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Viewpoint: The benefits of partial repowering

Clarification by the US government of tax credits for partial repowering in 2016 has inspired many in the wind industry to take a closer look at repowering options for their projects.

Full repowering entails complete decommissioning and removal of existing turbines, including foundations. The site is then redeveloped using modern technology. Partial repowering, on the other hand, involves leaving some portion of the existing structure and replacing the other portion with modern technology.

Electrical balance-of-plant equipment is only replaced or upgraded as required. This could mean as little as the foundation and tower remain, with everything else being replaced; or it could mean the foundation, tower, complete nacelle and hub remain, and the blades and blade bearings are the only components to be replaced.

The most compelling reasons for partial repowering can be financial, technological, or a combination of both.

These could include requalifying for tax credits when a project is approaching or has moved past the expiration of its original tax credits (which last ten years for new projects); projects where the winds are not as strong as expected; and projects using obsolete technology, where replacement parts are not available or the technology is otherwise difficult to support and maintain.

These situations could be prime candidates for a partial repower.

There are numerous potential benefits to partial repowering, though the degree to which a project can realise these can vary. Benefits include:

  • Greater energy capture from increased hub heights or rotor diameters
  • Lower operating costs and reduced supply-chain risks depending on the technology being replaced and scope of the repower, potentially due to a new warranty
  • Renewal of tax credits
  • Lower uncertainty for energy production assessments

Potential drawbacks should be considered early in the venture, as some may be dealbreakers. These are usually site-specific but may include:

  • Large capital investment required
  • Multiple contracts may need to be renegotiated, such as power purchase agreements, interconnection agreements and leases, though this may be seen as a benefit in some circumstances
  • Reusing components such as towers and foundations may increase risk of failure for the site.

Loads and risk-based inspections

When repowering, structural components that have accumulated some fatigue damage carry a greater risk of structural failure than the use of all new components.

However, for a lightly loaded site, the reliability level estimated after ten years of operation may still be well above the minimum accepted reliability level.

An analysis of how much structural loading a component has experienced to date, and how much more is expected post-repowering, will provide decision-makers with a basis to estimate how many more years of service may be expected from used structural components.

As an example, with all other factors remaining the same, if a turbine operated for ten years under inflow that perfectly reflected its design class and was then partially repowered with a technology imparting 80% of the original fatigue load on the tower, the tower could be operated for another 24 years before exceeding the design risk of fatigue failure.

In addition to using analysis to help inform decisions, risk-based inspections play an important role in ensuring the asset integrity meets expectations.

As the structural reliability of the turbines approaches the target level, structural inspections should become a routine part of good maintenance practices. The type and frequency of inspection is dependent on a number of site-specific factors, as well as the component itself.

Foundations are another concern for repowering. If the new equipment imparts lower loads on the foundation than the equipment being replaced, due to factors such as lighter materials and improved control approaches, then the chances of being able to continue to fatigue load the foundations for another 20 years is greater. Nevertheless, invasive inspections and monitoring during operation may be required for many repowering situations.

These challenges should be considered against the potential benefits of repowering, which could be significant, depending on the project's preexisting challenges that repowering is aiming to resolve.

Alex Byrne is a senior controls engineer at DNV GL-Energy

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