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Offshore wave energy converters

Exploitable results

Experimental values have been obtained for optimal (complex conjugate) control of an Edinburgh duck model in the presence of unidirectional monochromatic incident waves, in a 1-dimensional test tank of intermediate depth. These results are used to predict values at full scale in the presence of unidirectional Pierson-Moscowitz wave spectra in 100 m water depth. Four full scale configurations were considered. Force, velocity, acceleration and displacement for complex conjugate control in each configuration were calculated. Also, 2 suboptimal control strategies outlined, as were the coefficients required to achieve them. Estimates of efficiency were made for the implementation of these strategies in each of the 4 cases.
An experimental offshore point absorber 45 kW wave power converter was installed and tested on 30 m of water in the Danish part of the North Sea between 1989 and 1990 (phase 1). The most important experience gained from phase 1 is as follows: valves should be constructed as many, relatively small units instead of few large units; anchor lines should be designed in such a way that the rope and end fittings are not subjected to mechanical impacts (universal joints or bending strain reliefs are recommended); the components of a test plant should be dimensioned in such a way that failure of a single component does not lead to failure of others; large piston pumps require accurate construction, good wearability and low friction; submerged turbines and generators with fixed speed connected to the grid should be used. Fouling on the lining of the piston and pump tube did not take place while the plant was in operation. The bottom structure is founded directly in sandy sea bottom without any particular safety measures against scouring. Over 11 months only very little scouring was observed. On a sandy bottom it is recommended that a skirt is used along the periphery of the structure. Sedimentation in the pump housing itself was negligible. The construction experience has been considerable and very valuable for further development. A smaller, but further developed 1-2 kW pilot plant is being planned (phase 2). The plant is expected to be installed in the North Sea near Hanstholm in May 1994. The power converter will be equipped with a short term energy storage, a partly air filled buffer chamber, to create a smooth and steady flow through the turbine.
Free oscillation transient experiments have been carried out on a system of 2 oscillating water columns (OWC). The system is a model in scale 1:10 with operable air valves. Hydrodynamic parameters characterizing the system have been derived by analysing measured data from the transient oscillations of the water columns, excited by a pressure step applied to 1 of the 2 air chambers. The experimental data may also be used for setting up a time domain mathematical model. The excitation volume flux coefficient qe, has been derived from the radiated wave, by means of reciprocity relations. However, the measurements of the radiated wave were influenced by crosswaves in the flume. These crosswaves were not measured, so additional experiments have to be done in order to determine qe with sufficient accuracy. Preliminary tests suggest that the simulation model operates as expected. The fast Fourier transform (FFT) analysis will provide the correct values of a frequency spectrum only in a finite number of points, equally spaced in the frequency domain. Hence, significant information may be lost in regions where a quantity varies rapidly with frequency. This results in an uncertainty in the peak values of the spectrum. The use of transient experiments has proved to be an easy and powerful approach for retrieving information about the system. The hydrodynamic parameters found in this work show good agreement with results derived by incident wave experiments or by theoretical means. Experiments have shown that the determined hydrodynamic parameters to some degree depend on the initial applied pressure. This phenomenon provides information on nonlinear loss mechanisms in the system.
A wave power converter based on a float connected to a single working piston pump placed on the sea bed has been studied. In order to design such a system it is essential to have a model that is able to predict the motions of the float, the forces and angles of excursion in large waves. Key aspects of the system have been modelled and the assumptions and simplification relating to hydrodynamics have been considered. Also, the areas needing further attention have been noted. The research involved work on differential equations for describing the system and numerical methods used in the computerized solving of the nonlinear mathematical equations. The model can provide time series of float motion, wire forces and power production. Measurements of the wire forces in regular waves were compared to numerical time domain simulations. An experimental procedure has been developed. Average absorbed hydraulic power in different sea states are presented in terms of a semiempirical relation between the float diameter and the significant wave height. Increased energy absorption by implementing control are expected. Areas for future development for the numerical model and its application to related device types have been identified.
Large wave power plants consisting of a large number of wave energy converters constructed over a relatively short period, will require rationalized industrial production, installation and maintenance procedures. A wave power plant of 300 MW with a planned construction period of 5 years is considered. The plant consists of 2640 modules each of 135 kW. The modules are constructed of concrete and the industrial fabrication plant needed for the production and assembling of the structural elements has been discussed. As part of the wave energy research carried out in Denmark a preliminary assessment of the feasibility of 300 MW plants has been investigated at different offshore locations in Denmark, Ireland, Norway, Portugal and New Zealand. A wave energy converter suited for the Danish part of the North Sea and the production plant located in Denmark has been designed. It would be possible to site 2 plants of 300 MW each in the Danish part of the North Sea. Requirements with regard to the storage of converters being produced and equipment for launching and transporting the modules to sea have been discussed. Relations between daily production rates and storage capacity were presented as well as methods of installing devices on the seabed and connecting them to the grid. Aspects and methods of device maintenance were also analysed.
Modifications have been made to the capital cost, productivity and availability of the 2000 MW spine based duck scheme. The central cost estimate of the 1992 Thorpe review is 16 p/kWh, very close to the cost of wind energy in California in 1985 (an output of 671 million kWh from an investment of 1920m dollars) but certainly not low enough for any development in the United Kingdom. Despite the extra for bridges, links, higher torque and pressure accumulators, the effects of the removal of the gyro canisters, the use of thinner concrete, the reuse of shuttering and the coronet joints rather than Hooke's joints should reduce the capital cost by a factor of about 2. The extra productivity of complex conjugate control should give us an extra 1.5 times the energy at the generator terminals. Most important of all will be the development of methods to send electricity reliably along the spine during the productive months. This should deliver about twice the energy to land.
