1. Introduction
In the context of increasingly global energy demands and the vital importance of sustainable renewable energy sources, photovoltaic (PV) technology continues to play a pivotal role in meeting these challenges. Bifacial photovoltaic (bPV) technology has emerged as a promising solution, capable of converting sunlight from both the front and rear sides, potentially enhancing energy conversion efficiency compared to traditional PV modules. Two facts illustrate the relevance of bPV technology: the development of a specific IEC Standard to characterize bifacial devices (IEC 60904-1-2:2019 [
1]) and the fact that Task 13 of the International Energy Agency (IEA) Photovoltaic Power Systems Programme (IEA PVPS) published a detailed report on bPV in recent years [
2].
Production gains in the range of 6–10% under normal conditions have been reported [
3,
4,
5], but considerable increments can be obtained with a proper design (+15–30%) and can be as high as 40% with optimal design parameters [
6]. In order to take advantage of this potentiality, a thorough understanding of bPV technology is crucial, especially as a significant increment in worldwide bPV installation is expected. By 2030, the installation rate increased to 30%, while it was about 12% in 2020 [
2].
Bifacial technology presents a promising avenue for sustainable land use and renewable energy generation. This innovative approach combines the benefits of both systems, leveraging the dual-sided energy capture capability of bPV modules and the land productivity enhancements offered by agrivoltaics. By deploying bifacial PV panels above agricultural fields, agrivoltaic systems can harness sunlight from both the front and rear sides of the panels while simultaneously providing shade to crops or livestock below. This dual-use configuration increases the energy generation potential while optimizing land use efficiency [
7]. Additionally, the presence of vegetation beneath the solar panels can enhance the performance of bifacial modules by increasing the amount of reflected and diffused sunlight reaching the rear side of the panels [
8]. On the other hand, from an urban perspective, bifacial technology has a high potential of being integrated into energy-sustainable buildings [
9]. Bifacial modules can be used as transparent or opaque façades, skylights, sunshades, and curtains or integrated into rooftops [
9]. However, despite its architectural features, the energy production contribution of the bifacial modules requires a thorough analysis, paying special attention to the particularities of each installation.
Different parameters have been detected as crucial contributors to bPV performance. The main contributor is ground albedo (the percentage of incident sunlight that a surface reflects), as it proportionally increases the incident rear-side irradiance and, as a consequence, the rear power contribution. Daily albedo experiments [
10,
11] and seasonal variations [
11] can be as high as 0.9 (fresh snow) [
12,
13] or as low as 5% (gravel) [
11]. Another influential design parameter is the panel ground clearance height (GCH), which is the closest distance between the module and the ground. It has been found that its positive effect tends to saturate in the range of 0.4–1 m, depending on the location [
14,
15].
Module orientation (tilt and azimuth) can also have a positive effect, but the optimum is highly dependent on each specific site and may not coincide with the optimum for conventional modules [
6,
16]. Additionally, it has been highlighted that bifacial contribution increases when the diffuse to global radiation ratio is high [
10], i.e., in overcast conditions.
As a counterpart, the bifacial contribution is reduced by the non-uniformity of the rear-side irradiance. Since the performance of any PV cell string is heavily influenced by the worst cell in the string, the increase in said non-uniformity, at the parity of the average component, would reduce the performance of the worst cell, diminishing the overall operation. Standard deviations of 30% of backlighting may occur with low mounted modules [
14]. However, the effect can be reduced (ranging 2.8–4.1%) by increasing GCH [
14], which links the positive effect of increasing GCH to its intrinsic reduction in back non-uniformity.
In particular scenarios, not all the optimal system parameters can be achieved. This is the case of industrial-like rooftops, where roof structural resistance limits module height, as it would increase both dynamic and static loads (wind-pressure loads and PV weight structure, respectively), intrinsically augmenting the rear non-uniformity. This can produce a considerable reduction in bifacial contribution due to the self-shading factor being considerably high (12.3–20%), even in specially designed structures [
17]. The supporting elements (frames, beams, purlins, and piles) can obstruct backside irradiance about 20% in fixed tilted installations [
18].
As a possible countermeasure, cold roofs with high albedo have been proposed [
19]. As high albedo implies less radian energy translated into a local increment of ambient temperature, bPV modules will operate at lower local temperatures and higher back illumination. An albedo increment of 0.1 would imply an addition of 4.5% of energy production [
19]. A recent research has tested a nanomaterial with extremely high albedo and thermal emittance (96% and 80%, respectively), which would imply an additional cooling of 20 W/m
2 (compared to sand) and an additional cell photocurrent of up to 6.42 mA/cm
2 [
20].
