SPOWCON Tecnalia Infrastructure Access Report

Infrastructure Access Report

Infrastructure: TECNALIA Electrical PTO Lab

User-Project: SPOWCON

Spar OWC CONtrol

IDMEC/IST and Kymaner

Marine Renewables Infrastructure Network

Status: Final Version: Date: 28-Feb-2014

EC FP7 Capacities Specific Programme Research Infrastructure Action

Infrastructure Access Report: SPOWCON

Rev. 01, 28-Feb-2014 Page 2 of 23

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 2015. 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 www.fp7-marinet.eu 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)

InstitutFranais 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)

Queens University Belfast (QUB)

Plymouth University(PU)

Spain

Ente Vasco de la Energa (EVE)

Tecnalia Research & Innovation Foundation (TECNALIA)

Belgium

1-Tech (1_TECH)

NetherlandsStichting Tidal Testing Centre (TTC)

Stichting Energieonderzoek Centrum Nederland (ECNeth)

Germany Fraunhofer-Gesellschaft Zur Foerderung Der Angewandten Forschung E.V (Fh_IWES)

Gottfried Wilhelm Leibniz Universitt Hannover (LUH)

Universitaet Stuttgart (USTUTT)

Portugal Wave Energy Centre Centro de Energia das Ondas (WavEC)

Italy Universitdegli Studi di Firenze (UNIFI-CRIACIV)

Universitdegli Studi di Firenze (UNIFI-PIN)

Universit degli Studi della Tuscia (UNI_TUS)

Consiglio Nazionale delle Ricerche (CNR-INSEAN)

Brazil Instituto de Pesquisas Tecnolgicas do Estado de So Paulo S.A. (IPT)

Norway Sintef Energi AS (SINTEF)

Norges Teknisk-Naturvitenskapelige Universitet (NTNU)


Rev. 01, 28-Feb-2014 Page 3 of 23

DOCUMENT INFORMATION Title Spar OWC CONtrol Distribution Public Document Reference MARINET-TA1-SPOWCON

User-Group Leader, Lead Author

Lus Gato IDMEC/Instituto Superior Tcnico Av. Rovisco Pais 1049-001 Lisboa, Portugal

User-Group Members, Contributing Authors

Antonio Falco IDMEC/Instituto Superior Tcnico Joo Henriques IDMEC/Instituto Superior Tcnico Rui Gomes IDMEC/Instituto Superior Tcnico Pedro Vicente IDMEC/Instituto Superior Tcnico Jos Varandas Kymaner

Infrastructure Accessed:

TECNALIA Electrical PTO Lab

Infrastructure Manager (or Main Contact)

Eider Robles Sestafe

REVISION HISTORY

Rev. Date Description Prepared by (Name) Approved By Infrastructure

Manager

Status (Draft/Final)

1 28-Feb-2014 Joo Henriques/Rui Gomes


Rev. 01, 28-Feb-2014 Page 4 of 23

ABOUT THIS REPORT 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.


Rev. 01, 28-Feb-2014 Page 5 of 23

EXECUTIVE SUMMARY The present project concerns the development of the power take-off (PTO) control of an oscillating-water-column (OWC) spar buoy, possibly the simplest concept for a floating wave energy converter (WEC). The OWC spar buoy is an axisymmetric device (and so insensitive to wave direction) consisting basically of a (relatively long) submerged vertical tail tube open at both ends, fixed to a floater that moves essentially in heave. The oscillating motion of the internal free surface relative to the buoy, produced by the incident waves, makes the air flow through a radically new concept of a self-rectifying air turbine: the biradial turbine. This new patented turbine is also being developed and tested by IDMEC/IST and Kymaner. The turbine drives an electrical generator and the control of this set is the subject of current tests that were performed at TECNALIA Electrical PTO Lab. To reduce the overall costs of the PTO system, an electrical generator was adopted with a rated power twice the maximum expected average power conversion of the buoy. This level of generator rated power poses great challenges for the PTO control due to the irregular characteristics of the sea waves. The main objective of the experimental work is the assessment of control strategies of the turbine/generator set. The hydrodynamics of the OWC spar buoy and the aerodynamics of the air turbine are simulated in real time and coupled with the experimental model of the turbine/generator set. The instantaneous air turbine torque is emulated through the use of the electrical motor. In the present implementation, only irregular wave conditions were considered. The used hardware-in-the-loop simulator emulates in real-time the behaviour of the OWC spar buoyWEC. The simulator and data logger runs on a computer with Matlab xPC target real-time operating system, which connects the simulation model to the physical model. The simulation model input is the generator/motor (turbine) rotational speed. The computed turbine torque is supplied to the frequencyvariator that drives the motor. The experimental results allowed the characterization of the dynamic behaviour of the PTO, ensure the practical applicability of the proposed control algorithms and provide a basis for the validation of the numerical models.


