1. Introduction
Renewable energy sources (RES), in particular photovoltaic energy sources, have become increasingly popular. Ongoing research increases installations’ efficiency and develops new technologies. The energy yield can be improved with bifacial photovoltaic modules that use both direct radiation (incident on modules’ front side) and reflected radiation (incident on the module’s rear side) to generate electricity.
Figure 1a shows a bifacial module with two glass layers for absorption of sunlight on both sides of the module; radiation also reaches the rear side of the module. Radiation from around the module and passing the free spaces between the PV cells are reflected from the module’s surface.
Figure 1b shows a monofacial module whose rear layer is composed of an underlay impervious to solar radiation.
Due to additional absorption of the radiation incident on the rear side of the cell and the increased number of photocarriers generated, a bifacial module generates greater current and voltage than that of the monofacial module, making it possible to obtain more power. The additional energy yield depends on the amount of intense solar radiation reflected from the ground and reaching the module’s rear side. One of the most common bifacial cells structures is the
p+-
p-
n+ structure, based on an n-type semiconductor (shown in
Figure 2). Silicon oxide or nitride (SiO
2/SiNx) are used for front side and rear side passivation. The front side mesh is composed of silver or aluminum (Ag/Al), and the rear side mesh is composed of silver (Ag).
Experimental systems with low installed capacity achieve an efficiency range between 15% to 25% higher than standard modules in commercial solutions; however, the actual extra yield is in the 5–15% range [
3]. The systems’ operation depends on many factors, including the type of substrate below the module surface; the use of bifacial modules makes it possible to maximize the concentration of installed power. The use of bifacial modules offers another advantage—installations consisting of double-sided modules make it possible to obtain a greater energy yield from 1 m
2 of photovoltaic installation than single-sided modules [
4].
In most cases, bifacial modules are used in ground-mounted structures, on roofs-racks, or in BIPV installations (
Building Integrated Photovoltaics) systems for sunlight absorption on both sides of the module.
The International Technology Roadmap for Photovoltaic (ITRPV) predicts that the share of bifacial panels in the global photovoltaic market will increase to 60% by 2029 [
5]. The authors [
6] analyzed the two-sided panels market and predicted its growth from 3% in 2018 to as much as 40% in 2025; particular hope for greater use of bifacial panels is attached to BIPV and installations floating on water reservoirs [
7], mounted soundproof barriers [
8,
9], in photovoltaic farms [
10], and in solar-tracking systems [
11,
12]. Compared to single-sided modules, additional energy yield from two-sided modules is estimated to reach 30% or 40% in solar tracking [
13]. Moreover, the prices of double-sided panels are only slightly higher than those of single-sided panels. The use of a glass pane at the panel backing extends the panels’ service life as well as the warranty period offered by manufacturers.
The applicable IEC 60904 standard [
14] specifies the measuring conditions for single-sided photovoltaic modules; however, the existing standards do not specify the conditions for measuring the back irradiance intensity and measuring bifacial panels’ methods, which have gained increasing popularity on the photovoltaic market. There have been suggestions on measurement specification (such as the one presented in [
15]) for two-sided modules; however, the conditions set out in these documents are not yet precisely defined and do not offer general principles that ensure measurable results in all field conditions. Many researchers study bifacial panels, both in laboratory and field conditions. However, the research results are not consistent and usually cover a selected operation and applications area of panels that absorb light at the front and rear sides of the cell. For example, there are studies whose authors report as much as 50% additional yield from the rear side of bifacial panels compared to single-sided panels [
16]. Conversely, a 20% increase in power yield can be obtained with an optimal mounting height of the panel; this ensures that the direct solar radiation is reflected from the ground and reaches the rear side of the panel [
17]. Moreover, the change in the module mounting height and inclination angle, as well as panel-to-panel correlation, contribute to large variations in additional power yield in bifacial panels, as indicated in the following papers [
18,
19,
20]. The paper [
21] indicates the lack of precise tools for modeling energy yields from bifacial panels. However, the use of accurate radiation sensors with a breakdown of its individual components (direct, reflected, and diffused radiation) and accounting for the substrate surface’s albedo and the data related to the installation location in the available simulation models makes it possible to obtain the generated power values whose error is less than 1% for stationary systems and 2% for following systems. The use of double-sided panels introduces an additional approx. 0.5% uncertainty in the modeling process of photovoltaic installations.
