Joao C. C. Henriques

University of Lisbon | UL · Instituto Superior Tecnico

Boundary Element Methods (BEM) are widely used to calculate the hydrodynamic loads on marine structures. These methods, that implement developments initially made in the 1950s and 1960s by researchers such as Cummins, Ogilvie and Newman, have been included in different software tools. However, despite the high level of maturity of these tools, inac...

This work concerns the development of an offgrid oscillating-water-column wave energy converter (WEC) equipped with internal batteries designed for powering sensors and autonomous vehicles. Calculating the generator counter torque from electrical measurements is advantageous due to mechanical constraints with torque meters in sea-faring prototypes....

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 bet...

A fixed oscillating water column is a hollow structure open to the sea at its submerged part. The water column motion alternately compresses and decompresses the enclosed air above the inner free surface, which drives a self-rectifying air turbine. In 2003, Boccotti proposed the so-called U-OWC having in view the integration into breakwaters. In th...

This paper introduces a low-power off-grid oscillating water column wave energy converter with an internal battery bank. The research aims at the preliminary design and devising of the control strategy of a power electronics interface between the turbo generator and the battery bank. The converter comprises a spar buoy, a biradial turbine, a perman...

The greater wave energy content in deeper waters has initiated substantial research in offshore wave energy converters (WECs). However, the existing WECs are hindered by their high initial costs, primarily attributed to the demanding mechanical workloads they endure. This paper describes a novel wave energy conversion system developed at IST for de...

The oscillating water column is one of the most tested and used wave energy converter types. It requires the use of an air turbine connected to an electrical generator. The deep understanding of all physical quantities of the energy chain components is based on transducers. The electrical generator efficiency is a key input to wave-to-wire numerica...

Oscillating water columns (OWCs) are widely regarded as the simplest and most reliable type of wave energy converter. An economically viable OWC wave power plant requires maximizing the performance of the power take-off system, composed of an air turbine coupled to an electrical generator. Besides, the OWC power plant’s inherent nonlinearities and...

As the demand for sustainable and efficient energy solutions continues to grow, researchers worldwide are making significant strides in various domains of energy research. Recognising the future leaders of Energy Research is fundamental to safeguarding tomorrow’s driving force in innovation. This Research Topic aims to provide a comprehensive over...

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...

Open sea experience is essential to develop wave energy technology and reduce costs. One of the goals of the EU H2020 OPERA project was to advance the development of the biradial self-rectifying air turbine from TRL4 to TRL7. For this purpose, a 30 kW turbo-generation prototype was fabricated and tested at IST variable flow test rig, the shoreline...

The spring-like air compressibility effect in oscillating-water-column (OWC) wave energy converters may significantly affect the device’s power performance. Yet, this effect has rarely been accounted for in physical model testing, especially in floating OWCs. Quantifying this effect in the development and analysis of OWCs is crucial for i) making b...

The oscillating-water-column (OWC) wave energy converter with air turbine has been object of extensive development. The spring-like effect of air compressibility in the chamber is related to the density-pressure relationship. It is known to significantly affect the power performance of the full-sized converter, and is rarely accounted for in model...

Within the wave energy community, hydrodynamic coefficients obtained from boundary element methods (BEMs) are commonly used to predict the behaviour of wave energy converters (WECs) in response to incident waves. A number of commercially-available BEM solvers exist, with a number of open-source alternatives also available. While open-source solvers...

The development of control strategies that maximize power generation in Wave Energy Converters (WEC) is fundamental in making the exploitation of sea waves an economically viable activity. Model-based control techniques have limitations in achieving this goal, due to their dependency on modelling accuracy and inability to adapt to changing system d...

Within the wave energy community, hydrody-namic coefficients obtained from boundary element methods (BEMs) are commonly used to predict the behaviour of wave energy converters (WECs) in response to incident waves. A number of commercially-available BEM solvers exist, with a number of open-source alternatives also available. While open-source solver...

