Oscillating Water Column Wave Energy Converter Research Articles

Non-linear turbine selection for an OWC wave energy converter

Turbine-induced damping is a critical parameter affecting the performance of oscillating water column (OWC) wave energy converters. Therefore, selecting the appropriate turbine-chamber combination is an essential step in their design. In this work, a methodology is developed to determine the optimum turbine diameter for a given chamber, i.e., the diameter which maximizes the pneumatic energy capture of the chamber under an extensive set of wave conditions—covering virtually the entire range of wave conditions relevant for wave energy exploitation. This novel approach combines physical and numerical modelling with dimensional analysis. Importantly, it results in a turbine diameter that enables the turbine to operate at maximum efficiency. Through the different modelling techniques applied, the methodology accounts for air compressibility effects and other non-linear effects. It is applicable to non-linear turbines, with the study focusing on the promising biradial turbine. The results indicate that using the proposed methodology to select the turbine diameter significantly improves the capture-width ratio of the OWC, with increases of up to 100% for individual sea states. Two turbine diameters were identified as appropriate for the proposed OWC chamber design, 1.1 m for low-energy sites, and 1.4 m for mid- and high-energy sites.

Failure Consequence Cost Analysis of Wave Energy Converters—Component Failures, Site Impacts, and Maintenance Interval Scenarios

In the early stages of developing wave energy converter (WEC) projects, a quantitative assessment of component failure consequence costs is essential. The WEC types, deployment site features, and accessibility should all be carefully considered. This study introduces an operation and maintenance failure consequence cost (O&M-FC) model, distinct from conventional O&M models. The model is illustrated with case studies at three energetic Atlantic sites, each of which considers two types of generic floating WECs: a 300 kW point absorber (PA) with a hydraulic power-take-off (PTO) and a 1000 kW oscillating water column (OWC) with an air-wells-turbine PTO. This study compares 39 failure modes for PA and 27 for OWC in terms of direct repair costs and indirect lost production costs, examining the impact of location accessibility, capacity factors, and the mean annual energy production. The discussion revolves around the sensitive parameters. Recommendations for failure mitigations are presented, and the impact of planned maintenance (PM) during the operational phase is examined for 20 MW PA and OWC WEC projects. For a given WEC type, the method thoroughly evaluates how the location affects performance metrics. It offers a decision-making tool for determining optimal PM intervals to meet targets such as O&M costs, operating profit, or availability.

Experimental and computational analysis of elastomer membranes used in oscillating water column WECs

The study investigates the structural characterisation of flexible membranes used in oscillating water column (OWC) wave energy converters (WECs). Four commonly utilised elastomers – natural rubber, nitrile rubber, silicone, and latex – were subjected to a novel hyperelastic model selection process. A custom bulge test setup enabled the selection of second-order Mooney-Rivlin (SOMR) and Yeoh models for relevant accuracy (RMSE<0.018 MPa), stability and numerical validation. A 1:20 scale OWC model with latex was tested in a water tank to examine the effects of waves with a frequency range of 0.25–1.4 Hz and up to 0.24m amplitude. Water tank experiments demonstrated smooth frequency responses for OWC with membrane, beneficial for consistent power generation. A dry test rig was designed and built to replicate OWC inflation conditions and apply cyclic loadings up to 1.5 Hz, overcoming pressure limitations of the water tank, exploring wider material options, and validating numerical simulation. An optical motion capture system, Qualisys, supported the validation process by providing precise data on membrane deformation during experiments. Furthermore, finite element analysis (FEA) was utilised to conduct stress analysis and parametric studies, assessing the suitability of these materials for flexible OWC application.

Assessment of heat and entropy balance of an OWC wave energy converter

This paper deals with the thermodynamic processes governing an Oscillating Water Column (OWC) device, focusing on the entropy variation and the energy and heat budgets over the expansion–compression cycles. The influence of the thermodynamic performance on the Levelized Cost of Energy (LCOE) and the carbon footprint is analysed. The research deals with an experimental research on a simple OWC off-shore model for the purpose, with the open gas system inside the chamber formulated by a real gas model, and conceptually represented by an equivalent closed one in order to apply the First Principle of Thermodynamics appropriately. Analysed results show that the compression process is an active process, while the expansion process is a passive one. In addition, the observed non-adiabatic performance of the complete cycle implies a efficiency reduction, with consequences on the LCOE. Furthermore, an approach to emergy (embodied energy) analysis is considered, providing with concluding remarks on OWC renewability and possible impacts on the biosphere and GHG emissions. An utter approach on OWC energy and emergy assessment will be develop on future researches.

