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ENERGY AND POWER STORAGES

Pierre DuysinxUniversity of Liège

Academic Year 2010-2011

ReferencesR. Bosch. « Automotive Handbook ». 5th edition. 2002. Society ofAutomotive Engineers (SAE)C.C. Chan and K.T. Chau. « Modern Electric Vehicle Technology »Oxford Science Technology. 2001.C.M. Jefferson & R.H. Barnard. Hybrid Vehicle Propulsion. WIT Press, 2002.J. Miller. Propulsion Systems for Hybrid Vehicles. IEE Power & Energyseries. IEE 2004.M. Ehsani, Y. Gao, S. Gay & A. Emadi. Modern Electric, Hybrid Electric, and Fuel Cell Vehicles. Fundamentals, Theory, and Design. CRC Press, 2005.L. Guzella & A. Sciarretta. Vehicle Propulsion Systems. Introduction to modelling and optimization. 2nd ed. Springer Verlag. 2007.

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OutlineIntroduction

Energy sources characteristics

Electrochemical batteries

Ultra capacitors

Flywheels

Comparison

Characteristics of Energy Sources

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Energy source characteristicsEnergy and coulometric capacityCut-off voltage and usable capacityDischarging/charging currentState-of-chargeEnergy densityPower densityCycle lifeEnergy efficiency and coulometric efficiencyCost

Energy and coulometric capacities

Mission of EV energy sources: supply electrical energy for propulsionEnergy capacity EC [J] or [Wh]

v(t) and i(t) instantaneous voltage and currentCoulometric capacity [Ah]

For EV, energy capacity is more important and useful than coulometric capacityThe coulometric capacity (or capacity) is widely employed to describe the capacity of batteries

0

( ) ( )t

EC v i dτ τ τ= ∫

0

( ) ( )t

CC Q t i dτ τ= = ∫

4

Cut-off voltage and usable capacity

Energy capacity does not represent well the energy content of electrochemical sourcesBatteries can not be discharged down to zero voltage unless being permanently damagedCut-off voltage: knee of discharging curve at which battery is considered as fully discharged, so called 100% depth of discharge (DoD)Usable energy capacity and usable coulometric capacity before cut-off

Cut-off voltage and usable capacity

Discharge curves for Powersonic VRLA batteries

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Discharging/charging current

For batteries, energy capacity and coulometric capacity vary with their discharging current, operating temperature, and ageing

The battery capacity is determined with constant current discharge-charge testsIn a typical constant current test, the battery is initially fully charged and the voltage equals the open-circuit voltage V0c.A constant discharge current is applied. After a certain time tf, called the discharge time, the voltage drops to the cut-off voltage and the battery is empty.


Discharge curves for LiPo Ultimate PX-01 batteries

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The discharge time is function of the discharge current. The phenomenon is often described using the Peukert equation :

‘n’ is the Peukert exponent which varies between 1 and 1.5 (e.g. in VRLA n~1.35)

This equation also express the dependency of the battery capacity on the discharge current. If the capacity is Q2* for a current I2*, the capacity for a discharge current I2 is

2n

ft Cste I −=

1

0 2* *0 2

nQ IQ I

−⎛ ⎞

= ⎜ ⎟⎝ ⎠


Other more sophisticated models of current dependency can be found in literature:

Neural Network-based modelsModified Peukert equation for low currents (Kc a constant)

( )( )0* 1*0 2 21 1 /

cn

c

Q KQ K I I

−=+ −

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Instead of discharge current, a non dimensionsional value called C-rate is used

The C-rate current is defined as the current I0 that discharges the battery in one hour and which has the same value as the battery capacity Q0=CC.The C rate is often written as C/k where k is the number of hours need to discharge the battery with a C-rated current, i.e. c=1/k current, a current k times lower than the rated current I0.

