Landfill Site Transformed Into Clean Power Plant

Unique efforts to overcome little sun, snow cover

2013/11/11 13:05
Kenji Kaneko, Nikkei BP CleanTech Institute

Akita City is one of the heaviest snow zones in Japan. In winter, it has many cloudy days with little sunshine. In fact, it is very close to the bottom in the prefectural ranking for the amount of solar irradiation in Japan. There is even the fear that solar panels could be covered with snow.

The large-scale solar power plant Akita City constructed within the city was confronted by those meteorological drawbacks as well as the restrictions on land reclamation of landfill sites. Efforts are being made to conquer the "triple difficulty" of little sunshine, snow cover and a former landfill site and ensure adequate commercial returns.

Challenging construction in region ill-suited to PV power

The Akita City General Environment Center is a facility that accepts domestic waste from residents of Akita City. It incinerates and melts up to 460t of waste per day, reusing it in various ways such as slag.

Before the gasification melting furnace was introduced about ten years ago, the center burnt combustible waste in the incinerator and buried the incinerated ash and incombustible waste in the adjoining landfill site. After they are filled up, landfill sites are covered with soil and abandoned. Given the limited use of such sites, this site became a field of weeds.

On Oct 1, 2013, a 1.5MW solar power plant was completed and started power generation in an approximately 4.6ha idle site. As you pass through the facility's gate, a huge box-like building, which is the gasification melting furnace, can be seen up ahead.

Coming into view as you walk around to the building's rear side, with the adjoining recycling facility on your left, are the solar panels neatly arrayed across the gently sloping hill. The 9,170 1.6 x 1m polycrystalline silicon solar panels, tilted at 30° on the mounting systems, look like a modern art object made of huge plates with a navy and white striped pattern.

"Akita City is an ill-suited region for solar power generation because of the little sunshine it enjoys," said Masaaki Nasu, Global Warming Deputy Director, Environmental General Affairs Division, Environment Department, Akita City Municipal Government. "Moreover, former landfill sites have another issue: ground condition. Nevertheless, we boldly decided to construct a large-scale solar power plant following the implementation of the feed-in tariff (FIT) program."

"To energize the regional economy, we wanted local enterprises to be deeply involved with the solar power plant's construction and operation," he said. "To make it happen, we wanted to enhance our influence by producing power by ourselves, instead of just leasing land to power plants. However, it was difficult for Akita City to provide or raise the funds for initial investment in the solar power plant."

What Akita City decided upon after discussions by the municipal government was a lease method. The predecessor for this method was Ota City, Gunma Prefecture. In Ota City, the municipal government runs a power plant while Century Tokyo Leasing Corp (TCL) owns the solar power generation system facilities in the plant.

What differed from other conventional facility leases was the lease fee, which includes not only the construction cost but also regular inspection, maintenance and repair costs that would come about in accordance with the power plant's operation as well as the cost to purchase fire and other insurance policies. TCL calls contracts based on such a method "comprehensive facility lease contracts."

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Facility lease contract covers maintenance, repair

Based on the case in Ota City, in December 2012, Akita City called for proposals from the public on condition that the proposals follow the comprehensive facility lease contract business scheme. The applicants had to be experienced in solar power plant construction, be registered in Akita City's list of qualified bidders for construction and assume the participation of local enterprises.

As a result, the proposal by a corporate organization consisting of TCL, JFE Engineering Corp (Tsurumi Ward, Yokohama City) and Sejimo Construction (Akita City) was chosen at the review committee's meeting in January 2013.

The corporate organization consisting of the three companies was chosen as a result of appreciating not only the value of a lease fee but also how to secure the amount of power generation with little sunshine and what measures should be taken against snow cover, for example.

In the comprehensive facility lease contract, which was stipulated to be the business scheme foundation, Akita City receives sales of the generated power while the remaining part of the sales after deducting the lease fee paid to TCL would be the city's revenue. Akita City would bear the risk of lower-than-expected electricity sales due to fewer sunny days than average per year.

On the other hand, Akita City would not have to bear the risk of worse-than-expected power generation because it is the corporate organization's responsibility to maintain the expected performance of power generation.

TCL has, thus far, concluded three comprehensive facility lease contracts in the large-scale solar power plant market. In addition, TCL established Kyocera TCL Solar LLC (Chiyoda Ward, Tokyo) with Kyocera Corp and has launched large-scale solar power plants in four locations, expecting 60MW to 70MW of power generation business by the end of fiscal 2014.

Kyocera TCL Solar borrows land and runs its own power generation business. TCL established a framework to propose two options: whether to offer a lease contract or to borrow land and become a power producer itself, depending on the client's needs.

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Extreme "overload" to boost PV inverters' utilization rate

Meanwhile, how did the plant tackle the "triple difficulty" of snow, little sunshine and a former landfill site? Tomofumi Watanabe, assistant manager, Photovoltaic Generation Dept, Power Plant Division, JFE Engineering, which provided EPC (engineering, procurement and construction) services, looked back and said, "It was pretty difficult to ensure an annual power supply of 1.75 million kWh or more, which was stipulated in the public call for proposals, and maintain a commercial return while keeping costs low at the same time."

To generate more than 1.75 million kWh per year with solar panels of 1.5MW maximum output, the utilization rate should exceed the average in Japan of 12%. In the case of Akita City, the utilization rate was expected to barely exceed 11%.

Based on those circumstances, a method called "overload" was adopted to boost the amount of power generation. Because Akita City Solar Power Plant has set up PV inverters for 1.5MW maximum output, electricity that can be sold also amounts to 1.5MW at maximum. However, solar panels of up to 2.2MW maximum output have been installed on the site. Installing more solar panels than the capacity of PV inverters is called overload.

In such a case, the power surpassing the PV inverters' capacity generated by a sufficient amount of irradiation on sunny days would be wasted because it cannot be sold. However, the facility can maintain an amount of power generation almost equivalent to that of the PV inverters' maximum output even when the sun is weak, such as in the late afternoon, thanks to the large number of solar panels.

In other words, overload features the benefit of boosting the utilization rate of the PV inverters. Although large-scale solar power plants adopting overload are not rare, it is rare that the gap between the outputs of the solar panels and PV inverters is so large, as in Akita City.

"We could slightly increase the amount of power for sale by adding one more PV inverter to boost the capacity to 1.98MW, but that would lead to a rise in the facility cost," Watanabe said. "We decided on the current outputs of PV inverters and solar panels after repeatedly simulating the cost-benefit performance."

As a measure against snow cover, the solar panels were tilted at 30°C, and the lowest height of the panels was raised to 1.5m. This was aimed at making snow on the panels slip off and not accumulate. However, the panels' shadows grow longer with this angle when the sun is lower in the morning and late afternoon. The designated number of panels cannot be set up if spaced wide enough to prevent the shadow from covering the neighboring panel.

Hence, the deployment of the panels was designed assuming that the lowest row of panels would be shaded. The panels closer to the ground are parallel-wired in series, making one circuit. As a result, panels on the upper side can continue to generate power even when the output lowers due to shadows on the lower panels.

In addition, when constructing this large-scale solar power plant on the former landfill site, the development of which is restricted, a "tripod-type mounting system" was deployed on the sloping area. Four 4m pipes were radially driven into the ground at an angle of 40 to 45°, to which supporting posts to mount panels were locked with the fittings in the part above ground. By driving those pipes up to 2m into the ground on a skew, the strength to resist the pulling velocity of wind was said to be achieved.