Study of a New Wave Energy Converter with an Enlarged Double Turbine Wheel for Increased Rotation Speed

Highlights:

• A new device with a double turbine wheel and a belt drive system has been invented.
• The analytical model of the device is developed and validated through experiments.
• The prototype can achieve a power output of 26 MW and an efficiency of 11.57%.

Abstract: 

This study proposes a new wave energy converter featuring a double turbine wheelto to provide an enlarged rotation speed for the generator. The novelty of the system design lies in the counter-rotating turbine wheels connected to the generator shaft and rotor respectively through a belt drive system to amplify the generator’s rotation speed. This arrangement allows the power take-off system to be placed inside the buoy above the waterline. Additionally, the belt drive system can effectively absorb load fluctuations or vibrations caused by sea waves. An analytical model of the system’s lumped parameters is developed, where the Lagrangian principle is applied to analyze the system’s motion and computational fluid dynamics simulations are used to determine the system’s drag coefficients. The results obtained from the lumped parameter analytical model are validated through experimental test results. The developed lumped parameter analytical model allows for accurate performance prediction without the high computational cost of detailed simulations. The proposed research method can be applied as an effective tool to explore the science of wave energy conversion and improve the power absorption capacity and efficiency of the power take-off system, representing the originality of this paper and contributing to new knowledge in this field. For this prototype of the double turbine wheel wave energy converter with a total underwater volume of 0.002, the maximum harvested efficiency and power output under wave excitation conditions of 80 mm in amplitude and 0.3 Hz in frequency are 11.57% and 26.4 MW, respectively.

Graphical Abstract: 

Diagram of the working principle of the double turbine wave energy converter.

Introduction

: Ocean wave energy is one of the most promising renewable energy sources, offering significant advantages compared to other energy sources:

1. Higher energy density.
2. Limited environmental impact.
3. Waves can be reliably predicted in advance with low energy loss when traveling long distances.
4. The average availability of ocean waves is 90%, compared to 20-30% for wind and solar energy.

The global potential of ocean wave energy represented by waves hitting all coasts is estimated at approximately 2–3 TW [1], which is the same order of magnitude as the present worldwide electricity production. Numerous efforts have been made to develop wave energy conversion technologies. Various novel concepts and approaches have been proposed to capture the potential ocean wave energy and convert it into electrical energy. The feasibilities of these devices have been extensively studied and examined by numerical analysis such as Computational Fluid Dynamics (CFD) and experimental methods, general reviews of state-of-the-art wave energy converters are available in [2], [3], [4]. The types of these wave energy converters can be classified by location, working principle, and/or power take-off (PTO) system, the summary of the classification of wave energy converters is shown in Table 1. However, ocean wave energy technology has not converged into one technical stream while solar and wind energy techniques are slowly dominating the power grids around the world at the present stage of development. There are only a few models of ocean wave energy converters that have met the requirement of the real sea implementation [5], [6], [7], [8]. This is because many factors and challenges such as converting the slow and random oscillatory wave motion to drive the PTO systems to generate smooth electrical energy output, the adaptability of PTO systems as the frequencies may change due to various sea states or seasonable variations, the survivability under extreme weather conditions, and the high maintenance cost.

The most common type of WEC is the point absorber, it is a type of oscillating body system whose horizontal dimension is much smaller than the wavelength of the incident wave. Therefore, the point absorber WEC is more suitable for harvesting wave energy in the offshore regions as its operation is insensitive to the wave direction and the point absorber WEC has low manufacturing and installation cost compared to the OWC devices [9]. A point absorber WEC can be simplified as a mass-spring-damper model which can be studied and optimized using the frequency-domain method. However, most of these point absorber WEC devices have to operate at resonance to harvest the most energy from the ocean waves and improve their harvesting efficiency while ocean waves have a complex motion frequency spectrum, and the dominant frequency may vary over time. To improve the efficiency of the point absorber WEC devices, many types of research are conducted in optimizing the geometrical and physical properties of the WEC and frequency tuning strategies. The frequency tuning mechanisms are required to be employed for these devices to adjust the resonant frequency to match the wave excitation frequency where the wave prediction methods are required to estimate incident wave force and its frequency [10], [11]. These control mechanisms are composed of extra sensors, actuators, and other processor elements which add complexity to the WECs and can lead to reliability and maintenance issues. Other possible solutions for enhancing the harvesting efficiency of the point absorber WEC device are to increase the degree of freedom of the device by adding more submerged bodies to widen the harvesting bandwidth [12] and to apply the nonlinear power capture mechanism [11].

