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Response to wtmusic (Reply #61)

Mon Aug 12, 2013, 08:08 PM

65. Finally talking some sense about climate change....but then you digress

to making a patently false statement about retrievable energy in sun and wind....

"If you took the amount of sunlight that hits the Earth in one second and converted it into matter, how much would it weigh? How much energy is this in practical terms?
This is an excellent question, which puts the energy balance on Earth into perspective. Let me answer this question in two steps, and then let me compare the amount of energy from the Sun to the amount humankind is using right now.

The energy per time put out by the Sun is its luminosity, 3.8 x 1026 Joules per second (or Watts). Using Einstein's renowned formula that describes how much mass is transformed into energy, when energy is being produced, E = M * c2 (or: Energy = Mass * (Speed of Light)2), as 1 Joule = 1 kg m2/s2 and c = 300,000,000 m/s, the mass the Sun burns into energy every second is:


Mass/Time = 3.8 x 1026/(3 x 108)2 kg/s = 4.4 x 109 kg/s

or roughly 4 million tons per second.
At its distance of 1 Astronomical Unit (150 million km), the Earth is hit by the Sun's energy flux F = 1400 Joules/s/m2. We call this quantity the "solar constant", as this value averaged over each year is constant within better than 1% over time. With an Earth radius of approx 6400 km, the area, which is (pi * Earth's radius)2, with which the Earth intercepts sunlight is (pi * Earth's radius)2 = 1.3 x 1014 m2 making the amount of energy captured by the Earth each second:


F * (pi * Earth's radius)2 = 1.8 x 1017 Joules/s
According to the same procedure as above this makes the mass to produce this amount of energy per second:


Mass captured as sunlight per second = 1.8 x 1017 / (3 x 108)2 kg/s = 2 kg/s

This is about 4.5 lbs/s or close to 5 lbs/s.

To put these numbers into a perspective with highly practical relevance, on average, humankind is only using about 1/10,000 of that amount for its total energy consumption. In other words, sunlight seems to be a viable option for our energy needs, at least from the perspective of the total amount needed. Or from the point of view of mass, we are transforming about 20 kg of mass per day into energy for our energy consumption.

If we were to use much more energy, say a sizeable fraction of the amount that the Earth gets from the Sun, the Earth would have to heat up considerably in order to get rid of the waste heat. Every power plant needs a cooler to get rid of its heat; the Earth as a whole can only do this by getting hotter."

Dr. Eberhard Moebius
(January 2005)

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Purveyor Aug 2013 OP
Generic Other Aug 2013 #1
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