Abstract
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 concurrently in real-time the effects of flow and array geometry on structural loading and performance. Operating in this closely-spaced array was found to improve the power delivered by the rear turbine by 5.7–10.4% with a corresponding increase in the thrust loading on the rotor of 4.8–7.3% around the peak power operating point. The experimental methods developed and results arising from this work will also be useful for further scale-testing elsewhere, validating numerical models, and for understanding the performance and loading of full-scale tidal stream turbines in arrays.
Highlights
Full-scale tidal current turbines have been installed in small arrays, generating and exporting electricity to the local network for several years, both with individual unit capacity up to 1.5 MW [1], and smaller 100 kW units operated in an array in Shetland since 2016 [2]
Flow through the turbine array is influenced by the turbines, rotational speeds of the turbines are quoted in rpm, rather than tip-speed ratio (TSR), in all the results discussed in this paper
The tests provide valuable insight into the loading experienced by the rear turbine, both in the closely-spaced staggered array configuration that may be compared to measurements made on one unit on its own
Summary
Full-scale tidal current turbines have been installed in small arrays, generating and exporting electricity to the local network for several years, both with individual unit capacity up to 1.5 MW [1], and smaller 100 kW units operated in an array in Shetland since 2016 [2]. This paper presents the methods employed and results from a significant experimental campaign that shows how the flow conditions in the FloWave current and wave test tank (EP/I02932X/1) are modified by the presence of and energy extracted by an array of three 1.2 m rotor diameter 1/15 scale model tidal turbines. These turbines were made and tested at the University of Edinburgh, as part of the work of the EPSRC-sponsored SuperGen UK Centre for Marine Energy Research (EP/I027912/1), each identical with a 3-bladed horizontal axis turbine and four-quadrant controlled power take-off.
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