Also, the site selection for the wave energy converters (WECs) is critical for maximizing the energy conversion efficiency when the WECs are designed and deployed. Liang [13] proposed an accurate wave energy assessment equation to evaluate the wave energy resources before the wave energy converters (WECs) are deployed. The water depth is accounted for by introducing an explicit wave dispersion equation. It is a simple and accurate tool for estimating the wave energy fluxes as the equation is free of integration calculations and iteration computations. Parwal [14] proposed an energy management system with a dynamic rate limiter control to regularize the power generation from the WECs. The energy control strategy is equally as important as the conversion efficiency optimization because the power generation of the WECs is intermittent and power fluctuations can be harmful to the energy storage system or the grid. The performance of the wave energy converters is closely related to many factors in many disciplines such as power take-off system, dynamics, and control for adapting different sea states, hydrodynamic design, and so on. These parameters are not independent, which results in difficulties to optimize WECs.

To overcome the above-mentioned challenges, the unidirectional rotational type of PTOs has been developed by many researchers [7], [15]. They are mostly employed in the oscillating water column-based wave energy plants known as self-rectifying turbines. The main advantage of self-rectifying turbines is that they use the air as an intermediate to drive the generator for converting the wave energy into electrical power without contacting the corrosive seawater. Furthermore, these WECs with the unidirectional rotational PTOs only need vertical displacement of waves to operate, can operate in the sea state conditions of both the low and high wave amplitudes where there is no requirement for additional intermediate steps, thus they can be simple in design resulting in a robust system which is able to survive in the harsh ocean environments. Folley [16] studied a contra-rotating wells turbine installed in the OWC power station. He concluded that the contra-rotating wells turbine is less efficient than the biplane and monoplane wells turbines as it requires an additional generator or gearbox to reverse the rotational direction of one rotor which increases the complexity of the system and cost to be implemented. Yang [17] proposed an experimental study of a vertical axis rotor with cup-shaped blades that performs unidirectional rotation driven by the water flows. The rotor characterization has been investigated by experimental tests. Cong [18] studied the effects of multi-type floating bodies on power generation efficiency for the double-layered counter-rotating wave energy converter based on the Froude-Krylov method. Sun [19] performed a detailed CFD analysis for a counter-rotating wave energy converter and compared the analysis results with the experimental test results. Wu [20] carried out an experimental study for the same device in a wave tank. It was found out that the maximum power generation efficiency is 11.87% with a wave height of 0.3 m. Most of the studies regarding the counter-rotating turbine WECs were done by only either the Computational Fluid Dynamics (CFD) analysis or experimental study. The mathematical modeling of the counter-rotating wave energy converters for performance prediction and optimization has not been established. Despite the CFD approach does not require any simplifications to analyze the complex system, it is time-consuming and sometimes can be complex. The analytical simulation with a validated time-domain mathematic model for this type of wave energy converter is promising for revealing the system dynamics, energy losses and optimal electrical loads, compared to the physical experiments of high cost and time consuming.

