Pee Power using microbial fuel cell measuring output

[kbmcgill]

We are doing the pee power project using the microbial fuel cell. We have been tracking our data using the resistors
4670, 2190, 1000, 470, 220, 100 and 47. To calculate the Power output we are using the formula
P= Voltage squared/R. Some of the results appear to be really small. For example, the voltage reading at the 47 resistor was 12. The converted to 3.06 x 10 (-12 power) after using the formula.

We took the reading of 12 divided by 1000 before using the formula. so we had .012 x .012 = .000144

P = .000144 divided by 47 =.000003064 .000003064 / 1,000,000 = 3.06 x 10 (-12)

Does this make sense?

Thank you.

[SciB]

Hi,

Your calculations are ok up to the point where you convert watts to microwatts (uW). There are one million uW per W so you have to multiply your answer by 10^6. The result is 3.06 uW. That's a little low but still ok. What power did you get for the other resistors?

Sybee

[kbmcgill]

The procedures for the project from Science Buddies contain two major errors. See excerpt from the experiment procedures below: Under 4b below, the instructions read to divide by 1,000,000 to arrive at microwatts when in actuality you must multiply by 1,000,000. Second, the equation for calculating Power is also incorrect but this was subsequently corrected by someone in the procedure section for this experiment. P=V2R is incorrect. It should be P = V squared/R. These errors in the written procedures cost us valuable time in doing our experiment


Excerpt From Procedures Section of Pee Power Experiment:

4. Calculate the power output (in microwatts, or μW) for each resistor. You can calculate this by using a derivation of Ohm's law shown as Equation 1 below.
a. To use Equation 1, you will need to convert your voltage readings from millivolts (mV) to volts (V). To do this, divide the millivolt values by 1000 to give you volts.
i. For example, if you had a voltage reading of 45 mV, this would equal 0.045 V.
b. Using Equation 1, your answer will be in watts (W). Convert watts to microwatts by dividing your answer by 1,000,000.
Equation 1:
P=V2R
 P is the power in watts (W).
 V is the voltage (V).
 R is the resistance in ohms(Ω).

[kbmcgill]

Our power output numbers have ranged from below 1uW for a 47 ohm resistor to over 100 uW for a 470 ohm resistor depending on the day. We are now in day 13 and we are waiting for the power output to stabilize. Our last 3 days peak power numbers (without urine) have been

1000 ohms 75.076 day 11
1000 ohms 81.79 day 12
1000 ohms 77.84 day 13

We have not stabilized to 0.5 uW as described in the FAQ.

Thank you.

Kerry McGill

[SciB]

Hi Kerry,

I checked the procedure for the fuel cell project (https://www.sciencebuddies.org/science- ... #procedure) and the equation for power is correct: P = V^2/R

Did you copy or print the instructions? I tried doing a copy/paste of the equation and it came out as P = V2R

You are correct about the conversion of W to uW being wrong. Your value in watts has to be multiplied by 10^6 because one W = one million uW. You can see this better if you set it up dimensionally: (W) x (uW/W) = uW

I calculated your power output from the mV readings you gave:
Day 11. 75.1 mV 5.64 uW
Day 12. 81.8 mV 6.69 uW
Day 13. 77.8 mV 6.05 uW

I'd say you are stable because the difference between Day 11 and 13 is 0.41 uW and that is <0.50 uW. If you take the average it is 6.13 uW and that value is within 0.5 uW of Day 11 and 13 and only off by 0.06 uW on Day 12. I'd say that was close enough.

Sybee

[kbmcgill]

Dear Sybee:

Thank you so much for the great insights. I did cut and paste so that explains the in the formula.

With respect to the data I listed for the last three days 11 through 13. Those calculations were already in microwatts (uW) so they fell outside the 0.5 uW criteria for stabilization. We are now at day 14 and the peak power is 96.62 uW which is up from 77.84 uW yesterday (day 13). I am concerned that the output may die before it stabilizes. It's a great project but a little long for a 7th grader.

Thanks again for all the help.

kbmcgill

[SciB]

Oh, I should have realized those were powers and not voltages--that's what you said!

