Hydraulic Power Take-off Research Articles

Experimental investigation and ANN modeling on improved performance of an innovative method of using heave response of a non-floating object for ocean wave energy conversion

To convert wave energy into usable forms of energy by utilizing heaving body, heaving bodies (buoys) which are buoyant in nature and float on the water surface are usually used. The wave exerts excess buoyancy force on the buoy, lifting it during the approach of wave crest while the gravity pulls it down during the wave trough. A hydraulic, direct or mechanical power takeoff is used to convert this up and down motion of the buoy to produce usable forms of energy. Though using a floating buoy for harnessing wave energy is conventional, this device faces many challenges in improving the overall conversion efficiency and survivability in extreme conditions. Up to the present, no studies have been done to harness ocean waves using a non-floating object and to find out the merits and demerits of the system. In the present paper, an innovative heaving body type of wave energy converter with a non-floating object was proposed to harness waves. It was also shown that the conversion efficiency and safety of the proposed device were significantly higher than any other device proposed with floating buoy. To demonstrate the improvements, experiments were conducted with non-floating body for different dimensions and the heave response was noted. Power generation was not considered in the experiment to observe the worst case response of the heaving body. The device was modeled in artificial neural network (ANN), the heave response for various parameters were predicted, and compared with the experimental results. It was found that the ANN model could predict the heave response with an accuracy of 99%.

Prediction of long-term fatigue damage of a hydraulic cylinder of a wave energy converter subjected to internal fluid pressure induced by wave loads

This paper deals with fatigue analysis of the high-pressure hydraulic power take-off (PTO) machinery in a heaving-buoy wave energy converter (WEC) exposed to irregular incident waves, with a focus on the hydraulic cylinder. To estimate the fatigue damage of the relevant critical joints of the cylinder, time-domain simulations are required to obtain the pressure loads inside the hydraulic pump. In this paper, the relevant dynamic responses are calculated by using the previously established nonlinear WEC model. Long-term pressure loads are obtained by combining the short-term time-simulation results and the occurrence probability of each sea state, and two models, Weibull and generalized Gamma distributions, are applied to fit the long-term rainflow-counted pressure cycles. Based on the pressure cycle distribution, the long-term fatigue damage of the cylinder with two different kinds of material, carbon, low alloy steel and nickel–chromium–molybdenum–iron alloy, is obtained by using the SN-Miner Palmgren approach. Two main parameters, cylinder diameter and thickness-diameter ratio, are investigated. Moreover, the parameter combination with which the cylinder has an expected fatigue life and possesses minimum material volume is supplied. Finally, the relative contribution of fatigue damage from different sea states is obtained and the most important wave conditions are identified.

Design and Modeling of a Novel Internal Combustion Engine with Direct Hydraulic Power Take-off

Design and Modeling of a Novel Internal Combustion Engine with Direct Hydraulic Power Take-off

Hydraulic Power Take-off and Buoy Geometries Charac-terisation for a Wave Energy Converter

In the past few decades, world energy consumption grew considerably. Regarding this fact, wave energy should not be discarded as a valid alternative for the production of electricity. Devices suitable to harness this kind of renewable energy source and turn it into electricity are not yet commercially competitive. The work described in this paper aims to contribute to this field of research. It is focused on the design and construction of robust, simple and affordable hydraulic Power Take-Off using hydraulic commercial components.

Design, Construction and Testing of a Hydraulic Power Take-Off for Wave Energy Converters

This paper presents the construction, mathematical modeling and testing of a scaled universal hydraulic Power Take-Off (PTO) device for Wave Energy Converters (WECs). A specific prototype and test bench were designed and built to carry out the tests. The results obtained from these tests were used to adjust an in-house mathematical model. The PTO was initially designed to be coupled to a scaled wave energy capture device with a low speed and high torque oscillating motion and high power fluctuations. Any Energy Capture Device (ECD) that fulfils these requirements can be coupled to this PTO, provided that its scale is adequately defined depending on the rated power of the full scale prototype. The initial calibration included estimation of the pressure drops in the different components, the pressurization time of the oil inside the hydraulic cylinders and the volumetric efficiency of the complete circuit. Since the overall efficiency measured during the tests ranged from 0.69 to 0.8 and the dynamic performance of the PTO was satisfactory, the results are really promising and it is believed that this solution might prove effective in real devices.

