By Ashay Abbhi
Solar module costs have witnessed a rapid decrease over the past two years leading to a reduction in the overall costs. However, the on-field energy efficiency has remained stagnant especially in countries like India due to the extremely low adoption of experimental technologies. As a result, solar power technology is in a constant pursuit to lower its cost per energy unit generation as compared to traditional power generation systems.
Among the various developments in the solar power generation equipment segment, bifacial module technology is being seen as one of the most promising innovations. Bifacial solar modules produce solar power from both sides. These have transparent backsheets as opposed to the traditional opaque backsheet fitted mono-facial modules currently used everywhere. The transparent sheet allows sunlight to pass through the module, which gets reflected on to the surface underneath and falls on the other face of the bifacial module, thereby generating power from both sides. The performance of the rear-sheet is highly dependent on the albedo, the proportion of the reflected irradiance. The albedo varies with the surface on which the solar power plant is installed, resulting in a wide-ranging performance output of 5 per cent to 50 per cent.
Commercial white roofs, gravel, grass or shingles – each of these has different generation efficiencies as the amount and quality of sunlight reflected upon the surface to fall on the rear face differ. Therefore, the yield gain of bifacial modules over mono-facial modules is primarily determined by the amount and quality of irradiance incident and the conversion efficiency of the cells on the rear side.
Companies currently manufacturing bifacial modules include LONGi, Lumos Solar, LG, Prism Solar, Sunpreme, Silfab, Trina Solar and Yingli Solar. There are several global case studies that provide an insight into the quantified gains from bifacial modules over traditional solar modules.
154 kW Carport Rooftop, New York, USA
A rooftop solar power system with a total installed capacity of 154.3 kW in 532 Prism Solar bifacial modules of 290 W each was installed on a commercial building in New York. The system was installed on a white roof, with a high reflectance of over 75 per cent. The modules were installed on a rack, tilted at a 30° angle facing south, with the racking optimised to not shadow the rear of the modules. The total system was designed to generate bifacial gains of about 22 per cent over the traditional mono-facial solar power system, with annual energy generation of 1,650 kWh per kW. In the winter months, the system and the rooftop was regularly covered in snow. The generation continued to be significantly high despite the front face of the modules being covered with snow. The additional energy generated by the system during the winter months was estimated to be about 38-61 per cent, leading to gains of about 13-32 per cent over mono-facial modules during the same period. For the spring months, the gains were about 11-30 per cent.
1.25 MW Asahikawa Hakuto PV Plant, Japan
The bifacial system with a total capacity of 1.25 MW was installed in 2013, and uses 5,320 modules with EarthOn cells, made by PVG Solutions, and having a capacity of 254 W each. The modules are placed at a tilt of 40° and sit 1.8 metres above the ground, at the site spread over 8.7 acres. As compared to a mono-facial system that needs about 4.8 acres per MW, the Asahikawa bifacial plant requires nearly 7 acres per MW of area. The overall energy production in the first year was 1,378 kWh per kW, reflecting an increase in the generation by nearly 22 per cent as compared to the mono-facial systems. Also, the system recorded a higher rear-side generation of about 15 per cent during the winter months as compared to that in the spring months.
1.7 MW La Silla PV Plant, Chile
A bifacial solar power system was installed near the La Silla Observatory in Atacama Desert, Chile, with a total capacity of 1.7 MW. The system recorded gains of over 12 per cent with respect to standard mono-facial modules.
9.7 kW Carport System, Arizona, USA
An east-facing carport in Tuscon, Arizona, was fitted with a rooftop solar power system with a total capacity of 9.7 kW, using 36 Prism Solar bifacial modules with a bifacial ratio of 90 per cent. The system was installed on a light-coloured crushed rock surface with a tilt of 7° and a minimum height of 10 feet. It has a 90° deviation from the south-facing systems installed until now, significant to study the performance of the bifacial modules when installed to the east. The system recorded a performance increase of about 21.4 per cent.
While it has been established that albedo-dependent, bifacial modules enhance solar power generation by an average of 20 per cent, the design of the power plant has considerable implications on its performance as well. In this regard, test projects are currently under way to determine the yield gain of vertically mounted bifacial modules as compared to the traditional latitude tilt of the solar power plants. bifacial modules offer a unique design advantage suitable for vertical applications as these require both sides of the panel to generate power.
