The Performance and Hydrodynamics in Unsteady Flow of a Horizontal Axis Tidal Turbine

Tidal energy has clear potential in producing large amounts of energy as the world’s capacity exceeds 120 GW. Despite being one of the oldest renewable energy sources exploited by man, the technology is still in its pre-commercialisation stage and so lags behind other renewable sources such as wind and geothermal energy in terms of development and energy produced. One of the emerging energy extraction technologies in the tidal energy field is the Horizontal Axis Tidal Turbine (HATT) which harness tidal stream energy the same way Horizontal Axis Wind Turbine (HAWT) extract energy from the wind. While HATT has been the topic of many researches over the past decade, there is still a gap in the current literature in terms of its performance in unsteady flow which is closer to the typical environment where HATTs are installed.

This thesis looks at the hydrodynamics and performance of the Sheffield HATT, a turbine designed in the University of Sheffield, both in steady and unsteady flow through numerical simulations. The initial design of the turbine has been done using QBlade which is a Blade-Element Momentum solver with a blade design feature. Structural simulation of the blade using BEM data was conducted before a Computational Fluid Dynamics (CFD) model of the turbine was created to be first tested in steady flow. The performance curve of the Sheffield HATT was determined and compared with the BEM results. A peak coefficient of performance (CP) of 41.88% was obtained for the k-ε RNG case while it is 39.46% for the k-ω SST model, both happening at TSR=6 and both of them having values lower than that of the BEM simulation. The hydrodynamics of some tip speed ratio (TSR) on the turbine performance curve for the CFD cases were presented and were used to explain the complete response of the turbine.

An idealised unsteady flow boundary condition was used as the velocity inflow for the CFD simulation of the Sheffield HATT and the response of the turbine was presented in three TSR cases. The cyclic-average CP was shown to be less than the value with steady inflow at the same average TSR suggesting a negative effect of the unsteadiness to the turbine’s performance. In all cases, a hysteresis curve was observed showing that the turbine’s unsteady response does not follow the steady state curve at the amplitudes and frequencies investigated. The hydrodynamics of the turbine with unsteady flow was investigated to provide insight to the unsteady response of the turbine and explain the performance. The unsteady simulation at the turbine’s optimum TSR (TSR=6) was set to be the reference case for the other unsteady studies. The effect of varying the amplitude of the unsteady flow equation was conducted and it was found that as the amplitude increases, the cyclic-averaged CP of the turbine decreases. A frequency variation study was also shown where a drastic change in the turbine’s hysteresis curve when the frequency of the turbine is greater than 1 which also results to a lower cyclic-averaged CP. All of the cyclic-averaged CP for all of the cases simulated in this thesis has value less than the steady state CP value, with the base case unsteady flow results showing a decrease of almost 2% while a maximum difference of 8.03% was seen for the low TSR case. All of this suggesting a detrimental effect of unsteady flow in the performance of HATTs.