Counter-rotating horizontal-axis tidal turbine (HATT) based on blade element momentum (BEM) theory is designed in order to improve the condition of low energy efficiency of traditional HATT. The research found that the flow field characteristic of counter-rotating HATT is more ordered than traditional HATT. Dynamic performance, power performance and starting requirement of counter-rotating HATT is superior to that of traditional HATT. The optimal range of blade installation angle of counter-rotating HATT is from 57 degree to 65 degree. At an optimal L of near 0.225D (where L is the distance between the front and rear rotators and D is diameter of rotor), counter-rotating HATT showed best power performance among the 3 tested L.
The depletion of fossil energy and the environmental pollution problem motivate the pursuit of new environment-friendly alternative energy sources Segura et al. (2019). Ocean energy is an important alternative clean energy source besides wind energy, solar energy and biomass energy. The total reserves of ocean energy in the world are up to about 76.6 billion kilowatts, which is equivalent to the total power generation capacity of the world Reguero et al. (2015). Therefore, ocean energy has a potential to contribute to future sustainable development of human society Leeney et al. (2014). Tidal energy, one kind of ocean energy, has great development potential due to its regularity of occurrence cycle and large overall reserves across the globe Copping et al. (2014).
The horizontal-axis tidal turbine, HATT, has a higher power efficiency than other types of tidal turbines. The principles of design for HATT follow primarily those of horizontal wind turbines (Kim, B et al. 2011; Jeong et al 2012). Blade Element Momentum, BEM, theory plays a fundamental role in design and optimization of HATT (MacNeill et al.2017; Vogel et al. 2018). Kim, I. C et al. (2016) compared the performance of traditional HATT and counter-rotating HATT through experimental studies and concluded that performance of counter-rotating HATT is better than traditional HATT. Huang et al. (2015a, 2015b, 2016a, 2016b) conducted a more comprehensive study on counter-rotating HATT using CFD and experiments. The results showed that performance of counter-rotating HATT can be worse than traditional HATT under extreme conditions with flow separation and turbulent flow condition. In later studies, attempts were made to improve the performance of counterrotating HATT, where the pitch angle and the ratio of blade chord length to blade length, RCB, were optimized based on BEM theory. Lee et al. (2015, 2016) used CFD to study the effects of different blade installation angle of front and rear rotors on power coefficient under different flow velocity. In the later study, they also studied the effect of distance of front and rear rotor, L, on the power coefficient, Cp, by CFD. However, the optimal distance was not determined in their study.Wei et al. (2015) used wind tunnel experiments to evaluate the performance of counter-rotating HATT. They concluded that blade installation angle and distance between front and rear rotors can affect the Cp of counter-rotating HATT. They also compared two L and found that adding distance could improve the power coefficient. The experiments of Wei et al. were more comprehensive because they set up 8 control groups of counter-rotating HATT. However, the experiment input is wind rather than tidal current and only 2 groups of L were used for comparison. Furthermore, the effects of blade installation angle and distance were not studied independently. The rotational speed of the designed counter-rotating HATT is too high to work in tidal current conditions.