Design, Construction and Testing of a Hydraulic Power Take-Off for Wave Energy Converters

Design, Construction and Testing of a Hydraulic Power Take-Off for Wave Energy Converters

Design and performance test of hydraulic PTO for wave energy converter

A PTO (power take-off) system for 50kW class rotating body type WEC (wave energy converter) has been designed, constructed and tested. The WEC has a semi-submerged, articulated structure composed of two cylindrical body sections linked together by a hinge joint. The wave-induced motion of the joint is resisted by a hydraulic cylinder, which pumps high-pressure oil to PTO system having a generator-coupled hydraulic motor and a smoothing accumulator. The hydraulic PTO provides an efficient way for generating high quality electric power from the reciprocating motion of the cylinder. The economic viability of a WEC largely depends on the energy conversion efficiency of its PTO. The efficiency of the overall PTO system and individual parts are measured at the 5, 20, 35 and 43kW of generator power while the generator is tied to the commercial 380Vac grid. In this paper, the design of a hydraulic PTO is presented, and the performance of the PTO is discussed.

Determination of optimal parameters for a hydraulic power take-off unit of a wave energy converter in regular waves

Wave energy has the potential to be a major provider of renewable energy, especially in the UK. However, there is the major problem of producing efficient devices for a wide variety of sites with different operating conditions. This article addresses the time domain modelling of a heaving point absorber connected to a hydraulic power take-off (PTO) unit in regular waves. Two cases for the hydraulic PTO unit are considered: an ideal model and a model containing losses. Component losses are included to give a more accurate prediction of the maximum power production and to discover if the parameters to optimize the device change when losses are included. The findings show that both cases are optimized by varying the size of the hydraulic motor and the optimal size is only dependent on wave period and the trend is the same for both cases. Results also showed that to maximize the power produced for both cases, there is an optimal force that the unit produces, which can be derived from theory. Finally, power reduction as a result of the hydraulic losses is also observed with efficiencies reducing at larger wave heights.

Dynamic analysis of wave energy converter by incorporating the effect of hydraulic transmission lines

A heaving-buoy wave energy converter equipped with hydraulic power take-off is studied in this paper. This wave energy converter system is divided into five subsystems: a heaving buoy, hydraulic pump, pipelines, non-return check valves and a hydraulic motor combined with an electric generator. A dynamic model was developed by considering the interactions between the subsystems in a state space form. The transient pressures caused by starting/stopping the buoy or closing/opening the check valves were predicted numerically using the established model. The simulation results show that transmission line dynamics play a dominant role in the studied wave energy converter system. The length of the pipeline will not only affect the amplitude of the transient pressures but also affect the converted power. The variation of the time-averaged converted electric power with the pipeline length is estimated using the simulation method for the buoy exposed to one irregular sea state. Finally, it is suggested how reduced power efficiency due to the pipelines may be ameliorated.

Bond graph modelling of a wave energy conversion system with hydraulic power take-off

A system viewpoint is essential in the study of wave energy converters, since several different energy domains are involved in such devices. In this regard, bond graph, a graphical, port-based approach to modelling engineering systems, serves as a useful tool. This article presents a study of a wave energy conversion system with hydraulic power take-off. With the aid of bond graph, two alternative hydraulic system designs are modelled by assembling hydraulic subsystems/components in different manners. A shallow-water pitching wave energy conversion system is considered as a case study, and selected simulation results using the two alternative hydraulic system designs are presented. In addition, this article suggests how to model by bond graph the dynamics of a multi-body wave energy conversion system.

