Question

Just need someone to check my work please!
A total of about 180,000 terawatts of solar power reaches Earth’s surface, of which about 0.06% is used by plants for photosynthesis. Of the energy that goes to photosynthesis, about 1% ends up stored in plant matter (including wood). (Recall that 1 watt = 1 joule/s; 1 terawatt = 1012 watts.)

a. Calculate the total amount of energy that becomes stored in plant matter each second.
b. Suppose that power stations generated electricity by burning plant matter. If all the energy stored in plants could be converted to electricity, what average power, in terawatts, would be possible?

Answer 1

Would it be enough to meet world electricity demand, which is of order 10 terawatts?
c. Based on your answer to b, can you draw any conclusions about why humans depend on fossil fuels, such as oil and coal, which are the remains of plants that died long ago? Explain.

Answer 2

My answer to part a is this -
1.08 terawatts x (1012 watts/1 terawatt) x (1 joule/s / 1 watt)
= 1.08 x 1012 joule/s
Or 1.08 trillion joule/s

So I'm wondering, is the answer to part B simply 1.08 terawatts?

Answer 3

Since 0.06% x 180,000 terawatts
= 108 terawatts
From there, the question states that only 1% of what is used for photosynthesis actually ends up stored in plant matter. So the next step is to find 1% of 108 terawatts.
1% x 108 terawatts =
1.08 terawatts

Answer 4

If that's the case, than 1.08 terawatts is way below the necessary of order 10 terawatts for world energy consumption. So for part C I would speculate that it's not the wisest thing for us to depend on fossil fuels, and that's evident by all of the environmental problems we have. I just wanted to double check with someone else and see if all of these answers seem right/make sense.

Answer 5

Remember, "Power" is different from "Energy". In this case, though, there is 1.08TW of POWER per second stored as plant matter, and that is the same as 1.08TJ/s of ENERGY.

With 100% efficiency, clearly you can only realize 1.08TW of power. This is roughly 10x less than what is required.

Answer 6

So in that case, part B would have the same answer as part A?

Answer 7

Oh you said 1.08 terawatts of power - so 1.08 terawatts is correct?

Answer 8

Yes, I think they're trying to throw you on units. J/s = W = power. J = energy.

There is 1.08TW of power realized every second. This is a gross simplification, but this is why we say fossil fuels are "unsustainable". We use more than they are able to store.

Answer 9

Perfect. Thank you for the help.

Answer 10

Just wanted to mention that I received full credit for this problem/all calculations were correct. Thanks again for the help!