79MW Solar Plant Achieves Utilization Rate of 17.6% in Tohoku
2x areal efficiency, 1.6x overloading by setting up more than 1MW per 1ha
'Watari Lion Dance' performed to celebrate completion
Located about 26km to the south of Sendai and facing the Pacific Ocean where the Kuroshio Current flows, Wataricho in Miyagi Prefecture, Japan, is warm in winter and cool in summer. "Watari" is said to come from "passing over (wataru)" a river because the town is situated on the south bank of the Abukuma River. The town boasts fruit farming reflecting the mild climate and produces the most strawberries, in particular, in the Tohoku region.
On April 25, 2019, the completion ceremony of the "Watari Solar Power Plant," a mega- (large-scale) solar power plant with an output of roughly 80MW, took place in the town. A total of about 300,000 solar panels were set up across the approximately 75ha site on the coast, which was struck by the tsunami following the Great East Japan Earthquake (Fig. 1).
The solar panel capacity and the rated capacity of PV inverters total 79.548MW and 49.3MW, respectively, and they are the largest class among the mega-solar plants that are under construction or in operation along the tsunami-affected coast. Yamasa KK (Niimi City, Okayama Prefecture), a company engaged in developing and marketing amusement machines and airplane/vessel leasing, constructed the power plant and runs the power selling business using the feed-in tariff (FIT) scheme.
Along with guests including Hironobu Yamada, town mayor of Wataricho, and Shinya Endo, vice governor of Miyagi Prefecture, about 70 people related to Yamasa, the contractor and the town government participated in the ceremony. The town's traditional "Watari Lion Dance," which is reminiscent of the days of the town as the home of the Watari Date family, was performed (Fig. 2).
'Integrates accumulated design knowhow'
"It has already been eight years since the great earthquake," Yamada said emotionally. "This mega-solar project is a pioneer of the newborn Wataricho approach we are pushing forward with across the town. As I see the spectacular view of the panels set up across the site, I feel as if I can see the future of the town."
"We will strive to operate the plant stably every day for more than 20 years from now so we can contribute to local recovery and development through our mega-solar business," said Makiko Sano, vice president of Yamasa.
The project cost of the Watari Solar Power Plant amounts to about 20 billion yen (approx US$186 million). Yamasa started construction in June 2017 and began operation on March 2, 2019. Yurtec Co Inc provided engineering, procurement and construction (EPC) services.
Kyocera Corp's polycrystalline silicon-type (280W/unit) and Solar Frontier KK's CIS compound-type (175W/unit) solar panels, 1,667kW and 1,000kW PV inverters of Toshiba Mitsubishi-Electric Industrial Systems Corp (TMEIC) and SMA Solar Technology AG of Germany, as well as foundations and mounting systems manufactured by Schletter Japan KK were adopted (Fig. 3 & 4).
Yamasa entered the mega-solar development market in 2013. Although Yamasa is not a pioneer among those companies that embarked on solar power generation in the wake of the feed-in tariff (FIT) scheme, it has become Japan's largest class solar power developer, being engaged in projects with a total grid capacity of about 500MW or a total panel capacity of about 700MW including those being planned or constructed.
The Watari Solar Power Plant is the largest-class project among Yamasa's solar plants in operation. Furthermore, the plant has incorporated efforts to boost business performance as Shinichi Sano, president of Yamasa, describes, "We have integrated in this project the mega-solar design knowhow that we have accumulated thus far."
Areal efficiency doubled from normal level
In terms of mega-solar plants in Japan, it is generally said "nearly 2ha is required to set up solar panels equivalent to 1MW." For example, at the "Minamisoma Mano Migita Ebi Solar Power Plant," which also began operation in a tsunami-affected area just like the Watari Solar Power Plant, panels equivalent to about 60MW were set up on the approximately 100ha site.
At the Watari Solar Power Plant, on the other hand, solar panels equivalent to about 80MW were set up on the approximately 75ha site. The areal unit per 1MW of solar panels is less than 1ha, resulting in about double the areal efficiency per panel capacity at other solar plants.
Yamasa realized this through such efforts as solar panel array (unit of panel installation) structure and installation angle as well as the space between arrays.
Basically, at this plant, large-area arrays consisting of 12 sideways panels in 8 rows were tilted by 10° and mounted on pile foundations with the lowest part of each array's front side set up at 1.3m from the ground. In addition, the space from the array in front was shortened to about 1.4m (Fig. 5).
