Understanding Bifacial Gain in PV Power Plants

Unlike conventional PV modules that convert only front-side irradiance into electrical power, bifacial modules convert both front- and back-side irradiance into electricity. While the additional rear-side irradiance improves plant performance in terms of energy production, revenue and levelized cost of energy (LCOE), industry standards and best practices for predicting and quantifying these gains largely remain a work in progress. Here, I correct three common misconceptions related to bifacial gain in the real world. 

Courtesy TUV Rheinland

Misconception #1: The Engineer of Record Defines Bifacial Gain

During the project design phase, the engineer of record must determine a percentage of bifacial gain that it will use for sizing conductors and overcurrent protection. As with many other engineering analyses, this bifacial gain estimate includes a margin of safety. This conservative estimate is based on potential worst-case scenarios that do not reflect typical plant operating conditions. As a result, using the bifacial gain percentage defined by the engineer of record will overstate bifacial gain and plant performance in the real world.

Reality: Bifacial gain is not an input; it is a result.

From the perspective of the owner’s engineer (OE), bifacial gain is not a simple input to a software model but rather the result of multiple design and engineering decisions related to the balance of system (BOS) components. In order to estimate bifacial gain in the field, an OE must account for the impact of each of these design decisions. For example, what is the racking type and configuration? What is the height of the array or the ground coverage ratio? 

Each of these decisions will yield a result. For bifacial gain purposes, a taller array is better than a shorter array; a wider array spacing is better than a narrow spacing and so forth. While these relationships are often straightforward, they cannot be ignored. In order to predict bifacial gain and system production, the software model used to predict plant performance needs to characterize the impacts of BOS design and engineering decisions.

Misconception #2: The Design Drawings Determine Bifacial Gain

While design and engineering decisions have a major impact on bifacial gain, construction details are equally important. Wire management details and combiner box placement are potential sources of shade that can further erode bifacial gain. If the construction details in the field do not match those in the planset, the actual bifacial gain will not match the theoretical bifacial gain.

Reality: Construction details and rear-side shading from BOS matters.

It is very important that an owner’s engineer review and approve a so-called golden row, both on paper and in the field. This reference row should define wire management practices that minimize rear-side shading; it should also establish benchmarks for build quality, especially for tracker systems, which have moving parts. Once construction commences, the quality assurance and quality control (QA/QC) team can use this reference row as a template for its QA/QC activities to ensure that the as-built conditions match the planset and the reference row. Standardizing these construction details and practices will not only improve plant production but also the confidence of the production model, both in terms of operational considerations and rear-side shading expectations. 

Courtesy Hukseflux

Misconception #3: Albedo is a Static Value for Bifacial Gain Calculations

There are many inputs to a PVsyst model that are relevant to determining rear-side irradiance, shading and mismatch. Albedo, which characterizes the ground surface reflectivity, is one of the most important inputs to this model. This input is especially critical to the ASTM tests used for project acceptance. It is impossible to characterize bifacial plant performance and capacity without knowing the rear-side irradiance, which is a function of ground albedo.

Reality: Bifacial PV projects must have rear-side POA sensors.

Albedo varies throughout the day and the year. This variance is most acute in winter. As compared to native ground cover or a commercial roof surface, freshly fallen snow reflects considerably more irradiance. It is easy to overinflate this enhanced irradiance if you do not account for soiling. Front-side soiling is the direct result of snowfall; rear-side soiling is based on groundfall. The difference is that four days of snowfall equates to four days of front-side soiling whereas ground fallen snow may persist for two, three or four times longer. Over time, the snow on the ground degrades due to melting and soiling and its albedo diminishes. 

At present, the industry does not have a standard on how to treat rear-side snow soiling in production estimates. For capacity and performance testing purposes, however, a rear-side POA sensor allows project stakeholders to quantify rear-side irradiance under all conditions. This datapoint is essential for acceptance testing and operations and maintenance.

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