Technology
security lab, was commissioned to perform research in different energy spaces, including renewables.” The VAWT concept then became the focus of
Sandia’s renewable energy research, and led to its involvement in Project Éole in Quebec during the mid- 1980s – the tallest VAWT to date, at 110m in height, though it was shut down in 1993. “From the ‘70s through to the ‘90s, Sandia designed, studied and created tools for studying VAWTs, creating the largest body of knowledge for VAWTs, especially in terms of experimental data,” Ennis notes, until challenges around vertical-axis turbines and the limitations of the technology at the time led most of the industry to turn their focus to HAWTs. The US DoE, however, continued to hold a torch for
VAWTs, due in part to its potential use in areas where conventional wind turbines were unsuitable. While the US has great wind resources over land, these areas are often far away from the main population centres. Offshore wind, despite being considerably more expensive, could provide energy generation near key load centres – namely, US cities based on the east and west coasts.
In 2019, the DoE’s ARPA-E funding office launched a grant programme aimed at developing megawatt- scale vertical axis turbines for offshore wind farms. Sandia was announced as one of the awardees for their floating offshore VAWT project, dubbed ‘ARCUS’ – which takes the Darrieus model and makes some modifications to it, aimed at addressing the issues with traditional versions of the platform. VAWTs offer unique advantages for floating offshore wind, potentially reducing costs as they have no need for yaw systems, and boast improved aerodynamic efficiency and a lower-level placement of the turbine’s drivetrain that greatly reduces floating platform mass and associated system costs, according to ARPA-E. “Floating offshore wind is three to five times more expensive than land-based wind in the US,” says Ennis. “Part of the challenge is that you have a very complicated system. There are significant cost contributions from multiple components – from the turbine, the platform installation, operation and maintenance. […] You can’t really solve one cost in isolation, you have to come up with a design that minimises all of the costs to see meaningful reductions in the total levelised cost of energy (LCOE). VAWTs offer a lot of opportunity to minimise that cost.”
In with a VAWT Currently, Sandia’s ARCUS project has a similar swept area to 12MW and 14MW horizontal-axis turbines, but with a larger generator at 20MW – one of the benefits of a VAWT system is that you can capture higher wind resource energy. However, one of the main challenges holding back the development of modern VAWTs is that it requires switching the architecture of what’s been commercialised and industrialised over the past 20–30
World Wind Technology /
www.worldwind-technology.com
years. Not only do you not benefit from the lessons learned from decades of development that HAWT platforms boast, but you also don’t have the same level of validated design tools.
As a result, before Sandia could set its VAWT ideas in motion, it needed to build software capable of modelling the response of the turbine and floating platform to different wind and sea conditions to determine the optimal design of the whole system. Ennis and his team got to work and created a functional design tool to help Sandia in the design and optimisation of its floating VAWT system. “One of the reasons that we developed this tool is that we needed a couple of dynamics for a vertical-axis turbine,” Ennis explains. “Because it’s such a large system – we’re designing a utility scale, 20MW system – the aeroelastic effects are important; [as are] the couple dynamics, with the hydrodynamics and platform motion. It’s very critical to capture those because they’re very meaningful loads at that scale.” At the same time, Sandia needs to be able to trust the tool, which poses its own challenges. “That’s very tough when you don’t have the data – you haven’t iterated from design to design and validated your tool,” Ennis notes. “So, we’ve done code-to-code comparisons with horizontal-axis design tools to validate the hydrodynamics, the hydroelastic coupling. We actually pulled some old test data from the Sandia legacy VAWT experiments. And we then compared our predictions with a land-based 34m vertical-axis turbine.” The other thing Sandia needed the tool to provide was highly parallel optimisation. “Within our project, we’re performing the design in a different way than what’s done commercially,” Ennis notes. “We’re looking at a concurrent control co-design optimisation, where we’re designing the turbine platform and they’re controlled concurrently. So, to do that, it’s much more expensive computationally.”
As a result, Sandia needed the tool to be used easily on high-performance computing clusters with gradient- based optimisation. This helped Sandia’s ARCUS project to iterate on the traditional Darrieus VAWT format. “We call this design a ‘towerless vertical-axis turbine’. The traditional Darrieus VAWTs have a rigid tower that rotates with the blades as part of the rotor. As we were starting this project, we were trying to understand how we could remove mass that’s not directly capturing energy,” he notes. “For floating offshore wind, that’s a compounded effect, because that mass must be supported by additional mass on the platform itself.” Sandia’s towerless Darrieus VAWT replaced the central tower with tensioned guy wires, resulting in up to 50% lower rotor mass compared with earlier rigid-tower designs. Rather than eliminating turbine motion, Sandia’s design allows the oscillating turbine- platform system to operate safely under extreme weather conditions.
1 October
1926 Georges Jean Marie Darrieus patents the Darrieus vertical- axis wind turbine.
1971
US DoE assigns Sandia to investigate alternative
energy sources. 1973
Arab oil embargo creates further demand for alternative sources of energy.
Mid-1980s
Project Éole is constructed in Montreal, Canada – the largest VAWT in the world.
1993
Project Éole is shut down.
2019
US DoE’s ARPA-E funding office launches a grant programme aimed at developing megawatt-scale VAWTs for offshore wind farms.
2020 Sandia is
announced as one of the winners of this programme for its ARCUS project.
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