The Wells turbine is the most frequently used or proposed self-rectifying air turbine for oscillating-water-column wave-energy converter application, largely because of its conceptual and mechanical simplicity. Biplane Wells turbines allow a higher total blade solidity to be attained than monoplane turbines do, but this results in larger aerodynami...

Achieving cost reduction in wave energy conversion is seen as essential to enabling the progress of the sector. At utility scale in a wave farm, multiple devices are likely to be deployed in array configurations. Closely spaced, compact wave energy converter (WEC) arrays are a promising option for cost reduction, realising synergies in operation an...

The Digital Prototype (DP) procedure gets an approximation of the viscous and turbulent hydrodynamic forces that apply to full-scale floating offshore structures. It uses a Computational Fluid Dynamics (CFD) model validated against experimental tests at a lower scale, to get the best fit of a model based on Cummins' equation with hydrodynamic coeff...

Abstract The high variability and unpredictability of renewable energy resources require optimization of the energy extraction, by operating at the best efficiency point, which can be achieved through optimal control strategies. In particular, wave forecasting models can be valuable for control strategies in wave energy converter devices. This work...

Boundary Element Method (BEM) solvers are extensively used to obtain the hydrodynamic coefficients required to model hydrodynamic forces in floating marine structures. BEM solvers require the discretization of the submerged device surface as a mesh to compute the hydro-dynamic coefficients as radiation damping and added mass, response amplitude ope...

A fully-submerged bottom-standing 1.5 MW wave energy converter (WEC) equipped with a membrane-pump system is under development for installation at a test site off the coast of Pembrokeshire, Wales, UK. The system comprises several flexible-membrane air cells that are compressed and sucked by the action of the waves, rectifying valves, low-pressure...

The oscillating-water-column (OWC) spar-buoy is a type of wave energy converter that may exhibit undesirable large roll and pitch amplitudes caused by a dynamic instability induced by parametric resonance. The occurrence of this phenomenon not only reduces the power extraction but significantly increases the structural loads on the buoy, the turbin...

The development of wave energy converters is centred on the combination of two factors: cost-effectiveness of energy extraction and system survivability on extreme sea conditions. Despite advances in numerical modelling, wave tank model testing still presents the most reliable option to evaluate these two factors. This paper presents an experimenta...

A fully-submerged bottom-standing 1.5 MW wave energy converter (WEC) equipped with a membrane-pump system is being developed for installation at a test site off the coast of Pembrokeshire, Wales, UK. The system comprises several flexible-membrane air-cells that are compressed and sucked by the action of the waves, a system of rectifying valves, low...

This paper aims to maximize the performance of a U-type Oscillating Water Column power plant integrated into a breakwater to be built in Roccella Jonica, Mediterranean Sea, Italy. In this location, both prevalent mild waves and less frequent highly energetic sea states occur. The main challenge is to select a Power Take-Off (PTO) system able to ope...

The paper presents a new selection technique of wave spectra for the numerical simulation of offshore renewable energy (ORE) devices in the form of representative directional spectra, which conserves information about the distribution of wave energy along with frequency and direction classes. As a test case, an assessment of wave power resource for...

The current cost of electricity has been preventing the technological progress of wave energy converters. The use of compact wave energy converters arrays is seen as a technological breakthrough for reducing these costs. These array configurations aim to maximize the energy extracted per unit area of marine space and to promote the sharing of compo...

Ocean wave energy is a renewable energy which remains too costly for large-scale electricity generation. The oscillating water column (OWC) wave energy converter (WEC) is a promising device-type with a rectifying air turbine and generator which convert alternating airflow induced by the water motion into kinetic energy then to electric energy. Appl...

The quest for exploiting the ocean resources and understanding its behaviour has been a challenge with increasing needs for innovation and technology. Model testing is an essential step in offshore renewable energy technology development. It involves challenges that require experience and guidance. Costly mistakes might arise with the subsequent wa...

