Copied Text from Reddit Post on Solar Energy

[Request] "The energy stored in all the oil and gas in the Earth is the equivalent of just eight and a half days worth of sunlight hitting the surface of the planet." I love the eloquent beauty of the comparison and it really drives the message home, but how accurate is it?

Answer 1

I found a paper that estimates the total amount of burnable carbon in the Earth at 1.9x1016 tons, or 1.58x1021 moles.

The energy output of oxidizing carbon into carbon dioxide is 393.5 kJ/mol. So, burning 1.58x1021 moles of carbon will produce 6.22x1026 J.

The solar radiation hitting Earth is 1361 W/m2 ignoring atmospheric effects. The area of the Earth is 510.1 million km2, or 5.1x1014 m2. This comes out to 6.9x1017 W for the whole surface. To reach 6.22x1026 J, it would take just over 900 million seconds, which is 28.5 years.

If we consider known fossil fuel reserves instead of their total theoretical amount (using data from the same paper), we have:

2.24x1014 kg of oil 1.74x1014 kg of natural gas 1.1x1015 kg of coal (both types lumped together since they have very similar energy density) 9.4x1013 kg of lignite

Multiplying by corresponding energy densities, we get:

9.4x1021 J in oil 9.3x1021 J in natural gas 3.3x1022 J in coal 1.4x1021 J in lignite

In total, 5.3x1022 J of energy. To get to this using solar radiation, it would take 76800 seconds, or 21.3 hours.

I personally struggle to understand the difference between "reserves" and "resources" in the original paper. It seems that if the same math is done on resources instead of reserves, it would come out to around a couple days worth of soalr energy. I didn't do that because I don't entirely understand the meaning of the term.

Answer 2

One nasa article reckons 4.4 x 1016 W hits the earth. Another MIT article reckons 1.73 x 1017 W which is a suspiciously large difference.

The nasa value is based off a 342W/m2 irradiance, multiplied by earth's surface area is approximately MIT's value. So NASA did something I don't care to figure out.

Imma save you most of the working out, just trust me bro on this googling:

2.3 x 1014 kg of crude, 9.6 x 1013 kg of LNG, 1.07 x 1015 kg of coal.

Crude and coal release about 44MJ/kg and LNG 55MJ/kg.

6.238 x 1022 J of reserves. Which would take the sun about 100h or 4 days. So yea the 8day thing is approximately on point. There is probably twice as much fossil fuels in total as reserves.