10/27/2018. Introduction

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1 10/27/2018 Memorandum To: Mike Kozicki, CTO, Second Solar, Inc. From: Zach Hammond, Jerry Kyeremateng, Adly Khair Re: Study of bifacial modules for PV systems at potentially large deployment scales Introduction Currently, manufacturers in the solar industry are searching for solutions to increase module power and energy density to increase savings through high energy generation. Bifacial photovoltaic (PV) modules show promise in addressing this goal. This is mainly due to the unique capability of the module to allow light to be harvested from both, its front side and rear side. A portion of direct or diffuse light that gets reflected from the ground is scattered onto the rear of the module. This gives a higher energy yield than monofacial modules, provided that they have the same power rating under the same irradiance, temperature, and spectrum. In this memorandum, the economic and technological feasibility of deploying bifacial modules in large scale applications by our company, Second Solar, Inc. will be studied. This is done by discussing the module s science, technology, fabrication and manufacturing review, power and energy density, and SWOT (strength, weaknesses, opportunities and threats) analysis. Discussion Module science, technology, fabrication and manufacturing review Bifacial PV modules are fabricated from a dual solar cell that is encapsulated between two sheets of tempered glasses; one at the front of the PV module and one at the back of the PV module. The cells are held in place by Ethylene Vinyl Acetate, also known as EVA. Most bifacial PV modules are designed without frames, which improves aesthetics and reduces PV module cleaning. The comparison between the physical construction of bifacial modules and conventional monofacial modules can be seen below in Figure 1. Most bifacial PV modules use monocrystalline solar cells but have polycrystalline designs as well. Bifacial PV modules have contacts/busbars on both the front and back sides of their solar cells. Bifacial PV modules capture incident light from both sides of the PV module to generate electricity. Conventional crystalline silicon and thin-film monofacial PV cells typically use a fully metallized backside. This design needs a thick metal contact for reduced series resistance and is affordable to produce. In contrast, bifacial modules typically have lower amount of metal as the cells incorporate selective-area metallization schemes to allow light between the metallized areas [3]. This can be seen in Figure 1 under bifacial solar cell structure. Thus, the thin-film and silicon on the bifacial modules require tighter specs that are more expensive for manufacturing. The metallization scheme also raises series resistance concerns. In addition to that, different metals such as copper and nickel, and/or deposition

2 methods like plating or inkjet printing may be employed on bifacial cells. These employments require different equipment and add complexity to the manufacturing process. On the upside, the manufacturing cost can be reduced due to the typical frameless module design for bifacial modules. In addition to that, the module s manufacturing cost and complexity can be offset by the additional energy yield of cells. Figure 1: The composition of mono-facial and bifacial PV modules [4] Power and energy density of the modules In the case of bifacial modules, light is allowed on the front and rear side of the module. Given the same power rating under equivalent conditions, bifacial modules have a higher energy production and efficiency than a typical monofacial module. One example would be SolarWorld s bifacial cell with a recorded cell efficiency of 22.04%, which led to a new world record in 2016 [3]. For this study, we look more into bifacial modules that are landscape-mounted on flat roofs. The reason is because this mounting type does not require much row spacing between modules and hence, would allow more deployment of modules over a certain area. The additional boost of energy production for a bifacial module depends on two main factors, which are the installation height of the solar module and the albedo underneath the module. Figure 1 below shows the curve for added energy yield of a bifacial PV module for different installation heights and albedo percentages for a landscape-mounted module with 65% bifaciality, facing south at 30 degrees pitch and 2.5m row pitch. Note that bifaciality is the ratio of the max power output at the rear end and that of the front end. This information can also be seen in Table 1.

3 Figure 2: Added energy yield curve of a bifacial module for different installation heights and albedo percentages (source: SolarWorld 2016) Table 1: Added energy yield table of a bifacial module for different installation heights and albedo percentages (source: SolarWorld 2016) Thus, the findings show that the energy boost is higher when the installation height of bifacial module is increased. However, the actual installation height of the module is limited to wind loading factors. The recommended height of installation for bifacial modules is around one meter for ground-mounted systems and 0.3 meters for flat roof installations [3]. The additional energy yield is also higher when the albedo underneath the module is increased. In the case where the two factors, namely installation height and albedo is optimized, the energy boost can go up to 25% [3]. This added percentage can generate extra amount of electricity for the same rated power of system and thus, increase savings.

