Solar Plant on Former Eel Farm Avoids Floods But Suffers From Weeds

Weeding implemented in line with local events, vehicle-type weed cutter utilized

2019/08/16 10:59
Shinichi Kato, Nikkei BP Intelligence Group, CleanTech Labo

The "Tsu Kurimamachiya Mega Solar Power Plant," a mega- (large-scale) solar power plant with a solar panel capacity of 4.5408MW and a PV inverter capacity of 3.58MW, is located along a river near Ise Bay in Tsu City, Mie Prefecture, Japan (Fig. 1). It began operation in Kurimamachiyacho, Tsu City, Oct 7, 2015.

Fig. 1: Tsu Kurimamachiya Mega Solar Power Plant. Dragonflies mating above panels mistaking them for water, something often seen at mega-solar plants near ponds or rivers (middle photo). Rain at the site during interview. Panel surfaces immediately washed clean by rain (bottom photo). (top Sanko Real Estate, others Nikkei BP)

The power producer is Sanko Real Estate Co Ltd of Mie Kotsu Group Holdings Inc (Tsu City). The company has developed and is running a number of solar power plants with a total capacity of about 89MW at 24 locations exclusively in its home prefecture of Mie.

Sanko Real Estate has hired the same companies for construction and facilities at almost all of its solar power plants, deepening its partnerships with those companies. Chiyoda Corp has provided engineering, procurement and construction (EPC) services while Solar Frontier KK and Toshiba Mitsubishi-Electric Industrial Systems Corp (TMEIC) have continued to supply solar panels and PV inverters, respectively.

Mega-solar plant submerged 1st after water level rises

The Kurimamachiya Mega Solar Plant is situated along the watershed of rivers near the sea (Fig. 2). The site was previously used as a pond for eel farming (Related article: Riverside Solar Plant Absorbs Water of Flooded River).

Fig. 2: Mega-solar site sometimes becomes lower than river depending on tides and floods (source: Nikkei BP)

Located about 1km away from Ise Bay, the plant is adjacent to the junction of the Shitomo River and the Kenashi River at its south end, with the rivers running to the east and southwest, respectively. To the northwest the plant faces a ready-built housing complex. Sanko Real Estate started selling units in this housing complex in 1969 and has since maintained a close relationship with the residents.

Located near the sea, water levels around the plant shift with the tide. When the tide is high, the water level rises and water flows into the ditches built outside the levee and inside the housing complex.

Accordingly, Sanko Real Estate designed the site so water would flow into the mega-solar site first before the housing complex site floods as soon as the rivers rise to a certain level or higher and the amount of water increases to more than the ditches in the surrounding area can absorb.

The site has been reclaimed to gradually descend from the highest area adjacent to the housing complex in the opposite direction. At the same time, many areas in the plant site were built lower than the adjoining site on the side of the housing complex.

As a result, if the water level rises higher than expected, the mega-solar plant will be submerged before the housing complex. In other words, the power plant will play a role similar to a balancing reservoir. Moreover, water will pool in areas further away from the housing complex.

As for drainage measures inside the site, underdrainage, which is a measure to drain the stagnant water underground, was built in addition to a sand basin. The sand basin not only prevents sand from being discharged to outside by storing it, but also retains water. The underdrainage was set up underground connecting the sand basin and the ditches. The constructor buried pipes with holes in the ground and spread crushed stones all over the pipes. Water, after penetrating the ground, will enter the underdrainage through the holes and flow into the sand basin and the drainage ditches.

During the approximately three years and 10 months since the operation began, no solar panels and other power generation facilities have been submerged, proving the effect of the design.

When a high tide coincided with heavy rain, the roads inside the housing complex were apparently submerged from time to time. Even in such cases, however, only part of the mega-solar site's foundations seems to have submerged in the water near the south end, which is the lowest area in the site.

Considerations paid to neighboring area even when planting trees, weeding

To prevent the solar panels arrayed across the site from giving a rather lifeless impression to those who see the mega-solar plant from within the housing complex, trees were planted along the fences in the areas adjoining the housing complex (Fig. 3).

Fig. 3: Trees planted in consideration of housing complex. Compared with bottom photo shot in November 2015, trees have clearly grown (source: Nikkei BP)

As about three years and 10 months have passed since the operation began, the trees have grown and are suppressing the rather lifeless impression as expected. In addition, the plant implements weeding in the site near the housing complex shortly before the large-scale cleaning of the ready-built houses twice a year. The company weeds the solar site in order to give the same clean impression when the housing complex is cleaned and becomes cleaner than usual.

At the Kurimamachiya Mega Solar Power Plant, the weeding frequency has increased more than expected around arrays. Sanko Real Estate was planning to weed the site twice a year; however, weeding is needed at least four times a year, perhaps due to the site being a former eel farm (Fig. 4).

Fig. 4: Vehicle-type weed cutter utilized in site, vines on fences removed manually. Fences on river side (lower side in middle photo) covered by vines and blocked like walls with no open spaces. Measures taken to remove vines on fences at mega-solar site (bottom photo) (source: Nikkei BP)

As Sanko Real Estate, unlike the companies which operate a few power plants at remote locations, runs solar power plants at 24 locations in its home base Mie Prefecture, it can increase weeding efficiency by intensively weeding multiple power plants in series as planned, for example.

Sanko Real Estate adopted vehicle-type weed cutters ahead of industry peers and later an additional machine, which features a higher cutting height and is more suitable for use at solar power plants, to further boost work efficiency.

At Kurimamachiya Mega Solar Power Plant, with a site area of about 5ha and a solar panel capacity of roughly 4.5MW, it initially took four to five hours to weed the site using two weed cutters at the same time. Compared with the conventional strimmers, even this was a significant improvement in efficiency. By using a new vehicle-type weed cutter that features a higher cutting height, however, weeding now only takes four hours with one machine (Video).

Video: Cutting weeds by vehicle-type weed cutter with higher cutting height (source: Nikkei BP)

Partly because the operator is getting used to the operation of this vehicle-type weed cutter, the new model's characteristics are contributing to improving work efficiency. The weed cutter not only features higher cutting height, but also allows operators to raise and lower the cutting height just by shifting a lever while driving. These two characteristics reduce the risk of hitting crushed stones and other objects, so the number of times the operation stops or slows down decreases.

The lower risk of hitting crushed stones has also led to improved safety, namely the lower risk of injuring workers and damaging solar panels.

Facility overview