Experimental Validation of a Spar Buoy Design for Wave Energy Conversion

Transcription

1 Marine Renewables Infrastructure Network Infrastructure Access Report Infrastructure: NAREC Large Scale Wave Flume User-Project: SPAR BUOY WEC Experimental Validation of a Spar Buoy Design for Wave Energy Conversion IDMEC/IST, Tecnalia and Kymaner Status: Final Version: 01 Date: 02-May-2012 EC FP7 Capacities Specific Programme Research Infrastructure Action

2 ABOUT MARINET MARINET (Marine Renewables Infrastructure Network for emerging Energy Technologies) is an EC-funded network of research centres and organisations that are working together to accelerate the development of marine renewable energy - wave, tidal & offshore-wind. The initiative is funded through the EC's Seventh Framework Programme (FP7) and runs for four years until The network of 29 partners with 42 specialist marine research facilities is spread across 11 EU countries and 1 International Cooperation Partner Country (Brazil). MARINET offers periods of free-of-charge access to test facilities at a range of world-class research centres. Companies and research groups can avail of this Transnational Access (TA) to test devices at any scale in areas such as wave energy, tidal energy, offshore-wind energy and environmental data or to conduct tests on cross-cutting areas such as power take-off systems, grid integration, materials or moorings. In total, over 700 weeks of access is available to an estimated 300 projects and 800 external users, with at least four calls for access applications over the 4-year initiative. MARINET partners are also working to implement common standards for testing in order to streamline the development process, conducting research to improve testing capabilities across the network, providing training at various facilities in the network in order to enhance personnel expertise and organising industry networking events in order to facilitate partnerships and knowledge exchange. The aim of the initiative is to streamline the capabilities of test infrastructures in order to enhance their impact and accelerate the commercialisation of marine renewable energy. See for more details. Partners Ireland University College Cork, HMRC (UCC_HMRC) Coordinator Sustainable Energy Authority of Ireland (SEAI_OEDU) Denmark Aalborg Universitet (AAU) Danmarks Tekniske Universitet (RISOE) France Ecole Centrale de Nantes (ECN) Institut Français de Recherche Pour l'exploitation de la Mer (IFREMER) United Kingdom National Renewable Energy Centre Ltd. (NAREC) The University of Exeter (UNEXE) European Marine Energy Centre Ltd. (EMEC) University of Strathclyde (UNI_STRATH) The University of Edinburgh (UEDIN) Queen s University Belfast (QUB) Plymouth University(PU) Spain Ente Vasco de la Energía (EVE) Tecnalia Research & Innovation Foundation (TECNALIA) Netherlands Stichting Tidal Testing Centre (TTC) Stichting Energieonderzoek Centrum Nederland (ECNeth) Germany Fraunhofer-Gesellschaft Zur Foerderung Der Angewandten Forschung E.V (Fh_IWES) Gottfried Wilhelm Leibniz Universität Hannover (LUH) Universitaet Stuttgart (USTUTT) Portugal Wave Energy Centre Centro de Energia das Ondas (WavEC) Italy Università degli Studi di Firenze (UNIFI-CRIACIV) Università degli Studi di Firenze (UNIFI-PIN) Università degli Studi della Tuscia (UNI_TUS) Consiglio Nazionale delle Ricerche (CNR-INSEAN) Brazil Instituto de Pesquisas Tecnológicas do Estado de São Paulo S.A. (IPT) Norway Sintef Energi AS (SINTEF) Norges Teknisk-Naturvitenskapelige Universitet (NTNU) Belgium 1-Tech (1_TECH) Page 2 of 17

3 DOCUMENT INFORMATION Title Distribution Document Reference User-Group Leader, Lead Author User-Group Members, Contributing Authors Infrastructure Accessed: Infrastructure Manager (or Main Contact) Infrastructure Access Report: SPAR BUOY WEC Experimental Validation of a Spar Buoy Design for Wave Energy Conversion Public MARINET-TA1-SPAR BUOY WEC Luís M. C. Gato IDMEC/Instituto Superior Técnico Av. Rovisco Pais Lisboa, Portugal Antonio Falcão IDMEC/IST João Henriques IDMEC/IST Rui Gomes IDMEC/IST Pedro Vicente IDMEC/IST Pablo Ruiz-Minguela Tecnalia Pierpaolo Ricci Tecnalia José Varandas Kymaner Luís Trigo Kymaner NAREC Large Scale Wave Flume Jamie Grimwade REVISION HISTORY Rev. Date Description Prepared by (Name) Draft version Rui Gomes Approved By Infrastructure Manager Status (Draft/Final) Page 3 of 17