In this paper, a novel counter-rotational dual turbine wheel wave energy converter is proposed. Unlike the counter-rotating wave energy converter studied by Sun [19] and Wu [20], the power take-off system integrated with the belt-drive transmission system is placed inside the buoy, which is easy to access for maintenance and is kept away from corrosive seawater. Thus, the reliability and survivability of the device can be improved. The belt-drive transmission system is employed to transmit the mechanical energy captured by the dual turbine wheel to the generator and to increase the input rotational speed for enhancing the harvesting performance. This design has a feasible device structure and minimizes the volume of the submerged body. The floater should be designed and optimized to match its resonant frequency range with the income wave frequency range. The dual turbine wheel can continuously drive the generator to generate electricity on the condition that relative displacement occurs. Therefore, a large submerged body for the purpose of the resonant operation can be avoided. For a small-scale proposed WEC, the belt-drive transmission system is appropriate to transmit the torque generated by the dual turbine wheel to drive the generator. It requires no lubrication and minimal maintenance comparing to other transmission systems. The flexible timing belts can absorb the load fluctuations and the shock/vibration caused by the ocean waves. Furthermore, it is simple and economical to be built for different applications in various sea states. However, if the proposed WEC is built on a large scale for higher power output application, a gear drive system with flywheels is more suitable to operate under the high torque condition. Despite the initial cost of the gear drive system is much higher compared to that of the belt-drive system, the survivability of the large-scale WEC can be significantly improved.

In order to predict the performance of the wave energy converter, different from the conventional parameter models of the wave energy converter, a lumped parameter model has been proposed in this paper where the analytical equations of motion of the system are developed through the energy method using the Lagrange equation. A part of the model parameters can be obtained from the CFD simulation, the rest of the parameters are identified from the simulation and experimental modeling. For example, the average values of the drag coefficients instead of varying instantaneous values of the drag coefficients of the horizontally tangential and vertical directions of the blades should be derived from the CFD simulation results while considering the upper turbine wheel, blades, and outer transmission shaft as a whole part, as well as for the lower turbine wheel, blades and inner transmission shaft. These averaged drag coefficients for a certain maximum blade pitch angle are substituted into governing equations of the system to predict the voltage output. The output voltage of the WEC and the rotational speed of the dual turbine wheel WEC system can be predicted by solving the analytical equations of motion of the system through a Matlab Simulink code. The predicted results match well with the results measured in the experimental tests to identify the model parameters or coefficients of varying nature and their average values to simplify the analytical parameter model. The proposed lumped parameter model is then validated by the experimental study. It is simple in form, fast in computation, and reliable in prediction, which is useful for parameter study and design optimization. Therefore, the research method of the lumped parameter mathematic modeling can be applied as an effective tool to adapt this type of WEC to different applications in various sea conditions.

The paper is organized as follows: Section 2 describes the design concept and working principle of the dual turbine wheel wave energy converter and conducts lumped parameter mathematic modeling of the device. The CFD simulation details are presented in Section 3. Section 4 presents the experimental study and discussion of test results. Section 5 concludes our research work and Section 6 proposes our future work.

Section snippets

System structure

The proposed wave energy harvester as shown in Fig. 1 can be deployed in the ocean environment converting the heaving motion of the buoy into electricity. The operation range of the WEC in the horizontal directions can be restricted by evenly distributed mooring cables that are connected to the buoy of the WEC. Because of the mooring cables, the relative velocity exposed to the blades will be limited. However, if the WEC is deployed to power the remote ocean sensor which follows the surface of

CFD model simulation

In this section, a CFD model is developed to calculate the vertical and horizontal drag coefficients of the dual turbine wheel.

