The objectives of this paper are: (1) identify thermal issues of ultracapacitor cells and modules over a range of vehicle duty cycles to understand and minimize thermal impacts; and (2) identify improvements for ultracapacitor thermal management

Gonder, J.

Keyser, M.

King, C.

Lustbader, J.

Pesaran, A.

UNT Digital Library

Thermal Evaluation of a High-Voltage Ultracapacitor Module for Vehicle ApplicationsJason Lustbader(Jason_Lustbader@nrel.gov)Charlie KingJeff GonderMatt KeyserAhmad PesaranJuly 15th, 2008Advanced Capacitor World SummitSan Diego, CA July 14-16, 2008NREL/PR-540-435642Outline• Objectives• Cell Testing– Calorimeter testing– Thermal imaging• Module thermal testing• Observed self-cooling• Summary3Objectives• Identify thermal issues of ultracapacitor cells and modules over a range of vehicle duty cycles to understand and minimize thermal impacts• Identify improvements for ultracapacitor thermal management4Cell Description: Maxwell BOOSTCAP 3000-P • Voltage Range = 0 V – 2.7 V• Crated = 3000 F• Toperating = -40 C to +65 C• m = 0.55 kg• Carbon electrodes• Aluminum current collectors• Organic electrolyte (Acetonitrile)5Calorimeter Description• Cavity dimensions: 21 x 20 x 39 cm (WxHxL)• Heat rate detection: 0.015 W to 100 W• Minimum detectable heat effect: 15 J (at 25ºC)• Baseline stability: ± 10 mW • Temperature range: -30ºC to 60ºC• Large conduction calorimeter that measures heat generation and heat capacity 6Calorimeter Results: Heat Generation and EfficiencyCurrent Square Wave, 5 Cycles, Ttest = 30 C, Single BCAP3000-P CellAt 200 A, Requiv = 0.000350 Ω7Calorimeter Results: Heat Capacity, Ttest = 30°C, Single BCAP3000-P Cell050010001500200025000 0.5 1 1.5 2 2.5 3 3.5 4Time (hours)Heat Rate (mW)TmQC removedp ∆=Starting Cell T= 43.9oCEnding Cell T = 29.5oCAverage Cell T = 36.7oCMass = 0.545 kgEnergy = 8453 JHeat Capacity = 1079.6 J/kg-KThree test average resultsHeat capacity deviation  < 1%8Thermal Imaging9Thermal Imaging: Single BCAP3000-P Cell 200 A, Square Wave Cycle, Tambient = 24 C+ + +- - -10Thermal Imaging: BCAP3000-P Series String of 5 Cells200A, Square Wave Cycle, Tambient = 22°CCells coated for uniform emissivity+-Aborted test, terminal cell heating +-All images when target temperature was equal to 40°CSwitched positive end cells, retorqued +-Improved uniformity over aborted caseLarge bus bar on positive terminal-+Improved uniformity over re-torqued case1.2. 3.11Thermal Imaging: BCAP3000-P Series String of 5 Cells 200 A, Square Wave Cycle, Tambient = 22°C, Cells Switched +-12Module Thermal Testing Facility Description• ABC-1000 bidirectional programmable power supply– 420V, 1000A, 125 kW• Environmental chamber– 64 ft3– -45 C to 190 C • Independent data acquisition system13Maxwell Module BMOD0063-125 V • Early module design• 48 cells • Crated = 63 F• 0 V – 125 V• Toperating = -40 C to +65 C• Imax, cont = 150 A (Trise<=15 C)• Vfan = 13.8 V, Ifan = 6.55 A • In chamber air flow ~ 244 CFM • All clearances were greater than specified minimums Heat SinkCooling Air FlowCooling Air FlowHeat Sink14Module: Internal Thermocouple LocationsCell Detail InstrumentationBus Bar DetailTop FaceBottom FaceTop & Bottom FaceDetailedAir Flow•Thermal interface pad bottom•Thermal interface pad topIndicates positive terminal upHeat SinkHeat SinkThermal PadThermal NutBalance BarBus Bar15Module: Other Instrumentation• Voltages for every cell (48) attached to bus bars• Current• Airflow– Mapped flow as a function of pressure drop along fins– Used in-chamber pressure drop to estimate flow during chamber testsExternal Thermocouple Locations 16Thermal Performance Test Cycles• 20 A charge to 120 V immediately before cycling• 120 minutes continuous cycling• Square wave cycle– 60 V to 120 VSquare wave