A team at Oxford Brookes University has shown that wind farms could perform more efficiently by substituting traditional propeller-type designs for compact vertical axis wind turbines (VAWTs). When arranged in grid formation, these turbines can increase each other’s performance. Elly Earls speaks to two of the researchers behind the study, professor of engineering materials Dr Iakovos Tzanakis and former master’s student Joachim Toftegaard Hansen, to find out how to help speed the green transition.
In 2019, Joachim Toftegaard Hansen was in the final year of his mechanical engineering degree at Oxford Brookes University. He was casting around for a suitable dissertation topic, when Ørsted, the world’s leading developer and operator of offshore wind farms, downgraded their anticipated rate of return for several projects in Europe and Taiwan from 7.5–8.5% to 7–8%.
As the wind approaches the front row of turbines in a modern wind farm, turbulence is generated downstream, which is detrimental to the performance of the subsequent rows. And while the industry’s standard measurement model allows for this to some extent, Ørsted realised that wake turbulence had a higher negative effect on production than earlier models had suggested. Half a percentage point may not sound like much, but it added up to billions of pounds.
Not only did the announcement lead to a fall in the company’s shares, the management stressed this discovery could affect the entire industry. Hansen was intrigued. Was there a way to organise wind farms to minimise this effect and increase the amount of energy generated by each row? Or could a different turbine design be the answer?
Most wind farms today are made up of horizontal axis wind turbines (HAWTs) – the standard three-blade ‘pinwheel’ design. But what about vertical axis wind turbines (VAWTs), which spin around an axis perpendicular to the ground and, unlike HAWTs, can capture wind from any direction?
VAWT or not
The biggest VAWT that has been built to date is the 110m tall, 3.8MW EOLE turbine in Quebec, Canada, which was put out of service in 1993 when its rotor bearing failed under the 880t weight it had to support.
Since then, funding has largely been channelled into HAWTs, which have grown to 16MW offshore, positioning them as the clear favourite in the choice for wind power installations. But could the industry be missing a trick? Hansen consulted with his supervisor, Dr Iakovos Tzanakis, a professor in engineering materials whose research is focused on renewable energy, and they agreed to investigate.
“We really didn’t know where to start,” Hansen admits. “I had an idea that I wanted to do computational fluid dynamics (CFD) simulations, which are numerical calculations of how air moves around these wind turbines, but we were also thinking about doing experimental studies to back up our numerical model.”
They soon realised just how complex an undertaking this would be. “It would be a PhD project just doing one experimental measurement of [VAWTs],” Hansen laughs. “And we simply didn’t have time for that.”
So, they found a middle ground, carrying out numerical simulations at Oxford Brookes and backing this up with well-recognised data from the US. A team at Cal Tech had already carried out some wind experiments, which verified the Oxford Brookes team’s models.
The findings were striking. Using more than 11,500 hours of computer simulation, Tzanakis and Hansen demonstrated that wind farms could perform more efficiently by substituting traditional designs for compact VAWTs. When set in pairs, the VAWTs could increase each other’s performance by up to 15%.
“In wind farms made up of HAWTs, the efficiency of the front row depends on various factors including wind direction and turbulence. Then the efficiency of the subsequent rows depends on how much turbulence is generated by the front rows,” explains Hansen. “With VAWTs, they increase each other’s efficiency, but the front row also has an increased performance. So, all rows within the wind farm exhibit this performance augmentation.”
“While we know that HAWTs are more efficient on an individual basis than VAWTs, the thing that changed the game here is that when they operate in a farm scenario, one turbine next to the other at a specific angle and distance, the overall efficiency in the VAWT farm is better,” Tzanakis adds. “When working in harmony, one enhances the other.”
Engineering challenges
Since the 1990s, when the EOLE VAWT was decommissioned, most funding for wind turbine development has been channelled into HAWTs. “They’re easy to manufacture, you can make them very big and they’re easy to scale – you increase the diameter and get twice as much power,” Hansen explains.
Early VAWT projects tended to fail for a few reasons, which include problems with directly supporting such heavy weight on a single bearing and poor self-starting capacity. “At lower velocities, depending on their design, VAWTs may need a little push to get going because they do not generate enough torque to overcome the initial frictional forces,” says Hansen.
As Tzanakis stresses, these are engineering issues he believes can be solved with new designs and materials. “There are many small SMEs that have come up with some brilliant designs for VAWTs, but these machines are extremely expensive to manufacture,” he says. “There’s also a mindset issue. If something works well, like HAWTs, and big companies are in the routine of manufacturing these turbines, it’s difficult to change that.”
The pair hope their study may go a small way towards shifting that mindset. In fact, they’ve been shocked at the number of SMEs that have come forward since it was published, hoping to partner with them and have their VAWT designs included in further simulations. These include SeaTwirl, Vertax Wind, Headwind Technologies and KyneticEnergy.
Oxford Brookes has also developed a consortium of universities and companies interested in VAWT technology. The plan is to progress from 2D to 3D simulations of VAWT wind farms, as well as small-scale experiments in wind farm facilities.
Speeding the green transition
According to the Global Wind Report 2021, the world needs to be installing wind power three times faster over the next decade to meet net zero targets and avoid the worst impacts of climate change. Crucially, the goal of the Oxford Brookes study was never to advocate for the replacement of HAWTs with VAWTs; it was to speed the green transition and find a more cost-effective way to meet wind power targets.
One viable option could be hybrid wind farms. “Operators may consider having smaller VAWTs between HAWTs in existing wind farms because they can utilise the turbulence better,” Hansen suggests. VAWTs are also more suitable for deep sea regions than their horizontal counterparts as they frequently perform better in stressful weather environments.
They are easier to maintain than HAWTs because their gearboxes, generators, and most electrical and mechanical components are at or near sea level, avoiding the need for climbing gear, lifts and expensive cranes. They’re also easier to keep out of sight of the general public.
“I think wind turbines are a beautiful sight, but not everyone agrees. Because VAWTs can be positioned far out from the shore, the general public don’t have to look at big towers of white composite metal in the sea,” Hansen says.
However, there are still many unanswered questions about VAWTs. For example, what happens when they are scaled up in size; does this further increase efficiency or the opposite? “Currently what we’ve done is use 20m diameter rotors, but what if you had 100?
It’s a question we don’t have the answer to yet,” says Hansen. With just one 2D study under their belts, the researchers are a long way from the large-scale roll-out of completely new wind farm designs. However, their research has provided VAWT manufacturers with new hope for the wider use of their designs. Ørsted, and other wind industry leaders looking for ways to work around wake turbulence, have no doubt taken note.
This article first appeared in World Wind Technology Vol. 2 2021. The full publication can be viewed online here.
