrogen and oxygen, freeing hydrogen for fuel cells and hydrogen engines.

“The challenge is to continue to lower costs, ensure reliability and integrate solar electric power plants with the grid in an optimal fashion,” Hulstrom said. “When, not if, we are able to do this, then the potential is very attractive. In short, there is a tremendous opportunity beyond the challenges.”