Design an Optimum Air Duct for Oscillating Water Column Wave Energy Device

Oscillating Water Column (OWC) is a technology that harvests wave energy to generate electricity. It works by harnessing the wave-induced pressure oscillation in the opening mouth of OWC. The oscillation of the pressurised air is extracted using a turbine placed at the top of the OWC structure. Although OWC has been studied for many years, designing an optimum air duct for such devices is still an open research area. The air duct plays a crucial role in the energy conversion process, as it is responsible for directing the airflow towards the turbine, and its design can greatly affect the performance and efficiency of the system. The article aims to study how the air duct design impacts the power output form. Moreover, the result will be compared with the conventional air duct design. The OWC performance is analysed using Computational Fluid Dynamic (CFD) software to identify which shape of the air duct has high efficiency. Autodesk Inventor is used to design the OWC devices modified from the basis design. Then these designs are imported into CFD software to simulate and obtain the energy extraction result. The CFD software is chosen to simulate the hydrodynamics problem. In this study, the oval design shows significant improvement in the energy extraction performance compared to the basis design. The oval design managed to produce 9.81, 10.02, and 11.66 J/kg of mechanical energy compared to 9.81, 9.82, and 10.04J/kg as prompted by the basis air duct design at the wave height of 0.2505, 0.75, and 2.25m, respectively. The result shows that the non-edge produces higher air pressure than the sharp edge of the air duct shape. Thus, the oval air duct design is more efficient, and it may increase the performance of the OWC wave energy converter device at a low wave height of 0.75 m.

Experimental and numerical investigation on hydrodynamic performance of a 3D land-fixed OWC wave energy converter

The hydrodynamic performance of a land-fixed oscillating water column (OWC) wave energy device was thoroughly investigated under various wave conditions and geometric parameters through both experimental and numerical studies in a three-dimensional (3D) wave tank. The study involved the measurement and comparison of water surface elevation, air pressure in the OWC chamber, and hydrodynamic efficiency with the results obtained from the 3D Higher-Order Boundary Element Method (HOBEM). The influences of 3D effects, incident wave amplitude, front wall draught, and incident wave angle on the hydrodynamic performance of the OWC device were analyzed. The results demonstrate that, under actual sea conditions, the 3D effects significantly affect the hydrodynamic efficiency of the OWC, making it exceed unity near the resonance frequency. On the other hand, the incident wave angle was found to have minimal effect on the resonant frequency and varied trend of efficiency. However, the hydrodynamic efficiency decreases with increasing incident wave angle. These results highlight the importance of the consideration of the 3D effects and incident wave angles in optimizing the design and performance of fixed OWC wave energy devices.

Aerodynamic performance and noise emission of different geometries of Wells turbines under design and off-design conditions

The path to the World’s energy system decarbonisation requires the contribution of a mix of renewable energy sources. In particular, wave energy and oscillating-water-column (OWC) devices can be significant contributors. However, they are still in an early stage of development, and their levelized cost of energy (LCOE) is still too high to compete in the utility market. The Wells turbine is an attractive solution to reduce the LCOE of OWC devices. Completing the available information in the literature on the performance of different Wells arrangements is a necessary step towards decision-making on which configuration is most cost-efficient given the hydrodynamic characteristics of a particular OWC wave energy converter (WEC). Furthermore, there is a research gap on these devices’ acoustics, which needs to be filled, given the impact noise might have on surrounding ecosystems. This study investigated the aero-acoustics performance of four Wells turbine geometries, including monoplane and biplane turbines with and without guide vanes. Experimental tests of the turbine models were conducted in an open-circuit test rig under design and off-design flow conditions. The research provides accurate aerodynamic data on very high flow rate (pressure) coefficients, essential to turbine-generator control algorithms and OWC WECs wave-to-wire numerical models. A specific noise level definition was inferred by analysing the aerodynamic noise sources in Wells turbines. This allows the prediction of noise emission from geometrically similar turbines of different sizes and rotational speeds from the experimental noise data collected with the tested Wells turbines.

