Testing of a power take-off system for an OWC spar-buoy wave energy converter J. C. C. Henriques 1 , R. P. F. Gomes 1 , L. M. C. Gato 1 , A. F. O. Falcão 1 , J. C. C. Portillo 1 , E. Robles 2 and S. Ceballos 2 1 LAETA, IDMEC, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais,1049-001 Lisboa, Portugal 2 Energy and Environmental Division, Tecnalia Research and Innovation, Derio 48160, Spain System description and objectives I The OWC spar-buoy is an axisymmetric device consisting basically of a 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 new type of self-rectifying air turbine, the biradial turbine. I The work concerns the development of the power take-off (PTO) control of an OWC oscillating-water-column (OWC) spar-buoy wave energy converter. I The objective is to increase the PTO system efficiency and reduce the overall costs, an electrical generator was adopted with a rated power twice the maximum expected average power output of the buoy. I This level of generator rated power poses great challenges for the PTO control due to the irregular characteristics of the sea waves. I The tests were performed using an hardware-in-the-loop configuration. The hydrodynamics of the OWC spar-buoy and the aerodynamics of the air turbine were numerically simulated in real time and coupled to the physical model of the turbine/electrical generator set. The instantaneous air turbine torque is emulated through the use of the electrical motor. I The control of this set was the object of tests performed at Tecnalia Electrical PTO Laboratory, in Bilbao, Spain. I In the reported implementation, only irregular wave conditions were considered. I The experimental results allowed the dynamic behaviour of the PTO to be characterized, in order to ensure the practical applicability of the proposed control algorithms and provide a basis for the validation of the numerical models. a) b) c) OWC spar-buoy geometry and biradial turbine representation. NAREC large scale testing a) b) OWC spar-buoy at NAREC at 1/16 th scale. Tecnalia testing TECNALIA test rig Hardware-in-the-loop simulation and data logging PTO test configuration. (m 1 + A ∞ 11 ) d 2 x 1 dt 2 + A ∞ 12 d 2 x 2 dt 2 = -ρ w gS 1 x 1 + p at S 2 p * + F dr 12 A ∞ 21 d 2 x 1 dt 2 +(m 2 + A ∞ 22 ) d 2 x 2 dt 2 = -ρ w gS 2 x 2 - p at S 2 p * + F dr 21 γ (p * + 1) d dt (x 1 - x 2 )+(h 0 + x 1 - x 2 ) dp * dt = - γQ t S 2 (p * + 1) γ -1 γ d dt 1 2 I Ω 2 = P t - P g (1) In the hardware-in-the-loop tests, the rotational speed Ω of Eq. (1) is replaced by the experimental values. The floater is body 1 and the oscillating water column (OWC) is body 2, and the main vari- ables are: A ∞ ij - added mass hydrodynamic coefficient D - diameter of the buoy at the free surface F dr - excitation force plus radiation force g - acceleration of gravity p - p at - relative pressure between air chamber and atmosphere P g - generator power P t - turbine power S i - representative cross section of body i t - time V c and h 0 - air chamber volume, and air chamber height x i , v i , a i and m i - position, velocity, acceleration and mass of body i ρ at - air density at atmospheric conditions ρ w - water density Ω and I - rotational speed and turbine/generator set inertia Generator control Compared efficiency η t of the biradial, Wells and axial flow impulse (fixed guide vanes) turbines versus flow rate coefficient Φ/ Φ| η max , where Φ| η max is the flow rate coefficient at the point of maximum efficiency for each turbine. a) b) c) d) Generator power control laws. a) The basic control law, P A1 = a Ω b , 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, P rated gen . c) The curve used in a) with the maximum power clipped to the generator rated power. d) Modification of curve plotted in c), where the generator power increases smoothly from zero to the basic control law between 500 rpm and 800 rpm. Results Comparison of the results for tests 60, 62, 61, 63, 54 and 55, performed using control law A5 and valve control strategy T1, for three different significant wave heights, H s =6, 4 and 2 m, and two energy periods, T e =8 and 12 s. Time series Results obtained for tests 55 and 23, both for a significant wave height of H s =6 m and an energy period of T e = 12 s. Valve strategy for both tests was T1. Tests 55 and 23 use control laws A5 and A1, respectively. Conclusions I 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 twice the expected average annual power extraction. I Through the use of a relief valve in parallel with the turbine, it is possible to control the turbine/generator set rotational speed and the pneumatic power available to the turbine. I The use of a high-speed stop valve was not entirely successful since it resulted in large pneumatic power peaks. I The delay imposed by the control of the generator 