Local blockage effect for wind turbines

2015

Renewable Energy

As commercial scale tidal energy devices are shortly to be deployed in the 11 first arrays, the knowledge of how different array layouts perform is a key 12 and under-examined field. Here, the Momentum Reversal Lift (MRL) tur-13 bine, developed by the University of Exeter, is deployed in five different array 14 layouts utilising up to 15 devices. The use of dynamic turbines allows the 15 inclusion of analysis of the effects of flow direction in the wake.

Energies

Tidal stream turbines are subject to complex flow conditions, particularly when installed in staggered array configurations where the downstream turbines are affected by the wake and/or bypass flow of upstream turbines. This work presents, for the first time, methods for and results from the physical testing of three 1/15 scale instrumented turbines configured in a closely-spaced staggered array, and demonstrates experimentally that increased power extraction can be achieved through reduced array separation. A comprehensive set of flow measurements was taken during several weeks testing in the FloWave Ocean Energy Research Facility, with different configurations of turbines installed in the tank in a current of 0.8 m/s, to understand the effect that the front turbines have on flow through the array and on the inflow to the centrally placed rearmost turbine. Loads on the turbine structure, rotor, and blade roots were measured along with the rotational speed of the rotor to assess con...

Journal of Ocean Engineering and Marine Energy, 2018

CFD modelling of tidal turbines in arrays is described and assessed against experimental studies of turbines operating either at constant speed or constant torque. Rotor blades are represented by rotating actuator lines, whilst supports are represented by partially-blocked-out cells. For a single turbine the model successfully reproduces towing-tank measurements of thrust and power coefficients across a range of tip-speed ratios. For two turbines staggered streamwise, it is demonstrated that loads may be reduced or augmented, according as the downstream turbine is in the wake or bypass flow of the upstream turbine. When the downstream turbine is partially in the wake, individual blades are subject to large cyclic load fluctuations. Array performance is evaluated by comparison with experimental data, modelling up to 12 turbines in up to three staggered rows. The speed of each turbine is continuously adjusted in response to flow-induced torque. Distribution of thrust coefficients within the array is well reproduced, but there is greater discrepancy in angular speed. With actuator representation of blades, the choice of turbulence model has little effect on load coefficients for an isolated turbine or row of turbines, but a significant effect on the wake, and hence on downstream turbines in an array.

Renewable Energy, 2015

This paper investigates the influence of wake interaction and blockage on the performance of individual turbines in a staggered configuration in a tidal stream farm using the CFD based Immersed Body Force turbine modelling method. The inflow condition to each turbine is unknown in advance making it difficult to apply the correct loading to individual devices. In such cases, it is necessary to establish an appropriate range of operating points by varying the loading or body forces in order to understand the influence of wake interaction and blockage on the performance of the individual devices. The performance of the downstream turbines was heavily affected by the wake interaction from the upstream turbines, though there were accelerated regions within the farm which could be potentially used to increase the overall power extraction from the farm. Laterally closely packed turbines can improve the performance of those turbines due to the blockage effect, but this could also affect the performance of downstream turbines. Thus balancing both the effect of blockage and wake interaction continues to be a huge challenge for optimising the performance of devices in a tidal stream farm.

Journal of ocean engineering and marine energy, 2018

Theoretical models suggest that in order to maximise their collective power output, tidal turbines should be arranged in a single cross-stream row and optimally spaced to exploit local blockage effects. However, because it is assumed that the turbines within these arrays are identical, such models do not consider the possibility of enhanced power production through the exploitation of spanwise variations in local blockage and resistance. In this paper, we use depth-averaged numerical simulations to investigate whether the performance of a tidal turbine array can be further enhanced by varying solely the local blockage, solely the local resistance, or both local blockage and resistance together, across the array width. Our results suggest that for an initially uniform flow field, the optimal tidal turbine array is also uniform, that is to say that it comprises turbines of equal size, spacing, and resistance. This finding is encouraging because it is more cost-effective and much simpler to design each turbine to be the same and to operate in the same way. Together with earlier findings, these results also suggest a more general, and perhaps unsurprising, conclusion that tidal turbine arrays perform best when designed to match site-specific natural flow conditions. Tidal stream power • Tidal turbine array • Shallow water equations • Actuator disc • Non-uniform

