A Brief History of Bifacial Photovoltaics
To Space, Back Again, And Beyond
by Will Beutell, Application Engineer, EKO USA
While the first solar cell created in 1954 used an 'N-Type' (negatively charged) silicon wafer, PV crystalline silicon modules using 'P-Type' (positively charged) cells soon dominated. 'P-Type' cells were almost exclusively researched and used during the space race.
With so much money spent on space-related solar technology and one-upmanship, 'P-Type' cells improved quickly. It wasn't long before these advances trickled down to commercial and residential applications. Since then, over many years, 'P-Type' cells have come to lead the market, yielding more power, with lower manufacturing costs, than the original 'N-Type'. However, common 'P-Type' modules generally lack rear-side passivation, limiting their potential performance.
More recently, 'Passively Emitted Rear-side Contact' (PERC) module research, in tandem with progress in 'N-Type' cell development, sparked a surge of interest in bifacial photovoltaics (BPV). In 2016, manufacturing processes for both PERC and 'N-Type' cells finally became economical, opening the doors for BPV technology to be produced and deployed in earnest.
Many factors can drive market adoption of technologies within the solar energy industry, from the space race, through good old-fashioned regulatory changes, economics, and the availability of materials and resources through the supply chain. BPV is snowballing thanks to a combination of factors; better module efficiency, the increasing ease of manufacture, and the improving economic and clear environmental case for solar power, more generally, for example. However, another more opaque but crucial contributing factor has been the reduction in module degradation factors.
Photovoltaic (PV) modules are typically purchased with a warranty guaranteeing performance for 10 to 20+ years, depending on the brand and model. This average lifespan was driven initially by economic need; financiers required meaningful, long-term guarantees to build confidence. As early projects matured, however, some performance issues began to emerge. Many European power generation systems saw unexpected degradation that, for some time, was not well understood.
We now know that these degradation issues stemmed from the fact that European systems have long utilised a floating electrical ground, and many modules used standard glass containing small amounts of sodium ions. Differences in electrical potentials within the modules and at the junction of the module and frame would cause these sodium ions to either move into the PV cell or away from the cell into the metal framing. This degradation is referred to as Potential-Induced Degradation (PID).
For the USA and other countries, PID was not an issue, thanks to the convention of negative ground. However, inverter manufacturers modified their designs over time to incorporate a floating ground, thus exposing PID concerns to non-EU solar energy markets.
Eventually, it was discovered that removing the module metal framing and moving to a fully sealed glass frameless design mitigated the effects of PID due to reduced conduction points. Double glass-frameless modules were also proven to help. However, there were other systemic degradation issues to solve, all guide the PV market towards BPV.
Following initial exposure to radiation, the relatively standard 'P-Type' modules experienced higher than expected degradation, called 'Light Induced Degradation (LID), in part due to the use of Boron as the doping material. Other doping elements like Gallium for 'P-Type' cells were tried and successfully reduced the risk of degradation. However, supply issues due to the limited global availability of Gallium and higher costs made it impractical.
Meanwhile, N-type cells are nearly unaffected by LID; with increasing performance gains and reduced PID, bifacial has become not only cost-competitive but technically appealing too.
Today, bifacial continues to grow. A 2019 market research report published by Wood Mackenzie Power & Renewables (WoodMac) predicts that annual global bifacial module capacity will exceed 21 gigawatts by 2024; arguably, a conservative evaluation in light of recent election results and policy announcements, particularly in North America, while the International Technology Roadmap for Photovoltaics (ITRPV), published by German engineering association VDMA, forecasts that the growth in bifacial technologies will account for 80% of production by 2031.
For bifacial, the only way is up (and down)!
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