## 5 Bifacial Modules: Not a Simple Story about Higher Efficiency  
By revising solar cells' rear side electrical contact structure and replacing the opaque backsheet with glass, a PV module can convert light striking from the backside to electricity. This type of new products are referred as bifacial modules. A term 'bifaciality' describes how good the rear side efficiency is, compared with the front side, and is expressed as the ratio between the two:  
\[\beta=\frac{\eta^{rear}}{\eta^{front}}. \tag{2}\]  
The output from a bifacial module is expressed as the yield from both sides:  
\[P_{out}=P_{in}^{front}\cdot\eta^{front}+P_{in}^{rear}\cdot\eta^{rear} \tag{3}\]  
\[P_{out}=\left(P_{in}^{front}\cdot\eta^{front}\right)\cdot\left(1+\frac{P_{ in}^{rear}\cdot\eta^{rear}}{P_{in}^{front}\cdot\eta^{front}}\right)=P_{out}^{front} \cdot(1+Gain). \tag{4}\]  
The bifacial yield gain compared with monofacial is the product of bifaciality and the ratio between input power from the rear side and that from the front side:  
\[Gain=\beta\cdot\frac{P_{in}^{rear}}{P_{in}^{front}}. \tag{5}\]  
The second term in the above equation is referred to as the bifacial ratio, which is affected by many factors, including the reflectivity of the ground (or albedo), the angular distribution of light, orientation of the module, the height of the module, shading of the surrounding objects, etc. While some demonstration projects prove the bifacial gain can be higher than 20% [18; 19], those achievements occurred with highly reflective ground and the projects are of small sales, so the gains are not considered representative. There is no standard bifacial ratio when measuring bifacial modules, although the National Renewable Energy Laboratory and Sandia National Laboratories suggested 130-140 W/m\({}^{2}\) as the representative irradiation for the rear side while maintaining 1000 W/m\({}^{2}\) for the front side [20]. Applying a 13.5% bifacial ratio and 75% bifaciality, a 10% bifacial gain is derived. However, in the real market, developers are only accepting a 5% bifacial gain assumption in price negotiation, considering there are still uncertainties about the new technologies without enough field operation data.  
Figure 11: PV LCOE reduction vs. module efficiency and module price, 2018–2025. Note: The module efficiency compared with the benchmark in a specific year. For instance, 5% relative higher module efficiency in 2018 points to a module efficiency at 17.7%*1.05, or 18.6%. 15% relative higher module efficiency in 2025 points to a module efficiency at (17.7%*1.025”7)*1.15, or 24.2%. In this paper, we focus on efficiencies that are likely to be adopted by the market in the near future. However, much greater efficiencies based on tandem solar cells have been realized in lab research [16; 17].  
Although the bifacial gain is caused by more input power, rather than higher conversion efficiency, the effect is the same. According to the left-hand chart in Figure 11, bifacial modules that can be considered 5% relatively more efficient can ask for a price premium of up to 8% in utility PV projects in 2018. However, extra costs will be introduced at the system level in order to avoid shading to the rear side. Figure 12 shows the necessary changes for racking in a fixed-tilt scenario.  
Figure 13 shows two trackers designed for bifacial modules. In both cases, the rails are positioned in the space between neighboring modules to avoid shading to the backside. In order to achieve the same mechanical stability, the metal consumption increases, resulting in an additional cost for trackers or fixed racking systems. Therefore, bifacial modules only receive a 3-4% price premium, with the power rating defined by the front-side yield. Even that value is sufficient to cover the extra cost. Therefore, we expect bifacial modules will see a rising market share, starting from 3 GW global shipment, or a 3% share of total module shipment in 2018. BloombergNEF expects the global share to rise to 10% in 2020 and 40% in 2025 [21].