Reducing the Cost of Energy for Larger Wind Turbines
Current technical evolution within the wind energy industry seems to indicate an increase in the installed power per tower, thus leading to a need to progressively increase the length of the wind turbine blades. The IndeModular joint system is an innovative solution that overcomes the restrictions related to the manufacturing and transport of longer blades. It is a bolted connection located in the blade spar cap, where the main loads are transmitted. The joint system consists of cell units, which are pre-stressed after their assembly, resulting in a joint system under compression loads and allowing high fatigue resistance. Because of its geometry, IndeModular allows a higher supported and transmitted load than other conventional solutions, and, because of its modular concept, it can be easily integrated in the design and manufacturing of blades of different architectures and lengths by adding more or less cell units. IndeModular was successfully tested in a full-scale mechanical test to validate the joint system under the Germanischer Lloyd 2010 guidelines.
By Javier Sanz, CEO, and José Miguel Maruri, Product Manager, INDEOL, Spain
INDEOL has developed IndeModular, an advanced joint system that enables the separation of the wind turbine blade into two parts and its reassembly afterwards, therefore facilitating the manufacturing, logistics, installation and maintenance of the blade. It has been designed to have an optimal load transmission capacity and, because of its modular design, it can be integrated into any blade.
The Problem
The almost continual increase in crude oil prices and the direct impact on the cost of essential products such as electricity and transport has led to governments becoming deeply involved in the promotion and support of renewable (and sustainable) sources of energy. In recent years many countries have supported the erection of wind farms, most of them with wind turbines that produce less than 2MW (with blades smaller than 42 metres), which are now becoming obsolete due to the continuing high demand for electricity. Therefore, repowering them to more than 3MW should be a must. However, with wind turbine blades now in excess of 45 metres in length there are increasing challenges to be overcome in the transport and building phases of wind turbine construction.
The Sectional Blade
Sectional blades provide a feasible response to current limitations. Even though a slight increase in the manufacturing cost of less than 10% is expected, there is a compensatory reduction in the transport and logistics costs of more than 40%. Because shell and web joints are well known in the industry (basically riveting an aerodynamic blade-shaped cover and bolted plates respectively), most of the joint concepts are focused in the spar cap, where the principal loads that the system has to transmit are located. When evaluating which joint system to use for each particular blade structure, various important factors have to be considered, such as the integration of the joint system into the blade manufacturing process, increase in weight and cost, loads to be transmitted by the joint system, integration of blade systems such as lightning protection, and time required for in-field blade assembly. After construction, other factors will include minimising impact on energy production, possible difficulties with the accuracy of assembly, disassembly and reassembly of the blade for repairs, maintenance or inspections during operation, and, finally, safety during wind turbine operation.
The IndeModular Joint System
The IndeModular technology has been designed to overcome the drawbacks of the wind turbine segmented blade. The joint system is a bolted connection located in the blade spar cap. It is composed of various cell units and each one comprises four metallic inserts bonded to the spar cap, their respective bolts and nuts, and an intermediate plate. After all the cell units are assembled, they are pre-stressed, resulting in the cell units being under compression loads that allow high fatigue resistance. The innovative geometry accommodates a large number of bolts per cap width, resulting in at least a 15% increase in the supported and transmitted load per spar cap when compared to other bolted solutions. Due to its modular design, the joint system can be integrated into many different blade architectures (e.g. spar box, single and double spar webs) and blade lengths.
Structural Calculation
The loads to be transmitted by the joint system were calculated according to IEC61400-1 ed. 2 using the GH-Bladed Version 3.80 on a 5MW onshore class II-A wind turbine with a 120-metre tower height and 61.5 metres blade length. The metallic parts and the epoxy adhesive to bond the inserts to the spar cap were off-the-shelf. The structural calculation of the bonded joint was based on GL-2010, and analytical and finite element (FE) approaches were performed for the metallic parts. The FE model was used to calculate the load factor of the bolts, the contact loosening between the intermediate plate and inserts under extreme loads, the complex stress states at the intermediate plates and the bending effect on the bolts caused by the asymmetry of the joint system. The assembly preload was also calculated in order to prevent any separation between inserts and the intermediate plate during operation.
Risk Reduction Tests
A risk reduction test programme was also conducted to ensure the reliability of the joint system solution. First, drilling tests were performed to study the holes where the inserts are bonded to the spar cap: minimum drilling distances, defining the hole dimensions and the drilling tools needed. Then tests related to the spar cap-insert interface were conducted: processability, static and fatigue tests were performed on adhesives from different suppliers, and shear strengths and fatigue performances of the selected adhesives were characterised. After these tests were concluded the most suitable metal insert surface treatment was selected, and insert extraction tests were performed to verify the extraction force of the insert was higher than the design expected value. Static and fatigue tests were conducted, and it was confirmed that the bolt load factor coincided with that predicted by the finite element method (FEM) analysis. Finally, bolt tightening tests were carried out to verify both the resistant capacity and the manoeuvrability of the wrench inside the intermediate plate slots.
Test Validation
IndeModular was successfully tested in a full-scale mechanical test to validate the joint system under the Germanischer Lloyd (GL) 2010 guidelines. The test was divided into three phases: an initial static test with compression and tension loads up to 2MN, a fatigue test where fatigue loads of ±565kN were applied up to two million (2E6) cycles, and a final static test with compression loads up to 2.5MN and tension to failure. The test was performed using a 3MN universal Instron testing machine, and non-destructive inspections such as visual, eddy current and liquid penetration were done throughout the test. Each part of the specimen was instrumented with strain gauges to detect the force distribution throughout the assembly, the pre-tension level of each bolt and the bending effects, among other things. GL personnel evaluated, reviewed and verified the tests and tests analysis carried out to validate the system under their guidelines.
Major Features of the IndeModular Joint System
GL has validated the fact that each cell unit of IndeModular can transmit an axial force of 825kN and a maximum equivalent axial force of ±213kN at N = 2E6 cycles and R = –1.0. Nevertheless, the loads obtained during the test campaign were under a conservative scenario and the performance of the joint system will be enhanced by the use of a calibrated, non-manual tightening tool. The impact in cost and weight of implementing IndeModular in a blade has been calculated to be less than 10% of the total cost of the monolithic blade. Furthermore, it has been confirmed that the joint system has no significant impact on the dynamic behaviour of the wind turbine, since natural frequencies were very similar between monolithic and IndeModular sectional blades. INDEOL has therefore developed a reliable and economic solution for wind turbine blades over 45 metres length, which overcomes the major drawbacks of sectional blades.
Biography of the Authors
Javier Sanz is CEO of INDEOL, a company which provides green energy solutions ready to be implemented in the industry. INDEOL has also developed several technologies and patents which improve the performance of wind turbines.
José Miguel Maruri is Product Manager of INDEOL. He holds an MSc degree in Industrial Engineering and a Masters in Renewable Energies. Since 2005 he has been working in clean technologies, including both energy storage and renewable energy generation.
