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Energy Harvesting

Theoretical Estimation of generated voltage and current

Using the MFC for Energy Harvesting Applications

Moderator: TPD

Theoretical Estimation of generated voltage and current

Postby Thoski » Thu Feb 22, 2018 2:16 pm


I'm using MFC for energy harvesting in an application where the MFC is compressed by a surface strain of ~600 µm/m at around 20 Hz. I want to model the MFC as a current source with a parallel capacitor and resistor to simulate my conditioning circuit. I measured the values of R and C for the MFCs that I have.
In one of your presentations I found that the Generator characteristics of the 3-1 MFC in dependency of the strain should be 3250 pC/ppm. So I tried multiplying this value with my strain profile to get the generated charge Q. By using I = diff(Q) I calculate the current that I use to parameterize the current source in the simulation. This should lead to the voltage V = Q / (w * C). This seems to work quite fine for a M2814-P2 since the voltage I measured fits the voltage I calculated with small errors (measured 34 V peak vs. 38 V calculated).
However, this doesn't work at all for a M2807-P2. Its capacity is much lower and therefore the calculated voltage is much higher (73 V). But the voltage I measured is much lower (26 V). A lower voltage makes sense, since the MFC is smaller and therefore should generate fewer charges. But that doesn't show in the equation, since the generator characteristics and strain profile are the same. Do I miss something here? How else can I estimate the generated current/voltage/energy?
I'm very confused. I hope someone can help.
Thanks in advance.
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Re: Theoretical Estimation of generated voltage and current

Postby TPD » Thu Feb 22, 2018 2:49 pm

Hmmh, I think the important part to understand first is that the MFC (or any PZT used for EH) is NOT generating
a voltage but an electric charge.
You are measuring the voltage with an instrument which
has a certain internal impedance which is the same for both measurements.
As a (very) quick approximation the M2807P2 has only 50% of the PZT volume so the generated charge should be around 1600pC/ppm as the generator characteristics.

You need to include the internal impedance of the device measuring the Voltage being parallel to the MFC as well. Are you measuring with a Scope or DVM? Would recommend using a scope and measure Vpp.
Also, which I guess you have looked at, the impedance of the MFC changes with the frequency and if using a DVM you have to calculate the ESR of the MFC being parallel with the DVM impedance. ESR (equivalent series resistor)
ESR = tan δ / 2 πfC
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