The objectives of the European Union for the next years when it comes to achieving climate neutrality in 2050 and drastically reducing the emission of GHG (greenhouse gas) by 2030 are great opportunities for the wind sector for the next years.
To achieve the targets, the cost reduction of the new wind turbine models continues to be a crucial factor, also because the prices in energy auctions all over the world are decreasing drastically.
In the last decades of the XX century, the wind market was starting to grow therefore there was not big interest in optimizing the foundations. Most of the foundations were square shaped and with constant height. Plus, some OEMs used the steel ring system as the interface between the tower and the foundation.
In the first decade of the XXI century, there was a transition in the interface system from the steel ring to the anchor cage which was proved to be a more robust solution and the geometry was optimized from the square shaped to the circular variable height foundation.
After that, the race to reduce the LCOE started, and the foundation of the wind turbines was further optimized, and different sophisticated concepts were designed and patented.
In the context of the growth of wind turbine models, the foundations are also affected. Generally, as wind turbines become taller and heavier, there are various aspects of the foundations that need to be considered:
- The anchor cages due to sectorization, weight increase and special adjusting equipment needed (see example of sectorized cage).
- Higher complexity in the reinforcement and concrete pouring process, especially regarding additional resources that require a very strict coordination and planning to avoid issues in the most critical phase of the foundation execution.
- Since wind turbine’s foundations are massive concrete structures and the concrete volume is increasing with the scaling up, it is important to control the temperature at the core of the structure and the temperature gradient between the surface and the core, especially in locations with high ambient temperature.
If we talk about the foundation design, there are three factors that are different from a common foundation for other structure:
- The bending moment is the main load which implies that there is a high eccentricity of the load.
- The requirement related to the minimum dynamic rotational stiffness to mitigate the risk of resonance with the frequencies of the rotor rotation.
- Gap control to avoid the soil degradation due to repeated gapping cycles over the wind turbine lifetime.
Cost and alternative designs
Foundations of wind turbines represents approximately between 4% and 10% of the total wind farm cost depending on the geotechnical conditions, loads level and local constrains (such as material cost, location of the project, resources, experience of the civil contractor, etc…).
Since the foundation cost is significant, many engineering companies and the OEMs are trying to come up with new solutions to further optimize this component of the wind turbine.
The foundation typologies for conventional steel towers mentioned in the article are as follows: standard (circular shaped and variable height with anchor cage), octagonal shaped, EPS layer (soft spot), wall’s foundation, braced foundation, precast solutions, Patrick & Henderson and rock anchored foundation. Each link leads to a picture of the typology.
The table below includes an extensive comparative analysis of the above-mentioned solutions. There is no optimal solution for any load level and geotechnical conditions, a technical and economic study must be carried out to select the appropriate solution for each project.
There is also a promising solution called “Nabralift”, that is patented by the company Nabrawind, it is Self-erection system based on a three-column framed structure at the bottom of a conventional steel tower and it is supported by 3 special piles as a foundation system, known as Nabrabase.
Other concept, not mentioned in the article due to extension limitations, is the Airbase concept (by HWS). The concept consists of two precast beams that rest in four shallow isolated foundations or some piles.
Future
There is no clear consensus on which tower solution for wind turbines will be used most in the future. Currently, the solution being used when high hub heights are required is hybrid technology (concrete-steel).
The imminent evolution of tower technology will impact the foundations. Currently, the most common foundations are specifically designed for conventional towers, and in the future, the design will be deeply influenced by the tower design.
Life extension, repowering and sustainability
The extension of the operational life is a key aspect for OEMs and wind project operators. Regarding foundations, there is extensive research on fatigue safety monitoring and the evolution of crack width. Additionally, there is a lot of documentation on the failure mode of the steel ring and the reinforcement solutions and systems to address the issue.
Repowering will be also crucial in the future, as the best locations from wind resource perspective are, in general, already occupied by old wind projects, therefore installing new and more powerful wind turbines is a very interesting aspect to increase the energy generated and reduce the environmental impact.
When it comes to the foundation in repowering projects, if the existing foundations are demolished, the recycled aggregate resulting from the demolition can be reused for the construction of the new roads and hardstands. Other options include the manufacturing of new massive concrete structures or their use as lean concrete for the new foundations.
Conclusion
Finally, as a conclusion, simply note that it is necessary to do a technical and economic analysis to choose the best solution for a specific project considering the loads at tower base, the geotechnical conditions, and the local constrains.
It is important to consider the right balance between materials savings and the execution complexity when it comes to selecting a specific concept.
Access the full paper: Foundations of onshore Wind turbines: current situation and trends
Jiménez Toña, R., Cuadrado Rojo, J. & Rojí Chandro, E. (2024). Foundations of onshore Wind turbines: current situation and trends, 76(573), 6443. https://doi.org/10.3989/ic.6443
Cimentaciones de aerogeneradores onshore: situación actual y tendencias