cycle• Proprietary Oshkosh Heavy Hybrid cycle: Irms ≈ 225 A• Light-Duty HEV test cycle: Irms ≈ 90.4 A17Light-Duty HEV Test Cycle• NREL analysis shows significant HEV fuel savings are achievable with “low”-energy Ucap energy storage*• Power profile obtained from simulation to cycle this module:* Pesaran, A.; Gonder, J.; Brooker, A. “Factors & Conditions for Widespread Use of Ultracapacitors in Automotive Applications.” Proceedings of Advanced Capacitor World Summit 2007; July 23-25, 2007, San Diego, CA. • Vehicle Assumptions– Midsize car– Parallel HEV configuration– Vehicle mass = 1675 kg– Engine = 110 kW– Motor = 25 kW– US06 cycle – 80 Wh operating window – ~10% improvement in simulated fuel economy over comparable conventional vehicle on same drive cycle-60-40-200204060801000 100 200 300 400 500 600Time (s)Speed (mph), Power (kW) or Energy (Wh)Driving Speed (mph)Ucap Power (kW)Net Energy Use (Wh)18Module: Cell Terminal Temperatures150 A, Sq Wave, Ttest = 30 COffset due to fan heatingModule positive terminal cell heatingTemperatures close to steady state after 2 hours of cyclingPreheating due to initial charge prior to cycling19Module: Selected Temperatures150 A, Sq Wave, Ttest = 30 CPositive terminal lug hotter than cell, indicating lead wire heatingCoolest cell at air inlet corner (side cooling effect)20Module: Selected TemperaturesLight-Duty HEV Test Cycle, 12 cycles (120 min), Ttest = 30 C• Less than an 8 C rise after 120 minutes of cycling• Irms = 90.4 A 21Module: Cell Terminal Temperature Rise Over Ambient*Ttest = 0 or 30 C* Average over last five minutes of cycling, 115-120 min22Module: Cell Terminal Temperature Rise Over Ambient*Ttest = 0 or 30 CReduced fan, 221 CFM* Average over last five minutes of cycling, 115-120 min244 CFM23Module: Cell Terminal Temperature Rise Over Ambient*Ttest = 0 or 30 C* Average over last five minutes of cycling, 115-120 min244 CFM Reduced fan, 221 CFM24Module: Cell Terminal Temperature Rise Over Ambient*Ttest = 0 or 30 C* Average over last five minutes of cycling, 115-120 minReduced fan, 221 CFM244 CFMHH25Module: Cell Terminal Temperature Rise Over Ambient*Ttest = 0 or 30 C* Average over last five minutes of cycling, 115-120 minReduced fan, 221 CFM244 CFMHEVHH26Module: Estimated Temperature Distribution*150 A, Sq Wave, Ttest = 30 C • Average over last five                                   minutes of cycling• Bus bar temperature used when no cell data were available• Pos and neg cell terminals averaged• Missing data averaged and/or estimatedPositive Terminal Negative TerminalTemperature peak down centerline* Average over last five minutes of cycling, 115-120 min27Positive terminal heatingNegative terminalModule: Center Line Temperatures* 150 A, Sq Wave, Ttest = 30 C* Average over last five minutes of cycling, 115-120 min28Module: Cell 17 Detail, Exit Side Center150 A, Sq Wave, Ttest = 30 C• Capacitor midpoint significantly hotter than terminals, may be good place for maximum temperature measurement• ~40% temperature drop across fin• ~17% temperature drop from terminal to thermal pad• ~ 5% of the temperature drop across thermal pad (≈0.85°C)Heat SinkThermal PadThermal NutBalance BarBus BarFinal Average Temperature** Average over last five minutes of cycling, 115-120 min-+29Module: Observed Self-Cooling, Cell Temperatures Ttest = 30 C, Fans Off, Full Discharge•5.5 A charge to 125 V, open-circuit rest for 5 hrs, 20 A discharge to 0 V•Observed cell self-cooling to below ambient temperature30Module: Observed Self-Cooling, Fans OffCell 17 Detail (Air Exit Center Cell), Ttest = 30 C• Cooling trend from cell midpoint to outside environment• Cell surface midpoint cools ~ 0.82 C• Requiring ~ 483 JHeat SinkThermal PadThermal NutBalance BarBus Bar5 sec moving average