'Velocity deficits' equal to 10% drop in wind speed modelled by ArcVera at zones in US Atlantic in the frame for arrays built around ultra-large class, 12MW-plus units

Source: Recharge News | By Darius Snieckus

The wind industry’s Olympian drive to continue upscaling offshore turbine size could come under the microscope after new calculus concluded conventional engineering modelling of ‘wake’ – the turbulence created by rotor blades as wind moves through an array – has “vastly underpredicted” energy losses linked to so-called “external wakes” – particularly for the ultra-large-class machines now heading for the water.

The work by consultancy ArcVera, which studied long-range wake loss potential at project development zones off New York in the US Atlantic, found “velocity deficits” as high as one metre per second – equal to a 10% drop in wind speed – “could persist up to or greater than 100km downwind” of offshore developments using 12MW-plus models.

“This new study provides an important cautionary lesson as the wind industry proceeds to ever-larger wind turbine models with greater farm density across the globe,” Greg Poulos, CEO of ArcVera, said of the findings, which used high-fidelity Weather Research and Forecasting (WRF) numerical weather prediction modelling run with wind farm parameterisation (WFP) software to factor in the effects of turbines at a site

“WRF-WFP’s results here show that engineering wake... models currently under-predict long-range wake losses by a significant margin. Unexpected losses are likely to accrue from wind farms once thought to be too far away to be material to project performance.”

Mark Stoelinga, head of Atmospheric Science Innovation at ArcVera, added: “This [underestimate] is leading to long-range energy deficits much greater than expected by most subject matter experts in the industry.”

The research, scoped over a swathe of the New York Bight, where the state held a record-setting, $4.4bn leasing round this year, was sparked by the fact that engineering models commonly used in the wind industry to this point have been validated internal and “nearby” external wakes but not over long distances or for large nameplate, 12MW-plus, machines with 200-metre-plus diameter rotors.

ArcVera noted that WFP in WRF “has been validated against SCADA recorded production for an [in-house] onshore case, and it was accurate with respect to long-distance wakes within 16% at a 5km, 50-metre rotor diameter range”.

“In the onshore validation study that we conducted in Iowa, USA, wakes were found to travel over 40 km overland, in stable atmospheric conditions. Over the ocean, it is common for atmospheric stability to be enhanced, especially when warm air flow passes over colder underlying water,” said Stoelinga.

“We also surmise that the very large turbines used in the study produce unusually strong wakes that cannot easily recover their lost momentum, especially under enhanced atmospheric stability conditions.

Consultancy DNV in 2020 flagged the impact the so-called “blockage effect” and wake in general could have on the overall economics of offshore wind power.