One panel and a multitude of sessions dealt with issues such as gearbox failures; improved turbine designs; scaling up wind turbines even further; tower design; offshore wind; and reducing maintenance costs. Another session discussed the improvement in the US supply chain in recent times – there has been an increase in the share of U.S. sourced components from 20% to 50% over the last five years. But plenty of opportunity still remains for North American turbo machinery component suppliers who are willing to adjust to the peculiarities of this expanding industry.
“The large forging and casting capacity of the U.S. is much less than it was at the end of World War II,” said Bob Gates, Senior Vice President for Commercial Operations at Clipper Windpower. “While other countries may be able to build things more cheaply, American strength is innovation – coming up with better ways to do things.”
Another big debate concerned direct drive generators (DDG) versus geared turbines, with Siemens being the primary advocate. It has invested heavily in a direct drive model. After ten years of development it now has a 2.3 MW DDG turbine in production and operation with a 113 metre blade diameter, along with the SWT-3.0-101 DDG turbine, which is also in production and operation. This direct drive technology can be used for low to high wind speed conditions.
“We need direct drive to lower the cost of energy, removing losses in efficiency by not having a gearbox and reducing complexity,” said Michael Revak, Vice President of Sales and Proposals at Siemens. “DDG has 50% fewer components than geared turbines.”
Revak noted that Siemens continues to manufacture geared turbines and has no plans to discontinue them.
None of the other vendors present expressed any desire to investigate DDG. All remained committed to geared designs along with taller towers, large rotors, SCADA monitoring and better site selection.
“Gearbox designs are fundamentally good, but it is the components and quality control that cause the problems,” said Gates. “We are designing our turbines to work well in lower wind sites, while increasing reliability, and adding intelligence to head off problems before they require major repairs,” he added.
Turbine validation and testing
The wind industry receives significant Department of Energy (DOE) backing, in particular from the National Renewable Energy Laboratory (NREL). For example, Siemens' SWT 2.3-101 turbine is undergoing performance testing at NREL's site in Golden, Colorado. A study is looking into load response; structural characteristics; noise emissions; aerodynamic performance; and the ability to handle severe weather.
The complexity of wind turbine loads was emphasised further by a maths-heavy presentation by Emil Hedevang from Aarhus University in Denmark. He sliced and diced the numbers to prove that existing wind industry estimates of variations in power output were off by several per cent, due to variations in mean wind speed and turbulence.
He calculated a way to slightly adjust the standard deviation in order to more accurately reflect actual power output. His goal is to finalise an industry standard for the prediction of power curves which can be broadly accepted across the field. This will aid, he said, in turbine output comparisons.
NREL, in fact, has formed a Gearbox Reliability Collaborative (GRC) among manufacturers, owners and researchers. As part of its work, GRC is comparing in-field gearbox response with extensive gearbox tests in the NREL 2.5 MW dynamometer test facility.
The gearbox has been something of an Achilles heel for wind with several manufacturers having to spend millions changing out faulty gearboxes. While it is a couple of years now since the last flap of this kind, owner confidence in the gearboxes' supposed 20-year lifespan is not high.
“We are carrying out these tests so we can uncover problems before they get out into the field,” said Brian McNiff, a gearbox testing engineer with McNiff Light Industries. He is working with NREL on ways to better approximate actual field loads within the test facility:
“Rotor forces have an appreciable effect on ring gear and planet bearing load distribution,” said McNiff. Ongoing analysis and modelling efforts will cover the detailed interactions of gears; shafts; planet carriers; bearings; and housing structures.
Meanwhile, researchers at the University of Milwaukee have been delving into ways to reduce mechanical gearbox stress in variable speed wind turbines. As the gearbox is one of the most expensive components in the wind turbine, they aimed to address problems such as variable torque on the main shaft and the high-speed shaft which can cause vibration at resonant frequencies. They developed a stress reduction strategy based upon active damping via a stress damper controller that eliminates these resonant frequencies. This is achieved by using filters, each tuned to one particular frequency. The next step is implementing this approach on a turbine to see how it performs in the field.