Rev. 01, 28-Feb-2014 Page 6 of 23

CONTENTS

1 INTRODUCTION&BACKGROUND ...................................................................................................................... 7

1.1 INTRODUCTION .................................................................................................................................................... 7 1.2 DEVELOPMENT SO FAR .......................................................................................................................................... 8 1.2.1 Stage Gate Progress .................................................................................................................................... 8 1.2.2 Plan for This Access ................................................................................................................................... 10

2 OUTLINE OF WORK CARRIED OUT .................................................................................................................. 11

2.1 SETUP ............................................................................................................................................................... 11 2.2 TESTS ............................................................................................................................................................... 12 2.2.1 OWC Spar Buoy Configuration .................................................................................................................. 12 2.2.2 Power control laws .................................................................................................................................... 12 2.2.3 Test Plan .................................................................................................................................................... 14

2.3 ANALYSIS & CONCLUSIONS ................................................................................................................................... 18

3 MAIN LEARNING OUTCOMES ......................................................................................................................... 19

3.1 PROGRESS MADE ............................................................................................................................................... 19 3.1.1 Progress Made: For This User-Group or Technology ................................................................................. 19 3.1.2 Progress Made: For Marine Renewable Energy Industry .......................................................................... 20

3.2 KEY LESSONS LEARNED ........................................................................................................................................ 21

4 FURTHER INFORMATION ................................................................................................................................ 21

4.1 SCIENTIFIC PUBLICATIONS .................................................................................................................................... 21 4.2 WEBSITE & SOCIAL MEDIA ................................................................................................................................... 21

5 ACKNOWLEGEMENTS ..................................................................................................................................... 21

6 REFERENCES ................................................................................................................................................... 21

7 APPENDICES ................................................................................................................................................... 21

7.1 STAGE DEVELOPMENT SUMMARY TABLE ................................................................................................................ 21 7.2 SUMMARY TABLE OF THE PERFORMED TESTS .......................................................................................................... 23


Rev. 01, 28-Feb-2014 Page 7 of 23

1 INTRODUCTION & BACKGROUND

1.1 INTRODUCTION Offshore devices are the most appropriate for the extensive exploitation of the wave energy resource. The IDMEC/IST wave energy group is developing the OWC spar buoy, see Fig. 1a, a WEC adequate for large scale offshore energy production. Possibly, this is the simplest concept for a floating wave energy converter. 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, see Figs. 1b and 1c. The IDMEC/IST OWC spar buoy wave energy converter concept has already completed several important development stages, namely:

A numerical hydrodynamic optimization model based on linear potential theory has been developed to obtain the buoy geometry.

Tests at a 1/120 scale were carried out at IST small wave flume, allowing a preliminary validation of the concept.

Tests at a 1/35 scale have been carried out at the University of Porto wave tank.

A 1/16 scale model of the buoy with 1 m diameter and 3 m draft was built and tested at NARECs large scale wave flume within the framework of the 1st call of the FP7-MARINET programme.

The novel biradial self-rectifying air turbine has been designed, patented and experimentally tested at a 1/4 scale at the IST turbomachinery test rig.

An overview movie about the experimental work performed during these stages can be viewed on YouTube http://youtu.be/1Oa1fBC0_5I . The OWC spar buoy WEC is being developed as an integrated design, covering several energy conversion stages from the buoy hydrodynamics to the turbine aerodynamics. The hydrodynamic performance of the device is being completely characterised by the ongoing numerical simulations and from the results of the tests carried out at NARECs large scale wave flume, whereas the characteristic curve of the air turbine is known from experiments performed at the IST turbomachinery test rig and also from as Computational Fluid Dynamics (CFD) simulations. The IDMEC/IST group has already developed and calibrated the numerical tools to model the energy chain up to the turbine power output. The major task was the integration of this numerical model with the hardware available at TECNALIA, which was already fitted to be coupled with a hardware-in-the-loop simulator. The proposed control strategies were devised to improve the power absorption and ensure the best efficiency, as well as to choose the least expensive combination of PTO equipment. The present experimental work allows the assessment of different control strategies of the turbine/generator set. For this purpose, real-time simulations of the hydrodynamics of the OWC spar buoy and the aerodynamics of the air turbine are coupled with an experimental small-scale electrical motor/generator set, in a hardware-in-the-loop configuration. The air turbine is emulated through an electrical motor coupled to the generator, which imposes the simulation instantaneous torque.