Journal of Renewable and Sustainable Energy

The performance and economics of turbines in a tidal array are largely dependent on the power per turbine, and so approaches that can increase this power are crucial for the development of tidal energy. In this paper, we combine a two-scale partial array model and a one-dimensional channel model to investigate the effects of blockage, turbine arrangement, and channel dynamics on tidal turbines. The power per

2018

Due to the predictable nature of the tides, power generation from tidal streams could play a significant role in addressing future power network challenges by providing an auxiliary frequency balancing service. A requirement of such services is that power output may be rapidly adapted. This can be achieved by several alternative operating strategies applied to individual turbines, to entire farms or to clusters of turbines within a farm. The onset tidal flow speed to an array is sensitive to the net resistance provided by the array and so it is necessary to consider how such strategies would affect power output, net resistance and the flow onset to the farm. Investigations using a Reynolds-averaged Navier-Stokes Actuator Disc (RANS-AD) model and experiments in a shallow channel are conducted to assess the influence of rotor operating point on individual turbine loading and hence aggregate resistance of an array. Arrayand row-specific values for the local thrust coefficient and corre...

2014

Tidal currents are one of the most promising sources of power within the renewable energy sector, especially for countries with suitable marine conditions. The United Kingdom has the edge over the rest of countries in this type of renewable energy and leads the innovation race in order to keep the UK economy competitive in future years. Following these reasons, several types of marine turbines are currently being developed and tested. Nevertheless, one of the key issues, in the future development of marine hydrokinetic power generation systems, is to properly understand their prospective performance, not only for each power generation device alone but also for an array of devices as a whole. Recent theoretical studies have suggested that a dense cross-stream array of turbines (so-called turbine fence) is a promising way to extract power from marine currents; however, its optimal intra-turbine spacing depends on several physical factors, some of which are still uncertain. One of those uncertain factors is the effect of seabed friction causing vertical shear of the flow, which is difficult to study theoretically and requires 3-D CFD simulations. Also, little is known about the performance of multiple rows of marine turbines. Consequently, throughout this thesis, numerous arrangements of marine turbines (modelled as actuator disks) have been tested using ANSYS Fluent®, with the idea of assessing the effects of various parameters such as: intra-device spacing, number of rows and turbine resistance coefficient. These CFD simulations have been compared with existing theoretical models (two-scale actuator disk models) for the power extracted by the turbines with the aim of validating these theoretical models. Also, the results of CFD simulations have been analysed in detail to better understand the characteristics of flow past these turbine arrays. Keywords: Tidal Turbines, Optimal Spacing, Actuator Disk, RANS Simulations, Open Channel Flow, Marine Energy.

Renewable Energy (2016)

We present a comprehensive set of two-dimensional (2D) unsteady Reynolds-averaged Navier-Stokes (URANS) simulations of flow around a pair of counter-rotating vertical-axis wind turbines (VAWTs). The simulations are performed for two possible configurations of the counter-rotating VAWT pair, with various gaps between the two turbines, tip-speed-ratios and wind directions, in order to identify key flow mechanisms contributing to the enhanced performance of a pair of turbines compared to an isolated turbine. One of the key mechanisms identified, for the case of two turbines arrayed side-by-side with respect to the incoming wind, is the change of lateral velocity in the upwind path of each turbine due to the presence of the neighbouring turbine, making the direction of local flow approaching the turbine blade more favourable to generate lift and torque. The results also show that the total power of a staggered pair of turbines cannot surpass that of a side-by-side pair of turbines. Some implications of the present results for the prediction of the performance of single and multiple rows (or a farm) of VAWTs are also discussed. The local flow mechanisms identified in the present study are expected to be of great importance when the size of the farm is relatively small.