Example: C/5 rate for a 5 Ah battery means that kCn = 1/5*5=1A

CC (and also the EC) decreases with the increasing C rate

nI k C=

State-of-charge

The residual coulometric capacity is termed as the State-of-Charge (SOC)SOC defined as the percentage ratio of residual coulometriccapacity to usable coulometric capacity

SOC is affected by discharging current, operating temperature, and ageing

0

( )( )( )

Q tq t SOCQ t

= =

8

State-of-charge and energy capacity

Variation of the state of charge in a time interval dt with discharging current i approximated using the discharge current by charge balance

where Q(i) is the amp-hour capacity of the battery at current rate iThus state of charge SOC

0 ( )dSOC i dt

dt Q i=

00 ( )

i dtSOC SOCQ i

= − ∫

State-of-charge and energy capacityIn case of charge, the state of charge must take into account the fact that a fraction of current I2 is not transformed into charge due to irreversible parasitic reactions.It is often modeled using the charging or coulometric efficiency

The coulometric counting method is simple but requires frequent recalibration points.

The energy capacity can also be related to the current

2 ( )cQ I tη= −

( , ) ( )EC V i SOC i t dt= ∫

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State of charge and energy capacity

Energy density

Energy densities = usable energy density per unit mass or volume

Gravimetric energy density or specific energy: [Wh/kg]Volumetric energy density [Wh/l]

Gravimetric energy densityThe more important because of weight penalty on consumption and driving rangeKey parameter to assess suitability to EV

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Power density

Power density = deliverable rate of energy per unit of mass or volume

Specific power [W/kg]

Power density [W/l]Specific power is important for EV applications because of acceleration and hill climbing capabilityFor batteries, specific power vary with the level of DoDSpecific power is quoted with the percentage of DoD

Cycle life

Cycle life: number of deep-discharge cycles before failure Key parameter to describe the life of EV sources based on the principle of EV storageCycle life is greatly affected by the DoD and its is quoted with the percentage of DoD.

Example: 400 cycles at 100% DoD or 1000 cycles at 50% DoD

For energy generation rather energy storage, the life is definedby the service life in hours or kilohours

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Energy efficiency

Energy efficiency is defined as the output energy over the input energyFor energy storage source, the energy efficiency is simply the ratio of the output electrical energy during discharging over the input electrical energy during charging.

Typically for batteries in the range of 60-90%

Charge efficiency defined as the ratio of discharged coulometriccharge Ah to the charged coulometric capacity Ah.

Typically for batteries in the range of 65-90%

For EV, energy efficiency is more important than charge efficiency

Energy efficiency

The energy or power losses of batteries during charging discharging appear in the form of voltage losses. Thus the efficiency of the battery during charging / discharging can be defined any operating point as the ratio of the cell voltage V to the thermodynamic voltage V0.

During discharging:

During charging

0

VV

η =

0VV

η =

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Energy efficiency

The terminal voltage is a function of the current, the energy stored or the SOC. The terminal voltage is lower in discharging and higher in charging than the electrical potential produced by chemical reactions.The battery has a high discharging efficiency with a high SOC and a high charging with a low SOC. Thus net cycle efficiency is maximum around the middle range of SOC

Cost

Cost includes: initial (manufacturing) cost + operational (maintenance) cost

Manufacturing cost is the most important one

Cost is a sensitive parameter for EV sources because it is a penalty with respect to other energy sourcesCost in €/kWh or $/kWhPresently cost in the range of 120 to 1200 €/kWh

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Ideal batteries for EV

High specific energy great range and low energy consumptionHigh specific power high performanceLong life to enable comparable vehicle lifeHigh efficiency and cost effectiveness to achieve economical operation and maintenance free

Electrochemical batteries

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Batteries: principles

Basic element of each battery is the electrochemical cellCells are connected in series or in parallel to form a batteryBasic principle of batteries

Positive and negative electrodes are immersed in an electrolyte

Batteries: principles

During discharge, negative electrodes performs oxidation reaction and drives electrons to the positive electrode via external circuit. Positive electrode performs reduction reaction that absorbs electronsDuring charge, the process is reversed and electrons are injected in negative electrode to perform reduction on negative electrode and oxidation in positive electrodes

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Lead-acid batteries

Invented in 1860, lead acid batteries are successful product formore than one centuryLow price and mature technology even if new designs are still continued to be developed to meet higher performance criteria.

