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## lgw's Journal: Geothermal vs Solar Power1

Journal by lgw

Here are the basic numbers on aailable geothermal vs solar power (since this has come up in discussion more than once).

The surface area of the Earth is about 5.1 x 10^14 m^2. The cross sectional area is about 1.3 x 10^14 m^2 (one quarter of the surface area, of course).

Per this paper found as a cite on wikipedia, the total heat flow out from the Earth's interior is 4.42 x 10^13 W, or 0.0867 W/m^2. Of course, the available power is much less because it's only the subsurface-surface temperature difference that's available.

Total solar irradience is 1361 W/m^2 by NASA's latest estimate (so about 1.7 x 10^17 W across the entire cross section), or about 1000 W/m^2 on the surface at noon on a cloudless day. Averaged over the day-night cycle (surface area vs cross-section, so 250 W/m^2), and taking clouds into account that's about 180 W/m^2 (I can't find a solid source on that yet, but it looks close).

So, total solar power flow is about 4000 times as large as total geothermal flow. I'm not quite sure how to estimate the (ideal) available power as a percentage of the total geothermal power flow, but if we use a WAG of 50%, then the available power from solar is also about 4000 times per square meter more than geothermal - significantly more if we average solar power only across populated latitudes.

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