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Reply #24: 20% planetary accretion, 80% radioactive decay [View All]

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bananas Donating Member (1000+ posts) Send PM | Profile | Ignore Thu Jul-16-09 05:26 PM
Response to Reply #17
24. 20% planetary accretion, 80% radioactive decay
Geothermal gradient

The Earth's internal heat comes from a combination of residual heat from planetary accretion (about 20%) and heat produced through radioactive decay (80%).<2> The major heat-producing isotopes in the Earth are potassium-40, uranium-238, uranium-235, and thorium-232.<3>

It's still a trivial amount compared to solar influx:
Earth's energy budget

Incoming energy

The total flux of energy entering the Earth's atmosphere is estimated at 174 petawatts. This flux consists of:

* solar radiation (99.978%, or nearly 174 petawatts; or about 340 W m-2)
o This is equal to the product of the solar constant, about 1,366 watts per square metre, and the area of the Earth's disc as seen from the Sun, about 1.28 1014 square metres, averaged over the Earth's surface, which is four times larger. The solar flux averaged over just the sunlit half of the earth's surface is about 680 W m-2
o Note that the solar constant varies (by approximately 0.1% over a solar cycle); and is not known absolutely to within better than about one watt per square metre. Hence the geothermal and tidal contributions are less than the uncertainty in the solar power.

* geothermal energy (0.013%, or about 23 terawatts; or about 0.045 W m-2)
o This is produced by stored heat and heat produced by radioactive decay leaking out of the Earth's interior.

* tidal energy (0.002%, or about 3 terawatts; or about 0.0059 W m-2)
o This is produced by the interaction of the Earth's mass with the gravitational fields of other bodies such as the Moon and Sun.

* waste heat from fossil fuel consumption (about 0.007%, or about 13 terawatts; or about 0.025 W m-2) <1>.

There are other minor sources of energy that are usually ignored in these calculations: accretion of interplanetary dust and solar wind, light from distant stars, the thermal radiation of space. Although these are now known to be negligibly small, this was not always obvious: Joseph Fourier initially thought radiation from deep space was significant when he discussed the earth's energy budget in a paper often cited as the first on the greenhouse effect <2>.

We receive more energy each year from the sun than is stored in all the fissionable fuel underground even using reprocessing and breeder reactors:
World energy resources and consumption

The estimates of remaining worldwide energy resources vary, with the remaining fossil fuels totaling an estimated 0.4 YJ (1 YJ = 1024J) and the available nuclear fuel such as uranium exceeding 2.5 YJ. Fossil fuels range from 0.6-3 YJ if estimates of reserves of methane clathrates are accurate and become technically extractable. Mostly thanks to the Sun, the world also has a renewable usable energy flux that exceeds 120 PW (8,000 times 2004 total usage), or 3.8 YJ/yr, dwarfing all non-renewable resources.

When you look at how quickly we're burning through 0.4 YJ of fossil fuels, 2.5 YJ of nuclear fuel isn't going to last long, and that's assuming we can get breeder reactors to work reliably and economically. Without breeder reactors, it isn't even worth the trouble.

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