Wednesday, September 20, 2006

Bioethanol - The Math.

In my last posting, I worked out how much land and water would be required to grow the sugar crop and ferment from it sufficient hydrogen to replace the U.K. current use of oil-based fuels for transportation. I concluded that the option is a complete non-starter, without an extreme curtailing of transportation use, per se. I now present a similar calculation for ethanol production from sugar which though "better" than hydrogen, still requires using more than twice the amount of arable land there is in the U.K., and hence bioethanol production on this scale would overrun our means for food production.
I shall make a direct comparison between gasoline and ethanol, assuming they are both intended to be burned in internal combustion engines. The efficiency of ethanol in terms of "tank to wheel" might be improved using fuel cells, but this is still firmly in the experimental stage. Currently, the U.K. uses 54 million tonnes of oil (equivalent), which provides:

54 x 10*6 x 42 x 10*9 = 2.268 x 10*18 Joules of energy (J).

Ethanol may be considered as a partially combusted form of fuel (since it contains oxygen, with oil doesn't, being entirely hydrocarbon), and so it delivers less energy when burnt. Specifically, burning one mole of ethanol (46 grams) releases 326.68 kilocalories of energy, and so one tonne of ethanol would provide (10*6/46) x 326.68 x 4.18 = 2.967 x 10*7 kJ = 29.67 Gigajoules (GJ).
This may be compared directly with the figure of 42 GJ quoted for burning one tonne of oil equivalent. Hence we see immediately that ethanol packs around 30% less of a punch than gasoline does, or put another way, a tank full of ethanol will take the car 30% less miles than an equivalent tank filled with gasoline.

We need, therefore, 2.268 x 10*18/29.67 x 10*9 = 76.4 million tonnes of ethanol, which might be produced by fermenting sugar, according to the process:

C6H12O6 --> 2C2H6O (ethanol) + 2CO2.

The process is supposed to be CO2 neutral because the same amount of CO2 produced in the fermentation and ultimate combustion steps will be absorbed by next year's sugar crop (in essence, although in practice the situation is not that good). Assuming that the process is 100% efficient, we can expect to get (2 x 46)/180 - that is the ratio of the molecular weights of ethanol to sugar - or 0.511 tonnes of ethanol per tonne of sugar.

Sugar cane yields a crop of 87 tonnes per hectare (ha), that produces 19% of its weight of sugar, which is 87 x 0.19 = 16.53 tonnes. Hence this should give us 16.53 x 0.511 = 8.449 tonnes of ethanol. Since the density of ethanol is 0.789 kilograms/litre, this would occupy a volume equal to: 8.449 x 1000 /0.789 = 10,706 litres.

The actual production figure is around 6,718 litres/ha, and so the process is 6,718/10,706 = 63% efficient. Indeed this is similar to the efficiency of the fermentation process designed to produce hydrogen from sugar.

6,718 litres of ethanol weighs 0.789 x 6.718 giving a yield = 5.3 tonnes/ha. Hence the sugar crop would require 76.4 x 10*6/5.3 = 14,415,094 ha = 144, 151 square kilometers (km*2). Sugar beet comes in slightly better at 19.1 tonnes/ha and so an equivalent crop would need (16.53/19.1) x 144,151 = 124,755 km*2.

Since the area of arable land in the U.K. is about 65,000 km*2 even of we used all of it and grew no food, we could just about meet half our current fuel requirements from ethanol. Perhaps if we could "seed" more land, we would still need around half the entire area of the U.K. mainland of 244,000 km*2 to produce it!

The message is once again that without severe cuts in transportation use, the situation is hopeless. My figures are rough, and the situation will perhaps be improved by new technologies - but only slightly. Using "bio" fuels to break the hold that imported oil has on us, is really a non-starter, at our current levels of fuel consumption. It is these we need to reduce first and foremost, but that will entail living quite differently ... and probably far more frugally. The option of a Die-Off in human population as energy resources run-out is far more uncomfortable, however.


MCrab said...

Hi Chris,

Have you considered the effect of switching transport over to plug-in hybrid vehicles (PHEVs)? It has many efficiency advantages over the hydrogen cycle, with energy not being lost in the production, compression and transportation of hydrogen. Plus all the infrastructure is already in place.

A nice overview can be found here. To summarise:

1) Up to 44% well-to-wheels efficiency if modern combined-cycle power plants are used to produce the electricity. Could potentially result in almost a quartering of the energy used for automotive transport.

2) Electricity used can be generated by a variety of means, including low carbon nuclear and renewables.

3) Potential reduction in liquid fuel use of almost 80%. Even today, 30 years after its peak, the US still produces about half the amount of oil. Use of PHEVs could eek out the remaining reserves for decades, perhaps centuries if biofuels are used to suppliment.

4) Approximately 13% more electricity would need to be generated each year if all cars were suddenly PHEVs. This would not necessarily mean greater capacity was needed, as PHEVs could charge at night when demand is low.

5) A side benefit would be enhanced grid stabilization for renewables due to the energy sink represented by PHEVs batteries.

PHEVs seem to represent a way to continue with our current lifestyle while reducing both total energy and fossil fuel use. Those who advocate 'living quite differently ... and probably far more frugally' often don't take into account that they're selling a product that the majority of people have no interest in. This is a last ditch solution and others will be tried first, including PHEVs. I, for one, hope they succeed.

