An experimental investigation simulating flow effects in first generation marine current energy converter arrays
Highlights
► Multiple-row tidal turbine experiments conducted. ► High resolution flow mapping of array. ► Demonstrated fluid can be accelerated between adjacent devices. ► Available Kinetic energy 22% greater to downstream devices. ► Proves that synergy between devices can be achieved to increase array power output.
Introduction
Marine current energy conversion technology is presently at the commercial demonstrator stage where single devices of appreciable scale are operating at various tidal current sites around the world. Evidence of large amounts of available power at various European sites has long been known [1] with significant flow velocities measured around the British Isles, Philippines and far East, North East coast of North America and the West coast of Canada. There have been a number of studies conducted to attempt to quantify the fraction of extractable energy from generic and specific sites using a variety of methods for analysis [2], [3], [4], [5].
The next stage in the technology development path will be the installation of farms or arrays of Marine Current Energy Converters (MCECs) (Fig. 1). The layout of such arrays may well be site dependant as a number of variables will determine inter-device spacing. Many sites have bi-directional flow characteristics usually associated with tidal currents, sometimes referred to as tidal streams or marine currents. Therefore, carefully designed geometric patterns may proliferate for the layout governing optimised arrangement of devices within an array. Installation and maintenance vessels will vary in size and manoeuvrability thus affecting inter-device spacing. In addition, the flow field generated by the MCEC devices will affect one another in an array. Fluid passing through a horizontal axis MCEC will experience a reduction in velocity across the rotor plane. Downstream of the rotor this region of fluid is moving at a lower velocity than the free stream fluid (that passed around the rotor) and hence must expand in order to conserve momentum. This takes the form of a gradually expanding cone-shaped region downstream of the rotor that is more commonly known as the wake. Turbulent mixing in the boundary region between the wake and the faster moving free stream fluid serves to re-energise the wake, breaking it up and increasing the velocity. At a distance far downstream the wake will have almost completely dissipated and the flow field will closely resemble that which existed upstream of the rotor disk.
Some of the variables that may influence the wake structure for a single rotor disk are illustrated in Fig. 2. It will be a cost-benefit exercise for the developer of an array to decide at what lateral (cross flow) and longitudinal distances to separate MCECs; spaced too closely and device efficiencies will drop in the slower moving turbulent flow whilst over-spacing will not be an effective use of the tidal site which generally will have a compact spatial footprint.
The downstream wake generated by a MCEC will be significantly different to that of similar applications such as marine propellers or wind turbines. Principally due to the small total fluid height (rotor diameter may approach half total fluid depth), the pronounced velocity profile that exist in the sea and reduced flow re-energisation processes compared to the atmosphere. The principle variables that govern wake flow downstream of actuator disks in a vertically constrained flow were investigated by the authors and reported previously [6], [7].
There have been previous studies investigating the wake effects and energy losses within arrays of wind turbines [8], [9]. Results have shown that energy losses due to interaction effects may be significant. This study establishes the case for marine currents turbines through experimental quantification of fluid around actuator disks used to simulate various turbine array configurations. The experimental studies described herein attempt to provide a seminal base for the investigation of interaction effect for multiple MCEC devices operating in a vertically constrained flow. The work characterises the flow field of two actuator disks aligned perpendicular to the flow with variable lateral spacing coupled with a comprehensive flow mapping experiment investigating an offset dual-row array of devices.
Section snippets
Classification of MCEC arrays
It is likely that arrays will evolve in size and complexity as the technology develops. A useful concept for classifying arrays has been developed by the authors as part of an EU-funded project aspiring to formulate protocols for equitable testing amongst wave and marine current energy devices [10]. A key driver for nearly all types of MCECs will be the minimisation of negative interaction effects between devices whereby structural loading is increased and/or power production is reduced. Early
Experimental approach
Modelling horizontal axis rotors becomes impractical at very small scale due to disparate scaling laws. Previous studies have shown that rotor diameters less than 0.8 m introduce problems due to the low incident energy flux available at flow speeds sufficiently low enough to avoid large Froude numbers and associated changes in water surface elevation [11]. Thus porous ‘actuator’ disks may be used at appropriate Froude numbers with scaled length ratios and low blockage ratios to exert a similar
Data presentation
Horizontal axis wake recovery can be defined in terms of velocity deficit, which is a non-dimensional number relative to the free stream flow speed at hub height (U0) and the wake velocity (UW):
Horizontal and vertical shear stresses are defined as:Where u, v, w are the velocity components in the longitudinal (downstream), lateral (cross-channel) and vertical direction, ρ is the fluid density and the dash denotes the varying component of the velocity
Lateral device spacing for single row tidal arrays
For the first series of experiments a lateral array was considered with two actuator disks arranged perpendicular to the flow direction. Lateral disk separation, measured between the innermost edges of the actuator disks, was set to 0.5D, 1.0D and 1.5D. Despite the good width of the Chilworth flume increasing the number of disks would increase the blockage area of this model-scale array to unrealistic values therefore only 2 disks were used. The 0.5D lateral separation case was based upon a
Conclusions
A comprehensive flow mapping exercise around arrangements of multiple actuator disks has been presented. It has been demonstrated that flow can be accelerated between a pair of actuator disks arranged in a single row 1st-generation array. For the dimensionless length ratios used in this work an inner disk separation of 1.5-diameters in a water depth of 3-diameters flow was accelerated between a pair of rotor disk simulators. The resulting jet between the disks had a greater velocity than the
Acknowledgements
This work was part of a project funded by the Department of Business, Enterprise and Regulatory Reform (now the Department of Business, Innovation and Skills) on performance characteristics and optimization of marine current energy converter arrays, BERR Project number T/06/00241/00/00. The work addressing classification of arrays is part of ‘Equimar’, funded under the European Community’s Seventh Framework Programme FP7/2007-2013 under grant agreement number FP721338.
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