ULTRACAPACITOR BASED ENERGY STORAGE

ARAVIND G SROLL NO: 112S7 E1

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OBJECTIVES

• CONCEPT OF ULTRACAPACITOR

• MODELLING OF ULTRACAPACITOR

• MODEL IDENTFICATION OF ULTRACAPACITOR

• ENERGY ANALYSIS

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CONTENTS• INTRODUCTION

• COMPARISON WITH CONVENTIONAL BATTERIES

• ULTRACAPACITOR DYNAMIC MODEL

• MODEL IDENTIFICATION Impedance Spectroscopy Current Pulse Test

• ENERGY ANALYSIS

• APPLICATIONS OF ULTRACAPACITORS

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INTRODUCTION

• An electrochemical capacitor with relatively high energy density and power density in comparison to conventional capacitors

• Ultracapacitors of the same size as that of a conventional capacitor of capacitance, in the range of several tens of millifarads, will have a capacitance of several farads

• Ultracapacitors having capacities of upto 5000 farads have been developed

• The highest available ultracapacitor energy density is 30 Wh/kg

What is a ultracapacitor?

Contd….4

• In a conventional capacitor, the amount of charge stored per unit voltage is given by

• Ultracapacitors use "plates" that are in fact two layers of the same substrate, and their electrical properties, the so-called "electrical double layer", result in the effective separation of charge despite the vanishingly thin physical separation of the layers

• Existing ELDC’s have 1/10th energy-density but 10 to 100 times power-density in comparison to conventional batteries

Contd…. 5

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Comparison With Conventional Batteries

• Long life, with little degradation over hundreds of thousands of charge cycles.

• Environmental-friendly• Low cost per cycle• Extremely low internal resistance

(ESR) and consequent high cycle efficiency (95% or more)

• High output power• Improved safety, no corrosive

electrolyte and low toxicity of materials

• Simple charge methods—no full-charge detection is needed; no danger of overcharging

• The amount of energy stored per unit weight is generally lower than that of an electrochemical battery

• Highest dielectric absorption of any type of capacitor

• High self-discharge - the rate is considerably higher than that of an electrochemical battery

• Cells hold low voltages - serial connections are needed to obtain higher voltages. Voltage balancing is required if capacitors are connected in series

• Very low internal resistance allows extremely rapid discharge when shorted, resulting in a shock hazard

ADVANTAGES DISADVANTAGES

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ULTRACAPACITOR DYNAMIC MODEL

• In terms of dynamic behaviour, if an ultracapacitor and a conventional capacitor are considered, divergences regarding impedance and energy storage are observed

• The impedance of an ultracapacitor presents a non-linearity that mainly depends on the temperature and the operating point which affects the voltage level across it

• The diffusion phenomena causes a fractional behaviour which is described mathematically by means of fractional differential equations

Contd….8

• Consider a ideal conventional capacitor. The expressions for current and voltage in time domain are given below

• From (1) & (2), the ideal capacitor can be modelled with following impedance expression in Laplace and frequency domain as

• A more accurate expression is given below

Contd….9

• The previous model is able to describe very accurately the dynamics of a conventional capacitor

• But it is not accurate for the modelling of an ultracapacitor. For this the fractional behaviour and non-linearity should be considered

• So for modelling, a fractional integrator is used to better approximate the frequency response by means of the following expression

• A more precise expression with fractional poles and zeroes used is given below

• The above expression considers the dynamics and non-linearity 10

MODEL IDENTIFICATION

• An experimental setup shown below has been developed to perform the experimental identification and control of a commercial capacitor

• The setup consists of: a test circuit with a limiting resistor in series with the ultracapacitor; a real-time control platform dSpace 1103; a linear amplifier to supply excitation signal to the test circuit

Contd…. 11

• Various techniques can be performed for the experimental identification of an ultracapacitor

• The techniques employed in time domain are simpler. E.g:-use of current pulses to directly obtain the impedance model

• But a more accurate technique is impedance spectroscopy. It is in the frequency domain

• Other techniques widely employed are Power Cycling Charts (PCC) and Cyclic Voltammetry (CV).

