The Effects of Surface Waves and Submergence on the Performance and Loading of a Tidal Turbine

Tidal energy has the advantages of high predictability, high energy density, and limited environmental impacts. As tidal turbines are expected to be used in the most energetic waters where there might be significant waves, the assessment of unsteady hydrodynamic load due to surface waves is of great concern. The objective of this paper is to assess the effects of surface waves and submergence of the turbine on the power performance and loads ∗Address all correspondence to this author. of a tidal turbine by experimental approach. The experiments on a 1 : 25th model tidal turbine were carried out in a towing tank. A wide range of regular waves with periods from 1.0 s to 3.0 s at model scale were generated. Different submergence conditions were considered to investigate the effects of the presence of free surface. The cases with blade tip partly going out of water were also performed. The regular waves did not have significant influences on average loads and power output in the present experiments, but caused large amplitude of the cyclic variation of the loads. The amplitudes of the cyclic load were proved to be 1 Copyright c © 2017 by ASME proportional to the incident wave height, and to be sensitive to the wave frequency and submergence of the rotor. As the tidal turbine getting close to free surface, significant waves were induced by the underwater rotating blade. The effects of surface waves and submergence need to be taken into account in design. INTRODUCTION Tidal turbines can generate power from ocean current with the advantages of high predictability, high energy density, and limited environmental impacts. The concept of extracting power from ocean tides has a long history [1]. With the growing energy demand, tidal turbines have gained increased attention. In many parts of the world, tidal power presents an advantageous resource. About 61.3 TWh/year of tidal current energy technically is available in China. In the East China Sea, some excellent channels are most promising with energy density over 20 kWm−2 [2]. In the UK, the extractable resource was estimated to be up to 18 TWh/year [3]. Unsteady hydrodynamic conditions, such as turbulent inflow or free surface waves, would induce more complex loads on the submerged tidal turbines. Surface waves which can penetrate the water column to a depth of half of the wave length is an important part of the unsteady hydrodynamic loads. However the cyclic wave conditions are not considered in standard design procedure of tidal turbines. Tidal turbines nowadays are designed with an increasing size to produce more power from the whole cross-section of the channel. The oscillating wave loads will undoubtedly not only lead to increase of the extreme loads but also accelerate fatigue of the rotor and blades. As tidal turbines getting close to free surface to capture more energy, the dynamic effects on the loads and performance arise from the interaction between tidal turbines and free surface. On one hand, the presence of free surface would affect the power production of tidal turbines. On the other hand, the wake behind the turbine would induce waves and vortex on free surface in turn. The induced waves and vortex may bring safety issues to the nearby ships and structures. As tidal turbines are expected to produce power in the most energetic waters, further studies should be carried out to assess the effects of unsteady loads and free surface. Only a few experiments have been carried out to study the surface wave effect on tidal turbines. Barltrop et al. [4] performed experiments using a 400 mm diameter rotor in a towing tank with the presence of regular waves. The average thrust and torque were found to be independent with the wave frequencies or wave heights. However significant cyclic variation of the thrust and torque were observed. Similar model tests in towing tanks were reported by Lust et al. [5] and Luznik et al. [6]. They both confirmed that the average thrust and torque was not affected by the waves. Results from Luznik et al. [6] indicated a strong correlation between the measured torque and vertical wave particle velocity. Galloway et al. [3] investigated the effects of waves on an 800 mm 3-bladed horizontal axis TST device, whilst being towed at 0.9 ms−1. The regular waves with periods about 2.0 s and heights from 7.5 cm to 15.0 cm at model scale were used in Galloway’s experiments. They concluded that the waves caused the significant cyclic loading with the amplitudes of 37% and 35% of the mean for thrust and torque, respectively. Whelan et al. [7] suggested a correction for the effects of free-surface proximity and blockage on the performance of tidal turbines. The paper presents experiments on a 3-bladed tidal turbine which were performed in a towing tank to contribute to the understanding of the effects of regular waves and free surface on the performance of tidal turbines. Compared to previous studies, a wide range of regular waves with periods from 1.0 s to 3.0 s and heights from 5.0 cm to 15.0 cm at model scale were considered. The submergence of the turbine hub was from 1.0D to 0.45D with an interval of 0.05D (D is the turbine diameter). The cases with blade partly going out of water were firstly reported. The numerical prediction for comparison was made by Guo et al. [8] based on the modified BEM theory with an inclusion of dynamic wake model, added mass and wave excitation forces. EXPERIMENT SET-UPS Towing Tank The present experiments were carried out in the towing tank at Zhejiang Ocean University, China. The dimensions of the towing tank are 130 m × 6 m × 3 m. A flap-type wave generator is at the upstream end of the tank. A passive wave-absorbing beach is located at the other end with the damping grids for passive wave dissipation to absorb the wave energy. The carriage velocity is controlled and recorded by the computer. In the present towing test, the carriage velocity was at 0.56 ms−1 and 0.68 ms−1. Test rig The test rig mainly consisted of an airfoil section tank, a generator/motor, a dynamometer, and a 3-bladed model turbine. The generator/motor was equipped with close-loop controller to ensure precise speed control. The dynamometer was utilized to record the torque and thrust on the shaft. A 3-bladed tidal turbine as shown in Fig. 1 was used in the present experiments. Compared to marine propellers, the relatively slow rotating speed requires to choosing a larger size rotor for maximizing Reynolds number with moderate towing speed (about 1.0 ms−1). Meanwhile, the diameter is limited by blockage of the tunnel and processing difficulty of the aluminum alloy blade. The diameter was chosen to be 800 mm as a compromise to represent a 20 m-diameter, 1 MW prototype tidal turbine in the present experiments. The model blades were considered to be rigid bodies. Froude scaling is the dominant scaling parameter, and the scale ratio is 25. The tidal turbine with relatively low Reynolds number about 0.8×105 (at 75% blade radius) at model 2 Copyright c © 2017 by ASME FIGURE 1. PHOTO OF THE 1 : 25TH MODEL TURBINE scale would reduce the unsteady hydrodynamic performance compared to that expected at full-scale with Reynolds number of 1.0×106 [9]. In this paper, all the results from experiments and numerical prediction were shown and discussed at model scale. The hub diameter is 100 mm, and the blade sections were derived from NACA 63-8xx profile with varying thickness and pre-twist. The span-wise distributions of the chord, pre-twist and thickness are shown in Tab. 1 at model scale. TABLE 1. PARTICULARS OF THE TIDAL TURBINE BLADES AT MODEL SCALE r/R r (mm) Twist (deg) Chord (mm) t/c (%) 0.125 50 19.5 32.0 99.9 0.200 80 19.5 67.9 22.0 0.300 120 16.5 60.7 20.0 0.400 160 13.3 51.9 18.3 0.500 200 11.6 47.6 14.8 0.600 240 9.4 43.3 14.2 0.700 280 7.3 40.1 13.6 0.800 320 5.2 37.6 13.1 0.900 360 2.7 35.3 12.5 1.000 400 0.0 33.2 12.0 Two wave probes as shown in Fig. 2 were located in front of the tank to record incident waves and wake induced waves. The wave heights at the rotor plane were obtained by changing the phase of the measured incident waves 4.5 m ahead of the rotor. The whole test rig was connected with the main carriage which could move forward in the tank with a constant velocity. Meanwhile the whole test rig could be moved up and down to adjust the submergence of the turbine hub from 0.32 m to 0.96 m in order to satisfy different test conditions. All these mentioned instruments were carefully calibrated before the experiments. All the quantities were recorded with a sampling rate of 100 Hz. Rotor diameter 0.8m