What next?
In January 2006 XsunX began developing a high-efficiency opaque solar cell with exciting potential to make the non-glass portions of buildings productive. It, too, is flexible, so it can be applied to contoured surfaces on a roof or façade.
The technology combines concepts from Power Glass and a nanocrystalline solar cell to make a two-sided solar cell. Stacking techniques like this have been the subject of research to increase power output by trapping more of the light spectrum.
Stacked solar cells were initially built as a string with one positive and one negative terminal, similar to a storage battery. And, as with batteries, degradation in one cell degrades the entire string. This is because of a requirement known as current matching: The cell with the lowest current limits the current of the complete unit.
The XsunX stacked cell will have four terminals, instead of two. Djokovich predicts this design will eliminate degradation by overcoming the need for current matching. He explains:
"We can have a solar cell on one side of an insulator working to its maximum potential, and a solar cell on the other side working to its maximum potential, and combined they provide significantly more power. Out the back we run two positive and two negative terminals. With that we eliminate a condition that's plaguing other thin film devices, the need for current matching."
The market for opaque photovoltaics overshadows that of solar glass. With this technology, XsunX is aiming squarely at that larger market. The company's researchers expect to achieve at least 12 percent conversion efficiency, if not 15 percent (comparable to today's silicon wafer technology), at a lower cost.
"This is a product that will go right after the crystalline wafer market and offer an alternative, a lower price point, and more diversity of applications," says Djokovich.
He sees the next BIPV opportunity in the form of roofing materials and facades, particularly in prefabricated construction.
"The trend is toward modular panel construction -- where entire façades of a building are prefabricated offsite -- and integrating these PV components into the façades at the factory level. Then when they're installed they're all part of an engineered, integrated structure."
This panacea of BIPV may be two steps closer to reality.
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