As the PTO system of the proposed dual turbine wheel wave energy converter is placed inside the floater, the submerged body of the proposed WEC are the dual turbine wheel and two transmission shafts. Therefore, the simplified CFD model of the WEC is developed as shown in Fig. 4(a) which is accurately reproduced based on the prototype WEC as shown in Fig. 6. The SST k-ω

Experimental results and discussion

In this section, the prototype of the dual turbine wheel wave energy converter was built and connected to the MTS test machine through a cantilevered plate support platform as shown in Fig. 6. The distance between the upper turbine wheel and the lower turbine wheel is relatively close, which could lead to the wake interaction [35], [36] between the two rows of blades. In the last section, as the submerged body was precisely modeled in the CFD simulation, the effects of the wake interaction are

Conclusions

In this paper, a novel dual turbine wheel wave energy converter featured with the rotational speed amplification of the generator through its counter-rotating stator and rotor and a belt and timing pulley drive transmission is proposed and analyzed by both the lumped parameter model simulation and experimental test approaches. The design novelty of the system is that the generator stator and rotor are connected to the counter-rotational turbine wheels through a belt-drive transmission for the

Future work

Our future work will be to conduct the parameter sensitivity study and optimal design of the dual turbine wheel wave energy converter. Our future work will also study the dual turbine wheel wave energy converter connected to a submerged body to utilize both the resonant and non-resonant conditions of the two-body point absorber for boosting the wave energy conversion performance, which will be a hybrid of the dual turbine wheel wave energy converter and the two-body point absorber.

CRediT authorship contribution statement

Han Xiao: Conceptualization, Methodology, Writing – original draft, Data curation, Software, Visualization, Investigation, Validation. Zhenwei Liu: Conceptualization, Methodology, Investigation, Validation. Ran Zhang: Data curation, Software, Visualization. Andrew Kelham: Investigation, Validation. Xiangyang Xu: Supervision, Writing – review & editing. Xu Wang: Supervision, Writing – review & editing.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgment

The authors would like to thank the Australian Research Council Discovery Project grant DP170101039 for financial support.

source: https://www.sciencedirect.com/science/article/

References (36)

• K. Gunn et al.Quantifying the global wave power resourceRenewable Energy(2012)
• S.J. Beatty et al.Experimental and numerical comparisons of self-reacting point absorber wave energy converters in regular wavesOcean Eng(2015)
• E. Lejerskog et al.Experimental results on power absorption from a wave energy converter at the Lysekil wave energy research siteRenewable Energy(2015)
• C. Liang et al.Design, fabrication, simulation and testing of an ocean wave energy converter with mechanical motion rectifierOcean Eng(2017)
• D. Martin et al.Numerical analysis and wave tank validation on the optimal design of a two-body wave energy converterRenewable Energy(2020)
• S. Astariz et al.The economics of wave energy: A reviewRenew Sustain Energy Rev(2015)
• H.-N. Nguyen et al.Short-term wave force prediction for wave energy converter controlControl Eng Pract(2018)
• D. Son et al.Optimizing ocean-wave energy extraction of a dual coaxial-cylinder WEC using nonlinear model predictive controlAppl Energy(2017)
• Z. Chen et al.Performance evaluation of a dual resonance wave-energy convertor in irregular wavesAppl Ocean Res(2018)
• B. Liang et al.New equations of wave energy assessment accounting for the water depthAppl Energy(2017)
• A. Parwal et al.Energy management for a grid-connected wave energy park through a hybrid energy storage systemAppl Energy(2018)
• T. Setoguchi et al.A review of impulse turbines for wave energy conversionRenewable Energy(2001)
• M. Folley et al.Comparison of LIMPET contra-rotating wells turbine with theoretical and model test predictionsOcean Eng(2006)
• Y. Yang et al.A vertical-axis unidirectional rotor for wave energy conversionOcean Eng(2018)
• G. Wu et al.Experimental Analysis of a Novel Adaptively Counter-Rotating Wave Energy Converter for Powering DriftersJ Mar Sci Eng(2019)
• H. Joe et al.Development of a flap-type mooring-less wave energy harvesting system for sensor buoyEnergy(2017)
• R. Malki et al.A coupled blade element momentum – Computational fluid dynamics model for evaluating tidal stream turbine performanceAppl Math Model(2013)
• R. Lanzafame et al.Wind turbine CFD modeling using a correlation-based transitional modelRenewable Energy(2013)