The March 2011 Fukushima Daiichi nuclear accident damaged Japan’s energy infrastructure and caused the country to shut down the majority of its nuclear reactors and accelerate investment in renewable energy to close the resultant power gap.

Yet Japan’s mountainous, heavily forested geography poses a challenge for new energy site development. In the wake of Fukushima, the country has looked to one of its most plentiful resources, bodies of water, to aid in the development of wind and solar power resources.

The size of the projects being developed is tiny in comparison to the roughly 5,000 megawatts (MW) of nuclear generation capacity decommissioned at the Fukushima site, which largely survived the initial 9.0 magnitude earthquake, but was inundated by the resulting tsunami that flooded the plant and knocked out cooling systems that wrecked the reactors. Even so, they represent an effort by the Japanese government and leading manufacturers to develop electric power generating alternatives for this resource-constrained chain of islands that require neither uranium nor imported fossil fuels.

Challenges to Offshore Wind

While wind farms are relatively common in the North and Irish seas, the ocean floor off of Japan’s coasts discourages the use of traditional offshore turbines. Most European wind farms use fixed-bottom, subsea turbines installed into the sea floor using a monopile or tripile foundation. The Dutch Gemini wind farm off the Groningen coast, for example, places turbines 85 km (some 52 miles) offshore at a depth of about 30 m (98 feet). Placing fixed-bottom turbines at an equivalent distance off the Japanese coast is all but impossible as the Sea of Japan and North Pacific Ocean are around 5 times deeper than the North Sea.

Japan responded to these challenges by planning Fukushima FORWARD, a prototype floating wind farm. A 2 MW floating offshore wind turbine (FOWT), the government-funded project’s first, was completed and activated in November 2013. The turbine is located 20 km off the Fukushima coast and uses a six-piece catenary mooring system anchored to the seabed 120 m below the surface. FORWARD is novel because it involves not only floating turbines but also a 25 MVA floating substation that became operational along with the first turbine.

The Fukushima Offshore Wind Consortium is responsible for the project, and includes Mitsubishi Corp., Nippon Steel and Japan Marine United Corp. The project has faced multiple engineering and operational challenges that include mitigating the difficult metocean conditions created by Japan’s frequent earthquakes and typhoons, along with potential collisions with ships, environmental impacts and disruptions of the local fishing industry.

After surveying and estimating offshore wind and wave data, the consortium developed several float designs to minimize float motion. The completed 2 MW turbine employs a compact, semi-submersible floater designed by Mitsui Engineering & Shipbuilding Co. It uses steel beams to attach three vertical floats attached to the central turbine shaft and minimizes floater motion through ballast and turbine control optimization. The float is comprised of a high-tension steel manufactured using thermo-mechanical control process (TMCP) and ultrasonic impact treatment (UIP) to increase welding efficiency and improve fatigue characteristics.

The consortium has plans to complete two more 7 MW wind turbines by 2016. These projects involve different float designs to compensate for larger rotor diameters and heights that are 60% taller than the 2 MW turbine. The Japan Marine-designed 7 MW turbine uses a spar float consisting of a series of disc-like floating hulls attached to a central shaft. The bottom hull is filled with concrete, which lowers the station’s center of gravity and limits wave motion. The completed spar is fitted with a metocean measurement system consisting of meteorological, oceanographic and motion sensors to predict and compensate for float motion.

Mitsubishi Heavy Industries Inc. designed the other 7 MW turbine float as a V-shaped semi-submersible platform that consists of three columns connected by two steel beams. The float is moored using an eight-piece catenary system.

While deep-water offshore turbines eliminate many of the problems commonly faced by wind farms located on land, including visual issues, they have other disadvantages. For example, offshore wind generation is considerably more expensive. A floating turbine costs around twice as much as a traditional turbine. Fukushima FORWARD is projected to cost around $20,000 per kilowatt, eight times more than a comparable project on land.
According to a February 2014 IHS Energy report, "Japan Embarks on a New Renewables Path,"offshore wind is the most expensive renewable technology per megawatt hour in Japan, although this cost is projected to decrease to a level equal to the average retail price by 2020. And while consortium members say that the turbines have been designed to withstand Japan’s harsh oceanic conditions, their durability and longevity remain largely untested.

Floating Solar

Japan’s solar photovoltaic (PV) power has seen more widespread development due to incentives offered by the country’s feed-in tariff (FIT) program implemented in July 2012. Land suitable for PV systems is accordingly becoming scarce, necessitating the placement of PV plants on water. The country has numerous reservoirs used for agricultural irrigation and flood control, and many are considered to be well-suited for floating solar installations.

A floating solar PV facility could look like this artist's rendering. Source: Ciel et TerreA floating solar PV facility could look like this artist's rendering. Source: Ciel et TerreA number of floating solar projects have been initiated, including a 1.18 MW system in Saitama Prefecture and several more planned for construction in Hyogo. All three plants are based on Hydrelio float technology developed by French company Ciel et Terre, with PV technology by Kyoto-based Kyocera. The Hydrelio system is comprised of high-density polyethylene and consists of two types of floats: an angled main float to support PV modules, and a flatter secondary design to connect the main floats. The floats are conjoined using simple tab connections that result in a link resistance of 3,000 decanewtons (DaN).

This design is common among France’s floating solar installations and mitigates several anticipated problems with floating solar, such as establishing a mutually beneficial relationship between the solar installation and reservoir, according to Ciel et Terre. Floating PV systems are naturally cooled by the water they are placed on, reducing the need for a dedicated cooling system and improving power efficiency. By shading the water and reducing the water area exposed to air, the installation also results in reduced water evaporation from reservoirs and inhibited algae growth.

According to IHS Energy, Japan’s renewables forecast remains uncertain due to steadily decreasing feed-in tariff rates, as well as Japan’s recent decisions to restart nuclear reactors and expand the use of coal to meet incremental capacity. Japanese renewable energy projects also cost 30-90% more than those in Europe due to permitting requirements, geographic challenges and supply chain bottlenecks.

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Fukushima FORWARD