Simulation of the SEAREV Wave Energy Converter with a by-pass control of its hydraulic Power Take Off.

Transcription

1 Simulation of the SEAREV Wave Energy Converter with a by-ass control of its hydraulic Power Take Off. Aurélien Babarit, Hakim Mouslim, Michel Guglielmi & Alain Clément Ecole Centrale de Nantes, France 1

2 The SEAREV Wave Energy converter > Princile : A moving mass in a closed buoy The relative motion between the two bodies is converted in electricity > Advantages : The system is fully closed: all moving arts are inside Internal reference : slack moorings No ends stos : the moving mass is a cylinder with and off-centered gravity center. Yoke Hull > The relative motion is controlled Cylinder 2

3 General scheme of the SEAREV with a hydraulic PTO Hull Hydraulic motor Generator LP accumulator HP accumulators Hydraulic rams Hydraulic PTO : Cylinder Electric cable and moorings > Adated for large torques/forces and slow motion alications > Existing comonents. > Allow storage of the energy, then smoothing of the outut ower BUT : Low overall efficiency high need for maintenance 3

4 Motion ofthesearev WEC > Let : X = ( x, z,θ,α G G ) surge heave itch Relative motion > Equation of motion : Mass matrix ([ M ] + [ µ ]) X& = Fex [ δ ] I [ K ] X FPTO Wave-structure interaction Hydrostatic + mooring force PTO force > Solved numerically Assumtions : > Irrotationnal flow, unviscid and uncomressiblefluid > Small amlitude of the motion, small waves 4

5 PTO force without control Hydraulic PTO force Area of the rams F PTO = u v Pressure Without control : 3 > Constant PTO force > Coulomb daming Velocity Force -3 Time (s) Far from otimal daming oor efficiency need for control 5

6 Usual control for the PTO force > Several HP accumulators with different ressures and/or several rams in order to mimic a linear damer/continuous PTO force Linear damer Hydraulic force Time (s) Comlexity (several valves, several accumulators, ) 6

7 Declutching control of the PTO force > Disconnecting of the PTO by means of by-ass valve 3 Declutching Oerating Velocity Controlled force -3 Time (s) Simle way to control the PTO force, and hence the motion 7

8 Equations for the PTO,V q q m > Without control : q = u v α& q u v Ω m = m m V & = q + qm & = γv& / V F = u v PTO 8

9 Equations for the PTO,V q q u m > Control of the by-ass valve : u =1 oerating - u = declutching q = u v α& q u v Ω m = m m V & = q + qm & = γv& / V F = u v PTO 9

10 Equations for the PTO,V q q m u m > Control of the variable dislacement motor : q = u v α& q u v Ω m = m m V & = q + qm & = γv& / V F = u v PTO 1

11 Comutation of the control of the by-ass valve > Using otimal command method, with criterion maximisation of the mechanical absorbed energy : max E u t u v α& dt = f > One can show 1 if sign( & α ) α u = else ( & Λ) Declutching control is otimal F PTO = u v > PTO force should be or maximum available > Intermediate states are not otimal The by ass valve has to be switched each time the velocity vanishes 11

12 Time domain simulation of the motion of the SEAREV WEC with declutching control > With linear damer : + 31 % Wave (m) time (s) 15 2 > With declutching control : + 7 % Relative motion ( ) C PTO Energy roduction (J) E+7 2E+7 1E+7 Without control With Declutching control Simle PTO > PM sectrum > Peak eriod = 1s > Significant height = 2 m Simle PTO = PTO behaving like a linear damer FPTO = B PTO α& 12

13 Smoothing of the ouut ower by the HP accumulator HP volume (m 3 ) HP ressure (bars) time (s) Power (kw) Power from the ums Power to the grid > PM sectrum > Peak eriod = 1s > Significant height = 2 m Volume of HP accumulator = 2 m 3 13

14 Ratios of ower roduction with hydraulic PTO and with simle PTO Without control With declutching control 8 Ratio 8 Ratioc H s (m) H s (m) T (s) T (s) > Black line : isocontour 1 > Declutching control > Linear damer > without control 14

15 Conclusion > Declutching control : by-assing the hydraulic ums at some moments. > Declutching control is otimal : no need for several HP levels or several rams > For the SEAREV wave energy converter, efficiency imroved u by u to a factor 2 in random waves 15

16 Thank you for your attention 16

17 Control of the SEAREV motion Initially : Latching control Budal and Falnes [8] : > Latching control consists in locking the motion of the body at the very moment when it velocity vanishes at the end of an one oscillation, and waiting (during a certain delay to be determined) for the most favorable situation to release the body. > Highly efficient for a heaving oint absorber. > PTO behaving like a dash ot Babarit et al. [6] : > Latching control imroves the efficiency of the multi-dofs SEAREV Wave Energy Converter. > Otimal command method. > PTO behaving like a dash ot Falcao [8] : > Heaving buoy oint absorber with a hydraulic PTO > Highly efficient control is achieved without rediction of incoming waves Alternative : Unlatching or Declutching Salter [2] : > Unlatching consists in by-assing the Power Take Off at some moments. > As latching control, it is shown to be able to generate arametric resonance imrove efficiency of WECs 17