The trailing edge of wind turbine blades are commonly manufactured as an adhesive joint of the pressure-side and suction-side composite panels of the blade. Under some conditions, a lead-to-trailing (LTT) edgewise bending moment can induce buckling at the trailing edge adhesive joint, which may lead to early failure of the blade due to delamination. As a structural instability, buckling in wind turbines has been the focus of much research especially in full-scale tests and more recently at the sub-component level. These higher-level tests, however, are done on pre-manufactured wind turbine blades and require extensive preparation in order to adapt the testing rig to each blade section, as well as incurring into elevated costs.
An additional test level has been suggested for elements and details of wind turbine blades. It has been suggested that this level can fulfill many purposes: New concepts, modifications, material combinations and orientations can be tested, partial safety factors of larger scale tests can be reduced or even certifying minor details of the blade can be done at the element and detail level. As such, the focus of this project is to develop a testing method for a simplified trailing edge bonded joint with a custom designed hinged clamping system upon which a compressive moment can be imposed to induce buckling.
The design of this test will initially be based on a semi-analytical buckling plate model, where in-plane and out-of-plane displacements are coupled through the Von Karman strain-displacement relations. This semi-analytical tool is employed to quickly estimate the buckling loads for plates of varying dimensions. Strain-free imperfections can be included in the model for twisted/pre-bent plates in order to estimate their effect on reducing the load-bearing capacity of the structure. The semi-analytical tool is complemented with FE models for all the design parameters.
The semi-analytical and numerical results are compared to demonstrate the agreement of both approaches aimed to provide a sturdy base for the research. Next, the experimental buckling loads and force-displacement curves are shown against the predictions from the previous approaches with good agreement. Nevertheless, the observable discrepancies between the experimental and numerical results showed that the desired joint-fixity at the boundaries was not fully realized, therefore leading to a slightly different post-buckling behavior. In the end, suggestions are given to improve on the experimental clamping system in order to improve and expand the scope of this research.

The Kite Power Group of the Wind Energy Department of Delft University of Technology has developed a pumping kite system to harvest high altitude wind energy (HAWP). HAWP technology is one of many renewable energy solutions that are emerging in a time at which Earth’s non-renewable energy resources are becoming scarce. The kite power system operates in periodic pumping cycles, alternating between reel-out and reel-in of the main tether from a drum, which in turn drives a generator. Electricity is generated during reel-out and only a small portion of the produced energy is reused during the reel-in process, which gives a positive end result. The current system has a movable ground station and contains all basic elements required to operate the kite in flight. The technology of the kite control unit is still is at an early stage of development and is being further developed and extensively tested at this moment.
The future of the kite power technology depends heavily on the possibility of the kite power system to be fully automated and autonomous. In the current state of development of the system, a ground crew of at least three people is needed to operate the system which makes it highly inconvenient and costly in practice. As part of the Design Synthesis Exercise (DSE) a group of nine students was given the task to develop an automated launch, landing, and storage system for an upscaled version of the specific pumping kite power system which uses a kite of 70m2. Automation of the entire operation will enable the technology to be implemented on a larger scale and as such become economically profitable.
Going through a full design process that ended with a detailed design of a launch, landing, and storage system in a period of 10 weeks required a well structured planning. Systems engineering methods were used which divide the design process in three main parts marking their respective completion in specific milestones. The first part covered the entire project planning and the preliminary design phase with a baseline report as the first milestone in the design process. In a highly creative manner a large number of ideas was produced. Subsequently, each one of them was evaluated by means of a trade-off and obvious losers faced immediate elimination.
The second part of the design process was focused on the conceptual design of a limited number of concepts. At the start of this phase all ideas from the first part of the design process were integrated into full-system concepts. The feasibility of each of the integrated concepts was evaluated on a limited number of criteria after which another trade-off was performed. Four different concepts were chosen and were further developed in the remainder of the conceptual design phase. Multiple analyses were performed on topics such as functionality, technical details, sustainability and finance to obtain a good overview of the overall performance of each concept. After extensive research on all four concepts another trade-off was carried out and the final selection of one definite concept was made. At this point the conceptual design phase reached its milestone. Both the clients and kite power experts have confirmed the decision to take this concept into the detailed design phase.
