An optimal tuning strategy for tidal turbines

Abstract

Tuning wind and tidal turbines is critical to maximizing their power output. Adopting a wind turbine tuning strategy of maximizing the output at any given time is shown to be an extremely poor strategy for large arrays of tidal turbines in channels. This 'impatient-tuning strategy' results in far lower power output, much higher structural loads and greater environmental impacts due to flow reduction than an existing 'patient-tuning strategy' which maximizes the power output averaged over the tidal cycle. This paper presents a 'smart patient tuning strategy', which can increase array output by up to 35% over the existing strategy. This smart strategy forgoes some power generation early in the half tidal cycle in order to allow stronger flows to develop later in the cycle. It extracts enough power from these stronger flows to produce more power from the cycle as a whole than the existing strategy. Surprisingly, the smart strategy can often extract more power without increasing maximum structural loads on the turbines, while also maintaining stronger flows along the channel. This paper also shows that, counterintuitively, for some tuning strategies imposing a cap on turbine power output to limit loads can increase a turbine's average power output.

A note on the tuning of tidal turbines in channels

“…For the first strategy, k is adjusted at each time step to ensure that C P is maximised at each instant throughout the tidal cycle. Vennell (2016) has shown this 'impatient' tuning strategy, which is widely used in the wind energy industry, to be less beneficial for tidal turbines and so, here, the impatient strategy is used simply as a baseline for comparison.…”

“…By combining the simple tidal channel and bounded actuator disc models of Garrett andCummins (2005, 2007), Vennell (2010) showed that for turbines in channels, the maximum power depends not only on the power coefficient, but also on the thrust coefficient, which, together with the channel's natural dynamic balance, determines the throughflow velocity. Vennell (2010) also showed that large, optimally tuned turbine arrays can realise most of a channel's energy potential, and later demonstrated, following Adcock (2012), that varying the tuning over the tidal cycle can produce a higher power output, often without increasing the maximum loading on the turbines and whilst also maintaining a higher channel flow rate (Vennell 2016).…”

“…The works of Vennell (2010Vennell ( , 2016 provide key insights into the importance of tuning for tidal turbines in channels, but are, to a certain extent, limited by their turbine representation. Actuator disc theory provides a useful starting point for the analysis of tidal turbine performance, and has proven valuable in establishing upper bounds on the power available to rows of turbines placed at candidate tidal power sites (e.g.…”

“…In this paper, the works of Vennell (2010Vennell ( , 2016 are extended to incorporate a more realistic turbine representation, which is based on the blockage-corrected blade element momentum theory of Vogel et al (2018). This more advanced low-order model relates the flow through the turbine plane to the blade forces developed by the rotor, and may be used to consider different approaches to turbine tuning, which include varying the rotational speed and/or pitch angle of the turbine blades.…”

Assessment of the Malaysian tidal stream energy resource using an upper bound approach

“…The wake velocity coefficients are kept constant in time because it is assumed that the additional amount of power that may be gained by varying the turbine resistance over daily (Vennell and Adcock 2014;Vennell 2016) or spring-neap cycles ) is relatively small. Local blockage ratios of 0.1 and 0.4 are chosen to represent moderately sized and very large developments, respectively, and different combinations of rows are considered.…”