Jensen J.P.
Department of Planning and Development, Aalborg University, Aalborg, Denmark

Keywords: lifetime extension; wind turbines; environmental impacts; circular economy; lifecycle assessment.

Abstract: Resource depletion, resource efficiency and circular economy are all terms that have gained attention recently. Extension of the product lifetime is one of the key strategies to strengthen the circular economy. Wind turbines produce low carbon energy, but do at the same time contain large amount of materials. To be resource effective getting the most out of the materials is of high concern. The potentials for extending the lifetime of wind turbines are analysed by applying different circular economy initiatives being service/maintenance, reuse/redistribution and remanufacture/ refurbishment.

The performance level of a wind turbine has to be kept on a certain level to make it feasible to continue operate the wind turbine. The different approaches represent ways of doing this and thereby keep the materials in the loop for a longer period of time.

An assessment on the impact of extending the lifetime of a wind turbine shows environmental improvements can be achieved by extending the product lifetime. However, the lifetime of a turbine is also determined by several other factors, and the economic lifetime of the turbines is often shorter than the technical durability.

Extending product lifetimes

Circular economy as a concept has gained attention worldwide. One strategy of the circular economy is prolonging the lifespan of products thus thereby slowing down the throughput of resources in society and reducing the amount of waste.

Back in 1998 von Weizsäcker et al. (p. 70) argued that ‘durability is one of the most obvious strategies for reducing waste and increasing material productivity.’

Increasing material productivity or resource efficiency is on the political agenda, but depletion of resources and downcycling of materials has not been a main focus until recently. In fact, much of the materials are wasted as a result of the way consumption and production has developed (Bakker, et al., 2014). To reduce the throughput of materials and energy demands, Cooper claims that a strategy that goes beyond recycling and includes longer lasting products must be applied (Cooper, 2010).

The circular economy, as presented by Ellen MacArthur Foundation, highlights the ‘power of the inner circle’ and the ‘power of circling longer’. It refers to minimizing the material use by: 1) maintaining and repairing rather than reusing and recycling; and 2) prolonging the life cycles a product. A prolonging of the usage will substitute virgin material inflows (Ellen MacArthur Foundation, 2013).

Cooper highlights that:

‘A circular economy is a prerequisite for sustainability but may not be sufficient if resource throughput remains high. … A complementary approach would be to slow down the rate at which raw materials are transformed into products and the products ‘used up’ (Cooper 2010, p. 13).

Extension of product lifetimes requires support by the business model by maintaining value in the product (Bakker, et al., 2014) or as highlighted by Stahel (2010) that a shift in focus from resource throughput to asset management is needed.

Cooper divides the product lifespans into different categories e.g.:

  1. The technical lifetime being the maximum period a product has the physical threshold to function.
  2. The replacement or economic lifetime being the period from initial sale to the point where the owner buys a replacement regardless of the product functioning or not (Cooper, 2010).

Increased product life span is a more efficient use of materials and a slowdown of throughput. The reduction will probably not be offset by increased consumption as the ‘resources’ put into this is mainly renewable, being man hours for maintenance and repair (Cooper, 2010) (Stahel, 2010). However, some products will require new components and transport of the service technician.

Extending the product lifespan will not necessarily make a positive contribution towards sustainability. If the benefits of product improvements are outcompeted, it may be more favourable to replace it with a more efficient successor. The moment for replacement depends on the specific product and technology development (Bakker, et al., 2014). An industry that has integrated maintenance as part of the business model is the wind industry.

The lifetime of a wind turbine

A modern wind turbine is typically designed to work for approximately 120.000 hours throughout its estimated life span of 20 years (European Wind Energy Association (EWEA), 2014). However, some larger offshore turbines now have a projected lifespan of 25 years.

A wind turbine is a serial system. The reliability of the entire system is the output of individual sub-system reliabilities meaning that the failure rate of the system is the sum of individual sub- system failure rates (Ortegon, et al., 2014).

Specific components within the turbine are subject to more tear and wear. Generally, the moving parts are worn out faster than static parts, and exposed components are worn out faster than shielded components. Blades and gearboxes have historically been considered to wear out the fastest (WMI, 2014).

The actual lifetime of the turbine depends on the quality of all the components of the turbine, their assembly and the environment the turbines are placed within such as onshore, offshore, wind, turbulence, air density, humidity etc. The turbulence will in general be lower at sea as there are no obstacles (WMI, 2014).

The capacity factor

The capacity factor during the lifetime of a wind turbine is essential, when considering extending the lifetime, as it must remain at a certain level to make it feasible to run the wind turbines.

The capacity factor is calculated as:

‘the ratio of the amount of electricity actually produced by a turbine or wind farm over a period of a month or a year divided by the amount of output that would have been produced had it operated at full nameplate capacity for the entire period. This is expressed as a percentage, so that reported capacity factors lie between 0 and 100’ (Hughes, 2012, p.9).

