A revisit to solar radiation estimations using sunshine duration: analysis of impact of these estimations on energy yield of a PV generating system

Abstract

ABSTRACT The literature review reveals that though numerous models are available for estimating solar radiation; however, impact of these estimations on the energy yield of PV system and its economics has not been reported. With this background, present study is undertaken with the objective of evaluating the impact of estimated values of solar radiation (estimated using Sunshine duration method) on the energy yield and economics of a PV generator system. Models have been discussed for estimating/calculating: hourly solar irradiation received on a horizontal surface using sunshine hours; effective values of hourly solar irradiance; hourly energy yield of a PV generator; statistical parameters; payback period and levelized cost of energy (LCOE). Weather data of Trivandrum, India, has been considered for the present study. It has been observed that the values of MBE for hourly solar radiation estimations and the corresponding energy estimations, on yearly basis for the year 1998 are 8.75 and 8.05%, respectively; whereas, the corresponding values for the year 2004 are 12.29 and 10.95%, respectively. The values of RMSE for hourly solar radiation estimations and the corresponding energy estimations, on yearly basis for the year 1998 are 32.10 and 19.17%, respectively; whereas, the corresponding values for the year 2004 are 31.60 and 18.81%, respectively. The values of R2 for hourly solar radiation estimations and corresponding energy estimations, on yearly basis for the year 1998 are 0.8265 and 0.9351, respectively; whereas, the corresponding values for the year 2004 are 0.8471 and 0.9539, respectively. The values of t score for hourly solar radiation estimations and corresponding energy estimations on monthly basis, for the year 1998 are 0.9396 and 1.53, respectively; whereas, the corresponding values for the year 2004 are 1.4 and 2.37, respectively, which are lower than the critical t value of 2.718. The values of MBE and RSME indicate that there is good fitting between estimations and measurements. The values of R2 indicate that ratio of explained variation to the total variation is nearer to 1, indicating a lower unexplained variation. The t-scores for estimations are less than the critical t value, indicating that estimations are statistically significant. Thus, for all the cases considered, statistical parameters obtained for solar radiation and energy estimations broadly satisfy all the criteria, viz., MBE, RMSE, R2 and t-score, which are normally adopted to judge the accuracy of estimations. Further, the statistical parameters obtained for hourly energy estimations are superior to corresponding values for solar radiation estimations. The LCOE and payback period found from the estimated values of solar radiation are 7.5% lower than those obtained from the measured values of solar radiation for the year 1998; whereas, they are lower by 9.9% for the year 2004. Thus, a PV system designed on the basis of estimated values of solar radiation would be around 7-10% undersized. Consequently, it can be concluded that at locations where hourly solar radiation data are not available, solar radiation estimations using the method of sunshine hours can be effectively utilized for energy estimations and feasibility analysis of a PV generating system.