Optimising the placement of turbines within a wind farm can significantly increase energy extraction, but only up to a point.
Researchers at the Carnegie Institution for Science in Washington DC have revealed by computational study that power generation efficiency plummets once wind farms get beyond a certain size.
They investigated wind turbines’ effect on the airflow around them and, consequently, the ability of nearby turbines to extract energy from that airflow.
While small wind farms can achieve a power density of more than 10 watts per square metre, this can drop to a little as just 1W in very large installations
The first law of thermodynamics dictates that turbines must reduce the energy of the wind that has passed through them. Turbines also inject turbulence into the flow, which can make it more difficult for downstream turbines to extract energy.
“People were already aware of these issues, but no one had ever defined what controls these numbers,” said post-doctoral researcher Enrico Antonini.
In their research, Enrico and emeritus colleague Ken Caldeira used fluid dynamic models to examine the airflow over turbines.
First, they looked at the individual “wakes” of each turbine, studying different arrangements.
Reducing the turbine density in small wind farms provided a higher output per turbine. They also found that arranging the turbines in rows facing the wind or in a mosaic pattern provided a 56% greater power output than arranging them in wind-facing columns.
“Gains in energy generation can be 10, 20, even 30% if you have an optimal arrangement of turbines with respect to a random arrangement as you can reduce this wake interaction between the turbines,” said Antonini.
These gains largely vanished, however, as wind farms grew very large. To find out why, the duo used meteorological simulations of wind speed.
They found that, whereas in a small wind farm the principal cause of loss of output of a wind turbine is the wake of its near neighbours, which can be mitigated by careful design, in a large wind farm the surface wind speed is slowed down by the greater drag of that “region”.
“For large wind farms, there is a limit on how much energy can be replenished on the order of 1W/sq m,” says Antonini. “The turbines create a kind of uniform wake across the wind farm.”
The wakes of these large farms could extend tens of kilometres downstream, potentially affecting other wind farms.
Turning to floaters
DNV is leading a research project to explore the effects of wake steering on floating offshore wind farms, the first of which is installed off the Scottish coast.
Innovate UK in co-operation with the US National Offshore Wind Research & Development Consortium (NOWRDC) is funding the programme.
The objective is to reduce the levelised cost of energy (LCoE) of floating wind by investigating the effects of using wake steering, a wind farm control strategy.
In the UK, DNV, Durham University and Marine Power Systems are combining their expertise for the project, which is called CONFLOWS (CONtrol of FLOating wind farms with Wake Steering).
The US team, led by the National Renewable Energy Laboratory (NREL), in partnership with Cornell University and Equinor, will focus on specific regions of North America.
The consortium will share data and knowledge beneficial for the modelling of site-specific meteorological conditions and complex wind farm wake scenario.
“Steering” deflects each turbine’s wake away from downstream turbines, allowing increased overall power production, and a longer lifetime of the turbine through reduced fatigue damage.
DNV’s Pierre Sames says research is required in order to understand whether technology for onshore wind farms can deliver similar impacts on improving energy production of offshore floating turbines.
Game-changer…again
Time and again the expression game-changing pops up in technology development, not least when it comes to the hunt for the ultimate energy storage device – the super-battery.
There are many projects around the world where teams of scientists are seeking to trump everything that has gone before.
What is different about a battery research project currently going on at the Otago University in New Zealand is that it is focused on sharing battery use across communities.
© Shutterstock / petrmalinakAccording to Associate Professor Michael Jack, director of the energy programme at Otago’s physics department, reducing costs are encouraging a rapid build-up in household deployment of batteries, mostly aimed at storing solar and wind power.
But they could also could have a variety of uses in a future electricity grid.
“They could be used to feed energy back into the grid when there is a shortfall in renewable electricity supply,” Jack said.
“Or they could allow a house to reduce its demand on the grid during times of constraint, thus reducing the need for expensive new lines.
“As we move towards more renewable energy, and increase our use of electric vehicles, these services would be beneficial to a local community and the national grid, not just the individual house with the battery.”
The Otago team has been researching the capacity a battery would need to have to keep the peak demand below a certain value for both individual houses and a group of houses.
They considered both load smoothing around the average, and peak shaving, where the battery ensures grid power demand does not exceed a set threshold.
“Our key result is that the size of the battery required for this purpose is much smaller — up to 90% smaller — if the houses are treated collectively rather than individually,” Jack said.
“For instance, if peak shaving occurred for demand above 3kW per house, deploying batteries individually for 20 houses would require 120kWh of storage, whereas deploying them collectively would only require 7kWh.
“Sharing batteries or having one battery per 20 houses will be a much cheaper approach to providing these services.
“Another important finding is that, as peaks are mainly in winter, the battery would still be largely available for storing energy from solar cells in summer, so this would be an additional service and not competing with the main use of the battery.”
But there’s another twist.
In the future, many households may have batteries and be using these, or batteries within their electric vehicles, to provide services to the grid.
These batteries and other appliances in homes and businesses would have smart controllers that enable them to reduce demand or feed electricity back into the grid to accommodate the fluctuations of variable renewable supply and minimise the need for grid infrastructure.
People responding in this way would be paid for their services to the wider grid. It should enable a much lower cost, collective route to decarbonising energy systems worldwide.
Making waves in California
The US Department of Energy (DOE) will provide as much as $27 million in federal funding for research and development projects designed to advance the efficient conversion of wave-based energy into electricity.
Focal point will be the PacWave South test facility, which is the first accredited, grid-connected, pre-permitted US open water wave energy test facility. It was built in 2016 by the department and Oregon University.
The funding opportunity will focus on three topic areas:
• Testing wave energy converter (WEC) technologies at PacWave (up to $15 million in federal funding): This topic area focuses on testing of WEC systems intended for remote and microgrid applications, as well as open-source systems that aim to generate publicly available data and knowledge to benefit the entire converter sector.
• Advancing WEC designs (up to $5 million): This area will support the development of robust WEC systems to generate off-grid or grid-connected power. By the end of the award period, the systems designed would be ready for fabrication, deployment, and prototype testing at PacWave South.
• Wave Energy R&D (up to $7 million): This area will directly leverage the PacWave test facility to perform impactful wave energy R&D that will advance the marine energy industry as a whole. This topic area will support projects that advance converter systems, components, environmental monitoring technologies, instrumentation and prognostic health monitoring technologies, wave measurement equipment and other supporting technologies.
The various projects selected will be tasked with generating open-access data that will benefit the entire wave energy converter R&D community.
Data will include wave, wind, and ocean current resource measurements, geotechnical measurements, and environmental monitoring measurements.
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