[nero]

I found an interesting statistic today. Your typical tropical rain forest produces about 9000 kcal/m^2/year in net primary production (NPP). Doing some back of the envelope calculations I came up with the estimate that to replace the 84mbpd of oil used in the world would require the net primary production of the amazon rain forest. This was assuming no loss converting the cellulose to a usable energy form. If I understand it right the net primary production does not take into account the other consumers of the primary production ie other animals, bacteria and fungi so there is no way that we could every exploit the tropical rain forest that intensively anyway.

Does anyone have any idea what proportion of the net primary production we humans can take from field crops? What is the most efficient method to take solar energy in the form of biomass. I've read an estimate that says that to replace fossil fuels would require about a quarter of the net primary production of the world. My question is what percentage of the NPP do we actually take from field crops. Presumably there is an upper limit to the percentage of the NPP that we can take off a field since at some point the energy return on the marginal activity is negative (For example guarding the field from racoon competitors is probably not an energy positive activity).

This all ties back to the cellulose ethanol argument. I suspect we really don't have a hope of extracting enough biomass from the planet to be able to replace fossil biomass (oil) with renewable biomass, but I'm open to others saying I'm wrong. What do you guys think?

[rerere]

I Does anyone have any idea what proportion of the net primary production we humans can take from field crops?

Converting crop seed oil into biodiesel has been discussed elsewhere. 50-150 gallons an acre and palm oil at 650 gallons an acre per year.

What is the most efficient method to take solar energy in the form of biomass.

Using Algae you can get (in theory) 20,000 gallons an acre per year

[nero]

Converting crop seed oil into biodiesel has been discussed elsewhere. 50-150 gallons an acre and palm oil at 650 gallons an acre per year.

thanks rerere it's not exactly what I asked for but interesting info. This is probably an inefficient conversion process I was wondering about simply the harvesting of energy in agriculture. This is a very well developed technology and so there should be some hard numbers on what is the maximum amount of net primary production (pre any processing whatsoever and in all forms including cellulose) we can realistically take from a field. My guess is that with intensive agriculture we probably can't do much better than 33%. If that was the case then I could plug that number into my back of the envelope calculations and say that we would need a minimum of 3 amazon forests to sustainably replace the fossil biomass energy (oil) we use. This would be before we start talking about any new technologies. When you do start talking about new technologies the efficiency is a real moving target, so in the feasibility study it is useful to do an initial calculation to discuss the order of magnitude required to achieve the goal.

My gut sense so far is that if sustainable repacement of fossil fuels with biomass were to be achieved it would require us to be able to convert at least 20% of the biomass that enters the factory gate into a usable energy. Is that reasonable ? probably not I'm just speculating there.

[nero]

At 650 gallons per acre you need 49 million square kilometers of palm trees to replace fossil oil.

[rerere]

My gut sense so far is that if sustainable repacement of fossil fuels with biomass were to be achieved it would require us to be able to convert at least 20% of the biomass that enters the factory gate into a usable energy. Is that reasonable ? probably not I'm just speculating there.

No. Again, look at the biomass of Algae.

20,000 gallons an acre of Biodiesel is the theory. Your idea requires a process to make the product, whereas the Algae needs to have the oil removed from the body.

OH, and you get up to 50% oil by weight.

[nero]

20,000 gallons an acre of Biodiesel is the theory. Your idea requires a process to make the product, whereas the Algae needs to have the oil removed from the body.

OH, and you get up to 50% oil by weight.

I saw what you say about algae, but I don't know of any algae farms where this technology has been demonstarted so I took it with a grain of salt. You gave the "in theory" proviso, what is the practical experience? At 20,000 gallons per year per acre we are still talking a massive amount of land area but it isn't quite out of the question.

[rerere]

I saw what you say about algae, but I don't know of any algae farms where this technology has been demonstarted so I took it with a grain of salt. You gave the "in theory" proviso, what is the practical experience? At 20,000 gallons per year per acre we are still talking a massive amount of land area but it isn't quite out of the question.

The US Government had 'demonstration tanks' and based on the best yields, good sun, wind at their back while going downhill the researchers came up with 20,000 gallons per acre.

The report claimed 500 square miles of algae would satisify America's transportation energy needs.

This has been talked about here.....do a search on algae.

For what you are looking to do - grow plants to make fuel - Algae can do it. Oh, and you could feed the Algae - oh say - Human Sewage. Most crops won't do well on raw sewage.

[nero]

The report claimed 500 square miles of algae would satisify America's transportation energy needs.

That quote is hard to square with the 20,000 gallons per year per acre which is about 1.3barrel per day per acre. in other words needing 24000 square miles for america's 20,000,000 bpd. Of which probably over 50% goes to transportation needs.

But you probably did this math yourself so I don't know how you can use both those numbers. Something doesn't add up, one of them is wrong.

[Optimist]

Here's some interesting numbers:
It is estimated that the world's annual energy need is about 4 x 10^17 kJ/year, or 9.56 x 10^16 kcal/year ( http://www.cpast.org/Articles/fetch.adp?topicnum=13).

It is also estimated that about 600 "quads" or 1.5 x 10^17 kcal/year of biomass is used by humans for food, fuel and construction ( http://www.menomonieethanol.com/Pimentel.doc).

In other words, energy needs = 63% of total biomass production under human control.

So here's what we need to do: In the end, pretty much all biomass produced by humans (forestry and agriculture) would have to go to energy production. Pyrolysis is about 72% efficient (2.7 kcal out/[2.7 kcal + 1 kcal used to power conversion], http://www.menomonieethanol.com/Pimentel.doc) in converting biomass to oil. TDP claims 85% efficiency, which would be confirmed or disproven soon enough. Some of the food would obviously be recovered post-consumer (sewage sludge, trash, etc.). PEW! Saved by the fact that energy conversion (72% minimum) is greater than the 63% we need.

Of course, some energy would be "wasted" by consumers. So we would need to add some biomass not counted in the 1.5 x 10^17 kcal/year number, such as yard clippings, waste paper, etc.)

If worst comes to worst, we might even (Mr. Bush, I hope you are sitting down) have to increase our energy efficiency.

Conclusion: Sounds pretty workable.