Nevertheless, a notable increase in albedo could not necessarily imply a significant increase in production since non-uniform illumination induced by mounting structure can considerably reduce bifacial contribution [
21]. From a potential bifacial power gain (BPG) of up to 23% at high albedo, it could be reduced to 5% just for the unevenness introduced by support structures [
21]. Additionally, wide dispersion on measured backside irradiance has been observed to depend on both the sensor [
22] and module position (within an array of modules) [
23,
24]. The reported non-uniformity, as defined by IEC 60904-1-2 [
1], can be as high as 35% in outdoor conditions [
24]. Even though the presence of non-uniform back illumination should rarely produce hotspot issues in bPV modules [
22], there is at least one reported case of a considerable increase in local temperatures (+30 °C) caused by the torque tube of an horizontal single-axis tracker (HSAT) [
25]. This last fact highlights the importance of paying special attention to the role of mounting structures when dealing with bPV installations.
In order to investigate bifacial performance in the complex scenario of industrial-like rooftops, different tests were carried out. They include a comparison of a bifacial module with two different conventional PV devices (oriented at the optimal monofacial tilt, 30°) and a comparison of two bPV modules with slightly different backlight obstructions.
Accurate simulation tools for bPV modules, which can evaluate their power generation potential under different environmental conditions, are essential for predicting the energy yield of bPV-based solar generators and optimizing their performance [
26]. In terms of the electrical model, it is common practice to adapt conventional models for bPV applications. One approach is to incorporate a current source into PV-equivalent circuits to account for photogenerated currents from backside irradiance, resulting in four-, five-, or six-parameter models [
27]. Another proposal is to use two conventional models in parallel, one for each surface [
28,
29]. To fully represent the multiphysics behavior of bPV modules, it is necessary to integrate the electrical model with thermal and optical considerations [
30]. Experimental data, like those presented in this study, are critical to evaluate and validate these models or develop new ones.
The sections of this study are as follows:
Section 2 describes the experimental setup, the tests, and the methods that allowed us to obtain the results.
Section 3 presents and discusses the performance comparison of a bPV module against two different conventional devices and against itself (with an estimated monofacial performance) and, finally, the influence of the mounting structure on the power performance. The last section presents the conclusions of this article.
4. Conclusions
This study investigated the performance of bPV modules in a temperate-climate setting (Cfa, Köppen classification) and an industrial rooftop-like test environment characterized by low ground clearance height (GCH) and limited tilt angles. We elucidated that non-uniform rear-side illumination, stemming from both ambient conditions (e.g., low diffuse to global radiation ratio) and site-specific factors (e.g., mounting structure characteristics), can substantially diminish bifacial performance in these non-ideal operational conditions. Despite the challenges posed by non-uniform rear-side illumination, bPV modules demonstrated considerable advantages over their conventional PV counterparts. Comparative analyses against two different PV modules revealed a consistent power increase of 4.3–7.8% with bPV glass/glass frameless technology. Similarly, a self-comparison of bPV modules in the monofacial configuration, where the rear side was fully obstructed, indicated a bifacial additional power estimate of approximately 2% under clear-sky conditions, which could escalate to 15% in overcast scenarios characterized by a high-diffuse-to-global-radiation ratio.
This study delved into the influence of the mounting structure on bPV module performance across various tilt angles (0°–45°). It was demonstrated that even minor discrepancies in module alignment with structural beams can effectively negate any bifacial contribution. The absence of structural obstruction on clear days yielded an estimated additional power generation of up to 3.43% (+8.8 W), while less pronounced non-uniformity induced by the mounting structure still contributed to power enhancements of 1.3% (+4.13 W).
In essence, these findings underscore the complex interplay between environmental, site-specific, and technological factors in determining the performance of bifacial PV modules. Despite the challenges posed by non-uniform rear-side illumination and mounting structure influence, bPV technology offers performance advantages over conventional PV modules even in not optimized installations. Further research and technological advancements are warranted to address these challenges and fully unlock the potential of bifacial PV technology in sustainable energy generation applications.
Possible measures to reduce the production losses due to the effect of non-uniform rear-side illumination on industrial-like rooftops could include the use of half-cut cell modules and the proper separation of unavoidable blocking structural components, as well as the careful alignment of structural beams and rear cells. Their combination and alternative solutions still need to be investigated and optimized.