Climate change and global warming effect is currently one of the main threats that humanity is facing. The significant increase of greenhouse gas emissions since industrialisation has contributed to the global warming and therefore, the use of sustainable energy sources with zero greenhouse gas emissions has emerged as an urgent priority. Photovoltaic modules have become one of the world’s leading methods of generating electricity using solar energy. The share of solar PV electricity generation has significantly increased in the last decade and is expected to keep rising in the following years. However, the land area cost required for the development of photovoltaic solar systems is very high and it is therefore of foremost importance to increase the energy yield per unit area of PV modules. This is achieved using bifacial PV modules which make use of the irradiance incident on both sides of
the module leading to higher energy generation. This, combined with the prediction that bifacial modules will dominate the market in less than 6 years, a parameter used for the evaluation of PV modules performance in different climates needs to be defined namely ”energy rating”.
Energy ratings have been developed only for monofacial modules as described in IEC61853 standard and this work focuses on the extension of this standard to bifacial modules. For the calculation of the energy rating the energy yield and the irradiance incidence on both faces of the module have to be obtained. For this reason, an irradiance model is developed in MATLAB to calculate the irradiance on the rear side of the module using the data given in the standard IEC61853. Note that the front side irradiance is available in the existing data and as a result no further calculations for the front side of the module are required. The irradiance on the rear side of the module is obtained using 2D view factors taking into consideration the shading of the ground which can substantially affect the results. Also, the effect of the elevation of the module from the ground surface is examined and found that at sufficient height (≥ 1푚) the impact of ground clearance on the rear side irradiance is minimised. Finally, the ground reflected irradiance is obtained using the spectral reflectivity of different ground materials where it is shown that the use of improper albedo values can lead to inaccurate results.
The next step is the calculation of the energy yield of the bifacial module. For this, a similar methodology used to obtain the energy yield of monofacial modules in the standard IEC61853 is used. More specifically, the incident irradiance on the rear side of the module is corrected for the angle of incidence and spectrum effects to generate similar conditions as in the STC. Additionally, the operating module temperature is obtained and using the total irradiance on the front and the rear side of the module the energy yield is calculated. Finally, the energy rating of bifacial modules is determined using two different approaches. In the first approach, CSER of bifacial modules is obtained using the energy yield and the total irradiance on bifacial modules. This approach results in lower energy rating than unity but needs re-scaling of the monofacial module energy rating compared to the standard. The second approach use the energy generated from both sides of the module but only the irradiation on the front side of the module is taken into account leading to CSER values higher than one, while the energy rating of monofacial modules as obtained in the standard is used. In both approaches the energy rating of bifacial modules found to be up to 17% higher than that of monofacial depending on the bifaciality of the module and the climate conditions.
The developed model is validated using real outdoor measurements for a system located in Weurt, Eastern Netherlands. First, the in-plane irradiance on both sides of the module is obtained from the model and then, the simulated energy yield is calculated and compared to the measured energy yield of bifacial modules. The results show a small difference between the simulated and measured energy yield for the time period between October 2019 and June 2020 with a total variation of 4.65% in the total energy yield.