Control co-design of power take-off and bypass valve for OWC-based wave energy conversion systems

Wave energy is a significant source of renewable energy harnessed by wave energy converters (WECs). However, due to the relatively high levelised cost of energy (LCoE), wave energy has not attained a commercial stage yet. To minimise the LCoE, since the optimum (uncontrolled) WEC design typically differs from the optimum controlled WEC design, control co-design (CCD) techniques are essential. With CCD, the WEC control-related aspects are taken into account from the start of the WEC design phase and, ideally, the best control-informed WEC design is then achieved. This paper specifically focuses on CCD for an oscillating-water-column (OWC) WEC, equipped with a Wells turbine and a bypass valve. In essence, a parametric CCD approach is devised to find the optimum (control-informed) turbine rotor diameter, and bypass valve diameter, for the considered OWC WEC. In particular, the optimum design parameters minimise a ‘simplified’ LCoE, which is chosen as a suitable performance function. Despite the LCoE is primarily sensitive to the power take-off size, rather than to the bypass valve size, peak-shaving control with a bypass valve potentially increases the capacity factor and, consequently, can minimise the LCoE for small-to-medium sized turbines.

Power and dynamic performance of a floating multi-functional platform: An experimental study

Hybrid platforms which integrate a floating offshore wind turbine and a wave energy converter are a promising technology as the wave energy converter can lead to a more stable platform and potentially lower costs. In the present work, a semi-submersible floating offshore wind turbine platform integrating with an array of wave energy converters (WECs) in the form of an oscillating water column (OWC) is presented and tested experimentally. The hydrodynamic properties of the multi-functional platform were investigated under a series of regular and irregular wave tests considering both operational and survival sea states. The influence of various parameters (including incident wave directions and chamber opening ratios) on the dynamic responses and the wave energy capturing were systematically studied. The introduction of the oscillating water column can not only capture wave energy but also reduce the heave, roll and pitch responses, especially for long waves kh ≤ 2.0, kh ≤ 1.4 and kh ≤ 1.76 respectively. The maximum reduction in heave, roll and pitch are 35 %, 55.4 % and 13.8 %, respectively. The chamber air orifice size was analyzed and optimized. From the survival sea state tests, it is found that the introduction of the OWC WECs decreases the maximum amplitudes of the platform motions in all five freedoms and the nacelle acceleration.

Geometrical Analysis of an Oscillating Water Column Converter Device Considering Realistic Irregular Wave Generation with Bathymetry

Given the increasing global energy demand, the present study aimed to analyze the influence of bathymetry on the generation and propagation of realistic irregular waves and to geometrically optimize a wave energy converter (WEC) device of the oscillating water column (OWC) type. In essence, the OWC WEC can be defined as a partially submerged structure that is open to the sea below the free water surface (hydropneumatic chamber) and connected to a duct that is open to the atmosphere (in which the turbine is installed); its operational principle is based on the compression and decompression of air inside the hydropneumatic chamber due to incident waves, which causes an alternating air flow that drives the turbine and enables electricity generation. The computational fluid dynamics software package Fluent was used to numerically reproduce the OWC WEC according to its operational principles, with a simplification that allowed its available power to be determined, i.e., without considering the turbine. The volume of fluid (VOF) multiphase model was employed to treat the interface between the phases. The WaveMIMO methodology was used to generate realistic irregular waves mimicking those that occur on the coast of Tramandaí, Rio Grande do Sul, Brazil. The constructal design method, along with an exhaustive search technique, was employed. The degree of freedom H1/L (the ratio between the height and length of the hydropneumatic chamber of the OWC) was varied to maximize the available power in the device. The results showed that realistic irregular waves were adequately generated within both wave channels, with and without bathymetry, and that wave propagation in both computational domains was not significantly influenced by the wave channel bathymetry. Regarding the geometric evaluation, the optimal geometry found, H1/Lo = 0.1985, which maximized the available hydropneumatic power, i.e., the one that yielded a power of 25.44 W, was 2.28 times more efficient than the worst case found, which had H1/L = 2.2789.