Rev. 01, 28-Feb-2014 Page 8 of 23

a)

b)

c)

Figure 1 a) The IDMEC/IST OWC spar buoy geometry (not to scale). The device is equipped with a biradial turbine, a latching valve in series with the turbine and a relief valve in parallel with the turbine. The latching valve is shown in the closed position. b) The biradial turbine rotor and stator. c) Schematic representation of biradial turbine: cross section showing the axially sliding cylindrical latching valve. at the cal PTO Lab. Test

1.2 DEVELOPMENT SO FAR

1.2.1 Stage Gate Progress Previously completed: 9 Planned for this project:

STAGE GATE CRITERIA Status Stage 1 Concept Validation Linear monochromatic waves to validate or calibrate numerical models of the system (25 100 waves) 9 Finite monochromatic waves to include higher order effects (25 100 waves) 9 Hull(s) sea worthiness in real seas (scaled duration at 3 hours) Restricted degrees of freedom (DoF) if required by the early mathematical models 9 Provide the empirical hydrodynamic co-efficient associated with the device (for mathematical modelling tuning)


Rev. 01, 28-Feb-2014 Page 9 of 23

STAGE GATE CRITERIA Status Investigate physical process governing device response. May not be well defined theoretically or numerically solvable

9

Real seaway productivity (scaled duration at 20-30 minutes) 9 Initially 2-D (flume) test programme 9 Short crested seas need only be run at this early stage if the devices anticipated performance would be significantly affected by them

9

Evidence of the device seaworthiness 9 Initial indication of the full system load regimes 9 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 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 9 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


Rev. 01, 28-Feb-2014 Page 10 of 23

STAGE GATE CRITERIA Status Environmental impact issues Full technical and economic due diligence Compliance of all operations with existing legal requirements

1.2.2 Plan for This Access

1.2.2.1 Objectives The PTO control testing for wave energy converters can be performed in a laboratory environment using a hardware-in-the-loop simulator. The use of a test rig allows the repeatability of tests/working conditions and thus enables the test and comparison of several control strategies using real hardware measurements to be carried out. The present tests are performed with a scaled PTO and its control equipment available at TECNALIA, see Fig. 2. This equipment with appropriate control can run coupled to a hardware-in-the-loop simulator under real sea operating conditions. The control strategies, previously developed using purely numerical models, are tested with real equipment where physical limitations are present. Thus, the viability of concepts from the electric point of view can be analysed. The experimental results allow the team to characterise the dynamic behaviour of the device, assure the practical applicability of the proposed control algorithms and provide a basis for the validation of the numerical models.

1.2.2.2 Analysis of results Most of the analysis and post-processing were carried out during tests. The used simulator has two models that run simultaneously: a complete numerical model of the OWC spar buoy wave energy converter and a hardware-in-the-loop simulation. This approach allows a real-time computer screen plot comparison of the numerical and experimental data. Any deviation or malfunction that might be associated with specific constraints and/or unfeasible requirements is quickly spotted in the computer screen. The resulting data from the numerical and hardware-in-the-loop simulations is stored in computer files for additional post-processing. The time-series include, for both the complete numerical model and the hardware-in-the-loop simulation:

the vertical position of the buoy and the relative position of the oscillating water column; the pressure in the pneumatic chamber; the volumetric flow rate through the turbine and relief valve; the dimensionless volumetric flow coefficient, the dimensionless pressure coefficient and efficiency of the

turbine; the aerodynamic power and torque of the turbine; the generator torque; the rotational speed of the turbine/generator set; the relief valve and latching valve status; experimental measurements of the torque-meter.

For documentation purposes, all the Matlab/Simulink files used in each experimental test were stored. The objective of the collected data is the comparison of results obtained with different sea-state conditions, control strategies and tuning parameters. The remaining post-processing was left to a minimum and only includes mean values, standard deviations and exceedance curves of the instantaneous generator power, turbine power and rotational speed of motor/generator set.


