1.4MW Solar Plant in Toyama Directly Hit by Ferocious Winter Lightning
Annual power sale affected by snow accumulation
The "NiX Yatsuo Solar Power," a mega (large-scale) solar power plant with 1.416MW of output from solar panels and 1.26MW of output for grid connection, is in the mountains of Yatsuo Town in Toyama City, Toyama Prefecture, Japan. About four and a half years have passed since power sale was started Oct 1, 2014 (Fig. 1, See related article).
The mega solar power plant was constructed on a site where soil was collected to restore rice fields polluted by cadmium. The plant was developed by construction consultant NiX Co Ltd of Toyama City, using the former soil collection site owned by Toyama City. The power producer is NiX New Energy Co Ltd, which is a subsidiary of NiX and generates renewable energies.
NiX is engaged in the energy management business with a focus on renewable energy generation, setting the construction consulting business as its core.
3 negative factors including Hokuriku weather overcome
The company had to overcome three negative factors that lower the profitability in developing the mega solar power plant in Yatsuo Town. Two of the three factors were weather conditions unique to solar power plants in the Hokuriku region.
One of the factors is the lower intensity of solar radiation in the region, compared with other regions in Japan, due to the location of Hokuriku. The second factor is that the plant is in a snowy area.
The third factor is the configuration of the site, which consists of small plots of land extending from south to north (Fig. 2). The site is divided into the north block and the south block, and they are on both sides of a road. Moreover, many of the plots were not rectangular in shape. Because of this, it was difficult to arrange the solar panels effectively, and the total length of cables, as well as the total length of fences around the site, increases, resulting in a comparatively higher amount of equipment investment.
The land is leased for 25 yen per square meter. The total investment was approximately 390 million yen (approx US$3.6 million) and the power sale amount per year is estimated at about 44 million yen. The construction cost per equipment capacity was lowered to 275,000 yen/kW, which is equivalent to the construction cost of mega solar power plants on Japan's Pacific side based on the price at the time of construction, aiming to recover the invested amount in about 14 years.
PV inverters considered to be high capacity in those days, featuring 630kW of rated output and supporting 1,000V, were used. Because of the high capacity, the northern side (749.3kW) and the southern side (666.7kW) were respectively covered by one PV inverter. The number of panels per string (a unit of connected solar panels) was increased and the DC circuit from the panels to the PV inverter was designed to support 1,000V to reduce the number of combiner boxes and shorten the total length of cables. Power loss during power transmission was also minimized by this method.
Solar panels manufactured by Next Energy & Resources Co Ltd of Komagane City, Nagano Prefecture, were used. The plant has 4,800 polycrystalline silicon-type (295W/sheet) panels arranged on the site. PV inverters manufactured by Toshiba Mitsubishi Electric Industrial Systems Corp (TMEIC) were incorporated. The plant used PV inverters featuring 630kW of output and supporting 1,000V of DC input for the first time in the Hokuriku region.
The way it snows in Hokuriku
The annual power generation amount was higher than anticipated in the years after the start of operation, and the amount remains steady, according to the company. However, the power generation amount per year fluctuates by up to roughly 10% depending on the snow accumulation situation (Fig. 3).
Roughly speaking, the power generation amount tends to drop in January and February due to snow accumulation, so a moderate estimate is drawn up in the business plan. However, the amount exceeds the estimated amount when snow accumulation is low during those two months.
On the other hand, snow accumulation in the months from December 2014 to January 2015, the first winter after the start of operation, exceeded the estimation and the power generation amount in January was lower than the amount estimated in the business plan by 38%. Snow accumulated by about 30 cm per night for three to four consecutive days in ten days. Snow piled up on the arrays (units for fixing solar panels on mounting systems) and nearly reached the top of the solar panels.
The height of the bottom of the solar panels is set high at 1.5m, but there was a risk that the snow would reach the panels after sliding down the panels and accumulating. About 20 employees pulled down the snow and removed it in front of the arrays.
Possibly struck by 'winter lightning' unique to Hokuriku region
Damage rarely seen to solar panels occurred in the winter from December 2016 to January 2017, the year before the region was hit by heavy snow. Some cover glasses were cracked and significantly warped (Fig. 4).
During this season, the remote monitoring system frequently warned of ground faults in the DC circuits. This occurred once Dec 6, 2016, three times on January 5 and 6, 2017 and 12 times on January 12 of the same year.
When the warnings are transmitted, an electrical chief engineer visits the site and checks the situation. The severely warped solar panels were discovered by the licensed electrical engineer during a patrol. Based on the phenomenon that occurred later and weather conditions on these days, it is believed that the mega solar power plant was directly hit by "winter lightning" Dec 5 or 6 of 2016. The area suffered from a number of "winter lightning" strikes on those two days.
Winter lightning is a phenomenon unique to Japan Sea side regions and is generated in winter along cold fronts. The electrical energy of winter lightning is stronger than the electrical energy of summer lightning by more than 100 times.
Two scenarios are anticipated for the phenomenon that occurred at the mega solar power plant of Nix. One of them is winter lightning, which caused warping of solar panels and a temperature increase in the outer circumference of the warped solar panels, causing a vapor explosion and damage to the cover glass of the panels. The other is the electromagnetic force generated by winter lightning, which applied excessive stress in the outer direction of solar panel aluminum frames, causing the cover glass to break.
In both cases, breakdown was caused by the impact of the damage, causing warping of panels due to stress applied later by the weight of the snow.
Checked for aging by asking Hokuden Group to inspect by drone
Only three solar panels have been replaced so far due to cracking of cover glasses, including the damage caused by winter lightning. Panels are replaced as soon as cracking is discovered. The cost is covered by insurance.
The company also has a strong interest in age-related deterioration of solar panels. In August 2017, the company checked for abnormalities in the solar panels using a drone (unmanned compact aircraft) (Fig. 5).
The company checks the panels by measuring insulation resistance and I-V characteristics of each string via input terminals in the combiner boxes, in addition to visual exterior inspection. However, the company knew that panels with overheating of hot spots and deterioration in conversion efficiency could not be identified by these inspections. As a measure to solve the problem, the company entrusted an external company to inspect the panels with a drone.
The inspection detected temperature distribution abnormalities in 52 of the 4,800 solar panels. Heat distribution images of the 52 panels, in which temperature distribution abnormalities were discovered, were obtained by shooting the images with a hand-held infrared camera by walking around the site after the inspection using a drone (Fig. 6).
Images of the panels were shot from the back side of panels in the inspection performed on the ground. This was owing to the difficulty of obtaining appropriate data by shooting the panels from the front side, because panels at mega solar power plants with measures against snow accumulation are installed at a higher position and a higher angle. The height of the bottom of the panels at the mega solar power plant is 1.5 m and the installation angle is 30 degrees.
By shooting the images from the ground, significant temperature differences due to hot spots were detected in 21 of the 52 panels. However, the results of I-V characteristics measurement were not below the standard specified by the manufacturer, and, therefore, the panels did not fall into the replacement category covered by the manufacturer.