Number of Sheep Increased to Boost Weeding Efficiency at Solar Plant

Herbicide used proactively in areas with no sheep

2019/06/17 17:01
Shinichi Kato, Nikkei BP Intelligence Group, CleanTech Labo

On May 29, 2019, at the "Nikke Machinaka Power Plant Akashi Tsuchiyama," a mega- (large-scale) solar power plant with a total output of about 17MW in Inamicho, Kako-gun, Hyogo Prefecture, Japan, the five sheep being grazed within the plant site were sheared (Fig. 1).

Fig. 1: Sheep before shearing (top) and after shearing (bottom) (source: Nikkei BP)

The site had an idyllic atmosphere which is barely seen at typical solar power plants.

The Japan Wool Textile Co Ltd (Nikke), the power producer of this mega-solar plant, used a former short-distance golf course "Nikke Golf Club Tsuchiyama Center" as the site for the plant (Fig. 2, See related article).

Fig. 2: Utilizing former golf course in town. Pond of former golf course (middle row right), additional solar panels set up on adjacent former parking space (bottom left), shrine in site (bottom right). (source: top Nikke, others Nikkei BP)

The power plant consists of Phase 1 equivalent to about 11MW, which started operation in October 2013, and Phase 2 equivalent to about 6MW, which started operation in February 2014. At the Phase 2 area, additional solar panels equivalent to about 192kW were set up on an adjacent former parking space in 2018.

Nikke grazes its sheep in the site of this mega-solar plant in Akashi Tsuchiyama in the hope that the local residents may feel an affinity for the plant. Unlike most golf courses in mountain forests, this former golf course is located in town. Nikke feared that the power plant with 50,000 solar panels, which was transformed from such a green golf course, might give a cold inorganic impression.

Accordingly, Nikke decided to acquire some sheep and graze them in the power plant site. Among livestock, sheep are relatively meek and friendly. The company felt that local people would have an affinity with the mega-solar plant if they saw sheep walking around within the plant site.

The sheep also play a role in corporate PR activities, being used as the mascot character of Nikke whose original business is wool spinning. Furthermore, the sheep are expected to alleviate the company from the burden of weeding by eating the grass in the mega-solar plant every day.

Nikke currently has five sheep and shears them in late May every year. Because sheep have been bred as livestock for shearing, their wool does not naturally molt so continues to grow. If sheep wool grows too much in the hot season, it will interfere with the animals' body temperature control and health. For this reason, the sheep need to be sheared once a year from spring to early summer (Fig. 3).

Fig. 3: Sheared wool amounts to about 4kg per sheep (source: Nikkei BP)

Wool of the sheep at the mega-solar plant grows about 10cm per year. The staff of Rokkosan Pasture (Nada-ku, Kobe City), the sheep's previous home, visited and quickly sheared the five sheep. As soon as the sheep were sheared of about 4kg of wool each, they returned to the mega-solar site, looking refreshed, and started eating grass (Fig. 4).

Fig. 4: Start eating grass as soon as shearing finishes (source: Nikkei BP)

Sheep number increased to graze more-than-expected amount of grass

The number of sheep was initially three, but increased to five later. This was because the grass grew more than expected and the three sheep could not eat it all at the pace it grew.

The sheep are not grazed across the entire mega-solar site combining the Phase 1 and 2 areas. Of the about 220,000m2 site, the sheep graze in about 15,000m2 of the Phase 1 area. Nikke has left plenty of green from the former golf course partly in consideration of the landscape and adopted the sheep for grazing in this area.

According to Nikke, it has discussed increasing the number of sheep to more than five. However, if the number of breeding sheep increases to more than five in Hyogo Prefecture, they will be considered as a "business," such as farming, not in the "pet" category. A higher level management structure and facilities will be required in that case, which will be too much of a burden for the mega-solar business. For this reason, the company limited the number of sheep to five.

Measures against weeds, including the sheep management, at the mega-solar plant have been entrusted to group company Nikke Fudosan KK (Kobe City). Even in the area where the sheep graze, weeding by hand is still required. For example, there are some kinds of weeds such as dokudami (chameleon plant) that the sheep dislike (Fig. 5). Also, the sheep will not eat grass which has tough ends or that grows longer than a certain level.

Fig. 5: Sheep dislike dokudami (chameleon plant) and tough grass. (source: Nikkei BP)

Herbicide utilized for weeding in areas without sheep

Including for the additional sheep, Nikke has reportedly needed to undertake more work at a higher cost than it initially estimated. The company radically changed the measures against weeds in the areas where no sheep graze. Initially, Nikke was planning to cut the grass as it grew taller than a certain height. However, this method cost more every time the company weeded.

The areas with and without sheep are clearly different (Fig. 6). The area where the sheep graze is green, whereas the area without sheep is covered by crushed stones.

Fig. 6: Areas where sheep graze (right in top photo) and herbicide is used (left in top photo). No herbicide used around observatory (right in bottom photo) (source: Nikkei BP)

Laid under the crushed stones were weed barrier sheets. Even so, the effect was limited, and more weeds than expected grew. Given the circumstances, the company switched the weeding method and has effectively used herbicide for the past three years. Spraying herbicide three times a year, Nikke has succeeded in reducing manual weeding by limiting weed growth to a certain extent.

First, the company sprays herbicide in March before grass starts growing. This reportedly has a distinctive effect and limits growth when the grass starts growing. Then, the company sprays herbicide in July and between September and October. Since it began to use herbicide, it no longer needs to weed the site until July. After cutting grass around July, the company considers whether it implements weeding again according to the circumstances.

According to Nikke, over the last three years, it has gradually established an environment where weeds do not easily grow.

Large-scale PV inverter inspection and maintenance in 5th year

The year 2018 was the plant's fifth year since it began operation. In accordance with the business plan, Nikke overhauled the interior of 30 PV inverters at the Phase 1 and 2 areas. As this was first large-scale Nikke's first inspection, it was conducted in accordance with recommendations by the manufacturer of the PV inverters.

Although it turned out there were no serious malfunctions in this inspection, the PV inverters were more worn than expected, and the company even replaced some components.

The PV inverters are inspected every year in August. The company tries to minimize power selling loss even during the inspection period of five to seven days, starting inspection from early in the morning when the weather is bad and as late as possible on a sunny day, for example.

PV inverters are housed in a strong concrete hut (Fig. 7). Unlike metal facilities seen at many mega-solar plants, Nikke housed the PV inverters in a concrete hut.

Fig. 7: Concrete hut to house PV inverters. Equipped with a conductor rod and solar panels on rooftop (source: Nikkei BP)

Nikke built this concrete hut from a prudent perspective, taking into consideration the influence of heat and dust. Solar panels were also set up on its roof. The power generated through these solar panels is also counted as part of the power generated at the mega-solar plant.

In addition, some conductor rods have been set up on this hut and around the grid connection facility (Fig. 8). This is unusual for a mega-solar power plant in Japan. Nikke has taken more vigilant measures against lightning compared with other general mega-solar plants.

Fig. 8: Multiple conductor rods set up around grid connection facility (source: Nikkei BP)

Facility overview