Tidal Stream Turbine Modelling

At tidal energy sites large arrays of hundreds of
turbines will be required to generate economically significant
amounts of energy. Due to wake effects within the array, the
placement of turbines within will be vital to capturing the
maximum energy from the tidal current resource. This study
presents preliminary results of on-going work using Gerris, an
adaptive mesh flow solver, to investigate the flow through 4
different arrays of 15 turbines each, over a tidal cycle. The goal is
to optimise the position of turbines within an array in an
idealised channel. The turbines are approximated as regions of
increased depth averaged friction to enable numerical modelling
of multiple turbines in tidally reversing flow. This representation
of turbines as rectangular friction elements in the shallow water
equations does not show exactly the same flow structure as a
more accurate representation of a single turbine. However, the
goal of this paper is to represent the turbines and wakes in the
simplest realistic way in an adaptive mesh model at an array
scale so that the flow and power capture in tidally reversing flow
through large arrays can be studied. The effect of oscillating
tides is studied, with interesting dynamics generated as the tidal
current reverses direction, forcing turbulent flow through the
array. The energy removed from the flow by each of the 4 arrays
is compared over a tidal cycle and a staggered array is found to
extract 54% more energy than a non-staggered array. Further,
an array positioned to one side of the channel is found to remove
a similar amount of energy over a tidal cycle compared to an
array in the centre of the channel.

Abstract
Interest in the marine renewable energy devices, and particularly tidal stream turbines, has increased significantly over the past decade and several devices such as vertical and horizontal axis turbines and reciprocating hydrofoils are now being designed around the world to harness tidal stream energy. While tidal stream turbines are being developed at a high rate and getting closer to commercialisation, it is important to acquire the right tools to assist planners and environmentalists, not only in finding a right location for the turbines, but also in identifying their potential impacts on the surrounding marine and coastal environment.
In this study, a widely used open source depth integrated 2D hydro-environmental model, namely DIVAST, was modified to simulate the hydro-environmental impacts of the turbines in the coastal environment. The model predictions showed very good agreement with previously published 1D model results. Then, for demonstration purposes, the model was applied to an arbitrary array of tidal stream turbines in the Severn Estuary and Bristol Channel which has the third highest tidal range in the world. The model has shown promising potential in investigating the impacts of the array on water levels, tidal currents and sediment and faecal bacteria levels as well as the generated tidal power, which facilitates investigating the relative far-field impacts of the arrays under various climate change scenarios or different formations of the array.

Aspiring to build GW arrays of turbines is essential if tidal stream energy is to make a significant contribution to meeting the demand for renewable power.  Many channels have the potential to produce several GW.  Realizing one GW of this potential is much more complex than installing 1000 one MW turbines because large scale power extraction reduces tidal currents throughout a channel, changing the resource.  This work uses a 1D model to estimate the minimum number of turbines required to produce 1 GW in two example channels loosely based on the Pentland Firth and the Minas Passage. The minimum number of 400 m^2 turbines is 350-800, depending on how they are arranged into rows.
For some arrays  adding turbines increases the total output of the array, while simultaneously reducing the output of every turbine within it. For other arrays adding turbines increases both the array output and the output of every turbine within it, despite a reduction in tidal currents. Thus turbines in large arrays perform very differently to isolated turbines.  Large arrays have turbines which sweep more than 2-5% of a channel's cross-sectional area, which is only 40 turbines in the Pentland Firth.

