Landfill Site Transformed Into Clean Power Plant (page 3)
Unique efforts to overcome little sun, snow cover
Solar panels and PV inverter. The polycrystalline silicon solar cells were manufactured by Hanwha of Korea and PV inverters by Toshiba Mitsubishi-Electric Industrial Systems (TMEIC). (source: Nikkei BP)
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.