Three years later he developed a 30kW turbine on a farm where he did his civilian service (an alternative to compulsory national military service). In 1984, the first 25kW Aerodyn two-bladed turbine, developed with government financial support, was installed on the North Sea island Pellworm for field-testing.
"During a comprehensive 1.5-year measurement programme we accumulated more useful measuring data than the EUR50 million (German government) Growian turbine research project generated," says Siegfriedsen. "Results were used as inputs for Germany's first legislative framework on wind turbines."
Siegfriedsen estimates that Aerodyn-developed and licensed products account for a cumulative 31,000MW installed base, representing 12.3% of the world's total. This is second only to the 50,000MW installed by market leader Vestas. The company employs seventy staff in offices in Germany and China.
In 2005 Aerodyn was contracted by Bard Engineering to develop a conventional high-speed geared 5MW offshore turbine, which was completed in a record nine-month period and included rotor blades and tower. Bard 5.0 turbines are being employed at a major 400MW North Sea wind farm currently under construction.
Back in 1996 Aerodyn conducted initial studies for a new 5MW offshore turbine. Featuring a compact fully integrated low-speed drive system this innovative design, called Multibrid (Multi-megawatt and Hybrid), represents the world's first hybrid between traditional high-speed geared drivetrains and direct drive turbines (no gearbox). Introduced in 1997, it took two product owner-licencees several years - by late 2004 - before a M5000 Multibrid prototype was installed. Areva, current M5000 technology owner, installed six offshore-optimised turbines in the German Alpha Ventus project during 2008-9, and another 120 units are being built at the moment for two more offshore projects.
Reflecting on Multibrid, Siegfriedsen said: "Multibrid has proven to be a generally good design. We learned a lot from M5000 detail-engineering efforts conducted mainly in 2001, and extensive measuring programmes between 2004 and today, but with today's knowledge we would no doubt have done some things differently."
This statement refers to an ambitious development called super compact drive (SCD), with a full-size 2.0/3.0MW nacelle first displayed at the Husum 2007 wind fair. SCD turbines build on Multibrid technology. The designs share overall drive train compactness and permanent magnet generator (PMG) use.
One distinct difference is a shift from the Multibrid's single-stage planetary gearbox to a two-stage planetary gearbox in SCD turbines, enabling faster rotation and a smaller-size PMG with substantially reduced demand for rare earth materials. With the current high price of permanent magnets, this measure contributes to lower investment costs and cost of energy.
"Another main difference is that SCD gearboxes and generators are individual components featuring flanged housings with similar outer diameter," says Siegfriedsen. "These factors together enable cost-effective manufacture and easier component exchange. We fully own SCD drivetrain technology including all drawings, calculations and IP rights."
The two-bladed SCD turbines come in power ratings of 3MW and 6.5MW and Siegfriedsen speaks of substantial global wind market interest in both models.
"We have awarded Ming Yang an exclusive dual-licence for China and project licences for other countries," he says. "But before issuing additional licences in future we consider it crucial that all design, simulation, testing and verification processes are successfully completed. The SCD 3.0 features a rigid upwind rotor and in total eighteen units are in operation with another 12-15 turbines under construction or awaiting grid connection."
The 6.5MW SCD 6.5 is a downwind turbine with 140-metre rotor diameter and a head mass (nacelle + rotor) of 312 tonnes, compared with 350-400 tonnes for some competing latest generation 6MW turbines. A prototype is planned for early 2013.
"For both model versions we have chosen a rigid rotor design, as experiences with flexible teeter hubs have been shown to be problematic in extreme conditions," says Siegfriedsen. "China has a huge offshore wind potential, and especially the coastal stretch between Shanghai and Hong Kong is attractive due to a well-developed onshore grid network. But this region is also infamous for typhoons, and offshore turbines thus have to be made typhoon-proof. With a typhoon approaching the rotor is locked in horizontal position. Due to a combination of downwind rotor orientation and released hydraulic yaw-system a SCD 6.5 rotor can yaw freely and easily follow even rapid wind direction changes causing minimised structural loading."
Regarding the future of wind power, Siegfriedsen says he would love a first 10MW turbine customer, but simultaneously stresses the long period required to develop and optimise 5MW turbines.
"In the past 30 years huge progress has been made, but each next up-scaling step involves new and often substantial challenges," he says. "One potential issue is that with growing rotor diameters eigenfrequencies decrease while blade mass increases, and the combination could create stability issues. Structural components' deformation in very large wind turbines might potentially become another limiting factor. Whether 20MW as some believe could represent a technical upper limit and 10MW perhaps an economic limit remains to be seen, but so far most predictions have proven to be false."
EXPERIENCE AND KNOW-HOW - AERODYN'S INNOVATIONS FROM 1983 UNTIL THE PRESENT DAY
Since 1983 Aerodyn had assisted multiple wind industry third parties with complete wind turbine designs and main component parts - through licence agreements and product developments.
In 1989 Aerodyn developed a two-bladed 100kW turbine and a 250kW turbine. Larger 600/750kW models were introduced in 1994 after wind market upscaling. Both a 750/1000kW turbine and a first variable-speed pitch-controlled 1.5MW turbine came in 1998, followed by a still expanding range of technologically related 1.5/2MW products, which found their way to clients, particularly in Asia. During this time, Aerodyn also developed two direct drive kilowatt-class turbines for different clients.
The company now offers standardardised Aeromaster licensed products - wind turbines with 1.5MW and 2.5MW power ratings, and a recent 5MW model, each comes with two or more different rotor diameters. The Aeromaster turbines comprise a conventional main shaft with two bearings, a three-stage gearbox, and either a doubly fed induction generator or synchronous PMGs. The 5MW Aeromaster series has a 130-metre rotor diameter for IEC Wind Class I sites and 139 metres for Class II - it already has two customers from China and South Korea.
Aerodynblade rotor blades are available for 1.5MW, 2MW, 2.5MW and 3MW class turbines. The longest 3MW blade is 57.7-metre long, resulting in a 117-metre rotor diameter. Siegfriedsen has clear views on what could be 'ideal' ratios between rated power and rotor swept area with regard to turbine optimizing. He does not, for instance, fully agree with the current large blade trend, whereby the latest 6MW-class WC I turbines are fitted with rotor diameters in excess of 150m. He believes a 140-metre blade would work better with a 6MW without compromising on cost of energy performance.