Nabralift 200 meter hub height impact in LCOE

The following article has been published for GWEC Asia:

 

Current market trends are showing a clear path within the wind industry. The size and power of the wind turbines are greatly increasing. There are several facts boosting this trend. To sum up, as this is not the subject of this article, the wind industry is trying to reduce LCOE by these means.

In Navarra, for instance, a pioneering wind energy region in Spain, where the headquarters of Nabrawind are located, this trend is clear. The first wind farm was installed in 1994. This year, the projects under installation have, in average, four times longer blades, nine times more powerful turbines and three times higher towers.

The undeniable fact is these new wind turbines produce more energy, greatly thanks to the extra power and height offered by the new towers technology.

Nevertheless, this should not be the only issue to take into account. In most of the countries, for example, the best wind farms emplacement are far away from the main populated cities, which limits this kind of energy implementation. However, if we were to contemplate higher hights, it would turn out the suitable emplacements grow exponentially.

Technology Limitation

Nowadays, it seems to be a hub height barrier at 160 meters. Technology limitation makes not worthwile to go over that hight. However, Nabrawind has developed a new kind of self-erecting tower which solves this limitation.

Maximum hub height but tower technologyThis tower, named Nabralift, consists on a three columns structure installed under the uppermost part of a WTG tubular tower. It integrates the Self-Erection System (SES), that allows the installation of a full WTG without using large-size cranes regardless of the final hub-height.

The installation of the first prototype was completed in august 2018, taking advantage of the key characteristics of this tower. These are:

• Minimum assembly platform (1.200m²).
• Self-erecting system, avoiding the use of large cranes.
• Easy logistics. Transportation of the tower components by standard trucks.
• Pile foundation, that allows an 80% reduction of the concrete requirements.
• Elimination of frequency resonances.

Thanks to these characteristics, the increase in the cost of the tower is not proportional to the increase in height. In fact, Nabrawind has confirmed a positive impact of Nabralift in most of LCOE projects. Let’s check, then, some scenarios.

The first one shows a wind farm where the average wind speed is 7 m/s with a windshear of 0.15. Logically, the bigger the windshear, the bigger the Nabralift impact in LCOE, as it basically means the wind is stronger as you grow in height.

In this scenario it can be apreciated LCOE reductions from 3.5MW turbines and on. The impact is even more significant with more powerful turbines, such as the 5.5MW and 6.5MW ones. Optimum hub heights for each turbines are different, raging between 160 meters and 190 meters. If we were to increase the windshear, then the benefits are even greater.

In this case, LCOE reductions would range between a 3% and 6%, with optimal hub heights ranging from 170 meters to almost 200 meters. But the benefit would be maximum in Ultra-Low wind emplacements (6.5m/s).

In this scenarios, LCOE reduction could reach a remarkable 7% with hub heighs, again, up to 200 meters.

All this cases have been developed according to the cost installation of Nabralift. Consequently, it seems reasonable to explain in detail the Nabralift characteristics that make this possible.

Nabralift Tower Characteristics

This tower, made out of steel, consists on a three columns structure installed under the uppermost part of a WTG tubular tower. It also integrates the Self-Erection System (SES), that allows the installation of a full WTG without using large-size cranes regardless of the final hub-height.

The simplicity of this structure makes it a cost saving tower. In detail, the three columns structure is constituted by vertical columns and diagonals and horizontal nexus.

The vertical columns are, basically, one meter diameter tubes, a technology largely and successfully incorporated by the oil & gas industry. The flanges incorporated are just conventional flanges made of ring rolling, while the outer joints are traction bolts.

Regarding the diagonals, these are 400mm tubes, standardized and commonly used in several industries whereas the steel terminals have been specifically designed. Joints are accomplished by friction using lock bolts, which are maintenance free.

The other key aspect of this tower is the transition piece between the jacket type structure and the tubular tower. This piece hold the loads of the wind turbine and, then, distribute them through the Nabralift strcture.

Installation of the transition piece for the Eslava’s prototype and the updated versionAs it can been seen in the picture, the transition piece has been recently upgraded and new projects will be installed with the transition piece 2.0. This new version can handle higher loads without significantly increasing the cost of manufacturing. Its structure is composed by several parts which are the following:

• Upper Ring
• Monotubular tower segment
• Lower ring
• Strut
• Radial
• Corner connector
• horizontal

Nabralift Installation

But how does the installation process work with a self-erecting system and no large cranes? This is an exceptionally brief process which has been successfully proved during the installation of the first prototype of this kind of tower in Eslava, a village near Pamplona, an important wind industry hub in Spain.

The standardized system installation for this tower, and obviously the one followed and polished during the Eslava project, is conceived so as to install the lowermost sections of the tower at the last step of the assembly process. For this purpose, the self-erecting system is able to hoist the WTG in intermediate stages.

The self-erection process consists, thus, of the following steps:

1. The tower transition is assembled and fixed on the foundation interfaces by means of a small crane.
2. The tubular tower sections are installed on top of the tower transition by means of a standard crane.
3. The WTG nacelle and rotor are installed by means of a standard crane
4. The SES is installed (SES include an independent foundation to distribute load in the terrain surface).
5. A frame module to be installed is assembled close to the WTG structure
6. The SES jaws clamp the tower transition, that is then detached from the foundation. The SES elevates the WTG tower up to a height slightly higher than the frame tower module.

7. The frame module is guided to the lowermost part of the structure by means of a group of rails, and fixed to the foundation interface.
8. The WTG tower is lowered by the SES and fixed to the lowermost frame tower module columns.
9. SES jaws unclamps the tower and descend to the lower part of the frame module.
10. Steps 5-9 are repeated for every frame tower module.
11. The SES is uninstalled and moved to the next wind farm position.

Nabralift Benefits

Going through the specific benefits of this tower, the use of the concrete for the foundation is one of the parts in which the savings are great.

The Eslava’s prototype has confirmed that the assembly platform can be reduced to its minimum. In fact, compared to other solutions, this kind of tower only requires 1200 m² of assembly platform, reducing the foundation volume between 30-50%.

The outcome is that the assembly platform is finished in a record time of just one week. On the other hand, the concrete requirements are minimum, avoiding, thus, the emission of thousands of CO2 tones.

Even more, apart of this gravitational foundation, a pile foundation can be also used as a natural connection between the three columns structure and the ground. The savings with this pile foundation are 60 % of the conventional tower foundation cost, as the installation of another project in Morocco has confirmed.

On the other side, it is also worthy to note that most of the components of the tower can be transported in standard trucks, which highly reduces the transportation cost. It is also installed without large cranes, which are precisely the most expensive component of any WTG installation process.

On the other hand, the installation process in serial production will allow to install a +160m tower each 4 days. As a clear demonstration of that, this first prototype installation was concluded in just ten days, thanks not only to the fast assembly process at floor level, but also to the fact that the Self-Erecting System can work at higher wind speeds than conventional cranes (up to 15 m/s).

Finally, but not last, the enormous rigidity of the lower part of the tower moves the natural resonance frequency of the tower away from the rotor speed or blade passing frequencies, eliminating the risk of frequency resonance.

All in all, it seems clear that reaching 200 meters hub height has a positive impact on LCOE and, now, it is totally possible to do so.