Oscillating Water Column System Research Articles

Hydrodynamic performance of a land-based multi-chamber OWC wave energy capture system: An experimental study

Improving ocean wave energy capture in a cost-effective manner is a challenging task. Multi-chamber oscillating water column (OWC) devices are gaining favor due to their potentially efficient characteristics. This research experimentally investigated the hydrodynamic performance of a land-based OWC wave energy capture system with multiple chambers (ranging from 1 to 5). The free surface elevation, air pressure fluctuations, hydrodynamic efficiency, and reflection coefficient are considered. The hydrodynamic efficiencies of the overall, and sub-chambers in the OWC system with varying numbers of chambers are graphically presented. The influence of geometrical design parameters and wave conditions are also considered in evaluating the performance of the multi-chamber OWC system. It is found that the multi-chamber arrangement improves the hydrodynamic energy extraction characteristics compared to the traditional single-chamber setup. The sloshing mode of the chamber water column is utilized to effectively capture wave energy. As the number of chambers increases, the wave attenuation capability of the system improves for long waves. Yet, the overall efficiency declines when the number of OWC chambers exceeds 3. The chamber draft has the most substantial impact on the wave energy capturing capability, while the effects of opening ratio and wave height are attenuated due to multiple water-column interactions inside the chambers. The isometric sub-chamber structure demonstrates overall efficiency and wave attenuation advantages for short waves. The paper aims to guide the design and optimization of a multi-chamber OWC system.

Wave Energy Conversion through Oscillating Water Columns: A Review

An oscillating water column (OWC) is designed for the extraction and conversion of wave energy into usable electrical power, rather than being a standalone renewable energy source. This review paper presents a comprehensive analysis of the mathematical modeling approaches employed in OWC systems, aiming to provide an in-depth understanding of the underlying principles and challenges associated with this innovative technology. A prominent classification within the realm of wave energy devices comprises OWC systems, which exhibit either fixed or floating configurations. OWC devices constitute a significant proportion of the wave energy converter prototypes currently operational offshore. Within an OWC system, a hollow structure, either permanently fixed or floating, extends below the water’s surface, creating an enclosed chamber where air is captured over the submerged inner free surface. This comprehensive study offers a thorough assessment of OWC technology in conjunction with air turbines. Additionally, the investigation delves into theoretical, computational, and experimental modeling techniques employed for analyzing OWC converters. Moreover, this review scrutinizes theoretical, computational, and experimental modeling methodologies, providing a holistic understanding of OWC converters. Ultimately, this work contributes a thorough assessment of OWC technology’s current state, accentuating its potential for efficient wave energy extraction and suggesting future research avenues.

Hydrodynamic and energy-harvesting performance of an isolated oscillating water column device: An experimental study

Most previous experimental studies on the hydrodynamic and aerodynamic performances of an oscillating water column (OWC) wave energy converter were conducted in a wave flume. In this study, an experimental study on an isolated OWC plant is carried out in a wave tank. The interactions between the OWC system and incident regular and irregular waves are analyzed by measuring the surrounding wave fields using a wave gauge array. The wave distribution patterns and reflection ratios of the OWC model are presented. In addition, the free-surface elevation, air pressure varying amplitude, turbine torque output, and relative capture width ratios and efficiencies in regular and irregular wave scenarios are measured and analyzed. The operating performances of the OWC model are analyzed and compared to the results derived from the flume tests. An overall performance reduction of over 20% can be observed in the wave tank tests. In addition, the representative results of the overall capture width ratio of the OWC model derived from irregular wave tests are corrected from the scaling effects caused by the similarity contradiction.

Enhancing wave energy harvesting: Performance analysis of a dual chamber oscillating water column

This study investigates the hydrodynamic characteristics of a dual chamber oscillating water column (OWC) setup through a series of experiments, aiming to assess its potential for harvesting wave energy. The investigation involves varying frontwall drafts, power take-off (PTO) dampings, and incident wave conditions, while quantitatively evaluating the OWC system's performance using the capture width ratio (CWR) as the key metric. The findings indicate that the dual chamber configuration consistently exhibits enhanced performance for all combinations of orifice ratios, opening heights, and incident wave frequencies studied. As expected, increasing the opening height of the second chamber leads to improved performance of the OWC. Interestingly, the effect of varying applied PTO damping on improvement of CWR values is found to be relatively insensitive. The most significant improvements in CWR are observed for larger incident wave frequency values. For instance, for dimensionless wave frequency (Kh) values of 1.43 and 1.68, the CWR increases from 0.47 and 0.26 to 0.72 and 0.52, respectively, with the orifice ratio combination of τ1, 0.015, and τ2, 0.018. It is worth noting that the single chamber OWC operates most efficiently within the resonant frequency range of 0.94–1.23, while the dual chamber design extends this frequency range, enabling efficient wave energy capture over a wider spectrum of wave conditions. The dual chamber configuration offers considerable advantages for wave energy conversion applications by increasing the effective frequency bandwidth for optimal energy capture. The research findings suggest that this innovative design has the potential to significantly enhance the efficiency and adaptability of wave energy converters, contributing to the advancement of sustainable and renewable energy technologies.

