All the solar energy in the world...

Factual Questions
1

A while ago I was thinking about long term energy solutions for mankind. It occurred to me, as I’m sure it has to many others, that all of the energy we have on this planet came from the sun (‘ceptin’ geothermal of course). So, after we’ve depleted all of our fossil fuels, humanity will have to scrape by on whatever hits the earth every day from the sun.
There is the aforementioned geothermal energy, but compared to the sun, it’s a limited resource (bonus question: what would be the consequences of extracting the energy and cooling down the core of the earth?)

What is the amount of energy in watts that the earth recieves from the sun every day?

2

>> all of the energy we have on this planet came from the sun (‘ceptin’ geothermal of course).

I understand that the theroy that coal deposits are all fossils of living beings has been disproven and probably coal was there just like other minerals were there.

Oh, one more thing, how does nuclear energy come from the sun?

>> What is the amount of energy in watts that the earth recieves from the sun every day?

the earth receives 24.3 watts in summer, 18.5 in winter

3

Ah yes, nuclear energy, duhh. I personally don’t consider that an alternative as yet, but we can debate that in another thread.
Coal a mineral deposit? Interesting…if you can remember where heard/read that let me know. Regardless, its a limited resource.
“the earth receives 24.3 watts in summer, 18.5 in winter”
did you mean gigawatts? That wouldn’t light my bedroom!

4

I consulted online encyclopedia (duhh again) and found that the earth currently recieves “200,000 times the total world electrical-generating capacity” each day. This isn’t exactly an answer since I don’t know what exactly the the total world electrical-generating capacity is (qualitatively or quantitatively), but it does put my mind at ease a little.
If anyone has a more exact answer let me know though.

Aside: Somebody should let Brittanica know about coal being a mineral deposit they still claim it’s a fossil fuel.

5

I did this problem in college, I’m too lazy to find it or repeat the math, but IIRC the answer was about 500 W/m[sup]2[/sup]. That should be right within a factor of 10 or so. For some reason 411 W/m[sup]2[/sup] sticks in my brain - but I can’t believe I’d remember it so well after all these years.

Anyway, ignoring loss in the atmosphere, the surface area of the earth that is receiving the radiation is equivalent to a circle of the same radius of the earth. The entire hemisphere exposed to the sun is much bigger than a circle, but the parts that are not directly at right-angle to the sun receive less power, and it all averages out to be the same as if you had a big paper plate absorbing all the energy.

From JPL: Earth
Diameter: 12756 km

Thus, area of our paper plate: 5.1x10[sup]8[/sup] km[sup]2[/sup]. This is 5.1x10[sup]14[/sup] m[sup]2[/sup] - 1 million m[sup]2[/sup] in one km[sup]2[/sup].

So, the answer is (if I remember my figure right) about 2.5x10[sup]17[/sup] W.

Assuming your 200000 figure is correct, that would imply the earth’s man-made generation capacity is about 1.21x10[sup]12[/sup] W, 1.21 Terawatts.

What’d I say?
1.21 Terawatts!!

6

Ahh, better. Basic Solar Properties lists the luminosity of the sun at 3.83x10[sup]33[/sup] erg/sec.

1 W = 1 J/s = 1x10[sup]7[/sup]erg/s, so
L = 3.83x10[sup]26[/sup]W.

By the time that energy is passing the earth it is spread out over a sphere of radius 1 AU (1.5x10[sup]11[/sup] m).
Area of such a sphere: 2.8x10[sup]23[/sup] m[sup]2[/sup].

Thus, the power falling on the earth is 1354 W/m[sup]2[/sup]. I was off by a factor of 2 or 3 - fudge my previous calculations accordingly.

7

Ooh, enough to run a thousand time machines!

As for the effect of using all the solar energy, there wouldn’t be any. Instead of the light being absorbed by the ground and turned into heat, it will be turned into electricity, which will be consumed by some machine and turns into heat. Well, not if you use the electricity to power a searchlight pointed at the sky, but not too many people do that.

