Energy Storage in Advanced Vehicle Systems-Stanford University

Batteries
•High energy density (30-150 Wh/kg) •Moderate power (< .5 kW/kg at 95% effic.) •Relatively short shelf life (1 year w/o recharge) •Relatively short cycle life (1Kcycles, deep disch.) •Cycle life can dependent on the pattern of SOC •Variation of power characteristics with SOC
Energy Density and Peak Power Density Projections for Various Energy Storage Technologies
Ultracapacitors
•High power and low resistance (2 kW/kg, 95%) •Long cycle life (>100K cycles) •Cycle life independent of the pattern of SOC •Long shelf life (>10 years) •Relatively low energy density (< 10 Wh/kg) •Power characteristics independent of SOC
Energy Storage in Advanced Vehicle Systems
Andrew Burke Institute of Transportation Studies University of California-Davis Davis, California 95616
Presentation at the Stanford University GCEP Advanced Transportation Workshop October 10,2005
Power density examples
Prius battery Ni Mt. Hydride Panasonic EV Claimed peak power 1080 W/kg, V min =.7 EF= .9, 443 W/kg, EF=.95, 205 W/kg Saft Li-ion battery Claimed peak power 2000 W/kg, V min =2.7 EF= .9, W/kg=820, EF=.95, W/kg= 432 Panasonic Lead -acid battery Claimed peak power 360 W/kg, V min =1.48 EF=.9, 154 W/kg, EF= .95, W/kg=81
2.5
1200
1.0
1.2
2.3
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A key issue in making this judgement is the peak pulse power density assumed for a particular battery. In some applications, the efficiency of the charge and discharge is a key issue. This is the case for hybrid vehicles. The power density of the battery and ultracapacitor should be compared at the same efficiency. The voltage drop due to the current pulse is the best measure of the efficiency (EF=Vp/Videal) of the discharge. The high power quoted for ultracapacitors is for 90-95 % efficiency and that for batteries is in most cases for 65-70% efficiency.
Applications
Passenger cars, vans, and SUVs Light and medium duty trucks Transit buses
Advanced Vehicle Systems
Electric vehicles (ZEVs) Hybrid-electric vehicles •Charge sustaining (liquid fuel only) •Charge depleting (grid-connected) Fuel cell powered vehicles (hydrogen fueled)
Energy storage system characteristics
System designed for specified peak (pulse) power kW and useable energy storage Wh or kWh Either the peak power or energy stored can be the determining requirement that sizes (weight and volume) of the energy storage system The peak power is the critical requirement for most vehicle designs Trade-offs between power density W/kg and energy density Wh/kg become important for vehicle applications Variation of power density with SOC and charge/discharge can be important Very high cycle life is needed
Roles of energy storage on a vehicle
Primary energy storage (average power) • liquid fuel •Batteries on an EV •Hydrogen on a FCV Secondary energy storage (pulsed power) •Batteries or ultracaps on a hybrid •Flywheel or hydraulics
Summary of the characteristics of carbon/carbon devices
Device V rated 2.5 2.7 2.3 2.7 2.7 2.7 2.7 C (F) 2700 10 120 1800 3640 5085 1375 R (mOhm) .32 25.0 21.0 .55 .30 .24 2.5 RC (sec) .86 .25 2.5 1.00 1.10 1.22 3.4 Wh/kg (1) 2.55 2.5 3.8 3.6 4.2 4.3 4.9 Maxwell** Ness Ness (3) Ness Ness Ness Asahi Glass (propylene carbonate) Panasonic (propylene carbonate) Panasonic Panasonic EPCOS EPCOS Montena Montena Okamura Power Sys. ESMA W/kg (95%) (2) 784 3040 282 975 928 958 390 W/kg Match. Imped. 6975 27000 3700 8674 8010 8532 3471 Wgt. (kg) .70 .0025 .017 .38 .65 .89 .210 (estimated) .34 Vol. lit. .62 .0015 .010 .277 .514 .712 .151
Energy storage technologies
Electrical energy storage •Batteries •Ultracapacitors Mechanical energy storage • flywheels •Hydraulics •Compressed air Chemical storage •Liquid fuels •Hydrogen
Pulsed operation of the energy storage
Near peak power for short periods 5-30 seconds High power charge and discharge pulses High efficiency at high power is needed for both charge and discharge pulses Average power is much less than pulse power Hundreds of thousands of pulses over the lifetime of the devices Energy storage required (Wh) is use-cycle dependent SOC variation must be controlled
When does the use of Ultracapacitors make sense?
The battery can be sized by the energy storage or peak power requirement. If the battery weight to meet the power requirement is several times greater than that needed to meet the energy requirement or the battery design or its operation must be compromised to meet the power requirement, then it makes sense to consider the use of ultracapacitors either in place of the batteries or in combination with batteries.