Axial Flow Devices

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The limit on energy extraction by unblocked (e.g. wind) turbines is 59.3% of the kinetic energy flux through the turbine plane (Betz limit). In constrained tidal flows the sea surface, sea bed and neighbouring turbines can be used to provide flow blockage which can enhance the extractable power limit of each turbine by channelling more flow through each turbine and enabling a greater static pressure drop across each. In 2012 Nishino & Willden showed that the theoretical limit on energy extraction is lifted from the Betz limit to 79.8% of kinetic flux for a short fence of turbines arrayed normally to the flow direction in an infinitely wide channel (zero global blockage) through the use of carefully controlled inter-turbine spacing.

two_turbinesThe principle of turbine-turbine constructive interference has been demonstrated at small scale using fences of porous disk rotor emulators (Cooke et al., 2015) and numerical simulations of short fences of actuator disks and tidal rotors. Nishino & Willden 2013, Vogel & Willden 2017). Within a multi-rotor fence, device-device interference works to direct more of the flow through each device than would occur if the devices were more widely spaced; the ability of the flow to bypass the rotor being more restricted by the presence of neighbouring rotors. The closely-spaced multi-rotor configuration thus delivers higher performance per rotor.

rotor_designThe performance uplift afforded by constructive interference can only be achieved through redesigning the rotor for operation in closely spaced arrangements. It is not possible to simply spin conventional rotor designs faster to capture the high power available through interference due to cavitation constraints on operating tip-speed-ratio. This leads to noticeable differences in the chord distribution between wind turbine and tidal stream turbine designs.

sspa_1We have developed a suite of tools to design multi-rotor fences of axial-flow turbines to deliver the higher performance available through the use of a partial fence. We have recently conducted two significant experimental campaigns on two 1.2m diameter turbines designed for constructive interference at FloWave (February 2019) and SSPA (September 2019) to demonstrate the performance uplift available through exploiting constructive interference.

Constructive interference designed rotors not only offer the potential for a significant step change in LCOE, but also extend to lower speeds the range of flow speeds that may be commercially viable, and thereby may increase the scale of the overall technically extractable and economically feasibly resource.