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Durham University

Durham Energy Institute



Offshore wind energy yield wake-up call

(8 November 2019)

Durham University's Dr Majid Bastankhah has been quoted on his research into the impacts of wind farm blockage effects and farm-to-farm interaction in an article by REnews in connection with Orsted’s decision to downgrade group wide offshore wind production forecasts. The article highlights the importance of this area of research to the wind enegry sector.

You can read the full article here which has been republished with permission from reNEWS. 

The article was originally published in the subscription only renewable energy publication reNEWS issue 419


Offshore wind energy yield wake-up call

Orsted’s decision to downgrade group wide offshore wind production forecasts last week has left the industry racing to find new and better ways of calculating energy yields. The Danish developer said an under-estimation of blockage and wake effects led to a rethink of lifetime load factors across its entire global portfolio to around 48%, a cut of two percentage points.

“We believe this is an industry-wide issue because we have benchmarked our production estimates against external consultants’ estimates,” said Orsted chief financial officer Marianne Wiinholt.

“The models that we have been using are largely the same as those used by the rest of the industry and they have not been sophisticated enough, especially when taking the effects of neighbouring wind farms into account,” she added.

The developer said the un-levered lifecycle internal rate of return (capacity weighted average) has been reduced half a percentage point to between 7% and 8% for Borssele 1&2, Hornsea 2, German Cluster 1 and Gode Wind 3&4 in Europe, Greater Changhua 1&2a and Greater Changhua 2b&4 in Taiwan, and Revolution Wind in the US.

The changes affect some 5.5GW of projects due to start construction from next year. While a shock for the industry, the downgrades come as no surprise to the academic community where blockage effects and so-called farm-to-farm interaction have been a hot research topic since 2016.

“We have known about these issues for a while but are just starting to gain a fuller understanding of them,” Dr Majid Bastankhah, assistant professor in mechanical engineering at Durham University, told renews.

A paper published this year on farm-to-farm interaction suggests the effect of a wind farm’s wake can result in a 2% reduction in wind speeds 20km downstream, depending on atmospheric conditions. Owing to the cubic relationship between wind speed and power, this

translates to a 6% loss in output. Bastankhah added the distance needed for wind speeds to recover is larger than previously thought. Accounting for the phenomenon will add another layer of complexity to the planning and development of future offshore wind farms.

Crown Estate business development manager Jonny Boston said the issue is an “inevitable consequence of an ever-busier seabed” but added it will not impact site selections in the ongoing UK Round 4 leasing exercise. “However, we will continue to work with the market to gather data and evidence to inform future development,” he said.

DNV GL published a report into blockage effects in 2018 that challenged the prevailing industry logic about how turbines within an array interact with each other. “Previously it had been thought spinning rotor blades only really altered airflows in a way that affected turbines downwind,” explained principal researcher Jim Bleeg. The blockage effect is caused by air approaching a turbine slowing down as it diverges around spinning rotors.

“This upstream effect was previously considered observable up to two and a half rotor diameters in front of each turbine and discounted from calculations as standard spacing is five to eight rotor diameters,” Bleeg explained.

Estimates suggest the blockage affect can hit output from upstream turbines by up to 4% depending on a host of factors including project size and atmospheric conditions. The effect is likely to challenge developers to get creative with the shape and layout of future projects but more work needs to be done to understand its mechanics, Bleeg said.

“It is easy with hindsight to say things should have been discovered sooner but the interaction between the atmosphere and turbines is very complex and hard to predict. The industry is already working to correct existing models and build new ones,” he added.

Orsted has developed a new model that it used to calculate the production estimates unveiled last week. Wiinholt said the formula has already been partially applied to the developer’s existing asset portfolio. “It is applied to the degree that we feel captures the full impact. We will keep working on it but believe it will not continue to have a negative impact.

“With all this complexity there will be uncertainty that could go both ways. This is a new and very immature industry and we do not have the same experience that we have with onshore wind,” she added.

Other developers have played down the impact or are tight-lipped on the issue. The Blauwwind Consortium, developer of the 732MW Borssele 3&4 project next door to Orsted’s Borssele 1&2, said it has no plans to revise production estimates.

“The forecasts made during the development expenditure phase are considered to be sufficiently robust,” operations manager Michiel Spoor said. 

“There is bound to be industry-wide interest in this issue but it is... too soon to comment,” said a Vattenfall spokesperson. Innogy said financial forecasts for its renewables division remain unchanged for 2019 and declined to comment further.


Number crunching ‘must reach a whole new level’

Significant research capital is already being dedicated to understanding wake and blockage effects with experts saying new approaches to measurement are needed. At present, very detailed calculations of wind speeds within a project site are carried out before construction but expanding these over much larger areas is considered unrealistic.

“These high-fidelity calculations are done across a small area and that is already very computationally intense, which is expensive,” said Durham University’s Dr Majid Bastankhah.

In future, however, developers will have to consider what happens to wind speeds when neighbouring projects in the construction pipeline are brought online. These highly complex calculations will be a challenge.

“The wind is very heterogeneous and hard to predict but we need to build a simple, accurate engineering model that can account for what is happening,” added Bastankhah.

Sources working in wind farm modelling said Orsted’s decision to go public “has really lit a fire under attempts to solve the problem. When this industry puts its shoulder to the wheel it can achieve a lot”.

This article was first published in subscription renewable energy publication reNEWS issue 419

Find out more about Dr Majid Bastankhah's research and a article on his latest research findings