4 ABOUT THIS REPORT Infrastructure Access Report: SPAR BUOY WEC One of the requirements of the EC in enabling a user group to benefit from free-of-charge access to an infrastructure is that the user group must be entitled to disseminate the foreground (information and results) that they have generated under the project in order to progress the state-of-the-art of the sector. Notwithstanding this, the EC also state that dissemination activities shall be compatible with the protection of intellectual property rights, confidentiality obligations and the legitimate interests of the owner(s) of the foreground. The aim of this report is therefore to meet the first requirement of publicly disseminating the knowledge generated through this MARINET infrastructure access project in an accessible format in order to: progress the state-of-the-art publicise resulting progress made for the technology/industry provide evidence of progress made along the Structured Development Plan provide due diligence material for potential future investment and financing share lessons learned avoid potential future replication by others provide opportunities for future collaboration etc. In some cases, the user group may wish to protect some of this information which they deem commercially sensitive, and so may choose to present results in a normalised (non-dimensional) format or withhold certain design data this is acceptable and allowed for in the second requirement outlined above. ACKNOWLEDGEMENT The work described in this publication has received support from MARINET, a European Community - Research Infrastructure Action under the FP7 Capacities Specific Programme. LEGAL DISCLAIMER The views expressed, and responsibility for the content of this publication, lie solely with the authors. The European Commission is not liable for any use that may be made of the information contained herein. This work may rely on data from sources external to the MARINET project Consortium. Members of the Consortium do not accept liability for loss or damage suffered by any third party as a result of errors or inaccuracies in such data. The information in this document is provided as is and no guarantee or warranty is given that the information is fit for any particular purpose. The user thereof uses the information at its sole risk and neither the European Commission nor any member of the MARINET Consortium is liable for any use that may be made of the information. Page 4 of 17

5 EXECUTIVE SUMMARY This work concerns the experimental assessment of the energy extraction performance of the OWC spar buoy device equipped with a turbine simulator, under regular and irregular wave conditions. For this purpose, a 1:16 th -scale model with 1 m diameter and 3 m draft was built and tested at NAREC s large scale wave flume. The acquired data consisted in free surface elevations, pressure variations inside the OWC pneumatic chamber, buoy motion and mooring forces. The test programme comprised the testing of several model configurations: four turbine simulators, two buoy ballast, and two bottom slack-mooring connections. The device presented a peak efficiency for sea state with an energy period of 9 s. In regular wave tests was detected an undesirable dynamic instability when the period was half the roll/pitch natural period. However, this effect was not observed in irregular wave conditions. Large variations on the turbine damping coefficient do not represent large changes in the power extraction under irregular wave conditions.. Figure 1.1-1:16 scale model testing of the OWC spar buoy at NAREC, UK, Sept-Oct Page 5 of 17

6 CONTENTS 1 INTRODUCTION & BACKGROUND INTRODUCTION DEVELOPMENT SO FAR Stage Gate Progress Plan For This Access OUTLINE OF WORK CARRIED OUT SETUP TESTS Test Plan RESULTS ANALYSIS & CONCLUSIONS MAIN LEARNING OUTCOMES PROGRESS MADE Progress Made: For This User-Group or Technology Progress Made: For Marine Renewable Energy Industry KEY LESSONS LEARNED FURTHER INFORMATION SCIENTIFIC PUBLICATIONS WEBSITE & SOCIAL MEDIA REFERENCES APPENDICES STAGE DEVELOPMENT SUMMARY TABLE Page 6 of 17

