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Designing bifacial PV power plants - Getting the details right! Part 2

17th August 2021  

With almost all the new products (both cells & modules) being introduced in the market are bifacial, the technology is all set to dominate the PV market by the next decade. With such an advent, it is important for an end user to understand how optimally a bifacial module can be utilized for enhanced commercial returns. We in our previous blog titled "Designing bifacial PV power plants - Getting the details right! Part 1” introduced you to factors affecting the output of bifacial module & further also suggested the comprehensive range of those factors which can result in desiredoutput from a bifacial power plant. This part would introduce our end readers on the simulation study we carried out for bifacial power plant. It would educate its readers on the methodology we followed and assumptions we considered. In addition to it, this article would also point out the albedo dilemma in PVSyst and how can it be deciphered.

Methodology

There are lot of recorded studies which detail the performance of bifacial module around the world however only a few of them have India as a focus. India is amongst the fastest growing PV market in the world. Further it is also the 7th largest country & houses almost all the type of climatic conditions. Such varied conditions packed in a single country makes it a suitable choice for studying performance of any solar power plant. In order to clearly distinguish the performance of a power plant, we carried out a simulation study by considering a power plant of a fixed capacity. 6 different location each based in the 6 different climatic zones (refer Figure 1) were selected for simulation.

·Hot & dry climatic zone

·Warm & humid climatic zone

·Moderate climatic zone

·Composite climatic zone

·Cold & sunny climatic zone

·Cold & cloudy climatic zone

For further details on the description of climatic zones, kindly refer to our article "Mono vs Poly – An introspective simulation study! – Part 1”

The Albedo dilemma in PVSyst

Albedo as we discussed in the last article has direct impact on the bifacial gain. Thus it is important to have a power plant which utilizes ground with high albedo values. While there are studies citing the bifacial gain at a given albedo, they may not always factor in all the components correctly. It is important to note, albedo is not constant but it changes considerably as the season varies throughout the year. This makes it important to simulate the power plant with correct albedo factored in. While simulating any power plant in PVSyst, location of the plant is recorded at initial steps. The associated climatic conditions of the location can be very well approximated from the software. Albedo can be very well associated with the climatic conditions as the ground type utilized in a bifacial power plant gets a cover (water in rains, snow – both fresh & old during winter, etc.). However when we use PVSyst specifically for a bifacial power plant, it only considers a fixed albedo throughout the year. Further while there is input of manually entering the albedo and changing it, the effects of climate & its associated considerations in the software clearly seem to lack.

In our study we considered the ground to be of concrete which has an albedo of 30% ± 5% under clear climatic conditions. Taking example of cold & sunny climatic zone, the site located in this area has fairly clear weather from May to September & the concrete ground remains exposed most of the times. The winter here starts in October where one can experience drop in temperature, however the ground cover doesn’t change. From November to January (sometimes even February) the location receives heavy to very heavy snowfall and hence the ground has a fresh snow cover which requires the albedo values to be updated for those months. For the next 3 months, the location may receive occasional snowfall and the ground would mostly have old snow cover which requires the albedo of the ground to be updated again to lower values. A similar exercise was done for each location selected in the particular climatic zone which enables us to be close to the actual energy production from a power plant (refer Figure 2). The input to zone wise season variations were collected from various open source platforms. The associated albedo for a particular zone were further calculated from various open source reports (PVSyst database, NREL, ARL-NOAA Research Laboratories).

                                  

                              Figure 2: Graphical comparision of albedo variation between PVSyst and various climatic zones


Assumptions & considerations in the study

While we detailed the methodology of the simulation study we carried out, it is important to understand the assumptions & considerations we made. Firstly we carried out this study for both bifacial plants with fixed tilt & with trackers. This can be understood from the fact that utilizing bifacial modules with trackers lead to significant gain in energy output (when comparing it to bifacial power plant installed at fixed tilt). Further the market for bifacial trackers is expected to grow at a CAGR of 12%~30% in next decade which makes it a strong contender for the next decade (GTM Insights, Grand View Research). Secondly, as we mentioned earlier the ground was considered to be made up of concrete. Concrete was considered because it is easily available, has fair albedo and can very well have a power plant built upon. Further the varying seasons and its effect on albedo values was also considered as mentioned above. Next for easing the calculations on GCR & land requirements the bifacial module was considered to be in 1P orientation for both fixed tilt & tracker based power plants.

Specific to mounting, for trackers we have considered trackers with backtracking option. The tracks the sun for the entire day which enables energy boost. However there may be times when there is mutual shading of tracker due to position of sun. Thus when the mutual shadings begin, the tracker does no more follow the sun, but it goes back to its original position so that no more further shading occurs. Further as the module are in 1P orientation, there would be structure shading specifically on trackers onto the module. We have considered the structure shading factor of 5% in our study (Soltec). The shed transparency factor represents the amount of light that passes through cells and in between modules. With the current module technology & the 1P tracker, the shed transparency factor is considered at PVSyst default of 0%.

While this part gave you an outline to our study, the albedo dilemma & the important assumptions we considered in the study, the next part of the study would give detailed analysis of gains (both technical & commercial) one can expect when installing a bifacial plant throughout the country. Keep looking onto this space for out next article.

Let us all pledge to make solar energy the primary source of energy in the near future.

RAHE ROSHAN HAMARA NATION


                                                                            

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