Evidence is growing that life extension for megawatt-class turbines makes sense financially in most cases — as long as a turbine has been operated and maintained as recommended by the original equipment manufacturer (OEM), say experts. And products are being developed that will address the practical aspects of life extension.
For the most part, modern wind turbines and their major structural elements - the foundation, tower, hub and bedplate — are reliable. Wide design margins are incorporated, notes Alex Byrne, a senior engineer at Danish certification and classification organisation DNV Kema, which has conducted research on the economics of extending turbine life. Non-structural components — including blades — can generally be replaced or upgraded, although the cost can be substantial, especially offshore.
According to Byrne, every year of life extension benefits the internal rate of return (IRR) and the net present value — the difference between the current value of an investment's future cash flow and the size of the original investment. This benefit shows in the data regardless of how the life was extended.
The DNV study gave three options for extending turbine life: using targeted inspections to deliver more frequent checks; de-rating a turbine during high-loading events such as severe turbulence to cut fatigue; or operating the turbine with advanced controls such as lidar (light detection and ranging) to predict wind energy flow, or state estimation, an advanced algorithm-based approach to turbine control.
In the study, a turbine was decommissioned when the foundation, tower or bedplate became cracked. The researchers found the risk of fatigue failure for steel components rose slower than expected after year 20, provided the turbine was operated in the conditions for which it was designed.
If targeted inspections were used - meaning key structural components were checked more frequently - the researchers found that a turbine decommissioned after 25 years of operation would give a 14% higher lifetime IRR compared with a turbine decommissioned after 20 years — the standard assumed lifetime of a wind turbine (base rate) — even with O&M and inspection costs included. Thirty years of operation would give a 23% higher IRR, while 35 years would produce a 28% improved IRR.
Modifying turbine operation by a higher percentage from the start, for example reducing blade-flap fatigue, will extend turbine life further and give a higher IRR. Similarly, using either lidar or state estimation to reduce the mean fatigue load from when the wind farm is first commissioned would produce a longer life span and higher IRR overall than implementing it after ten year of operation (see chart, below).
Combining the three approaches is most beneficial, says Byrne. But the best mix will vary, she adds, because owners, or perhaps their insurers, have different risk appetites. And some turbine sites are more benign and less fatiguing. DNV strongly recommends against continuing to run an ageing fleet without analysing accumulated fatigue.
Old versus new technology
Smaller turbines have been operating for more than 20 years in Denmark and areas such as California's San Gorgonio Pass and parts of Minnesota in the US, and many have not been replaced, even by owners with deep pockets, points out John Dunlop, O&M consultant for the American Wind Energy Association (AWEA).This is despite the fact that they could be repowered with far larger and more productive machines.
Dunlop, however, cautions against comparing "over-engineered" vintage turbines — "basically tractors on a pole", as he calls them — with today's complex, finely tuned machines. In order to cut the cost of energy, today's design margins in turbines are narrower, he says. "I do not expect (today's turbines) to have the same durability," he adds.
Even so, O&M is also a different beast nowadays. The difference comes through long experience, newer techniques such as condition monitoring, as well as the sophisticated data analysis offered by major O&M firms or OEMs, such as at Vestas' global performance and diagnostic centre in Aarhus, Denmark.
Most independent experts suggest at least following a turbine manufacturer's O&M recommendations and ensuring a thorough end-of-warranty inspection. Few will be pinned down on the specifics of how O&M can extend life expectation. "What is end of life? We just don't know how long turbines can last," says Kevin Alewine, renewable energy director at wind turbine maintenance provider Shermco Industries and co-chair of AWEA's O&M working group. He emphasises the importance of knowing the equipment and its service history to identify weaknesses and plan inspections.
Peter Wells, CEO of Upwind Solutions, another US O&M provider, suggests reviewing which components should be upgraded or retrofitted, while Christian Venderby, chief operating officer of Vestas-American Wind Technology, recommends reducing fatigue with checks and controls such as condition monitoring, borescoping or infrared cameras to detect heat. "It's always a trade-off between long life and cost," he says.
Targeted inspections of key structural components starting a few years before the end of design life, could include inspections of foundations every five years; annual inspection of all blades from the ground, with selected blades also receiving rope-access checks; and annual assessment of some welds and all the hubs and bedplates, DNV's research suggests.
Research & development
As interest in life extension grows, so do the related products in development. Upwind is working on an upgrade to its predictive software to help clients work out how to extend life.
London-based Lloyd's Register is testing a pilot of its Arivu dynamic software on an offshore wind project. It updates a risk profile based on a turbine's age, use and operating experience and offers a maintenance and inspection plan. Input is from condition monitoring, visual inspections, past maintenance and failure causes. "It could help you determine whether it's appropriate to continue using (the) equipment," says Mark Spring, a senior loads-analysis engineer at Lloyd's.
And 18 months ago, Siemens started investing a "significant" amount in life extension research and development in Brande, Denmark, says Tim Holt, CEO of service renewables at Siemens Energy. The research includes which components should be upgraded and what testing and analysis is best, he says.
Holt says Siemens is likely to recommend customers start implementing the life-extension package in year 12 or 15 - when the turbines have probably been paid off, and a customer can start thinking ahead. But he stressed that life extension is still a relatively new field: "The more the industry matures, the better our recommendations will be."