The article aims to select a Power TakeOff (PTO) system for a fixed Oscillating Water Column (OWC) based wave energy converter installed in low energy areas. Specifically, the study considers a breakwater embodying U-type OWC caissons that is planned to be constructed in Roccella Jonica, Mediter-ranean Sea, Italy. The site is characterized by a wav...

Self-rectifying impulse turbines are a popular alternative to the Wells turbine for oscillating-water-column wave energy converter converters. Self-rectifying impulse turbines have two sets of guide-vanes, one set placed symmetrically on each side of the rotor, instead of a single set as in unidirectional turbines. The efficiency of self-rectifying...

An active control method, to suppress the onset of pitch/roll parametric resonance on an oscillating water column (OWC) spar buoy, is proposed in this paper, which utilises a pressure relief valve at the top of the OWC air chamber. The paper examines the hypothesis that by opening the relief valve, to reduce the air chamber pressure difference and...

The mooring system of floating wave energy converters (WECs) has a crucial impact on power generation efficiency, cost of delivered energy, proper operation, reliability and survivability. An effective design, addressing such competing objectives, requires appropriate mathematical models to predict mooring loads and dynamic response. However, conve...

The wave energy sector has faced enormous technological improvements over the last five decades, however, due to the complexity of the hydrodynamic processes, current numerical models still have limitations in predicting relevant phenomena. In particular, floating spar-type wave energy converters are prone to large undesirable roll and pitch amplit...

The paper presents a numerical study on the aerodynamics of a biradial turbine for wave energy conversion. The turbine is symmetrical with respect to a plane perpendicular to its axis of rotation. The two inlet/outlet rotor openings are axially offset from each other facing the radial direction and two rows of radial-flow movable guide-vanes surrou...

Ocean waves are a huge, largely untapped energy resource, and the global wave energy potential is estimated at about 32,000 TWh/year. Due to its great potential to provide renewable and sustainable electricity without carbon dioxide emission, wave energy conversion has attracted extensive attention from multi-disciplinary communities. Compared with...

The self-rectifying biradial air turbine was proposed to equip wave energy converters of the oscillating-water-column type. There are two configurations: with fixed guide vanes and a radially more compact one with axially-sliding guide vanes. The added complexity of the movable guide-vane mechanism represents a drawback in offshore environments. Th...

Maintaining a setpoint to maximize energy harvest of wave energy converters (WECs) with the uncertainties induced by the ocean waves irregular excitation can be ensured by using sliding mode control. In practice, the infinite switching frequency in the sliding mode is limited by the actuators bandwidth. This work compares multiple second-order slid...

The development of devices for extracting wave energy from the ocean is largely supported by numerical models, as they allow the simulation of different configurations without the large costs of tank testing. From the different available options, time-domain models offer a very good combination between accuracy, flexibility and computational time....

The oscillating-water-column (OWC) wave energy converter with air turbine has been object of extensive research and development effort, including the deployment of floating and fixed-structure full-sized prototypes into the sea. It consists of a hollow (fixed or floating) structure, open to the sea below the water surface. Wave action alternately c...

This paper presents a complete mathematical model of an array of three oscillating water column (OWC) wave energy converters (WECs) and the design of a direct generator torque control strategy using a sliding mode control (SMC) to maximize the output power of doubly-fed induction generators (DFIGs) attached to bi-radial turbines that are driven by...

The self-rectifying axial-flow impulse turbine is a popular alternative to the Wells turbine for oscillating-water-column wave energy converter applications. For increasing flow rate coefficient, the peak efficiency point is followed by a gradual decline, which contrasts with the sharp drop that characterizes in general the Wells turbines. This all...

During experimental test campaigns, it is important to estimate the quality of the numerical methods developed during the design stages of a wave energy converter. One of the most used methods for this purpose is to measure the diffraction and radiation forces experimentally. The present work addresses the experimental assessment of fixed Oscillati...

The OWC (oscillating water column) is a class of wave energy converters (WECs) whose power takeoff system typically comprises a tur-bine/generator set. Since its introduction in 2011, the biradial self-rectifying air turbine has set a new performance level against all other turbines for this type of applications. The biradial turbine designed and c...

