Rising tide: how can simulation software help the tidal power sector?

Dr I Jones, ANSYS

Wave and tidal power generation is receiving significant attention, but many believe that the industry is where wind power was 15 years ago, on the cusp of a flurry of activity set to accelerate development in the sector.

Many commentators point out that the wave and tidal industry is where the wind power was at least 15 years ago – at the bottom of a very steep curve of adoption.

Despite the challenges ahead, there is profound interest in the sector. And it is clear to see why: In the UK alone, The Carbon Trust estimates that the country could capture just under a quarter of the global marine energy market – worth to up to £76bn by 2050, and generating over 68,000 jobs. The analysis, the most in-depth of its kind, found that total marine energy capacity in the UK could be 27.5GW by 2050, capable of supplying the equivalent of over a fifth of current UK electricity demand to the grid.

More immediately SSE Renewables and Scottish Power Renewables have signed lease agreements to develop sites, which “represent potential for up to 600 MW of capacity and a step change in the industry”. £6.4m in funding from the European Regional Development Fund was recently confirmed for Tidal Energy Limited to manufacture its 1.2MW DeltaStream device for deployment in Wales.

Canada and the U.S. have also increased efforts to exploit tidal resource. The US Water Power Program mission has a projected spend of US$50M for 2012, and plans for 23-38GW output by 2039, supported by the Production Tax Credit (PTC) system and the Marine and Hydrokinetic Renewable Energy R&D Act. In Canada, the world’s first feed-in tariff for tidal projects has been set at US$0.78/kWh.

In addition to these individual projects there has been a strategic shift in attitudes towards tidal and wave generation. The barrage-based approach that demanded huge, expensive dam-like structures to be built has now been joined by a focus on less intrusive underwater turbines that can tap this repeatable, predictable, non-polluting source of energy.

What are the technical challenges that remain?

Against this backdrop, there are some specific technical challenges to be overcome.

Firstly there are the challenges of developing and installing large scale machinery in one of the most hostile environments on the planet. This is a gauntlet that has been thrown down to engineers hungry to develop energy generation technologies.

Then there are the more familiar concerns such as reducing the cost of manufacturing of equipment, not to mention maximising its yield. This is a challenge for businesses (and even Governments) looking to find and profitably exploit the next sustainable energy source. Lastly there are of course environmental concerns to be addressed.

These challenges can be broadly classified into one of three areas – turbine design, turbine placement and additional environmental factors.

Turbine Design

There are many different types of turbine design. A horizontal axis turbine extracts energy from moving water in much the same way as wind turbines extract energy from moving air. These can also be mounted on a vertical axis to extract energy in a similar fashion. Devices may also be housed within ducts to concentrate the flow experienced by the turbine.

Alternatively an oscillating hydrofoil uses a hydrofoil attached to an oscillating arm. The motion is caused by the tidal current flowing either side of a wing, which results in lift. This can then drive fluid in a hydraulic system to be converted into electricity.

Though these systems suit different needs they all face the same issues as a result of the unchartered, hostile underwater environment. The tidal streams that can create this power are necessarily fast and the forces exerted on the blades, support structures, mountings and the drivetrains can be immense. As a result there is a high incidence of failure and high maintenance costs.

Simulation software – specifically mechanical and stress analysis simulation - can predict the physical stresses on the various components of the turbine: the blades, gear box, drive train and support structures. This can then provide evidence for the selection of one design over another based on its likely resilience.

As well as being long-lasting, the turbines must extract as much energy as possible from the tidal streams. Fluid dynamics simulations can predict the energy yield from a turbine design, and also the pressure loading on the turbine components, which is required by the stress analysis simulations.

These simulations are a key technology. They can be used to guide the optimisation of the turbine components and assess the impact of design changes (that may range from increased costs and delayed installation to drastically improved yields or more resilient equipment) and help to increase the likely ROI of the entire installation.

Turbine placement

Turbine placement is also a critical factor. The depth of the water will define many aspects of a turbine design – though as a rough guide turbines are suitable for water of around 20-50 metres deep.

However, assuming a given design and even a given location, there are then issues of how to place the turbine to best capture tidal energy, reduce installation costs and minimise interactions effects.

Interaction effects arise when the flow of water around one turbine impacts the flow around a different turbine through its wake – potentially restricting the number and layout of turbines that can be situated in a single location. In addition the support structures disrupt the flow and impact the amount of energy generated.

Simulation is critical here as it is not an option to move the turbines in-situ just to assess the best position or least interactive site. Building on the mechanical and CFD assessment of the design itself, simulation can first show the most cost effective options for installation.

The technology can then look at the interaction of the turbines across the entire installation and give an estimate of the output a given configuration or placement will yield. And of course, different configurations can then be compared. Critically this is all done before committing resources.

This mixture of stress analysis and fluid dynamics simulations means that expensive prototypes that can cost millions and take months to develop and test can be avoided. It also means that improvements in design can be tested and “verified” in the virtual environment before resources are committed.

Simulation is a key technology in finding these answers and helping to capitalise on the opportunities presented by the industry, especially in these early stages before inevitable consolidation.

Simulation can demonstrate the dynamic development of stress on the underwater structure and show the likely output of a given field of turbines, as well as highlighting the interaction effects of different configurations. This information is critical is developing a realistic, cohesive business case for the investment in wave and tidal power generation.

Combined with the predictable nature of tidal power, the more reliable estimations mean that energy companies have a realistic base line of performance that they can proactively manage. Whereas wind and solar power can be sporadic, the tides are reliable and repeatable. As such the expected ROI and output from a turbine field can be identified in advance.

Taking this one step further, simulation can enable energy businesses to experiment with different configurations of turbines and installations to ensure that it is competitive with alternative forms of energy.

And simulation is not limited to these commercial factors. It can also help minimise environmental disruption – be this disturbance to fish stocks or the damage caused by scouring of the sea bed. In the UK this is critical as many of the turbine sites are in Scotland where the fisheries industries are a vital source of employment.

On the global scale, there has been concern that inserting turbines into the tidal streams responsible for regulating the temperature of the earth may have detrimental impact. Simulation offers a critical insight into these changes and can help ensure that the development of a new turbine field does not damage existing economies and employment.

However in order to deliver these benefits, simulation must be incorporated now – at the point of design and development. This is critical, not only to optimise the operational elements of the turbines but also to enable accurate management and optimisation of the installations themselves.

By simulating the design of the individual turbines as well as the entire field, a cohesive and comprehensive assessment of the output can be developed, demonstrating the return as compared to other sources. As renewable energy faces increasing scrutiny it is clear that with the right tools, tidal power remains a force to be reckoned with.

Click here for more information on Ansys simulation solutions.

About the author: Dr. Ian Jones is the Head of ANSYS-CFX Technical Services and a visiting Professor at Nottingham University in the UK. Dr. Jones has participated in many EU projects including ALESSIA, BloodSim, @neurIST, GEMSS and COPHIT. ANSYS-UK Ltd is a UK subsidiary of ANSYS, Inc (www.ansys.com), a leading engineering analysis software company.

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Policy, investment and markets  •  Wave and tidal energy  •  Wind power