Scientific or mystical? How to understand the performance of bifacial modules

May 18, 2022 Christophe Campistron, partner at Everoze

Today, on the one hand, bifacial technology complicates the estimation of performance – and therefore of producible – of PV power plants and, on the other hand, there is no consensus on the method to be adopted. Everoze has identified three critical elements regarding bifacial modules. The key is to simplify the definition of performance in EPC and O&M contracts to limit, simplify – or ideally avoid – lengthy discussions and complaints between the parties concerned.

We can consider that the appearance of bifacial modules is a recent technological leap. This technology is now widely recognized and adopted, because the additional cost is generally offset by a gain in production. For some developers of ground power plants, it is even the default technology. Many publications, works and tools deal with the calculation of the bifacial gain. Estimating the performance of a bifacial module is extremely more complex than for a module where only the front face is active. It is necessary to take into account the albedo, the shadows on the back face of the PV field and other criteria which are difficult to evaluate precisely. All these factors lead to increasing the uncertainties of producible studies for bifacial projects. But at this point, despite – or perhaps because of – the size of the gain, there is no consensus. Each technical advisor, engineering office, manufacturer or financier may have a different vision on the same project.

Furthermore, the albedo, which was previously a not (or little) discussed entry in the PVsyst energy evaluation for monofacial modules (with a standard of 20%), has a much higher impact on the bifacial performance gain. One way to address this 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 evaluation.

Market practices

Despite an increase in the number of on-site measurement campaigns, market practices around proposed installation procedures for on-site albedo measurement continue to evolve. It is not uncommon to see short term campaigns with an albedometer on site for a few days. On the correlations between satellite data and on-site measurements that Everoze carried out for this type of campaign, the results do not seem promising. So this approach does not seem to be the solution to the problem.

In order to better understand the importance of the impact of albedo, Everoze performed a simple analysis on several projects using bifacial technology. The analysis – performed using PVsyst – demonstrated a linear correlation between producible 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, a 10% change in albedo means going from an albedo of 0.2 to 0.18 or 0.22. So, with this sensitive correlation in mind, a 40-50% error in the albedo figure leads to a non-negligible impact on the return on investment in the financial model.

Promote simplicity

Finally, the question must be asked of the definition, installation, measurements and corrections of the performance of bifacial modules within the framework of construction contracts, all of this to arrive at the implementation of guarantees. As the performance guarantee is meant to protect the owner or lender from poor performance over the life of their asset, this aspect is critical.

On albedo measuring stations, the good news is that the latest IEC 61724 standard allows you to check if the station is properly installed and meets industry standards. However, it is also important to ensure the correct location of the station, which should be representative of the “average plant albedo”. Only for truly homogeneous projects with the same inter-row spacing in desert climates, for example, can this be straightforward. It should also be borne in mind that the albedo of the first year may not be representative of the long term, since the works have changed the ground conditions which will regenerate after the first years of vegetation regrowth. As performance tests are limited to one or two years, they may not be representative of long-term plant conditions.

The higher the complexity of a photovoltaic contract, the greater the chance of never agreeing on the PR obtained by the manufacturer. Given the financial stakes, resolving this discussion can be long and costly. Therefore, we consider it best if possible not to include an albedo correction (as with temperature correction) in the EPC contract, and instead find an appropriate albedo figure before signing a contract. This is either by setting up long-term measuring stations or by agreeing on an albedo figure that is not the default figure of 0.2.

The assessment of bifacial gain and albedo clearly adds complexity to the definition of warranty in the EPC contract when it is not supported by standards or widely adopted market practices. All project stakeholders must be aware of the uncertainty associated with an estimate of the solar producible, which is even greater in projects with bifacial modules. Choosing pragmatic and simple definitions may be the only way out while waiting for more clarity and consensus on the subject.

About the Author

Christophe Campistron has a broad and deep understanding of the technical and commercial realities of solar photovoltaic projects, combined with experience in the construction and operation of projects. He is just as comfortable advising at a strategic level as he is digging into the details of on-site technical issues.