While Bifacial Panels are becoming a trends, maximising Albedo is extremely crucial

Solar cell technology is a fast evolving technology with new advancements and improvements seen almost every day. Solar panels are the most important part of a solar energy system, which need to improve, evolve and increase its functioning efficiency to become a reliable source of energy. One such innovation is the invention of the bifacial panel, which is expected to play a key role.

This technology has been around since the 1970s where it was used for space applications. However bifacial panels have become increasingly popular only in recent times as is becoming increasingly cost-effective, as shown in the figure below.

Fig 1: Cost gap between Bifacial and Conventional Modules
Fig 1: Cost gap between Bifacial and Conventional Modules

How does Bifacial work?

Bifacial panels essentially are able to generate electricity from the front as well as the back face of the module, which allows it to generate 7-15%[4](depending on certain factors of the project) more energy as compared to mono-facial panels. This results in higher energy generation, which is the main reason for its rising popularity. 

Fig 2: Illustration of bifacial vs mono-facial functionality
Fig 2: Illustration of bifacial vs mono-facial functionality

There are certain additional factors that need to be considered while designing the project with bifacial panels, which can boost the yield of the plant. However, there is one factor that needs to be exploited in order to make the most out of the bifacial panel project!

Albedo

Albedo is defined as the reflectivity of the surface, or the amount of sunlight the surface can reflect. This is an extremely important parameter to maximise the amount of energy each panel can generate. To give an idea, if a surface has an albedo of 0.7, it means that it can reflect 70% of the sunlight falling on it. Below are the albedo’s of some surfaces.

SurfaceAlbedo
Fresh Snow[2]0.8 to 0.9
Desert Sand[2]0.4
Grasslands[2]0.25
Bare soil[3]0.18

It is the most challenging factor to understand, and one that has among the most significant influences on total energy yield. Measuring albedo is complex for several reasons. Firstly, it cannot be taken as a constant value[5]. Even slight changes in the color of the ground can have a significant effect. So for projects installed above grass or mud, there will be considerable seasonal changes, and even above concrete or gravel, rain or dust can bring about changes in albedo, along with the actual light conditions.

In a white paper published earlier this year, Spain-headquartered tracker manufacturer Soltec[6] sought to provide a practical guide to albedo measurement, based on data gathered at its Bifacial Tracking Evaluation Center (BiTEC) in California. The company notes that measurement points need to be representative of conditions across the whole site. Albedo Meters should be inspected and cleaned daily and recalibrated according to manufacturer guidelines. Soltec calculates a height between 1 and 2 meters as ideal for albedometer installation, with the option to adjust to ensure that the height from the surface is constant, even with snowfall or vegetation growth.

Soltec’s research reveals a linear dependency between albedo and bifacial gain. It divides surfaces into three broad categories of albedo to demonstrate this. White surfaces, with measured albedo of 55.6%, afforded a bifacial gain of 15.7%, while gravel with an albedo of 29.5% saw a 9.6% bifacial gain. For grass (seasonal) with 19.9% albedo, the bifacial gain was 7.3%. The figures (presented in the chart to the upper right) are based on one year of data from BiTEC installations utilizing Soltec trackers[6].

Fig 3: Albedo's of different surfaces
Fig 3: Albedo’s of different surfaces

Conclusion

With analyst predictions going as high as 40 GW for annual bifacial demand by 2023[7], it is imperative for stakeholders across the industry to understand the behavior of these modules in the field. Ground albedo and the way that diffuse and reflected light hits the rear side of the modules are the key concerns, and need to be maximized to get the highest yield out of the project.

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Source: @Solar_Edition 

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[1]: https://www.pv-magazine.com/2020/08/19/bifacial-modules-the-challenges-and-advantages/

[2]: http://www.climatedata.info/forcing/albedo/

[3]: Gascoin, Simon & Ducharne, Agnès & Ribstein, Pierre & Perroy, E. & Wagnon, Patrick. (2009). Sensitivity of bare soil albedo to surface soil moisture on the moraine of the Zongo glacier (Bolivia). Geophysical Research Letters – GEOPHYS RES LETT. 36. 10.1029/2008GL036377. 

[4]: Rodrı́guez-Gallegos et al., Joule 4, 1514–1541 July 15, 2020 a 2020 Elsevier Inc. https://doi.org/10.1016/j.joule.2020.05.005

[5]: https://www.pv-magazine-india.com/2020/01/25/the-long-read-all-about-albedo/

[6]: https://soltec.com/wp-content/uploads/2019/11/BiTEC-whitepaper-4_en.pdf

[7]: Solar Power Europe. Global Market Outlook for Solar Power (2019-2023). URL: https://www.solarpowereurope.org/wp-content/uploads/2019/07/SolarPower-Europe_Global-Market-Outlook-2019-2023.pdf