The hose pump wave energy converter has been tested in Lake Lygnem South of Goeteborg (1980-1981). The 5 converters were interconnected by a main pipe drawn ashore to a Pelton turbine and generator. This system gave good results and experience to increase the size and move into the sea. During 1983 to 1984 a demonstration plant consisting of 3 converters with a main pipe and turbine/generator ashore was tested in Kattegatt. In 1986/1987 a hose pump lightbuoy was developed and sold to Pharos Marine Ltd, United Kingdom. This buoy is now undergoing preserial production tests. The results from the trials have been used to verify a 1-dimensional time-domain mathematical model that simulates the behaviour of 1 converter. The input to the model is converter data and 1-dimensional wave time series generated by software. This software gives the vertical coordinate of the sea surface from a given spectrum and a given significant wave height/mean wave time period pair. The mathematical model is used to calculate the possible energy capture on different sites. Along with the mathematical modelling a method for estimating yearly energy production has been developed. Economic evaluations of wave power plants designed for different sites have been undertaken. The most thorough one is the Bremanger study which involves: energy production, electrical system, electrical losses, installation procedure, operation and maintenance, and cost evaluation. A similar study has been done for the Baltic Sea. The last 2 studies have been scrutinized by the Swedish State Power Board. Due to the lack of government funding a hose pump wave energy power plant has not yet been a reality in spite of the low energy cost per kWh.
The motion of a solo duck wave energy absorber has been investigated using 3-dimensional linear water wave diffraction theory. Numerical predictions of hydrodynamic coefficients were compared with analytical results for a hemisphere, and experimental results for a solo duck. A 3-dimensional linear wave diffraction computer program can be used as a productive design tool for the linear regime. Singularities in the damping matrix gave rise to large excursions under complex conjugate control, even in short waves. The control strategy is modified by the application of motion constraints. Even when the constraints are imposed, the duck is able to absorb power from outside the width of sea it occupies. A 10 m diameter, 29 m wide, solo duck has a relative capture width of a little over 2 for the centre of the South Uist wave spectrum in power levels of about 35 KW/metre. The efficiency of the 6 degrees of freedom system is larger in short oblique waves than in short head on waves. This suggests that the performance of the duck may be improved by modifying its geometry.
A horizontal, submerged cylinder oscillating in heave and surge has been investigated. Preliminary measurements indicate that superposition of results obtained with the cylinder oscillating in pure surge and pure heave do not necessarily yield correct results for the case of oscillation in both modes at the same time. On deep water, a difference in the amplitude of the x and z (horizontal and vertical direction) components of the excitation force of 30% has been observed. This agrees with the difference of 65% in radiation resistances in surge and heave, found from measurement of radiated waves. With the cylinder in circular (but nonspinning) motion, it seems that the radiated waves cannot be calculated by superposition of waves found in experiments where the cylinder oscillates in only one mode at a time. Further, measurements of forces which are in phase with the cylinder acceleration, indicate that the added mass associated with the x and z components of the cylinder oscillation are different from those found when the cylinder is oscillating in only one mode (surge or heave).
In order to improve the prospect for wave energy, it is argued that only a small fraction of the average incident wave should be converted into useful energy, since an optimally designed wave energy converter (WEC) converts a maximum fraction of the incident wave energy in rather moderate seas only. Secondly, each WEC unit should be of a relatively small size. Thus a large wave power plant should consist of many equal mass produced units. The design principles advocated have relatively less variation in the power output than there is in the natural wave energy transport. This reduces the required size of the short time energy store which, due to the variation of wave groups, is needed for WECs which deliver electricity to a grid. Because of the evening of the power variation, the primary mechanical energy delivered to the energy store has a higher quality, and hence higher value, than for similar larger WEC which operates at its full design capacity a smaller fraction of its lifetime, and hence has a less power evening ability. A larger and more costly WEC would produce more energy, however with increased power fluctuation. A design study, including economic considerations, is required in order to determine the best design dimensions for a wave power plant with a given location and wave climate. The result of such a study will probably be a design where the design constraints come into play during a significant portion of the lifetime of the wave power plant. This necessarily means a relatively small size of the WECs.
An optimal (complex conjugate) control strategy has been simulated for a 0.1 m diameter duck model. Experimental conditions are such at the model operates at very low Keulegan Carpenter numbers. The experimental results agree well with predictions based on experimentally derived coefficients. The power absorbed by the duck is inferred from the incident, reflected and transmitted wave amplitudes, and from the device forces and velocities. The power extracted from the waves appears to be approximately 15% greater than the power absorbed by the device. Evidence suggests that the missing power results from very small errors in the relative phases of force and velocity, from bearing losses in the pitch axis and from viscous losses at the model water interface which are several times larger than those predicted by Stokes-Wang approximations.
The conversion of ocean wave energy requires a technology able to meet thedestructive forces during storm conditions and yet produce energy at a reasonableprice over a long service period. Many different ideas for the ideal technology havebeen proposed over the years; the main objective here was to focus on a suitabletechnology to act as both a test bed for components and for the demonstration ofwave energy power conversion. The wave energy conversion technologies studiedwere composed of floats on the surface of the sea activated by waves, convertingthe power through hydraulic systems driving an electrical generator. Several methodswere compared using numerical tools developed as part of the project. Project Work

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