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Energy storage and applications with supercapacitorsDr. P. Barrade/A. Rufer

Supercapacitors: Summary

• Generalities on Supercapacitors– Principle– Model of a supercapacitor– Series connection of supercapacitors

• Sizing of a supercapacitive tank– Sizing method– Energy efficiency and power availability

• Applications and converters– Voltage drop compensation for weak distribution networks– Energy buffers for elevators– Uninterruptible power supplies

• Conclusion


Generalities on Supercapacitors

• Principle

– Supercapacitors: electrochemical double layer capacitor

– High energy density, together with a high power density

– Energy stored by charge transfer at the boundary between electrodes and electrolyte

– Amount of stored energy is a function of :• Electrode surface,• size of ions,• level of electrolyte decomposition voltage

Separator Electrolyte

Electrodes Currentcollector

+ -

-+-+-+-+-+-+-+-+-+-+-+-+

-+-+-+-+-+-+-+-+-+-+-+-+

-+-+-+-+-+

-+-+-+-+-+-+-+-+-+-+-+-+

-+-+-+-+-+-+-+-+-+-+-+-+

-+-+-+-+-+



• Principle : technology– Electrodes: activated carbon

• High surface area part• Define the energy density

– Current collectors• High conducting part

– Membrane• Avoids electronic contacts between the

electrodes• Allows the mobility of the charged ions

– Electrolyte• Supplies and conducts ions• Dissociation voltage of organic electrolytes

less than 3V• Organic electrolytes have a lower ionic

conductivity (reduced power capability)

Separator Electrolyte

Electrodes Currentcollector

+ -

-+-+-+-+-+-+-+-+-+-+-+-+

-+-+-+-+-+-+-+-+-+-+-+-+

-+-+-+-+-+

-+-+-+-+-+-+-+-+-+-+-+-+

-+-+-+-+-+-+-+-+-+-+-+-+

-+-+-+-+-+



• Model of supercapacitors

Rs

Co

ic

ucCuu

r1 r2 rn

c1 c2 cn

Rl

– Voltage dependant capacitance– Series resistor

– Leakage resistor– rc sub-circuits (relaxation phenomena)



• Series connection of supercapacitors– Voltage sharing solution, using power electronics solutions to offer a high efficiency.

C1

C2

C3

D2

D3

D1

Ns1

Ns2

Ns3

Np

I

i1

i2

i3

T

C1

C2

C3

L1

L2

T1 D1

T2 D2T'2 D'2

T'3 D'3

I

i1

i2il1

il2

i'2

i3C1

C2

C3

T1

T2

T3

D2

D3

D1

Np1

Np2

Np3

Df

Nf

I

i1

i2

i3

forward dc-dc converterwith distributed primary

association of buck-boostdc-dc converters

Centralized flyback dc-dcconverter with distributed

secondary


Sizing of a supercapacitive tank

• Sizing method– Energy stored in a supercapacitor

2M M

1W CU2

=

– Voltage discharge ratio: the minimum voltage during the discharge has to be limited for efficiency reasons

m

M

Ud 100U

=

– The usable energy is then only part of the maximum stored energy2

u MdW W 1

100⎛ ⎞⎛ ⎞= −⎜ ⎟⎜ ⎟⎜ ⎟⎝ ⎠⎝ ⎠

– Number of supercapacitors for a given usable energy

us 2

2M

2WNdCU 1

100

=⎡ ⎤⎛ ⎞−⎢ ⎥⎜ ⎟

⎝ ⎠⎢ ⎥⎣ ⎦



• Sizing method– Example

Wu=0.55MJ (5.7kW.h) C=1800F, UM=2.5V, IM=200A

Ns d (%) Volume (m3) Weight (kg) E (kw.h)4872 50 1.46 1948 7.6 5709 60 1.71 2283 8.92 7164 70 2.15 2865 11.2

The needed energy isEu=20.55MJ(5.7kWh)



• Energy efficiency and power availability– Due to the series resistor, energy efficiency of supercapacitors has to be taken into

account during the sizing of the supercapacitive tank– Energy efficiency has also an influence on the power availability– Two cases have to be investigated:

• Energy efficiency for a constant current charge/discharge• Energy efficiency for a constant power charge/discharge

Ue

L Rs

C ucic

Rs

Cuc

ic

Iu

u

IP=Cte



• Energy efficiency and power availability : Example of a 2600F/2.5V/0.7mΩ ScapCharge Discharge

– Time for loading energy has to be kept up to 10s for a 90% energy efficiency (Ic<320A or P<700W)

– Time for unloading energy has to be kept up to 10s for a 90% energy efficiency (Ic<320A or P<400W)

– Taking into account the necessity of 90% of energy efficiency, the current or the power for charging/discharging have to be limited: the power density is only 806W/kg (instead of 4300W/kg)


Applications and converters

• Voltage drop compensation for weak distribution networks– Using supercapacitors to provide/absorb the energy needed to maintain at a constant level

the end-of-line voltage.

