Monthly Archives: August 2013

Shaping array design of marine current energy converters through scaled experimental analysis


A.S. Bahaj and L.E. Myers – Energy, August 2013

Abstract

Marine current energy converters or tidal turbines represent an emerging renewable energy technology that can provide a predictable supply of electricity. Single devices are in operation around the world with aspirations to deploy farms or arrays of multiple devices.

We present an experimental study that has characterised the downstream wake flow around a 1/15th-scale turbine in a large circulating water channel and a series of experiments involving static actuator disks at 1/120th-scale allowing simulation of multiple-device layouts.

Our analysis demonstrates that the near wake is highly turbulent with structures generated by the rotor and support structure. This region of flow may prove difficult to numerically simulate with a high degree of accuracy. In the far wake the performance of static actuator disks can be matched to mechanical rotors reducing scale and cost facilitating replication of complex array geometries. Here the ambient turbulence and geometric properties of the device/channel drive the wake recovery towards free stream conditions.

Devices operating downstream of others will be subject to a non-steady flow field making comparative performance difficult. We discuss the possibility of unequal device specification and rated power within an array (unlike wind farms) providing a more representative measure of array performance.

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The Impact of Tidal Stream Turbines on Circulation and Sediment Transport in Muskeget Channel, MA


Aradea R. Hakim, Geoffrey W. Cowles, and James H. Churchill – Marine Technology Society Journal, August 2013

Abstract

The Finite-Volume Community Ocean Model (FVCOM) is configured to evaluate the potential impact of the proposed Muskeget Tidal Energy Project on circulation and sediment transport in the surrounding region. The extraction of tidal kinetic energy from the water column is modeled by augmenting the momentum equations with additional drag terms parameterized using local flow velocities and parameters specific to the installed turbine farm. Model-computed power output compares well with estimates based on velocities derived from a shipboard acoustic Doppler current profiler (ADCP). Total extracted power from the proposed installations during a spring ebb tide represents roughly 9% of the natural power in the deep section of the channel and 30% of the natural tidal dissipation in the turbine installation region. Due to this low level of extraction, turbine installations at the proposed transects result in relatively minor differences in the tidal current magnitude (2.5%), water level (0.8%), sediment flux (0.6%), and bed level (9%). Computations also indicate that the proposed installation generates minimal impacts to the tidal harmonics (3.3% change in amplitude and 1-min delay in phase) and tide-induced depth-averaged residual currents (2.8%). Model-computed extraction at increased levels is associated with greater perturbations to the natural conditions.

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Modeling Hydrokinetic Turbine Performance in the Mississippi River


Patrick Karalekas, Gregory J. Kowalski, and Edward Lovelace – Marine Technology Society Journal, August 2013

Abstract

Free Flow Power has developed a renewable energy technology that can convert the kinetic energy flowing in a river to electricity without the use of dams. The company plans to install a specially made turbine directly into the flowing stream. This process, known as hydrokinetics, is an innovative approach that provides energy at a reliable and predictable rate as opposed to other intermittent renewable energy sources. There are currently two dozen hydrokinetic projects in the licensing process along the Mississippi River, which will account for 4,000 MW of power-generating capacity. Hydrokinetics could develop into a $1 billion a year industry.This article describes the modeling tool developed for Free Flow Power’s hydrokinetic sites along the Mississippi River. The performance models compare river velocity, power generation, reliability, maintenance costs, and finance options to establish a likely performance profile for a proposed site.These models calculate the expected returns for Mississippi River projects and can be used to perform a sensitivity analysis on all of the major variables for hydrokinetics. The unique aspect of this performance model is the incorporation of a reliability calculator, which estimates the lost revenue resulting from component failures. It can be used to develop the maintenance strategy for the array and to evaluate the total cost of reliability for components. The modeling tool described provides Free Flow Power with the ability to compare different design scenarios and quickly gives an estimate of a site’s performance.

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Subsurface Mooring Stability in the Presence of Vertical Shear


Howard P. Hanson – Marine Technology Society Journal, August 2013

Abstract

Observations in the Florida Current reveal significant vertical shear that can occur on the scale of turbine rotors and that can reverse. Because ocean current turbines in the Florida Current will require anchored moorings, the dynamics of the moored systems in the presence of shear become a factor in design and operation. Here, a simple stability analysis shows that the case of negative shear (slow current near the surface) is dynamically unstable for moored turbine systems.

