Simulation & Prediction of Performance of Oscillating Water Column based Wave Energy Plant

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Project by:�Akshay Antrolia - M1810002

Prasad Chettiar - M1810062

Pushkar Raut - M1810073

Project guide: Dr. Balwant Bhasme

B.Tech. Project Presentation - Semester 8

(with National Institute of Ocean Technology,

Ministry of Earth Sciences, Government of India)

External guide: Mr. Prasad Dudhgaonkar � (NIOT)

Presentation Flow

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• Introduction to Ocean Energy
• Existing Commercial Technologies Around the World
• Literature Review
• Oscillating Water Columns
• Previous OWC Project at NIOT
• Problem Statement
• Summary of Stage 1 (Agenda 1 & 2)
• Agenda 3 - Sea Wave Simulation in Caisson
• Agenda 4 - Porous Media Simulation
• Agenda 5 - Preliminary Turbine Simulation
• Agenda 6 - Caisson-Turbine Integration
• Conclusion and Scope for Improvement

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Introduction-Ocean Energy

• Energy is harnessed from waves using devices like oscillating water column(OWC), oscillating wave surge converters, surface attenuators, buoys, etc and tides using devices like barrages, tidal stream generators, etc.

• Ocean energy refers to immense energy carried by oceans in form of waves, tides and ocean thermal gradient.

Wave Energy Devices

Tidal Barrage

Existing Commercial Technologies Around the World

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5. Sihwa Lake tidal barrage station, South Korea

1. OWC at Islay, Scotland

2. Pelamis attenuator, Scotland

3. Eco Wave Power, Israel

4. Orbital Marine Power at Orkney, Scotland

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Literature Review

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Literature Review

Oscillating Water Columns

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• A type of Wave Energy Converter (WEC) that harness energy from the oscillation of the air inside a hollow chamber caused by the action of waves�
• The rise and fall of sea waves powers a turbine via which electricity can be generated from a completely renewable energy source�
• This type was preferred as it was more economical and had lesser environmental interference

An animation on OWC. Credits: Voith Hydro

Problem Statement

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• This computational model will help in better performance prediction of the OWC
• It will help in testing out the varying sea conditions to aid in a better design
• In a broader perspective, it is an opportunity for us to contribute to indigenous technology for India’s clean energy transition

• To develop a CFD-based computational model for an entire OWC plant
• Build a model to help NIOT to predict the performance of OWC based Wave Energy Plants designed in future

Previous OWC installed at Vizhinjam, Kerala

NIOT may undertake future projects on OWC based wave energy plants.

Thus our objective is to:

Our motivation to take up this project:

Summary of Stage I - Agenda 1 & 2

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Wave Simulation using Volume of Fluid Method

Agenda 1 - Determination of Porous Media Characteristics

1. To simplify caisson simulations, a turbine has been replaced by a porous media
2. Pressure drop across chosen porous media was compared with the experimental data of actual turbine at Vizhinjam
3. We obtained a pressure difference of 6.87 kPa across the medium which was a reasonably good match to the behaviour of the actual turbine
4. Shown beside is the expression for pressure drop in the medium. �Alpha is the permeability and C2 is known as the inertial resistance

Agenda 2 - Preliminary Wave Simulations

1. Using Multiphase Volume of Fluid method, waves have been simulated in ANSYS Fluent
2. This would serve as a basis for further simulations coupled with a caisson and the turbine. Thus it would help simulating the entire system

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Pressure drop v/s Flow Rate

Agenda 3 - Sea Wave Simulation in Caisson

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Having developed seawaves in a random geometry, the next step was to actually produce them inside the Caisson structure (Reinforced concrete chamber).

A CAD geometry of the caisson was developed with drawings of the plant.

OWC Structure Internal View

Caisson Drawings of NIOT Plant

Agenda 3 - Sea Wave Simulation in Caisson

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Geometry: The entire geometry was created on CAD as per the dimensions provided by NIOT.

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Here you can see an internal cross-section.

After a few cleanups in the CAD, the geometry was taken forward for CFD simulation.

Views of Caisson

Internal Cross Section View

Agenda 3- Sea Wave Simulation in Caisson

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Geometry

Mesh Settings

1. An enclosure was created around the caisson leaving enough space in the vertical direction above the turbine.

2. The caisson was removed by boolean to create the fluid domain.

1. Skewness was maintained as low as possible so that the simulation won’t diverge.
2. Lesser mesh elements would ensure less use of computing power.

Enclosure with Caisson Boolean/ Fluid Domain

Details of Mesh

Sliced view of domain mesh

Skewness of domain

Agenda 3 - Sea Wave Simulation in Caisson

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Boundary Conditions

Models

Cell Zone Conditions:

The following conditions were simulated.