Control of multiple PTOs in single OWC air chambers

The paper presents the study of multi-PTO interaction in single-air chamber oscillating-water-column (OWC) wave energy converters (WECs). This strategy seeks: i) tunning system's performance to sea states, ii) increasing reliability through modular designs that are easier to install and replace, and iii) increasing conversion efficiency through better matching between available power and electric conversion equipment. The strategy was assessed through a novel non-linear time-domain model for OWC WECs implemented in the multi-physics object-oriented language Modelica. The model considers multiple degrees of freedom associated with the various PTOs. Different case studies are presented to show the proposed approach's versatility, some of the constraints expected in real implementations, and potential pathways to overcome them. The wave-to-wire model considers air compressibility and non-linear power take-off systems, which is fundamental to assessing the damping level variation required for each sea state considered. The cases presented are for a fixed-structure coaxial-duct OWC WEC, but the results are generalisable for floating structures. Results show a significant increase in mean annual power, and control strategies are proposed for power maximisation. Furthermore, some critical points are shown in the operational conditions' envelop for the system under a selected Portuguese wave climate. The identification of these critical values is important for the control of the OWC WECs. This study represents an advance in the control strategies considering multiple PTOs for a single OWC air chamber to foster innovation actions.

Wave-to-Wire Control of an Oscillating Water Column Wave Energy System Equipped with a Wells Turbine

Wave energy is a significant and relatively untapped source of renewable energy [1], which can considerably contribute to decarbonization. The oscillating-water-column (OWC) [2] is one of the most promising wave energy converters (WECs) for harnessing wave power, especially due to its relatively simple operating principle and the fact that all the moving parts are above the water level.&#x0D; To improve the commercial viability of WECs, the levelised cost of energy should be minimised and, to this end, comprehensive control strategies to maximise electric energy are essential [3]. Due to the important issue of turbine efficiency, the vast majority of OWC control strategies [4] focus on a simplified control objective, namely turbine efficiency maximisation. While maximising turbine efficiency is a primary focus of OWC control strategies, it is important to note that rotational speed control impacts generator performance. Additionally, for Wells turbines [5], rotational speed control also affects the hydrodynamic performance, specifically the wave-to-pneumatic energy conversion process. Therefore, Wells turbine rotational speed should be ideally modulated to improve the overall wave-to-wire (W2W) efficiency of the OWC system [6], rather than just turbine efficiency.&#x0D; In this paper, a control strategy for maximising W2W efficiency of a fixed OWC WEC equipped with a Wells turbine is proposed. A schematic of the W2W power train of the OWC WEC considered in this paper is depicted in Figure 1. The proposed control strategy comprises of two parts. Firstly, a `complete' setpoint, which considers the entire OWC W2W model (WEC hydrodynamics, Wells turbine, and generator dynamics), is derived. Secondly, a Lyapunov-based nonlinear controller is designed to track the aforementioned setpoint. Preliminary results from the numerical simulation show that, in comparison to the somewhat traditional turbine efficiency maximising control approach, the proposed W2W control strategy leads to a significant improvement in the electric energy production for all the considered sea states.

Effects of projected wave climate changes on the sizing and performance of OWCs: a focus on the Mediterranean and Atlantic European coastal waters

Reliable estimations of the Annual Energy Production (AEP) achievable with a certain Wave Energy Converter (WEC) are among the fundamental elements for a sound evaluation of the related Levelized Cost of Energy (LCOE) that plays a crucial role in the investment decision-making process. The lack of reliability in estimates of the device productivity can be a result of the uncertainty in the assessment of the available wave energy resource since the geometry of the device which maximizes the AEP is dependent on the specific wave climate at the foreseen installation site.&#x0D; The Climate Data Store of the Copernicus Climate Change Service delivers projections of the wave climate along the 20 m bathymetric contours of the whole European coastlines, covering the period 2040-2100, under two Representative Concentration Pathway (RCP) scenarios (RCP4.5 and RCP8.5) [1].&#x0D; This work addresses the effect of such long-term wave climate changes on the optimal sizing and performances of Oscillating Water Column (OWC) WECs to be installed along the Atlantic European coastline, expanding a previously published work in which the Mediterranean coastline was investigated [2]. The capture width of the OWC WEC under different wave conditions is computed using an empirical model [3] capable of predicting the device performance with acceptable accuracy and limited computational time.&#x0D; The results show that the optimal geometry of the OWC WEC varies significantly in the different geographical locations, and that the long-term changes in the wave energy resource could cause a slight modification of the optimal geometry in each potential installation site. The AEP of the OWC WEC in the projected wave climate scenario can be significantly different compared to the present one, potentially contributing to reducing the LCOE of this wave energy conversion technology in some geographical locations.&#x0D; REFERENCES:&#x0D; [1] Caires, S., Yan, K., 2022. Ocean surface wave time series for the European coast from 1976 to 2100 derived from climate projections. (Accessed on Feb. 2022) DOI: 10.24381/cds.572bf382. Copernicus Climate Change Service (C3S) Climate Data Store (CDS)&#x0D; [2] Simonetti, I., Cappietti, L., 2023, Mediterranean coastal wave-climate long-term trend in climate change scenarios and effects on the optimal sizing of OWC wave energy converters, Coastal Engineering, 173, 104247.&#x0D; [3] Simonetti, I., Cappietti, L., Oumeraci, H., 2018. An empirical model as a supporting tool to optimize the main design parameters of a stationary oscillating water column wave energy converter. Appl. Energy 231, 1205–1215.

