Today, bifacial technology makes it increasingly difficult to define plant performance, and so far no industry consensus on the methodology has formed. When it comes to evaluating, installing and evaluating installations with bifacial modules, we at Everoze have identified three critical challenges to overcome. After all, simplifying the definition of plant performance in EPC and O&M contracts is perhaps the best way to limit – or better avoid – lengthy discussions and complicated claims for all parties involved.
The bifaciality of PV modules has led to a technological leap in recent years. The technology is now widely used, since the cost difference is generally well compensated by the energy gain. For some developers of ground-mounted photovoltaic plants, this is now the default technology. Many publications, books and tools deal with the calculation of the bifacial gain. The complexity of evaluating the gain on the back side of modules has increased significantly compared to measuring the front side alone. Assess albedo (i.e. ground reflection of irradiation), estimate back shading – due, for example, to structures or other components – and consider other factors , is not easy. A careful energy evaluator will need to increase the uncertainty associated with the back side gain of the project. But at this point, despite – or perhaps because of – the size of the gain, there is no consensus. Each technical advisor, design office, builder or financier may have a different perspective on the same project.
But that’s not enough of a challenge. Albedo, which was an indisputable input in the PVsyst energy rating for monofacial modules (20%), impacts the bifacial performance gain to a much greater degree. One way to solve this problem is to undertake albedo measurements early in the development of new projects. The objective of these measurement campaigns is to increase the precision of the albedo values and therefore that of the energy balance.
Although more and more on-site measurement campaigns are conducted, market practices around the proposed installation procedures for on-site albedo measurement are still evolving. For example, some experts offer short-term campaigns with an albedometer on site for a few days. Everoze has analyzed the benefit of correlating long-term satellite albedo data with one- or two-day on-site measurements, but the results don’t look promising. So this approach doesn’t quite solve the puzzle yet.
In order to better understand the importance of the impact of albedo, Everoze carried out a simple analysis on several projects using bifacial technology. The analysis – based on PVsyst – demonstrated a linear correlation between energy yield and albedo. Sensitivity analysis revealed that a 10% variation in albedo results in an energy variation of approximately 0.2% to 0.3% depending on plant design and location. To be clear, 10% change in albedo means going from an albedo of 0.2 to 0.18 or 0.22. Thus, given this significant correlation, an error of 40 to 50% on the albedo figure, which is not improbable, has a significant impact on the return on investment of the financial model.
Finally, we come to the challenge of defining how to install, measure and correct the performance of bifacial modules in the EPC contract and its guarantees. This is important because the performance bond is meant to protect the owner or lender from a decline in performance, over the life of their asset.
At least there is good news, since the latest IEC 61724 standard provides guidance to check if the albedo station is properly installed and meets the correct industry standards. But he
It is also important to ensure the correct location of the albedo station, which should be representative of the “average plant albedo”. Only for truly homogeneous projects with the same spacing between rows, for example in desert climates, will this not be a problem. And keep in mind that the albedo of the first year may not be representative of the long term, because the work has changed the conditions of the ground which will regenerate after the first years of regrowth. Any test based on this transient albedo condition may not be representative of the long term.
The higher the complexity of a PV contract, the better the chances of never agreeing on the PR achieved by the contractor. Given the financial consequences, resolving this debate can be long and costly. Therefore, we think it’s best not to include any albedo correction (like temperature correction) in the EPC contract, if possible, and instead find an appropriate albedo figure before signing with long-term measuring stations or to agree on an albedo figure that is not the standard 0.2.
The assessment of bifacial gain and albedo clearly adds complexity to the definition of warranty in the EPC contract when not supported by standards or generalized aligned market practices. Being aware of the uncertainty associated with an estimate of solar power, an uncertainty that has only been increased by the complexity of implementing a bifacial module, will soften discussions. Choosing pragmatic and simple definitions may be the only way out until there is more clarity and agreement.
About the Author
Christophe Campistron has a broad and deep understanding of the technical and commercial realities of solar PV projects, combined with hands-on experience in building and operating projects. He is just as comfortable advising at the strategic level as he is delving into the details of on-site technical issues.