Hydraulic design of a very low head axial turbine for clean energy option at Tana-Beles sugar irrigation canal in Ethiopia

Abstract

This paper envisages the hydraulic design of a Very Low Head Turbine (VLHT) based on an existing dual-purpose project, which consists of an upstream hydropower plant that feeds water to the downstream irrigation system. The design aims to develop a VLHT that can generate 500 kW of electricity using discharge and head in a primary canal near a rural community.

Crucial flow parameters and overall dimensions of the VLHT were determined through one-dimensional (1-D) analysis and free-vortex flow assumption. The VLHT design consists of nonmoving guide vanes and a fixed blade runner, both oriented in an axial direction. Thirty-two meters of downstream energy dissipator and seventeen meters of upstream canal coupled with a complete three-dimensional VLHT were the domain of interest. Subsequently, the finite volume method was applied to mesh all the domains of interest. Numerical flow simulations were performed using the Reynolds Averaged Navier Stokes (RANS) equation and the shear stress turbulence (SST) model, simultaneously capturing the interaction between the VLHT and canals. Two Computational Fluid Dynamics (CFD) simulation approaches were employed to assess their design performance prediction capabilities: steady and unsteady single-phase simulations.

A full-fledged design was realized for VLHT inclined 15.27° from the horizontal, characterized by optimal energy conversion, acceptable operating ranges, and minimal energy losses with no cavitation. The flow fields in the rotor-stator assembly, upstream canal, and energy dissipator were favorable toward the intended hydraulic design and justified with facts from the general fluid dynamics principle. At the best efficiency point, the mechanical power outputs for the steady and unsteady approaches were 562.31 kW and 582.73 kW, respectively, with the associated losses within the rotor-stator of 11.35 % and 10.32 %. Furthermore, at the same operating point, the corresponding VLHT discharge rates stand at 22.87 m³/s and 22.82 m³/s, and the recorded slit overflows are 5.13 m³/s and 5.18 m³/s for the steady and unsteady approaches, respectively. Quantitative and qualitative evaluations of the results provided excellent insight into the flow behavior within the VLHT and the region of flow interaction between the canal and the turbine.

The hydraulic efficiency and mechanical power output for the unsteady simulation surpassed that of the steady simulation. The observed difference was approximately 20 kW and 1 %. The rotor-stator interface loss was 2.51 % for steady simulation and 0.162 % for unsteady simulation, expressed as percentages of the average net heads between the two simulations. In terms of significance, this tailored-based research outcome can give hydraulic practitioners a design clue on the adaptability of similar sites for low-cost, low-impact propeller turbine concepts and provide great insight for Ethiopia's regional and federal governments to adopt clean energy technology options from irrigation canals. Exploiting this multi-purpose hydro energy resource for off-grid applications helps the nation to achieve sustainable development goals by providing affordable, reliable, and modern energy services for rural societies through its National Electrification Program (NEP).