The capacity factor for wind power has historically been assumed in the range of 30– 35 % of the name plate. Some studies have however shown examples of mean values below 21 % (Boccard, 2009), but new wind turbine parks are however often calculated with an expected capacity factor between 37 – 40 % (Siting Specialist, 2015).

Different factors affect the capacity factor. Three better understood are:

  1. Machine availability: Downtime of the turbines or the electrical infrastructure can affect the output by 4 to 7 % in decline (Staffell & Green, 2013).
  2. Operating efficiency: Sub-optimal control systems, misaligned components and electrical losses within the farm can reduce the output by 2 % of the turbine (Staffell & Green, 2013).
  3. Wake effects: Wind farms are affected by power loss as neighbouring turbines increase turbulence and reduce wind speeds. The output can drop in the area of 5 to 15% (Staffell & Green, 2013)

and two less understood are:

  1. Site conditions: Imperfections in the local environment like e.g. turbulence intensity and terrain slope will impact the output. These are site specific and will vary, but are estimated to reduce output by 2 to 5% plus 1 % per 3% increase in turbulence intensity (Staffell & Green, 2013).
  2. 2. Turbine ageing: Different factors of decline in output as the turbines age. (Staffell & Green, 2013).

The capacity factor over time

A thorough study on wind turbines in UK and Denmark by Hughes concludes a significant decline in the average capacity factor (adjusted for wind availability) as the turbines ages. It concludes that the capacity factor in UK decline with 0.9 percentage points per year the first 10 years of operation starting at approx. 24 % and falls to 11 % at age 15 (Hughes, 2012).

Another study by Staffell and Green that analyses the same data concludes that the decline with age is 0.45 percentage points per year, which is quite different from Hughes. (Staffell & Green, 2013).They further claim that farms built before 2003 have a decline rate two to three times higher than turbines built after 2003 (Staffell & Green, 2013), which is indicating a more reliable technology today than earlier.

An analysis by McKinsey & Company finds that performance is unrelated to the age of wind parks (and of the manufacturer). However, factors such as e.g. dirty blades can prevent the wind force from being transmitted to the blades and the generator, which results in lower output (McKinsey & Company, 2008).

There are however strategies to maintain a high performance and thereby make a basis for lifetime extension. Lifetime extension can follow different strategies. These are analysed below and the environmental impact of lifetime extension is assessed.

Routes for lifetime extension

The following section will analyse the present approaches of 1) service/maintenance, 2) reuse/redistribute and 3) refurbish/ remanufacture, to maintain a certain level of capacity factor.


A study by Ortegon, Nies and Sutherland present that the number of failures, the downtime and the cost will be drastically reduced with regular maintenance (Ortegon, et al., 2014).

The service concept has been gradually integrated in the business model of the OEMs (Original Equipment Manufacturers) in the wind industry.

Supervisory control and data acquisition (SCADA) systems are integrated parts of modern wind turbines, which makes it possible to remotely monitor information such as electrical and mechanical data, operation and fault status, meteorological data and grid station data constantly. It regulates the active power output of the turbine and is an essential part in keeping the capacity factor as high as possible. Further, turbine condition monitoring systems are now available, which makes it possible to perform precise condition diagnostics based on vibration, which can give an early warning if any components are having problems and thereby reduce maintenance costs and optimize energy output (Siemens Wind Power, 2014).

The diagnostic tools are getting more and more advanced and are able to prevent problems before they occur. The field is developing and lately has a system to monitor the transistor been developed, which detects failures before it overheats, which can prolong the lifetime of the wind turbine (Frandsen, 2014):

The service business of the OEMs has grown rapidly over the last years and has proven to be a good business case for both the OEMs and the wind turbine owners.

Recently, the service concept has been even more sophisticated and presents refurbish or reuse options offered by the different OEMs (see below).

Reuse/Redistribution (new location)

Reuse/redistribution can be a good option, when e.g. a wind turbine reach its economic life at one site (time period with subsidies), but not its technical life.

Several private companies are ‘brokers’ of this service e.g. Repowering Solutions, Hitwind etc, but also an OEM as Vestas has launched it’s ‘Wind for Prosperity’-programme in late 2013, which deals with this life extension option on a larger scale (Vestas Wind Systems A/S, 2013).

The ‘Wind for Prosperity’ aims at ‘combatting energy poverty and deploy green technology in developing countries’ by committing to source and factory-refurbish a selection of wind turbines that have favourable dimensions for transportation and erection’ (WindforProsperity, 2014).

As the main market for ‘Wind for Prosperity’ is third-world countries, Vestas mainly focus on ‘small’ wind turbines (WindforProsperity, 2014). Vestas has teamed up with ABB to deliver wind power to local communities (Wang, 2014).

The business model behind Wind for Prosperity can act as a pilot project for other and larger types of turbines, so technical well-functioning wind turbines can have an ‘afterlife’ on another location.

