Optimization and Performance of Bifacial Solar Modules: A Global Perspective

Abstract — With the rapidly growing interest in bifacial photovoltaics (PV), a worldwide map of their potential performance can help assess and accelerate the global deployment of this emerging technology. However, the existing literature only highlights optimized bifacial PV for a few geographic locations or develops worldwide performance maps for very specific configurations, such as the vertical installation. It is still difficult to translate these location- and configuration- specific conclusions to a general optimized performance of this technology. In this paper, we present a global study and optimization of bifacial solar modules using a rigorous and comprehensive modeling framework. Our results demonstrate that with a low albedo of 0.25, the bifacial gain of ground- mounted bifacial modules is less than 10% worldwide. However, increasing the albedo to 0.5 and elevating modules 1 m above the ground can boost the bifacial gain to 30%. Moreover, we derive a set of empirical design rules, which optimize bifacial solar modules across the world, and provide the groundwork for rapid assessment of the location-specific performance. We find that ground-mounted, vertical, east-west-facing bifacial modules will outperform their south-north-facing, optimally tilted counterparts by up to 15% below the latitude of 30o, for an albedo of 0.5. The relative energy output is reversed of this in latitudes above 30o. A detailed and systematic comparison with data from Asia, Africa, Europe, and North America validates the model presented in this paper. An online simulation tool (https://nanohub.org/tools/pub ) based on the model developed in this paper is also available for a user to predict and optimize bifacial modules in any arbitrary location across the globe. I. INTRODUCTION Solar photovoltaics (PV) has become one of the fastest growing renewable energy sources in the world as its cost has dropped dramatically in recent decades The present levelized cost of electricity (LCOE) of large-scale PV is already lower than that of fossil fuel in some cases [1]. New technological innovations will lower LCOE further. In this context, bifacial solar modules appear particularly compelling [2], [3]. In contrast to its monofacial counterpart, a bifacial solar module collects light from both the front and rear sides, allowing it to better use diffuse and albedo light, see Fig. 1(a). For example,
Cuevas et al. [4] have demonstrated a bifacial gain up to 50% relative to identically oriented and tilted monofacial modules. Here, bifacial gain is defined as
Bifacial Gain = (𝑌𝐵𝑖−𝑌𝑀𝑜𝑛𝑜)/𝑌𝑀𝑜𝑛𝑜, (1) where 𝑌𝐵𝑖 and 𝑌𝑀𝑜𝑛𝑜 are the electricity yields in kWh for bifacial and monofacial solar modules, respectively. Moreover, the glass-to-glass structure of bifacial modules improves the long-term durability compared to the traditional glass-to-backsheet monofacial modules. Also, many existing materialsthin-film PV technologies (e.g., dye-sensitized [5], CdTe[6], CIGS [7]) are readily convertible into bifacial solar modules. Due to the high efficiency and manufacturing compatibility into the bifacial configuration, silicon technologies, e.g., Si heterojunction cells, have received most attention [3]. This process compatibility, extra energy produced by the rear-side collection, reduced temperature coefficient, and longer module lifetime can potentially the

Fig. 1 (a) A schematic of a bifacial solar module with absorption of direct (Dir), diffuse (Diff), and ground-reflected albedo light (Alb). Equations used to calculate these irradiance components are labeled here. E and H denote the elevation and height (set to be 1 m in paper) of the solar module, respectively. (b) The three parameters discussed in this paper to optimize Alb (eqns. 9 - 12) Dir (eqn. 4) E Diff (eqns. 5 - 8) Elevation (E) Azimuth Angle ( ) Tilt Angle ( ) (b) (a) S N W E S N W E S N W E 2

installation cost as well as the LCOE significantly [8], [9]. This overall economic advantage persists despite that manufacturing bifacial solar modules can be more expensive than monofacial ones due to additional materials (e.g., dual glasses) and processes (e.g., screen-printing rear contacts). Based on these considerations, the International Technology Roadmap for Photovoltaic (ITRPV) anticipates the global market share of bifacial technology to expand from less than 5% in 2016 to 30% in 2027 [10]. The 50% bifacial gain for idealized standalone modules predicted by Cuevas et al. [4], however, is not always achievable in pr

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