Also, remember that if we ever need more energy than that, we can just as easily launch solar power satellites and beam the power down as microwave radiation. In fact, that’s probably a better idea than having lots of solar power stations on earth anyway - much less ecological damage, for one thing.

8

Also, remember that if we ever need more energy than that, we can just as easily launch solar power satellites and beam the power down as microwave radiation. In fact, that’s probably a better idea than having lots of solar power stations on earth anyway - much less ecological damage, for one thing.

I know that works in SimCity, but it seems to me that microwaves would get absorbed pretty quickly by the water that is in the atmosphere. (At least, that’s what happens in my kitchen). Can somebody who knows more about this please tell whether transmitting power in radiation is really feasible? It was my understanding that to transmit enough power to be useful, one would have to use small (ie high energy) wavelengths, which would be easily absorbed by the earth’s atmosphere

9

I got that. Just thought you’d want to know that someone did.

10

Nuclear power is based on uranium or plutonium fussion. The former is created in supernova explosions, the latter in breeder reactors using uranium. So ultimately, nuclear power did originate in stars.

Also, it’s theorized that the heat in our planet is stoked by radioactive decay of elements. These elements, too, were generated in supernovas.

11

Yeah, water absorbs microwaves–microwaves at a frequency of around 2.5 Gigahertz (and some other frequencies, too). (Interesting fact: microwave ovens also work because fats and sugars absorb microwaves, too.)

There are microwave “windows” in the Earth’s atmosphere: ranges of frequency where the atmosphere is more or less transparent. This is why microwave communications, microwave radar, microwave astronomy, and beamed microwave power are possible.

On coal as a mineral deposit, check out Thomas Gold’s “Deep Hot Biosphere.” I saw him give a talk on it, and it was pretty intriuging. However, stating that the fossil origin of coal and petroleum has been disproved is overstating it. That is certainly NOT the consensus in the geological community.

12

doughlips said:

"Thus, the power falling on the earth is 1354 W/m2. I was off by a factor of 2 or 3 - fudge my previous calculations accordingly."

Spot on! That’s the sort of figure people use when working out how big to build their space-based solar power stations.
They have to build them twice the size though - microwave power transmission does work but is only 50% efficient.

On the Earth, you have to factor that the light is falling onto a hemisphere rather than a disc, which doubles the area and so halves the average power. Then if you take the hemisphere in the shade into account, you’re down to 338.5 W/m[sup]2[/sup], on average, over a day. Then you’ve got to consider that you lose a lot of the UV and higher frequencies to the ozone layer, and some energy is scattered away by the atmosphere.

I have an insolation map of the Earth based on surface measurements, which gives the average power as 240-260 W/m[sup]2[/sup] in the tropics down to 80 W/m[sup]2[/sup] at the poles.

The Sun supplies vastly more energy to the Earth than we generate, and solar power advocates like to point out that we could supply our needs by devoting similar land area to solar power that we do to agriculture. It will all come down to cost-benefit however: - nuclear power will use a lot less land to supply the same energy, and the cost of the infrastructure will be much less.

Some other alternatives:

OTEC - still solar power, but you use the whole sea as your collector. Uses the difference between warm surface water and cold dep sea water to run a heat engine. Requires massive construction at sea to get reasonable amounts of energy out. Some designs produce fresh water as a byproduct. I believe a small OTEC plant is being used to supply fresh water to Hawaii, but it produces no net power.

ICETEC - again, still uses solar power. This time you’re using the temperature difference between the tropics and the poles to power a heat engine. Rather than construct something spanning a quarter of the globe, you bring the cold of the poles to the tropics by towing icebergs into warmer waters. We know how to tow iceburgs, it’s occasionally done to stop them bumping into oil platforms.
When you get them there, run a heat engine on the temperature difference between surface sea water and the ice.

The Moon - another source of energy apart from solar, geothermal and nuclear! Currently, energy from the Moon is generated using tidal barriers. I saw a design for a tidal energy power station in New Scientist many years ago which could be built out at sea - it was basically a huge concrete tank which filled when the tide rose and emptied through a turbine when it fell.