7 1 INTRODUCTION & BACKGROUND Infrastructure Access Report: SPAR BUOY WEC 1.1 INTRODUCTION Offshore devices are the most appropriate for the extensive exploitation of the wave energy resource. The wave energy converter that is being developed for large scale offshore energy production by the IDMEC/IST wave energy group is a Spar Buoy. It is an axisymmetric device (insensitive to wave direction) consisting basically of a submerged vertical tail tube open at both ends, fixed to a floater that moves essentially in heave. The air flow displaced by the motion of the oscillating water column relative to the buoy drives a self-rectifying air turbine. Tests of a 1:150 th -scale model were already carried out at IST wave flume (20m x 0.7m x 0.5m). A 1:35 th -scale model was also build and tested in Portugal at Faculty of Engineering of the University of Porto wave tank (28m x 12m x 1m with 1.5m deep central pit). Due to the limited water depth and the use of a central pit, it was not possible to replicate the full scale conditions. In Portugal, wave testing facilities are limited in dimensions and do not allow the testing of the Spar Buoy model at a scale higher than 1:35. The testing of the model at NAREC's Large Scale Wave Flume (NAREC-LSWF) would overcome those problems besides providing a more realistic physical modelling by using a 1:16 th -scale model Access to NAREC- LSWF would allow the team to validate the numerical models and benefit from the experience of the staff of a world-class wave testing facility. 1.2 DEVELOPMENT SO FAR Stage Gate Progress Previously completed: Planned for this project: STAGE GATE CRITERIA Stage 1 Concept Validation Linear monochromatic waves to validate or calibrate numerical models of the system ( waves) Finite monochromatic waves to include higher order effects ( waves) Hull(s) sea worthiness in real seas (scaled duration at 3 hours) Restricted degrees of freedom (DofF) if required by the early mathematical models Provide the empirical hydrodynamic co-efficient associated with the device (for mathematical modelling tuning) Investigate physical process governing device response. May not be well defined theoretically or numerically solvable Real seaway productivity (scaled duration at minutes) Initially 2-D (flume) test programme Short crested seas need only be run at this early stage if the devices anticipated performance would be significantly affected by them Evidence of the device seaworthiness Initial indication of the full system load regimes Stage 2 Design Validation Accurately simulated PTO characteristics Performance in real seaways (long and short crested) Survival loading and extreme motion behaviour. Active damping control (may be deferred to Stage 3) Device design changes and modifications Mooring arrangements and effects on motion Status Page 7 of 17

8 STAGE GATE CRITERIA Data for proposed PTO design and bench testing (Stage 3) Engineering Design (Prototype), feasibility and costing Site Review for Stage 3 and Stage 4 deployments Over topping rates Stage 3 Sub-Systems Validation To investigate physical properties not well scaled & validate performance figures To employ a realistic/actual PTO and generating system & develop control strategies To qualify environmental factors (i.e. the device on the environment and vice versa) e.g. marine growth, corrosion, windage and current drag To validate electrical supply quality and power electronic requirements. To quantify survival conditions, mooring behaviour and hull seaworthiness Manufacturing, deployment, recovery and O&M (component reliability) Project planning and management, including licensing, certification, insurance etc. Stage 4 Solo Device Validation Hull seaworthiness and survival strategies Mooring and cable connection issues, including failure modes PTO performance and reliability Component and assembly longevity Electricity supply quality (absorbed/pneumatic power-converted/electrical power) Application in local wave climate conditions Project management, manufacturing, deployment, recovery, etc Service, maintenance and operational experience [O&M] Accepted EIA Stage 5 Multi-Device Demonstration Economic Feasibility/Profitability Multiple units performance Device array interactions Power supply interaction & quality Environmental impact issues Full technical and economic due diligence Compliance of all operations with existing legal requirements Status Plan For This Access Objectives The scientific objectives of this work concern the experimental assessment of the energy extraction performance and validation of numerical models, under regular and irregular waves, of the OWC spar buoy equipped with a turbine simulator. The IDMEC\IST WEG has already developed the numerical tools to model the energy conversion chain, from the hydrodynamics to the turbine power output. The hydrodynamic optimization was performed by solving the time domain equations of motion and using a boundary-element method to compute the hydrodynamic coefficients. Tecnalia wave energy group used dynamic simulation and analysis tools to validate the design the device's mooring system. These tests allowed the team to simulate an array of equally-spaced devices positioned perpendicularly to the incident waves. For this purpose, a 1:16 th -scale model with 1 m diameter and 3 m draft was tested at the NAREC- Page 8 of 17