Battery principle:Negative electrode: metallic leadPositive electrode: lead dioxideElectrolyte: Sulfuric acidElectrochemical reaction:

2 2 4 4 22 2 2Pb PbO H SO PbSO H O+ + ↔ +

Lead-Acid batteries: electrochemical reactions

Discharging:Anode (negative electrode): porous lead

Cathode (positive electrode): porous lead oxide

24 4 2Pb SO PbSO e− −+ → +

22 4 4 24 2 2PbO H SO e PbSO H O+ − −+ + + → +

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Lead-Acid batteries: electrochemical reactions

ChargingAnode (negative electrode)

Cathode (positive electrode)

Overall

24 42PbSO e Pb SO− −+ → +

24 2 2 42 4 2PbSO H O PbO H SO e+ − −+ → + + +

2 2 4 4 22 2 2Pb PbO H SO PbSO H O+ + +

Lead-Acid batteries: Thermodynamic voltage

Thermodynamic voltage of the battery cell is related to energy released and the number of electrons transferred in the reactionThe energy released is given by the change of Gibbs free energy ∆G usually expressed per mole quantity

In which Gi and Gj are the free energy of products i and reactant species j.

Products Reactantsi jG G G∆ = −∑ ∑

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In reversible conditions, all the ∆G is converted in electric energy

F=96495 F, the Faraday constant, the number of coulombs per mole and Vrev the reversible voltage

At standard conditions T=25°C, p=1 atm

revG n F V∆ = −

00

rev

GVn F∆

= −


The change of free energy and thus cell voltage are function of the activities of the solution species

The Nernst relationship gives the dependence of ∆G on reactants activities

R universal constant R=8,314 J/molK and T absolute temperature

0 (activities of products)ln

(activities of reactants)rev rev

RTV Vn F

⎡ ⎤= − ⎢ ⎥

⎢ ⎥⎣ ⎦

∏∏

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Lead-Acid batteries: Specific energy

Specific energy is defined as the energy capacity per unity battery weight (Wh/kg)The theoretical specific energy is the maximum energy that can be generated by unit total mass of reactants

For lead-acid batteries: Vr=2,03 and ΣMi=642g

Actual specific energy (with container, etc.):

(Wh/kg)3,6 3,6

th rspec

i i

nFVGEM M

∆= − =

∑ ∑

170 Wh/kgthspecE =

45 Wh/kgthrealE =


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Best specific energy is obtained with the lightest elements: H, Li, Na… for negative electrode reactantsHalogens, oxygen, S… for positive reactants

Best electrode design for effective utilization of the containedactive materialsElectrolytes of high conductivity compatible with the materials & compatible with materials in both electrolytes

Aqueous electrolytes at room temperature

Choice of metallic reactant:Aqueous electrolytes prohibits alkali which react with water

other metals with a reasonable electro negativity: Zn, Al, Fe and which are rather abundant not to be too expensive

Lead-Acid batteries: Specific power

Specific power is defined as the maximum power per unit battery weight that can be deliveredSpecific power is important for battery weight especially in high-power demand applications such as HEVThe specific power depends mostly depends on the battery internal resistance

Rint represent the voltage drop which is associated with the battery current

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int4( )peakc

VPR R

=+

20


The Rint represents the voltage drop which is associated with the battery currentRint depends on two components:

Reaction activity

Electrolyte concentration

IL = limit current

logAV a n I∆ = +

ln 1CL

RT IVnF I

⎛ ⎞∆ = − −⎜ ⎟

⎝ ⎠


The voltage drop Increases with increasing in discharge currentDecreases with the stored energy

Specific energy is high in advanced batteries, but still need to be improvedSpecific energy of 300 W/kg is still an optimistic targetSome new developments with SAFT

HEV batteries with 85 Wh/kg and 1350 W/kgEV batteries with 150 Wh/kg and 420 W/Kg

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Energy sources categoriesRechargeable electrochemical batteries

Lead Acid (Valve Regulated Lead Acid=VRLA)Nickel based: Ni-Iron, Ni-Cd, Ni-Zn, Ni-MHMetal/air: Zn/air, Al/air, Fe/airMolten salt: Sodium-β: Na/S, Na/NiCl2, FeS2

