Recently I was analyzing various v-i curves of different types of fuel cell. While simulating a fuel cell stack I could figure out an v-i curve (whose .jpg file is attached) which is bit different from the conventional v-i curves of fuel cell. I find the voltage is going to negative value, but generally we do not find a v-i cuve of fuel cell whose voltage goes to negative, v-i cuve with positive voltages are generally seen in books, or research papers.
Can any one tell me whats is happening exactly in the fuel cell so that the voltage is shown negative?
This is a simulated curve, I think you're saying. I believe the model is saying that the max short circuit current the fuel cell stack can provide is about 140 amps. And then, beyond 140 amps, another power source in the mesh can drive the current value higher. Which will reverse the voltage across the fuel cell stack, as it behaves more like a resistor.
To be clearer, the model should be stopped when the fuel cell stack reaches 0 V.
The Following References may be of some help to you.
1. http://ase.kuas.edu.tw/green/info/solar/Operational_Fuel_Cell_Voltage.pdf
The key point about the plot of the cell voltage against current density:
• The open circuit voltage is less than reversible OCV.
• There is a rapid initial fall in voltage.
• The voltage then falls less rapidly, and more linearly.
• There is sometimes a higher current density at which the voltage falls rapidly.
For a typical solid oxide fuel cell (SOFC) operates at about 800oC, the V-I curve can be shown as following. The key points are:
• The open circuit voltage is equal to or only little less than the theoretical value.
• The initial fall in voltage is very small, and the graph is more linear.
• There may be a higher current density at which the voltage falls rapidly, as with
lower-temperature cells.
the reversible voltage is lower for the higher temperature, the operating voltage is generally higher, because the voltage drop or ireversibilities are smaller.
2. http://blog.genport.it/?p=93
Effects of Temporary Reverse Voltage on Proton Exchange Membrane Fuel Cell Performance:.
Polymer electrolyte membrane fuel cells are gaining great attention as a ... value, particularly when the stack is operating at high current). These cells can reach negative voltage, long before other correctly supplied cells.
Water condensation in the flow field, which can increase the input/output pressure difference, can prevent homogeneous hydrogen distribution in one or more cells (homogeneous gas supply in a fuel cell stack is very important in keeping the voltage of all cells at a similar value, particularly when the stack is operating at high current). These cells can reach negative voltage, long before other correctly supplied cells. Depending on the duration and degree of negative voltage, the fuel cells can recover or be rendered completely ineffective. The surface area loss of the cathode platinum particle by cell reversal was also detected.
P.S.
Dear Amit Kumar,
I suggest you to check what it will be in your experiment, if instead of the solar cell, you take a linear resistor and a battery (like a Thevenin equivalent of some active linear circuit, or simply a model of a REAL voltage source) and make from it a load for a similar, but stronger circuit, i.e. the one including a stronger battery. Why shouldn't you reveal the "negative branch" also in this case? You simply "insert by force" the current into your cell, forcing it to consume electrical energy. Incidentally, in a real ARRAY of many solar cells, you can come to the situation where some of the cells CAN CONSUME energy from the other cells. This will be so, if, because of the different light on the cells, some of the cells become weak, i.e. become loads for the other cells.
The two attached pieces, furst of all the CASN one, can be of some iterest for you here.
Yours, Emanuel.
Dear Emanuel Gluskin Sir,
Thank you for paying special attention to my question and helping me with your valuable suggestions.
Sir, in your reply I find your explanation regarding the negative voltage with respect to a solar array/module where a cell can become load to other cell.
But sir my question is regarding Fuel cell, shall I also understand the answer by considering individual cell from a fuel cell stack ?
Yes, Amit. Certainly. What I once did for myself many years ago (writing some instructions for a laboratory with solar cells) can be done for any connections of one-ports each possessing a known v-i characteristic.
You can do the following (either physically, or on a millimetric paper, using/drawing the v-i characteristics).
STEP 1: Take three sells, one leave as it is, and the other two connect either in parallel or in series. Thus you obtain, in whole, TWO ONE_PORTS, one of which is composedof two sigle cells.
STEP 2: Connect these TWO ONE_PORTS, obtained in STEP 1, so that either the two parallel will be in series with one, or the two series in parallel with one. (This depends on what you have prepared in STEP 1)
STEP 3: Apply Kirchhoff's laws GRAPHICALLY, using the individual v-i charcateristics of the SINGLE elements. (You can assume the single elements to be the same, but for two in parallel, or in series, the characterustucs are already different from that of the single one, -- check this already in STEP 1)
You WILL find the situations when one sell will consume the energy from the two!
But, again, check then also the case of linear real sources!
Regards, Emanuel.
Thank you very much Dr. Pisupati Sadasiva Subramanyam sir for the links, now my confusion is clear...thanks !!
In fact, there exists no negative voltage in the physical power system. The reason is that the MATLAB didn't set limits for voltage calculation algorithm.
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