taken atcell midpoint minimum31Investigation of Self-Cooling,Endothermic Calorimeter Response on Discharge: BCAP 3000-P Cell, Ttest = 30 C• Preliminary data (insufficient rest periods)• Endothermic response measured• Lead heating will decrease measured endothermic response for the inner chamber• On the order of 345 [J] of cooling for a 1.3 V discharge• If linear with voltage change – ~628 [J] of cooling on full discharge – giving ~1.06 C of cooling for a cell• Consider the heat gain from the environment and additional thermal mass in the module (terminal nuts, bus bar, and heat sink)Insufficient rest periodsLead heatingCharge and clampDischarge from Vmax to 0.5 VmaxTest description: charge at 5.5 A to 2.6 V, clamp voltage for 3 hrs, discharge to 1.3 V at Idis, rest 3 hrsPossibly caused by self-discharge10 A 20 A 30 A 50 A 70 A 90 APreliminary DataIdis32Explanation of Self-Cooling, Reversible Heat Effect:Entropy Theory Compared to MeasurementC: Cell capacitance [F]e: Elementary Charge [C]k: Boltzmann constant [J/K]Q: Heat [J]S: Entropy [J/K]t: time [s]T: temperature [K]U: Potential [V]VH: Helmholtz Layer Volume [cm^3]Vo: Total electrolyte Volume [cm^3]Charged State Discharged StateReversible heat, entropy model*Simplified Helmholtz Layer Assumption* Schiffer, J., et al. (2006). "Heat generation in double layer capacitors.” Journal of Power Sources 160(1): 765-772.• Assume– Entropy model suggested by Schiffer et al.– A specific capacitance of     6.5-30 μF/cm2– Vo ≈ 200 cm3– dHelmholtz = 0.8 nm– T change can be neglected for entropy calculation• Found– Qdischarge = -203 [J] to -515 [J] • For a 1.3 V discharge• T = 30 C– Agrees reasonably well with the Q ~ -345 [J] measured in the calorimeterdtdUVVeCkTdtdsTdtdQoHmeasrev−=−= ln2,33Conclusions: Cell• Thermal efficiency decreased approximately linearly with current• Heat generation increased approximately with the square of current• Thermal imaging showed that – Positive cell terminals tend to heat faster, possibly because of cell construction– Cell terminal connections are important for thermal performance34Conclusions: Module• With ~80% of rated unrestricted air flow, the tested module was less than 2.8 C above its rated 150 A continuous current temperature• Vehicle environmental temperatures (-30 C- 52 C*) and power demands are highly variable, requiring an understanding of ultracapacitor temperatures as a function of these variables• The current level must be limited to prevent cells from reaching high temperatures that reduce life and reliability • The module had less than an 8 C rise after 120 minutes of continuous cycling on a simulation based HEV US06 drive cycle• Peak temperatures occurred near the module center and at the module positive terminal• Understanding module temperature distribution is critical to design effective thermal management systems and properly locate sensors • Preferential cooling of the module centerline and reduction of lead wire heating would be beneficial• Capacitor self-cooling was observed on discharge both at the module level and in the calorimeter*USABC FreedomCAR requirement35Acknowledgements• Supported by the Vehicle Technologies Program of the U.S. Department of Energy• Review and Technical Discussions– Oshkosh Corporation: Michael Bolton, Lorenzo Pisan– Maxwell Technologies: John M. Miller, Uday Deshpande, Jeremy Coperthwaite– JME: John R. Miller, Arkadiy Klementov– NREL: Bob Rehn, Ken Kelly, Rob Farrington, Barb Goodman, Terry Penney

Thermal Evaluation of a High-Voltage Ultracapacitor Module for Vehicle Applications

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Thermal Evaluation of a High-Voltage Ultracapacitor Module for Vehicle Applications

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