IN DEPTH: Fishing for energy
In an island nation where local fishing co-operatives have the rights to steward coastal waters and can stymie any moves to share them, it is a good idea for offshore wind developers to keep them onside.
Two groups of Japanese technology designers have taken that notion one step further — effectively applying the motto “if you can’t beat them, join them” — by incorporating fish-farming infrastructure into their wind- and marine-energy machines.
Mitsui subsidiary Modec — best-known in offshore circles for its floating production vessels for the oil industry — unveiled its 1MW Skwid last year. The unit marries a 24-metre-diameter direct-drive Darrieus vertical-axis wind turbine (VAWT) with a keel-mounted Savonius ocean-current energy device and a purpose-built spur export line for firing up the high-intensity banks of lights used to lure squid to the surface.
“To begin with, we are hoping to sell the first units to the fisheries co-ops but the final goal of ours is to build it for micro-grid-connected power,” says Takuju Nakamura, Modec’s general manager for new business development.
“It is very difficult to get the consent of the fisheries associations for this sort of project. And it does not make commercial sense if we have to ask to use an area — this would result in maybe one project a year.
“So to make it commercial we have to have a system that [the fisheries] want for power production, for an additional revenue stream as well as an alternative source of power for their lighting systems.”
The Skwid’s 36-metre-long aluminium blades have been shown in model tests to capture twice the energy from their swept area as a similarly dimensioned conventional onshore turbine.
Below the water line, its S-shaped split-cylinder ocean-current turbine does treble duty — generating power, jump-starting the wind turbine in currents as slow as 0.6 metres per second (m/s), as well as acting as a self-righting ballast to keep the machine stable.
The floater’s steel hull, moored by a six-line spread with drag-embedded anchors, houses the power train’s gearbox and generator, which are mounted on a set of rubber gimbals at deck level to dampen the impact of waves.
The Skwid, which can produce electricity while tilting up to 25° from vertical, is calculated to have an output capacity of 500kW in winds of 13m/s, rising towards 1MW in velocities of 16m/s, with production flow smoothed by the overlapping outputs from its two turbines.
The fully assembled 1MW prototype was on its way to start a testing programme last year off Karatsu, Saga prefecture, when the ocean-current turbine — which has a draft of 18 metres — snapped off as the unit’s U-shaped installation barge tried to navigate under a low bridge in the shallows of the Kanmon Straits.
Modec is making a second attempt at deployment this summer, using a re-engineered two-stage installation technique where the VAWT will not be fitted to the floating hull until it reaches its 50-metre-deep installation site.
Two years’ testing at the chosen location — where winds blow at an average 7m/s over waters with a 1.7m/s current — is slated to begin in September, with output sold to regional utility Kyushu Electric.
“The site is very good for our purposes in terms of wind and current speeds,” notes Nakamura, “but the key point was the fisheries associations’ willingness to allow us to install it here.”
In the longer-term, the Skwid — which is designed to be scaled up to a 5-10MW model suitable for water depths down to 2,000 metres — could be installed in arrays, particularly for small onshore networks or remote micro-grids that otherwise rely on fossil-fuel generators.
“Offshore energy is still very expensive, so I don’t think [the Skwid concept] will compete with nuclear soon, but it could compete with diesel,” says Nakamura.
The WindLens concept under development at Kyushu University is being tailored as a multi-turbine wind/marine power station — complete with fish farm enclosures built into the underside of the hull.
A 140-tonne micro-scale version of the design, made up of a pair 3kW WindLens turbines and a 2kW PV panel on an 18-metre-wide hexagonal floating platform, has been operating in 4m/s winds in Hakata Bay off Fukuoka city, since late 2011.
“After model tests, this was the real test of the technology,” states WindLens inventor Yuji Ohya, director of wind engineering research at Kyushu University. “It continues to perform very well, with better than 50% availability, and it has even survived a typhoon that knocked down many buildings in the area.
“Also, we found that the mooring lines and underside of the floating platform have grown seaweed, so it has become a kind of artificial reef. And many fish have been feeding here on the marine growth, so the fishermen are happy.”
The WindLens, a passive-yawing downwind turbine, features a broad aerodynamic ring encircling the rotor blades called a “brimmed diffuser” or “shroud”, which turns the wind flow into curling vortices of air, forming a low-pressure area immediately behind the rotor. This near-vacuum pulls the airstream over the blades faster, raising revolutions per minute and, with it, energy production levels by a factor of two to five.
For a full-scale model, three WindLenses — powering direct-drive permanent-magnet generator transmission systems — would be fitted to triangular “super-carbon fibre” or hybrid-concrete semi-submersible foundations that could be honeycombed together to form giant integrated energy/aquaculture farms.
Ohya is hoping to receive government funding this year for a 180-metre all-steel 1-2MW model, featuring a trio of 350kW WindLenses, several ocean-current turbines and PV panels, to be moored in 80 metres of water in the Genkai Sea off Fukuoka.
“Each power source is too dilute on its own, so we are looking at integrated utilisation designs to improve the efficiency and output of the unit,” says Ohya.
“[The Hakata Bay] project has been very successful at proving the viability of the technology but it remains too high-cost so we will be working very hard to bring the costs down for the stage-two project.”
On the far horizon, the floating concept could be scaled up to create a “super-WindLens turbine” of 5-10MW class, while a 1GW wind plant in near-shore waters is not outside the realms of possibility, though “a long, long way in the future”. A 10MW model would have a rotor spanning 112 metres, compared to conventional 5MW offshore turbine rotor diameters of about 120 metres.