Although the curtailment of solar energy in Japan remains at an appropriate level by international comparison, it is often considered serious because of the possibility that the amount of loss of energy due to curtailment may affect business operations. On the other hand, analysis of time-series data from more than 200 small-scale solar power plants compiled by the authors indicates that the amount of loss of energy due to improper maintenance is not negligible and has reached a level more serious than curtailment at some power plants. This report quantitatively evaluates the lost power generation from these two perspectives, proposing cumulative curtailment ratio as a novel indicator.

As a basis for the use of natural energy, we have elucidated the factors that cause supercooled water to start freezing from the outside of a cylindrical pipe to which an electric field is applied. The experiment focused on the effect of the wettability of the inner surface of the pipe on the ease with which supercooled water begins to freeze. The pipe with an oil and water repellent coating on the inside was used. The electric field was generated by applying a high voltage of 25-35 kV from the outside of the pipe using a pair of thin metal wire electrodes. Specifically, the effect of the coating on the onset of freezing of supercooled water was investigated by observing the three-phase boundary under the electric field. Comparing the displacement of the three-phase boundary formed at each inner surface and the oil-water interface in the PVC-C pipe in this study and the PVC pipe in the previous study, there was no significant difference in the displacement between the two. On the other hand, the onset of freezing occurred in a shorter time than in the previous study, which was observed in the first large displacement of the three-phase boundary after the application of the electric field. In addition, freezing relative degree 1 was obtained in less than half the time in the present study when compared to the results of previous study with the same supercooling and voltage application conditions.