Offshore load warnings, bullish bids turn spotlight on models

Offshore load factor downgrades are spurring more rigorous wind modelling as developers assume higher factors for future projects.

In October, Orsted announced it had been overestimating the load factors of its offshore wind farms by underestimating the impact of wake and blockage effects.

The world's leading offshore developer, Orsted will have an installed capacity of 7.5 GW by the end of 2020. The group plans to reach an installed capacity of 15 GW by 2025.

The wake effect refers to the slowing of wind after hitting a turbine while blockage describes how wind slows down as it approaches a turbine. New models showed greater losses from these effects within a single windfarm as well as a greater impact between neighboring wind farms, Orsted said.

As a result, Orsted lowered its estimated lifetime load factor for a defined portfolio of European projects, either operational or under development, from 48%-50% to 48%.

Orsted's announcement follows a report by DNV GL in 2018 which showed blockage effects cause greater upstream wind speed reductions than commonly assumed. The research group also found that front row turbines generally produce less energy than they would operating in isolation and adjusted its energy assessment methodology accordingly.

Revealed through growing operational datasets, the latest findings highlight the need for improved modelling of highly complex wind dynamics. For its part, Orsted is developing new proprietary models to better calculate the interplay between turbines and between neighboring wind farms.

"Most third-party production estimates have been trending towards a more positive view than ours,” Orsted said.

“Therefore, we believe that underestimation of blockage and wake effects is likely to be an industry-wide issue," it warned.

Developers Iberdrola and SSE have checked their models and found no reason to change their project fundamentals, the companies told the Financial Times.

                        Owners' share of installed offshore capacity (end 2018)

                                                               (Click image to enlarge)

Source: WindEurope, February 2019

The recalibration of load factor assumptions comes as developers eek out competitive advantages to pursue aggressive growth strategies.

Developers are assuming higher load factors for future projects to lower bid prices in competitive tenders, as demonstrated in the U.K.'s Round 3 offshore tender completed in September.

The UK government awarded 5.5 GW of new offshore wind projects, at contract for difference (CFD) prices of between 39.65 and 41.61 pounds/MWh, around 30% lower than prices awarded in 2017.

Due online in 2023-2025, the Round 3 projects assume a turbine interaction loss of around 10%, compared with 15% in projects selected in 2017, DNV GL said in report for the UK government, published in October.

Loading up

According to DNV GL, UK average offshore wind load factors will increase from about 40% at present to around 50%-55% by 2035.

The main drivers will be larger, more efficient turbines and the development of far-shore sites with higher average wind speeds than earlier near-shore sites, Ray Thompson, Head of UK Business Development at turbine supplier Siemens Gamesa, told New Energy Update.

Indeed, UK Round Three projects are being built further offshore and at higher hub heights than previous rounds, resulting in a higher average speed of about 10 m/s, Simon Cox, Offshore Wind Business Manager at DNV GL, told New Energy Update.

In one example, Equinor and SSE Renewables have selected GE's latest 12 MW turbines for their 1.2 GW Dogger Bank Round 3 project.

“The high wind speed and turbine technology will result in these projects achieving load factors in the range of 50 to 55%. The load factor is part of the record low price," Cox said.

Larger turbine capacities and wider spacing between units is helping developers expand wind farm capacity while limiting losses, Cox said.

“Maintaining the number of turbines and increasing turbine spacing results in a reduction in turbine interaction [wake and blockage] losses," he said.

Technology solutions could also help mitigate wake loss impacts. Siemens Gamesa recently unveiled a new wake steering product for offshore wind operators. By adapting to site conditions, the wake steering solution can reduce losses by up to 10% and increase annual energy production by up to 1%, the company said.

Better operations

Greater turbine reliability and improved operations and maintenance (O&M) practices are also helping operators increase load factors, Cox said.

“This has been achieved through improved handling of unscheduled maintenance, in which minor failures are rectified at scheduled maintenance visits,” he said.

Offshore service vessels (OSVs) which allow direct "walk to work" access to turbines can significantly boost uptime, Cox said. Average turbine availability in the UK when using OSVs is around 97%, some 1.5% higher than using other vessels, he said. Greater transmission grid availability is also supporting higher load factors, he added.

Other drivers of record low project bids include supply chain savings, more efficient O&M networks and assumptions of longer operational lifetimes, which are backed by lessons learned on earlier projects, Cox said.

Neighbor issues

Strong buildout in leading markets like the UK could have further ramifications for load factors, even as sites move further offshore.

The UK currently has 8 GW of installed offshore wind capacity and plans to tender for 2 GW per year of new capacity over the coming decade.

                   Europe installed offshore capacity by country (end 2018)

                                                         (Click image to enlarge)

Source: WindEurope, February 2019

Further modelling is required to capture the impact between neighboring wind farms, Orsted said in its recent statement.

Orsted is developing a new model that can more accurately predict wake effects over longer distances, factoring in "more extensive offshore wind buildout, it said.

The new model, which is still being refined, suggests a slower wind speed recovery and higher wake effects than previously thought, Orsted noted.

"As the global offshore wind build-out accelerates, the whole industry will see higher wake effects from neighboring wind farms," it said.

By Neil Ford