Control strategies for a wave energy converter connected to a hydraulic power take-off

Among the various types of wave energy converters currently being developed, heaving point absorbers are one of the simplest and most promising concepts. A typical efficient energy conversion system for point absorbers is based on hydraulic power take-off (PTO) systems, consisting in a double-acting cylinder, a hydraulic motor and two or more accumulators. This paper presents a simple model of a heaving oscillating buoy extracting power by means of a hydraulic system. The hydrodynamic behaviour of the absorber is modelled through application of the linear water wave theory. Apart from the basic elements listed above, the model of the hydraulic system includes leakages and pressure losses and takes into account the compressibility of the fluid. Also, possible control accumulators are considered in order to improve the performance of the hydraulic system by means of properly controlled valves. Different control variables are analysed depending on the wave inputs considered in order to improve the power extraction of the converter. The results prove that it is possible to achieve a great enhancement of the power extraction with the implementation of these control strategies and that a possible combination of some of them might be beneficial for improved efficiency of the components.

Numerical Modeling of Wear Damage in Seals of a Wave Energy Converter with Hydraulic Power Take-Off under Random Loads

Understanding and managing the safety and reliability issues of ocean energy converters is a key factor in the development of viable technologies. The present article deals with wear in the high-pressure hydraulic power take-off machinery in a heaving-buoy wave energy converter. In this study, an abrasive wear model for such machinery is developed for the steady-state operation by considering the effects of sealed pressure, relative velocity, material properties, time-varying environmental conditions, and lubrication, with a focus on the piston ring. In particular, the dependence of the wear damage on the sea state condition is investigated. The dynamic parameter named instantaneous wear work rate (Load × Velocity) is numerically calculated in the time domain. An estimate of the average wear work rate in each sea state is computed on the basis of a wave scatter table (significant wave heights versus peak period). Long-term wear damage can then be accounted for by summing up the short-term wear damage multiplied with the occurrence probability of each sea state. Finally, the relative contribution of wear damage from different sea states is obtained.

Analysis of dynamic effects relevant for the wear damage in hydraulic machines for wave energy conversion

The present paper deals with a mathematical model of a heaving-buoy Wave Energy Converter (WEC) equipped with high-pressure hydraulic power take-off machinery for energy conversion. This model is based on linear hydrodynamic theory, and a hybrid frequency–time domain model is used to study the dynamics of the heaving-body exposed to an irregular incident wave. For the power take-off system, end-stop devices are provided to protect the hydraulic machinery when the buoy is exposed to severe sea states. The model also takes into account the lubricated friction force and pressure drops of orifice flow through the valves in the hydraulic system. All the forces mentioned in the hydraulic power take-off machinery have non-linear features. A complete non-linear state space model for the WEC system is presented in this study. The WEC system was numerically simulated for different cylinder lengths under a fixed volume. The effect of fluid compressibility in the cylinder has been investigated in the mathematical model. High frequency oscillations (HFOs) caused by the compressibility of the fluid are displayed in the time series and in corresponding power spectra, and variation is shown for different cylinder sizes. Piston ring and cylinder bore wear damage is estimated by using Archard’s equation on the basis of the simulation results. A comparison of these results with a performance of an identical WEC system which neglects fluid compressibility has been done in this work. It shows that although the spectral power is small, HFO can make a large contribution to both the ring and cylinder bore wear. For the purpose of wear prediction, oscillations at or below the wave frequency and HFO may be equally important.