The difference in power generation of south-facing bifacial panels compared to unilateral panels is about 21%, as reported in the studies conducted in the north of Alaska [
22]. Conversely, the vertical installation of double-sided panels in the east-west configuration resulted in the same annual energy generation as in the case of south-facing panels. However, the generation profiles were different, which may also be of great importance for energy consumption for current needs without the need for energy storage. A comparison of power generation curves in single-sided and double-sided panels in different assembly configurations is shown in
Figure 3.
The comparison of the generated power values in photovoltaic installations located in Poland: rated installed power of 5.04 kW (one-sided panels) and 6.1 kW (two-sided panels) installed on grassy ground, a 10% or 28% greater energy efficiency was obtained for double-sided panels in high and low insolation, respectively. The calculated value of CO
2 emission reduction with two-sided cells was 16% higher compared to unilateral cells [
24]. In the installations located in Korea [
25], the year-round additional energy yield from double-sided panels was 10.5% when installed over a concrete floor whose albedo was 21%. With an albedo of 79% (white membrane), the increased yield reached 33.3% compared to single-sided panels. The uniaxial tracker increased energy generation by 15.8% compared to a stationary structure with single-sided panels in the same research period. Energy yields modeling in solar-tracking systems must take into account additional yields from the generation in the rear side of bifacial panels [
26,
27]. However, the solar tracking system determines the optimal setting for the front side of the panel.
In [
28], its authors determined the 130–140 W/m
2 irradiance value reaching the rear part of the bifacial panel in the test conditions specified in IEC 60904, with an irradiance of 1000 W/m
2 at the front of the panel (the mounting height of the panel—1 m above the ground, and the albedo coefficient for light soil is 0.21). The value of the back irradiance was observed to depend on many factors, such as the light transmittance degree of the panel surface, the panel’s mounting height and angle, and the substrate’s reflectance. A new method used to characterize bifacial photovoltaic cells in STC is presented in [
29]; a cell’s front and rear are simultaneously illuminated in order to make it possible to perform electrical measurements of the front side (efficiency, open-circuit voltage, short-circuit current, and fill factor) and the short-circuit current of the rear side below the STC conditions. The authors indicate that the output current of the bifacial panel is the total of the currents generated in the front and rear side of the panel, and the irradiance reaching the panel rear side is always lower than that reaching the front side. The compensated-current method (the sum of the panel rear and front side parts’ currents) is also used by the authors of [
30] in order to model energy generation from bifacial panels. The [
29] authors introduced the two new parameters (i.e., the product of bifacial yield and additional yield) in order to describe the bifacial panels in greater detail. Bifacial monocrystalline panels placed on a supporting structure directly on the ground at an inclination of 30° were tested for four different values of the albedo coefficient (0.1, 0.2, 0.3, and 0.6). The measurement results were compared with the data obtained from the simulation of generated power, and the maximum error was 6.7% with an average error of 1.86% [
7]. These tests reports included the date of the test, as it impacted the direct and scattered irradiation components and thus irradiation reaching the rear side of the panel. In other works [
31,
32], the authors also indicate the energy yields dependence from two-sided cells on the region where these panels are installed and latitude affecting environmental conditions. However, greater energy yield from bifacial panels changes the charging and discharging cycles of energy storage facilities, which results in an alteration of the values related to the stored energy. This also affects the battery service life and self-discharge losses [
33].
Air purity, assembly orientation of panels, and the size of the surface reflecting solar radiation also affect the work of bifacial panels, which has been confirmed by the research results reported in [
34]. An increased concentration of PM10 dust in the air by 100 μg/m
3 resulted in the module’s efficiency dropping by 0.9%. Moreover, a double surface area of the substrate with the tested albedo increased the efficiency by 1.6%. Conversely, compared to the horizontal one, the vertical assembly of the panel has contributed to the shading of a bigger area under the panel and resulted in the short-circuit current value decrease by 14.3%.
The optical model used in [
35] and the possible, additional power yields from the bifacial panels were determined in panels that had the rear coatings of various materials (a white primer, glass, and glass with mesh) for internal reflections from the back substrate to the rear side of the PV cells. The analysis demonstrates that the rear side base type affects impacts of not only additional power yield but also the cells’ temperature. Therefore, a white mesh in the rear side, transparent base with a width distance equal to that between the PV cells, has proven to be the optimal solution. Moreover, the power yields obtained, resulting from the modified base, exceed the power losses associated with the additional cells’ heating.