Ambient temperature lithium: Li-polymer, Li-ions

SupercapacitorsUltrahigh speed flywheelsFuel cells

Lead-acid batteries

Voltages:Nominal cell voltage: 2,03 V (highest one of all aqueous electrolytes batteries)Cut-off voltage: 1,75 VVoltage depends on sulfuric acid concentration, so that voltage varies with SOCGassing voltage (decomposition electrolysis of water at 2.4 V)

Energy/ power densitySpecific energy density: 35 Wh/kgEnergy density 70 Wh/lSpecific power density: 200 W/kg

Energy efficiency: >80%Self discharge rate: <1% per 48 hoursCycle life: 500-1000 cyclesCost: 120-150 $/kWh

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Lead-acid batteriesVoltage depends on sulfuric acid concentration, so that voltage varies with SOC

Nickel-based batteries

Family of different kinds of electrochemical batteries using nickel oxyhydroxide (-OOH)

Ni-Fe invented by EdisonNi-Cd is known from 1930iesNi-MH is used in modern HEV vehicles (1990ies)Ni-Zn is still under development

Use Nickel oxy hydroxide as the active material for the positiveelectrode

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Ni-Cd batteries

Developed in the 1930ies, it is used in heavy industry for 80 yearsBattery principle:

Negative electrode: metallic cadmium CdPositive electrode: nickel oxyhydroxideNiOOHElectrolyte: KOHElectrochemical reaction:

2 2 22 2 2 ( ) 2 ( )Cd NiOOH H O Cd OH Ni OH+ + +

Ni-Cd batteries

Voltages:Nominal cell voltage: 1,2 V Cut-off voltage: 1,00 V

Energy/ power densitySpecific energy density: 56 Wh/kgEnergy density 110 Wh/lSpecific power density: 80-150 W/kg

Energy efficiency: 75%Self discharge rate: 1% per 48 hoursCycle life: ~800 cyclesCost: 250-350 $/kWh

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Ni-Cd batteries

Discharge curves of Ni-Cd

Ni-Cd batteries

Advantages:Medium specific energyMedium cycle lifeFlat curveGood performances in low temperaturesEasiness of charge

DisadvantagesCd environmentally not friendly and carcinogenicityMemory effect sensitivitySelf discharge

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Ni-MH batteries

Developed and marketed in the 1990ies, it is a rising star used in many modern HEVSimilar principle to Ni-Cd but it uses hydrogen absorbed in a metal hydride for the active material in negative electrode

Battery principle:Negative electrode: hydrogen absorbed in metal hydride: MHPositive electrode: nickel oxyhydroxide Ni(OH)2

Electrolyte: KOHElectrochemical reaction:

2( )MH NiOOH M Ni OH+ +

Ni-MH batteries

Key compound is MH, the metal alloy that is able to absorb / deabsorb hydrogen with a high efficiency with a high number of cycles.

AB5 rare-earth such us Lanthanum with NiAB2: Ti or Zr alloys with Ni

Voltages:Nominal cell voltage: 1,2 V

Energy/ power densitySpecific energy density: 70 - 95 Wh/kgEnergy density 150 Wh/lSpecific power density: 200 – 300 W/kg

Energy efficiency: 75% (70 90) Cycle life: 750-1200 cyclesSelf discharge: 6% per 48 hours Cost: 200-350$/kWh

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Ni-MH batteries

Discharge curves of Ni-MH

Ni-MH batteries

Advantages:High specific energyHigh specific powerFlat discharge curveFast chargeLow sensitivity to memory effect

DisadvantagesDo not withstand overchargingDetection of end of chargeLittle better life cycle than Ni-Cd

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Lithium-based batteries

Lithium is the lightest metallic element which allows for interesting electrochemical properties:

High thermodynamic voltageHigh energy and power density

Two major technologies of electrochemical batteries using lithium

Lithium-polymerLithium-ions

Li-ions batteries

First developed in the 1990ies, it has experienced an unprecedented raise and it is now considered as one of the most promising rechargeable battery of the futureAlthough still at development stage, it is already gained acceptance in HEV and EV.

