Winds of change

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Advanced simulation tools provide a platform to develop advancements in energy generation technologies, writes Robert Roe

Through the use of cutting-edge simulation, software engineers are developing innovative new wind energy technology and increasing the efficiency and durability of wind turbines. The drive to find sustainable energy sources is one of the most pressing concerns facing society. The development of advanced sustainable energy sources offers not only an increase in power and efficiency of energy generation, but also an alternative to fossil fuels.

Fossil fuel resources are becoming scarcer and so renewable sources such as wind and solar power are providing more power than ever before. The UK, for example, has set a target of 20 per cent of its total energy being produced by renewable sources by 2020.

Increasing the efficiency of technologies like solar panels and wind turbines is a key challenge to the success of renewable energy sources, along with production and manufacturing costs.

The use of modelling and simulation software offers path towards innovation as large-scale simulation can be employed to help design new systems and increase the efficiency of existing technologies to make them more economically viable.

At the UK Altair Technology Conference (ATC) 2019, David Yáñez, co-founder of Spanish tech-startup, Vortex Bladeless, presented the company’s design for a new wind energy technology. One of the key characteristics of this system is the reduction of mechanical elements that can be worn by friction. The company developed the technology using CFD tools provided by Altair, which helped the company study both the fluid-structure interaction and the behaviour of the magnetic fields in the alternator. The results are then compared with experimental results obtained both in wind tunnel and in real application environments.

Vortex Bladeless is a vortex-induced vibration resonant wind generator. It harnesses wind energy from a phenomenon of vorticity called vortex shedding. Vortex shedding is an oscillating flow that takes place when a fluid such as air or water flows past a bluff at certain velocities. In fluid mechanics, as the wind passes through a blunt body, the flow is modified and generates a cyclical pattern of vortices.

Once the frequency of these forces is close enough to the body’s structural frequency, the body starts to oscillate and enters into resonance with the wind. This is also known 
as vortex induced vibration (VIV).

Vortex’s mast geometry is designed to achieve maximum performance based on the average observed wind velocities. It is able to adapt very quickly to wind direction changes and turbulent airflows commonly observed in urban environments.

The disturbance of the downstream wind current is why regular turbines need to be installed at a certain distance to each other. However this is not the case for the VIV system, as any limitation associated with the ‘wake effect’ is avoided. Furthermore, the company expects Vortex devices to work better together, causing feedback and increasing the velocity of the vortices if they have the proper free space around them, which is estimated to be half of the total height of the device.

For regular wind turbines, this free space is usually five times the total height of the device.

The bladeless technology consists of a cylinder fixed vertically with an elastic rod. The cylinder oscillates under certain wind conditions, which then generates electricity through an alternator system.

‘Resonance is a great way to transfer energy from a fluid to a structure. We obtain a resonance when two frequencies are close together, for example the natural frequency of a structure and, in this example, the frequency that is created by these vortices,’ explained Yáñez.

‘In Spain today we produce as much as 20 per cent of our energy from the wind but when we speak about the distribution of energy the king is the solar panel.

‘We are designing a new tool to collect energy from the wind and we tried to increase, the resonance phenomenon that appears,’ added Yáñez.

Vortex technology

The outer cylinder of the Vortex Bladeless system is designed to be largely rigid and has the ability to vibrate, remaining anchored to the bottom rod. The top of the cylinder is unconstrained and provides the maximum amplitude of the oscillation. The structure is built using resins reinforced with carbon and/or glass fibre, the same materials used in conventional wind turbine blades.

The rod’s top supports the mast and it’s bottom is firmly anchored to the ground. It is built of carbon fibre reinforced polymer, which provides a fatigue resistance and has a minimal energy leakage when oscillating. The design of this bladeless induction system is quite different from a traditional turbine. Instead of the usual tower, nacelle and blades, the Vortex systems use a single mast of lightweight materials over a base. Traditional wind turbines such as HAWT (horizontal axis wind turbines) and VAWT (vertical axis wind turbines) work by rotation where the Vortex Bladeless device works through oscillation.

The development process requires careful examination of the device and an understanding of its behaviour in different wind conditions. The resonance of the mast and the vortices that are produced as wind passes across the device must be similar frequency for the oscillation motion to occur and generate energy.

‘We have to start to visualise our device and here one key factor is Altair. To help us understand how our structure interact with the wind,’ stated Yáñez. When the frequency of the vortices is close to the resonance frequency of our mast then we begin to produce energy. We work a lot with AcuSolve and with HyperMesh to build this mesh.’

