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Original post by Raving Lunatic on 01//08

 

A weed called switchgrass produces more ethanol than corn - up to 5 times the input required to grow it. I wonder how much they could get out of pot, er, I mean hemp? Oh, never mind, we can't use hemp because it's naughty.




Posted by mrwilder

Hemp may be is substantially more potent than corn, but whatever it's energy value is can't have the density of petroleum. Due to the constraints of the most basic physics - it is almost positively completely impossible that anything which we could possibly grow on Earth could ever be useful in the manner that popular media is describing.

If you factor out all the fossil fuel energy used in growing *INSERT GROWN FUEL SOURCE HERE*, along with the production energy needed to make the machines, produce the insecticides and herbicides, and the food used to feed the people who are growing the corn, the remaining energy GOING INTO the system is from the soil and sun. Of the energy in the soil, 100% of it came from the sun, so, ultimately, every bit of energy you're going to get out of a system operating at 100% efficiency boils down to, at a MAXIMUM, the amount of energy in sunlight that falls on the surface of the earth during the period after planting and before harvest. And finally, it is known that at best, the Kreb's cycle and cellular respiration in general is 40% efficient and that photosynthesis and the entire process from "sunlight to biomass" is a net 5% gain.

Let's look at some numbers:

barrel of oil = 42 gallons = 5.8 million BTUs

Sulight: 425 BTU/sq. ft./hour

lets say corn uses 2 sq ft. of ground and takes about 75 days to mature. You can pick your own numbers for your plant.

days = 24 hours so, the maximum sunlight aborbed before harvest if the sun shied 24 hours a day 365 days a year is 425 * 24 * 2 *75 = 1,530,000 BTU * .05 for the energy used in biomass conversion = 76500 BTU *40% cellular efficiency = 30,600 BTU produced in 75 days.

Now, in the United states, we use about 20,000,000 barrels of oil per day, or, 115 million million (U.S. Trillion) BTUs per day.

If one ear of corn produces 30,600/75 BTU per day, we're getting 408 BTU in a day from 2 sq ft of corn. We'd need:

115,000,000,000,000/408 ears of corn or 280,000,000,000 sq. ft.

There are 43,560 sq ft in an acre, so we need 280,000,000,000 /43650 sq/ft acres or 6,500,000 acres of arable land (which in the real world has to lie fallow some times)

Currently, there are only about 13,000,000 TOTAL of irrigated cropland acres in the United States - which I understand is about 1.8 arable acres of land for person right now. Based on current technology we require at least 1.2 acres per person just to eat at our current "required" nutritional levels.

And I've been very generous with those numbers. Petroleum is so potent because it had billions of years worth of solar energy stored in it... not THREE MONTHS worth! While there's a gigantic fudge factor here, organic fuel sources don't seem to be the most likely solution.

Posted by mrwilder

Hehe... yep, look at that LAAAaaaast little sentence, barely noticeable in it's obscurity:

"Low input systems are just not going to be able to get the energy per acre needed to provide feed, fuel and fiber."


   
January 23, 2019


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