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. I Although not apparent from the shown results, simulations of systems at smaller scales would be expected to reveal self-start problems due to the level of friction torque of this particular test rig, since power losses would not be scaled as net power. I Future work should be focused on the use of the high-speed stop valve to perform latching control, as well as on the application of Model Predictive Control to determine the valve opening and closing instants, while taking into account limitations of the generator. Acknowlegements The research was partially funded by the European Community’s Seventh Frame- work Programme under MARINET initiative. This work was also funded by the Portuguese Foundation for Science and Technology (FCT) through IDMEC, un- der LAETA Pest-OE/EME/LA0022 and contracts PTDC/EME-MFE/103524/2008 and PTDC/EME-MFE/111763/2009. The second author was supported by post- doctoral fellowship SFRH/BPD/93209/2013 from the FCT. ICOE 2014 - International Conference on Ocean Energy - Halifax, Nova Scotia, Canada [email protected] // http://www.facebook.com/OWCsparbuoy // Report at: http://waves.tecnico.ulisboa.pt/SPOWCON2013/MARINET-SPOWCON.pdf
Testing of a power take-off system for an OWC spar-buoy wave energy converter
Nov 11, 2015
The OWC spar-buoy is an axisymmetric device consisting basically of a 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 new type of self-rectifying air turbine, the biradial turbine. The work concerns the development of the power take-off (PTO) control of an OWC oscillating-water-column (OWC) spar-buoy wave energy converter. The objective is to increase the PTO system efficiency and reduce the overall costs, an electrical generator was adopted with a rated power twice the maximum expected average power output 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 tests were performed using an hardware-in-the-loop configuration. The hydrodynamics of the OWC spar-buoy and the aerodynamics of the air turbine were numerically simulated in real time and coupled to the physical model of the turbine/electrical generator set. The instantaneous air turbine torque is emulated through the use of the electrical motor. The control of this set was the object of tests performed at Tecnalia Electrical PTO Laboratory, in Bilbao, Spain. In the reported implementation, only irregular wave conditions were considered. The experimental results allowed the dynamic behaviour of the PTO to be characterized, in order to ensure the practical applicability of the proposed control algorithms and provide a basis for the validation of the numerical models.
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Testing of a power take-off system for an OWC spar-buoy wave energy converterJ. C. C. Henriques1, R. P. F. Gomes1, L. M. C. Gato1, A. F. O. Falco1, J. C. C. Portillo1, E. Robles2 and S. Ceballos2
1LAETA, IDMEC, Instituto Superior Tcnico, Universidade de Lisboa, Av. Rovisco Pais,1049-001 Lisboa, Portugal2Energy and Environmental Division, Tecnalia Research and Innovation, Derio 48160, Spain
System description and objectives
I The OWC spar-buoy is an axisymmetric device consisting basically of asubmerged vertical tail tube open at both ends, fixed to a floater thatmoves essentially in heave. The oscillating motion of the internal freesurface relative to the buoy, produced by the incident waves, makes the airflow through a new type of self-rectifying air turbine, the biradial turbine.
I The work concerns the development of the power take-off (PTO) control ofan OWC oscillating-water-column (OWC) spar-buoy wave energy converter.
I The objective is to increase the PTO system efficiency and reduce theoverall costs, an electrical generator was adopted with a rated power twicethe maximum expected average power output of the buoy.
I This level of generator rated power poses great challenges for the PTOcontrol due to the irregular characteristics of the sea waves.
I The tests were performed using an hardware-in-the-loop configuration. Thehydrodynamics of the OWC spar-buoy and the aerodynamics of the airturbine were numerically simulated in real time and coupled to the physicalmodel of the turbine/electrical generator set. The instantaneous air turbinetorque is emulated through the use of the electrical motor.
I The control of this set was the object of tests performed at TecnaliaElectrical PTO Laboratory, in Bilbao, Spain.
I In the reported implementation, only irregular wave conditions wereconsidered.
I The experimental results allowed the dynamic behaviour of the PTO to becharacterized, in order to ensure the practical applicability of the proposedcontrol algorithms and provide a basis for the validation of the numericalmodels.
a)
b)
c)
OWC spar-buoy geometry and biradial turbine representation.
