WTG dynamics in soft & stiff towers beyond 160m hub height
The latest generation of wind turbines announced by the leading OEMs will shift wind turbines to rotor diameters of 150m and blade tip heights above 240m. As a consequence of this, the transport and installation ofmassive blades and towers becomes one of the great challenges in the renewable energy industry over the next decade.
Nabrawind Technologies has developed two solutions to break these logistic and installation barriers: a self-erecting tower and a modular blade joint, both simple to integrate in all existing and future wind turbines.
NABRALIFT, THE CRANE-LESS SELF-ERECTING TOWER
Towers with heights over 120m are now a reality in all markets, reaching already heights up to 178m in Europe. In 2016, more than 2GW of towers higher than 120m were installed and it is expected this will grow to more than 10GW a year within 5-10 years (more than 15% of the total wind energy market). In fact, all major producers of wind turbine generators have included towers with heights beyond 160m in their catalogues.
However, the challenges of these large scale towers are limiting its installation in many cases: exponential increase in cost, difficulties with transport and/or logistics design constraints, scarce availability and high mobilisation and rental costs of cranes, increased time required for the installation of the wind farm and lastly an increase in the tower slenderness that complicates the compatibility between the tower stiffness and the rotor speed.
For several years there have been a variety of proposed alternatives to solve one or several of these problems. However, no solution has been able to solve all problems at once like Nabralift, Nabrawind’snew crane-less tower.
Nabralift consists of two different parts. The upper part of the tower, with a length equal to that of the blade, is a tubular steel tower, which is a conventional technology that is widely recognised and very competitive if the aforementioned barriers didn’t exist. In the lower part, a framed structure is introduced with three main columns of approximately one metre diameter, connected diagonally, providing the required torsional and shear rigidity. These columns have a distance between each other of between 14m and 18m, which provides high stiffness and a significant weight and costreduction compared to other tower solutions.
Its straight profile and constant distance between the tower columns simplifies the self-erection, which is carried out with conventional hydraulic devices widely used in the heavy lifting sector. This self-erection is carried out once the nacelle and the rotor are installed, eliminating the need for large cranes during the installation and operating stage of the wind turbines. The next image shows the differences between the conventional lifting of a wind turbine and the new system.
This solution overcomes all the existing problems with large scale towers:
- The logistics of all components can be carried out with standard ISOshipping containers (all components are less than 12m long).
- The installation on site does not require large cranes, hence the cranes required are widely available. In addition to the benefits of not using expensive cranes, the surface required for the installation is also reduced significantly.
- The self-erecting system is sized to work at higher wind speeds than conventional cranes (up to 15 m/s). This almost halves the time lost due to inefficiencies when components cannot be lifted using conventional lifting methods. This, combined with the simple tower structure and the use of smaller cranes allows a significant reduction in the time required to install the wind turbine generators.
- The high stiffness of the lower part of the tower shifts the natural resonant frequency of the tower away from the rotor speed or blade passing frequencies, eliminating the risk of frequency resonance that is a design-driver of tubular steel towers above 120m in height.
- As a consequence of all this, the cost of the tower is reduced in all its components. The foundation volume is reduced by 30-50% due to the use of three individual footings located in a very efficient layout. 20% of the mass in the lower section is saved, and therefore also its cost. Transport in conventional trucks also minimizes logistical costs, and installation costs are minimized due to the self-erection process without large cranes and the smaller installation pad.
Putting all this together, the savings for a turbine of 120-140m can be higher than 100k€, and this saving increases for higher towers.As a consequence of this, the Cost of Energy (CoE) can be reduced by 5% in wind farms with a wind resource appropriate for high hub heights.
Tower cost evolution and cost of energy in relation to height
Certification process and a real full-scale prototype of the first model of Nabralift (160m height tower, designed for the 3-3.5MW range wind turbines) will be completed in the first half of 2018. First commercial deliveries are expected for 2019.
SOFT vs STIFF XXL TOWER DYNAMICS
As previously mentioned, one of the main advantages of the Nabralift tower is its increased stiffness compared to other metallic solutions. The impact of the performance of the wind turbine due to this characteristic of increased stiffness is presented below.
Stiffness-based tower classification
Towers are defined as soft-soft or soft-stiff depending on the natural frequency of their first mode. Soft-soft towers have a frequency that is lower than the rotor’s natural frequency when it rotates at nominal speed (called frequency 1P). Therefore, these towers enter into resonance with the rotor at some point between start-up of the wind turbine and its nominal speed of operation.
