Germany

Germany

WindTech: Master classes in offshore turbine design

Students on a wind engineering master's course worked together to develop an impressive 12.5MW offshore turbine. Eize de Vries spoke to them and one of the professors about what makes their design different from current offerings.

Radical… The OptimusXL’s medium-speed gearbox and generator assembly is mounted behind the tower
Radical… The OptimusXL’s medium-speed gearbox and generator assembly is mounted behind the tower

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At 12.5MW, the Optimus 200XL is the most powerful single-rotor offshore turbine concept introduced today.

It has a medium-speed drivetrain and a 200-metre rotor diameter with a competitive 398W/m2 specific power rating as well as innovative design features to drive down lifecycle Opex and, ultimately, the levelised cost of energy (LCoE).

The Optimus 200XL was launched on 31 January — not by a major OEM, but by a group of 32 international students completing a wind engineering masters degree at the Flensburg and Kiel universities of applied sciences (Fachhochschulen or FH) in northern Germany.

The launch was attended by several wind-industry companies, some of which had provided active design and other professional support to various master student teams.

Firms supporting the students included Aerodyn for drivetrain and fatigue-load calculations, Senvion for blade design, Liebherr for crane and hoisting technology, and DNV GL for load and dynamic simulations.

There is also plenty of industry experience behind the six professors and senior lecturers made available from both universities in support of the student teams.

Peter Quell has been heading the offshore technology department at the FH Kiel’s mechanical engineering faculty since October 2012 and is one of the initiators and driving forces behind Optimus.

Before joining the university, mechanical engineer Quell headed the research and development department at Senvion, or Repower as it was then, from 2001-11.

He was actively involved in the development of the offshore-dedicated 5MW Repower 5M (2004) and the 6.2MW successor model introduced in 2009.

Predecessors

Prior to the Optimus 200XL, students developed the 3.5MW Optimus 150 onshore turbine, with a 150-metre rotor diameter and a 198W/m2 specific power rating, Quell said.

"The concept featured a conventional three-point gearbox support and a medium-speed drivetrain combination, incorporating a compact two-stage planetary gearbox and permanent magnet synchronous generator (PMSG) with a flanged connection between.

"The latter returned as a key drive element in our initial conceptual ideas for a 10MW Optimus turbine, the Optimus 200 with a 200-metre rotor diameter and 318W/m2. The overall solution was developed as a concept by masters students in 2017."

The current Optimus 200XL is at a much more advanced development stage, Quell added.

It again operates with pitch-controlled variable speed and has a state-of-the-art 90m/s rated tip speed.

And, right from the start, the overall turbine configuration focused on dealing with high-wind North Sea IEC 1B conditions.

The students had to work on their thesis project under stringent conditions, including a timeframe of only one three-month semester.

They therefore made the most of the break between the second and third semester to develop ideas for upgrading and optimising the Optimus 200 to the 200XL.

"The Optimus 200 and 200XL medium-speed geared drivetrain concepts are functionally similar, but the upgrade to 12.5MW without changing the rotor size increases the input torque by 25%," said project leader Ahmed Abdel-Moneim Hassan.

"This required an extensive evaluation of all drivetrain elements, especially on substantial machine bed frame and gearbox strengthening and switching to a matching higher rated generator.

"Medium-speed has become a main offshore trend, and, in our view, currently represents the highest efficiency at moderate investment costs."

It also offers enhanced reliability performance due to the elimination of the high-speed parallel gear stage, and therefore should reduce Opex, contributing to a competitive LCoE level, Hassan added.

The unusual Optimus 200XL rotor support consists of a wide cast housing located directly above the six-metre tower top. Inside is a hollow shaft that is seven metres long, with a two-metre outer diameter and two bearings.

"The front bearing is a CARB type, a self-aligning solution from SKF capable of allowing modest shaft deflections like a spherical roller bearing plus limited axial play like a cylindrical roller bearing," according to Hassan and head of system integration Ahmed Ashour.

"The rear rotor bearing is a common double taper-roller type that absorbs both axial and radial rotor-induced loads. This overall solution, in our view, offers the highest reliability performance."