Bearing the above in mind, the study’s authors conducted the examination for bifacial modules to determine the parameters used to estimate additional electricity yield, depending on the installation settings and environmental parameters, at the same time. The parameters which characterize bifacial modules have been determined. The analyses may be used to determine the most favorable operating conditions for such modules and, in particular, the module inclination angle with a specific substrate. The method for determination of irradiance value incident on the module has been presented; this is used to determine the module power generation without the information of the share generated by the rear part of the module. The convergence of the calculated parameters with those determined through experimental methods for various types of substrate and the module inclination angles were demonstrated.
The studies presented by the authors concern a special method of mounting bifacial panels directly on the ground, which has not been analyzed in the cited publications thus far. When installing the panels above the surface, the angle of inclination is not as important as it is when installing directly or low above the ground. The authors proved that the greater angle of inclination, the greater the surface reflecting solar radiation behind the module, and it constitutes an important element determining the value of additional energy yield.
This article has the following structure. The second section presents the test stand, measurement conditions, and analyzed parameters. The third part contains the research results related to the influence of the inclination angle and the substrate type on the modules’ parameters. The fourth part includes the discussion and analysis of research results. The article ends with a summary.
2. Materials and Methods
The research objective was to determine the current voltage and power characteristics of the LG 390N2T-A5 bifacial module of the 390 W nominal power, whose electrical rating is presented in
Table 1. In order to make it possible to compare the module operation in different weather conditions, the module temperature, ambient temperature, and insolation on the module surface were also measured. The measurement system (shown in
Figure 4) consisted of the devices indicated in
Table 2.
The module edge rested on the ground at each of the measurements. Measurement of solar irradiance intensity on the module surface was conducted each time before voltage and current measurements. The solar irradiance intensity was measured at 12 points and then averaged. These values might have changed during the measurement, as the tests were performed in natural outdoor conditions. In central Poland, the measurements were performed in the vicinity of Poznań (52°05′ N, 16°54′ E) in September and October 2020.
Figure 5 demonstrates the total energy of solar irradiance in particular locations in Poland. The average annual total energy in the indicated location was approx. 1080 kWh/m
2. The average monthly radiation energy was 103.7 kWh/m
2/month in September and in October 59.2 kWh/m
2/month. The average monthly temperature in September was 14.93 °C, in October—9.33 °C [
37]. The measurements were performed on days with different ambient temperatures and solar irradiance, which made it possible to obtain a spectrum of various atmospheric conditions, ranging from high cloudiness when the radiation density was approx. 60 W/m
2, to an insolation of up to 1100 W/m
2.
The research focused on determining the angle of inclination and the type of substrate (different albedo coefficients) that influences the bifacial photovoltaic modules’ operation. The tested angles were in the 15–90° range with 15° increments.
The substrates used for measurements were grass, white substrate, and foil, which well reflects solar irradiance. Grass’ reflectance is in the range between 0.15 and 0.26 [
39]. The white material imitates concrete or white sand, with the respective albedo of 0.2–0.4 and 0.6 [
39]. The foil used for tests was a thin sheet of metallized plastic, which has a high radiation reflectance, ranging from 0.8 to 0.9.
Figure 6 demonstrates the module inclinations (selected angles).
Figure 6a presents the test object on a grassy substrate, and
Figure 6b shows the system with the under-module surface covered with a white substrate;
Figure 6c shows a foil substrate.
When determining the bifacial module parameters, assume that the short-circuit current of the front and rear side of the module depends linearly on the irradiance reaching the given part of the module. Moreover, the total short circuit current of the bifacial module is assumed to be the total of the front and rear side currents. The selected values to compare the bifacial module work in different conditions:
- 1
Power generated by the module, P [
40]:
- 2.
The filling factor of photovoltaic module, FF determines the maximum value ratio achieved by a power module vs. the theoretical maximum power of a module [
40]:
- 3.
The module efficiency,η, i.e., the module’s generated power ratio vs. the energy which reaches the module [
40]:
- 4.
BGE index indicates the additional power generated by the rear side of the module related to that produced at the front side of the module [
1]:
- 5.
BGEIsc index determines the current efficiency ratio of the rear side of the module to the current efficiency of the front side [
1]:
- 6.
The total value of irradiance that reached the module, EE [
1,
28]:
- 7.
Short-circuit current of the bifacial module, Isc,bi [
1]:
- 8.
The open-circuit voltage of the bifacial module, Uoc,bi [
1]:
- 9.
Pseudo fill factor, pFF, which does not consider the losses generated by series resistance [
29]:
- 10.