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Dynamics of a Floating Platform Mounting a Hydrokinetic Turbine


Tobias Dewhurst, M. Robinson Swift, Martin Wosnik, Kenneth Baldwin, Judson DeCew, and Matthew Rowell – Marine Technology Society Journal, August 2013

Abstract

A two-dimensional mathematical model was developed to predict the dynamic response of a moored, floating platform mounting a tidal turbine in current and waves. The model calculates heave, pitch, and surge response to collinear waves and current. Waves may be single frequency or a random sea with a specified spectrum. The mooring consists of a fixed anchor, heavy chain (forming a catenary), a lightweight elastic line, and a mooring ball tethered to the platform. The equations of motion and mooring equations are solved using a marching solution approach implemented using MATLAB. The model was applied to a 10.7-m twin-hulled platform used to deploy a 0.86-m shrouded, in-line horizontal axis turbine. Added mass and damping coefficients were obtained empirically using a 1/9 scale physical model in tank experiments. Full-scale tests were used to specify drag coefficients for the turbine and platform. The computer model was then used to calculate full-scale mooring loads, turbine forces, and platform motion in preparation for a full-scale test of the tidal turbine in Muskeget Channel, Massachusetts, which runs north-south between Martha’s Vineyard and Nantucket Island. During the field experiments, wave, current, and platform motion were recorded. The field measurements were used to compute response amplitude operators (RAOs), essentially normalized amplitudes or frequency responses for heave, pitch, and surge. The measured RAOs were compared with those calculated using the model. The very good agreement indicates that the model can serve as a useful design tool for larger test and commercial platforms.

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The Use of Numerical Modeling to Optimize a New Wave Energy Converter Technology


Brandon E. Green and Daniel G. MacDonald – Marine Technology Society Journal, August 2013

Abstract

A numerical model of a new point-absorber wave energy converter (WEC) technology was designed for simulation purposes using Python. The governing equations were defined to take into account the relevant forces on the buoy in an ideal wave environment as well as any opposing forces due to damping, the power take-off (PTO) mechanism, and alternator. These equations of motion were solved using a high-order iterative process to study the linear kinematics of the buoy, the behavior of the PTO, and the associated power output in an ideal ocean wave environment. The model allows for the adjustment of relevant parameters to explore the behavior of the WEC and optimize system efficiency depending on the wave conditions. The numerical model was designed to run single simulations for a specified time interval; however, an optimization routine was implemented to optimize the mechanical parameters that greatly affect power output. The optimization portion of the model was implemented to study the response of the virtual WEC to a variety of input conditions pertaining to the buoy, PTO, and wave dynamics. This paper explains the development of the prototype WEC and the associated numerical model, in addition to evaluating the response of the WEC to a variety of input conditions. The output of the numerical model is discussed for the associated wave field used for simulation purposes. The design and implementation of the numerical model provides insight into changes in design components to maximize system power output and efficiency. The results of the numerical model and examples of data output for specific input conditions are investigated.

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Theoretical Assessment of Ocean Current Energy Potential for the Gulf Stream System


Xiufeng Yang, Kevin Haas, Hermann M. Fritz – Marine Technology Society Journal, August 2013

Abstract

The Gulf Stream system features some of the fastest and most persistent currents in the Atlantic Ocean and has long been identified as a promising target for renewable ocean current energy. This study investigates the theoretical energy potential of ocean currents for the Gulf Stream system. A simplified analytical model is calibrated and utilized to represent the quasi-geostrophic balance in the North Atlantic subtropical circulation. The effect of turbines is included in the model as additional turbine drag force. The energy equation in the system is derived and analyzed both locally and basin-wide. Basin-wide, energy production from surface wind stress is balanced by energy dissipation from natural friction and turbines. However, the pressure gradient is playing an important role in redistributing the energy in the local energy balance. It is found that increasing turbine drag does not necessarily increase total energy dissipation from turbines. The maximum energy dissipation by turbines is estimated to be approximately 44 GW, although electrical power output will be significantly reduced due to various engineering and technological constraints. The turbine drag has significant impact on the circulation system. The reduction of energy and volume fluxes in the circulation is featured for different levels of turbine drag. It is found that residual energy flux along the western boundary can be significantly reduced under the peak energy dissipation by turbines, while reduction of volume flux is less extreme.