Experimental and numerical investigations on the solitary wave actions on a land-fixed OWC wave energy converter

The dynamic performance of a typical land-fixed oscillating water column (OWC) wave energy converter (WEC) under solitary waves (SW) is experimentally and numerically investigated. Both two-dimensional (2-D) and three-dimensional (3-D) numerical models are considered in the present study. The air orifices of the OWC chamber in 2-D and 3-D models are represented by a narrow rectangular opening and a circular opening, respectively, with the same opening ratio (i.e., the cross sectional area ratio between opening and the air chamber). It is found that the overflow of the orifice occurs when the wave height exceeds a threshold, which results in a larger wave force on the structure compared with that induced by the SW crest. The 2-D numerical model fails to predict the overflow phenomena due to the full submergence of the orifice. The overtopping and roof impacting phenomena are induced when the wave run-up exceeds the front wall height. A portion of the SW energy is released during this process. Hence, total wave forces are reduced. The effect of wave heights on force components exhibits different behaviors due to the coupling of air and water inside the chamber. Both the impacting force and overtopping discharge increase with incident wave heights.

Technology, Geometry, Performance and Challenges in Wave Energy Converters

Recently, the worldwide energy scarcity raised the demand for integrated renewable energy in the modern grid. Considering the potential and ocean areas, wave energy-based power generation becomes an essential sustainable source for industry and academia. Notably, the wave energy converter (WEC) is the most common solution for extracting power from wave energy and convert into electricity. Wave electricity generation is the main method of using wave energy, and wave energy converters that are the core of wave energy technology have been widely and rapidly evolved. In this paper, the state-of-art WECs are reviewed with the classification based on different ways and different principle of operation of capturing energy of waves, including oscillating water column WEC (OWC-WEC), oscillating buoy WEC (OB-WEC) and overtopping WECs (O-WEC) are systematically reviewed in terms of operation principles, geometric structure and power take-off (PTO) strategies. Additionally, the energy conversion efficiency between different technologies is compared and summarized. The challenges and optimization aspects are delivered.

Enhancing Power Generation Stability in Oscillating-Water-Column Wave Energy Converters through Deep-Learning-Based Time Delay Compensation

Oscillating-water-column wave energy converters (OWC-WECs) are gaining attention for their high energy potential and environmental friendliness. However, their irregular input energy characteristics pose challenges to achieving stable power generation, particularly due to high peak power compared to average power. This study focuses on stable rating control to enable continuous power generation in the presence of irregular wave energy. It is difficult to precisely configure the existing rated power controllers due to physical time delays; this impacts system stability and utilization. To address this, we propose a rated power controller that compensates for system time delays using a deep learning algorithm. By predicting the valve control angle in advance and analyzing the input data for angle estimation, we successfully compensate for the physical time delay. The performance of the proposed rated power controller, incorporating the deep learning algorithm, is evaluated by analyzing the algorithm’s error rate. The results demonstrate that the proposed method improves power generation under various wave conditions by compensating for the unavoidable time delay of OWC-WECs, leading to a significant increase in annual power generation. In conclusion, the proposed method achieves approximately 31% higher annual power generation compared to the time delay controller.

Analysis study of the ratio of wave height to column diameter in OWC wave energy converter

Hydro energy is one of the potentials of renewable Energy in Indonesia. Indonesia has a vast sea, in the amount of 3,544,743.9 km2 making Indonesia a suitable country to utilize Oscillating Water Column (OWC) type power plants. The study was conducted to determine the effect of column diameter on the performance of OWC devices, especially in speed and dynamic pressure. There are 5 variations of the diameter of the column studied, namely 0.5 m, 0.75 m, 1 m, 1.25 m, and 1.5 m. Plus there are also variations in the wave period, namely 3s, 4s, and 5s. The result of this study is the column diameter of 0.75 m has an average speed of 0.389 m/s and a dynamic pressure of 2.676 N/m2, making it the best column diameter ratio of the five column diameter ratio options available.