Fixed type-oscillating water column front wall angle variation and impact on chamber performance: CFD numerical wave tank assessment

This study investigates optimizing the front wall geometry of an Oscillating Water Column (OWC) chamber to enhance turbine performance and output efficiency when harnessing energy from progressive waves with consistent periods and wavelengths. Numerical Wave Tank (NWT) adopted simulations carried out using ANSYS-Fluent ® package, VOF method with multiphase flow (air-water) where 2-D wave motion of NWT implemented using a C+ computer code. Frontal wall angle of the air/water chamber changed from 0° to 80°, for 10 cases. OWC system was most effective in harnessing and converting wave energy at 0° angle, reaching the maximum power output of 605.08 W/m. In contrast, 80° angle exhibited lower efficiency, with the lowest power output of 26.55 W/m. The average power output over time reflects consistent energy conversion, with the 0° angle demonstrating the highest average power output of 123.72 W/m, while the 80° case exhibited less efficient pneumatic power potential with an average of 3.5 W/m. A uniform cross section provides higher pneumatic power when with PTO in OWC. The efficiency of power generation can be increased by approximately 10 to 20% by keeping 0 to 10° angle of front wall without any appendages fitted with into the wall.

Bell-Metha Power Augmented Savonius turbine as Take-off in OWC Systems

The present paper deals with the study of ducted Savonius turbine performance to be used as power take-off in oscillating water column systems equipped with a power booster. Two converging sections with a Bell-Metha profile have been used to increase the mass flow rate on the active side of the turbine while masking the resistant part. The Bell-Metha profile was chosen with the aim of reducing vortices immediately upstream of the turbine, avoiding both disturbance and energy loss. A laboratory scale Savonius turbine with a diameter of 0.09 m with an aspect ratio of 1 and an overlap ratio of 1/3 was tested. Performance was tested using a laboratory scale oscillating flow simulator. All tests were performed at fixed air velocity of 5 m/s. Based on the results, it is possible to state that ducted turbine with power augmenter performance is higher than plain ducted turbine ones, at all tip speed ratios.

Numerical investigation on the hydrodynamic performance of a land-based OWC system with multi-chamber modules

Modular design of wave energy converters (WECs) can offer a promising scheme to improve the economic competitiveness of wave energy by reducing the levelized cost of energy. In this study, the hydrodynamic performance of a land-based oscillating water column (OWC) system with multi-chamber modules is evaluated to optimize the layout and design of the system, the OWC structure is extended to five-chamber modules. A two-dimensional wave flume is developed using the high-order boundary element method to simulate the interaction of the incident waves with the OWC device based on the fully non-linear potential flow theory. The proposed numerical model is validated by published experimental data. It is concluded that the two-chamber installation provides superior hydrodynamic performance compared with the single-chamber OWC. The five-chamber OWC system is more suitable for energy capture in low-frequency wave conditions, where the first and last chambers are dominant in energy extraction. From the perspective of energy conversion efficiency and economy, the two- and three-chamber systems are most suitable for targeting low and high frequency waves in the tested conditions, respectively. Additionally, the resonant frequency of the single-chamber module plays a decisive role in the energy capture of the OWC system, and a chamber with a low resonant frequency and a small orifice opening ratio should be selected as the module unit. This paper provides valuable guidance for the modular application of OWCs and the arrangement of multiple chambers.