Rev. 01, 28-Feb-2014 Page 11 of 23

Figure 2 Overview of the configuration used by the IDMEC/IST groupat the Electrical PTO Lab test rig. Team

at the cal PTO Lab. test

2 OUTLINE OF WORK CARRIED OUT

2.1 SETUP Figure 2 presents an overview of the configuration used by the IDMEC/IST group at Tecnalia test rig. The test rig can be divided into three main parts: the motor, the generator and the hardware-in-the-loop simulator and data logging. The motor and the generator are coupled through a shaft. To increase the system inertia, a flywheel was rigidly attached to the shaft. The motor part is composed by the motor and frequency converter which can be used to control the output torque via an analogue input signal. The aim of these components is the simulation of the turbine torque under the prescribed sea conditions. The motor torque is an output of the hardware-in-the-loop simulator. The generator part includes the generator, the back-to-back power converter to control the generator and the grid connection, and a PLC with the generator control software. The IDMEC/IST control laws were directly coded in a subroutine of the PLC main code. The PLC control programme was supplied by TECNALIA. This part represents the real equipment that could be connected between the WEC and the grid. The hardware-in-the-loop simulator part emulates the behaviour of the OWC spar buoy WEC. The simulator and data logger runs on a computer with Matlab xPC target real-time operating system, which connects the simulation model to the physical model. In the present implementation, computer simulations of the OWC spar buoy hydrodynamics and turbine aerodynamics are simulated in real-time for different sea-state conditions. The inputs are the generator/motor (turbine) rotational speed and an analogue signal from a torque-meter. The real-time computed turbine torque is supplied as reference torque to the frequency variator that drives the motor. The motor frequency converter was supplied by Leroy-Somer. It is rated at 18 kW and allows peaks of power up to 28 kW. It is suitable to control motors with a maximum speed up to 3000 rpm. It can be controlled remotely via external 4/20 mA signals. Both speed and torque control modes are available. The motor was also supplied by Leroy-Somer. It is a 2 pair of poles squirrel cage induction motor with the following specifications: 15 kW nominal power, 1460 rpm nominal speed and a maximum speed of 1800 rpm.


Rev. 01, 28-Feb-2014 Page 12 of 23

The generator is a squirrel cage induction generator from ABB with the following specifications: 11 kW nominal power, 768 rpm nominal speed and a maximum rotational speed of 1000 rpm.The generator is connected to a 400 V isolated grid by means of a back-to-back bidirectional converter supplied by ABB. This converter is rated at 11 kW (heavy-duty use) and permits a flexible remote control of the generator torque or speed via analogical signals. In the current application, a torque reference calculated by the generator control software is supplied to the frequency converter. The generator control software has been programmed in a PLC from Beckhoff with similar characteristics to the one that may be used in a real system. The controller and the generator frequency converter communicate through several analogical and digital inputs/outputs. In this way the generator rotational speed is fed into the PLC. The PLC executes a state machine that handles the possible errors and, in case of no error, carries out the control algorithms. The output of the controller is the electrical torque that needs to be applied to the generator. This torque reference is sent to the power converter through a 4/20 mA analogue output terminal. The controllers embedded in the commercial frequency converter will follow this torque reference and will control the DC bus voltage and the power injected into the grid. In order to manage the PTO system, the state machine has been implemented in the PLC controller. The state machine uses only 6 states to control the whole system.

2.2 TESTS

2.2.1 OWC Spar Buoy Configuration The IDMEC/IST OWC spar buoy configuration used in the simulations is shown in Fig. 1. To limit the rotational speed of the turbine/generator set and/or to prevent overloading the electrical generator, the buoy was tested with two configurations, see Fig. 1:

a latching valve mounted in series with the biradial turbine and; a relief valve mounted in a parallel configuration with respect to the turbine.

The valves are never operated simultaneously. Due to the stator geometry, the latching valve is actuated with a vertical sliding motion with typical amplitude of less than 10 cm. The latching valve can be used with two purposes: i) to cut the mass flow that feeds the turbine when the rotational speed exceeds a given threshold and/or ii) to perform phase control of the buoy in order to increase the overall power output. The relief valve is used to control the pneumatic pressure in highly energetic sea conditions. By reducing the pneumatic pressure, the turbine shaft torque is reduced thus avoiding excessive speed of the turbine/generator set. This approach reduces the possibility of overloading the electrical generator to limit its rotational speed. The typical diameter of this type of valve, for the current configuration of the OWC spar buoy, is 1.5 m.