Diffuser-augmented wind turbines are known for the potential improvement in power extraction in comparison with open wind turbines. Despite the large number of research works dealing with this subject and unlike the open rotor case, an optimum ducted rotor model is still missing. Since the Joukowsky (free-vortex) optimum rotor exhibits the best power coefficient in the open configuration, this paper presents a newly developed ring-vortex free-wake approach for the performance evaluation of an optimum Joukowsky rotor enclosed in a duct of general shape. The method, which is extensively verified, relies on the exact solution of the steady, incompressible, inviscid and axisymmetric flow, and it naturally takes into account the wake divergence and rotation. The procedure is used to obtain, for the first time, the maximum-power-coefficient/tip-speed-ratio characteristic curve for a diffuser augmented wind turbine. The proposed ducted rotor beats the Betz limit by 14.5% when the power coefficient is referred to the device frontal (exit) area. Additionally, the device experiences a slower decrease in performance with the reduction of the tip-speed ratio, thus extending the design range of ducted rotors in comparison with the open ones. Finally, taking into account the mutual influence of the disk and duct, a new rotor design strategy, capable to evaluate the optimum distribution of the chord and pitch-angle along the blade span, is also proposed. A complete design exercise is carried out and the rotor geometry is obtained for three different values of the nominal tip speed ratio. The paper also proves that a two-dimensional design procedure, which strongly couples the duct and the rotor induced flow, is mandatory to properly evaluate the optimum rotor geometry.

A procedure for the optimisation of hydrokinetic turbine array layout through surrogate modelling is introduced. The method comprises design of experiments, computational fluid dynamics simulations, surrogate model construction, and constrained optimisation. Design of experiments are used to build polynomial and Radial Basis Function surrogates as functions of two design parameters: inter-turbine longitudinal and lateral spacing, with a view to approximating the capacity factor of turbine arrays with inline and staggered layouts, each of which having a fixed number of turbines. For this purpose, two scenarios have been used as case studies, considering uniform and non-uniform free-stream flows. The major advantage of this method in comparison to those reported in the literature is its capability to analyse different design parameter combinations that satisfy optimality criteria in reasonable computational time, while taking into account complex floweturbine interactions and different turbine types.

A metamodel simulation based optimisation approach for the tidal turbine location problem is introduced. The method comprises design of experiments, computational simulations, metamodel construction and formulation of a mathematical optimisation model. Sample plans with different number of data points are used to fit 2nd and 3rd order polynomial as a function of two design parameters: longitudinal and lateral spacing, with a view to approximating the power output of tidal turbine farms with inline and staggered layouts, each

Concern over global climate change has led policy makers to accept the importance of reducing greenhouse gas emissions. This in turn has led to a large growth in clean renewable generation for electricity production. Much emphasis has been on wind generation as it is among the most advanced forms of renewable generation, however, its variable and relatively unpredictable nature result in increased challenges for electricity system operators. Tidal generation on the other hand is almost perfectly forecastable and as such may be a viable alternative to wind generation. This thesis paper shows the important of Tidal power over other sources and it’s generation using DFIG. 
This thesis is analyzed based on the following configuration;
A 6 MW tidal power site having four, 1.5 MW tidal turbines connected to a 25 kV distribution system which ensures power to a 120 kV grid through a 30 km, 25 kV feeder line. A 2300V, 2 MVA plant with a motor load (1.68 MW induction motor at 0.93 pf) and of a 200 kW resistive load which is also linked into the same feeder. Both tidal turbine and motor is attached to a protection system which was used to monitor voltage, current and machine speed. DFIG DC link voltage is also monitored. Tidal turbines used an induction generator having a wound rotor and an AC/DC/AC IGBT based PWM converter. The stator winding is also linked directly to the 50 Hz grid and the rotor is fed at variable frequency through the AC/DC/AC converter. DFIG technology was also used for extraction of maximum energy from the tides for low tidal speeds through optimization of turbine speed, while lowering turbine mechanical stresses during heavy tides. The turbine’s optimum speed used to produce maximum mechanical energy for a given tidal speed which is directly proportional to tidal speed. The rotor runs at sub synchronous speed for tidal speeds will also be lowered at 10 m/s and at super synchronous speeds for higher tidal speeds.
The simulation of 6MW tidal site was carried out for normal condition and at different types of faults introduced on the 25kV line. They are carried out as follows;
a) For normal conditions, at a tidal speed of 13m/s.
b) For a phase A to ground fault in the 25 kV line for t=5 to 5.1 seconds.
c) For a phase A and B fault in the 25 kV line t=5 to 5.1 seconds.
d) For a phase A and B to ground fault in the 25 kV line t=5 to 5.1 seconds.
e) For a symmetric fault in the 25kV line for t=5 to 5.1 seconds