P.S. There is scope for similar energy savings in the heating sector. Use of geothermal heatpumps with the latest combined-cycle power plants could conceivably halve the gas used for heating.

Professor Chris Rhodes said...


thanks, and I will look into this further!

I quite agree, I don't want to return to the stone-age either, but
it annoys me to see fuel being squandered on kiddie-runs in fleets of SUV's. We walk! Sure: it is a real vote-loser to suggest "frugality", and no government will, until that "last ditch" arrives. That is the tipping-point where our technology has failed us!

However, I feel that some other kind of fuel-cut - if that can be
done (you mention 80%) - would take us a long way toward a sustainable scenario, when biofuels (ethanol seems best in terms of tonnage of fuel per hectare) might make a significant contribution to the final "mix".

Overall, using "fuel" in the form of "electrons" seems to be a lot
more practical than hydrogen. Hydrogen really loses out in the well-to-wheel stage, i.e. it is fairly inefficient to make (in practice anyway). I gather that, as is the problrem with storing electricity on a large scale made from solar-power, implementing sufficient "battery technology" is at issue.

But, either way, the situation needs to be taken seriously, and if
our world governments don't do so... we are headed for, probably not the
stone-age immediately, but say the rural-economy of 1850's England (or its U.S. "pioneer" equivalent)!

It's good to get some decent feedback - much appreciated!


MCrab said...

Hi Chris,

Thanks for the response, and sorry if my last comment was a bit snippy - spend too long reading the peak-oil websites and debating with those salivating over the prospect of a die-off and you start to become a little less well disposed towards your fellow man yourself. :)

I enjoy popping in on your blog, Chris, as it seems to be an earnest attempt to grapple with the problem that should concern all of us - namely fossil fuel depletion.

In my own muddled way I've been reading around the subject for the last few years and it seems to me there is no one silver bullet solution to either climate change or peak oil. Rather there are a whole host of small solutions that, while not sufficient on their own, can reinforce the impact of each other to give an effect larger than their individual sum.

Professor Chris Rhodes said...

Hi mcrab,

no "snippyness" taken at all! My point in part is simply to get people aware of the sheer "scale" of the problem facing us, in terms of just how much fossil fuel we would need to replace at current levels. Certainly, my overall intention is, as you say, to try and arrive at some practical solutions. I am also coming around to the idea that there is no "one fix", and probably there will be more emphasis on "micro-generation". So, energy might be made locally according to whatever particular resource there is, e.g. a river. Interstingly, Her Majesty The Queen has had a couple of (micro) hydroelectric turbines installed in the river Thames below Windsor Castle, so she is leading the way here! I am sure that there will be an ultimate "mix" of technologies. I have read your "link" about the PHEV and I must say it looks very promising. However, I wonder whether there is sufficient infrastructure to provide the required electron-storage capacity ('batteries' if you will), on such a grand scale (factories and raw materials, e.g. cadmium is quite a scarce mineral). I don't know how many cars there are in the world, or in the U.S. I believe (from memory) there are about 20 million of them in the U.K., for a population of about 60 million. But, I think that this may be the limiting factor in the inauguration of electric vehicles on the grand scale. What do you think?
I am also fed-up with the doom and gloom, and I feel in a way fairly optimistic. Sure, we are going to have to live differently - in some way, but I don't think there will be an overnight crash. Probably the changes will come more gradually, and they will be ushered-in by economic drivers. So, things in general will become more expensive. That is bad news for those who carry heavy debts. I saw something on the news this morning that the U.K. is the heavist population of debtors in Europe! So, if a lot of people in this situation lost their job, for instance - and companies might go to the wall in times of economic recession - they would join the ranks of the inevitably growing poor.
It all needs some planning. At present the West seems to be "in denial". Maybe there should be a 12 step programme for the fiscally intemperate!

Please keep popping-in!

Kind regards,


Kevin Scott said...

What about BioDiesel as opposed to ethanol and Hydrogen as a fuel. At least it has close to the same energy content as normal diesel?

Is the math any better?

Kevin Scott said...
This comment has been removed by a blog administrator.
Professor Chris Rhodes said...

you make a good point, and one which I addressed in the posting immediately following ("Biofuels - a Comparison of Practicality.") In fact, at a biodiesel yield of 2 tonnes per hectare, the math is even worse!

You would need about twice the land area to grow a biodiesel crop as is required to produce enough bioethanol to match the energy output of 54 million tonnes of oil. i.e. around four times the area of arable land there is in the U.K. There are crops e.g. palm oil that give a greater yield (about twice) so on the basis of acreage they could just about match the yield from bioethanol (but still need twice the arable area of the U.K. to grow the sugar crop to make it!). Then there are "super" crops which if grown in the best soil can yield about twice that again (around 8 tonnes per hectare), but this is the agricultural equivalent of "going downhill with a following wind!"

However, this would require "only" about the entire area of national arable land, and so using such "super-crops", and not growing any food at all, but turning our agriculture entirely over to their production, we could conceivably run our current fleet of cars,lorries and planes on biodiesel!

What is worrying is that the rain forest is being chopped down (slashed-and-burnt) to grow these crops (not in the U.K., obviously - we destroyed our forests centuries ago for shipbuilding and to make charcoal for smelting iron!), therefore losing much of any real advantage, and contributing to further environmental catastrophe, since the forests themselves become major emitters of CO2 when maltreated in this way!