Model Parameters

Boundary Condition Parameters

Cell Zone Parameters

Agenda 3 - Sea Wave Simulation in Caisson

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Governing equations:

Momentum equation

Continuity equation:

Volume of fluid method equation:

Agenda 3- Sea Wave Simulation in Caisson

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Results: Timestep calculations can be seen here. Two different settings were used for 5 sec each to quicken the simulation.

We animated a video of the seawave simulated within the caisson. Volume Rendering of the same can be seen here.

Timestep Parameters

Volume Rendering

Agenda 4 - Porous Media Simulation

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Porous Media characteristics had to be added in order to simulate a pressure drop. However, in the Volume of Fluid Method, there isn’t an option to input porous media. One has to create a UDF (User Defined Function) to input Porous Media. Since this option wasn’t available, we were forced to skip this and jump to the turbine simulation.

Cell Zone Parameters in VOF method

Agenda 5 - Preliminary Turbine Simulation

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Having developed seawaves in the caisson, the next step was to simulate a turbine rotating and measure the power produced by it.

Case 1 Turbine’s specifications are as follows:

NACA 0021

0.38 m chord (constant)

Tip dia = 2m

Hub dia = 1.2m

Blade material = stainless steel

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This Wells turbine was selected by NIOT due to its unidirectional direction of rotation with regards to bidirectional flow of air in the caisson.

Turbine CAD

Agenda 5 - Preliminary Turbine Simulation

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Geometry

Mesh Settings

1. An enclosure was created around the turbine to create a dynamic mesh

2. Another larger enclosure was created to define the fluid domain

3. A boolean was created around the turbine

1. Skewness was maintained as low as possible so that the simulation won’t diverge.

2. Lesser mesh elements would enable a smoother simulation.

Enclosure with Turbine Boolean

Mesh Settings

Sliced View of Domain Mesh

Skewness of Domain

Agenda 5 - Preliminary Turbine Simulation

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The Dynamic Mesh method was utilized to simulate the rotating turbine. �Six DOF was given to the turbine, while defining it’s moment of inertia, it’s axis and the center of rotation.

Dynamic Mesh Properties:

�Rigid rotating body

1. External body contact surfaces with the turbine body

2. Main turbine body geometry

3. Internal of turbine geometry

4. Turbine walls

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Stationary body

1. Internal of geometry other than turbine

Six DOF Parameters

Dynamic Mesh Parameters

Agenda 5 - Preliminary Turbine Simulation

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Results: Following are the Power and Moment plots obtained in Fluent.

Results: On extracting individual datapoints at all timesteps from Fluent, we imported that data to Excel and plotted the same curves. They can be seen as follows.

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These values are only test values to ensure reliability of the simulation method.

Results: The animation of the turbine simulation can be seen here.

Pressure Contour Rendering of the Turbine

Power vs Time Graph

Moment vs Time Graph

Agenda 6: Caisson - Turbine Coupling

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The Caisson-turbine geometry meshing encountered multiple problems of high skewness, very high number of elements and the subsequent failure of simulation.

This was found to take place due to small faces and sharp edges on the turbine geometry which led to a high number of elements and high skewness.

Repairing of this was done in SpaceClaim by the tool of Small Faces and Merge Faces.

Turbine Geometry Issues

Cleaned Geometry

Agenda 6 - Caisson - Turbine Interaction

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Geometry: A coupled turbine and caisson geometry was then developed.

Mesh Details

Agenda 6 - Caisson - Turbine Interaction

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Models, Cellzone Conditions, Boundary Conditions, Dynamic Mesh settings have been kept in this simulation as mentioned in the above sections. Timestep Calculations and Solution Methods can be seen here.

Results: We animated a video of the seawaves simulated within the caisson with the dynamic mesh on the turbine. Volume Rendering of the same can be seen here.

Solution Methods and Timestep Calculations

Volume Fraction Rendering

Agenda 6 - Caisson - Turbine Interaction

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Results: The following graphs were plotted with the simulation data in Fluent.

Agenda 6 - Caisson - Turbine Interaction

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An attempt was made to develop a structured mesh of the caisson-turbine domain to obtain a better solution. This helped in reducing solution time and skewness.

Steps:

1. Body Split into 3 parts
2. Joining bodies as Fluid domain parts
3. Body Sizing was given to the 3 bodies
4. Skewness of the geometry was checked

Structured Mesh Method

Agenda 6 - Caisson - Turbine Interaction

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The following results were obtained in the form of graphs from Fluent over the entire 20 sec simulation.

