Higher up and further out
Dr Axelle Viré is Assistant Professor at the department of Aerodynamics, Wind Energy & Propulsion (AWEP). Her work focuses on the numerical modelling of floating wind turbines and airborne wind energy devices. “The future of wind energy lies in moving higher up into the sky and further out at sea. This will open up new markets and sites that are still left unexplored,” she says.
Models for floating wind turbines and airborne wind energy devices need to take into account the so-called fluid-structure interactions. “In the case of floating wind turbines the system is moving as it interacts with the wind, the ocean currents and the waves”, explains Dr Axelle Viré. “The structure of a kite is a flexible membrane that deforms through the surrounding wind flow. In both cases we want to know how these interactions impact on the performance of the systems.”
Existing fast models can simulate the dynamics of such systems in real time, or over the expected lifetime of a system. However, such models usually only address linear phenomena, involving simple motions like small waves. “We are more interested in non-linear events, such as strong winds or large waves”, explains Viré. “Once you have a design for a kite or a floating turbine, you want to make sure it can also withstand severe conditions. We want to calculate for example how breaking waves impact the structure of floating wind turbines.” Another issue is the misalignment of wind and waves. “Waves do not always propagate in the direction of the wind. The combination of non-extreme winds and waves may also lead to extreme loads. Fast models usually assume that wind and waves are aligned and not severe.”
“We need more refinement in our models”, concludes Viré, who is leading the development of the high-fidelity numerical tools necessary to achieve this. Among others, she is looking at the modelling of the behaviour of airborne devices, which involves interactions between aerodynamics, structural dynamics, and flight dynamics. “We already have some simple models for this, but we want to couple these with computational fluid dynamics (CFD) to refine them. This is still at a very fundamental level and has not been done yet anywhere in the world as far as we know.”
Such high-fidelity simulations are slow, high computational cost models, but the results from them could also be used to improve the quality of the faster models. “We cannot use a high-fidelity model to look at the entire lifetime of a system, but we can look at specific conditions and how the system behaves under them. This will then give us more accurate values that we can then put back in our fast models”, Viré explains. “For example, in our kite models we now use crude approximations of the lift and drag of the wing. With the new method we are developing, we could refine that.” That is quite another challenge, though. “You will then need some kind of representation of your detailed model in your larger model, but how do you bridge the two? That is a typical challenge with multi-scale problems.”
As a postdoctoral research fellow at Imperial College London, Viré worked on the numerical modelling of floating offshore wind turbines, a subject that was still missing when she came to Delft. “I aim to develop floating wind as a research field at TU Delft. As a Wind Energy group, we should be looking into this”, she states. “Floating wind is actually closer to commercialisation than kite power is. There are already prototypes that are connected to the grid and delivering power continuously.” The world’s first floating wind park is being built by Statoil in Norway, who installed their first Hywind turbine in 2009. In Portugal, the WindFloat Atlantic (WFA) project is planning to have 3 or 4 floating wind turbines operational by 2018. “Norway and Portugal are regions where you have deep water close to shore. That is ideal for floating turbines. It is too deep to place turbines on monopiles, but close enough to shore to make it easier to connect to the grid.” Outside Europe, Japan has already installed three floating wind turbines near Fukushima, to replace the closed-down nuclear plant.
Until recently, there was limited interest for floating wind turbines in the Netherlands. Possibly, the Dutch thought they did not need it, because the North Sea is shallow enough for monopiles. Obtaining research funding on this specific topic at a national level was difficult. “This is changing now. The Netherlands have recently published a market study on floating wind, and Dutch businesses now also see it as an opportunity”, says Viré. And rightly so, because although the market for floating wind is in deep water, you can still develop the knowledge as an export product, she says. That message seems to have sunk in now. There could be local advantages too. “With floating platforms, you can go further away from shore and avoid a lot of public opposition. It opens up a much wider range of sites than are available today.”
When wind energy first moved off-shore, onshore designs and concepts were used, but that is not without its problems. “Traditionally, onshore turbines are monopiles. To install them offshore, you have to extend the pile to the seabed and know the soil condition,” says Viré. “Even if we can build something that goes that far down, it is not cost-effective. However, if you build a floating platform that you moor to the seabed, you only need cables and anchors. That is much less costly than a full structure, and has less impact on the environment.” In fact, Viré foresees both that both fields, floating wind and kite power, will merge in the future. “Kites are cheaper than conventional turbines. That is attractive when you are working offshore, where other costs are higher. You need ships to access the turbines, and in case of extreme weather that is difficult, meaning the availability windows are smaller than onshore. So launching kites from floating platforms might be the most cost-effective solution.”
Viré finds the disciplinary aspects of her research particularly interesting: “It involves so many fields. We work with the CEG faculty on the substructures, and with EEMCS and 3mE on the control and the electrical engineering. DUWIND is a good platform to bridge all these fields.” She also coordinates a course on offshore wind for professionals. This is a seven-week course that will start in May, taught by lecturers from all various disciplines. ”Because it is such a multidisciplinary field, people usually have a background in only one of these fields. The objective of the course is to get them acquainted with the other fields, and teach them how to integrate all these subjects when designing a windfarm.”
Viré still maintains close links to her former research group at Imperial College, London, where she is an honorary fellow. “We try to apply for funding together, for example, and exchange staff for short visits. They can benefit from our expertise on wind, because historically the group there is more involved in ocean and tidal energy.” She also recently wrote a funding proposal for a consortium including the Norwegian and Portuguese floating wind projects. “Even though our research is still fundamental, we still have to show the societal impact and link to practice. It is becoming ever more difficult to get fundamental research funded without involving industrial partners.” However, that is not the only reason to want to involve the Portuguese and the Norwegians. “The added advantage is the data they have. In a field that is so new, it is sometimes difficult to approach commercial parties. They won’t share their company secrets with us, or their patentable knowledge. But we can still simulate the design they share with us, whether it is a more generic design or a final one. The more data we have, the better.”
“My work also has strong links to applications”, Viré continues. “That always drove me, even though I am looking at more long-term solutions.” Whereas industry usually has a more short-term perspective, Viré believes you should always keep looking further ahead and at more high-risk solutions. “If you only focus on improving what currently exists, you are not going to make any step changes or breakthroughs.” The subjects she is looking at are still more about rethinking current concepts. “That way, we are opening new markets and more sites offshore. However, if we really want to move forward and have more impact we need to innovate further.”
Looking back is also an option here. “Solutions that were dropped years ago may prove to be interesting again,” says Viré, citing vertical access wind turbines as an example. “These are very scarcely used due to their lower performance. But if you look at floating wind, they might be interesting, as they have a lower centre of gravity,” she explains. “You can imagine that a moving turbine on a floating platform is dynamically not very stable. If you can move the mass of the system further down, the stability will increase.” Unfortunately, industry will not be cueing up to adopt this idea yet, as all current turbines, and thus all design and production facilities, are based on horizontal technology. “We are now researching what is the best scale of floating platform for the existing wind turbines. We can take knowledge from the oil and gas industry but scales in our field are very different. The question is how does the platform scale with the wind turbine rating? What is the optimal floater design? Changing the whole concept might provide the answers. That is still applied research, though it may never be put into practice. That is why we need more funding for high-risk research.”