The third and final part of the systems engineering approach is the detailed design phase. In this phase the chosen concept was extensively researched and upgraded to a detailed design over a period of three weeks, and the results are contained in this technical report. Prior to research on the technical properties of the detailed design a detailed design overview was produced. The functionalities of the system were fully explored and put on paper. Furthermore the automated system was divided into subsystems and corresponding mechanisms. This enabled performing detailed research on specific elements of the system in subsequent steps of the design process. At the start of this third phase, pertinent background studies were performed to acquire a thorough understanding of both the aerodynamics and the commercial, environmental, and legal frameworks. The detailed research was first performed on all the subsystems, whereafter the integrated system got a closer look. The analytical approach was hereby always set prior to any calculations. The analysis of the subsystems was divided into structural integrity, energy consumption, actuators and sensors, operation time and cost. The integrated system analysis comprised research on the operations and maintenance, sustainability, soft- and hardware, and RAMS characteristics as a few examples. During this process many challenges arised. To resolve these challenges and optimize the design an iteration phase was implemented after the research was finished. In this phase the compliance matrix was used as a guide and solutions that improve compliance of the detailed design with the requirements were implemented. The detailed design phase ended with an investigation on post-DSE topics such as a financial analysis including profitability and Return on Investment (RoI), verification and validation and a project design and development logic.
As a final result of the design process a solution for automated launch, landing, and storage of the kite power system has been produced on a detailed level. This system is based on a tall vertical boom of 35m (comparable to the length of a Boeing 737) on which a horizontal beam can slide up and down. The kite is launched from and landed upon this horizontal beam, when it is positioned at the top of the vertical boom. Both the horizontal beam and vertical boom can be tilted to put the system at an optimal angle for the launch and landing process. A storage unit has also been designed on a ground level. The horizontal beam can slowly lower the kite into the storage, in which a folding mechanism has been added to prevent entanglement of the tethers and the bridle system. The system has been designed in such a way that it can operate automatically and autonomously for a period of three months; a decision-making tool has been produced that ensures operation only within the kite’s operating limits (no lightning and a limited range of wind speeds). The research and development process as described in the above paragraphs have ensured that the detailed design has been explored on a widespread range of topics and has pushed to project group to achieve a sound level of depth within the available resources.
The various results that are produced in this report lead the way for several conclusions. The most important conclusion is that the performance of the automated launch, landing and storage system is considered to be a success. It was shown that the the design can autonomously operate a pumping kite system within the boundaries of the requirements. Furthermore, it was shown that the system can land the kite in a wind speed range of 4 to 25m/s and launching can be performed between 5 and 25m/s. However, this performance fully relies on the capabilities of the control unit. The system was required to not exceed the limit of using 0.1% of the energy produced by the kite for the launch, landing, and storage procedure. As the system uses 0.036% of the average produced energy, it is considered to have met this requirement.
As the team aimed to develop a sustainable design, recyclable materials were implemented. Besides the beam subsystem, steel is used in the design of the automated launch, landing and storage system. Steel is found to be recyclable for 93% and has highly convenient properties in case of structural application as well as costs. As for some parts no renewable materials could be used for structural or durability reasons, the goal to make the system 100% recyclable is not met. The client has stated a maximum cost requirement of Euro 20,000. Unfortunately this requirement is exceeded by 167% as the total estimated costs were estimated to be about Euro 53,500. If the client would reach break even after 20 years of operation, the energy price of 1kWh will have to reach an average price of Euro 0.285, which is unfeasible in comparison with the current price of Euro 0.06. The main reason for this is the replacement of the kite and main tether after each three months.
The most important recommendations that follow from the conclusions are related to cost reduction, profitabilityand funding. The currently designed system increases its cost budget of Euro 20,000, reaching a total of Euro 53,451. It is suggested to use cost estimation relationships for assembly to obtain a more accurate representation of the total cost. A second point of interest is to investigate the need for storage of the kite. Possibly this need can be eliminated or reduced to a partial-cover if the kite and tether durability are increased. The regular kite and tether replacement (every three months) are the largest cost drivers of the entire system and an increase of their durability and lifespan will most certainly help the system to move towards a commercially feasible state. Other recommendations to improve the profitability of the system are related to upscaling the existing model and exploration of an offshore implementation, such that the system can generate more power. The last two propositions on the financial area are focussed on funding. The Kite Power Group is advised to start with the acquisition of subsidies as soon as possible and also look into the prospects of attracting business partners and sponsorships.
A second set of recommendations is related to sustainability. On one hand hand the amount of recycled and/or recyclable materials can be further increased. A second suggestion poses the idea of promoting the kite power system as the true symbol of renewable energy and innovation to increase public interest. The kite power generation could be combined with a wind turbine on top of the tall vertical boom and a storage roof that is covered with solar panels. One final recommendationn follows from the contact with SkySails during this project. At this point such company is already commercially active on the energy market with a kite power system that has comparable features to the proposed detailed design in this technical report. The company has informed that they would gladly support the research with technical feedback and as such it is recommended to keep close contact with this company in further development stages.