Improvement of advanced design tools

◀ Back to projects overview P201203-006-ECN

Wind energy has to compete with other sources of energy mainly based on costs. For this reason there is an ongoing effort required to reduce the cost of energy from wind farms. Within this project, there were several objectives all expected to result in a reduction of the cost of energy due to reducing in the cost of the turbines, the O&M costs, as well as a possible increase in power production. The objectives were to reduce Costs of Energy by:

A. Develop advanced control strategies for active loads reduction in the wind turbine structure and transfer this knowledge to the industry.

B. Develop and verify advanced integrated design tools for increased accuracy in design load predictions.

Results obtained
Both main objectives in the project were rather separated, and therefore the approach and results are described below per objective.

1. Advanced control:

Conventional wind turbine controllers contain control algorithms for regulating the power production and rotor speed, but lack control features for the reduction of fatigue and extreme loads on blades, tower, drive-train etc. The implementation of newly developed load reduction algorithms into the controller allows to achieve cheaper turbine designs per installed capacity by cutting down in material costs or by designing larger rotors. With the constantly increasing rotor sizes, there is an increasing demand from industry for the implementation of such loads reduction algorithms into their wind turbine controllers.

For this purpose, within the FLOW program ECN developed and verified a tool for the design of cutting-edge industrial wind turbine controllers, that allows the industry to achieve an optimal balance between maximal power yield and minimal loads. The new developed tool is given the name Advanced Control design Tool (ACT). Innovations included in ACT are control algorithms for:

Reducing fatigue loads on blades, tower, drive-train, nacelle, hub and yaw system
Reduction of the number of shutdowns due to strong wind gusts
Reduction of extreme loads on tower and blades during emergency shutdowns due to failures (loss of grid connection, failures in pitch mechanism).

In this FLOW project ACT is developed and applied to the 2-bladed 6MW wind turbine 2B6 of project partner 2BE. In the connected FLOW project P201204-003-ECN the controller is designed and fine-tuned for a 3-blased wind turbine XD115 of XEMC Darwind and verified in the field.

To this end, the ACT controller is implemented on the XD115 prototype situated at ECN’s test site in Wieringermeer, and tested extensively during long period of time. The measurements have been performed by ECN in accordance with IEC standards, and compared to the measurements from a previous measurement campaign during operation of the original XD115 controller. A comparison indicating that the ACT controller achieves a huge reduction of damage loads on the blades: 20.1%. An example is shown in figure 1.

Figure 1: Measurement results for blade root loads with original XEMC Darwind controller (blue) and ECN new designed controller (red) [14].

The design tool ACT has already been used in several other commercial wind turbine projects, helping turbine manufacturers to improve their controllers. The tool is available also in the form of a license, and one turbine manufacturer already expressed interest in obtaining it.

2. Advanced Integrated design:

Within this project WMC has developed and performed an automated optimization of a representative jacket support structure for offshore wind turbines, using FOCUS6. Diameters and wall thicknesses of members of the structure were parameterized, and the connectivity of the joints and members were kept as defined in the original design. The results comparing key parameters for the optimized and baseline configuration are summarized in the following table:

	Baseline	Optimized	%
Support structure mass	1	0.997	-0.27
Max. joint fatigue damage	1	0.812	-18.81
Max. joint ultimate load	1	0.891	-10.91
Max. member ultimate load	1	0.878	-12.15

All maximum loads within the support structure, as presented in this table, are reduced compared to the baseline design, indicating that failure is less likely. The results show that running an automated optimization in FOCUS6 can help wind turbine designers to find ways of decreasing fatigue loads on support structures without increasing material costs. Therefore failure of the support structure joints is less likely and the lifetime of the support structure may be extended.

Within the advanced integrated design activity, ECN focused on the accuracy of the currently used state-of-the-art tools for loads analysis of drive train, yaw system and pitch system.

As a highlight: the industry-standard state-of-the-art approach to aerodynamic modeling, based on the Blade Element Momentum (BEM) theory, was compared to the much more detailed free vortex method AWSM. The comparison illustrated that the loads predicted by the conventional BEM method were higher. Furthermore, the BEM method predicted also that the rotor speed at which high vibrations can occur in the blade due to classical flutter is lower as compared to the AWSM prediction, which is also conservative. This implies that blade designs performed using the standard BEM approach might be unnecessarily conservative, and that material cost reductions might be possible by basing the design on the more advanced AWSM model. Still, the calculation time is currently a restrictive factor for using AWSM for performing all design load calculations, since one single simulation takes several days on a single computer. However, even a small sized computer cluster may allow to simulate the relevant design load cases within acceptable time.

This example and items concerning: yaw control, IPC (individual pitch control), tower damping control, detailed gear box analysis, show that specific load calculations on these main components have room for improvement and could lead in the future to less heavy structures.

Contribution to FLOW targets
This project contributes to the reduction of the overall cost of energy by influencing the capital expenses (CAPEX), O&M costs, and the energy production. The results enable either a CAPEX reduction or an OPEX reduction, or even both. Reducing loads and increasing accuracy can lead to either a reduction of the mass of the total structure or it can prevent early failures and required replacements.

CAPEX can be reduced due to the reduced extreme and fatigue loading on the main wind turbine components, as realized by the new control tool ACT, allowing for material reduction. Furthermore, the improved design tools enable a more accurate prediction of loads. The resulting reduction of design uncertainty enables reduction of the safety factors used in the design, and therefore material costs savings. Finally, the integral support structure design approach deliver yet another (direct) contribution to lowering the material costs.

O&M costs are lowered more indirectly by the results of this project. By reducing fatigue loads on the main components, the controller effectively reduces the damage induced by structural vibrations. This decreases the failure rates and, consequently, unplanned maintenance costs. An additional contribution to the O&M cost reduction is given by the increased reliability of the loads predictions, achieved by the improved design tools.

Finally, the energy production is also expected to increase, primarily due to the reduction in the number of failures of mechanical components, and thus overall reduction of the downtime.

All these effects combined could result in a reduction of 4.8% of the LCoE. It should be pointed out that this concerns the combined effect together with the sister FLOW project P201204-003-ECN.