When the frequency of the tower is superior to the 1P but inferior to the frequency of the blades passing the tower, 3 times the frequency of the rotor (called frequency 3P), it is classed as a soft-stiff tower.
Standard tubular towers of heights greater than 110-120m are no longer soft-stiff and become soft-soft, however this does not occur in concrete towers or in the Nabralift solution which maintains its first mode above 1P (see Figure 1). These definitions are explained graphically in the Campbell Diagram of Figure 1, which includes:
- Two prohibitive frequency bands that are associated with the 1P and 3P frequencies at nominal speed.
- The rotation frequencies 1P and 3P as a function of the rotor speed.
- The frequencies of two tower models selected to show the comparison of the dynamic behaviour of the wind turbine, with a hub height of 160m: a conventional metallic soft-soft tower and a Nabralift soft-stiff tower.
Figure 1 – Comparison of Tower Height vs. Frequency and a Campbell Diagram
To evaluate the influence of the stiffness in the dynamics of the Wind Trubine, the design of the first model of the Nabralift tower of 160m in height is compared with the predesign of a steel soft-soft tower. For the same hub height (160m), same rotor diameter (130m) and same nominal power (3.5MW), a difference in mass of almost 200T is seen between the two technologies. Furthermore, due to the difference in stiffness of the two towers, the deflection of a standard tower under a static load of 1MN is more than double that of the Nabralift tower, see Table 1.
Table 1 – Definition of the selected models of tower
Dynamic Simulation Results
Dynamic simulations of both tower models have been undertaken using the Bladed® software (DNV-GL). The comparative study was carried out using the IEC 61400-1 standard. More than 2,000 simulations were made to determine ultimate load state and more than 400 simulations were used to determine fatigue loads.
To verify the influence of each technology on the behaviour of the wind turbine, the deflection at the top of the tower and the bending moment at the root have been analysed (see Figure 2).
Figure 2 – Definition of the simulation results compared
The most interesting results of the analysis are seen when comparing the behaviour of the wind turbine in critical design scenarios:
- At nominal operation with maximum head thrust and turbulent winds (DLC1.2 in the standard), the wind turbine installed on a soft-soft tower operates with a significantly large tower head deflection (see Figure 3.a) and high oscillation. This causes an increase in loads in the root of the tower, as seen in Figure 4.a.
- When the wind turbine operates under turbulent winds and a failure occurs (DLC2.4 in the standard), the aerodynamic loads to suddenly decrease at the moment of failure (the pitch system acts quickly to prevent the rotor from accelerating). The braking of the rotor creates a high inertia load causing the head of the tower to oscillate (see Figures 3.b and 4.b). In this scenario, the stiffness of the tower is very significant and causes a great difference in the behaviour of the model, leading to very large deflection and load cycles in the soft-soft tower. Normally these types of cases are determining the static design of the tower and its foundation and have an high influence in the fatigue design spectrum.
- Finally, in the scenario of the wind turbine stopped with the rotor in idling and very high winds (DLC6.4 in the standard) there is very little difference in the behaviour of the two models. The only difference is in the deflection due to the difference in rigidity of the two systems (see Figure 3.c and 4.c), but no relevant load differences are observed.
|Figure 3.a – Case A: Tower top deflection|
|Figure 3.b – Case B: Tower top deflection|
|Figure 3.c – Case C: Tower top deflection|
|Figure 4.a – Case A: Bending moment at tower base. Loads for ULS and FLS.|
|Figure 4.b – Case B: Bending moment at tower base. Loads for ULS and FLS.|
Figure 4.c – Case C: Bending moment at tower base. Loads for ULS and FLS.
The comparison of the overall results (all the load cases required as part of the IEC standard for ultimate limit state static and fatigue loads) shows a considerable difference between the two tower models in terms of extreme bending loads at the tower base (see Figure 5). The difference is even greater when considering the equivalent fatigue load outside the range of resonance frequencies. This comparison highlights the advantages for the tower and foundation design that Nabralift gives by allowing the possibility of a metallic soft-stiff tower above standard heights.
Figure 5 – Global Results: Bending moment at tower base. Loads for ULS and FLS.
Therefore, the load analysis comparison concludes that:
- The flexibility of the tower increases both the static and fatigue loads for high towers, with a moderate increase under normal operation simulations but a significant increase in shutdown loadcases (becoming critical for soft-soft towers).
- The increase in loads increases the effort necessary in the design of soft-soft towers and their foundations, since higher design loads can lead to tower reinforcement that can shift the tower natural frequency too close to the 1P forbidden band.
CRISTINA PÉREZ GARCÍA, Senior Engineer