2.2.2 Power control laws A simple control law for the generator that has been proposed and tested numerically, based on the hydrodynamics of the wave energy absorption by the OWC spar buoy and on the aerodynamics of air turbines [1,2], is of the form

, (1)where is the generator resistive power and and are constants to be determined for the type and geometry of the OWC device and air turbine. The exponent is exactly equal to 3 if the time-averaged turbine aerodynamic efficiency is to be maximized, and was found to slightly exceed this value for maximum overall (OWC plus turbine) time-averaged efficiency, i.e. if the power output of the turbine is to be maximized [1,2]. In the present work, constants and are obtained using a least-squares exponential regression of the maximum power output values obtained by an optimization algorithm, for a given set of sea-states characteristic of the wave climate off the Portuguese Western coast. In the optimization algorithm, the rotational speed was assumed not to be constrained by such factors as centrifugal stresses, occurrence of shock waves or electrical generator limitations.


Rev. 01, 28-Feb-2014 Page 13 of 23

Figure 3 - Generator power control laws. a) The basic control law, , obtained from exponential regression of the maximum power extraction computed for a set of sea-states characteristic of the wave climate off the Portuguese west coast. b) Basic control law combined with two hysteresis loops where the maximum value is the generator rated power, . c) The curve used in Fig. 3a with the maximum power clipped to the generator rated power. d) Modification of curve plotted in Fig. 3c, where the generator power increases smoothly from zero to the basic control law between 500 rpm and 800 rpm.

The tested dimensionless generator power control laws are plotted in Fig. 3. The basic control law A1 is given by

,

where is given by Eq. (1) and is the generator rated power of the prototype. The major drawback of the basic control law is the lack of limitation of the generator power if the rotational speed goes above a certain threshold. In control law A2, see Fig. 3b, the basic control law is combined with a hysteresis loop where the upper limit value is the generator rated power. In order to avoid unstable behaviour, a hysteresis exists between the switch-on and switch-off threshold. Control law A3, shown in Fig. 3b, is similar to curve A2, but the hysteresis loop is applied in a narrow rotational speed interval. Due to the existence of the discontinuity, control laws A2 and A3 imposed fast transients in the generator electrical output. To avoid the generator overload, control law A4 is clipped to the generator rated power, , see Fig. 3c. Performed experimental tests showed that, in cases when the energy level of the incoming waves, and hence the turbine aerodynamic torque, remain small over a substantial period of time (say several wave periods), the turbine-generator set would stop due to friction torque. It was found that the Tecnalia test rig has a non-negligible friction when performing tests in the smaller scales (tests at a scale below 1/4.7, see Table in subsection 7.2). Nevertheless, this value of friction does not invalidate the results nor the conclusions found. To prevent the turbine/generator set from stopping in the less energetic sea conditions, control law A5 was introduced, see Fig. 3d. This imposes zero extracted power from the system for rotational speeds less than 500 rpm. Above this rotational speed, the generator power increases smoothly (with continuous first order derivative) from zero to the basic control law at 800 rpm. For that, a third order polynomial control law is applied


Rev. 01, 28-Feb-2014 Page 14 of 23

,

such that

0,

,

0,

.

Above therotational speed , the control law follows curve A4.

2.2.3 Test Plan The experiments performed at the TECNALIA test rig were carried out using the generator power laws shown in Fig. 3. The experiments consisted in the control of the maximum rotational speed of the turbine/generator set and/or to prevent overloading the electrical generator. For this purpose, the buoy has two valve configurations. A latching valve mounted in series with the biradial turbine and a relief valve mounted in a parallel configuration with respect to the turbine, see Fig. 1. To keep the rotational speed and generator power within safe limits, three types of valve control strategies were devised:

Type T1 - Opening a relief valve when the speed or power reaches maximum allowable values. Type T2 - Closing a valve in series with the turbine when the speed or generator power reaches maximum

allowable values. Type T3 - Latching control using the valve mounted in series with the turbine. In extreme wave conditions

close the valve. A 15-day test-plan has been envisaged. Only irregular waves were considered. Sea state conditions referenced for testing were based on the following operating conditions:

Optimum efficiency range: 2 and 4 m and 6, 8, 10, 12 and 16 s. Extreme loading conditions: 6 m and 6, 8, 10, 12 and 16 s.

All tests were performed for a time-length of 3800 s for the prototype (full-scale), corresponding to a (real) time duration of

. A total of 84 tests were performed. In all tests, the Pierson-Moskowitz spectral distribution was adopted. A summary of the more relevant tests is presented in section 7.2. The most important results are plotted in Figs. 4 to 8. The dimensionless turbine power output, used to plot the curves in the figures, is defined as

,

and, analogously, for the dimensionless generator output power

.