Optimization of the design of an oscillating water column for the Pacific Ocean conditions using the response surface methodology

Wave energy possesses a significant potential as a sustainable and renewable source of power due to its abundant presence in the Earth’s oceans. This research focused on the assessment of the hydrodynamic efficiency (ɛ) of a resonant chamber within an OWC (oscillating water column) system. Four geometric design parameters of the resonant chamber (h1/h, b2/h, d/b2 and α) were selected to optimize ɛ, being h the depth of the water; h1 and α correspond to the vertical length and angle of inclination of the front wall, respectively; b2 is the width of the resonant chamber, and d represents the diameter of the air outlet hole. To analyze the system, a combined approach utilizing CFD with the VoF (volume of fluid) method and the response surface methodology (RSM) was adopted. The RSM employed a FCCD (face-centered central composite design) to optimize the factors, while the VoF method enabled to simulate in a detailed way the fluid behavior. The ANSYS-Fluent software was employed to perform a transient simulation. A second-order regression model was constructed and validated based on the simulation results. The quadratic regression model exhibited a high Radj2 value (82.81%) and had a p-value associated ¡0.05 (4.66 × 10-8). Therefore, the model was highly significant in accurately representing the ɛ of the OWC resonant chamber. The highest ɛ value (67.01%) was obtained for h1/h, b2/h , d/b2 and α equal to 0.7786, 0.4455, 0.0863, and 89.38°, respectively. The numerical results were validated with the Stokes second-order wave theory and a good correlation was observed with the numerical findings. This study demonstrates that the designed and optimized OWC can be effectively utilized in the Colombian Pacific Ocean to generate electrical energy, promoting economic development in the area.

The Evaluation of WaveSAX Power Generation to Support Port Energy Self-Sufficiency

Remarkable advancements have been made in wave energy converters, notably the innovative WaveSAX technology—an oscillating water column system conceptually designed for coastal structures. Proven effective in existing installations, particularly at Civitavecchia Port, Italy, where the WaveSAX-1 prototype was tested in 2018 and the WaveSAX-2 was updated in 2021. The device’s power generation capacity was evaluated using a Simulating WAves Nearshore (SWAN) model that simulated 30 years of wave conditions. Validation with radar and Acoustic Doppler Profiler instruments showed excellent performance in wave height simulation. Results revealed higher wave power generation near the harbor breakwater, surpassing offshore levels, especially in central and northern sections. WaveSAX-2, featuring an improved Wells turbine, demonstrated a remarkable 37% increase in average efficiency compared to WaveSAX-1. If a linear WaveSAX array were installed, it could produce 4 GWh annually, satisfying 20% of the port’s energy needs and reducing around 2800 tCO2e/year. Doubling arrays and incorporating triangular modules could significantly enhance sustainability for port operations, offering a promising path toward self-sufficiency and environmental responsibility.

Discussion on “Influence of incoming wave conditions on the hysteretic behavior of an oscillating water column system for wave energy conversion” by J. Peng, C. Hu and C. Yang

Recently, Peng, Hu and Yang presented a lumped parameter model to quantify the hysteresis in Oscillating Water Column systems. We noticed that the model they presented is remarkably similar to the one we introduced in some of our previously published works. The similarity extends not only to the assumptions, derivation and methodology used to obtain an analytical solution, but even to the almost totality of the symbols chosen for the many model variables. None of the papers where we introduced the model and its solution were referenced by Peng and his coauthors, who therefore claimed for themselves the credit due to the original authors of the model. Peng and his coauthors have then applied the lumped parameter model to a test case different from the one that we had validated it on. This gives further confirmation of the validity of the model, which we feel the responsibility to reestablish the scientific property of.

Reply to Discussion on “Influence of incoming wave conditions on the hysteretic behavior of an oscillating water column system for wave energy conversion” by J. Peng, C. Hu and C. Yang

Reply to Discussion on “Influence of incoming wave conditions on the hysteretic behavior of an oscillating water column system for wave energy conversion” by J. Peng, C. Hu and C. Yang

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.
 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.
 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.

Investigate air turbines performance for power generation by tidal waves in the river: A review

The phenomenon of climate change resulting from the increase of global warming has become one of the main problems facing the world. Where researchers and specialists have worked for many years to find a solution that reduces this phenomenon and limits its risks. Clean energy is likely an alternative to fossil fuel sources, which are the main source of global warming. One of the clean energy sources is ocean wave energy, which is a huge and untapped energy source, despite the possibility of extracting large amounts of energy from waves. This paper focuses on the study of deep-sea turbines and their results. A study was conducted on the capture chamber. The turbines are driven and generate electricity by using a wave energy harvesting system (oscillating water column). Reaction turbines (impulse and Wells turbines) are tested and evaluated for performance in an oscillating water column system. Reaction turbines have a narrow operating range, and higher peak efficiencies. In contrast, turbines that have a greater operating range and lower peak efficiency are impulse turbines. Wells stator turbine has the highest efficiency while a variable pitch axial impulse turbine has the widest operating range. Better efficiency possessed by turbines with optimized geometry than other turbines.

