Bifacial Solar Cells and Modules

Bifacial modules are one of the most popular topics in the field of PV module advancements. It is a simple step away from the traditional reflective backsheet and replacing it with a transparent layer,  allowing light to enter the backside of the module. Depending on a number of factors such as mounting conditions, tilt angle, site albedo, module bifaciality and module design, the gains range from 5% to 30% increase in power output. The International Technology Roadmap for Photovoltaics (ITRPV) also predicts that true bifacial modules will gain 50% of the world’s PV module market share by 2029[2].  Furthermore, bifacial PV modules are predicted to gain 60% of the global market share by 2029. Now, nearly every module provider has at least 1 bifacial design and nearly every cell manufacturer is also conducting bifacial cell research.

Figure 1 – Prediced global market share of true Bifacial modules Source [2]: ITRPV

Glass-glass module technology is an important driver for bifacial module design, this is due to the increased reliability and more importantly, its transparency provided to allow more light to enter the back of the module. However, another important driver for the uptake of true bifacial modules is the transition to higher efficiency solar cell architectures, which are intrinsically bifacial. Furthermore, although PERC cells are not bifacial by nature, the process can be easily changed to produce them as bifacial cells. This cost-effective approach for bifacial PERC is an important step for the transition to bifacial modules, especially since PERC cells have boomed in the industry, dominating the global market in 2019. 

On top of using a glass-glass design, which does not feature any gains in performance, bifacial modules can also be combined with techniques such as half cells, mutli busbar (MBB) and larger wafer formats. Therefore, manufacturers have begun to provide a tougher module with longer warranties (30 years) and higher power output due to the combination of sturdier glass-glass structure, bifacial cells with MBB, half cells and larger wafer formats.

Figure 2 – Jinko Solar’s SWAN bifacial module with 5 busbars, larger wafer format, half cells. “Up to” 435W and uses a transparent tedlar backsheet instead of glass. Source [3]: Jinko Swan Brochure

Although bifacial cells are becoming the norm, bifacial modules still require a number of design changes in manufacturing. Aside from the transparent backside, the junction box design must be changed, alternative encapsulants must be considered and different cell interconnection methods are needed to maximise the benefits of the bifacial module. There are also the associated costs of the double-glass design and extra mounting support required for the heavier modules. 

Encapsulant changes

Almost every bifacial module manufacturer has begun opting for polyolefins (POE) encapsulant instead of the traditional EVA. This is to maintain the enhanced reliability of the double-glass design. The traditional encapsulant is known to release free radicals during processing, which are trapped by the double-glass and then can become detrimental to solar cell performance. Furthermore, POE can also limit potential induced degradation (PID), featuring about 10 times more barrier capability. This is achieved by hindering the ionic current flow through the encapsulant.[3]

Transparent Baskside, Glass or Tedlar?

The double-glass design offered by manufacturers only maintains the selling point of higher reliability and subsequently, longer warranties. However, the higher costs of glass and associated costs of mounting structures for the heavier design have caused teething problems for bifacial modules. Attempting to drive down these costs and weight, frameless designs have been offered but these introduce another wave of challenges such as reduced reliability and a difference in mounting structure requirements. An alternative to the glass backside is therefore, a transparent backsheet. 

An interesting alternative to a glass backsheet is a transparent tedlar backsheet which is for example available from DuPont. This backsheet allows the same features of glass but at a reduced weight. On top of being lighter, the manufacturing process is similar to traditional mono-facial production and the backsheet enables better heat dissipation. However, the transpartent backsheets have a lower transparency, durability and require a frame. The lower durability also once limited the warranty of bifacial modules with transparent tedlar backsheets to 25 years, prompting installers to choose the 30 year double-glass design. This has since changed with products like Jinko Solar’s SWAN module (figure 2), which is bifacial, uses a transparent tedlar backsheet and has a 30 year warranty. Furthermore, a lighter module will enable fewer costs in mounting equipment, particularly for naturally heavier, 72 cell module design.

Rear Escape Losses

Bifacial modules incur a larger rear escape loss mechanism compared to traditional mono-facial designs. With a transparent backside, light that passes through the gaps of the cells or not absorbed by the cells are free to ‘escape’ out the back. With higher efficiency technologies, this loss, although minimal, has an increasing effect on the cell to module (CTM) losses. Traditional mono-facial modules obtain some CTM gains using a reflective backsheet, a similar approach for bifacial modules is printing a reflective pattern across the glass or tedlar backsheet. The pattern leaves the area of the cells uncovered such that light can still enter while the areas between the cells are covered with a reflective coating. This reduces rear escape losses, enabling more light to be reflected back into the module.

Junction Box Design

Furthermore, junction box suppliers have begun offering junction boxes that can be attached to the corner of modules. This avoids the excess shading and subsequent mismatch of 1 or 2 cells that a traditional junction box on the backside would cause.

Figure 3: patterned backsheet in a lab Source [4]: Taiyang news article

Bifacial Module Rating

Further teething problems for Bifacial modules were the lack of an international standard for qualifying them. This void in standards lead to uncertainties in power rating, subsequent yield simulations and hence pricing and bankability. However, the International Electrotechnical Commission has since released a document explaining how to perform the “Measurement of current-voltage characteristics of bifacial photovoltaic (PV) devices” [5], a step forward in reaching a more robust system to accurately rate the performance of Bifacial PV. Bifacial modules also have an additional characteristic, bifaciality, which is the ratio of front power to rear. P-PERC cells have a bifaciality of 70% while HJT boasts over 90% bifaciality. Bifacial models such as the HJT 72 cell design from Sunpreme has a 390 W power rating and bifaciality of 95%. However, the 390W power rating would only be based on standard testing conditions, with varying expected outputs for rear illumination.

References:

[1] – Shravan K. Chunduri, Michael Schmela, “Surprising Developments Leading to Significantly Higher Power Ratings of Solar Modules”, TaiyangNews report on Advanced Module Technologies, 2019. Available: http://taiyangnews.info/reports/advanced-module-technologies-2019/

[2] – International Technology Roadmap for Photovoltaics, 10th Edition, 2019, pg. 45, fig. 43. Available: https://itrpv.vdma.org/

[3] – López-Escalante, M, Martín, L, Caballero, F, Gabás, A, Cuevas, J & Ramos-Barrado 2016, ‘Polyolefin as PID-resistant encapsulant material in PV modules’, Solar Energy Materials and Solar Cells, vol. 144, pp. 691–699.

[4] – Michael Schmela 2019, ‘TaiyangNews First Combined Market Survey on Solar Backsheets & Encapsulation Shows New Products & Trends That Have Convinced Module Makers to Offer 30 Year Performance Warranties’. Available: http://taiyangnews.info/opinion/innovation-in-module-backsheets-encapsulation/

[5] – International Electrotechnical Commission, ‘IEC TS 60904-1-2:2019 – Photovoltaic devices – Part 1-2: Measurement of current-voltage characteristics of bifacial photovoltaic (PV) devices.’ Available: https://webstore.iec.ch/publication/34357

[6] – Max Mittag, Karin Schneider, image of a bifacial module, 2019. Fraunhofer ISE. Available: https://www.ise.fraunhofer.de/en/press-media/news/2019/analyzing-bifacial-modules-under-realistic-conditions.html