4 Strength, weaknesses, opportunities and threats (SWOT) of bifacial modules An analysis of strengths, weaknesses, opportunities, and threats (SWOT) was studied to better understand how bifacial modules fit into the current photovoltaics market. To start, bifacial modules have several strengths. They typically generate 5-25% greater power than their monofacial counterparts. Most bifacial modules have a glass-on-glass composition, providing increased durability and a lower permeability to water. Furthermore, this glass-on-glass design takes away the need for an aluminum frame altogether, lowering manufacturing and raw materials costs. With balance of system (BOS) costs becoming a larger portion of system costs, a higher efficiency from bifacial modules is becoming more valuable to developers who wish to minimize the area of their mounting systems [2]. Given that bifacial modules have just recently gained traction in the PV industry, they do not have a record of reliability in the field. This will require more accelerated lab testing and field observation. The increased complexity of these modules makes it difficult to provide modeling software for estimating expected production and ROI, causing some plant designers and owners to avoid bifacial systems. Finally, specialized mounting systems and manufacturing methods are required for bifacial modules, increasing up front system costs. There are many opportunities available for bifacial modules to grow their market share. Electricity rate structures in the United States are trending toward time-of-use schedules to better reflect the true cost of electricity generation, transmission, and distribution. Monofacial modules produce most of the energy between the hours of 9am and 3pm, during which there is little demand for most households. Bifacial modules, on the other hand, smooth out the production profile, allowing for more generation in the early morning and late afternoon. This allows solar owners to receive higher sale prices on excess electricity sent to the grid and reduces the need to purchase electricity from the grid during on-peak hours. As balance of system (BOS) costs continue to make up a higher percentage of overall system costs, developers are working to reduce the size of their mounting systems. By using bifacial modules, developers can produce higher power without increasing their mounting costs. As manufacturers decrease the costs associated with bifacial modules, some designers may start choosing bifacial modules as a substitute for high-efficiency monofacial modules. The current system for manufacturing PV modules has, up until now, hindered the growth of bifacial modules. Most manufacturers today rely on low operating margins due to oversupply and pressure from consumers to lower prices. Therefore, some companies just do not have the capital available to invest in the manufacturing processes required for producing bifacial modules. Moreover, it is difficult for manufacturers to advertise the benefits of bifacial modules due to performance rating restrictions. Currently, STC ratings only take into consideration power produced from the front side of the module. Without proper ratings and modeling software available, it is challenging to convince investors to choose bifacial modules over standard monofacial options [1].

5 Conclusion Despite the obstacles related to improper ratings and heightened manufacturing costs, bifacial modules have become increasing popular in the PV market due to 5-25% gains in power production. Bifacial modules alter the daily production curve by producing power at high angles of incidence (AOI), meaning that the back side of the module has production peaks during the early morning and late afternoon. This brings additional value to customers on time-of-use plans, providing a strong opportunity for bifacial modules in the worldwide market. Additionally, due to the module s additional energy yield, the overall power density for a utility-scale bifacial PV power plant would be much greater. Hence, this leads to a higher return on investment (ROI) for power plant owners. Appendices Figure 3: Fabrication of bifacial PV module [5] Figure 4: Landscape mounting for bifacial PV modules on flat roofs (source: SolarWorld 2016)

6 References [1] Bifacial Solar Photovoltaic Modules, Electric Power Research Institute, Accessed 20 Oct [2] Bifacial PV Systems. Solar Professional Magazine, ation&nopaging=1#.w8ambghkhpb. Accessed 24 Oct [3] Calculating The Additional Energy Yield of Bifacial Solar Modules, Solar World, Accessed 25 Oct [4] Guo, Siyu; Walsh, Timothy; Peters, Marius. Vertically Mounted Bifacial Photovoltaic Modules: A Global Analysis, Vol 61. Pp , Accessed 20 Oct [5] Double Sided Power Production, Discount Solar Supply. wer-systems-back.htm. Accessed 22 Oct. 2018