A case in point is the test project at University of Grenoble that was analysed for the potential gains of vertical bifacial modules. The researchers analysed a mix of east-west, equator-oriented tilted mono-facial and bifacial modules, and north-south oriented vertical bifacial modules. The energy gain was measured to be in the range of 10-30 per cent over a year. Moreover, the reduced soiling due to the vertical design resulted in an additional gain of 10-20 per cent, especially in dusty areas.
bifacial module technology can also be used for building-integrated photovoltaic (BIPV) applications. These can be deployed as exterior walls over buildings with direct solar irradiance to provide additional yield as compared to mono-facial BIPV. Moreover, projects are under way to estimate the energy gains by using bifacial modules along with tracker technology.
Due to the lack of standards available for bifacial solar modules, their pricing is highly variable and depends upon their applications and composition. Moreover, since there is no standard for including the rear-sheet into the nameplate rating of the modules, most manufacturers price bifacial modules based only on the front-sheet standard test condition flash ratings. Therefore, the price of bifacial modules continues to be non-standard and often incomparable with other bifacial modules unless all conditions and assumptions are constant.
The price of bifacial modules in the US varies largely. In 2016-17, the prices fell to $0.7-$1.35 per W, based on vague nameplate ratings of the front-sheet only. As a general rule, these are considerably more expensive than mono-facial panels that are priced in the range of $0.3-$0.4 per W. However, the high price is often justified with the energy yield gains and unique design applications offered by bifacial modules. Meanwhile, it must be noted that the technology is currently at the development stage with no consistent standards. Moreover, there is lack of long-term field performance data and the available energy yields may not be comparable as they vary owing to wide-ranging design parameters.
For BIPV applications, the prices of bifacial modules were $1.2-$1.65 per W in 2016-17. These are typically more expensive considering that aesthetics are a primary price driver and that bifacial modules are able to displace high-priced architectural glass. To increase their adoption in utility-scale projects, bifacial modules need to be competitive, falling in the range of $0.5-$0.7 per W.
Although bifacial module technology is slowly becoming mainstream, high costs continue to pose a challenge. Manufacturers need to bring down the costs for mass commercial acceptance of the technology, especially in the price-sensitive developing countries. Higher generation and low costs will help deliver an even lower levellised cost of electricity (LCoE) from solar power, thereby decreasing off-taker risks and increasing investor interest in the segment.
Considering that bifacial solar module technology is at a nascent stage, the manufacturing equipment needs to be customised for the application. Therefore, the cost of manufacturing is high whereas the financial return on the process and manufacturing asset investments is low. Until bifacial module manufacturing achieves mass production and economies of scale, the cost of production is likely to remain higher than mono-facial solar modules and, as a result, the adoption may remain low. Limited energy yield data for bifacial modules is also considered to be a challenge as their performance is highly site dependent.
There is no standardised nameplate rating method available for bifacial modules. This puts the technology at a significant quality risk, which may result in misleading performance information. Moreover, there are only field tests and pilot projects available for bifacial modules, with scattered on-ground performance record. Innovative designs to enhance the rear-side light capture may put the balance-of-plant equipment at a considerable risk due to the lack of standard designs and corresponding performance data.
Technology experts anticipate the adoption of bifacial modules to increase up to 30 per cent across the world by 2026, driven primarily by the need for more efficient solar power generation. bifacial cell and module manufacturing requires unique and expensive manufacturing equipment. Considering the low adoption of the technology, few manufacturers are willing to park investments of such scale in the manufacturing of bifacial modules. Moreover, in the present scenario of oversupply of solar modules in the market, which has significantly reduced the manufacturers’ margins, investments in new and more expensive technology will not find many takers, making it a challenge for manufacturers to raise capital for the technology.
Unique and innovative technologies such as bifacial solar modules are severely misaligned under the present solar manufacturing market conditions and business models. For greater technology adoption, cost parameters of bifacial modules must be effectively controlled to make it more competitive. Meanwhile, information from pilot and field test projects needs to be analysed to validate the performance enhancement levels and standardised. Standards regarding nameplate rating, manufacturing cost, and design and price parameters will need to be put in place to be able to provide project developers with uniform information regarding the bifacial technology.