Bifacial Solar Panels
Bifacial solar panels generate solar power from both direct sunlight and reflected light (albedo), which means they are essentially double-sided panels.
That's a big difference from the more common monofacial solar panels, which generate power only from the sun-facing side.
Bifacial solar is not new. In fact, the first solar cells produced by Bell Laboratories in 1954 were bifacial.1 However, despite their potential for increased efficiency, bifacial solar panels do not have the widespread adoption of monofacial solar panels, due in part to their relative cost as well as the more specific environmental conditions they require.
How Bifacial Solar Cells Work
By capturing albedo as well as direct sunlight, the amount of electricity generated by each bifacial panel increases, meaning fewer solar panels need to be installed.
Unlike monofacial solar panels, they are made of transparent glass, which lets some of the light pass through and reflect off of the surface below. To further increase the amount of light passing through, they use glass instead of metal frames or grid lines to hold them in place. The glass is tempered glass reduce scratching. Otherwise, they perform exactly as other photovoltaic (PV) panels work, using crystalline silicon to absorb sunlight and convert it into an electric current. The backside of a bifacial solar panel usually shares its circuitry with the front side, thus increasing the efficiency without increasing the circuitry.
Bifacial vs. Monofacial Solar Panels
Bifacial panels can generate up to 9% more electricity than monofacial panels, according to recent research by the National Renewable Energy Laboratory (NREL), a division of the U.S. Department of Energy.2 As is the case with higher efficiency monofacial panels, this means that fewer panels need to be installed—as well as the associated hardware like panel mounts, inverters, and cables—reducing both hardware costs and labor costs.
Solar PV technology is less efficient at higher temperatures, which gives bifacial panels another advantage. Because they are made of glass without the heat-absorbing aluminum backing of monofacial panels, they have lower working temperatures, which adds to their efficiency.
Bifacial panels don't need to be grounded, since they lack metal frames that might potentially conduct electricity. And since their construction makes them more durable, they often come with longer warranties—30 rather than 25 years for monofacial panels.
Because bifacial panels rely more on diffuse solar radiation, they are more efficient than monofacial panels in cloudy climates, or anywhere there is less direct sunlight and a greater percentage indirect, diffuse insolation. For the same reason, bifacial panels are more efficient for longer periods of the day, when there is still diffuse sunlight but none directly shining on the panels.3
Bifacial panels can also better benefit from solar trackers to follow the sun throughout the day. With tracking, the electricity generated has been shown by one study to increase by 27% over monofacial panels, and by 45% over fixed-tilt bifacial panels.4 Another study with similar results determined that bifacial panels on solar trackers decreased the cost of electricity by 16%.5
Where Are Bifacial Solar Panels Typically Installed?
Bifacial solar panels are best suited above highly reflective surfaces such as sand, concrete, or snow. With their minimal tree cover, deserts like the Atacama Desert in Chile pictured above, have high albedo rates, as do regions where grass turns brown in the summer, such as on California hillsides.
NREL has built a database comparing the levels of reflectivity of different materials and made it available on the DuraMAT website. Solar installers can use the data about an area's humidity, average cloud cover, type of ecological biome, wind speed, and other parameters, to calculate the relative efficiency of locating bifacial solar panels in different sites.
The same applies to higher latitude regions with long periods of snow cover. Solar panels typically produce about 40-60% less electricity during the winter, yet solar panels are more efficient in the cooler temperatures and decreased atmospheric interference of higher latitudes. In wintry climates, capturing reflected sunlight from snow improves that efficiency during a season in which they are best able to convert light to electricity.
Generally speaking, bifacial panels are not well-suited for residential rooftops, for a number of reasons. To reduce shading underneath them, bifacial solar panels usually need to be situated higher off the reflective surface below, so they cannot be installed close to a roof surface. Even if they could, dark-colored roofs absorb rather than reflect light. Bifacial panels are also heavier which makes them more difficult to install and limits their use cases. Older roofs may also not be able to support the added weight or accommodate the support structures that bifacial panels require.
Finally, bifacial panels are more expensive and labor costs are higher, making the higher total upfront costs prohibitive to many smaller-scale residential customers. Still, the added cost of the panels is less than 10%, according to the same NREL research cited above, so it is offset by the added efficiency of the modules.6 If a homeowner has a roof that will support bifacial solar and the ability to finance the investment, it is well worth the cost.
Other surfaces, however, are ideal locations. Flat-roofed buildings that are painted with lighter colors can have bifacial panels mounted on them, as can parking canopies, pool patios, decks, pergolas, porches, awnings, and other shade structures. Ground-based systems which cover light materials like concrete, sand, gravel, or tile, are strong candidates as well.
What may be a limitation may also be a virtue. Requiring higher mounts than monofacial solar panels, bifacial solar panels can be more easily made part of an agriphotovoltaic system, which combines farming with solar energy generation. Crops can be more easily grown around the higher mounts, while grazing cows and sheep can benefit from the shade the panels provide, making the land 60% more productive by combining the two functions.
Outlook
According to NREL, “Bifacial PV is becoming mainstream with gigawatts of installed projects.”2 Market forecasters expect bifacial solar to have a compound annual growth rate of 15% during the forecast period 2020-2027. And NREL predicts that by the end of the decade, bifacial solar panels will represent 60% of the solar PV market, up from roughly 15% in 2019.8 As solar energy scales up with increasing market demand and government support, and as climate change increases the need to electrify everything everywhere, space constraints and increasingly contentious issues of land use and may favor fewer, more efficient, bifacial panels.
As with solar technology in general, the cost of bifacial panels is bound to decline as volume of production increases, with predictions that price parity with monofacial solar will soon tip the market in favor of bifacial panels. The cost of solar electricity dropped by 90% between 2009 and 2020, according to Lazard's Levelized Cost of Energy.9 This makes bifacial panels especially attractive to utility-scale and community solar farms, where economies of scale mean the increased energy yield comes at only marginally increased cost.10
The cost difference would also decrease if the U.S. government eliminated the tariffs put in place in 2018. So far, the Biden administration has supported the tariff as it seeks to promote American-made production of solar panels over imports from China, with the support of some U.S.-based solar manufacturers. To date, no such lifting of the tariff is in the works, as the issue winds its way through the court system. Already, however, the levelized cost of bifacial systems is competitive with monofacial systems. NREL predicts that “post-tariff, bifacial is a clear winner.”8
Our Future Is Now
Unlike other attempts to increase the efficiency of solar panels, such as perovskite solar cells, bifacial technology exists now, is deployable at scale, and is deployable quickly. As the urgency of action on climate change gets clearer and clearer, bifacial solar technology presents one of the most effective means of reducing carbon emissions in the energy sector.
While bifacial panels aren't for every rooftop or even every ground mount, their increased efficiency allows utility-scale solar developers to get a quicker return on their investment—and thus attract investors looking for short-term gains. Their smaller footprint compared to monofacial panels allows apartment building owners to efficiently bring solar electricity to their tenants, and allows community solar farms to be built close to where customers need it, all without the need for large electricity transmission upgrades. Bifacial
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