Rev. 01, 28-Feb-2014 Page 15 of 23

Table 1 summarizes the configurations used for the curves plotted in Figs. 4 to 8. Figures 4 to 7 concern four wave spectra, all with a significant wave height 6 m and energy periods 8, 10, 12 and 16 s, respectively. These figures show most of the capabilities and limitations of the proposed control strategies under highly energetic sea states. Figure 8 compares power law A5 with valve control strategy T1 for three different significant wave heights, 6, 4 and 2 m, and two energy periods, 8 and 12 s. This figure shows the performance of the best control under lower energetic sea states.

Table 1 Summary of the configurations used in the tests plotted inFigs. 4 to 8.

Figure Significant waveheight, [m] Energy

period, [s] Test

number Generator

control Power

law

4 6 8 54 A5 T1 50 A4 T1 41 A1 T2 21 A1 T1

5 6 10 39 A3 T1 47 A4 T1 42 A1 T2 22 A1 T1

6 6 12 55 A5 T1 48 A4 T1 43 A1 T2 23 A1 T1

7 6 16 57 A5 T1 49 A4 T1 44 A1 T2 24 A1 T1

8

6 8 55

A5 T1 12 54

4 8 63 12 61 2 8 62 12 60

Figure 4 Results for tests 21, 41, 50 and 54, performed for 6 m and 8 s. Mean power output a), exceedance curves for the generator power b), turbine power c) and rotational speed d).


Rev. 01, 28-Feb-2014 Page 16 of 23




Rev. 01, 28-Feb-2014 Page 17 of 23


Figure 8 Comparison of the results for tests 60, 62, 61, 63, 54 and 55, performed using power law A5 and valve control strategy T1. Mean power output a), exceedance curves for the generator power b), turbine power c) and rotational speed d) for three different significant wave heights, 6, 4 and 2 m, and two energy periods, 8 and 12 s.


Rev. 01, 28-Feb-2014 Page 18 of 23

Figure 9 Results obtained for tests 55 and 23, both for a significant wave height of 6 m and an energy period of 12 s. Valve strategy for both tests was T1. Tests 55 and 23 use power laws A5 and A1, respectively. a) Experimental instantaneous generator output power. b) Comparison between the experimental and numerical rotational speed of the turbine/generator set.

2.3 ANALYSIS & CONCLUSIONS The results from tests performed under a sea state described by a Pierson-Moskowitz spectrum with 6 m and

8 s are plotted in Fig. 4. The best results in terms of generator output power were obtained for test 41. However, test 41 is 12% of the time above the generator rated power and 2% well above this limit. Test 21 follows the trend of test 41 but it is only 7% of the time above the generator rated power. Tests 50 and 54 are almost identical, showing only a small difference for rotational speeds bellow 600 rpm, as expected from applying power laws A4 and A5, see Fig. 3. For these two tests, the generator rated power is never exceeded. Another interesting result is the lower power output resulting from applying power laws A4 and A5 in comparison with power law A1. Figure 5 presents the results for a sea state with 6 m and 8 s. The curves of tests 22, 42 and 47 follow closely the trends of tests 21, 41 and 50 plotted in Fig. 4, respectively. For this sea state condition, a test for the power law A5 was not performed. Test 39 used power law A3. The hysteresis loops used in power law A3 (and also in A2) imposes a discontinuous behaviour on the generator output power. It was found that the generator controller was unable to match the required dynamics of power law A3. The extracted power for this case was significantly less than the others (see Fig. 5a). Power law A3 (and also A2) has been discarded. Figures 6 and 7, both with 6 m and 12, 16 s respectively, follow basically the trends of Fig. 4. Comparing the studied combinations of power laws and valve control strategies, it may be concluded that the best option for limiting the rotational speed and the generator peak power is power law A5 and valve control strategy T1. Figure 8 shows the performance of this combination for three significant wave heights, 6, 4 and 2 m, and two energy periods, 8 and 12 s. Figure 9 compares a time-series of the results obtained for tests 55 and 23, both for H 6 m and T 12 s. For both tests, valve strategy T1 was used. Tests 55 and 23 use power laws A5 and A1, respectively. In Fig. 9a, the experimental instantaneous generator output power is plotted for both test cases. As expected for this highly


