Lifetime Extension for Wind Turbines

In detail, preparation of an expert analysis for continued operation of a wind turbine is subdivided into three parts. An evaluation report is drawn up based on the results of the analyses performed. If desired, multiple turbines at a single site can be analysed together and the results presented in a combined evaluation report.

1. Practical part: Inspection of turbine
   
   In the practical part, the current technical condition of the wind turbine is inspected and recorded. This on-site inspection by an expert engineer is taken as a basis for determining whether the condition of the turbine is sufficient to allow continued operation beyond its design life. Cracks, loose bolts and other issues related to single components are often discovered. Certain turbine types have well-known design-related issues, which are additionally considered at the inspections on site.
   
   Critical factors for potential continuation of operation are the condition of the components in the main load path, e.g. foundations, tower, machine carrier and hub, as well as other load-bearing structures and rotor blades. Additionally, all other safety-relevant parts and components are examined during the inspections, e.g. lightning protection system (LPS), pitch drives (where applicable for older turbines) and electric and electronic systems.
   
   A well-maintained wind turbine usually does not show serious issues. Nevertheless, when serious issues that directly impact on the structural integrity of the wind turbine are detected, immediate shutdown of the wind turbine operation is recommended to protect both the wind turbine itself and the surrounding infrastructure and population. Minor issues or damage which can easily be fixed by simple maintenance measures are more frequently discovered during the inspections. On the turbine itself, these could be damage from weathering or corrosion, or damaged coatings or worn-out wiring. Where the rotor blades are concerned, small cracks or surface pitting are common.
   
   
2. Wind conditions: Determination of wind conditions at the site
   
   Detailed knowledge of the wind conditions at the site throughout the turbine’s operating life is an essential basis for exact estimation of the turbine loads and the remaining margins. In this process, the site-specific wind data and the operation data (SCADA data) of the wind turbine are evaluated and the turbine documentation is reviewed. This enables mean wind speeds, extreme wind events and turbulence intensities to be tracked over the turbine’s operating life. These detailed data are subsequently used to calculate the loads on the wind turbine over the operating life in the third, analytical part.
   
   
3. Analytical part: Calculation of turbine loads and margins of components
   
   The information collected concerning the wind conditions at the site forms the basis for the final analytical part and calculations in the expert analysis for continued operation. These calculations, which are related to load simulations, take the requirements of the IEC 61400-1 standard (and also DIBt:2012 and others if necessary) as well as the original design conditions into consideration. The resulting time series can be summarised to give damage equivalent loads (DEL) for all relevant components and parts. The continued operation time is calculated on the basis of these values, taking into account the load level under design conditions for the relevant components in the main load path.

When an expert analysis of continued operation and evaluation report are drawn up, various operation and (energy) production data for the wind turbine are considered together with the evaluation reports as part of the Type Certificate. The analysis process further takes into account any documentation related to the erection, commissioning and operating approval of the wind turbine together with its conditions and requirements, as well as maintenance, repair and inspection reports.

The outlined documents and data can only be provided by the operator and owner of the wind turbine. To ensure an efficient workflow, these should be provided before the start of evaluation. If further documents are required they will be requested by our expert engineers in the course of the evaluations. If data or documents are not available, TÜV SÜD will work to find reliable alternative solutions and approaches to clarify the relevant areas in order to be able to issue an evaluation report for continued operation.

When a wind turbine moves into a period of continued operation, it is usually no longer covered by the regular payment schemes set up by governments and authorities worldwide. The point of departure from the payment scheme often, but not always, coincides with the (common) 20-year design life of a wind turbine. Some newer turbines have a longer design life, while some national renewable energy schemes do not guarantee payments for 20 years. Some wind parks may even fall outside any national scheme because a PPA was concluded or a different situation applies.

