InnoTip - Innovative offshore tips to improve wind farm yield

Background
InnoTIP (Innovative rotor blade tips to improve offshore wind farm yield) is a collaboration project between Energy research Centre of the Netherlands (ECN) and LM Wind Power (LM) that aims at reducing the Levelized Cost of Energy (LCoE) of offshore wind by improving turbine yield as a result of an improved blade tip geometry.

Objective
InnoTIP aimed at developing and demonstrating three innovative blade tip shapes designed for offshore wind turbines. The innovative offshore blade tips aims to increase efficiency and power and in addition increase turbulence in the wake of the turbine so that recovery of the wake is improved.

Short description of activities
InnoTIP is aimed at developing three different tip shapes, which have later been manufactured as retrofits for testing on ECN’s 2.5 MW test turbines in EWTW (ECN Wind turbine test field in Wieringermeer). During the conceptual design phase, a number of software tools have been used to find an optimum design for each of the 3 concepts (turbulators, winglet and conventional tip).

For each concept, a number of parameter variations have been studied. The goal was to find the most optimum tip shape that results in the most beneficial LCoE decrease (aimed at increase in yield) within the specified restrictions. Once the conventional tip has been designed, the winglet has been designed to be comparable to it. Some off-design conditions have also been evaluated before choosing the final tip shape. Finally, the performance of the configurations for winglet and conventional tip have been confirmed by CFD analyses. The turbulator geometry has been designed using the lifting line method LLM, since in this tip configuration the wake is of main interest. 

In the detailed design phase, the issues regarding the lightning protection, structural strength, attachment and drainage have been solved for the final tip concepts. The inputs from a large number of stakeholders have been gathered and used in the detailed design phase to optimize the design and implementation methodology. Once the designs reached a final state, prototypes of the tips have been made in full scale and installation and removal tests on a scrapped blade tip have been performed in a controlled environment. Following the installation test, a structural test for the extreme loads has been performed to verify the structural strength of the tips and the connection method. Finally, the tips have been removed and the original blade tips have been successfully restored. 

The field tests have been performed at EWTW, where five Nordex N80 2.5MW turbines are available for use. Besides the SCADA data of the turbines, ECN’s calibrated instruments have been installed on these turbines to collect high quality data. The experiments are combined with MetMast measurements , which has been used for the verification of the wind and weather conditions. The turbulators and conventional tips have been installed and tested simultaneously in a side-by-side configuration to reduce the uncertainty coming from the monthly and seasonal changes in the wind characteristics. To quantify the effects of the new tips on the turbine performance, the changes in power production of the turbines with the new tips are compared with the changes of power production of the turbines without the tips for the periods before, during and after the test campaign. 

Results
Three tip concepts have been designed and evaluated in the project; the conventional tip, winglet and turbulator. For testing, the conventional tip and winglet have been manufactured to be installed on top of the blade tips on the 2.5MW wind turbines in the test field. The turbulators have been glued to the trailing edges of the existing blade tips. Due to the time restrictions of the testing period, the winglet was not installed, therefore only the turbulators and the conventional tips have been tested in the field. The results of the experiment showed 2 to 9% increase in power with the conventional tips and 2 to 10% increase with the turbulators.  

Using these results in the cost analysis indicated that the new innovative tips as designed and applied in Innotip project, can reduce the LCoE at least 1% excluding the blade length extension. This is does not reflect the full potential of the innovative tips due to the design limitations included in the implementation phase coming from the fact that the new tips were applied as retrofit solutions and the tip designs were not integrated in the blade designs. In reality, when the new tip designs are integrated in the actual blade designs, more potential is expected; at least 35% more by looking at the earlier analysis performed in the project.