It's always a pleasant surprise to go to a lecture at MIT or Harvard and find somebody who not only knows the subject deeply but is having fun playing with that knowledge, has real enthusiasm, and can communicate that excitement as well as the relevant data. Jeffrey Gordon of Ben-Gurion University is one of those people. He spoke at MIT on 8/12/09 about his work with concentrating light for solar electricity.
He is working with Solfocus to make small scale high concentration solar electric systems. Already in commercial use is a 1 cm2 cell with 31 cm parabolic mirror. The 1mm2 version is still being tested.
At the cm2 size, the high efficiency triple junction solar cells [Indium Gallium Phosphide, Gallium Arsenide, Germanium] can be passively cooled even at concentrations of 100s of suns, 1000s of suns at mm2 size.
Abstract
Unprecedented capabilities for ultra-efficient yet affordable solar electricity generation at high concentration stem from the confluence of progress in multi-junction photovoltaic technologies and advanced optical design. Several generations of new optics that approach the thermodynamic limit to concentration and optical tolerance, and have been tailored to the exigencies of the latest generations of concentrator solar cells, will be presented, some of which already comprise megawatt-scale installations. In addition, experimental results from novel localized irradiation probes that use ultra-intense concentrated sunlight for investigating concentrator solar cells will be reviewed. The findings include elucidating cell solid-state properties that are of value in designing future generations of higher- efficiency solar converters.
Solfocus is using a multijunction cell with efficiencies around 40%. At high concentration, this may mean 1W per cm2. Passive cooling becomes possible if cells are small like cm2 or mm2. Commercial PV arrays now have a concentration at 100-300 suns. The Solfocus concentrator uses 2 reflectors, one to produce a focus, another at the focus to reflect light back to the PV cell. Each concentrator is sealed The commercial version, already available, uses 31 cm dish aperture with 1 cm2 cell. At 500 suns, cell is 20º C above ambient temperature with no loss of efficiency.
In the 1 mm2 cell, the concentrator aperture is 31 mm in diameter. This configuraton is in prototype now at Solfocus with concentrations up to 10,000 suns. 3 junction cells are 50x more expensive than regular PV. 20-30% efficiency for the system overall. Metallization (wires to carry off current) is usually 12-13% of the surface of a PV cell. Gordon has a micro-optical solution that will direct light away from the metallization to the cell surface which allows other efficiencies: no loss from metallization and higher proportion of metallization to cell. He has also generated fullerenes and nanotubes with concentrated sunlight, including some compounds that have not been produced with other methods, and done laser "surgery" with non-coherent light.
This was a great lecture by a great experimenter and teacher talking about remarkable research with great promise. He was there to collaborate with other MIT energy researchers and clearly has a lot to offer.