Electroactive polymers asGenerators: How can we use polymer materials to generate electricity form mechanical energy. Generator mode

By, Gadam Srikanth 0901EE08.

E.P.A.M

Electroactive polymers (EAPs) convert electrical energy to mechanical work and vice versa.

EAPM has a very simple structure comprised of polymer films (elastomers) sandwiched bytwo compliant electrodes made of a flexible and elastic material, and can operate as an electric control generator and actuator.

EAP’s are lighter ,cheaper and can be made in many different forms.

What is an Electroactive Polymer?

Structure of an Electroactive Polymer:

Stretched State

Contracted state

Many Types of EAPs

Artificial Muscle

Conducting PolymersDielectric elasromers

Gels

Thermal and Others

Electrostrictive Polymer

Nanotubes

Dielectric elastomers are

particularly promising

Assumptions: The Electrodes have zero resistance. The Dielectric has Infinite resistance. The Polymer is perfectly elastic and there will be no mechanical losses. The volume of the polymer is constant during the change of state.

With these assumptions the electrical and mechanical aspects of the system are separately and together lossless in the sense that energy is conserved during changes of state(from stretched to compressed and vice versa).

A

A + d A

+++++++++ +++++

_______________

+++++++++++++++++++++____________________

Charge=Q

Charge= Q

Z

Z-d Z

Principle of operation:-

Consider a general case, Let the electrostatic energy “U” stored in the initial stage is U=(Q^2/2C) where C=єA/Z where є is the permittivity of the dielectric.As the dimensions changes there will be change in “U”.

V2

V1 = V bias

“U” is a function of Q and C i.e. U is a function of Q,Z,A.Differentiating “U” with respect to its variables we have dU = (∂U/∂Q)*dQ+ (∂U/∂Z)*dZ+ (∂U/∂A)*dA .

=>dU=(Q/C)*dQ+ (Q^2/2єA)*dZ - (Q^2*Z)*dA/(2єA^2)

=>dU=(Q/C)*dQ+ (Q^2/2C)*(dZ/Z) –(Q^2/2C)*(dA/A)

=>dU= (Q/C)*dQ + U(dZ/Z - dA/A)

Here the first term on R.H.S represents V*dQ (as Q/C=V).is just the change in dU caused from an external electrical power source during the incremental change in state and gets added(subtracted) to the Electrostatic Energy.

The Second term in the dU comes from the Mechanical Energy of the polymer.Consider the situation when dQ=0(That is there is no external electrical power source during the change of state).

Then we have dU= U*(dZ/Z-dA/A).As for an elastomer volume is constant we have volume =KA*Z=K => A*dZ + Z*dA =0 =>dZ/Z = -dA/A substituting this relation in above relation we have dU = U*(2dZ/Z) or dU= -U*(2dA/A).

From the above relations we can say that the Electrostatic energy increases if dZ is ‘+’ that is Z increases, in other wayElectrostatic energy increases if dA is negative ie A decreases..

That is Electrostatic Energy is increases if we go from initial stretched state to final contracted stage(As Z increases in this case and dZ is positive)

This increased Electrostatic Energy can be utilized forthe power generation purposes and can be harvested by connecting a load across EAPM capacitor.

For continuous power this process must be repeatedly done. Rectifier circuits can used for further waveform modification.

Que: From Where Does this Energy come? It takes some positive work for any stretching force to stretch the EAPM , when an EAPM is stretched by that force it stores the work done by that force as mechanical energy in it.(It is analogous to that of stretched spring).Now when the force is removed it will convert this mechanical energy to Electrostatic energy that we can use for power generation.

(1) Film is stretched (2) Electrical energy U is placed on film in stretched state by connecting a battery(bias voltage) to the electrodes. (3) Battery is disconnected to maintain constant charge. (4)Now the film is contracted ,hence Electrical Energy on the film increases to some U+u (as shown previously). (5)Now this capacitor is connected to a load through which it discharges giving an electrical energy U+u to it. (6)Total Electrical Energy given to the capacitor is ‘U’ by the battery initially. (7)Total Electrical Energy taken by the load is ‘U+u’ .So where is this additional energy u from? It comes from the mechanical energy stored in EAPM during the stretching process. Hence net generated Electrical energy is = (U+u)-(U) =u. (8)Hence in one cycle we generated amount ‘u’. (9)Now u depends on bias voltage and material taken as dielectric as shown next.