All these systems have the problem of low power density, in that you have to build huge plants to generate/collect the power. The cheapest way to collect solar energy on a really large scale may well turn out to be plant trees, grow them and burn them. Which I would guess cause something of a crisis for Greenpeace.

13

Thanks everyone, for the great posts!

According to the trusty online encyclopedia, plants only absorb about 1-2% of the energy they recieve. Solar power on the other hand acheives 7-11%.When you consider that solar panels could be generating during the 30 years or so of growth that the trees need, I wonder if its less costly.

As for nuclear power being less costly, it seems like half the plants I know of were never finished because of cost overruns. On top of that, there is the disposal of the waste. As far as I know, we still don’t know what to do with the stuff. When it has a half life that will probably outlast the human race, why keep making the stuff?

14

If coal is a mineral deposit then why does it have imprints of leaves and clumps of vegetable matter in it?

Also, we will need some other method of harnessing solar power besides solar cells. The last I heard, it took 10 years for a typical solar cell to have a net increase in energy. In other words, it takes 10 years of sunshine to make up for all the energy it took to make the cell. I am sure, with all the advances in Solar Cell Efficiency, that this number is down to 5 years or so, but it is still too long.

There are other methods. In the California Desert near Barstow there are a couple of pilot Solar Power Plants. One uses a circular array of mirrors that point to a central tower. That tower circulates liquid sodium that is boiled by the solar heat. I once saw the plant in operation and the central tower was glowing white hot. It was visible maybe 50-100 miles away in the small plane I was in.

15

m3 said:

** “According to the trusty online encyclopedia, plants only absorb about 1-2% of the energy they recieve. Solar power on the other hand acheives 7-11%.When you consider that solar panels could be generating during the 30 years or so of growth that the trees need, I wonder if its less costly.”**

True. But if your talking about 100 square miles of trees compared with a square mile of photovoltaic cells, I’m guessing the cells will cost you more. Depends how cheap your land is. The thirty years wait is a start-up delay, then you can plant and harvest continously.

There are other designs, solar concentrators generating heat to boil water or run big stirling engines, or solar “chimneys”, which may be more competitive than photovoltaics or burning trees. Or maybe we’ll boost the efficiency of trees using GM, which again will probably upset Greenpeace!

Overall though, I’d still bet that nuclear power will be increasingly adopted as fossil fuels become scarce.

16

There is also the possibility of nuclear fusion energy, but the technology isn’t available right now, but it’s only a question of time…

17

At the risk of hijacking my own thread, I gotta respond to the nuclear power issue. Even if nuclear power CAN be made safely, does that mean it WILL? Three mile island and Chernobyl are chilling associations that folks have with nuclear power.
We tried nuclear power here in Washington state. We thought that there would be so much growth we would need more energy. I think we started about 15 or so plants. The result? Only a couple were actually finished and rest were stopped due to incredible cost overruns. Washington electric co-ops are now billions of dollars in debt and with nothing to show for it. We’ll be paying that debt off until 2011 and we’ve been paying it for a long time.
This isn’t the only story I’ve heard along this line. Nuclear power might look cheap on paper, but its more costly than you think.

18

Well, I’d prefer to see OTEC plants than nuclear, but the market will decide.

I’d really like to see orbiting solar power stations, the spin-offs in launcher technology and orbital construction techniques would open up the whole solar system to mankind, but that is a whole other thread…

19

Carl Sagan postulated an extremely advanced civilization that built a containment sphere around its star to collect ALL the starlight, and used it to power their civilization.

A massive engineering feat, but the ultimate free lunch.

Regards,
Shodan

20

Since this exact question was asked at the Tommy Gold talk I attended, I can give you his answer, which was if coal is made by the decomposition of organic matter, the last thing you’d expect to see in it is a perfect little undecomposed leaf.

I am not a geologist, I can’t tell you whether or not that’s a good argument, but it made an impression (if you will) on me.