The Nataional Renewable Eenrgy Lab (NREL) reports on super-efficient concentrated solar cells and the keys to low-cost solar power They describe the Amonix 770 concentrated solar power system which can be manufactured at about a third to one half of generators using crystalline silicon or thin-film approaches. 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. Production cells never work quite as well as cells produced in the lab. But 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.
The 7700 also keeps down costs by integrating the lenses, the cells and the mounting structure into a single unit that eliminates most of the parts and costs associated with other concentrator designs. The seven MegaModules that make up the 53-kilowatt system can be hauled on two flatbed trucks, then assembled in the field in hours, rather than weeks.
Those cost-slashing measures, together with the Amonix 7700's large-scale capacity, are catching the interest of utility companies from California to Colorado. Twenty Amonix 7700s, erected on just five acres of desert, can generate more than a megawatt of rated capacity, enough to power 750 homes. That's half the space typically needed to generate that much power.
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.
Cells made from gallium, indium and other elements from the III and V columns of the periodic table are more expensive to produce today, but also can be more efficient at converting the sun's photons into usable electrons for electricity.
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 two-axis tracker is the only moving component on Amonix's CPV systems and has been designed for reliability and minimum maintenance. The energy needed to move the two-axis tracker amounts to less than 1 percent of the power output.
The system has just 12 subassemblies, which are shipped to installation sites for deployment. Once the site is ready, an Amonix system can be installed very quickly, within hours. By contrast, some systems require shipment of thousands of parts to the installation site.
Cost savings were factored in every step of the way — from foundry to grid — said Bob McConnell, who worked at NREL before he left the lab in 2007 to join Amonix and help bring the research to market.
The result is a generator manufactured at about a third to one half of generators using crystalline silicon or thin-film approaches.
Multi-junction cells can operate at higher ambient temperatures than traditional PV cells, making them ideal for sunny and dry climates in the southwestern United States, and ripe for future cost reductions.
Utilities expect their generators to last 50 years. The Amonix 7700 can reach that target with proper maintenance and timely replacement of certain parts
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."
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