TL

675Vdc

SupplyStation

3 kmVal-VertStation

BellefontaineStation

DC

DC

End of lineSupercapacitive

Tank

Val-VertStation



• Voltage drop compensation for weak distribution networks– Typical waveforms on the line No7 (Lausanne, CH)

Power ProfileAt the “Bellefontaine” station

Voltage at the end of line“Val-Vert” Station



• Voltage drop compensation for weak distribution networks– It is necessary to use a double stage power converter to interface the supercapacitive tank

to the end of the line.

SupercapacitiveTank

End ofLine



• Voltage drop compensation for weak distribution networks– Special control algorithm have to be implemented

Voline

Cinter Cscaps

PWM PWM

Vscaps_max

DOUBLE-STAGE DC/DC CONVERTER CONTROL

+

-

G1

-1

Vinter_ref

Iref_charge_scaps_max

+

+

+

-+

-

+

-

+

-

RECTIFIERSUBSTATION SUPERCAPACITORS-BASED SUBSTATIONLINE

(CATENARY)

R_Vendline

R_Vinter

R_Iinject R_Iscaps

R_Vscaps

Vendline Iinject Iscaps VscapsVinter

Vendline_ref-

+Itransfer

ymaxIline_r G2RlineDeltaVline

L2 RL2L1 RL1



• Voltage drop compensation for weak distribution networks– Simulation results

0 200 400 600 800 1000 1200 1400 1600 1800-800

-600

-400

-200

0

200

400

600

800

t (s)

U end-of-line (V)

U Scaps (V)

I Scaps (A)



Principle of operationMG M

RECUPERATION

Diesel

Scap

ConvertisseurAC - DC

unidirectionnel

ConvertisseurDC - AC

bidirectionnel

ConvertisseurDC - DC

bidirectionnel

Elémertsstockeursd’énergie

• Application in transportation : reduction of emissions and energy savings– Diesel-electric trains: reduction of emissions and energy savings– Partner : Stadler Rail/ ABB



The track

Instantanous value of power

0 2000 4000 6000 8000 10000 12000 14000 16000 180001000

1200

1400

1600

1800Altitude trajet COMPLET

temps (s)

Alti

tude

(m)

0 2000 4000 6000 8000 10000 12000 14000 16000 18000-2

-1.5

-1

-0.5

0

0.5

1x 106

Puissance des moteurs de traction trajet COMPLET

temps (s)

Puis

sanc

e (W

)

P netteP moyPmoy = 42.46 kW

Altitude

• Application in transportation : reduction of emissions and energy savings



0 100 200 300 400 500 6000

100

200

300

400

500

600

700Réduction de la puissance max des moteurs diesel en fonction de la quantité d`énergie stockée

Energie stockée (MJ)

Réd

ucio

n de

la p

uisa

nce

des

mot

eurs

die

sel (

kW)

• Application in transportation : reduction of emissions and energy savings– Reduction of the maximal delivered power of diesel motors in dependency of the amount

of stored energy



• Energy buffers for elevators

I D1I s

Us

Ud

Uc

Id

3

I D2



Speed, Position for a 10-floors up/down run Power and Energy for a 10-floors up/down run

The needed energy is 220kJ (61Wh)

• Energy buffers for elevators : – simulations regarding a real system

⇒car weight : 720kg, counter weight : 1440kg, load : 1400kg



Scaps voltage/current (10-floors up/down run) Powers for a 10-floors up/down run

• Energy buffers for elevators– Simulations regarding a real system

• The power provided by the network has only to compensate the losses of the lift drive


Conclusion

• Supercapacitors are new components for energy storage– High energy density (even if lower than batteries)– High power density

• Model of supercapacitors have to take into account– The voltage dependence of the capacitance– The series/leakage resistors– Relaxation phenomenon

• Energy efficiency– Power availability of supercapacitors is affected by their energy efficiency

• Applications– Main applications use supercapacitors as energy buffers– In applications where supercapacitors are used as main energy source (UPS), the reduced

energy density is compensate by their high power density, combined with an increased lifetime compared to batteries.

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