'World's Largest' Floating Solar Plant Operates on Reservoir

Flood-prevention reservoir to supply income, emergency power at times of disaster

2013/11/17 17:02
Shinichi Kato, Nikkei BP CleanTech Institute

While speaking about large-scale solar power plants, most people generally consider that they generate power from solar panels bedded over a vast, flat site. Amid such circumstances, a method that floats solar panels on water was realized at "Solar-on-the-Water Okegawa" on the reservoir of Okegawa Tobu Industrial Park in Okegawa City, Saitama Prefecture (Fig. 1).

Approximately 4,500 solar panels are floating on a surface area of about 12,400m2 of the 30,000m2 reservoir, which was built to adjust the amount of water in nearby rivers. From fall to winter, various kinds of birds including migrating birds and waterfowl rest on the floating mounts of the solar panels (Fig. 2).

The method that floats solar panels on the lake and reservoir has a lot of benefits. It gives the lake/reservoir owner the opportunity to earn rent and business tax, which could not be imagined before. For power plant operators, on the other hand, it allows them to rent the property cheaply with no land reclamation cost and relatively lower costs for weeding and other maintenance and management.

Furthermore, even if the panels used are crystal silicon solar ones whose power generation efficiency tends to decrease due to high temperatures in summer, the floating method keeps their temperature from rising and enables the system to more easily secure the required amount of power generation than the on-land method, thanks to the cooling effect produced by the water surface. However, considering the large cost for installation, the issue is how to absorb the rise in cost and secure profitability at the same time.

Solar-on-the-Water Okegawa's maximum output is approximately 1.2MW, which is "probably the largest for on-water solar power plants in the world," said Executive Director Toshihisa Nagashima of West Holdings Corp. The plant started power generation in July 2013 with an expected power generation capacity of about 1.25 million kW per year, which corresponds to the amount of electricity used by roughly 400 4-people families.

The owner of the reservoir is Okegawa City, while the West Holdings Group constructed the plant and became its operator. As such, Okegawa City succeeded in meeting its goal to effectively use its asset, a balancing reservoir, following the construction of the power plant.

Okegawa City will earn ¥1,859,000 (approx US$18,555) per year in rent from the water surface for the next 20 years. It is paid by Japan Mega Solar Power Co (Shibuya Ward, Tokyo), which is affiliated with West Holdings (the operator of the power generation business). Previously, the reservoir was only used for water storage, costing the city in terms of maintenance/management, and being left unused except for times of emergency. Hence, it is a great benefit for the city to start earning income from the reservoir.

This reservoir was built primarily as a measure against flooding of the many rivers in this area such as the Naka and Ayase rivers and their side streams. It prevents flooding by pooling some of the river water that has increased due to torrential rain. There are many reservoirs for the same purpose scattered in the plain spreading across the eastern part of Saitama Prefecture including Okegawa City.

Set up on municipal property, the power generation facility features, upon request by Okegawa City, a stand-alone operation capability and removable secondary batteries. This is for the purpose of using the batteries as regional emergency power supplies during times of emergency and blackouts. A 250kW-output PV inverter was introduced for these emergency power supplies in addition to the 500kW model to generate power for sale under usual conditions. Both are products manufactured by Toshiba Mitsubishi-Electric Industrial Systems Corp (TMEIC).

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Method used to float solar panels on reservoir

How did they manage to float approximately 4,500 solar panels on the reservoir? Instead of metal mounting systems used with the on-land systems, plastic members similar to rafts attached to the solar panels are floated on the reservoir (Fig. 3). Even though the plastic mounting systems are floating on the water, they still have to prevent the solar panels from moving excessively and the solar panels from tilting due to changes in the water level.

Construction is carried out by connecting the raft-like plastic members, carrying them to the designated position by ship and mounting 20 solar panels on each row. This process is repeated, and the solar panels are stabilized (Fig. 4). Those floats are subsequently anchored on four corners so that the overall system cannot be taken away by the wind or water movement. On windy days, the devise applied to the member connection area can absorb the welter generated by waves on the reservoir surface.

According to West Holdings, in a cross-sectional view, the reservoir is shaped like a basin with an almost flat bottom and a wall climbing up on a slant toward the shore. What matters to this reservoir is the descent of the water level, rather than the ascent, the company said. Should the solar panels be floated in a widespread pattern that they almost reach the shore, the panels close to the shore could affect the overall power generation performance by moving on to the ground when they touch the bottom of the reservoir due to the low water level.

Based on this situation, the solar panels are designed to float away from the shore where they can adapt to a 9m gap in the water level. The 9m gap was set up as an index considering that the water level is not likely to lower more than a certain level as there are springs at the bottom of the reservoir, the company said (Fig. 5).

The member to attach solar panels is hollow and is called a "float." This member is a product of Ciel et Terre International of France. It was employed because solar power plants floating on reservoirs, etc, are more advanced in Europe and the US than in Japan. Even so, it was unusual to apply this method to large-scale plants like the one in Okegawa, West Holdings' Executive Director Nagashima said. Therefore, another member jointly developed with the West Holdings Group was introduced.

West Holdings deployed solar panels manufactured by JA Solar Co Ltd of China, which Ciel et Terrre recommended as the optimum product for its float. The solar panels are tilted at 15°, which was determined to minimize the impact of wind pressure and ensure the amount of power generation at the same time. Given the little amount of dust on the reservoir, the setup at this almost flat degree is not likely to lower the amount of power generation that much due to dirt, compared with the models set up on land.

To generate power with solar panels floating on water, other technologies are also important: technology to block the moisture that would penetrate from the backside and side surfaces of a solar panel, technology to prevent short circuits caused by moisture penetrating the electric joints, and technology to prevent electricity loss due to lower insulation resistance in the humid environment.

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Diffusion depends on construction time, introduction cost

Regarding the issue that it costs more to introduce an on-water-type solar power generation system than an on-land system, West Holdings has not revealed information such as the investment amount. Compared with the West Holdings Group's existing on-land systems, however, "The installation cost about 30% more partly because this was the first adoption for a large-scale solar power plant, but the cost will decrease from now," said President Hidehisa Onda of West Energy Solution Inc.

It is easy to bring about a volume-production effect with this system. Therefore, the cost to purchase floats is expected to decline as this method is increasingly adopted.

The West Holdings Group announced that it can complete the construction of an on-land-type large-scale solar power plant of 1MW output in one month. On the other hand, it took three months to set up the on-water plant of about 1.2MW this time. According to Executive Officer Kenji Araki of West Holdings Corp, the construction period is estimated to shorten to about two months, from the next case onwards.

The construction period is longer with an on-water system because a series of processes to set up the solar panels, such as mounting solar panels onto floats and connecting the floats together, is carried out by engineers on boats. Given this process, West Holdings Corp explained that it is difficult to construct a plant on water in the cold period of winter.

Operation and maintenance is still uncertain in some respects. Although it requires no weeding, the algae that grow on the floats and panels need to be removed. Regarding this issue, the optimum schedule will be determined after one year of observation. Even though it is an artificial reservoir, it has water inflow from the streams around, and fish are living in it. West Holdings said it is also planning to monitor and give consideration to the plant's impact on such an ecosystem.