This paper addresses the hydrodynamic optimization of a floating oscillating water column (OWC) wave energy converter. The device, with an axisymmetric geometry and a draft much larger than the horizontal dimension, is known as the OWC spar buoy device. The design of the converter geometry, mass distribution and turbine damping is carried out throu...

This paper deals with the issues of scaling the air compressibility effects in OWCs. For this purpose, use was made of a coaxial-duct (CD) oscillating-water-column (OWC) wave energy converter (WEC) designed at Instituto Superior Técnico (IST). A small-scale model (1:100) of a fixed CD-OWC was built and tested at the wave flume of the Hydraulic Labo...

Highlights: Comparative study and assessment of six control strategies developed for the biradial turbine in the Mutriku OWC plant. --- Two are adaptive controllers, other two are latching controllers, one uses a RL scheme and one is a non-linear MPC. ---- Numerical simulations are done on a variety of sea states to support a performance and reliab...

The paper presents a detailed analysis of the dynamics and control of air turbines and electrical generators in oscillating-water-columns (OWCs). The aim is to explain the performance of an OWC device based on the physical behaviour of each system component. The Mutriku wave power plant was chosen as the test case. The power plant is a breakwater l...

The REWEC3 (Resonant Wave Energy Converter) is a fixed oscillating water column (OWC) wave energy converter (WEC) incorporated in upright breakwaters. The device is composed by a chamber containing a water column in its lower part and an air pocket in its upper part. The air pocket is connected to the atmosphere via a duct hosting a self-rectifying...

Backward bent-duct buoy Frontward bent-duct buoy Optimisation Performance assessment Tecno-economic assessment A B S T R A C T This work focuses on the investigation of bent-duct buoy oscillating-water-column (OWC) wave energy converters (WEC), which have been generally studied in their backward configuration (BBDB). This work studies both backward...

The performance assessment of industrial marine energy converters involves the integrated treatment of their hydrodynamic design and the optimization of their device hulls. Nowadays, such tasks require extensive experimental work and simulation plans, consuming considerable resources and time. In this comprehensive review of integrated approaches t...

A 30 kW fixed-guide-vanes biradial turbine-generator set was designed by Instituto Superior Técnico (IST) and manufactured by Kymaner, within the framework of the H2020 European project OPERA, for open sea testing at the Mutriku breakwater oscillating water column (OWC) power plant and at the IDOM MARMOK-A-5 OWC spar-buoy, deployed at BiMEP, Basque...

Oscillating water column (OWC) devices equipped with an air turbine are the most extensively studied and possibly the most reliable wave energy converters (WEC). Self-rectifying air turbines are usually employed, being the key element of the power-take-off system. Experimental testing the air turbine in realistic operating conditions is a fundament...

Fluid Mechanics Course.

Fluid Mechanics Course.

The paper presents an experimental study of a novel self-rectifying twin-rotor air turbine to equip an oscillating water column wave energy converter. The turbine is based on a pair of conventional radial inflow rotors mounted on a common shaft, complemented by the corresponding inlet guide vane rows, by a curved-duct manifold arranged circumferent...

To be competitive against other renewable energy sources, wave energy needs to reduce the associated levelised cost of energy. In literature, it has been proven that advanced control algorithms can increase power production and device reliability. They act throughout the power conversion chain, from the hydrodynamics of wave absorption to the power...

Quantitative reliability, availability, and maintainability (RAM) assessments are of fundamental importance at the early design stages, as well as planning and operation of marine renewable energy systems. This paper presents an RAM framework adaptable to different offshore renewable technologies, conceived to provide support in the choice of the d...

This work focuses on the initial performance assessment of an array of coaxial-duct (CD) oscillating-water-columns (owc’s) with potential to be used as multipurpose platform for the creation of value in a diverse range of offshore economic activities. The coaxial-duct owc (CD-owc) is an axisymmetric oscillating-water-column wave energy converter th...

