A.S. Bahaj

Fundamentals applicable to the utilisation of marine current turbines for energy production

The potential of electric power generation from marine tidal currents is enormous. Tidal currents are being recognised as a resource to be exploited for the sustainable generation of electrical power. The high load factors resulting from the fluid properties and the predictable resource characteristics make marine currents particularly attractive for power generation and advantageous when compared to other renewables. There is a paucity of information regarding various key aspects of system design encountered in this new area of research. Virtually no work has been done to determine the characteristics of turbines running in water for kinetic energy conversion even though relevant work has been carried out on ship’s propellers, wind turbines and on hydro turbines. None of these three well established areas of technology completely overlap with this new field so that gaps remain in the state of knowledge. This paper reviews the fundamental issues that are likely to play a major role in implementation of MCT systems. It also highlights research areas to be encountered in this new area. The paper reports issues such as the harsh marine environment, the phenomenon of cavitation, and the high stresses encountered by such structures are likely to play a major role on the work currently being undertaken in this field.

Tidal energy resource assessment for tidal stream generators

Abstract This article is a review of the current understanding of tidal energy resources in the context of the emerging technology of tidal stream power generation. The geographical focus is on the north-west European continental shelf, the scope of a number of published reports on exploitable tidal stream energy resources. These estimates are reviewed as are some analytical models of energy extraction by tidal stream generators.

Measurements and predictions of forces, pressures and cavitation on 2-D sections suitable for marine current turbines:

An investigation has been carried out into the lift, drag and cavitation characteristics of two-dimensional foil sections, which may typically be used as a starting point in the design of blade sections for marine current turbines. Cavitation tunnel experiments and numerical predictions using a panel code were carried out on four representative sections derived from the NACA series 4415, 6615, 63—215 and 63—815. The experimental lift and drag results show reasonable correlation with published wind tunnel data. The sections were modelled numerically using the two-dimensional panel code XFoil. The numerical cavitation predictions in most cases showed satisfactory agreement with the experiments and it is considered that such predictions could be used with reasonable confidence for predicting cavitation at the preliminary design stage. Overall, the results of the investigation provide detailed information that should assist in the design and operation of marine current turbines.

Accuracy of the actuator disc-RANS approach for predicting the performance and wake of tidal turbines

The actuator disc-RANS model has widely been used in wind and tidal energy to predict the wake of a horizontal axis turbine. The model is appropriate where large-scale effects of the turbine on a flow are of interest, for example, when considering environmental impacts, or arrays of devices. The accuracy of the model for modelling the wake of tidal stream turbines has not been demonstrated, and flow predictions presented in the literature for similar modelled scenarios vary significantly. This paper compares the results of the actuator disc-RANS model, where the turbine forces have been derived using a blade-element approach, to experimental data measured in the wake of a scaled turbine. It also compares the results with those of a simpler uniform actuator disc model. The comparisons show that the model is accurate and can predict up to 94 per cent of the variation in the experimental velocity data measured on the centreline of the wake, therefore demonstrating that the actuator disc-RANS model is an accurate approach for modelling a turbine wake, and a conservative approach to predict performance and loads. It can therefore be applied to similar scenarios with confidence.

Tribological design constraints of marine renewable energy systems

Against the backdrop of increasing energy demands, the threat of climate change and dwindling fuel reserves, finding reliable, diverse, sustainable/renewable, affordable energy resources has become a priority for many countries. Marine energy conversion systems are at the forefront of providing such a resource. Most marine renewable energy conversion systems require tribological components to convert wind or tidal streams to rotational motion for generating electricity while wave machines typically use oscillating hinge or piston within cylinder geometries to promote reciprocating linear motion. This paper looks at the tribology of three green marine energy systems, offshore wind, tidal and wave machines. Areas covered include lubrication and contamination, bearing and gearbox issues, biofouling, cavitation erosion, tribocorrosion, condition monitoring as well as design trends and loading conditions associated with tribological components. Current research thrusts are highlighted along with areas needing research as well as addressing present-day issues related to the tribology of offshore energy conversion technologies.

