4 Types of Foundations Adopted for 15MW Solar Plant
All possible measures taken for water discharge using permeable anti-weed sheets
"DREAM Solar Higuchi Nagasaki," a mega- (large-scale) solar power plant with an output of about 15MW, is located in Togitsucho, Nishisonogi-gun, Nagasaki Prefecture (Fig. 1). Togitsucho is situated to the north of Nagasaki City's central area, facing Omura Bay.
The power plant was built on the former "Higuchi Togitsu Country Club" golf course. The location is about 19km from the Nagasaki Tarami Exit of the Nagasaki Expressway, about a 10-minute drive from Michinoo Station on the JR Nagasaki Main Line. The former golf course was built leveraging the mountain slopes. Despite the short total length of 6,005 yards, it was a mountain course with a relatively wide range of heights.
The power producer is Daiwa Lease Co Ltd (Osaka City) of the Daiwa House Industry Group. The Daiwa House Group has been engaged in everything from development to operation of the power plant while Daiwa House Industry Co Ltd provided engineering, procurement and construction (EPC) services and Daiwa Lifenext Co Ltd (Minato-ku, Tokyo), a building and condominium management group company, is responsible for its operation and maintenance (O&M).
The golf course commenced business in 1983 and terminated business in June 2014. The Daiwa House Group rented the site from Mitsudomoe Co Ltd (Nagasaki City), which had owned and run the golf course, and built the mega-solar power plant there. It started construction in August 2014 and power sales Nov 2, 2015. All generated power is sold to Kyushu Electric Power Co Inc.
Facility layout optimized by modifying the initial plan
The engineering and construction required special measures that could only be applied to a golf course in the mountains. There were almost no flat areas while the direction, undulations and angles of the terrain varied in the site. Some areas were affected by the shadows of trees.
In terms of not only solar panels but also PV inverters, intermediate step-up transformers, and switches and cubicle for grid connection with the extra-high voltage power transmission line, the Daiwa Group gradually altered the initial plan for each facility and optimized their installation in light of the actual geographic circumstances (Fig. 2).
Sharp Corp and Toshiba Mitsubishi-Electric Industrial Systems Corp (TMEIC) supplied 60,088 solar panels (output: 250W each) and 24 500kW PV inverters, respectively.
The Daiwa House Group arrayed the solar panels in accordance with the terrain, keeping the original undulations without full-fledged land reclamation.
Some trees were cut down in the site. Following the Forestland Development Permission System, the former golf course had preserved forests inside a certain ratio. Because this ratio had to be kept, even after the purpose of land use was altered from a golf course to a solar power plant, the Daiwa House Group cut the trees within the range to meet this requirement.
By cutting down some trees on the side slopes within the course, the company could array solar panels even in such areas. The areas where some trees were cut down amounted to roughly 40,000m2. The amount was larger on the south side and smaller on the north side.
Water discharge capacity maintained using anti-weed sheets as permeable as the ground
All areas where the solar panels would be set up were covered by anti-weed sheets (Fig. 3). Daiwa House has basically, thus far, covered all the areas upon which to set up solar panels with anti-weed sheets at the mega-solar power plants to which it has provided EPC services. The same method was also adopted at this plant.
The Daiwa House Group did not implement full-fledged land reclamation in the former golf course because of the need to limit its development within the "development activities" permitted when building the golf course. This also benefited the group by alleviating the development cost and in terms of water discharge measures.
At golf courses, water discharge systems are optimally structured in accordance with the undulations and other conditions of the terrain. Drainage ditches, for example, are provided in areas where the natural water discharge function is insufficient. Without full-fledged land reclamation, additional water discharge facilities can be kept to a minimum.
The water discharge function includes the land's water retaining capabilities. In the case of this mega-solar power plant, the ground surface was to be changed from the grass of the golf course to anti-weed sheets. Accordingly, the company adopted Unitika Ltd's "AG200" as an anti-weed sheet, which can realize almost equivalent permeability to that of the previous grass on the ground surface.
At the explanatory meeting for local residents, interest was particularly high in the water discharge measures because the meeting was held immediately after the landslide disaster in Hiroshima Prefecture that followed a severe rainstorm in August 2014.
The Daiwa House Group said it had gained an understanding at the meeting after explaining that the plant would maintain the same water discharge environment as that during the time of the golf course through the existing water discharge facilities and the adoption of highly permeable anti-weed sheets, and so forth.
The plant varied the way of using the anti-weed sheets in accordance with the geographic circumstances.
In flat areas, crushed stones were laid over the anti-weed sheets (Fig. 4). The crushed stones, along with the anti-weed sheets, prevent weed growth. The stones also prevent the anti-weed sheets from deteriorating and being blown away. So as to prevent the stones from rolling out of the areas, the plant built embankments on the perimeter of each area.
The areas with crushed stones accounted for less than 10% of the entire site. The plant selected fine particle-sized stones that looked good and would not easily roll away.
In the sharply sloping areas where no crushed stones could be spread, the ground surface was covered by nothing but the anti-weed sheets. Piles were driven in the ground from the upper side of the anti-weed sheets, preventing the sheets from being blown away.
Four kinds of foundations used according to area
The plant used four different foundation methods in accordance with the slope and terrain of the area to set up panels (Fig. 5).
The most basic was the concrete secondary product (precast) foundation for flat areas (Type A). When the area was sloping too much to use this foundation, spiral anchors were added to the small precast foundation for slopes (Type B). Rock anchors were used for side slopes where the ground was sloping even more and both the above methods could not be used (Type C).
The plant was initially planning to use only these three types of foundations. However, some areas, to which the precast foundation for flat areas (Type A) had been applied, were found to slope too much to build the base structure for the foundation. As a result, Daiwa House decided to use a fourth foundation, that is, the steel tube-based pile foundation (Type D), in those areas.
When adopting the pile foundation, there was a challenge concerning the method of driving the piles into the terrain. The conventional excavation method called "earth auger," which excavates bedrock with a special cutting edge at the tip, allowed for no more than two to three piles to be driven a day while the cutting edge of the driving machine easily wore out.
Consequently, Daiwa House adopted a method called "down-the-hole hammer" that crushes bedrock and boulders by striking them with a hammer and driving the pile foundations into the terrain (Fig. 6).
On the side slopes, solar panels were set up at the minimum height from the ground. The Daiwa House Group installed the panels, lifting and supporting them with cranes after attaching the solar panels with the rails that had been assembled on the mounting systems in advance on a nearby flat area (Fig. 7).
When working on steep slopes including the side slopes, holding fixtures with ropes were set up using pickets at the top with the aim of securing the safety of the workers (Fig. 8). The holding fixtures were left even after construction was completed and the plant started operation. The fixtures are used to secure the safety of operation and maintenance workers.