In order to de-risk wave energy technologies and bring confidence to the sector, it is necessary to gain experience and collect data from sea trials. As part of the OPERA H2020 project, the Mutriku Wave Power Plant (MWPP) is being used as a real condition laboratory for the experiment of innovative technologies. The plant is situated in the North s...

The oscillating-water-column (OWC) wave energy converter consists of a hollow (fixed or floating) structure, open to the sea below the water surface. Wave action alternately compresses and decompresses the air trapped above the inner water free-surface, which forces air to flow through a turbine coupled to a generator. The spring-like effect of air...

This paper presents the experimental tests of a 16th-scale model of the Spar-buoy oscillating water column (OWC) carried out at a large scale wave flume. The model is slack-moored to the flume floor by two mooring lines. The turbine effect is replicated using calibrated orifice plates. The device six degree-of-freedom motion, inner free surface dis...

The oscillating water column (OWC) with an air turbine is a major class of wave energy converter that has been object of extensive research and development effort over many years, including the deployment of prototypes into the sea. In an OWC, the air alternately flows from the chamber to the atmosphere and back. Unless check valves are used, the t...

A high-order Discontinuous Galerkin (DG) finite element time-stepping method is applied for the numerical solution of optimal control problems within the framework of Pontryagin’s Maximum Principle. The method constitutes an efficient and versatile alternative to the well-known Pseudospectral (PS) methods. The two main advantages of DG in compariso...

The self-rectifying biradial air turbine was proposed to equip wave energy converters of the oscillating-water-column type. There are two versions: (i) with axially-sliding guide vanes and (ii) with fixed guide vanes. Version (i) was model tested and found to be the most efficient self-rectifying turbine, with peak efficiency about 78%. The drawbac...

The focus of this paper is the design and optimization of the guide-vane system of a self-rectifying biradial air turbine for use in oscillating water column systems for wave energy conversion. Since the turbine is required to be self-rectifying, there are two sets of guide-vanes, placed symmetrically on both sides of the rotor, instead of a single...

The reliability and performance of the oscillating-water-column (OWC) have made it one of the most studied wave energy converter options. The present work aims to present a new wave-to-wire model of a spar-buoy OWC that includes a detailed cost analysis. The major contributions of the paper for the current state-of-the art of the OWC technology are...

This work has been focused on an innovative method of increasing the energy extracted from ocean waves, using point absorber type Wave Energy Converters (WECs). This method involves the reduction of a WEC's natural frequency, such that its excitation response better matches the resource spectra of typical wave climates. The operating principle of t...

One main problem with OWC wave energy converter is the energy conversion efficiency of the air turbine. The reciprocating air flow generated by the waves does not allow conventional air turbines to be used unless rectifying valves are employed. Alternatively, rectifying air turbines have been developed for converting the pneumatic power into mechan...

This study concerns with the potential applicability of using wave energy converters, specially oscillating water columns, as monitoring buoys for operational oceanography. The concept of operational oceanography and the network of infrastructure and collaboration behind the concept is introduced, identifying the main sectors and stakeholders invol...

As the development of a wave energy converter (WEC) approaches the sea trials, the handling of the power peaks becomes a more important issue. Among the various types of WECs, the oscillating-water-column (OWC) is probably the most capable to control or dissipate the excess of energy available to the power takeoff (PTO) system that may occur in med...

The oscillating water column (OWC) is a major class of wave energy converter that has been object of extensive research and development effort over many years, including the deployment of prototypes into the sea. Unless rectifying valves are used, the turbines are self-rectifying. Several types of such special turbines have been developed. The axia...

Parametric resonance is a dynamic instability commonly observed in spar-type wave energy converters that use their vertical motion to extract energy. This problem can occur due to instantaneous variations of hydrostatic restoring moment, or due to nonlinear pressure variations induced by the wave on the instantaneous converter wetted surface. Durin...

