Bifacial PV Systems
Inside this Article
Conventional PV modules are monofacial, meaning that their electrical power output is a function of the direct and diffuse radiation captured on the front side of the module only. By contrast, bifacial modules convert light captured on both the front and back sides of the module into electrical power. Bifaciality improves PV system energy capture—dramatically in some cases—and rewrites conventional system design rules in interesting ways.
This article is an introduction to bifacial PV systems. After briefly reviewing the history of bifacial PV cells and providing a high-level overview of bifacial cell technologies, I summarize the potential benefits of bifacial PV modules and systems. I then focus on best practices and applications for designing and deploying systems that integrate bifacial PV modules. Finally, I consider some challenges to adoption and important efforts under way internationally to unlock the full commercial potential of bifacial PV systems.
Research on bifacial PV cells dates back to the dawn of the solar industry, according to Andrés Cuevas’ oft-cited article, “The Early History of Bifacial Solar Cells” (see Resources). Japanese researcher H. Mori proposed a bifacial PV cell design as early as 1960 and had successfully developed a working prototype by 1966. Russian and Spanish researchers proposed uses for bifacial PV cells around the same time. It was the Russians, however, who first deployed bifacial PV modules in the 1970s, as part of their space program. A major milestone occurred in 1980, when Cuevas and some of his colleagues in Spain documented the ability of light-colored surfaces to direct reflected light (albedo) to the back of a bifacial PV cell and increase its power output by 50%.
Due to the high cost of producing bifacial PV cells, the first terrestrial applications for this technology were relatively late to emerge. One of the best-documented early field applications is a north-south–oriented vertical photovoltaic noise barrier that Swiss researchers deployed in 1997 along the A1 motorway in Zurich using 10 kW of bifacial PV modules. The first signs of commercialization, at least in North America, appeared roughly a decade later when Sanyo introduced its first UL-listed HIT Double bifacial PV modules. Though Panasonic, which acquired Sanyo, subsequently discontinued the bifacial product line, as of January 2017, at least eight manufacturers offer bifacial PV modules certified for use in North America (see Table 1).
Cell technology. While bifacial PV cells currently make up an insignificant percentage of worldwide PV cell sales, the technology is in some ways a continuation or logical extension of standard monocrystalline silicon (mc-Si) cell technology. Depending on whether the semiconductor material contains a relative abundance or deficiency of electrons, the industry broadly categorizes mc-Si cells as either n-type or p-type devices, respectively. It is possible to fabricate bifacial cells out of both p-type and n-type wafers, given high-quality silicon material, although the process requires some additional manufacturing steps compared to producing conventional monofacial cells.
In practice, more than 90% of the PV cells sold worldwide are based on a p-type architecture, while the vast majority of the bifacial products in Table 1 are n-type devices. This underscores the fact that many n-type PV cells, which are primarily found in niche high-efficiency modules from companies such as LG, Panasonic and SunPower, are inherently bifacial. (Some people trace the history of bifacial PV cells all the way back to Bell Labs, since its first practical solar cell in 1954 was an n-type device.) P-type devices dominate the market because they are cost-effective to fabricate at scale. While n-type bifacial cells offer the highest efficiency, companies such as SolarWorld are betting that p-type bifacial cells can provide a good balance between performance and cost.
Regardless of the specific cell technology, the rear side of a bifacial PV cell needs to be able to act as a collector, which requires advanced architectures and manufacturing techniques. The authors of the informative Electric Power Research Institute (EPRI) Bifacial Solar Photovoltaic Modules (see Resources) explain: “Today’s crystalline silicon and thin-film monofacial PV cells commonly use a fully metallized backside. This feature involves a moderately thick metal contact for reduced series resistance and is relatively inexpensive to produce. By contrast, bifacial cells incorporate selective-area metallization schemes to allow light between the metallized areas.”
Though thin-film manufacturers are still working out the material science issues necessary for bifacial thin-film modules, many mc-Si manufacturers have successfully produced bifacial cells, which often incorporate thin-film layers, such as the rear passivation layers of amorphous silicon in Figure 1. The next challenge is adapting these technological advances for mass manufacturing. The EPRI report continues: “The lower amount of metal changes how cell performance is optimized, potentially requiring tighter (more expensive) specs on the silicon and thin-film material used and also increasing series resistance concerns. Furthermore, bifacial cells may employ different metals, such as copper and nickel, and/or deposition methods, such as plating or inkjet printing, which, in part, requires different equipment and entails a potentially more complex manufacturing process. Consequently, the backside metal represents a nontrivial impediment to manufacturing bifacial cells with high performance and low cost. This added complexity and cost needs to be offset by the performance gains from increased light collection.”