NAREC large scale testing
a) b)
OWC spar-buoy at NAREC at 1/16th scale.
Tecnalia testing
TECNALIA test rigHardware-in-the-loop simulation and data logging
PTO test configuration.
(m1 + A11)d2x1dt2 + A
12
d2x2dt2 = wgS1 x1 + pat S2 p
+ F dr12
A21d2x1dt2 + (m2 + A
22)
d2x2dt2 = wgS2 x2 pat S2 p
+ F dr21
(p + 1) ddt (x1 x2) + (h0 + x1 x2)dpdt =
QtS2
(p + 1)1
ddt
12I2 = Pt Pg (1)
In the hardware-in-the-loop tests, the rotational speed of Eq. (1)is replaced by the experimental values. The floater is body 1 andthe oscillating water column (OWC) is body 2, and the main vari-ables are:
Aij - added mass hydrodynamic coefficientD - diameter of the buoy at the free surfaceF dr - excitation force plus radiation forceg - acceleration of gravityp pat - relative pressure between air chamber and atmospherePg - generator powerPt - turbine powerSi - representative cross section of body it - timeVc and h0 - air chamber volume, and air chamber heightxi, vi, ai and mi - position, velocity, acceleration and mass ofbody iat - air density at atmospheric conditionsw - water density and I - rotational speed and turbine/generator set inertia
Generator control
Compared efficiency t of the biradial, Wells and axial flow impulse(fixed guide vanes) turbines versus flow rate coefficient /|max,where |max is the flow rate coefficient at the point of maximumefficiency for each turbine.
a) b)
c) d)
Generator power control laws. a) The basic control law, PA1 =ab, obtained from exponential regression of the maximum powerextraction computed for a set of sea-states characteristic of thewave climate off the Portuguese west coast. b) Basic control lawcombined with two hysteresis loops where the maximum value isthe generator rated power, P ratedgen . c) The curve used in a) withthe maximum power clipped to the generator rated power. d)Modification of curve plotted in c), where the generator powerincreases smoothly from zero to the basic control law between500 rpm and 800 rpm.
Results
Comparison of the results for tests 60, 62, 61, 63, 54 and 55,performed using control law A5 and valve control strategy T1, forthree different significant wave heights, Hs = 6, 4 and 2m, andtwo energy periods, Te = 8 and 12 s.
Time series
Results obtained for tests 55 and 23, both for a significant wave height of Hs = 6mand an energy period of Te = 12 s. Valve strategy for both tests was T1. Tests 55and 23 use control laws A5 and A1, respectively.
ConclusionsI It was shown that it is quite challenging to simultaneously control the
rotational speed and the instantaneous power of the generator with a ratedpower of twice the expected average annual power extraction.
I Through the use of a relief valve in parallel with the turbine, it is possible tocontrol the turbine/generator set rotational speed and the pneumatic poweravailable to the turbine.
I The use of a high-speed stop valve was not entirely successful since itresulted in large pneumatic power peaks.
I The delay imposed by the control of the generator should be taken intoaccount for highly energetic sea conditions, where fast transients may occur.The value of the delay was found to be 150ms.
I Although not apparent from the shown results, simulations of systems atsmaller scales would be expected to reveal self-start problems due to thelevel of friction torque of this particular test rig, since power losses wouldnot be scaled as net power.
I Future work should be focused on the use of the high-speed stop valve toperform latching control, as well as on the application of Model PredictiveControl to determine the valve opening and closing instants, while takinginto account limitations of the generator.
Acknowlegements
The research was partially funded by the European Communitys Seventh Frame-work Programme under MARINET initiative. This work was also funded by thePortuguese Foundation for Science and Technology (FCT) through IDMEC, un-der LAETA Pest-OE/EME/LA0022 and contracts PTDC/EME-MFE/103524/2008and PTDC/EME-MFE/111763/2009. The second author was supported by post-doctoral fellowship SFRH/BPD/93209/2013 from the FCT.
ICOE 2014 - International Conference on Ocean Energy - Halifax, Nova Scotia, Canada [email protected] // http://www.facebook.com/OWCsparbuoy // Report at: http://waves.tecnico.ulisboa.pt/SPOWCON2013/MARINET-SPOWCON.pdf
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