This paper applies a Discontinuous Galerkin (DG) finite element time-stepping method for the numerical solution of optimal and sub-optimal control problems within the framework of the Pontryagin's Maximum Principle. The local nature of the piecewise polynomial approximation used in the DG method handles easily the case of a large number of switchin...

This paper applies a Discontinuous Galerkin (DG) finite element time-stepping method for the numerical solution of optimal and sub-optimal control problems within the framework of the Pontryagin’s Maximum Principle. The local nature of the piecewise polynomial approximation used in the DG method handles easily the case of a large number of switchin...

The Mutriku breakwater wave power plant is located in the Bay of Biscay, in Basque Country, Spain. The plant is based on the oscillating water column (OWC) principle and comprises 16 air chambers, each of them equipped with a Wells turbine coupled to an electrical generator with a rated power of 18.5 kW. The IDMEC/IST Wave Energy Group is developin...

Pseudo-Spectral (PS) methods are one of the most important and popular tools to compute solutions of optimal control problems (OCPs). In the PS methods, the state and control trajectories are approximated in the entire domain by single polynomials where the unknown coefficients are computed at the Legendre-Gauss-Lobatto (LGL) or Chebyshev-Gauss-Lob...

The OWC spar buoy is possibly the simplest concept for a floating oscillating water column (OWC) wave energy converter [1]. It is an axisymmetric device consisting basically of a submerged vertical tail-tube-fixed to an axisymmetric floater that oscillates essentially in heave. The air flow displaced by the water motion inside the tube drives a sel...

This paper studies the dynamics and power extraction of a U-Oscillating Water Column (U-OWC) device. A three-dimensional hydrodynamic model, based on the boundary integral equations method, is implemented. The U-OWC width and wave direction are analysed and compared with a two-dimensional model based on the eigen-function expansion method. Results...

The oscillating-water-column device equipped with an air turbine is regarded by many researchers and developers as the simplest and most reliable wave energy converter. It has been object of extensive development effort, including the deployment of prototypes into the sea. The maximization of the produced electrical energy involves the control of t...

PART 1 * Generator feedback control: Mutriku test case * Power take-off system: biradial vs Wells turbine *Mathematical model * Generator feedback control law: computation and sensitivity analysis PART 2 * Model predictive latching control: Spar-buoy OWC test case * Power take-off system: biradial turbine equipped with an HSSV * Changes to mathemat...

The Spar buoy is possibly the simplest concept for a floating oscillating water column (OWC) wave energy converter. It is an axisymmetric device consisting basically of a submerged vertical tail-tube-fixed to an axisymmetric floater that oscillates essentially in heave. The air flow displaced by the water motion inside the tube drives a self-rectif...

The oscillating-water-column wave energy device equipped with an air turbine is widely regarded as the simplest and most reliable, and the one that was object of the most extensive development effort. The aerodynamic performance of the air turbine plays a major role in the success of the technology. A case study was selected to investigate these is...

The use of an electrical test infrastructure is crucial for developers of ocean energy technologies to success in prototype testing phase. It offers numerous advantages to improve product reliability and technology readiness at each stage of the development of new concepts for wind, tidal or wave energy. The aim of the paper is to propose a methodo...

The oscillating-water-column wave energy device equipped with an air turbine is widely regarded as the simplest and most reliable, and the one that was object of the most extensive development effort. The maximization of the produced electrical energy involves the control of the rotation speed, which affects the hydrodynamic process of wave energy...

The quest for conquering the ocean and understanding its behaviour has been a challenge with increasing needs for innovation and technology investments in many areas of strategic value for the promotion, growth and competitiveness of the marine economy worldwide. Current oceanographic buoy systems are limited to low power levels and intermittency o...

For large-scale exploitation of the offshore wave energy resource, floating wave energy converters should be deployed in arrays with inter-body mooring connections, reducing the number of bottom slack-mooring lines needed and consequently the project total cost. The present paper concerns the experimental investigation of the dynamics, energy extra...