9 LSWF. The tests consisted in the measurement of the system motions and forces under regular and irregular wave conditions. The acquired data consisted in measurements of: the water free surface elevation in several positions (including the measurement of the water column); pressure variations inside the pneumatic chamber; model motion in 6-degreeof-freedom; mooring forces. Seven different configurations were tested by introducing changes in turbine damping coefficient, buoy ballast and slack-mooring stiffness. The regular wave tests were comprised a large range of frequencies and three different wave heights (small, moderate and high) for the assessment of the nonlinear effects. The irregular wave tests were performed based in a Bretschneider wave spectrum (two parameters: significant wave height and energy period), for a range of energy periods and three significant wave heights (low, moderate and high) Analysis of results The experimental data time series were analysed using classical time- and frequency-domain tools developed by the IDMEC\IST WEG. The acquired data was used to compute: flow rate through the turbine simulator; instantaneous and time-averaged power; compressibility effects in the pneumatic chamber; incident time-averaged wave power per meter of wave crest; comparison between specified and measured wave parameters values; floater motion using a 6-degree-of-freedom motion tracking system and a inertial measurement unit; mooring cable loads. The compiled results were compared with the numerical simulations obtained by the IDMEC\IST and Tecnalia wave energy groups. 2 OUTLINE OF WORK CARRIED OUT 2.1 SETUP The NAREC-LSWF has a 60 m length, 5.2 m width and a water depth of 7.1 m. It is equipped with an Edinburgh Designs multi paddle (flap-type) wavemaker acted by a hydraulic system. The wavemaker is hinged at a position above the flume bottom. The dissipative beach consists of a ribbed steel plate with an approximately 10 deg slope. The flume uses sea water. The following equipment was used in the experiments: an ultrasound sensor to measure the internal free surface motion (water column); 2 fast-response differential pressure sensor to measure the pressure inside the pneumatic ; 4 orifice plates (with different orifice diameters) to simulate different turbine s quadratic pressure drop versus flow rate curves; video motion tracking system, which consists of three infrared cameras developed by Qualisys, to measure the buoy motion (see 6-degree-of-freedom inertial measurement unit to measure the buoy motion; 4 magnetostrictive probes to measure the free surface elevation,, one placed aside the model and the others to measure wave reflection; 2 load cells to measure the mooring cables load; a data acquisition system. Figure 2. presents a schematic representation of the position of the instrumentation in the wave flume. One pressure sensor is placed on the model, and the other on the bridge over the flume. The ultrasonic sensor and pressure sensor signals are acquired by the data acquisition system on the buoy. The remaining data is acquired by other data acquisition system placed aside the flume. Page 9 of 17

10 Figure Schematic representation of the model, free surface elevation probes and anchoring points in the wave flume. Figure Schematic representation bottom slack-mooring connections. Figure 2.4 CAD model of the 1:16 th -scale model of the OWC spar buoy. The 1.16 th -scale model of the OWC spar buoy was based on an optimized geometry obtained from previous studies by the IDMEC\IST wave energy group (Gomes et al., 2012). The model was made from steel plate and welded in order to keep each part watertight. The lower part of the model was made of concrete. A small compartment in the lower part of the model was reserved to small removable ballast modules to adjust the specified water level and allow tests with an increased buoy mass. The physical characteristics of the models were computed through the use of a three-dimensional computer-aided design (CAD) software (moment of inertia, centre of buoyancy and centre of Page 10 of 17

