DC circuit question

[sbtelecom]

I recently did a question about DC circuit and I was confused by the answer given from the book. Can anyone convince that my answer is right?

Q) Suppose a single solar cell is capable of supplying exactly 81mW of DC power when exposed to direct sunlight. If 3 of these cells are connected in series, the resulting set of solar cells can theoretically provide

My Answer - 243mW of DC power in direct sunlight.

Text's answer - 719mW of DC power in direct sunlight.

I think the answer is 243mW because the current drawn from the cells are the same when they are connected in series. Recall that P = IV , so we have P = I(3V). Therefore, the total power supply by 3 cells is 243mW.

 

[Ratchit]

sbtelecom,

If the answer is 719 mW, then each cell would have to supply about 240 mW of power. Since each cell is only capable of supplying 81 mW of power, how can the text answer be correct?

Ratch

 

[crutschow]

Your answer is correct. The text is obviously incorrect.

The author apparently thought that with 3 cells in series, the voltage will triple and, since power in a resistor is proportional to the voltage squared, the power would go up by a factor of 9. But the power from the cell is proportional to volts times amps. Since the current does not change, the power goes up by a factor of 3, which is consistent with an answer of 243mW.

 

[RCinFLA]

Max generated current will be same, max available voltage will triple. Answer is 3x power.

Single silicon cell at 25 deg C will be about 0.52v and (0.081watt / 0.52v) = 156 mA at max power point loading.

Three in series will be 156 mA x ( 3 x 0.52v) = 156 mA x 1.56 vdc = 243 miliwatts.

( Quality silicon cells are about 35 mA per cm^2 of area at full sunlight)

 

[sbtelecom]

Thank you guys, I will try to report the problem to the publisher.
By the way, I have another question.

Is it possible to store the energy from the lightning stroke?
I've learned that a lightning stroke consist of about 100KA of current,
which is very destructive.

 

[crutschow]

No. But even if you could, the random and infrequent nature of lightning strokes would make it difficult to make a collector that would attract more than a few strokes a year, at most, at any one location.