• We confine our focus to the first two i.e. Impedance Spectroscopy Current Pulse Test

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IMPEDANCE SPECTROSCOPY

• A frequency sweep between 0.001 Hz and 50 Hz has been carried out by applying a sinusoidal current with variable frequency and capturing the voltage in the capacitor terminals.

• The voltage operating point has been fixed to 1V and 9V by previously charging the ultracapacitor to these levels before the frequency sweep is performed.

• Figures obtained from the test show the divergence of real values from ideal values caused by the non-linearity due to the voltage dependency

Contd…. 13

• The above figure shows the comparison of the ideal capacitor and experimental results obtained at two operating values

Contd….

• The above figure shows the comparison of the ideal capacitor and experimental results obtained at two operating values

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• The representation of Nyquist diagram is more useful to observe the particularities of ultracapacitor impedance

• In an ideal capacitor the Nyquist diagram is a straight line parallel to imaginary axis and starts in real axis at value of series resistance, but in a ultracapacitor it is not so.

Contd….15

• In the previous figure we observe that there is a special region called Waburg region, where the slope of the Nyquist plot is approximately 45º, hence fractionality (α)=.5

• Also at low frequencies α is slightly less than one.

• Thus at these two regions ultracapacitor shows fractional behaviour

• The results obtained in the impedance spectroscopy have been used to adjust the model, which is able to reproduce the frequency response of an ultracapacitor more accurately.

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CURRENT PULSE TEST

• It is a technique in the time domain for experimental identification of the ultracapacitor. It is simple and fast.

• The response of ultracapacitor to a pulse of constant current is used to determine the parameters and develop the model

• For example, the test performed shows a complete discharge of the ultracapacitor from 9V to 0V by applying a current of -2A until the ultracapacitor is completely discharged

• It is seen that voltage slope is not constant and increases as the voltage decreases. Thus it can be deduced that equivalent capacitance decreases with the reduction of the voltage

Contd….17

• The capacitance can be determined for different voltage levels by measuring voltage gradient

Contd….

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• Capacitance can be approximated by a linear equation as

where C-capacitance; C0-capacitance at 0V; Kc-derivative of capacitance with respect to voltage

• A good identification of capacitance is necessary to obtain an accurate model of the ultracapacitor since variations depending on the voltage operating point are significant

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ENERGY ANALYSIS

• The energy of an ideal capacitor is given by the expression:

• From the following graph which is obtained from the current pulse test, we understand that the energy stored in the ultracapacitor differs from ideal expression

Contd….20

• The maximum error of the energy estimation, considering an ideal capacitor, is found at 5.6 V where the energy is 9.3% overestimated if above expression is used

• A more accurate expression is given as

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APPLICATIONS OF ULTRACAPACITORS

• Used in electrical vehicles providing energy recovery during braking and energy burst during accelerations due to their high power density

• Used in telecom systems

• Used in lighting equipment in combination with photovoltaic system

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REFERENCES[1] J. M. Carrasco, L. Garcia Franquelo, J. T. Bialasiewicz, E. Galvan, R. C. Portillo, M. A.

Martín, J. I. León, N. Moreno, “Power-ElectronicSystems for the Grid Integration of Renewable Energy Sources: A Survey”, IEEE Trans. On Ind. Electr., vol. 53, no. 4, August 2006, pp. 1002–1016.

[2] K. Sahay, B. Dwivedi, “Energy Storage Technology for Performance Enhancement of Power Systems”, Electrical Power Quality & Utilization EPQU Magazine, vol. 4, no. 1, available online www.leonardo-energy.org, March 2009

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[3] A. Burke, “Ultracapacitors: why, how and where is the technology”, Journal of Power Sources, no. 91, 2000, pp. 37–50.

[4] L. Gao, R. A. Dougal and S. Liu, “Power Enhancement of an Active Controlled Battery/Ultracapacitor Hybrid”, IEEE Trans. On Power Electr., vol. 20, no. 1, January 2005, pp. 236–243

[5] J. Moreno, M. E. Ortúzar and J. W. Dixon, “Energy-Management System for a Hybrid Electric Vehicle, Using Ultracapacitors and Neural Networks”, IEEE Trans. On Ind. Electr., vol. 53, no. 2, April 2006, pp. 614–623.

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THANK YOU

Questions....

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