Practical Approach :The cycle for Power Generation:

Practical approach:

EAPM in stretched state connected to battery

EAPM in contracted state supplying power to load.

Factors effecting the power generated ‘u’:

As shown previously we have dU= U*2dz/Z =>∫dU/U = 2* ∫dZ/Z (integrating under suitable limits) =>ln (U2/U1) = 2*ln (Z2/Z1) =>U2/U1= (Z2/Z1)^2 U2=U1*(Z2/Z1)^2 now we know U2-U1=u; u=U1* ( (Z2/Z1)^2 -1) we know U1 =Q^2/(2*C1) As Q=C1*V (V=V bias) we have U1 = ½ C1(V^2) and we know that (Z2/Z1)^2=C1/C2because C1/C2 =A1*Z2/A2*Z1,put A1=K/Z1 and A2=K/Z2.Substituting these in the above equation for u we have u=0.5*C1*(V^2)*((Z2/Z1)^2) -1) u=0.5*C1*(V^2)*((C1/C2) -1)

Factors effecting the power generated ‘u’:From the above equations we can see thatThe Electric energy generated by the EAPMis directly proportional to the square of bias Voltage V bias. u œ V^2.

And we can also notice that u œ (Z2/Z1)^2This ratio depends on the elastic properties of the material like stress , strain relations. If a material is more elastic than Z2/Z1 increases as now it can be compressed more so as Z1 decreases and Z2/Z1 increases and u increases.

If a material has more energy storing ability per unit volume then that will give more generated energy ‘u’

We had assumed there are no losses any where in the system but in reality there will be some losses because of finite resistances of electrodes, some leakage current through Dielectric material ,Hysteresis loss in the polymer etc.

Circuit for the realization of the generator:

Bias

⁺⁺

⁻

Bias

Circuit for the realization of the generator:

In the circuit they have used a diode to prevent backward flow of charge during contraction and in contracted state, in this way we can eliminate the need for disconnecting battery (voltage source) every cycle.

After contraction the EAPM capacitor discharges through drain resistor shown until the voltage across it’s terminals reaches V bias from the peak value of V2.(V2=V*(C1/C2)).

To get the waveform of the voltage oscilloscope is used.The results are shown next:

V2 .

V=bias voltage

Discharging capacitor through drain resistor

As expected we can see the exponential decay of voltage.

The voltage wavegenerated for one impact in the case of EPAM it is on the order of 150-200 ms [time]. This is one of thesignificant characteristics of EPAM. The long pulse duration and the highenergy density of

EPAM in power generation mode helps in mechanical energy to be be effectively converted to electric energy even at low frequencies

Experimental result matching the theoretical expectations:

V2

.

Thus we can use EAPM as a medium for power generationfrom mechanical energy with efficiency as high as(80%).

There are many areas where this technology has given fruitful results for power generation.

Some of them include Heel strike generators , Power fromocean waves , wind etc.

Research is still on for the commercial use of these technologies.

SIR international is working in this field and now they are in the process of developing polymer engine.They had successfully developed generators and actuators.

Developments and advances in EAPM technology:

V2

.

DARPA developed a heel-strike generator to capture free energy while walkingDemonstrated up to 0.8 J per heel strikeDeveloped multi-layer polymer fabrication techniquesDemonstrated 15 layer device

Defense Advanced Research Projects Agency’s(DARPA) Heel strike generator:

Heel-Strike generators are expected to produce 1W of

power under normal walking conditions

EXPANDED

+Vout

Dielectric Elastomer (EAP)

+Vin_

__

_

_+

++

+

+

Compliant Electrodes Base

+Vin_ _ _ _ _++ + +

++Vout

CONTRACTED

.

Other significant contributions from EAPM technology :Potential applications of EAPM’s cover areas like actuators sensors , robotics, in medicine as an alternative for the muscle etc.

actuators sensors

Applications in robotics for electrically controlled muscle like movements.

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THANK YOU

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References:IEEE journal :Current Status and Future Prospects ofElectric Generators UsingElectroactive Polymer Artificial Muscle.

IEEE journal: Innovative wave power generation systems using Electroactive Polymer Artificial muscles.

SPIE journals.

Book : Dielectric Elastomers as Electromechanical transducers. Edited By: Federico Carpi , Danilo De Rossi, Roy Kornbluh ,Ronald Perline and Peter Sommer-LarsenScientific American article –OCT 2003. Google Search engine.