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India, due to its widespread in geography can be viewed as a nation where vivid range of weather condition exists simultaneously. With such varied conditions the albedo (the ratio of reflectivity of light from a surface to the incident light) is expected to vary from place to place. In power plants where mono-facial (single sided) solar modules are usedthe reflected light from the ground merely increases the internal temperature of the module (generally) resulting in lowered performance. Hence, the need of effective utilization of such reflected light can be clearly felt. This is where bifacial solar cell (and module) comes into play.
The history of bifacial solar celldates back to 1960 when Japanese researcher H. Mori proposed the design of bifacial PV solar cell and had developed the working prototype by 1966. A typical structure of a bifacial solar cell is shown in the figure below. Comparing it to the typical solar cell used in current market, the bifacial solar cell has textured surface (enables more absorption of incident light) on both sides. The bifacial solar cell has an additional surface passivation (enables increased current collection) on its rear surface. Additionally, instead of full metallic back surface, the bifacial cell has finger grid at rear side.

Figure 1: Comparison between typical solar cell (on top) and bifacial solar cell (on bottom) (Source: EPRI)

Bifacial modules uses these bifacial cells (as shown in figure above) encapsulated with transparent materials on both side for maximum light absorption. This enables the effective use of light reflected from the ground surface. Additionally, it also helps in reducing the internal temperature of module bettering its performance. Below are few advantages of using these modules:
1) Enhanced performance: An average power daily curve from a study carried out by Sandia National Laboratories is shown below. The study was carried out using mono and bifacial modules at different angles and orientations. It is interesting to note that bifacial modules outperform mono facial modules in all the conditions. This would enable more power output from same footprint. Additionally, it would also lead to a slight reduce on Balance of System (BoS) cost when compared to cost from mono-facial modules.

Figure 2: Performance analysis of bifacial module (Solid line) versus mono-facial modules (dashed lines) (Source: SOLARPRO)

2) PID free modules: This can be attributed to the fact that the current modules available in the market are frameless. Aluminium frame is known to be a conducting medium which allows the exchange of ions and electrons causing PID.
3) Lower Levelized cost of electricity (LCOE):The LCOE of the system due to its high energy generating capacity leads to lower LCOE. Additionally it also leads to slightly higher Return on Investment (ROI) while comparing it to the mono-facial module system.
4) Other benefits: Other benefits include higher linear performance warranty of plants (30 years) by all the leading module manufacturers (LG, Trina, etc.). Currently only glass (on both front and rear side) are used in the modules. This enables a reduction in cost as the glass which are currently used are 20~30% cheaper than the transparent sheet.

While there are advantages of such system, there are few problems which havehad significant impact on utilizing the benefits of this technology. Below are the few problems:
1) High upfront cost: The first and foremost barrier to use the system is the high upfront cost. This is because the bifacial cells are expensive compared to mono-facial cells.Additionally the manufacturing of these modules require additional steps adding to its cost. India markets value cost the most and it shall not be an easy task to make the consumers accept such modules compromising on cost.
2) Name plate rating of solar module: Currently the name plate capacity determination happens by flashing the module with 1 kW/m2 (which reflects actual field conditions)and measuring its power output. However for bifacial module the irradiance at which the rear end should be flashed is not known. This is because the irradiance received at the rear end changes location wise and day wise. In India, such discrepancy affects the projects where the AC capacity and/or the nominal ratio (the ratio between name plate DC power capacity to AC power capacity) are fixed. Thus the uncertainty in DC name plate capacity would lead to uncertainty in the nominal ratio of such plant.
3) Varying weather conditions:The variation causes the ground surface to vary from grass in rainy seasons to dry soil in summer seasons. This variation insurface causes variation in albedo (refer Figure 3) and hence varying power output. Such seasonal (and sometimes daily) variation in energy output. Such variation causes problem in energy modelling for the system. Additionally, with both the central and state regulator(s) bent on implementing scheduling and forecasting for renewable energy generators, such variation in energy output may cost the generators a significant amount of money.

Figure 3: Albedo factors of different surfaces (Source: SOLARPRO)

4) Optimizing height of under structure: The optimum height at which the module has to be placed has always been an interesting topic of research. One simulation study from SolarWorld suggests that the energy boost is steepest (around 15%) between 0 to 0.2 meters and the point of inflection is 0.5 meter where the energy boost of around 20% can be realized. However, after 0.5 meters height the energy boost curve is almost flat.These studies however do not account for additional cost incurred in strengthening the system against wind loads when increasing the height of under structure. Some studies also suggest that keeping the module at lower height causes backside shading leading to loss of power output. Hence a clear idea on optimum height of module under structure height is yet to be known.

Figure 4: Optimal under structure height (Source: SOLARPRO)

While we presented both the advantages and disadvantages of using bifacial modules, the fact that they shall be the future of tomorrow cannot be ignored. The figure below shows the world market penetration of bifacial cell technology where a clear indication of increase in uptake in use of bifacial solar cells from early 2021 can be seen. However, such uptake shall require support from the government and high acceptance rate from consumers. Waaree Energies Ltd. has high quality bifacial solar modules manufacturing capability and is ready to take forefront in this continuously changing and growing PV market.

Figure 5: Market share of bifacial solar modules by 2025 (Source: ITRPV 2017)

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

RAHE ROSHAN HAMARA NATION