Historically, solar PV modules have been monofacial, meaning that they use the light shining on just one side of the module to generate electricity. Bifacial modules use light shining on the front and reflecting on the rear of the module to generate electricity.
Monofacial modules generally have a glass front sheet and polymer backsheet, whereas most bifacial modules use glass front and back sheets, although some bifacial modules instead use a transparent polymer rear backsheet. The choice of materials affects the structural performance of the module, resistance to extreme weather events, and operational parameters such as heat dissipation, operating temperature, and long-term degradation.
The main reason to use bifacial modules is that they provide a performance increase (gain) relative to monofacial modules of the same front-side capacity. The bifacial module typically sees maximum power (W) increases of 5 to 25%. The performance increase strongly depends on the amount of light reflected onto the rear side of the module, which is governed mainly by the surface albedo (ground reflectivity). A higher albedo surface reflects more light from the ground and increases the gain of bifacial modules. Clear dirt ground may have an albedo of approximately 10-15% reflecting 10-15% of the incoming light. Some other common ground surfaces include grass at 15-25%, light gravel and sand at 25-35%, or snow at 60-80%. Performance gains are also affected by plant design parameters such as module height, row distance, inverter technology and mounting structure type.
Use of bifacial modules with a reasonable albedo ground surface and well-designed plant can result in plant yield (kWh) increases of similar magnitude to the power increase of the modules. Large increases require a highly optimized plant design and specific operational conditions.
Bifacial modules generate more power relative to a monofacial module of the same front-side capacity, therefore fewer need to be installed to achieve a specific capacity for a given power plant (for example, 100 MW). Fewer modules mean less balance-of-system (BOS) components are required, such as cables and connectors. This reduces potential points of failure, overall BOS cost, plant cost, and levelized cost of energy (LCOE).
Bifacial modules that use glass on both sides of the module, instead of glass and a polymer backsheet as found on a monofacial module, tend to show lower degradation over their operating lifetime and eliminate some module failure modes associated with polymer backsheets. This results in increased plant performance over the lifetime of the project.
As of November 2020, data from PVXChange indicates bifacial modules cost 0.31 EUR/Wp compared with high-efficiency mono modules at 0.30 EUR/Wp. This difference is around 0.01 EUR/Wp (3%). This is a significant reduction from just five years ago when the cost difference was nearer 20%. Bifacial and high-efficiency modules are expected to remain similarly priced.
Exhibit: Solar PV price module trends
Depending on the design and operational philosophy, bifacial plants can have other additional cost increases such as trackers, terrain preparation, or increased cleaning needs. However, the increase in CAPEX and OPEX are typically lower than the performance gain and subsequent financial gain experienced by a project using bifacial modules instead of monofacial modules. Simply, a lower levelized cost of energy can typically be achieved with bifacial modules.
As the price of bifacial modules has decreased, and the levelized cost of energy that can be realized becomes lower, bifacial modules have gained considerable traction in the marketplace. They have become the desirable module technology for new plants, with their deployment limited mainly by supply constraints.
Cell production lines have transitioned from producing single-digit percentage of suitable bifacial cells in 2015 to almost producing entirely bifacial-suitable cells in 2019. Bifacial technology is expected to dominate for the foreseeable future.
As mentioned previously, the performance increase of bifacial modules is strongly coupled to the albedo of the local area. However, the local albedo is often not characterised and understood as well as classical irradiation data used for monofacial plant performance forecasts. Consequently, there is a higher uncertainty associated with the yield contribution from the rear-side of the modules, and more generally with bifacial projects.
This increased uncertainty means a lender may discount the rear-side yield relative to the front-side yield, resulting in increased financing costs for a bifacial project relative to a monofacial project. Additional contractual provisions could be used to transfer risk away from the financier, such as higher contractual performance metrics, higher performance liquidated damages, or extended acceptance test and performance warranty periods. However, reducing risk for all parties will drive lower LCOE values. Better understanding of the local albedo remains an important and ongoing area of research for the industry .
Bifacial modules are increasingly dominating the solar PV industry. Project CAPEX and OPEX costs will be slightly higher, however with the correct plant design and operational conditions, bifacial modules offer increased yield and lower levelized cost of energy over the life of the plant. Increased uncertainty does remain with respect to the rear-side performance and albedo. Contractual provisions can provide some transfer of risk away from the lender, but ultimately decreased finance costs should stem from the additional understanding gained from ongoing industry efforts to research albedo and operational performance.
Photo credit: Asia Chang on Unsplash