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Li-ions battery principle

Negative electrode: lithiated carbon intercallated LixCPositive electrode: lithiated transition metal intercalation oxide Li1-xMyOz (ex LiCoO2)Electrolyte: liquid organic solution or a solid polymer

Electrochemical reaction:

for example

1x x y z y zLi C Li M O C LiM O−+ ↔ +

( ) 6 [1 ( )] 2 6 (1 ) 2y x y x y yLi C Li CoO Li C Li CoO+ − − −+ +

Li-ions batteries

Discharge: Li ions are released from negative electrodes, migrate via the electrolyte and are taken up by the positive electrode.

Possible positive electrode materials are Li1-xCoO2, Li1-xNiO2, and Li1-xMn2O4 that are stable in air, high voltage, reversibility for lithium intercalation reaction.

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Li-ions batteries

For LixC / Li1-xNiO2 battery (C/LiNiO2 or nickel based li-ion battery)Voltages:

Nominal cell voltage: 4 V

Energy/ power densitySpecific energy density: 80-130 Wh/kgEnergy density 200 Wh/lSpecific power density: 200-300 W/kg

Energy efficiency: 95% Cycle life: > 1000 cyclesSelf discharge rate: 0,7% per 48 hoursCost: 200 $/kWhHigher performance for LixC / Li1-xCoO2 but also higher cost due to Cobalt

Li-ions batteries

Advantages:High specific energy and high specific powerNo memory effectLow self dischargeNo major pollutants

DisadvantagesDo not withstand overcharging, May present dangerous behavior when misusageShort circuit due to metallic Lithium dendritic growth

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Li-polymer batteries

Li-ions battery principle:Negative electrode: lithium metalPositive electrode: transition metal intercalation oxide MyOz

Electrolyte: thin solid polymer electrolyte SPEElectrochemical reaction:

The layered structure of transition metal oxide MyOz allows lithium ions to be inserted and removed for charge and dischargeDischarge: Li ions that are formed at the negative electrodes migrate through the SPE and are inserted into the crystal structure at the positive electrode.

y z x y zx Li M O Li M O+ ↔


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Li-ions batteries

Thin solid polymer electrolyte: improved safety and flexibilityCapability of fabrication in various shapes and sizes and safe designsPossible positive electrode material are Vanadium oxides: V6O13

Battery Li/SPE/V6O13

Voltages:Nominal cell voltage: 3 V

Specific energy density: 155 Wh/kg Specific power density: 315 W/kg Energy efficiency: 85% Self discharge: 0,5% per month


Advantages:High specific energy and high specific powerNo memory effectLow self dischargeNo major pollutantsVarious shapes and sizes

DisadvantagesLower power and energy density than Li-ionsMore expensive than Li-ionsSpecific energy density: 155 Wh/kgCharging must be carefully conducted to avoid inflammation

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Supercapacitors

Capacitors and SupercapacitorsCapacitors = simple devices capable of storing electrical energy

Capacitors = electrostatic capacitorsEssential component in electronicsCapacitance ~ pF to µF

Electrolytic Double Layer Capacitors (EDLC) or ultra / supercapacitors

Capacitance ~ F – kFWorking principle: double electrolytic layer by Helmotz

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Capacitors and SupercapacitorsOne must distinguish the electrostatic capacitors from supercapacitors that use the Helmotz double layer principle

Supercapacitors: principle

Supercapacitors are based on the double layer technologyWhen carbon electrodes are immersed in aqueous electrolyte, the ions will accumulate at the porous interface of the electrode. The ions and the current carriers make a capacitors with a gap of a few nanometers

Separator

Electrolyte Electrode in porous carbon

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EQUILIBRIUM CONDITION OF MATERIALS

WITH DIFFERENT ELECTRICCONDUCTIVITY

EQUILIBRIUM CONDITION OF A SYSTEMCOMPRISING OF TWO DIFFERENT MATERIALS

DOUBLEELECTRIC

LAYER


Uncharged state Charging

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Charged stateDischarging

SupercapacitorsAn electric double layer occurs at the boundary of the electrodeand the electrolyte.The electric double layer works as an insulator when voltage stays below the decomposition voltage of water.Charge carriers are accumulated at the electrodes and one measures a capacitance

Because of the double layer, one has two capacitances in series

1 ²2

E CV=

1 2

1 1 1C C C= +

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Electric double layer capacitor principles