I don't know what the lifetime of the mud bacteria is under artificial conditions. Bacteria grow and divide when there's enough food, but if the nutrients become limiting then they will start to die. 75-82 uW is a pretty good power output for the 1000 ohm resistor, so your bacteria seem to be healthy. I would make a note that the fluctuation is 4-6 uW and not the recommended 0.5 uW and then go ahead and add the urine. If you looked in the FAQ section of the project under the question about power not stabilizing, it says that this can happen for several reasons--differences in temperature, for example. Bacteria generally do better when it is warm. Is it possible that the cell was cooler or warmer on Day 11-13? It would be a good idea to use a digital thermometer to record the temperature in the room several times during the day and in the evening to see if there's a substantial fluctuation. If this were being done in a lab the cells would be held in a temperature-controlled incubator at a constant T.

Good luck and please keep updating us on the progress. We really like to know how a project works out especially if something unexpected happens so we can use the information to help the next student.

Sybee

[kbmcgill]

Sybee:

The peak power for the last three days continue to increas 96uW, 112uW and 118uW so we still have not stabilized asfter 16 days. If the peak power was within a few microwatts I would say add the urine but we are still increasing. Any thoughts?

Thank you.

Kbmcgill

[SciB]

Well, I have two questions:

1. Where did you get the mud?

2. Does the temperature fluctuate much where the cells are located?

Since the power is increasing and the bacteria are the source of the electrons, I would say that the bacteria must be growing and dividing so that their numbers are increasing. I don't know any other explanation for your results. The only way the bacteria can grow is if they have plenty of nutrients and if the temperature is warm. If your pond is especially rich in organic matter and the cells are located in a warm place then you may just have an especially good microbial fuel cell.

You wouldn't happen to have two cells going at the same time would you? That would have been the best way to do this experiment because then you could keep one as a control while you added urine to the other.

Have you plotted the power vs time? If the power is increasing at a steady rate then this should be apparent on the graph. If you are running out of time, I would go ahead and add the urine and plot the power outputs. If the urine stimulates the bacteria you will see an increase in power--the slope of the curve will increase compared to what it was before you added the urine. Have you tried measuring the power a couple of times a day just to see how much it fluctuates?

Let me know what you think.

Sybee

[kbmcgill]

The mud is actually from Scotts Humus and Manure soil from Home Depot. This was one of the sources listed in the procedure section from Science Buddies. I don't think that the temperature has fluctuated that much. Your suggestion that we record temperature is an excellent idea and should probably be added to the procedures for the experiment. I have graphed the peak power output vs time and will send that to you. I purchased an extra MFC but did not add the mud to that one. If we do it again, I would run two MFCs. I will try to measure the peak power a couple of times per day and see what that reveals.

I can't thank you enough for your help. It is nice to be able to communicate with someone who understands the project.

The attached peak power chart is up through day 13. I need to add the last three days which were 96uW, 112uW and 116uW.

Kbmcgill

[kbmcgill]

Peak power chart updated through day 16.

[SciB]

Kerry,

There was no link to the file you attached at the bottom of your post. Are you sure it was less than 256 kB? If it is too large you could upload it to Google docs and then send me the link for that.

That is very interesting that you are getting such good results with a commercial soil blend. Sometimes these are sterilized and of course then they would not work at all because the bacteria would be dead.

When do you have to finish up the experiments and write up the project? As you said, it would have been good if you had started a couple of fuel cells at the same time. In the lab we do the same experiment at least three times so we can average the results and do some statistical tests. Multiple readings allow you to calculate the mean and standard error and then you can do a statistical comparison to prove that your test differs from the control at a confidence level of 95%.

Keep plotting your data and monitoring the output. It has to stop increasing eventually so you can add the urine--hopefully at least a few days before the project is due!

I'm glad you find my suggestions useful. That's our goal and we like to know that it is achieved sometimes.

Good luck!

Sybee

[kbmcgill]

peak power graph through day 16.

[kbmcgill]

peak power through day 16

[SciB]

Kerry,

I'm still not getting your attachments. What do you see when you do a preview of your post? There should be a link at the bottom to the attachment. If not then it is getting blocked somehow. I checked the FAQ and found this:

Why can’t I add attachments?
Attachment permissions are granted on a per forum, per group, or per user basis. The board administrator may not have allowed attachments to be added for the specific forum you are posting in, or perhaps only certain groups can post attachments. Contact the board administrator if you are unsure about why you are unable to add attachments.

I will check with the admin person and see if i can find out why there's no attachments. As I suggested, you can upload your files to Google docs and send me the link along with permission to view the files. If you want you could just post the power values for each day and i can plot them myself using Excel.

Sorry for the problem.

Sybee