Study on Design and Performance Evaluation of a Cross Flow Turbine to be utilized in a Floating OWC wave energy converter

Abstract

In the modern era the adverse effects of human activities has imparted a major threat on the earth’s habitable environment. With the cast technological advancements taken place in the past century led new doors open to global warming and environmental pollution. The carbon emission and other greenhouse gases emissions are the major reason for climate change as well as ozone layer depletion. In addition the rising demand for energy hassled many countries due to exponential decay of fossil fuel resource, the consequences made the man kind look for new, renewable and environmental friendly energy sources. Wave energy offers such a solution to the energy need. Wave energy is the most consistent of all intermittent renewable energy sources.

The present study focuses on harnessing offshore wave energy resource by utilizing a direct drive cross flow turbine and also design of a novel floating structure as a supporting fixture for the designed CFT (Cross Flow Turbine) to work. The wave energy resource site selected estimating the annual mean wave power density. The 3.0m significant wave height at 9.0s wave period yields the highest energy bins which occur at the highest number of times per year. The CFT was designed to match these wave conditions. CFT design was based on original preliminary design criterion. Turbine has a 2.0 m diameter and 1.36 m internal diameter, runner has 18 blades and rotates at 35 rev/min. The turbine nozzle was designed to enhance the efficiency of the turbine. Four nozzle shapes are designed varying the nozzle entry arc angle by 20 deg. In each step. To analyze the performance of turbine ANSYS CFX 17.6 commercial code used to simulate the turbine. Turbine width wise 1/15th scale strip of 0.5m used in the numerical simulations to reduce the computational effort, cost and time. Steady state simulations carried out for geometric optimization, the nozzle entry arc angle having 150 deg. preforms best of out of other three nozzle angles. The base model in steady state reached a maximum of 54.33% With 33.366 kW of power output at 35 rev/min with a 3.0m of head. The base model was then analyzed under bi-directional flow simulation with time averaging (Transient) calculation method. Under the effect of bi-directional flow with 3.m of head two rotational speeds were analyzed. The peak cyclic efficiency recorded at 35 rev/min of 56.83% and lowest of 11.55% the average cyclic efficiency was 36.52% with a 36.4 kW of average power output. The flow behavior through the runner and nozzle was analyzed under steady state condition (Numerical calculation computes the fully developed solution that does not change with time, such that the mean values are computed)

The 2nd study of design and simulation of a floating structure was carried out. The initial weight estimation was 1200 Tons, and dimensions selection. The initial stability calculation carried out. The model was then taken to ANSYS AQUA 17.6 foe hydrostatic stability parameters to obtain. The COG 9.34m, COB 5.018 m, the Metacentric height (GM) recorded as 4.33m. The intact stability and dynamic stability criterion was satisfied. The model was then taken to SIMEMNS Star CCM+ CFD platform for hydrodynamic behavior calculation. The wave generation with 3.0m height with 9.0s period and the simulated water depth 100m. The model uses options in the simulation physics such as VOF (Volume Of Fluid) waves for generation of waves, Elulerian multiphase for air and water, also Dynamic Fluid Body Interaction (DFBI) model for floating structure to couple with environment and floating body kept in station keeping by four body couplings of catenary mooring type with specified cable stiffness. The Structure and the Turbine was complex to simulate in a single domain, in which an orifice plate was designed to match the turbine pressure and mass flow damping effect to match.

The floating structure was then fixed with the orifice plate was simulated. The Floating body damping coefficient was also found by carrying out the free decay test by using numerical method to enhance the stability and power extraction capability. The orifice inside the floating body was examined at 9.0s wave period having a significant wave height of 3.0m, and the orifice potential power and efficiency was calculated based on wave energy availability. The simulations were run on two conditions as the floating structure was inserted with and without the viscous damping coefficient to analyze and compare the dynamics stability and the orifice performance. The damped structure placing the orifice plate performed with a higher potential power of 29MW’s with an orifice efficiency of 21%. The undammed system performed with a lower power potential of 18.4MW at an efficiency of 13.26%. And the system was then simulated Floating body was stable in both hydrostatic and hydrodynamics conditions satisfying the requirement to utilize as a floating type OWC (Oscillating Water Column) WEC (Wave Energy Convertor) to house the designed CFT for the specified wave recourse location.