11 gravity). These computations were necessary to guarantee the device s stability requirements. The model was connected to the flume floor through two slack-mooring three-section lines, each one with a weight and a buoy, as represented in Figure TESTS The experiments carried out at the NAREC-LSWF comprised the testing of several configuration of the OWC spar buoy model Test Plan The base configuration of the model refers to an optimized configuration of the OWC spar buoy, which is being the subject of research by the IDMEC\IST wave energy group. The variations from the base configuration are done to assess the system sensitivity. The major quantities to be evaluated are the instantaneous and time-averaged power as a function of the turbine simulator damping. Four quadratic pressure drop curves were considered to simulate the characteristic damping of impulse turbines (including the biradial turbine). Additional tests will cover a different draft (by adding ballast), and second mooring configuration (by changing the mooring buoy and weight). The sensitivity analysis is performed for a smaller sub-set of regular and irregular waves. For the experimental setup it was considered the following configurations: four turbine simulators (Turbine A, B, C, D) two ballasts configurations (Configuration A, B) two moorings (Mooring A, B) Regular wave tests were carried out for a set of equally-spaced wave periods, ranging from 1.55 s to 3.87 s. Three different wave heights were considered in order to simulate small (0.100 m), moderate (0.200 m) and high (0.333 m) wave heights. Not all wave heights were tested for each configuration. Each test have run for 400 s. Irregular wave conditions were based in the two-parameter Bretschneider spectrum (significant wave height and energy/peak period). As in the regular wave tests, test were carried out for a set of wave energy periods. Three significant wave heights were considered to simulate low (0.100 m), moderate (0.200 m) and high (0.333 m) energy sea states. Each test have run for 1800 s. The eight parts of the testing programme are described in Table 2.1. The model s configuration A corresponds to the optimum design obtained from numerical simulations. Turbine simulator Mooring type Ballast Wave conditions Configuration A B A A Regular & Irregular Configuration B C A A Regular & Irregular Configuration C D A A Regular & Irregular Configuration D A A A Regular & Irregular Configuration E B A B Regular & Irregular Configuration F C A B Regular & Irregular Configuration G A A B Regular & Irregular Configuration H B B B Regular & Irregular Table Different configurations used in the experimental tests. 2.3 RESULTS The experimental results presented here concerns the irregular wave tests. The graphs vertical axis present the capture width, a measurement of the efficiency of wave energy converters, which represent the ratio between the power extracted and the incoming wave power per meter of wave crest. The capture with is presented in its nondimensional form by dividing by the maximum measured capture width. In the horizontal axis is represented the energy period of the irregular wave sea state. Page 11 of 17

12 Figure 2.5 presents the effect of the significant wave height on the capture width. These curves capture the nonlinear effect introduced by the quadratic effect of the turbine simulator, and of the nonlinear effect of the buoy and mooring system dynamics. The influence of the turbine quadratic damping coefficient on the capture width is shown in Figure 2.6. The turbine damping effect increases from A to D. Figure 2.7 presents the effect of two different ballast (different buoy s mass and draft) on the capture width. Ballast A corresponds to the optimum device configuration and B to an increase of the buoy s mass. The influence of the mooring configuration is presented in Figure 2.8. Mooring A corresponds to the base configuration and mooring B has higher mooring buoy s buoyancy and higher weight mass. Mooring B corresponds to an increase of the mooring system s stiffness. Figure 2.5 Effect of the significant wave height (low, moderate and high) on the capture width against the sea state energy period, for configuration A. Page 12 of 17

13 Figure 2.6 Effect of the turbine damping (turbine simulator A, B, C and D) on the capture width against the sea state energy period, for a moderate energy sea state. Figure 2.7 Effect of the ballast configuration on the capture width against the sea state energy period. Page 13 of 17

14 Figure 2.8 Effect of the turbine damping (turbine simulator A, B, C and D) on the capture width against the sea state energy period, for a moderate energy sea state. 2.4 ANALYSIS & CONCLUSIONS These experiments consisted in the analysis of the OWC spar buoy dynamics and power extraction under regular and irregular wave conditions. The regular wave test allowed a more focussed analysis on the dynamics of the device. It was observed the advantageous two-peak power spectrum characteristic of the two-body system devices (buoy-tube set and water column), which set a broader range of periods with an efficient power extraction. The more marked feature of the regular wave results was the occurrence of a dynamic instability when the wave period was approximately half the natural pitch/roll period. This effect is caused nonlinear hydrostatic effect that is not captured by the usual linear numerical models, however was already expected to occur by the previous experiments. This effect causes the buoy to engage in a large amplitude roll/pitch motion, which removes energy from the desirable heave motion. As a result, a reduction on the power extraction is observed. It was shown that the moorings and buoys mass influence this effect. Under irregular wave conditions this effect was not so evident due to the presence of several wave periods that would not excite the pitch/roll motion. The power extraction presents a peak at the energy period of 9 s. When compared with the small-scale test results, it was also evident a reduction of the scale effect, expressed by the higher nondimensional capture width. The results have shown a decrease of efficient with the wave height increase. This effect is related with the quadratic turbine behaviour (pressure drop versus flow rate) and with nonlinear effects due to fluid viscosity and large buoy motions. Large turbine damping variations (half and twice the design value) present a small influence on the capture width. An excessively damped turbine (turbine D) presented a decrease of 30% in capture width. The increase of the buoy s ballast, with the objective of increasing the buoy metacentre height, and therefore its stability characteristics produces a decrease of 10% at the peak energy period. Above the energy period of 10 s, the differences are negligible. The modification of the slack-mooring system stiffness do not produces significant changes in the power extraction. Page 14 of 17