The fill factor of the bifacial module, FFbi [
1]:
4. Discussion
The project’s measurements made it possible to test the bifacial photovoltaic module operation in various conditions. The factors which have been examined and found to influence the module’s performance are the module’s inclination angle in relation to the substrate and various substrate types under its module.
For grass-covered ground, the highest power was recorded for the module’s inclination angle 60° and 75° in relation to the ground. The lowest values were reported with the module’s inclination at 15° angle, the highest efficiency with the 90° angle. In the case of the white substrate, the maximum power was recorded with the 90° inclination angle; the module mounted at the 75, 60, and 45° angle in relation to the ground also proved to be a beneficial option. In the case of the foil substrate, the test was conducted under changing weather conditions; rapid radiation changes are reflected in photocurrent fluctuations. The results show that the 90, 75, and 60° angle in relation of the module to the ground were advantageous. These values were optimal for each of the tested substrates. The optimal inclination angle for a single-sided module mounted in Central Europe was within the 25–70° range, depending on the period; in the spring and summer period, 30 to 45° angle proved to be the most optimal [
8]. A greater inclination angle for double-sided modules indicates that the rear side cells absorb more solar radiation, owing to the radiation reflected from a larger substrate surface than that in the panel with a smaller inclination angle.
Subsequent measurements made it possible to compare the influence of the particular substrate on the module’s operation. The tests were conducted with the 75 and 60° inclination angles and with similar irradiance values. The best results were achieved with 75° inclination with the foil substrate. The white substrate and the grassy substrate had similar results. For the 60° inclination angle, the maximum power was achieved with the white substrate. The module assembled above the surface covered with a radiation-reflecting foil or a white-coated substrate had a positive effect on the generation of electricity in the module; this was caused by a high albedo coefficient of the substrates and the increased solar energy reaching the rear PV cells of the bifacial module.
The characteristics presented in the second part of this paper demonstrates the operation-related data of both the front and rear side of the module and only its rear side. This illustrates the additional energy yield from the rear side of the module. The recorded values of photocurrent were measured when the module’s front side reached up to 0.7 A. The maximum rear side power of the module is demonstrated in
Table 12 and
Table 17; the values are 18.3 W and 18.8 W, respectively. With a heavy cloud cover, when the irradiance value was below 100 W/m
2, the module’s rear side proportion of energy was more noticeable than intense incident radiation.
Table 14 compares the module’s front and rear side yields. The power at the maximum power point for the rear side was 25.6% of the total power generated by the whole module.
In order to determine the parameters characteristic for the bifacial panels, calculations were performed with the use of Equations (4)–(10). The results are presented in the tabularized form in
Table 18. The data demonstrates that the additional energy yield in the bifacial module depends on the inclination angle and the type of substrate; the greater the inclination angle and the higher albedo, the greater the
BGE increases. There is a wide variation in the results; they range from 2.4% to as much as 35.5%, which was caused by the total increase of short-circuit current of the module, from 5% to as much as 42%. This results from the fact that the rear side cells can generate the same current and voltage values as the front side cells; thus, the vertical orientation of the bifacial panels in the east-west direction will result in symmetrical power gains (with constant irradiance) maintained during the day.
The particularly important parameter can be used for simulation tests is the GE indicator, which informs about the value of irradiance incident on the front side of the module and which will generate the power equal to the power value when the bifacial module operates in certain conditions. Thus, it is not necessary to know the value of the irradiance reaching the rear of the module and the inclination angle and the substrate albedo. The determined cells’ fill factor FF also indicates the good quality of the cells.
In order to determine the unit cost of electricity generation from various photovoltaic installations, based on bifacial and traditional photovoltaic modules, analysis of the discounted unit cost of energy generation was conducted. Due to most of the financial outlays incurred today (mainly investment costs) and the effects observed in the future, in a longer time horizon, the conducted economic calculations should be standardized to a common time point using the discount account (discount rate
df). The UNIPEDE method proposed by the International Union of Producers and Distributors of Electricity consists of analyzing the valuation of current expenses and future income using the discount factor. The discounted unit cost of electricity generation for an installation whose construction period does not exceed one year can be calculated from the relationship [
41]:
where:
kjf—discounted unit cost of electricity generation (EUR/kWh),
KI0—investment expenditure (EUR),
KUt—maintenance and repair costs in a given year (EUR),
KP—cost of fuel used to generate a unit of electricity in a given year (EUR),
Et—the amount of electricity produced in a given year (kWh),
df—discount rate in the year of the investment (-).