Remanufacturing/Refurbishment (same location)

Remanufacturing of wind turbines is another possibility. Interest in refurbishment from an owner perspective can come from permitting e.g. height restrictions on some properties (Dvorak, 2014). Experience with other remanufactured products indicates that when costs of a remanufactured product exceed 70% of a new product, the new product is preferred.

Studies show that the effective age of a remanufactured wind turbine is estimated to additional six years (Ortegon, et al., 2014).

The Spanish wind turbine manufacturer, Gamesa, proposes an ‘aging fleet solution’ offering a service program that focuses on lifetime extension of wind turbines. The wind turbine life-extension program consists of a series of structural reforms and a monitoring system designed to prolong the useful lives of wind turbines (Gamesa, 2014).

The programme includes both some Gamesa turbines, but also turbines from competitors. The estimation is to add a 10 year life extension. The focus is mainly on <1MW turbines as they are reaching the 20 years design life at the moment, but Gamesa plans to include larger turbines. Gamesa expects the cost to be half of the extra revenue generated by the life extension (Dvorak, 2014). Gamesa is taking part in the European Union-sponsored project, SafeLife-X that seeks to develop effective solutions for minimizing the ageing of industrial infrastructure (Gamesa, 2014).

Other OEMs has also started to look into this side of the business. Examples are:

In May 2014 Vestas introduced their PowerPlusTM programme (Vestas Wind Systems A/S, 2014), which focus is upgrade of existing turbines to increase annual energy output (AEP). The programme offers upgrades within three different areas and depending on the turbine and the number of upgrades, the AEP is estimated to increase by 2.3 % – 6.8 %. It can potentially make it feasible to keep old and smaller turbines running for some extra years (Dvorak, 2014).

General Electric have developed an upgrade on the blades, so their 77m blades can be replaced with 91m, which will increase the swept area by 40% and can boost the AEP by approximately 20 % (Dvorak, 2014).

The routes for lifetime extensions are becoming digitalized by companies like ‘Spares in Motion’, which just have received the award ‘Best Industry Newcomer (Froese, 2015). It acts as a e-trading platform for the services used for lifetime extensions. It reduces the complexity, cost and lead time for these services and thereby does this innovative business model improve the possibility of lifetime extensions.

The different approaches all present possible ways to optimize the turbine during its lifetime and thereby making it attractive to extend the lifetime of the turbine. The larger the turbine is, the more revenue is created by the upgrade.

The wind turbine industry is still in a developing phase, which complicates the access to spare parts as it changes over time and from producer to producer. A standardisation of some components across the industry could help ease the maintenance and remanufacture possibilities.

The environmental impact of lifetime extension

The best approach from an environmental perspective is not straightforward. The routes for lifetime extensions have to be done on an individual basis as different turbines on a wind farm see different types of loading, which leave them at different stages after e.g. 15 or 20 years (Dvorak, 2014).

In 2012, Kenetech chose to repower 235 turbines after an analysis, whereas EDP Renewables reviewed their 153 wind parks and chose to extend the project life of the parks from 20 to 25 years, which highlights the different strategies (Houston & Marsh, 2014).

An assessment of the environmental impact on extending the lifetime can be seen below in Table 1. The calculations is based on a life cycle assessment for a 3.2MW onshore wind power plant by Siemens Wind Power, and the data behind the lifetime calculations is provided by Siemens Wind Power.

The assessment of the contribution to global warming shows that the operation and maintenance impact is small. An extended lifetime will have a positive impact on the carbon footprint and the amount of times the energy is paid back will increase significantly.

kg CO2 per MWH and times the energy is paid back for different lifetimes of wind turbines


The way of doing business is changed by moving away from traditional ’take, make and dispose’ pattern towards focusing on product durability by integrating asset management. Specifically, it becomes possible for the manufacturer to add value through the lifetime of the product and opens up new business possibilities.

Different studies show as indicated different results regarding performance over age for wind turbines, but it is possible to maintain a high performance and even upgrade the wind turbine over time. In the case of wind turbines different routes of asset management are operational to obtain a long product life being 1) service/maintenance 2) reuse/redistribution or 3) refurbish/remanufacture. The options include different management strategies and the addition of non-renewable resources differs, but the gain from extending the lifetime will often outcompete the added energy and materials.

The OEMs have entered the business of service and are getting more and more sophisticated and have at this stage shown different potentials of prolonging the lifetime and thereby improving environmentally and economically.

The lifecycle assessment of a scenario with regular service/maintenance and replacement of some components shows that the environmental benefit from prolonging the lifetime is significant. Environmentally, it is worth maintaining the wind turbine to reach its technical lifetime, the question is how to make the economic incentives support this.


The author would like to thank Kell Øhlenschlæger, Tine Herreborg Jørgensen (Siemens Wind Power) and Arne Remmen (Aalborg University) for providing data and contributing to this paper.


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