Re: It's the End of the World as We Know It
From: Robert (RB_at_..)
Date: 08/18/04
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Date: Wed, 18 Aug 2004 02:29:56 GMT
On Fri, 13 Aug 2004 18:14:51 -0700, "Eric Gisin" <ericgisin@graffiti.net>
wrote:
>"Robert" <RB@..> wrote in message
>news:19nah0hagg3d70bpjh5k2hh7pqn21i3po9@4ax.com...
>> On Mon, 02 Aug 2004 08:01:37 -0500, Tequila <tequila@mockingbird.com> wrote:
>
>. >A factoid,
>> >world sugar trades on its value
>> >as conversion to fuel-ethanol.
>>
>No sugar is converted to ethanol, it is too valuable as food.
>
>> At present consumption rates, the average vehicle would require about 3
>square
>> kilometres of prime agricultural land to keep it going.
>>
>> One billion vehicles means 3 billion kms^2.
>>
>> There are only 150 million kms^2 on planet Earth.
>>
>Absolute bullshit.
>
>A hectare produces 5000l of corn ethanol. A modern car consumes 10,000l/year.
>You are off by a factor 150 times. Methanol from biomass produces 2-5 times
>the fuel.
It might be indeed be possible to produce 5000 litres of ethanol from selected
individual hectares of the best soil, if water and fertilizer are poured into
that land. That would require an overall efficiency of about 0.2% (converting a
constant 200 W/m2 solar energy to chemical energy).
However we cannot afford to re-allocate all our best agricultural land just to
produce car fuel. The human race has to eat as well......and we cannot live
solely on corn or sugar.
Your figure is totally unrealistic for large scale ethanol production. You
obviously didn't take into account all the other energy used for planting
harvesting and processing your corn into ethanol. Your end product is being
heavily subsidized by coal and oil.
My figure is based on available solar energy and the efficiencies of both the
natural photosynthetic process and conversion of biomass to ethanol.
************
Plants use only about 3% of available sunlight in the process of
photosynthesis. The process itself is very inefficient particularly at colder
temperatures. Maybe only 1% of that 3% of solar energy ends up as biomass
energy. Just watch how slowly a tree grows.
Some plants appear to grow quite rapidly but they are 90% water.
Consider a location where the year round average 24 hr insolation is 100W/m^2.
That amounts to around 3 billion Joules per year per square metre.
What figure should we take for typical efficiency of converting of cellulose to
Ethanol?
Let's say 5%.
Multiply 3 billion by 3% x 5% x 1% and we are down to 45000 joules/m^2/year.
************
Say, 1 litre of petrol produces about 30 megajoules of energy.
A vehicle using 10 litres fuel per day expends 110 billion joules per year.
Divide 110,000,000,000 by 45000 .
That gives 2,400,000 m2 or 2.4 km2 per car.
************
Let's do the calculation another way.
The total world consumption will be around 15 billion litres oil per day in a
few years time.
That amounts to a staggering energy figure of 1.6 x 10^20 joules per year.
Dividing by 45000 gives the area required to replace oil with ethanol as 3650
million kms2.
There are about 150 million kms2 of land on planet Earth.
Even if my estimates are out by a factor of ten, there is still no way ethanol
can ever replace oil.
Once again one has to remember that all the energy used in the whole process
form start to finish (converting sunlight into ethanol at the bowser) has to
come from the crop itself. There will be no oil (or eventually coal) to fall
back on.
So if my figures are out a little, it certainly isn't by very much.
I welcome you will check them.
The conclusion is that we will have to forget ethanol powered personal
transport in hte future. Soon we will all be driving 'electric golf carts' with
solar cells on the rooves which help top up the batteries while in the parking
lot.
Our houses will all face towards the equator and be literally covered with PV
cells.
Problem is, where is all that pure silicon going to come from.
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