Capacitance

Distance d of the layer ~1 nmC ~ 0,1 F/m²

Increase S: surface area of the electrode / electrolyte specific surface

For activated carbon, S depends on the micropores amount (<2nm)S~1000 m²/g 1000 F/g

SupercapacitorsCapacitors with double layers are limited to 0,9 volts per cell for aqueous electrolyte and about 2.3 to 2.3 V with non aqueous electrolyte Because the double layer is very thin (couple of nanometers), capacity per area is high: 2,5 to 5 µF/cm²With material with a high active area (1000 m²/g) like activated carbon one gets a capacity about 50 F/g

1000 m²/g x 5µ F/cm² x 10000 cm²/m² = 50 F/g

Assuming the same amount of electrolyte as carbon, one gets a capacity of 25 F/gHowever, the energy density is rather low: <10 Wh/kg (remind VRLA ~ 40 Wh/kg)

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Supercapacitors: equivalent electrical scheme

ССDELDEL ССDELDEL

RRleckleck RRleckleck

RRIsolIsol

RRelel

ССDEL DEL –– CapacityCapacity of Double of Double ElectricalElectrical LayerLayer

RRleckleck –– Leakage ResistanceLeakage Resistance

RRelel –– Electrolyte ResistanceElectrolyte Resistance

RRisolisol –– Isolation ResistanceIsolation Resistance

Supercapacitor Technologies

separator layer

electrode layermantle

electrode layer

mantle

separator layer

wound supercapacitor single cell (schematic)

stacked cells

Traditional - wounded cells

up to 300 -

1000 V

BALANCING SYSTEM

No balancing systemrequired

D. Sojref, European Meeting on Supercapacitors, Berlin, D. Sojref, European Meeting on Supercapacitors, Berlin, DecemberDecember 20052005

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Comparison of SuperCaps technologies

SuperCaps: manufacturing technology

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SuperCaps: manufacturing technology

Working environment:- Modular assembly and connection- Thermal exchange,- Electrical insulation- Failure mode

Supercapacitors: stacked constructions

Structure of Newcond GmbH supercaps

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SuperCaps Manufacturers

SuperCaps Manufacturers

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Super capacitors and batteries

Supercapacitors are distinguished from other class of devices for storing electrical energy such batteries

Absord / release energy much faster: 100-1000 times: (Power density ~ 10 kW/kg)Lower energy density ( Energy density < 10 kWh/kg)Higher charge / discharge current : 1000 ALonger life time: > 100.000 charge dischargeBetter recycling performances


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C.J. Farahmandi, C.J. Farahmandi, AdvancedAdvanced CapacitorCapacitor World World SummitSummit, San , San Diego,CADiego,CA, July 2008, July 2008

* * 10-3 to 30 s

> 500.000> 500.0001.000Life (cycles)

10-3 to 10-6 s0,3 to 30 s*1 to 5 hrsCharge Time

< 100.000< 7000< 1000Specific Power (W/kg)

0,11 to 1010 to 100Energy Density (Wh/kg)

CapacitorSupercapacitorBattery

Supercapacitor

Envisioned applicationsWhenever a high power is requested for a short time (electric and hybrid vehicles, tramways, diesel engine starting, cranes, wind turbines, computers, lasers,…)Capacitors are generally combined with another energy source to increase power, to improve economic efficiency and to preserve ecological demandCapacitors can be used as buffer for later charging of a battery

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Envisioned applications

Major component to low carbon economy:Enhancing energy efficiency

Mobile applications: Hybrid propulsion systemsAutomobile, railway

Stationary applications:Clean Electrical Power Smart grids including renewable energy sources

New applicationsMechatronic applications: All electric systemsMedical applicationsDefense applicationsSafety applications:

Emergency systems

Transport applicationsStop & Start of engines 2020

Fast start devices of large Diesel enginesStart & stop of automobile engines

Braking Energy recovery 2020-2030KERS systems

Peak power source for acceleration 2020-2030Mild hybrid

Sub-station stabilization 2020-2030Reduction of peak power stress

Short range autonomy of railway vehicles 2020-2030Tramway and trolley in historical cities