At most solar power plants in Japan, each array consisting of sideways panels in four rows is tilted by 10 to 20° and set up at 1 to 1.5m from the ground. The arrays are usually spaced 2 to 3m from each other.
Space to stop arrays shading behind on spring and autumnal equinox
If panels are set up in such a conventional style, arrays will not shade other panels between 9am and 3pm, when the most power is generated, even on the winter solstice (when the sun's culmination altitude is the lowest during the year). In other words, arrays are spaced so they will not shade the panels between 9am and 3pm on the winter solstice at most solar power plants. This is because, in this way, no panels are shaded from 9am to 3pm, when the most power is generated, throughout the year.
If the conditions to "keep arrays from shading panels between 9am and 3pm on the winter solstice" is applied to the Watari Solar Power Plant, where each array consists of panels in eight rows, the space between the arrays will reportedly be as long as "about 3.8m." This is because the height of an eight-row array, whose width (depth) is about 8m, reaches 2.7m on the highest rear side even if it is only tilted by 10°.
However, the space between the arrays is "1.4m," less than half of the Watari Solar Power Plant. In fact, the space between the arrays at this power plant corresponds to "keeping the arrays from shading panels from 9am to 3pm on the spring and autumnal equinox." As the sun's culmination altitude on the spring and autumnal equinox is in the middle of that on the midsummer and winter solstices, some panels are shaded half a year from the autumnal equinox day to the winter solstice through the spring equinox (Fig. 6).
Yet Yamasa adopted such a design because it analyzed, as a result of computer simulations, the amount of annual power generation rises by boosting the areal efficiency to increase the number of solar panels despite the loss from the panels shaded for half a year.
The company estimates the annual power generation will total 75,880,000kWh and the utilization factor of the PV inverters and the grid facilities will reach 17.57%. Compared with the utilization factor of 15 to 16% estimated for other new mega-solar plants in Japan, this plant outperforms the national average by several points despite being located in the Tohoku region.
Compared with the adjacent mega-solar plant with a conventional design, it is so evident how narrow the space is between the arrays is and how high the areal efficiency is at the Watari Solar Power Plant. In the drone photo shot from above, no ground can be seen as the panels appear to be connected with each other at the Watari Solar Power Plant, whereas the ground between the arrays can be seen at the conventionally designed mega-solar plant (Fig.7).
'Failures fixed collectively'
That said, for panel replacement and other maintenance work in high positions, it is a challenge as the eight-row arrays as high as 2.7m require a lot of work and cost.
"Replacing defective panels and doing other maintenance work at the mega-solar plant would not increase cost so much if we collectively fix them no more than once or twice a year," said Sano, commenting on this point (Fig. 8).
The Watari Solar Power Plant tries to find failures at an early stage through remote control and data collection, adopting a string monitoring system of Sumitomo Electric Industries Ltd, for example.
"Although it is very important to swiftly understand where the failures are, cost efficiency is not good if we repair each failure every time we find one," Sano said. "Mega-solar operation requires a concept of running it without making too much effort even if there are some small failures and efficiently fixing multiple failures collectively."
Overloaded by '1.6x'
Adopting "eight-row arrays, installation angle of 10° and 1.4m space between the arrays" based on such an operational method, the Watari Solar Power Plant could set up panels equivalent to about 80MW on an approximately 75ha site. As a result, the overloading ratio rose to 1.61 times in comparison with the grid capacity of 49.3MW. This is an unusually high ratio in Japan where the ratio is 1.2 to 1.3 times at many mega-solar plants (Fig. 9).
The effect of "overload" to set up more solar panels than a grid capacity is generally said to "boost business performance by increasing power generation in the morning and evening even though power generation around noon on a sunny day is slightly reduced." The limited grid capacity can be effectively used as the graph of alternating power generation (power sales) per day is a "trapezium," and the utilization factors of PV inverters and grid facilities improve.
However, "If the overloading ratio were about 1.2 to 1.3 times, the power generation of solar panels would scarcely exceed grid capacity even at noon on a sunny day," Sano said, based on his experience in mega-solar operation thus far. "To boost utilization factors through peak cut effects, setting up more panels and boosting the overloading ratio to 1.3 times or more is necessary."