Researchers from the Solar Energy Research Institute of Singapore (SERIS) at the National University of Singapore (NUS) and from the International Solar Energy Research Center Konstanz (ISC Konstanz) in Germany have developed the world's first full-size interdigitated back contact (IBC) bifacial solar module.
ISC Konstanz developed the low-cost bifacial IBC solar cell, called ZEBRA, while SERIS created the interconnection and glass-glass module. The first prototype used bifacial ZEBRA IBC solar cells with efficiencies as high as 22%, fabricated using standard industrial 6-inch n-type Cz monocrystalline silicon wafers. This, according to the team, represents the first time the bifacial glass-glass module technology was married with a 6-inch IBC ZEBRA cell, whose cost is comparable to conventional cells.
Two-faced but hard-working
The module absorbs light from the front and from the back, with a bifaciality of 75%. As a result, the double-glass module can generate as much as 30% extra power from the reflection of sunlight from the ground (“albedo”) towards the module's rear surface. What is more, because the sunlight that is reflected from the ground unto the back is converted into energy, the IBC cells overheats less, which mitigates degradation. A customized electrical junction box prevents shading of the rear surface of the bifacial IBC cells.
Added structural stability and protection from encapsulating the IBC bifacial solar modules’ cells between two planes of glass could extend a module’s warranted lifetime beyond 30 years. “Large ground-mounted bifacial systems consisting of glass-glass ZEBRA modules will have the lowest LCOEs [levelized cost of electricity] ever and the longest durability,” confirms Dr Radovan Kopecek, Managing Director Advanced Cell Concepts and member of the Executive Committee at ISC Konstanz, adding that the modules are also well-suited for building integration (BI).
“With this innovation we have created an extremely low-cost approach to fabricating IBC modules with a high power of more than 400 W, depending on the installation in the system,” says Kopecek. “Compared with standard technology, e.g., monofacial PERC or Al-BSF, the system LCOEs with ZEBRA modules can have 1-2 cent/kWh lower costs.”
Performance of the IBC bifacial solar module
Kopecek, who says his team prefers to talk about power rather than efficiency as that is what counts in the end, says, “The front side efficiency using 22% ZEBRAs 60 cell 6-inch M2 modules at STC is 320+ W. With a 25% gain from the rear, this module has a power of 400+ W. In the field ZEBRA module powers of close to 500 W are expected because in many places you have more light from the front side than under STC. So for the first time, we are approaching 0.5 kW-powerful models.”
The researchers say the next step is to achieve 350 W frontside power in the short term and to get to 380 W in future. Kopecek expects this progression to be “quite easy” using M4 wafers and going to 23% ZEBRA efficiency and putting small reflectors in between the cells. His colleague from SERIS, Dr Yan Wang, is presenting that idea at SNEC 2017 in Shanghai, where Kopecek himself is also speaking.
The team, who is in the process of talking with several cell and module manufacturers, believes the technology is ready for production as the industrially feasible solar cell and module fabrication process (screen-printing) and equipment enable the module to achieve high efficiency at lower cost.
Written by Sandra Henderson, Research Editor, Solar Novus Today