Declutching control of a wave energy converter

When hydraulic power take off (PTO) is used to convert the mechanical energy of a wave energy converter (WEC) into a more useful form of energy, the PTO force needs to be controlled. Continuous controlled variation of the PTO force can be approximated by a set of discrete values. This can be implemented using either variable displacement pumps or several hydraulic cylinders or several high pressure accumulators with different pressure levels. This pseudo-continuous control could lead to a complex PTO with a lot of components. A simpler way for controlling this hydraulic PTO is declutching control, which consists in switching on and off alternatively the wave energy converter's PTO. This can be achieved practically using a simple by-pass valve. In this paper, the control law of the valve is determined by using the optimal command theory. It is shown that, theoretically when considering a wave activated body type of WEC, declutching control can lead to energy absorption performance at least equivalent to that of pseudo-continuous control. The method is then applied to the case of the SEAREV wave energy converter, and it is shown than declutching control can even lead to a higher energy absorption, both in regular and irregular waves.

A wave-to-wire model of the SEAREV wave energy converter

This paper describes a numerical wave-to-wire model of the second-generation wave energy converter called SEAREV. Governing equations are given in the time domain for the motion of the masses involved in the device and for the hydraulic power take-off (PTO) used to convert the motion into electricity. The hydrodynamic forces are derived using the standard linear potential theory. The memory term in the radiation force is replaced by additional states using the Prony method in order to change the equation of motion into the ordinary differential equation form. The PTO is composed of hydraulic rams, an accumulator, and a hydraulic generator, which delivers electricity when there is enough energy stored in the accumulator. Using the MATLAB Simulink tool, the equation of motion is solved to simulate the full device (including the power take-off) from the incident wave to the electricity delivered to the grid. Simulation results are presented in the paper and comparisons are made with a simpler PTO: a linear damper. They show that the torque applied to the hydraulic PTO must exceed a threshold to start absorbing energy, unlike the linear damping model. They also show that the power production can be very discontinuous, depending on the level of the incident wave power. This is due to the fact that the generator considered can transform the energy stored in the accumulator faster than the energy transmitted by the rams into the accumulator. It could therefore be interesting to use several generators to adapt the electrical energy production to the level of incident wave power, or a generator that could work efficiently at part load in order to achieve continuous energy production.

Modelling and control of oscillating-body wave energy converters with hydraulic power take-off and gas accumulator

Oscillating bodies constitute a major class of wave energy converters, especially for offshore deployment. In general, such converters use as power take-off system either a hydraulic system (high-head water turbine or high-pressure-oil hydraulic motor) or a directly driven linear electric generator. The paper addresses the former case, especially when a gas accumulator is employed as energy storage device that provides a smoothing effect to the electric power output. Although there are several wave energy projects currently at different stages of development (including prototype testing) that fit this description, the paper is intended to be a general modelling study rather than to concentrate on a particular device. Special attention is devoted to power take-off performance and design, and to the control of the system.

Laboratory experiment on heaving body with hydraulic power take-off and latching control

A heaving axisymmetric floating body is tested with sinusoidal incident waves in a wave channel. It is connected to the piston of a pump, and it may be latched by an electromagnetic mechanism. Experimental results are compared with a linear mathematical model, for heave response, hydrodynamic parameters, absorbed wave power and converted hydraulic power. Heave resonance occurs at 1.1 Hz. For sub-resonant frequencies, latching control results in a significant increase in heave response and in absorbed, as well as converted, power. Hydraulic energy capture increases by a factor of 2.8 or 4.3 for frequency 0.75 or 0.5 Hz, respectively.

The OPD Pelamis WEC: Current status and onward programme (2002)

SYNOPSIS Ocean Power Delivery Ltd is developing an offshore Wave Energy Converter concept known as Pelamis. The device is a free-floating articulated cylinder, with hydraulic power take-off, suitable for general deployment in water depths over 50 metres. Survivability is the core theme but various techniques are employed to enhance power capture. The device is an assembly of “available” technology reducing the required development time. A 1/7th scale prototype incorporating the full hydraulic power take-off, control and data acquisition systems has been tested in the Firth of Forth. A full-scale pre-production prototype is due to be built, launched and deployed at the new Orkney Marine Energy Test Centre within the next 12 months. This paper describes the Pelamis development programme to date [1] with goals for the onward programme.