CFD An Indispensable Tool For Wave Energy

Figure 1: Wave Energy Convertor CETO 6, 1MV capacity unit. Image credit – Carnegie Wave Energy.

995 words / 5 minute read

As the world scrambles for alternative sources of energy, extracting renewable energy has been high on agenda, for most countries. With oceans covering nearly 70% of earth, wave energy has been the most conspicuous forms of renewable energy. Over the last couple of decades, many innovative ideas on extracting energy from waves have promised to deliver, but most of them fizzled out in the rough seas and the rest were not as effective as they seemed on paper.

Amidst many Wave Energy companies cowering in the last decade, CETO from Carnegie Clean Energy, has realised the vision of large scale utilisation of ocean wave energy. Extensive progress made in the research and development of CETO technology, has proved it to be unimpeachable in making wave energy an economically viable renewable energy source.

Figure 2: Deepwater location And offshore power production. Image credit – Carnegie Wave Energy.

Good Vibes Happen on the Tides
Taking on the name of the ancient Greek goddess CETO, this wave energy convertor is designed to convert the constant motion of ocean waves into usable electrical energy and also to produce fresh water for domestic consumption.

The device consists of a fully submerged buoy, called Buoyant Actuator (BA), which moves in response to the waves. This physical motion drives an onboard hydro-electric system producing electricity. Part of the power generated is used for desalinating the water and the rest is delivered back to the shore through subsea cables.

This simple yet robust technology is very flexible with ability to operate at various water depths, swell directions, tides and seafloor conditions. Being fully submerged and possessing efficient energy damping capabilities, they can survive even large storms. In addition, they can co-exist with marine life, have minimal visual impact and being far-off from into the sea, they do not create inconveniences to beachgoers.


Figure 3: CETO 6 technology design concept. Image credit – Carnegie Wave Energy.

Need for CFD as a Design Tool
Decade of extensive research and development conjured up the present state of product maturity. Constant testings in onshore wave tanks and onsite ocean beds along with numerical analysis of the various designs has helped to make the product robust, efficient and economically viable.

Hydrodynamic analysis of these highly non-linear systems using CFD softwares like OpenFoam coupled with GridPro has helped to test out various design variants in quick succession, understand the underlying flow physics and come-up with better designs.

Previously, hydrodynamic analysis of WECs were done using linear wave theory. Though in actuality the flow dynamics around a submerged body is non-linear, this simplification of the actual problem is acceptable when the waves are assumed to be of small amplitude, relative to wavelength and water depth and the associated motion of the body in fluid is also assumed to be small.

But fluid-structure interactions in reality are more complex. In cases like a fixed submerged body in the vicinity of the free surface or incident waves of relatively high steepness, wave shoaling may occur, which might result in breaking at the top of the body. Depending on the shape of the body, complex highly non-linear interactions happen among the reflected waves, incident waves, radiated waves and the transmitted waves.

Video 1: Wave interaction with a wave energy converter. Video credit – Carnegie Wave Energy.

These intricate wave-wave interactions become more intense in magnitude for a body in large amplitude of motion and simple linear wave theory fall short in predicting these fluid motions. Therefore, it becomes essential that sophisticated non-linear predicting tools like CFD be used in modelling in-water devices like CETO for calculating accurate and reliable estimates of power, loads and motions.

Carnegie designers make use of open-source CFD software OpenFoam to simulate the flow past various CETO models under moderate to extreme conditions. CFD has helped to improve the performance and survivability of the CETO devices at relatively low cost when compared to the traditional wave tank testing approaches. This has led to the development of better CETO designs which shows 50 percent higher efficiency compared to CETO 6 design and stronger survivability under extreme conditions.


Figure 4: GridPro mesh for CETO 6 device. The device is an articulated multi-body system consists of the BA and the Pump. Image credit – Carnegie Wave Energy.

“Any Grid for CFD” is Just a Semblance
Optimal functioning design of prototypes depend on accurate and reliable CFD predictions, which relies on the underlying grid to provide accurate and stable solutions. Earlier Engineers at Carnegie used cartesian trim meshes, but later collaborated with GridPro to create high quality structured multi-block grid for these simulations. Body-fitted flow aligned grids from GridPro not only helped to maintain high grid quality around complex geometric features, but also contributed to reducing the computational cost by over forty percent.” says Ashkan Rafiee, a hydrodynamics Engineer at Carnegie.

Figure 5: Zoomed views showing the surface mesh displayed as colour by blocks. Image credit – Carnegie Wave Energy.

GridPro’s ability to handle rigid body’s transformation ensures high grid quality even for simulations which involve rotation of the pump around a hinge point. Carnegie Engineers claim this has aided in maintaining the solver robustness and stability during the simulation.

Figure 6: Two fully submerged CETO 6 devices. Image credit – Carnegie Wave Energy.

Concluding Remarks
A herculean problem the  scientists face when the model is used in a real world setting, are the cost and effort necessary to build a prototype. A challenging wave climate makes it hard to construct a device and in some cases even destroys it during construction. CFD modelling proves to be an unimpeachable tool for development and cost reduction of new complex technologies like CETO. Unlike the conventional lab and onsite testing, CFD helps to evaluate various prototypes at a low cost, and in a short time frame.

References:
1. https://www.carnegiece.com/
2. Rafiee, A., and Fiévez, J., “Numerical Prediction of Extreme Loads on CETO Wave Energy Converter”, In: Proceedings of EWTEC 2015, The 11th European Wave and Tidal Energy Conference, Nantes, France 2015.

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