Wind forecasts ‘biased’ as blockage ignored, warned DNV GL

Wind forecasts ‘biased’ as blockage ignored, warned DNV GL

Experts at DNV GL warned last year that ‘blockage effects’ – one of the factors cited by Orsted in its downgrade of long-term offshore wind production forecasts – were being ignored, skewing forecasts on the upside.

A paper from the global energy group on efforts to better understand the impact of blockage effects said dynamic flow modelling to date has almost exclusively concentrated on wake effects between turbines.

Blockage effects – where wind slows in front of a turbine – are tougher to observe and measure, said DNV GL in a 2018 paper on the topic.

“Compared with wakes, blockage effects, and the gradual wind speed changes associated with them, resist direct observation. Nevertheless, a growing collection of evidence points to the pronounced influence of these effects,” said DNV GL.

“Blockage effects also cause front row turbines to produce less energy than they each would operating in isolation. Wind energy prediction procedures in use today ignore this effect, resulting in an overprediction bias that pervades the entire wind farm,” it added.

DNV GL’s warnings were cited by Orsted as a prompt for the major review of impact of both wake and blockage effects that led it to downgrade long-term production forecasts and project return expectations.

“We can see that the modelling used up to now has not been sophisticated enough,” admitted CFO Marianne Wiinholt as the offshore wind leader shaved 2% off lifetime load factor forecasts for part of its European portfolio.

Philip Totaro, CEO of specialist wind technology and IP consultancy Totaro & Associates, said the industry will over time find solutions to better predict power output, with Orsted now benefiting from data gathered from major projects.

Totaro told Recharge: “Today there are approximately 19 separate research projects underway globally, most of which are collaborative between universities and industry, on wind turbine wake effect mitigation for both onshore and offshore wind.

“Orsted themselves have been investigating this issue since 2014, but they now have more sufficient data from parks which have been commissioned in the past few years.”

For blockage effects, Totaro said: “There is really nothing you can do about it. It is a fundamental characteristic of a compressible fluid such as air. So from this perspective, you can only model it in a more accurate way.”

Totaro added: “The implications of this are that power production will be a bit lower than predicted, so having a more sophisticated model will be helpful.

“The real problem is that project finance levels, including the term and tenor of project Capex loans, were fixed using the old power production methodology, and this will have a potential impact on the payback period and could diminish subsequent investor interest in existing projects.”