Assuming the very common situation that a wind turbine has left the national payment scheme and the design life has expired, the energy generated very often has to be sold at currently available market prices. The original prices per kWh at the time of erection and commissioning of the turbine are usually no longer reached. Measures to maintain an optimised energy price during production (e.g. operation of turbine only when high energy prices are guaranteed) are possible, but require the support of the chosen grid operator and energy seller.

In addition to the efforts made in producing the expert analysis of continued operation, further actions, e.g. replacement of components, may be required for the turbine, either immediately or after a few years of operation.

Even if the economic situation of turbine operation in the continued operation period is characterised by decreasing income and earnings, there might still be identifiable potential savings. This mainly involves savings related to rent (e.g. when rental contracts come up for extension, an earnings-related variable rent may be feasible that would save rent in the continued operation periods and potentially offer the lessor a later option to increase rents if repowering with larger turbines becomes possible). Insurance payments as well as maintenance and other service provision might offer further potential for savings, considering that none of them would generate any further earnings if the turbine was dismantled.

The extent to which continued operation of a wind turbine makes sense and is economically viable must be judged on a case-by-case basis. Beside pure economics, it also depends on the owner and operator’s expectations of income. An additional attraction for some operators might be to secure a site for repowering (on the basis of a changed legal situation in the future) while initially keeping the existing turbine operating on a decreased income and replacing it by a much larger turbine later.

When an operator decides not to continue operating a wind turbine for economic reasons, a last alternative to dismantling is the sale of the turbine to another operator specialising in continued operation.

Financial considerations may mean shutdown and dismantling for many older wind turbines. TÜV SÜD’s services can help to extend the operating life of your wind turbine and keep your turbine available as a renewable energy source. Just ask our experts for details.

The decision as to which of the four options is suitable for which operator and which turbine at which site must be founded on detailed analysis. Irrespective of the most attractive option for you, TÜV SÜD supports you along the whole life cycle throughout every phase of operation of your wind turbine.

Continued operation and the related necessity of setting up new contracts and dealing with other issues needing clarification might seem to involve enormous efforts at first glance. Nevertheless, this option is often actually the one with the least effort and fewest changes: the turbine still remains in operation and the number of involved parties (authorities, insurance etc.) is still low compared to the other options.

An expert analysis for continued operation is often also required when the turbine is sold. Furthermore, detailed Due Diligence Checks are necessary. If desired, we can also offer our services for this.

If dismantling of the turbine is the chosen option, many other topics will need clarification; additionally, heavy equipment is necessary.

In repowering, old turbines are replaced by a lower number of newer turbines with increased nominal power and efficiency. This usually results in increased hub heights and rotor diameters. In sum, higher annual energy production by the wind park is usually possible afterwards. In addition to the dismantling operations involved, planning and preparation for the erection of the new turbines often goes on in parallel, sometimes resulting in individual synergy effects. Replacement of a single turbine by a larger one with greater annual energy production and higher efficiency is often reasonable and feasible. Due to the political boundary conditions in many countries worldwide, not many wind parks can currently be repowered feasibly and cost-effectively. With an eye to local ongoing political debate, continued operation of a wind turbine or wind park could be worth considering – even if only moderately economically viable – with the hope of changes to political boundary conditions in the near future.

In the situations described above, the question might arise as to what fundamental boundary conditions need to be fulfilled for continued operation of a wind turbine to become a viable option.

In summary, continued operation of a wind turbine may be feasible in principle if

   - A positive evaluation report for continued operation is available

   - Continued operation is economically viable

   - There are no opposing legal interests

   - Repowering is not possible or economically viable

Continued operation is most likely to be possible when the wind turbine

   - Has not faced any extreme events (strong hurricanes, “storm of the century”)

   - Is located in a wind park with >5D in main wind direction

   - Has undergone regular service and maintenance together with appropriate corrective
     actions and repairs

   - Was inspected regularly as part of recurring inspections and the last inspection report
     does not show any serious faults (report in 18th or 19th year of operation)