Today, there are more than 34.000 onshore wind turbines which are 15 years or older, with a total capacity of 36 GW (WindEurope, 2020). Among those, 9 GW belong to turbines that are 20 to 24 years old and 1 GW to turbines that are 25 years or older.

The average life expectancy of a properly maintained wind turbine ranges between 20 and 25 years. At the end of its operational cycle, a decision needs to be taken with regards to the future of the project. The wind farm can be repowered or be completely dismantled. Other factors that can derive on the decision to dismantle a WTG are an end of the land use contract, irreparable damage in the wind turbine that compromises safety, legal problems with operation or the use of land, or economic problems related to the exploitation of the wind farm.

Despite the lack of European standards for the decomissioning of wind turbines, an increasing number of countries have regulated the issue with national legislation. In Denmark and the United Kingdom, the decomissioning process needs to be set upon the approval of the project. Other legislators, such as Italy or Germany are more lenient, but often require returning the site to its original condition at the end of the life cycle of the project.

Because of this, a decomissioning plan needs to be laid out in the business case for upcoming projects. The dismantling operation will have both economic and environmental consequences, that will depend on the characteristics of the wind turbine. Both costs are not unrelated, since the breach of environmental standards or the introduction of decomissioning standards based on environmental concerns may have a significant impact on the economic cost of a project.

Following the current needs of the market, higher WTGs are being erected, with heights over 140 meters. One of the main solutions for this height range is a hybrid steel and concrete tower. The dismantling process will depend on the constitution of the tower, with a differentiation between “dry joints” technology (that allows disassembly of concrete segments) and “grouted joints” technology (that does not allow disassembly of concrete segments).

Nabralift is one of the other leading market solutions for this height, with a significant advantage in certain areas such as the LCOE reduction and the craneless installation process.

This note compares the economic cost and environmental implications of decomissioning these alternatives. Additionally, a case study (6 WTGs with a hub height of 165 meters in Germany) has been analysed

Hybrid tower and Nabralift tower

Wind turbine decommissioning process

Tower disassembly

The first step in the decommissioning process of hybrid towers consists of the removal of the rotor, the nacelle, and the upper steel tower section. For both dry and grouted joints hybrid towers, a crane as big as the WTG is necessary for this purpose. The mobilization of large cranes does not only represent a major cost for this process, but also may be a problem in terms of their availability depending on the wind farm location.

Once the steel segments of a hybrid tower are removed, the disassembly process changes for the dry joints and grouted joints hybrid towers:

• For towers with dry joints, the large crane keeps operating by removing one concrete segment at a time.
• For towers with grouted joints, the concrete segment needs to be blasted. Tower blasting operations might cause important inconveniences to the local environment and the use of land nearby the wind turbine location.

Concrete tower blasting vs Nabralift deinstallation. Sources: LA Ingenieros & Nabrawind

Nabralift towers can be disassembled with a simpler process that provides several competitive advantages:

• First, Nabrawind’s Self-Erecting System used for lifting the tower can be used in reverse, removing frame modules from the bottom section of the tower.
• Once all the frame modules have been removed and the transition piece is attached to the foundation, a standard crane is used to remove the rotor, nacelle, and tubular tower. This crane is much smaller and standard than those used for the decommissioning of hybrid towers.

This procedure avoids the usage of big cranes and explosives to uninstall very tall WTGs, representing major economic and environmental advantages.

Foundation dismantling

In most of the cases, after the decommissioning of the turbine, the terrain shall be returned to its original condition. For that purpose, tower foundations must be dismantled either completely or to a certain depth below the top edge of the terrain in the case of deep foundations.

Concrete hybrid towers are installed on gravitational foundations with massive use of concrete and steel, while Nabralift towers integrates deep foundations based on small piles or anchors, as shown in the following figure:

Shredding and transport of tower and foundation components

The next step in the dismantling process includes the separation and transportation for recycling or disposal of the scrap components. The cost of the process depends on the nature of the tower and its disassembly process, as well as the foundation type and volume:

• The components of the Hybrid towers with dry joints are transported to the recycling plants. The process is not complicated, but it comprehends transportation of a large volume of parts (>1000T), with a large logistic cost.
• Hybrid towers with grouted joints needs to be shredded locally, since the materials of the tower are spread nearby due to the blasting process. Concrete and steel reinforcement have to be separated on site, requiring a large time.

Tower concrete after demolition. Source: LA Ingenieros

• Nabralift towers are fully composed by metallic elements designed to maximize modularity and remove transportation barriers in the industry. The disassembly process of Nabralift allows all tower components to be transported in the exact same way (standard small trucks) as they were transported 20-25 years ago to the wind farm.

Recycling of tower and foundation materials

The steel used for the WTGs can be easily reused and recycled. On average, new steel products are made of 37% recycled steel, most of which is recycled at a local level. Steel can be recycled infinitely without losing its properties. Nowadays, there is not enough steel scrap to meet the demand for new steel products, with more than 650 million tonnes of steel recycled every year (Worldsteel, 2021).

A Nabralift tower of 165 metres uses around 500 tonnes of steel, as compared to 150 tonnes for a hybrid concrete tower. Considering a reference steel scrap sell price of 100 euros per tonne, the income obtained for the Nabralift tower surpasses the income from hybrid towers by several tens of thousands of euros.

Steel recycling plant. Source: Siderúrgica Sevillana

Recycling concrete is also viable, but less prevalent than steel due to its economic value, since the recycling process is more expensive and the materials that are obtained do not have a high cost.

The cost of decommissioning hybrid towers may increase in the next years due to the introduction of targeted legislation to hamper the disposal of concrete waste in landfills.

Case study results

The examined case study consists of a wind farm in Germany with 6 WTGs of hub heights equal to 165m. Three tower alternatives have been studied: Nabralift, a hybrid tower with dry joints and hybrid using grouted joints. Nabralift tower is formed by 4 frame modules, the transition piece, and a tubular tower the length of the rotor’s blade. Hybrid towers with a concrete segment of 100m have been examined as an alternative.

The following tables show a direct qualitative comparison of the 3 tower options in terms of cost and environmental impact following the processes described in the previous section.

 

A calculation of the decommissioning cost has been done as per current market prices for cranes, steel scrap, recycling concrete and trucks. The following figure shows the results of the estimated cost per decommissioned tower in this case study:

As shown in the figure Nabralift towers are on average 67% cheaper to decommission than hybrid towers of the same hub height, with a total saving in the range of 250 000€ to 300 000€. This result is consistent with the operational advantages previously exposed:

• Big cranes or explosives are not needed.
• Foundation dismantling is simplified by its small volume.
• The transport of tower components is optimized by means of a logistics-oriented design of the frame and transition structures.
• Tower and foundation materials maximize the steel scrap sold and minimize the concrete to be recycled.

Taking everything into consideration, Nabralift towers represent the best alternative in terms of cost and environmental impact for the decommissioning phase of WTGs with very high hub heights. In a wind turbine market that is now assuming all the life cycle costs and a society that is pushing the economy to greener and more circular products and processes, decommissioning advantages may play an important role on the decisions to be taken for new projects.