Two innovations

A key benefit of the main bearing layout is that the complete medium-speed gearbox/generator assembly could be mounted behind the tower, according the students.

The rear of the Optimus 200XL rotor shaft housing therefore has a drivetrain-housing mounting flange.

The rotor shaft is attached to the gearbox input shaft via an elastic Geislinger Compowind coupling, a design measure aimed at preventing rotor-induced non-torque loads entering the gearbox.

"The 3.3kV PMSG comes with a full converter, and the generator voltage for the Optimus 200XL is stepped up to 66kV in an oil-cooled transformer.

"Both components are located inside the nacelle to minimise power transport losses down the tower and to the wind farm’s offshore high-voltage power collection station," said Hassan.

This overall drivetrain layout inspired the student teams to introduce two major innovations before they started the Optimus 200XL project.

The first is moving the converter and MV-transformer from inside the nacelle rear to two elevated levels atop the rotor-bearing housing close to the tower axis.

A main benefit is that it minimises accelerations and dynamic excitations of these vibration-sensitive electrical components, whereby a separate intermediate mounting platform was created for the MV-transformer due to its overall size and height.

The second main innovation involves several interconnected elements. "The basis is a welded fabricated steel machine bed creating the second higher elevated level, which accommodates the converter," Ashour explained.

"It is, in parallel, used as a crane boom-mounting platform for a permanently installed service-support crane with two main functions.

"The first is deployment for various service tasks, including regular O&M activities and rotor blade inspections.

"It will be used in parallel for the temporary installation of a main crane-hoisting system with 150-tonne capacity for exchanging the gearbox and the generator."

Inside… A second elevated level houses the converter, the service-support crane and acts a crane boom mounting platform

Crane solution

An electric winch forms an integral part of the crane solution. This device is fitted at a tower bottom mounting area, while the pulley hooks and all fastening gear will be temporarily installed on top of the crane boom.

The innovative lifting concept allows the lowering and lifting of a complete gearbox/generator assembly in case of a major failure.

A key claimed benefit of the lifting solution for main drivetrain components is eliminating the deployment of costly jack-up vessels in the harsh marine environment.

Hassan and Ashour say they gained much from the project and now know a great deal more about how a large-scale wind turbine functions.

They have some clear ideas regarding the future of the Optimus 200XL, especially the potential for further optimising.

"We were operating a good-functioning controller, but it lacks advanced recent features like enabling individual pitch control.

"Therefore, we had to work with relatively high design loads, resulting in a 745-tonne head mass.

"That means there is still a lot of potential for mass reduction. Also, the current blades weigh 65 tonnes each due to a decision not to use carbon."

A modified controller and the use of carbon could cut 10-15 tonnes from the weight of each blade. They believe an additional 60-80 tonne mass saving in the nacelle’s structural part is realistic as well.

Working together

Team work was a key aspect of the project. Project managers were elected and pre-determined project tasks, such as electrical system, innovative lifting solution and drivetrain upgrade, were assigned to individual teams of up to four members, according to Hassan and Ashour.

Of the 32 students, nine came from Germany and the rest from Asia, Africa and Brazil, so adapting to different cultural backgrounds when tackling various design-related issues was an important part of the team work and the project as a whole.

Before joining the course, Egyptian-born Hassan and Ashour completed a five-year mechanical-engineering bachelor with specialisation in energy and renewables in their country.

Egypt envisages a much greater role for renewables, including wind power, which means many experienced professionals are needed in the short term. This led the pair to enlist on the course in Germany.

Potential

Quell said the Optimus 200XL is an economical and potentially attractive technological concept for future offshore wind, although a lot of work clearly still needs to be done before it can be turned reality and, ultimately, a competitive, market-ready product.

"For the FH Kiel and Flensburg, the Optimus project continues to offer a great learning experience for international engineering students and a promising basis for future research projects.

"The positive responses from outside support our view that we are on the right track with the turbine concept and ideas to further lowering offshore LCoE", he concluded.

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