Influence of incoming wave conditions on the hysteretic behavior of an oscillating water column system for wave energy conversion

The performance difference of a Wells turbine in an oscillating water column (OWC) system in the acceleration and deceleration phases of the incoming waves is known as its hysteresis characteristic. In the present work, a laboratory OWC system is simulated numerically with deformable grids to simulate the air–water interface, and a lumped-parameter model is used to reproduce the interaction between the air flow and the Wells turbine in the OWC system. The model results are compared with experimental and numerical results, with excellent agreement, then the model is used to quantify the hysteresis intensity of the hysteresis loop under different incoming wave conditions and turbine rotation speeds. After separating the hysteresis of the OWC system from that of its Wells turbine, the lumped-parameter model shows that the former is due largely to the compressibility of the air in the chamber. When the compression ratio of the OWC system decreases from 4.40 to 2.00, the hysteresis intensity of the hysteresis loop decreases by 64.13%, and when the rotation speed of the Wells turbine increases from 3600 rpm to 4800 rpm, the hysteresis intensity increases by 34.53%. The present work contributes to predicting the performance of Wells turbines and quantifying the hysteresis intensity for maximum extraction of sea-wave energy.

Reliable control of turbine–generator set for oscillating-water-column wave energy converters: Numerical modelling and field data comparison

Global energy strategy has quickly shifted to a new paradigm. Countries understand the importance of a near-zero carbon energy mix and the need to reduce their energy imports and become energy independent. One immediate solution is the re-enforcement of renewable energy infrastructures such as wind, solar and hydro generation. However, certain sectors, particularly the Blue Economy, may need a different solution. The oscillating-water-column (OWC) wave energy converter is a proven concept, many prototypes of which are already being deployed on the open sea, and can help bridge this gap. Energy harvesting from this system will be improved if more efficient (and reliable) power take-off (PTO) systems and control algorithms are used. A novel control algorithm for a turbine–generator set was developed based on the physical interactions between the PTO and the OWC system. The results were compared with experimental data from real operation at sea. The algorithm does not need to predict the sea state conditions in order to maximise power generation. Additionally, it protects the PTO in extremely energetic sea states and minimises overspeeding and the use of a safety valve. Comparing the operating results of the 30kW biradial turbine–generator set at the Mutriku wave power plant with the corresponding numerical results provided by the novel control algorithm showed a 6% increase in annual electricity production.

The hydrodynamic performance of a shore-based oscillating water column device under random wave conditions

The testing of scale models is crucial if we are to continue the development of wave energy converters. Investigations have examined the interaction of shore-based Oscillating Water Column (OWC) devices with regular water waves that impact the device perpendicularly in flumes or wave basins. However, these simplistic experiments might misrepresent the effectiveness of an OWC system. This study examines the interaction of irregular, oblique, water waves with a shore-based OWC device in terms of its hydrodynamic performance. In a spectral wave basin, a series of experiments were performed on a scale model of a single chamber of the Mutriku Wave Energy Plant. Wave propagation conditions were examined to see their impact on the hydrodynamic performance of a fixed OWC system. The wave amplification factor, hydrodynamic efficiency, non-dimensional air pressure inside the chamber, and non-dimensional water pressures on the chamber walls were evaluated. The results show that when the free surface is close to the front wall lip, the hydrodynamic efficiency increases, while the effect of significant wave height was seen to be more important in the wave amplification. Finally, it was found that the hydrodynamic efficiency under random waves is lower than that in experiments under regular waves reported previously.

Wave-to-Wire Efficiency Maximisation for Oscillating-Water-Column Systems

Wave energy is a significant source of renewable energy which is harnessed by wave energy converters (WECs). However, due to the relatively high levelised cost of energy, wave energy has not attained a commercial stage yet. One of the key pathways to achieve commercialisation of WECs is to design effective control strategies to optimise the overall wave-to-wire (W2W) energy conversion process. This paper particularly focuses on W2W efficiency maximisation for oscillating-water-column (OWC) WECs. In OWC systems, the displacement of a water column compresses/decompresses a volume of air, consequently generating a bidirectional air flow. The air flow is typically used to drive a self-rectifying air turbine, which is directly coupled with a suitable electric generator. Due to the demanding issue of turbine efficiency, current OWC control strategies aim to maximise turbine efficiency by controlling the turbine rotational speed, albeit ignoring hydrodynamic performance. However, for Wells turbines, variations in the rotational speed affect the hydrodynamic efficiency (i.e., the wave-to-pneumatic energy conversion process) of the OWC system. Furthermore, the generator performance also depends on rotational speed and, therefore, rotational speed should be ideally modulated to improve the overall W2W efficiency, rather than just turbine efficiency. To this end, this paper investigates the benefits of W2W efficiency maximisation through Wells turbine rotational speed modulation, for a fixed OWC system. Results from numerical simulation show that, for Wells turbines, appropriate rotational speed control can further improve the overall OWC W2W energy conversion process, especially due to the impact of rotational speed on the hydrodynamic performance.

