Tildy Bayar, Associate Editor, Renewable Energy World
June 19, 2013 | 1 Comments
LONDON -- Small wind turbines (SWTs) less than 50 kW are playing a growing role in electricity generation, and are now being deployed in domestic, commercial and agricultural scenarios. The World Wind Energy Association (WWEA) predicts that the global SWT market will increase from 95 MW in 2011 to 700 MW in 2020, growth of approximately 25 percent per year. But market share has remained small when compared to domestic-scale photovoltaics (PV).
Dr Nigel Jakeman of UK inverter manufacturer GenDrive believes this limited market share has restricted supply chain options for SWTs compared to large wind turbines. To address this problem Jakeman has spearheaded Optiwind, a new research project with the aim of improving the performance of SWTs through inverter optimisation.
Optiwind's research programme will be carried out by an EU research consortium including GenDrive and research partners the UK Intelligent Systems Research Institute and Fundacion Tecnalia Research & Innovation of Spain. Jakeman says the research partners will be supported by a consortium of companies representing all aspects of the small wind supply chain, including turbine manufacture and installation through to inverter and subassembly manufacture.
SWT Inverters Must Come Into Their Own
The small wind inverters that are currently available, says Jakeman, are effectively PV inverters; they are subject to the vagaries of the larger solar market and, crucially, are not designed for wind. "The control you get on most SWTs is appropriate to solar cells, not wind turbines," he explains. "Optiwind's purpose is to come up with a control system that's more appropriate for wind turbines."
Power change for a solar panel is a relatively slow and not particularly dynamic process. When the sun rises in the morning the panel's inverter turns on, varying at a level of operation that's consistent with the day's level of solar radiation, and in the evening it turns off – and that is the extent of the change its control system needs to manage. Wind is much more locally variable: a cloud passing over and temporarily obscuring the sun is a relatively slow event compared to the wind changes experienced at blade point.
A wind turbine speeds up and slows down as quickly as the wind gusts, so its inverter needs to be much more responsive in order to track the wind. "Control systems are all about the speed and accuracy of how you follow a rapidly changing input," says Jakeman.
On a SWT the primary control method is implemented within the inverter software which controls power flow from the turbine. Of course you need the right hardware platform to get the best from the turbine, says Jakeman, but Optiwind will focus on the software. Key intellectual property and performance enhancements are executed within the software; the hardware must simply be able to deliver what the software is designed to do.
"It's about improved dynamics, really – how the inverter responds to a change of wind condition and how it controls its response, how fast it can do it," Jakeman continues. "What you're really trying to do is track an optimum performance curve. There's always an optimum performance point regardless of wind condition; it changes and you try to keep to it. If you don't have that dynamic of control you move away from optimum performance and compromise energy capture."
Optiwind is a two-year project due to end in 2014, and is currently at the initial research stage. "At the moment we're in simulation mode, so we're doing a lot of field trials to benchmark our simulations with real-world data. Then we'll optimise the control we want to achieve, and then put that on a turbine and see what the improvement will be with the control algorithm," Jakeman says.
When asked to quantify the efficiency improvements expected from Optiwind's research Jakeman said, "We have a target to improve SWT performance by 10-20 percent. It's tough to know how realistic that is at this stage, without [having finished] the testing. But for return on investment it's pretty significant."
The key difference between SWTs and high-value, full-sized turbines is that operation of the latter is supported by optimising technology – active pitch control systems, for example, and a variety of wind-tracking sensors – whereas a SWT is "a lot more of a compromise," says Jakeman, because the cost per kW rises as turbine size shrinks. SWTs are largely passive-response systems; many lack blade pitch control and will instead be on a hinge or spring. This means the system's intelligence needs to be located in the inverter – while, at large wind scale, improved aerodynamic control systems are affordable. "SWT solutions are a lot cruder," Jakeman says, "and therefore the inverter needs to be a lot more understanding and intelligent about how it calls power off the system."
According to Jakeman, Optiwind will also save the installer the work of setting up the inverter, moving it closer to being a plug-and-play device, and will help alleviate maintenance issues. "If the inverter is effectively working out the characteristics of the turbine, it can also monitor the turbine," he says. "If you can monitor how optimum performance changes, you can also monitor how potential performance could be degrading and service it appropriately."
Jakeman says Optiwind will offer are three core benefits: first, improved energy yield, which will increase returns for the end user by 10-20 percent; second, time saved for the installer at the site, up to an hour at a time or 5 percent of standard electrical setup time ("not hugely significant but it helps in a competitive market," says Jakeman); and third, service costs avoided. Service cost reductions are difficult to quantify in advance, but Jakeman says "It's all about trying to manage the time a service agent has to spend at the site. Rather than relying on scheduled maintenance, he would be able to tell when a service event is needed, and the optimised inverter will help him determine that."
Lead image via OptiWind