Structured Mesh Method

Agenda 6 - Caisson - Turbine Interaction

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Having obtained success in developing and simulating a structured mesh, with much lower runtimes and low skewness numbers, we decided to run a longer simulation and observe its effects.

Structured Mesh Method (Iteration 2)

Console scripts at the end of the simulation

Moment vs Time and Pressure vs Time graphs

Residuals and Velocity at the end of the blades

Conclusion

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1. The results didn’t give us the expected values because mesh sizes are around 1m in size.

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• The physics of the problem has been captured properly which could be seen in the results obtained.

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• We have successfully created a computational model on which future models and designs can be build upon.

Scope of Improvement

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1. Number of Mesh elements were kept low so as to obtain results quickly, so access to higher computational power would ensure better quality results.

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• Time step size could be decreased and number of timesteps could also be increased to capture time gradients properly and for longer duration by having access to higher computation power.

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• More practical Sea-wave theories like Jonswap Spectrum could be used to capture sea waves more accurately.

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Thank You

Equations

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Momentum eqn

Continuity eqn

Vol of fluid method

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• They are a type of Wave Energy Converter (WEC) that harness energy from the oscillation of the air inside a hollow chamber caused by the action of waves.

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• The rise and fall of sea waves powers a turbine via which electricity can be generated.

Oscillating Water Columns (OWCs)

Credits: Voith Hydro

OWCs were focused upon in the project due to: �

• Lesser environmental interference than �tidal barrages and submerged turbines.

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• More economical as there are no submerged moving parts or transmission lines.

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Construction of an Oscillating Water Column

Construction and Working

• Waves oscillate inside the reinforced- concrete chamber/caisson, these oscillating action forces air to pass through guide vanes.

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• Guide vanes increase the kinetic energy of the flow which in turn rotate the turbine at higher speeds.

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• The turbine unit will be coupled to a generator to produce electricity.
• Various turbine like Wells, Hanna and impulse turbines are used.

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Project Objectives

• Developing a CFD based computational model to predict flow field inside OWC based wave energy plant at Vizhinjam, Kerala.�
• Predict performance of OWC for estimation of electrical power output of the plant.

Flow & Agenda

• Simulating air flow over porous material to emulate the resisting behaviour of the turbine.
• This is done in order to simplify the simulation while predicting performance.�
• Simulating wave flow in the Caisson.
• Sea waves in the caisson would be simulated using multiphase CFD models.�
• Simulation of coupled system of OWC: Caisson, waves and turbine(porous media).
• Simulation and calculations to check whether the desired electric power is obtained.�
• Modifying design parameters and simulating the turbine to obtain higher efficiency, based on system capabilities

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Agenda 1-Porous Media

• In order to simplify caisson simulations, a turbine has been replaced by a porous media whose parameters have been determined by us to give the same pressure drop as a turbine.�
• Pressure drop for various flow rates are there from experiments done at Vizhinjam, these values were used to predict parameters of porous media viz inertial resistance and viscous resistance via CFD simulations.�
• The values are shown below

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• The pressure drop between the medium is expressed as shown below. Alpha is known as permeability whereas C2 is known as inertial resistance.

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CFD Details for Porous Media Simulations

Mesh Details

Boundary Conditions

Geometry Details

CFD Results

Volume Rendering

Pressure Variation across axis

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Results

• The porous media seems to be emulating the turbine well qualitatively as seen from the fairly matching trends.

• The trends also show a reasonably good match quantitatively as can be seen from the close match in the region under focus.

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• Prediction of pressure drop across chosen porous media was compared with the experimental data.

• Once the porous media characteristics were fixed, mass flow rates were varied and compared to an actual turbine at Vizhinjam.

Agenda 2- Wave Simulation

• Using Multiphase Volume of Fluid method, waves have been simulated in CFD Fluent. �
• This will be coupled with a caisson and the entire system will be simulated.�
• Details of the simulation are shown on the next slide.

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CFD Details for Wave Simulation

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Geometry Details

Mesh Details

Boundary Conditions

Solution Details

CFD Details for Seawave Simulation

Summary

• OWC was determined to be a viable replacement for the existing hazardous power sources based on the previously mentioned sources.�
• Turbine has been successfully replicated by porous media.�
• The required multiphase simulations would be done in order to predict OWC plant's performance.�
• The future path of the project has been mentioned in the following slide.

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Progress & Future Course of Work

Aug-Sep

2021

Sep-Oct 2021

Oct-Jan 2021/22

Jan-Mar 2022

Feb-Apr 2022

• Wave
• Caisson geometry
• BC Waves
• Porous in caisson
• Turbine
• Turbine in caisson
• Dynamic mesh
• Interpret