The experience of the TÜV SÜD experts has shown that even in the case of different WEC types, wind farms can often be assessed as a total complex

The experience of the TÜV SÜD experts has shown that even in the case of different WEC types, wind farms can often be assessed as a total complex (Quelle: TÜV SÜD)

TÜV SÜD experts know what is important in the Assessment and Verification of the Lifetime Extension of Onshore Wind Energy Converters (LTE) and how to leverage opportunities.

Roughly four years ago, the German Wind Energy Association (Bundesverband für Windenergie) published the second edition of its Basic Principles for Performing an Assessment and Verification of the Lifetime Extension of Onshore Wind Energy Converters (LTE). The document clarified – at least in theory – that individual WECs, but also entire wind farms may stay connected to the grid even after the end of their projected service life. However, in practice the managers of larger wind farms in particular find themselves facing critical questions, such as: when and how can I easily determine whether my WECs can and may continue to produce electricity?

The answers to these questions are primarily technical in nature. Other relevant issues include profitable marketing strategies for electricity following expiry of the feed-in tariff funding under Germany’s Renewable Energy Act (EEG) and the possibility of extending a construction and operation permit under the German Building Code (see info boxes 1 and 2). At the same time, these and other questions can only be answered if a WEC fulfils the technical criteria for lifetime extension. The load-bearing components, from the rotor tip to the tower foundations, must be able to reliably absorb the forces arising during operation and must not fail during their extended lifetime.

Determining load reserves

Using the LTE principles as a basis, the experts examine the condition of the WEC and its parts and components and assess whether the installation still has load reserves at the end of its design life (generally 20 years). The assessment is composed of a theoretical and a practical part. The experts start with a detailed examination of the conditions at the site, such as wind turbulences, and compare them against the type certificates or WEC design.

If analysis of the wind and weather data shows that the turbine has not been exposed to stress beyond its design limits, the experts will progress to the practical part of the assessment. In an on-site inspection, the experts perform visual testing of the entire installation and review the documentation. They focus in particular on the actual condition of the turbine and on whether any maintenance and repair work has been carried out correctly, expertly and professionally. At the end of the process, the experts prepare a test report summarising whether the WEC has load reserves, identifying which components may have to undergo maintenance, repair or replacement, and describing the conditions under which continued operation of the turbine may be possible and for how long. In this context, costs must always be proportionate to the benefits. In many cases, expensive measures such as the replacement of costly components can be delayed by shortening inspection intervals.

Making use of synergistic effects

Managers of larger wind farms, stakeholders of larger portfolios and WEC operators at nearby sites may benefit from the synergistic effects of LTE. The analytical part, for example, requires extensive data records – primarily the operational data and records of the WEC, which realistically map the wind conditions at the respective site. 

However, downtime, defective measurement instruments and other situations such as replaced components or deleted data often result in incomplete or even worthless data records. Completing and validating these data records is easy in cases where the data of nearby turbines or wind farms are available for LTE. The greater the volume of data included in analysis and simulation, the more precisely the wind and weather conditions in the previous years of operation can be determined and applied to the individual WECs in the area. This saves both time and money. “Clustering” also produces highly reliable predictions for possible lifetime extension.

Performance of LTE on a larger number of WECs also offers advantages if a site includes many turbines of the same type or manufacturer. This also applies to WECs with different owners and different years of construction as well as WECs for which continued operation has not yet even been taken into consideration. Specific turbine types or models from a manufacturer frequently show similar outcomes in continued operation.

With this in view, it often pays to focus not only on WECs that are nearing the end of their design lives, but also look at more recent converters nearby. In many cases this increases the odds for lifetime extension later on as it can enable typical defects and damage to be prevented easily and cost-effectively. Owners and managers who have good neighbourly relations should come together, discuss lifetime extension and initiate LTE jointly as a wise move for the future.

Finding the right time for LTE

TÜV SÜD recommends initiating and carrying out LTE in good time before the end of the design life. Basically, LTE can be carried out at any time, as it often represents a type of value report. A good practice is for managers to initiate LTE in the last regular year of operation. This ensures that they will obtain the assessment report six months before expiry of their equipment’s design life at the latest. This leaves them with enough time to establish the formal conditions for continuing the operation of their WEC or wind farm in compliance with the law (e.g. applying for permits or licences, updating insurance policies, initiating repairs and the future marketing of the electricity).

However, specialist expertise is required to make optimum use of this roughly six-month period and ensure the LTE is performed smoothly. At TÜV SÜD, LTE assessment reports are drawn up jointly by experts from multiple disciplines, working as a close-knit fixed team to deliver testing continuity and clear communication with the client. The largely automated reporting process fulfils all formal and technical LTE requirements. The experts use established industrial software to run computer simulation and load computations. An app developed especially for this purpose supports on-site inspection and documentation and automatically prepares a test report from the data entered.

Expertise is important

The final report is available within six to eight weeks of placement of order. But the standardised procedure of TÜV SÜD offers more than cost-effectiveness and time-efficiency. It also gives WEC owners and managers predictability. The clearly laid-out assessment report firstly ensures transparency of the capital expenditures required for continued operation, and secondly facilitates the approval procedure.

The experience of the TÜV SÜD experts has shown that even in the case of different WEC types, wind farms can often be assessed as a total complex. This clustering produces the synergistic effects described above. Ideally, managers need only one assessment report to cover the entire wind farm.

What are the cases where lifetime extension pays off?

Lifetime extension or continued operation makes good sense if

  • a positive lifetime-extension assessment report exists,
  • a marketing concept ensuring profitable continued operation is available,
  • there are no legal concerns, and
  • repowering is impossible or uneconomical.     

Long lifetime extension is likely if the WEC

  • has not been exposed to site conditions close to the WEC design,
  • has undergone regular servicing and appropriate repair,
  • has received periodical inspections, and is free from major defects according to the most recent inspection report (in year 18 or 19).

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