15 3 MAIN LEARNING OUTCOMES 3.1 PROGRESS MADE Progress Made: For This User-Group or Technology The tests at NAREC were carried out using standard measuring equipment, commonly used in the laboratory, and redundant equipment more adequate for field conditions. The 6-degree-of-freedom inertial measurement unit, which is being developed by the IDMEC/IST group, presents a good solution for the motion measurement of large buoys. The data acquisition system located onboard has shown to be good solution for measurements in the sea. For the first time, the forces on the slack-mooring lines were measured. Its behaviour as shown to be in agreement with the assumptions used in the numerical models used by the IDMEC/IST wave energy group (Vicente et al., 2009, 2011). The results of the system motions and power extraction revealed a smaller influence from the scale effects, namely the viscous effects, whose similitude was not respected in preference to the Froude scaling criterion. These largescale test results have given confidence on the already expected good hydrodynamic efficiency of the OWC spar buoy device Next Steps for Research or Staged Development Plan Exit/Change & Retest/Proceed? The tests carried out at NAREC have fulfilled the expectations set by the research group. They have provided guidelines for the development of more advanced numerical methods, namely the introduction of hydrodynamic nonlinear effects. The next step in the research will be focussed in the analysis of device dynamics under extreme sea conditions and the study of solutions for increasing the reliability of the device. The behaviour of an array of devices is another focus of research. The group it is already planning the model testing of an array of OWC spar buoy in a wave tank, under moderate and extreme wave conditions. In the meanwhile, there is a continuous study on control strategies for the turbine that could increase largely the hydrodynamic efficiency Progress Made: For Marine Renewable Energy Industry From the test results it was shown that considerably large variations on the turbine damping creates a relatively small influence on the time-averaged power extraction under irregular waves. This implies that turbines may not be specifically designed of a given device in order to present a good extraction efficiency. 3.2 KEY LESSONS LEARNED Regular wave testing results revealed the occurrence of a dynamic instability when the wave period was approximately half the natural pitch/roll period. Page 15 of 17

16 4 FURTHER INFORMATION 4.1 SCIENTIFIC PUBLICATIONS List of any scientific publications made (already or planned) as a result of this work: R.P.F. Gomes, J.C.C. Henriques, L.M.C. Gato and A.F.O. Falcão. Tank testing of a 1:16th scale model of the OWC spar-buoy model with slack moorings. In preparation for submission in an international conference. 4.2 WEBSITE & SOCIAL MEDIA Website: YouTube Link(s): LinkedIn/Twitter/Facebook Links: Online Photographs Link: 5 REFERENCES Gomes, R. P. F., Henriques, J. C. C., Gato, L. M. C., Falcão, A. F. O., Hydrodynamic optimization of an axisymmetric floating oscillating water column for wave energy conversion. Renewable Energy 44, Vicente, P. C., Falcão, A. F. de O., Gato, L. M. C., Justino, P. A. P., Dynamics of arrays of floating point absorber wave energy converters with inter-body and bottom slack-mooring connections. Applied Ocean Research 31, Vicente, P. C., Falcão, A. F. de O., Justino, P. A. P., Slack-chain mooring configuration analysis of a floating wave energy converter. In: Proc. 26th International Workshop on Water Waves and Floating Bodies. Athens, Greece. 6 APPENDICES 6.1 STAGE DEVELOPMENT SUMMARY TABLE The table following offers an overview of the test programmes recommended by IEA-OES for each Technology Readiness Level. This is only offered as a guide and is in no way extensive of the full test programme that should be committed to at each TRL. Page 16 of 17

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