In order to perform a comparative unit cost of electricity generation from a photovoltaic installation based on traditional PV panels and bifacial modules, the following costs (collected in
Table 19) and data were assumed:
annual energy yield from 1 kW of installed power at the level of 1000 kWh/year;
for installations with bifacial modules, the data presented in
Table 18 calculated
BGE indices, indicating increased energy yields,
a linear decrease in the installation capacity of 0.7% per year was assumed;
installation lifetime of 25 years;
KI0 investment outlays include photovoltaic modules, an inverter, accessories, and labor;
the KUt maintenance costs include the replacement of the inverter with a new one in the 13th year of the installation’s use, inspection, and cleaning of the installation;
KP fuel costs 0 EUR;
the “Accessories” item (
Table 19) includes the cabling, required protections, etc.;
discount rate df = 7%.
Installation based on bifacial panels is characterized by higher investment costs (as the price of bifacial modules is approx. 65% higher than the price of traditional PV panels). In certain cases, an inverter with a higher power should also be used due to the higher values of the generated power. For bifacial modules, a higher maintenance cost was also assumed, e.g., due to the need to wash the back side of the module (EUR 65.93 instead of EUR 43.96). The remaining elements of the installation, such as electrical installation equipment (protections, limiters, disconnectors, etc.) are comparable in both cases. To calculate the unit costs of electricity generation, the power of the installation was referred to 1 kW.
The results of the calculations of the discounted unit cost of electricity generation, in accordance with relationship Equation (11), for the analyzed types of photovoltaic installations are presented in
Table 20.
The obtained values of the discounted unit cost of electricity generation from photovoltaic installations using traditional PV panels are approx. 0.1055 EUR/kWh. The costs of generating energy in a photovoltaic installation using bifacial modules range from 0.1385–0.1033 EUR/kWh. As the ratio of additional energy obtained from the back side of bifacial modules increases, the costs of power generation decrease, and at the highest values of the ratios obtained they are slightly lower for traditional modules. The development of photovoltaic systems’ technology and the increasing efficiency and awareness of scientists, producers, and investors result in a decrease in the prices of installation components, which translates into lower energy production costs.
5. Conclusions
Research on new technologies and increasing the efficiency of the solutions used now make it possible to further develop renewable energy sources. RES use is important in improving the natural environment’s quality and making it possible to discontinue the use of non-renewable, exhaustible sources.
Electricity generation from photovoltaic modules has increased over time; in addition to standard single-sided modules, bifacial modules are also used.
This paper focused on the influence of selected parameters, i.e., the angle of inclination of the panel to the substrate and the type of this substrate, on the operation of bifacial photovoltaic modules. The comparison to classic, one-sided modules may be of particular interest.
The latter makes it possible to increase the energy yield with the same surface the cells are mounted on. Bifacial modules’ work depends on, inter alia, the inclination angle. The optimal inclination angle in relation to the ground of the bifacial module is greater than that of single-sided modules.
In the case of single-sided modules, for a given latitude, the key parameter is the module inclination angle in relation to the ground. In Poland, this angle is on average 37° to the south and is within the range of the most frequently used roof slope angles of single-family houses (20–45°). For bifacial modules, the optimal angle is larger (60–75°), which implies the use of these modules in installations mounted on flat roofs or on the ground. A consequence of this larger angle is also the possibility of placing more modules, and hence the installed power, per unit area.
Another conclusion from the conducted research is the answer to the question of whether a several percent increase in the generated power from a single module is sufficient from an economic viewpoint for the use of this type of module in a PV farm. In most cases, apart from the base composed of aluminum foil, the cost of producing 1 kWh of energy is higher for the bifacial module. A key component of this cost is the price of the module itself. However, in the case of high costs of obtaining land for a PV farm, bifacial modules may be a better solution.
The type of surface under the module is also important, as substrates with a higher albedo make it possible to generate higher powers in double-sided modules than when using substrates with a lower reflectance value. There is another argument favoring bifacial technology; the increasing popularity of “cool roof” systems covered with a light coating that reflects solar radiation well. The roof’s white top layer reflects radiation of greater power directed to the rear side of the module. A further increase in solar radiation sources to generate electricity in the energy mix on a global scale is expected. The use of solar-tracking systems as supporting structures for bifacial panels will result in a significant increase in yields compared to traditional solar panels.
Among the directions of further research on the use of bifacial modules, in our opinion, the analysis of the impact of the size of transparent gaps between individual cells on the efficiency of the entire panel deserves special attention.