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Mechatronic applications

Criteria for power application in transport

Intrinsic performances :high cell voltage, energy density, max temperature 60°C (…80°C…)new generation aqueous and stacked supercapacitors have to be push for high voltage and mecatronicApplications

lowest ration Rs / C (reduced losses)today: 200-300µΩ for 2600F-2,7V - 25°C (DC method) 100-200µΩfor 5000F-2,7Vtomorrow: < 100µΩ for 9000F-2,7V

power cycling reliability on vehicle mission profiles

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Criteria for power application in transport

Assembly performances :lower total serial resistance (reduce contact resistance), stacked technology

simplification of cells balancing

Safety : identified failure modes (gas leakage, …sealing box, neutralisation gas) : opening case due to over pressure, or internal electrical disconnection

Systems interface : high voltage DC bus level … high voltage assembly …chopper link

Maintenance strategy : predictive solution, replacing cell strategy


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Supercapacitors: ECON

4,861,020,28175,0036,75364202300,70000,151000700,036/700

2,001,280,36100,0064,00506602300,40000,801000400,064/400

3,131,250,35100,0040,00323802300,40000,501000400,040/400

2,640,630,1876,5618,38293102300,40000,301000350,018/350

1,972,370,6675,0090,00385702300,30002,001000300,090/300

1,501,010,2860,0040,50404902300,37500,901000300,040/300

3,130,830,2375,0019,80242002300,30000,441000300,020/300

1,131,180,3356,3359,15506302300,30001,751000260,060/260

1,392,500,7050,0090,00365502300,20004,501100200,090/200

1,511,700,4756,0062,72373802300,0035160,00400028,060/28

1,371,570,4435,6440,77263002300,0055104,00400028,040/28

0,750,880,2410,8912,74151302300,0045130,00135014,012/14

0,820,930,268,179,3110952300,006095,0067014,09/14

(KW/Kg)(Kj/Kg)(Wh/Kg)(KW)(Kj)(Kg)(mm)(mm)(Ohm)(F)(A)(V)

Specific Power

Specific Energy

Specific Energy

PowerEnergyWeightHighDiameterInt. resist

CapacityCurrentVoltageType

Automotive applicationsComparison of Supercaps with respect to batteries will be demonstrated using the eco efficiency approach

Ecoefficiency: global index which accounts for both:Environmental impacts: CO2 + emissions + noiseUser satisfaction criteria (US): composite index that assesses the ability of the vehicle to meet customer’s expectations about his transport needs: Performance + Daily Cost…

Fair comparison between antagonist constraints is provided by using an optimization approach

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Automotive applicationsFind best designs by solving optimization problem

Min Environment Impact; Performance Indexdesign variables: size of energy storage, ICE engine, electric machines

Subject to:Design constraint on mass, acceleration, cost, etc.

Choice: mild hybrid busHybrid Electric with NiMH batteriesHybrid Electric with Supercap (Maxwell)Hybrid hydraulic bus

Application: bus optimizationModelling & Simulation: ADVISORVanHool A300 Bus

Typical 12 meters bus used by public operators in Belgium

Reference propulsion systemICE Man diesel engine 205 kW (here Detroit Diesel engine from ADVISOR)Number of passengers: 33-110

here 66 passengers

Driving CyclesSORT 2 Bus Drive Cycle by IUTPCommercial speed: 17 kph (urban driving situation)

Figure 5: SORT 2 drive cycle, for easy urban cycle

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Hybrid Electric Vehicles

Hybrid Hydraulic Vehicle

New reversible hydraulicmotor /pump: Low drag, high efficiency, fluid=water

Parker P2 or P3 series

Hydraulic accumulators (HP) / Reservoir (LP): high efficiency (95%)En.: 0,63 Wh/kg Power ~ 90 kW/kg

Hydac

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ECOEFFICIENCY: AN OPTIMIZATION APPROACH

A parametric study is made in BOSS QUATTRO to construct some response surface approximations of US and Ecoscore

The ecoefficiency design optimization problem is solved using a multi objective genetic algorithm (MOGA) available in BOSS-QUATTRO based on response surface method

Parametric models (scaling factors) inADVISORSimulation of performances and fuel consumption & emissions against driving cycles

Optimization: Problem Statement

Mathematical multi objective design problem statement:Minimize:

With respect to

Subject to:

max

max

201005%

20000

500000€150 200

50 100400 800

acc

engine

motor

MB

t sv kphpm kg

CP

PN

≤

≥

≥

≤

≤≤ ≤

≤ ≤

≤ ≤

coscore1 2( ) ( ( ) ; ( ) 1/ )F X f x E f x US= = =

/( , , )engine motor bat scX P P N=

50

0,7

0,72

0,74

0,76

0,78

0,8

0,82

0,84

0,86

1 1,05 1,1 1,15 1,2 1,25 1,3 1,35 1,4

Minimiser l'impact environnemental (SE)

Min

imis

er (1

/SU

)

Bus_NiMH Bus_SPCAPS Bus_Hydr

Numerical Application

Figure 5: SORT 2 drive cycle, for easy urban cycle

1

1,5

2

2,5

3

3,5

4

4,5

5

5,5

6

1 1,2 1,4 1,6 1,8 2 2,2 2,4 2,6Min (impact environnemental)

Min

(1/U

S)

Bus_NiMH Bus_SC Bus_HYDR

Numerical Application

51

Hybridization of Energy Sources

Hybrid energy storage systems

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Flywheels

FlywheelsPrinciple: Storing energy as kinetic energy in high speed rotating disk

Idea developed 25 years ago in Oerlikon Engineering company, Switzerland for a hybrid electric bus

Weight = 1500 kg and rotation speed 3000 rpm

Traditional design is a heavy steel rotor of hundreds of kg spinning at ~1.000 rpm.

Advanced modern flywheels: composite lightweight flywheels (tens of kg) rotating at ~10.000 rpm

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Flywheels

A rotating flywheel stores the energy in the kinematic form

Jf= moment of inertia and ωf= rotation speed

The formula indicates that enhancing the rotation velocity is the key to increasing the energy storage. One can achieve nowadays rotation speed of 60,000 rpmFirst generation flywheel energy storage systems use a large steel flywheel rotating on mechanical bearings.

Newer systems use carbon-fiber composite rotors that have a higher tensile strength than steel and are an order of magnitude lighter.

212f f fE J ω=

Flywheels

Pentadyne flywheel

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FlywheelsA typical system consists of a rotor suspended by bearings inside a vacuum chamber to reduce friction, connected to a combination electric motor/electric generator.

FlywheelsWith this current technology it is difficult to couple directly the flywheel to the propelling system of the car.One would need a continuous variable transmission with a wide gear ratio variation range.The common approach consists in coupling an electric machine to the flywheel directly or via a transmission: One makes a ‘mechanical battery’The electric machine functions as the energy input / output portconverting the mechanical energy into electrical energy and vice-versa

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Flywheels

Principle of flywheels

Comparison of energy storage systems for electric and hybrid vehicles

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Energy and peak power storages

Different types of batteries:Lead-Acid: developed since 1900 with a high industrial maturityNi-Cd : developed from 1930ies, with an industrial maturityNa Ni Cl (Zebra): since 1980ies small seriesNiMH: since 1990ies, industrial productionLi-Ions: still under industrial development

Peak power sources:Double layer super capacitorsHigh speed flywheels

Energy and peak power storages

Performance criteria for selection (decreasing importance):Specific energy (W.h/kg)Specific power (W/kg)Number of charge cyclesLife timeSpecific costCharge discharge efficiencyVoltageVolumeRecycling

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Traction batteries

200200-350250-350120-150Cost [$/kW.h]

1000+1200750-1200800500-1000Life time [cycles]

90-95857075>80%Charge discharge efficiency [%]

200-300148200-30080-150150-400Specific power [W/kg]

80-1307470-9550-6035-50Specific energy [W.h/kg]

Li-IonsZebraNi-MHNi-CdLead acidBatteries

Problem of batteries w.r.t. fuels

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Problem of batteries w.r.t. fuels

8421001420Specific energy at wheel [W.h/kg]

801812Mean conversion efficiency in vehicle [%]

10511.66711.833Specific energy / PCI [W.h/kg]

Li-IonsDieselGasolineFuel

Facteur 200!

Energy density vs power density

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Energy density vs power density

Energy per volume / per weight

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Discharge characteristic time

EP

τ =

Discharge characteristic time

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Efficiency vs life cycles

Investment cost

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