Bifacial solar cells
A bifacial solar cell (BSC) is a photovoltaic solar cell that can produce electrical energy from both front and rear side. In contrast, monofacial solar cells produce electrical energy only when photons are incident on their front side. Bifacial solar cells and solar panels (devices that consist of multiple solar cells) can improve the electric energy output and modify the temporal power production profile compared with their monofacial counterparts.
In conventional installations, such as fixed-tilt equator-facing solar panels or panels mounted on solar trackers, bifacial solar cells allow additional energy production due to more effective use of albedo irradiation. The albedo (reflectivity) of the ground, the bifaciality factor of the solar panels (defined as the ratio of rear efficiency to the front efficiency subject to the same irradiance), and the technical installation parameters define how high this production boost is. In cases where the additional irradiance on the rear side is lacking, such as common parallel-to-roof solar panels, bifaciality does not provide additional electricity yield.
Another way to use bifaciality of solar cells is to install them vertically, so that the sides are facing east and west. This installation provides two-humped production profile, where the electricity production peaks in the morning and in the evening. Compared with a conventional solar energy production profile that has a single high production peak at noon, the vertical bifacial production profile allows to produce more electricity when the consumption is high and to avoid the overproduction of solar energy around noon. Especially, in the power system with high share of solar energy like Germany, vertical bifacial solar cells allow to mitigate the duck curve issue.
Vertical bifacial solar cells are especially attractive in high-latitude locations, where they can reach higher annual production than their monofacial, equator-facing counterparts. Combining vertical installation to agricultural land (agrivoltaics) allows efficient land use: sparse spacing of vertical solar panels allows machinery to move between the rows. Moreover, vertical panels are less vulnerable to soiling than panels with lower tilt angles, which is beneficial e.g., in desert environments. However, vertical bifacial solar cells are vulnerable to shading from surroundings: due to vertical installation and low solar elevation angle when the sun is at east or west, vertical solar panels require open space in the direction where the panel is facing.
Efficiency of solar cells, defined as the ratio of incident luminous power to generated electrical power under one or several suns (1 sun = 1000W/m2), is measured independently for the front and rear surfaces for bifacial solar cells. BSCs and modules (arrays of BSCs) were invented and first produced for space and earth applications in the late 1970s, and became mainstream solar cell technology by the 2010s. It is foreseen that it will become the leading approach to photovoltaic solar cell manufacturing by 2030 due to the shown benefits over monofacial options including increased performance, versatility, and reduced soiling impact.