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Testing the WET-NZ Wave Energy Converter Using the Ocean Sentinel Instrumentation Buoy


Terry Lettenmaier, Annette von Jouanne, Ean Amon, Sean Moran, Alister Gardiner – Marine Technology Society Journal, August 2013

Abstract

This paper describes ocean testing of the half-scale Wave Energy Technology-New Zealand (WET-NZ) prototype wave energy converter (WEC) using the Ocean Sentinel instrumentation buoy during a 6-week deployment period in August-October 2012. These tests were conducted by the Northwest National Marine Renewable Energy Center (NNMREC) at its Pacific Ocean test site off the coast of Newport, Oregon. The WET-NZ is the product of a research consortium between Callaghan Innovation, a New Zealand Crown Entity, and Power Projects Limited (PPL), a Wellington, New Zealand private company. The Oregon deployment was project managed by Northwest Energy Innovations (NWEI), a Portland, OR firm. NNMREC is a Department of Energy sponsored partnership between Oregon State University (OSU), the University of Washington (UW), and the National Renewable Energy Laboratory (NREL). The Ocean Sentinel instrumentation buoy is a 6-m surface buoy, developed in 2012, that provides a stand-alone electrical load, WEC generator control, and data collection for WECs being tested. The Ocean Sentinel was deployed and operated for the first time during the 2012 WET-NZ tests. During these tests, the operation of the WET-NZ was demonstrated and its performance was characterized, while also proving successful deployment and operation of the Ocean Sentinel.

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Experimental Evaluation of a Mixer-Ejector Marine Hydrokinetic Turbine at Two Open-Water Tidal Energy Test Sites in NH and MA


Matthew Rowell, Martin Wosnik, Jason Barnes, and Jeffrey P. King – Marine Technology Society Journal, August 2013

Abstract

For marine hydrokinetic energy to become viable, it is essential to develop energy conversion devices that are able to extract energy with high efficiency from a wide range of flow conditions and to field test them in an environment similar to the one they are designed to eventually operate in. FloDesign Inc. developed and built a mixer-ejector hydrokinetic turbine (MEHT) that encloses the turbine in a specially designed shroud that promotes wake mixing to enable increased mass flow through the turbine rotor. A scaled version of this turbine was evaluated experimentally, deployed below a purpose-built floating test platform at two open-water tidal energy test sites in New Hampshire and Massachusetts and also in a large cross-section tow tank. State-of-the-art instrumentation was used to measure the tidal energy resource and turbine wake flow velocities, turbine power extraction, test platform loadings, and platform motion induced by sea state. The MEHT was able to generate power from tidal currents over a wide range of conditions, with low-velocity start-up. The mean velocity deficit in the wake downstream of the turbine was found to recover more quickly with increasing levels of free stream turbulence, which has implications for turbine spacing in arrays.

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Transmision Shaft Design for Hydrokinetic Turbine with Reliability Consideration


Gouthan Pusapati – Missouri University of Science and Technology, 2013

Abstract

Hydrokinetic energy, a relatively new kind of renewable energy, can be generated from flowing water in rivers or oceans. Hydrokinetic turbines (HKTs) are a major system for hydrokinetic energy, and the reliability of the HKTs is critical for both their lifecycle cost and safety. The objective of this work is to apply advanced methodologies of reliability analysis and reliability-based design to the transmission shaft design for a horizontal-axis, non-submerged HKT. The deterministic shaft design is performed first by considering failure modes of strength and deflection using distortion energy, maximum shear and deflection theories. Then the reliability analysis of the shaft designed is performed by using Sampling Approach to Extreme Values of Stochastic Process method (SAEVSP). Finally reliability-based design is applied to the transmission shaft design, which results in the minimal shaft diameter that satisfies the reliability requirement for a given period of operation time. Since the time-dependent river velocity process is involved, the time-dependent reliability method is used in the reliability-based design. The methodology for the shaft design in this work can be extended to the design of other components in the HKT system.

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