[permaculture] energy/ethanol
Toby Hemenway
toby at patternliteracy.com
Fri Aug 31 11:45:19 EDT 2007
Nathan Czuba wrote:
> The total solar energy available to the earth is approximately 3850 ZJ (zettajoules) per year.
> Oceans absorb approximately 285 ZJ of solar energy per year.
> Winds can theoretically supply 6 ZJ of energy per year.
> Biomass captures approximately 1.8 ZJ of solar energy per year.
> Worldwide energy consumption was 0.471 ZJ in 2004.
>
It's those last two numbers that are the most revealing. All the life on
Earth, after about 3 billion years of evolution, blanketing nearly the
whole land surface and much of the ocean, can capture 0.05% of the
sunlight that hits the planet. So how much do you think humans can
capture? Doesn't that suggest that referring to total solar energy
hitting the Earth is meaningless? What matters is not how much energy
reaches the planet, but how much can be usefully converted. All of life,
all over the planet, can only capture about 4 times more solar energy
than the energy humans use in the form of petroleum. That gives you an
idea of how much hardware we'd need to keep up the current rate without
oil. We already redirect for our needs about 40% of current biomass
produced on the planet, according to some figures. So it's going to be
tough to increase that amount without even more seriously disrupting
ecosystem function.
I'm curious about the figure for wind. Is that how much energy all the
winds on the whole planet produce? If so, it's meaningless. Remember,
you have to build the hardware, and keep it running, to catch energy,
which drops the EROI. That's why plants are only about 7% efficient. PV
panels themselves are more efficient, but once you add in embedded
energy costs, transmission and conversion costs, etc., PV drops to worse
than plants. PV panels take years just to pay back their embedded
energy, while plants pay as they go.
Here's a thought problem: if a PV panel takes years to pay off its cost
of construction, how many PV panels will it take to build one PV panel?
I think if we used most of the petroleum left to build PV panels and
wind generators, we could keep up our current lifestyle for a while
after oil gets scarce. But we're not doing that.
Toby
http://patternliteracy.com
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