Flow optimization for unconventional approaches to hydrokinetic energy conversion

School of Engineering, Brown University

Energy from flowing water in rivers, termed riverine hydrokinetic energy, is a renewable source of energy. In the US alone, the theoretical resource of riverine hydrokinetic energy is estimated to be 1381 TW hr/yr (Electric Power Research Institute. Tech. rep. 1026880. 2012), which constitutes about 36% of the 2014 national electricity consumption of 3764 TW hr (U.S. Energy Information Administration, Electric Power Annual 2014, 2016). Hydrokinetic turbines, operating individually or in arrays, have been proposed to convert this energy to electricity, however, commercial viability of such operations remains questionable. I direct a project spanning activities from fundamental fluid dynamics to business development for successfully commercializing hydrokinetic energy conversion.

We propose two unconventional approaches to hydrokinetic energy conversion, one about the device and a second about the array configuration. Our innovations circumvent some of the critical reasons underlying the failure of previous attempts to commercialize hydrokinetic energy. I present results from flow optimization arising from our approach on the device and array scale.

On the device scale, we propose an oscillating hydrofoil for harnessing hydrokinetic energy. A hydrofoil pitches and plunges in a manner that extracts energy from the flow. An oscillating hydrofoil has many practical advantages over conventional rotary turbines. We ask which of the periodic, but otherwise arbitrary, pitching-plunging trajectories of the hydrofoil maximizes the energy conversion efficiency. To answer this question we couple an experimental realization of the oscillating hydrofoil to an optimization routine and systematically explore the space of periodic trajectories. We find that, despite the nonlinearities in the fluid dynamics, a finite-amplitude sinusoidal pitching-plunging trajectory maximizes the energy conversion efficiency.

On the array scale, we propose installing hydrokinetic devices in a row aligned with the freestream. Conventionally, installing turbines in the wake of upstream ones deteriorates the performance of the array, and limits the density of turbines that can be installed in a given area. We envision an asymmetric operation of the turbines so that the wake from upstream turbines is deflected around the downstream ones. Using a systematic framework to abstract the process of flow deflection and energy conversion, we show that the maximum possible power density of a linear array is at least an order of magnitude greater than that of conventional arrays.