The presentation described the development of mesh for the mast, which breaks the shape down into a number of cells. Yáñez described how the growth of these cells is very important to understand if the results of the computer testing can be verified in a real-world test. ‘We need to how close we are to the real conditions and with AcuSolve and FieldView we are able to understand the results, which allows us to transport the knowledge that we have obtained with this simulation into our devices,’Yáñez continued.

Initial testing found some issues with the design that the team were able to overcome with some out of the box thinking. ‘We saw that the performance of our device was not what we expect. One day I started to study another area, which was an area of science where people were studying the vortices created by the tails of fish and in the wings of birds,’ comments Yáñez. ‘I took their formulas and mixed them with the formula used by structural engineers, and we obtained a new formula that led us to develop another geometry. With this new geometry we increased our performance.’

The changes to the mast design allowed the engineers to increase the size of the mast furthering development towards a full production sized system. ‘A few months ago we started five devices of 2.5 metre height that have more that would be suited to produce energy in homes. But we saw in real conditions that these devices are able to adapt very quickly to changes in the wind direction and velocity because we do not have any kind of spin or momentum,’ Yáñez concluded.

While two-dimensional simulations are useful, VIV is a 3D phenomenon and as such it requires the large scale CFD simulations that have been developed by Yáñez and his colleagues. Since this is a new technology, a lot of work has to be done to ensure that the devices behave as expected and produce energy with the required efficiency. This means creating new models that must be validated. These 3D simulations are based on the Reynolds number, an important dimensionless quantity in fluid mechanics used to help predict flow patterns in different fluid flow situations

These simulations require a large amount of computational resources so the engineers have been pareterning with Altair and the Barcelona Supercomputing Center (BSC) to find the best way to achieve optimum results in an affordable manner.

Simulating growth

Another reason for large-scale simulation of wind turbines is to stay competitive in an increasingly difficult market. The global renewable energy market is expected to grow at a 13.1 per cent annual compound rate from 2018 through 2024, according to Envision Intelligence. This huge potential for growth drives competition. As a result, companies are searching for ways to stay one step ahead of competitors.

Earlier in 2019 Ansys announced details of its partnership with WEG, a Brazilian engineering company looking to take advantage of the growth in the energy sector. The company choose Ansys due to its ‘pervasive simulation’ which enables companies to iterate and innovate quickly across every aspect of a design life cycle.

In a blog post, Ahmad Haidari, global industry director at Ansys, noted that ‘WEG chose Ansys’ pervasive simulation to assess the structural, electromagnetic, thermal and fluid performance of all of its products.’

‘WEG engineers are developing a 4mW direct-drive wind turbine with high-efficiency and low-maintenance requirements. By almost doubling the output of its current 2.1mW platform, WEG hopes its new design will keep up with increasing demands. The engineers use a variety of pervasive simulation tools to test and develop its designs throughout their life cycle,’ continued Haidari. The engineers in this project made use of several Ansys tools including Ansys Mechanical, Ansys Maxwell and Ansys DesignXplorer.

The increased power output involved in doubling the performance of a wind turbine causes high dynamic loading on the structural components. WEG engineers use Ansys Mechanical to evaluate the various load cases throughout the structure.

‘The nacelle tower-top adapter, which sits on top of the concrete tower and bears the weight of the turbine blades mounted on its front, must withstand extreme loads while avoiding plastic deformation and slippage. Engineers use structural simulation to evaluate stresses at the neck and at welding points. To complete their fatigue failure analysis, engineers use Ansys nCode DesignLife,’ added Haidari.

‘Critical welding spots throughout the structure are potential regions of structural weakness. Using Mechanical and DesignXplorer, WEG engineers evaluate these spots to ensure they can withstand the largest loads they would experience,’ Haidari continued.

WEG engineers use Ansys Maxwell to simulate the low-frequency electromagnetic fields produced by the turbine during normal operation. These simulations evaluate torque, induced voltage, losses and magnetic core saturation.

‘Minimising harmonic currents between the generator and the power converter is critical for safe and optimal wind turbine performance. To maintain low, total harmonic distortion, engineers used Maxwell simulations to analyse magnet positioning, determine the generated voltage and assess the harmonic spectrum,’ stated Haidari.

‘Pervasive simulation has made its way into every aspect of the design of WEG’s wind turbines. The same can be said about the other products made by WEG such as its turbo generators and hydrogenerators.’

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