Oscillating-water-column wave energy converters: A critical review of numerical modelling and control

Wave energy is a significant untapped renewable energy source which can be harnessed by wave energy converters (WECs). The oscillating water column (OWC) is one of the most promising WECs, due to its relative simplicity of operation and relatively small number of moving parts, all located above the water level. OWC power take-off systems also have lower levels of mechanical stress and more easily dissipate excess wave power, compared to other types of WECs, helping to increase the capacity factor. To improve the economic viability of OWC WECs, the performance of the associated energy-maximising control system is a major determining factor. However, energy maximisation alone does not necessarily imply that the economic return is maximised, since: (i) capital and operational costs of the OWC system should be considered and (ii) the additional goals of power quality and device integrity are also important. Indeed, to maximise return on investment, the optimisation pathway should ideally minimise the levelised cost of energy (LCoE). This critical review aims to: (i) provide a comprehensive analysis of the OWC control problem, (ii) offer an exhaustive review of available control strategies for OWCs, (iii) identify unexplored control and optimisation possibilities, and (iv) suggest future directions for OWC control. Ultimately, this review highlights that, to date, OWC control mainly focuses on turbine control, especially due to the importance of operating the turbine around its maximum efficiency point. However, comprehensive control strategies should maximise the overall, or wave-to-wire (W2W), efficiency of the device. To this end, control-oriented (complete and computationally simple) W2W OWC models should be considered in model-based control strategies. Finally, control co-design techniques should be adopted to guarantee optimal control-informed WEC design and to take into account cost-related aspects.

Numerical investigation of a dual cylindrical OWC hybrid system incorporated into a fixed caisson breakwater

In this paper, a hybrid Oscillating Water Column (OWC)system combining with a heaving floater Wave Energy Converters (WEC) was investigated. The traditional cylindrical-type breakwater consists of dual cylinders with an opening inlet located at the outer wavefront wall, allowing a ring-type wave chamber formed between two cylinders. The oscillating buoy OB is hinged at the front of the OWC device. The WAVE Chamber formed between the two breakwater cylinders is to be used as OWC. Based on the Computational Fluid Dynamics (CFD) software Star CCM+, A three-dimensional numerical wave tank is developed to investigate the hydrodynamic performance of the hybrid system, and the numerical model is validated with published experimental results. The power take-off (PTO) damping performance and the hydrodynamic efficiency affected by water conditions and geometrical dimensions of the device are discussed. Results show that the after combining the floater to the breakwater-type WEC, a great improvement on frequency bandwidth was achieved and the efficiency increase to 83.28% compared to the case of a single device, which was of 57%. Furthermore, the effects of the opening inlet height and the volume ratio of the OWC chamber were studied to optimize the certain geometrical parameters. Lower height ratio of the opening inlet of the OWC chamber should be avoid for larger water depth condition while designing such a hybrid system.

Benchmarking marine renewable energy technologies through LCA: Wave energy converters in the Mediterranean

The present work evaluates the environmental performance of three wave energy converters including on-shore oscillating water columns and oscillating floaters embedded in piers, and near-shore seabed-based buoys in the Mediterranean Basin. The life cycle assessment methodology was used to account for their potential environmental impact, in terms of carbon footprint (t CO2eq), considering four main phases, i.e., manufacturing of material components, assembling and installation on site, maintenance in time, and decommission end of life. Results show the greenhouse gas emission from different lifecycle processes, based on the inventory of main energy inputs and materials, highlighting the major impact of the manufacture of the structural components (52 %), especially due to the limited durability of materials. In order to compare the performances of the three different wave energy converters, the carbon intensity of electricity was evaluated considering a range of electricity production per technology based on data available in scientific literature. The results obtained for a single device (203–270 g CO2eq‧kWh−1 for the oscillating water column system; 94–374 g CO2eq‧kWh−1 for oscillating floater and 105–158 g CO2eq‧kWh−1 for the seabed-based buoy) highlight that wave energy converters are promising solutions to harvest wave energy, showing lower carbon intensity of electricity values compared to fossil energy sources; nevertheless, technological improvements are needed to increase efficiency and achieve the performances of other renewable energy sources. Moreover, the combination of wave energy converters with other solutions, such as offshore wind turbines, represents a valuable option in the future to increase productivity and foster energy transition of the Mediterranean regions.