Rev. 01, 28-Feb-2014 Page 19 of 23

energetic sea state, test case 53 is always below the generator rated power which is not the case for test case 23.Since test 53 has the generator output power limited, the rotational speed and the instantaneous power decay later, in comparison with test case 23. This behaviour can be seen, for example, in the power curves inside the magenta rectangle, Fig. 9a. Below 800 rpm, the instantaneous generator power for test case 53 is smaller than for test case 23 due to the used power law, see Fig. 9a and Fig. 3. As stated before, the used simulator has two models that run simultaneously: a complete numerical model of the OWC spar buoy wave energy converter and a hardware-in-the-loop simulation. Figure 9b shows a comparison between the numerical and the experimental rotational speed values. In test case 23, the match between the experimental and computational speed is very good. In test case 53, there is a significant difference between the numerical and the experimental values when the instantaneous power is well above the rated power. We think that this behaviour is a result of the generator controller since we are getting closer to its upper limit. Comparing the rotational speed of test case 53 and 23, when the generator power is limited the resistive torque is smaller and the rotational speed increases. From the analysis of the results and considering the three valve control strategies adopted, the following conclusions may be drawn:

The relief valve is more effective than the latching valve for controlling the upper limits of both rotational speed and generator power.

The valve control strategy T2 should be changed. The valve installed in series with the turbine opens and closes depending on the rotational speed. The valve opening order should take also into account the current relative pressure of the pneumatic chamber to reduce the turbine instantaneous output power to acceptable values.

The implemented control strategies were found to be more effective for system configurations with low inertia.

High inertia systems respond slower to fast turbine power transients but have problems with self-start, for the adopted scales in the tests, due to the level of friction torque of the test rig.

The delay imposed by the control of the generator from ABB should be taken into account for highly energetic sea conditions, where fast transients may occur. The value of the delay was found to be 150 ms.

3 MAIN LEARNING OUTCOMES

3.1 PROGRESS MADE

3.1.1 Progress Made: For This User-Group or Technology The IDMEC/IST wave energy group successfully implemented a hardware-in-the-loop PTO simulator. The tests at Tecnalia test rig were carried out using only standard equipment commonly used in laboratory. The numerical models used in the hardware-in-the-loop simulator were validated and tested against a PTO with a real electrical generator. The numerical simulator was able to cope with the typical signal-to-noise ratio found in a real experimental setup. The hydrodynamic and aerodynamic numerical models used in the hardware-in-the-loop simulator were implemented for a full-scale prototype, while the PTO simulator was running at a prescribed model scale. To perform this type of modelling, the time-step of the full-scale equations was decreased, accordingly to Froude scaling, since the time-step of the Tecnalia test rig cannot be changed. The coupling between the hydrodynamic and aerodynamic numerical models (full-scale) and the PTO test rig (model-scale) is done by power and torque values. The scaling of power and torque is performed in real-time. A complete description of the used numerical models will be described in detail in a forthcoming paper. This approach and the need of running the real-time simulations in the Simulink/xPC Target result in a complete re-writing of all the code previously developed by the IDMEC/IST group.


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The results have shown the capabilities and limitations of the proposed control algorithms. Additionally, the tests provided valuable insight for the group about the real behaviour of an electrical generator and its power electronics. The software of the power electronics that controls the electrical generator has a delay of about 150 ms and slower than expected dynamics. The knowledge about this delay is very important especially for dealing with fast transients such as latching (valve control T3) or a pressure increase due to a wave train with a wave height much higher than the mean value. For the case of the valve controls T1 and T2, a correction for the delay was as already introduced in the numerical models that are being used to simulate/design the IDMEC/IST OWC spar buoy.

3.1.1.1 Next Steps for Research or Staged Development Plan Exit/Change & Retest/Proceed? In the reported testing programme at Tecnalia PTO laboratory, the knowledge of instantaneous values of rotational speed and air pressure in the OWC chamber was assumed. These control variables have been used to perform the overall system control. The causal latching control used during these tests was not very effective for improving the energy extraction. By knowing the behaviour of the system in advance, the optimization of the instantaneous generator torque and the selection of the ideal actuation instant of latching valve will be possible to obtain through model predictive control (MPC). In framework of the fifth FP7-Marinet call, a model predictive control (MPC) strategy will be tested again in the Tecnalia test rig. The control will try to tune the WEC with the incoming waves by controlling a fast-actuated valve positioned in series with the turbine flow. The fast-actuated valve will be used to almost instantaneously block the air flow and place the system near resonance conditions (latching control). The biradial turbine presents a very simple and effective solution for applying a fast-actuated valve since this can be done by a single moving part with a relatively small stroke. Additionally, the rotational speed, the generator rated power and the turbine instantaneous peak power will be the system constraints for the MPC. This will be important for normal operating conditions but also, and most importantly, when the system has an excess of energy. The practical implications of this strategy on the generator and electrical equipment will be analysed. Another important aspect of the hardware-in-the-loop simulator that should be improved is how to emulate inertia for the small scale model that is larger than the physical installed inertia in the test rig. This can be done by correcting the angular acceleration according to the inertia that is being simulated. In the current tests, the scale of the tests was changed.

