San Diego, Calif. — Bifacial PV modules have been in the market for some time, but with limited usage. The BIPV applications such as awnings and canopies have been a niche market for bifacial PV modules because of glass-on-glass, see-through, aesthetic looks. They have also been used, because of their vertical install-ability, as railings or sound barriers at highways and railways or other places where space for installing standard PV modules is limited. But, are bifacial PV modules ready for larger deployment in the growing PV industry?
When bifacial modules are installed vertically and facing east-west, they can yield a very similar amount of energy produced by monofacial PV modules installed south-faced at the optimal tilt (~30 degree). Dr. Shinya Obara, Professor of Kitami Institute of Technology, pointed out that bifacial PV modules installed vertically have two peaks — one in the morning and one in the afternoon. Yusuke Kaneko, Senior Managing Director of Tokyo Solar Building Materials (TSBM), a bifacial PV module maker in Japan, said, “the vertical install-ability of bifacial PV modules provides more flexibility in installation design and minimizes space for installation.”
Although bifacial PV modules have certain advantage under unique installations, Kaneko stated that they produce the largest output when they are installed in the same way as monofacial modules — on the ground or roof with a tilt. Since bifacial modules produce electricity from both front and back sides of the module, the total energy output of bifacial modules is a sum of output from the front side and backside. For regular modules, we know avoiding shades over the modules helps energy output. For bifacial modules, we need to pay attention what goes under the modules to maximize their benefits, bringing more electricity generation per module area.
Prism Solar Technologies, a U.S. bifacial module maker, states in its Design Guide that the increase in output power can be achieved by installing the modules over surfaces with a high albedo, or surface reflectivity. One of the key components of optimizing output of bifacial PV modules is the usage of “reflectors” that amplify the amount of light captured by the backside of the module. White concrete surfaces have a high reflective rate of 80-90 percent, compared to the 30 percent reflective rate of regular gray concrete or gravels, according to testing conducted by bSolar, a producer of crystalline bifacial PV cells in Israel.
Another bifacial PV system test is currently taking place in Kitami city, Hokkaido, Japan. Hokkaido is located at the north end of Japan where the climate is cool during summer and very snowy during winter. Two 3-kW ground-mount systems were installed (south-faced with a 35-degree tilt): One system was built over finely crushed scallop shells while the other was built over a grass field. Scallop shells were used because they are inexpensive and abundant in the region, but must be washed (cleaned) or replaced annually in order to maintain opitmal surface reflectivity. During winter, snow serves as a reflective surface for both systems.
Based on the data collected from the snow season to the summer, researchers found that the average performance ratio (the actual amount of solar power produced by a module in comparison to the maximum possible power output) for the system over the scallop shells was 13.5 percent higher than the grass surface, and 23.6 percent higher than the monofacial modules. Between February and March when the ground was completely covered by snow, the average PR of bifacial PV systems was 23.9 percent higher than that of monofacial PV modules. Snow is proven to be a superior surface reflector for bifacial modules.
The distance between modules and reflectors also largely influences the output of bifacial PV modules. The available albedo light that hits the back of the module is directly related to the height and tilt of the module installed over the surface. Jerry Hughes, Director of Sales and Marketing of Prism Solar Technologies, stated that placing the modules too close to the surface creates a self-shading, which can block available albedo light and reduce sunlight reflected to the backside. Because of the self-shading effect, he recommends elevating modules above the mounting surface as much as possible.
Better Cost Performance at Optimal Installation
The cost of bSolar’s bifacial PV cells is very competitive — almost the same as that of the conventional monocrystalline PV cells, according to Yossi Kofman, CEO of bSolar in Israel. bSolar manufactures crystalline bifacial PV cells with standard equipment and production processes. The main differences in their cell technology are usage of (1) boron, instead of aluminum, for the back surface field and (2) p-type silicon, instead of n-type silicon.
The main differences in their cell technology are usage of (1) boron, instead of aluminum, for the back surface field and (2) p-type silicon, instead of n-type silicon. Usage of conductive and transparent boron, replacing the traditional opaque aluminum, allows the panel to gather additional reflected light on both sides. bSolar claims that its bifacial cells provide the lowest levelized cost of energy (LCOE) based on the fact that bSolar’s cell can produce significantly more output (plus 10-30 percent) than monofacial PV at a cost very similar to the monofacial.
Panasonic, a maker of bifacial modules in Japan, states that its bifacial modules known as HIT Double cost about 10 percent more than regular HIT modules. Kaneko of TSBM said that even if bifacial modules cost twice as much as monofacial modules, the bifacial will yield a much better ROI than monofacial because of the higher annual output during 20 plus years of the system life.
Bifacial PV modules have started gaining momentum. This year PVG Solutions provided its bifacial PV modules for two large ground-mount systems, Kuranuma Solar Project (250 kW) and Hokuto Solar Project (1,250 kW), in Asahikawa City, Hokkaido. Koichi Sugibuchi of PVG Solutions said that both systems use the tilt of 45 degree, which is 10 degree higher than that of the field test in Kitami city to prevent snow accumulation on the modules. Hokuto Solar Project, with 5,320 bifacial PV modules, was completed at the end of November. Nishiyama Takeda Electric, the owner of these two projects, chose bifacial PV modules because of their superior energy yields with snow.
Further expansion of the bifacial PV market requires more people need to understand and identify their optimal installation conditions and locations, stated Dr. Obara.