16 February 2011
The Amonix 7700 Concentrated Photovoltaic (CPV) Solar Power Generators are being tested for reliability and performance at the SolarTAC facility in Aurora, Colo. Credit: Dennis SchroederAmonix and NREL are jointly developing a solar power concentrator that generates electricity at prices competitive with natural gas.
The Amonix 7700 Concentrated Photovoltaic (CPV) Solar Power Generator 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 at the annual event honoring the greatest breakthroughs in technology, often called "The Oscars of Invention."
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% 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% efficiency at the module level and 27% at the system level in the field, the highest ever achieved for an operating CPV concentrator.
That unprecedented efficiency opened the door to reducing costs and reducing land use — both key for solar electricity to reach cost-parity with fossil fuels.
Seeing the potential for game-changing cost cuts, Amonix, with technical support from NREL's High-Performance PV Project and financial support through DOE and its Solar Energy Technologies Program, redeveloped its flagship CPV system using the multi-junction cells.
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 7700 already has driven the price of electricity from solar down to the price of electricity from natural gas, according to the California Market Price Referent, which establishes a proxy price for electricity generated by a new state-of-the-art natural gas plant. Solar power is at or near price parity in six other states that share California's sunny and dry climates — Arizona, Nevada, Utah, New Mexico, Colorado and Texas.
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.
Utilities take notice
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 Southern Nevada Water Authority and California Polytechnic Institute in California are among those that have purchased the Amonix 7700. The DOE and Amonix are paying for testing of the 7700 at the Solar Technology Acceleration Center (SolarTAC) in Aurora, Colo., to validate the reliability of the system.
The key breakthrough that lifted the 7700 to a 50% 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. NREL scientists had developed a high-efficiency multi-junction indium gallium phosphide PV cell that had been used previously for energy for spacecraft.
To offer up the more efficient multi-junction cell as a possible replacement for the silicon cells used in most PV concentrators, NREL issued a request for proposals for projects designed to accelerate multi-junction cell development and their integration into CPV solar systems. NREL awarded Amonix $1.2 million for a project that began in 2004 and concluded in 2008. At the end of the NREL project, Amonix was able to demonstrate close to 31% efficiency for a one-square-meter module — a world record at the time.
Martha Symko-Davies, a senior supervisor at NREL, recalled that most concentrator companies could not see the benefits of switching to new-generation solar cells, but Amonix was different, conducting research and development with NREL to overcome stiff challenges.