Continuous radionuclide recovery from wastewater using magnetotactic bacteria

Magnetotactic bacteria (MTB) can be magnetically removed and harvested from samples collected from ponds and streams. This is achieved by placing a permanent magnet at the sediment/water interface of a sample container. The bacteria swim along field lines, accumulating at regions close to the pole of the magnet. This is the basic principle of Orientation Magnetic Separation (OMS), where the applied magnetic field is utilised to orientate the bacteria to swim in a specific direction. This paper describes the use of MTB for bioaccumulation and radionucleide removal from wastewater using an OMS system.

A blade element actuator disc approach applied to tidal stream turbines

At a commercial scale tidal stream turbines are likely to be installed in multi-device arrays to maximise power output from a tidal site. This paper demonstrates a computationally efficient method for predicting the performance of an array. The approach parameterises the turbine by deriving momentum source terms using blade element (BE) theory. The source terms are added to Reynolds-averaged Navier-Stokes (RANS) momentum equations. The RANS+BE method is similar to blade element momentum (BEM) theory but can predict the flow field as well as the performance of the rotor. This investigation first verifies the RANS+BE method against BEM theory and shows good agreement, although tip-losses still need to be included. Secondly, it compares the RANS+BE approach to a parameterisation based on a uniform resistance coefficient, over a range of ambient turbulence values and thrust coefficients. The RANS+BE method generates increased azimuthal velocities in the near wake compared to the uniform approach. Finally the investigation compares results of a model of an infinitely wide array of turbines with five rows. The RANS+BE model can predict the performance of each turbine. and shows more rapid wake velocity recovery within the array. The investigation provides a detailed insight into the applicability of wake modelling techniques for the configuration of tidal stream turbine arrays.

Student project allocation using integer programming

The allocation of projects to students is a generic problem in many universities within the UK and elsewhere, not only in engineering but also in various other disciplines. This paper defines the student project allocation problem explicitly by an objective function and a number of constraints. Two integer program models are presented, the first of which is a dynamic program. A general purpose solver is used to solve the models, and the input files are included in the Appendix. The models are computationally efficient and easily solved on a PC. Important issues in interpreting the model outputs are highlighted. As with any optimization problem it is possible for constraints to be too tight to permit any feasible solution. Application of the models is demonstrated by using data from the Department of Civil and Environmental Engineering, University of Southampton, for the academic year 2001-2002. The model has been used successfully to allocate Individual Project and Group Project to students and is likely to become the defacto method of allocation of projects in the future. This paper demonstrates how operations research techniques used widely in optimizing use of resources can be applied in education.

Influence of turbulence on the wake of a marine current turbine simulator

Marine current turbine commercial prototypes have now been deployed and arrays of multiple turbines under design. The tidal flows in which they operate are highly turbulent, but the characteristics of the inflow turbulence have not being considered in present design methods. This work considers the effects of inflow turbulence on the wake behind an actuator disc representation of a marine current turbine. Different turbulence intensities and integral length scales were generated in a large eddy simulation using a gridInlet, which produces turbulence from a grid pattern on the inlet boundary. The results highlight the significance of turbulence on the wake profile, with a different flow regime occurring for the zero turbulence case. Increasing the turbulence intensity reduced the velocity deficit and shifted the maximum deficit closer to the turbine. Increasing the integral length scale increased the velocity deficit close to the turbine due to an increased production of turbulent energy. However, the wake recovery was increased due to the higher rate of turbulent mixing causing the wake to expand. The implication of this work is that marine current turbine arrays could be further optimized, increasing the energy yield of the array when the site-specific turbulence characteristics are considered.

Photovoltaic roofing: issues of design and integration into buildings

In the last decade, the development of photovoltaic roofing elements has been exceptional. These efforts were not limited to merely improving the integration methods with standard PV products but were also directed towards the production of appropriate elements that could replace roof tiles or shingles. This paper reviews some of the competing technologies for photovoltaic roofing and addresses the issues raised in the total integration of PV into roofing structures. The paper also discusses the general requirements of an idealised photovoltaic roofing system and some of the relevant variables that are required for the satisfaction of the end users. It has been shown that current strategies available for the true integration of PV elements as roof tiles or shingles utilising the sloped building fabric are limited. There are however, major on-going activities to research and develop roofing systems based on the foot-print of a roof tile or shingle.