wave energy conversion: basic principles and implications .... bacelli_sandia... · wave energy...
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Sandia National Laboratories is a multi-mission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525.
Wave Energy Conversion:Basic Principles and Implications for Design
Giorgio [email protected]
Control Co-Design for Wind and Marine-hydro-kinetic Energy SystemsARPA-E WorkshopJuly 26th, 2018
SAND2018-8611 PE
mailto:[email protected]
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Wave energy converters
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No standard configuration yet
Wave spectra and bathymetry are very different around the world:there will likely be multiple designs, depending on the location and end users
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Power flow through a WEC
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Energy transfer through an oscillating body wave energy converter
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(Basic) Control problem of a WEC
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PTO BuoyControl system
Wave forces (disturbance)
𝑌𝑌𝑖𝑖 𝜔𝜔
𝑢𝑢
�1 𝑌𝑌𝑖𝑖 𝜔𝜔 = ⁄𝑢𝑢 𝑦𝑦 = 𝑍𝑍𝑖𝑖 𝜔𝜔 = intrinsic impedance of the device
𝑥𝑥 = WEC state (e.g. generator’s voltage, or buoy’s velocity)
𝑢𝑢 = PTO input (e.g. generator’s current, or buoy’s force)
WEC 𝑍𝑍0
𝐺𝐺𝑍𝑍𝑙𝑙
Thevenin Equivalent circuit(WEC model)
Load impedance(Control system)
Electrical circuit analogy
Optimal control problem Impedance matching problem
𝑍𝑍𝑙𝑙=𝑍𝑍0∗
𝑥𝑥
𝐺𝐺 𝜔𝜔
𝐺𝐺=−𝑍𝑍𝑖𝑖∗
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Control problem of a WEC
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Device non tuned
Device absorption characteristics – non tunedSea state spectral characteristics
Device absorption characteristics – tuned
Frequency
Power spectral density
Frequency
Power spectral density
Device tuned
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Considerations about power
Available wave power
P ∝ T P ∝ H2
Band limited 4 ≤ 𝑇𝑇 ≤ 20Most of wave power is in a limited frequency range
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Example: Floating cylinder
⁄1 4 the displacement of an aircraft carrier!!(≈ 50000 𝑚𝑚3)
R=20m
D=20m
𝑇𝑇0 ≈ 2𝜋𝜋 �𝑚𝑚 𝑘𝑘 ≈ 9𝑠𝑠
Natural period increases with increasing size
Trying to design a naturally resonatingcould be VERY EXPENSIVE!
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Energy storage and reactive power
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Device tuned using control and PTO design
Required stored energy for each cycle for optimal tuning
Naturally tuned device
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LCOE ESTIMATIONReference Model Project
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Reference Model 3
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Reference Model 5
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Reference Model 6
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SANDIA ADVANCED WEC DYNAMICS AND CONTROL PROJECT
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Device overview
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Control and PTO potential
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0 1 2 3 4 5 6 7
T [s]
0
0.2
0.4
0.6
0.8
1
Theo
retic
al C
aptu
re R
atio
, , [
--]
theoretical limit
theoretical limit
with viscous losses
Resistive Control (baseline)
Wave period [s]Natural period
Potential improvement withcontrol and PTO design
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Controllers performance
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CC is the theoretical optimum (non causal)
Naturally tuned WEC(large device)
Peak frequency of waveSpectrum smaller than
WEC’s natural frequency(Small device in long waves)
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Reducing device size
What we know Can improve absorbed
power for small devices with advanced control and PTO
Structural costs are dominant
Assumptions Device size = ½ original Same AEP Similar PTO costs Only considering effects on
structural cost
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What happens to LCOE if we reduce device size and apply control co-design?
Estimated 20% reduction in LCOE
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Conclusions
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Control Co-Design
Allows improvements in all the aspects of the design of a WEC
Good design of the system ≠ good design of individual subsystem
Wave Energy Conversion:�Basic Principles and Implications for DesignWave energy convertersPower flow through a WEC(Basic) Control problem of a WECControl problem of a WECConsiderations about powerEnergy storage and reactive power LCOE estimationReference Model 3Reference Model 5Reference Model 6Sandia advanced wec dynamics and control projectDevice overviewControl and PTO potentialControllers performance Reducing device sizeConclusions