Using Battery Modeling as a Sizing Tool
Transcript of Using Battery Modeling as a Sizing Tool
Using Battery Modeling as a Sizing Tool
IEEE ESSB Summer MeetingChicago – June 12, 2016
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IEEE battery sizing
– Sizing standards• IEEE Std 485-2010 for vented and valve-regulated lead-acid• IEEE Std 1115-2014 for nickel-cadmium
– Both use modified Hoxie method• Hoxie, E. A., “Some discharge characteristics of lead-acid
batteries,” AIEE Transactions (Applications and Industry), vol. 73, no. 1, pp. 17–22, Mar. 1954.
• Divide duty cycle into successive periods with constant load• Size by section with progressively more periods
o Section 1 with first period onlyo Section 2 with first two periods o Etc.
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IEEE cell sizing worksheet (IEEE 1115)
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Limitations for the IEEE method
– Cumbersome for numerous load steps– No consideration for temperature changes during
operation• Ambient changes during prolonged duty cycles• I2R heating effects
– No consideration for ramping loads– No consideration (yet) for new technologies
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Battery modeling – an alternative approach
– Various platforms depending on need. Examples:– COMSOL Multiphysics
• ‘Micro’ modeling e.g. electronic & ionic charge transfer• Useful for cell design
– Matlab-Simulink• ‘Macro’ modeling of electrical and thermal characteristics• Scalable to battery level for interactions with dc system• Models for advanced technologies may include battery-
management algorithms
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Saft Li-ion Matlab-Simulink models
– Models run same algorithms as battery management systems
– Exactly mimic real battery behavior, including contactor management
– Include aging inputs and outputs
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Example 1 – from IEEE 485
– Model requires random load to be in correct location
– Assume ambient temperature of 10 °C
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Example 1 – model results
BOL100% SOH
EOL80% SOH
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Example 2 – PJM regulation
– PJM Interconnection runs the grid in 13 states• Biggest Independent System Operator in the USA
– PJM power market includes fast regulation service (RegD) typically supplied by battery energy storage systems• Approx. 300 MW in service or being built
– RegD signal is energy neutral and changes every 2 sec.– Sample day:
0 1 2 3 4 5 6 7 8 9
x 104
-1.0
-0.8
-0.6
-0.4
-0.2
0
0.2
0.4
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Time (s)
P.U
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Example 2 – model results
0 2 4 6 8 10
x 104
-2000
0
2000
Time (s)
I bat
tery
(A)
0 2 4 6 8 10
x 104
600
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800
Time (s)
U b
atte
ry (V
)
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Time (s)
Bat
Tem
p (°
C)
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-1000
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Time (s)
Pow
er (k
W)
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Time (s)
SoC
(%)
0 2 4 6 8 10
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Time(s)
Ene
rg. e
ff. (%
)
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So is modeling the future of battery sizing?
– For Saft, the answer is ‘yes’• But not every battery manufacturer has a model• Most are less sophisticated
– Tabular discharge data does not exist for Saft Li-ion cells• The model is the source of all performance characteristics
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Where do we go with sizing standards?
– The IEEE sizing standards have their origins in the time before computer-driven modeling• Needed to avoid use of different sizing methods for the same
project• Continue to be vital for the nuclear industry
– Why do we need sizing standards if models exist?– Is validation an issue?
• How is ‘trust the model’ different from ‘trust the tabular discharge data’?
– Food for thought…
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‘Bonus’ material – cycle counting
– Question• How many cycles?• …of what depth?
– Answers are important for folks who want to compare cycle count with published cycle-life curves
– The following slides show the Saft method• Others may follow a slightly different approach
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Cycle counting technique
– Cycle must be ‘closed’• SOC must return to starting point
– Example with 2 closed cycles• From 80% SOC to 20% SOC and
back again (60% DOD)• From 55% SOC to 50% SOC and
back again (5% DOD)
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Modified example
– Still 2 closed cycles• From 65% SOC to 20% SOC and
back again (45% DOD)• From 55% SOC to 50% SOC and
back again (5% DOD)– Cycle starting from 80% still
waiting to be closed• If charging continued (after
pause) up to 80% SOC... • 45% cycle would become 60%
DOD cycle
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Another modification
– Now 3 closed cycles• From 65% SOC to 20% SOC and back again (45% DOD)• From 55% SOC to 50% SOC and back again (5% DOD)• From 80% SOC to 30% SOC and back again (50% DOD)
– With Li-ion we count only delta-DOD cycles• Cycle between 20% and 10% SOC counts same as one
between 90% and 80% SOC• Hence no 60% DOD cycle above