The Nezzy concept incorporates a two-bladed downwind super compact drive (SCD) 8MW turbine with fixed yaw angle mounted atop a guyed tubular steel tower that tilts ten degrees backward in the wind-flow direction. The reason for tilting is to create sufficient cable clearance for adequate load transfer.
The key benefit of the guyed design is that it virtually eliminates bending moments in the tower and the tower base, which results in significant mass and cost saving. The cables, which must cope with continuous exposure to seawater and general harsh marine conditions for 20 to 25 years, will cost around EUR 200,000, according to product inventor and Aerodyn director Sonke Siegfriedsen.
The tower itself has a droplet shape with the rounded part facing the wind, a measure that is claimed to reduce wind resistance (drag) from about drag coefficient = 1.3 (tube) to only Cd = 0.3-0.4. "Here, our aim was to enhance the structure's overall stability by limiting nacelle and rotor movement during gusty weather conditions," says Siegfriedsen.
Y-shape structure
The tower bottom is attached to a three-legged Y-shape hollow concrete support structure. The 60-metre leg is manufactured in two pieces, with the two shorter single-piece legs measuring 30 metres each. The outer ends are sealed with a metal bracket, with the longest leg's bracket incorporating a rotating mechanism and chains for structure anchoring and rotation in following the prevailing wind direction.
"A concrete specialist assists us with the support structure development," Siegfriedsen explains. "Currently each rectangular shaped leg is a constant height, but the width increases like a cone from an outer measurement of three metres to nine metres near the centre and is attached to a central interface. Each individual leg is further post-tensioned with the aid of special marine-proof steel cables inside," he adds. "The central interface also serves as the tower bottom flange mounting."
"I got the initial support structure ideas from a conference-call telephone with a Y-shape foot during a brainstorm session," Siegfriedsen recalls.
While towing a fully assembled SCD Nezzy turbine to a wind farm site, the legs are sealed and, being filled with air, enhance buoyancy and stability. "During operation the legs fill-up with seawater, whereby the bottom structure section settles at about 18-metre water depth," says Siegfriedsen. "The minimum required water depth is around 40 metres, which qualifies Nezzy for deep-water North Sea locations too."
Coning floaters
Another unusual design feature can be found in the three coning plastics-composite floaters, individually incorporating eight sealed compartments, with each floater connected to the support structure via cables or chains.
Total investment costs for a support structure plus chains and anchors is estimated at around EUR 2 million, according to Siegfriedsen, making it competitive with state-of-the-art fixed-bottom foundations.
The SCD 8.0 turbine features a 168-metre rotor diameter and is a further development of the 6MW SCD 6.0 with a 140-metre rotor. The first 6MW prototype was installed in China in September by Aerodyn's SCD licensee Ming Yang.
Visually, as well as being larger than the 6MW turbine, the 8MW Nezzy has no helicopter-landing platform, which had to be eliminated for operating safety reasons. Inside, the 6MW drivetrain design was mechanically adapted to the acceleration forces imposed by wave-induced structure and nacelle movements, Siegfriedsen says. "Switching to taper-roller bearings for the two planetary gearbox stages and the generator was the main SCD 8.0 modification for the Nezzy. The rotor bearing design is still under consideration," he adds.
No PMG
Another major SCD 8.0 design modification, but unrelated to the floating turbine concept, is the switch from a synchronous permanent magnet generator (PMG) to a synchronous generator with brushless electrical field excitation.
"There were several reasons that made us switch generator technology," says Siegfriedsen. "The first is linked to ongoing difficulties with handling and assembly of the magnets. The second reason is that warming-up the PMG from stationary non-operating condition must be performed very carefully in order to avoid over-voltage issues. Finally, this type of generator is much cheaper.
"The main penalties are a slightly bigger and heavier generator, and the generator excitation losses, but this is limited to 20kW only during operation at rated power output level." For an 8MW turbine that represents only 0.25%.
The SCD 8.0 features Aerodyn-designed blades with carbon fibres incorporated in the load-carrying spar caps, a measure that is aimed at curbing blade deflection. The maximum chord (largest width of the blade) is about 6 metres, qualifying these blades to be one of the widest currently developed. "The SCD 8.0's 361W/m2 specific power rating represents a good value for IEC class I turbines, from both yield and cost of energy optimising perspectives," says Siegfriedsen.
The product is currently a concept and Aerodyn is looking for international interest.
