# Power Prediction of Bifacial Si PV Module with Different Reflection Conditions on Rooftop
## 3 Proof of Theoretical Approach
### Test Results  
The output power (\(P_{exp.rear}\)) of the rear side was estimated through the modeling by using the actual measured output power on the test position and environment. The power output is obtained using the Profitest from Gossen in Figure 9. The measurement duration for separate measurement of individual modules of Profitest is higher than 20 ms (approximately 100 pairs of measured values), and thus the capacitive characteristics of the device under test have no influence on measurement. The software shows the visualization, evaluation and documentation of measured characteristic curves with database shown in Figure 10. Raw data obtained from the Profitest were used for the measurement analysis of bifacial module. In Experiments 1-4, the minimum errors between the in-situ output power and the predicted output are 1.58%, 1.5%, 0.8%, and 0.4%, respectively. The maximum errors are 4.45%, 4.45%, 6.7%, and 4.8%. In the modeling proposed in this study, the experimental error rate in the TEST BED was  
Figure 10: I–V test results. Figure 9: Measurement tools using IV-Checker (Gossen Metrawatt, Nuremberg, Germany). Figure 14 shows the result graph for Experiment 4. Experiment 4 shows that the influence of the solar irradiation on the day of experiment is significant, even though it considers that there is little reflection on the backside of the module. In addition, it can be concluded that the error rate of the experiment is small when the solar irradiation amount is high and _DNI_ amount is large. Using this experimental method, the modeling output power is compared with the output power obtained throughout the day in a normal soil environment. Figure 15 shows the solar irradiation measured at 5-min intervals from 10:15 to 15:00 on 2 September 2017. Figure 14: Experimental data of Exp. 4. Figure 13: Experimental data of Exp. 3. Figure 16 shows the error rate by comparing the modeling output with the measured data at every 5-min interval from 10:15 to 15:00 on 9 February 2017. From the results, when the solar radiation is high, the error rate between modeling data with experimental data is as low as less than 5%. The lower is the solar radiation amount, the larger is the error rate. The solar irradiation system used Profitesst from Gossen (Nuremberg, Germany) with a pyranometer (Gossen, Nuremberg and Germany) having calibrated monocrystalline irradiation sensor with integrated Pt1000 temperature sensor [28], as shown in Figure 15. The pyranometer is then installed to examine the amount of irradiation incident on the rear surface, as shown in Figure 17.  
Figure 16: The error rate by comparing the modeling output with the measured data. Figure 15: The solar irradiation measured at 5-min intervals from 10:15 to 15:00 on 9 February 2017. Figure 18 shows the irradiation data obtained from the measurements of the front pyranometer, the rear pyranometer and the modeling from 07:00 to 19:00 on 18 July 2018. Since the solar module follows the smallest output value, the amount of irradiation incident on each cell is required on the rear surface, but it is assumed that only one irradiation is representative in the experimental environment. In Figure 18, the output of the module takes a relatively larger ratio when the irradiation amount is low rather than high at the front. It can be assumed that there is a saturation value of the rear irradiation which can be received at the maximum. Figure 10.0 Pyranometer installed on the back of the modules. Figure 10.1 Irradiation data obtained from the measurements of the front pyranometer, the rear pyranometer and the modeling from 07:00 to 19:00 on 18 July 2018. Figure 19 is a graph showing the difference in the values of the irradiance measured through the rear pyranometer and predicted through the modeling. The solar radiation on the front is injected directly from the sun. Most of the incident irradiance into rear side at high irradiation condition, however, comes from the directly reflected irradiance resource around the environments.