3.1.2 Progress Made: For Marine Renewable Energy Industry From the test results, it was shown that it is quite challenging to simultaneously control the rotational speed and the instantaneous power of the generator with a rated power of about twice the expected average energy extraction. Due to the irregular characteristics of the sea-waves, in highly energetic sea-states, trains of waves could reach the WEC with an instantaneous power much larger than the generator rated power. This wave behaviour can accelerate the turbine/generator group well above rotational speed limit since the generator does not have enough capacity to absorb this excess energy. The main reason to use the smallest possible generator is to reduce the overall costs and the weight of the PTO. The weight of the PTO is of extreme importance for hydrodynamic stability since it is installed on top of the buoy. Through the use of a relief valve in parallel with the turbine, it is possible to have a PTO control that limits the turbine/generator set rotational speed and the pneumatic power available to the turbine. The use of a latching valve was not entirely successful since it resulted in large pneumatic power peaks. The proposed strategy for the latching valve, based only on the rotational speed, must be changed to cope with large pressure differences that could occur between the pneumatic chamber and the atmosphere. The valve should be open only if the rotational speed is below a specified threshold and if the relative pressure of the pneumatic chamber is also within a safe range. This could decrease the overall system efficiency. The latching valve is clearly more interesting, both from simplicity and economic point-of-view, than the relief valve to control the PTO. The latching valve is considerably smaller than the relief valve and is cheaper to build. It is installed in series with the turbine and allows the air duct to close and prevent green water from reaching the turbine. It should remain closed in stormy conditions.


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3.2 KEY LESSONS LEARNED A preliminary visit to the Tecnalia test rig was very important to fully understand the capabilities and

limitations that the group would encounter in the tests. During this visit, a first version of the Matlab/Simulink model was tested. It was found that the model did not work and it was completely re-written.

The IDMEC/IST team used their own data acquisition board to fully test the Matlab/Simulink model at IDMEC/IST before going to Tecnalia for the complete set of tests. Testing the complete model only at Tecnalia would be very time-consuming and error prone.

The Matlab/Simulink state-spaces used in the simulation needed to be coded in the C language in order to decrease the execution times and allow real-time control.

The use of a complete numerical simulation that runs in parallel with the main hardwarein-the-loop model allows a rapid and real-time comparison between the IDMEC/IST numerical model and the hardware in the loop simulations. The hardwarein-the-loop simulator comprises two systems of 26 ordinary differential equations. This approach allows the team to identify the physical limitations of the real hardware during the execution of the tests. This is one of the main reasons for performing hardware in the loop simulations.

4 FURTHER INFORMATION

4.1 SCIENTIFIC PUBLICATIONS List of scientific publications made planned as a result of this work:

J.C.C. Henriques, R.P.F. Gomes, E. Robles, S. Ceballos, L.M.C. Gato and A.F.O. Falco. Hardware-In-the-Loop test of a Power Take-off system for an OWC spar buoy. In preparation for submission to a journal.

4.2 WEBSITE & SOCIAL MEDIA YouTube Link(s): http://youtu.be/1Oa1fBC0_5I Online Photographs Link: http://waves.tecnico.ulisboa.pt/SPOWCON2013/index.html

5 ACKNOWLEGEMENTS The IDMEC/IST group acknowledges all the support from Tecnalia, namely Eider Robles and Salvador Ceballos. The friendship of all members of Tecnalia during the IDMEC/IST group stay at Bilbao was much appreciated.

6 REFERENCES [1] Falco, A.F. de O., 2002. Control of an Oscillating Water Column Wave Power Plant for Maximum Energy Production, Appl. Ocean Res., vol. 24, pp. 73-82. [2] Falco, A.F.O., Henriques, J.C.C., Gato, L.M.C.,Gomes, R.P.F., 2014. Air Turbine Choice and Optimization for Floating Oscillating-Water-Column Wave Energy Converter, Ocean Eng., vol. 75, pp. 148-156. [3] Henriques, J.C.C., Falco, A.F.O., Gomes, R.P.F.,Gato, L.M.C., 2013. Latching Control of an Oscillating Water Column Spar-Buoy Wave Energy Converter in Regular Waves. J. Offshore Mech. Arctic Eng.-Trans. ASME, vol. 135, 021902.

7 APPENDICES

7.1 STAGE DEVELOPMENT SUMMARY TABLE The following table gives 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.


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7.2 SUMMARY TABLE OF THE PERFORMED TESTS

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