Investigation of the Possibilities to Improve Hydrodynamic Performances of Micro-Hydrokinetic Turbines

Marina Barbarić,Zvonimir Guzović

doi:10.3390/en13174560

Abstract

Horizontal axis turbines are commonly used for harnessing renewable hydrokinetic energy, contained in marine and river currents. In order to encourage the expansion of electricity generation using micro-hydrokinetic turbines, several design improvements are investigated. Firstly, optimization-based design of rotor blade is used to get as close as possible to the efficiency limit of 59.3% (known as Betz limit), that counts for bare turbine rotors, placed in the free flow. Additional diffuser elements are further added to examine the potential to overcome the theoretical efficiency limit by accelerating water at the axial direction. Various diffuser geometrical configurations are investigated using the computational fluid dynamics (CFD) to obtain insight into hydrodynamics of augmented micro-hydrokinetic turbines. Moreover, the turbines are compared from the energy conversion efficiency point of view. The highest maximum power coefficient increase of 81% is obtained with brimmed (flanged) diffuser. Diffusers with foil-shaped cross-sections have also been analyzed but power augmentation is not significantly greater than in the case of simple cross-section designs of the same dimensions. The power coefficients’ comparison indicate that considerable power augmentation is achievable using brimmed diffuser with higher value of length-to-diameter ratio. However, the impact of diffuser length increase on the power coefficient enhancement becomes weaker as the length-to-diameter ratio reaches a value of 1.

Highlights

Hydrokinetic technology has gained increased attention as an alternative for renewable energy production, especially at locations where conventional hydropower technology cannot provide a feasible solution
The highest maximum power coefficient increase of 81% is obtained with brimmed diffuser
The impact of diffuser length increase on the power coefficient enhancement becomes weaker as the length-to-diameter ratio reaches a value of 1

Summary

Introduction

Hydrokinetic technology has gained increased attention as an alternative for renewable energy production, especially at locations where conventional hydropower technology cannot provide a feasible solution. The main barrier to wider commercialization is relatively low efficiency, limited by so-called Betz limit, which indicates that hydrokinetic turbines cannot capture more than 59.3% of energy in open flow, regardless of the turbine design. Water depth at location limits generated power that is usually rather small. To approach the problem of relatively low efficiency, the concept of ducted or diffuser-augmented turbines can be used. These elements serve as turbine casing that causes pressure drop behind the turbine and accelerate the water in axial direction.

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