Peak shaving control in OWC wave energy converters: From concept to implementation in the Mutriku wave power plant

Control algorithms for wave energy conversion technologies with air turbines require contingency tools to prevent the rated power and maximum rotational speed from being exceeded. Survival problems limit energy conversion when the available power is too large compared to the turbo-generator-rated power. Short-term predictions require information from local wave-measuring buoys, which raises reliability concerns. Wave groups in the ocean result in extremely high wave power peaks that cannot be smoothed by low-inertia power-take-off (PTO) systems, leading to survival problems. The most common strategy to cope with this situation is to set the rated power of the PTO several times more than the annual average produced electrical power. The PTO is shut-down when the available power is excessive. These two engineering decisions have a detrimental impact on the capacity factor and the energy cost. The paper shows a new control algorithm for wave energy oscillating-water-column (OWC) devices that can control the shutter position of a fast-acting valve to dissipate the pneumatic energy excess. Hence, the PTO can operate even when the available power is significantly larger than the generator’s rated power. The algorithm was validated with the 30 kW biradial turbine prototype from the OPERA H2020 European project under real sea conditions at Mutriku’s wave power plant. Results show the algorithm’s effectiveness in real situations and during a generator failure simulated during the experimental campaign. This new concept may reduce the energy cost from wave energy and open a new field of design possibilities for OWC wave energy converters.

Design and Analysis of Oscillating Water Column Wave Energy Converter Using Contra Rotary Self-rectifying Impulse Turbine

Background: The global energy demand is rising day to day. To cover the energy gap, governments around the world are pursuing a variety of strategies to meet energy demands. However, according to the long-term strategy, all nonrenewable energy plants must be replaced with renewable energy sources. Existing technologies are enhanced, and new sources are being investigated for this purpose. The ocean covers 70% of the planet so there is a huge potential to extract energy from this source. Objective: This research aims to optimize the design of an oscillating water column (OWC) wave energy converter to extract energy from ocean waves. Methods: The total efficiency of an OWC wave energy converter is determined by the turbine's design configuration and the chamber's partially submerged water column. The various designs of wave energy turbines were examined before simulating the contra rotary impulse turbine with middle vanes at specific angles. The efficiency of the turbine was determined using ANSYS FLUENT workbench simulations. Additionally, the design of an OWC was simulated to determine the optimal chamber slope. To conduct experiments, a 3D-printed contra rotary self-rectifying impulse turbine with middle vanes was generated as a demonstration model. Middle vanes used in contra-rotary turbines can assist in reducing turbulence between contra rotors. Results: The results demonstrate that the turbine with middle vanes has a higher torque coefficient Ct than the turbine without middle vanes. Similarly, the input coefficient Ca is higher for the turbine which has middle vanes. Therefore, the turbine with a middle vane shows better efficiency than the turbine without a middle vane in low flow coefficient as well as higher flow coefficient. Conclusion: The efficiency of the contra rotatory turbine is greater than the single rotatory turbine and the efficiency increases further when middle vanes are added.

Measuring and investigating the effect of sloshing on the performance of an OWC wave energy converter by means of a novel experimental approach

Renewable energy development could be an appropriate method to tackle the problems caused by fossil fuels. In this regard, an Oscillating Water Column (OWC) with a scale of 1:10 has been studied in this research. Sloshing is an unwanted phenomenon in OWC. Therefore in order to study sloshing, A Novel experimental approach for measuring free surface profile inside OWC by considering the effect of wave characteristics and sloshing was investigated experimentally in this paper. Eight wave gauges were installed at the top of the converter to study the hydrodynamic behaviour. The results showed that an increase in frequency raises the possibility of sloshing. However, up to the frequency of 0.37, the TFS (Top Free Surface) angle increase is insignificant. As frequency approaches 0.37, suddenly TFS angle rises, and sloshing occurs at frequencies of 0.37–0.80. Also, dead volumes are formed on TFS (some positive and negative volumes formed on free surface which cancel each other). Therefore if TFS angle is more than 3.85°, the sloshing is happened, and the converter's performance decreases severely. In the best conditions, efficiency and power are 53.7% and 192.2 W, respectively, and in sloshing area, efficiency and power are 8.3% and 35 W, respectively.