F.A. Farm

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More Analysis of Internal Energy in a Litre of Petrol

In regards to Rob Hopkins’ claim of “1 litre of petrol equaling 35 days of hard manual human labor,” I looked at his source, which is FEASTA, The Foundation for the Economics of Sustainability. (Thanks to David McLeod of Transition Whatcom for pointing this out to me.) Here is the website link and the relevant paragraphs, which evidently date from 2007: http://www.feasta.org/documents/energy/emissions2007.htm

The energy in a kilogram of oil is equivalent to the output of about 24 working days or just under 200 hours of human work. That makes a day's human work equal to about 40 grams of oil, a couple of desert-spoons full. Another way of looking at it is that a 40 litre fill-up at a petrol station is the equivalent of about four years of human manual work.

Put another way, if an averagely fit person pedaled a generator, they could light a 70 watt bulb though their efforts. This means that, for every hour that they spent in continuous physical labour, they could achieve 3,600 x 70 Joules of work. (A watt is a joule per second - so a 'watt hour' is calculated from the number of seconds in the hour, which is 3,600.) 3,600 seconds x 70 Joules is 252,000 J per hour, the amount the average worker could achieve.

There are no references for FEASTA’s claims and there is a major theoretical problem in their calculations. If we take the first claim of 200 hours of human work for 40 grams of oil, we don’t know whether they mean crude oil or diesel or gasoline. Presumably they mean diesel or gasoline (which are not equivalent in mass, by the way) since they mention a 40 litre fill-up at a gas station. They also state the 40 litres as equivalent to about four years of human work. At 2000 hours per year that calculates to 8000 hours for 40 litres, or 200 hours of work per litre. If we divide this by an 8-hour day, that means a litre of petrol would be equal to 25 days of human manual labor. If Rob is using this method of calculation, he simply made a math error. (The paragraph above actually states 24 days, which is probably because they didn’t round up to 200 hours in their original calculations.) 

If, on the other hand we look at the second method of calculating human work used by FEASTA, we quickly uncover their theoretical error. They first postulate “an averagely fit person” pedaling a generator that lights a 70 watt bulb. Their actual calculations are spot on and I appreciate their conversion to 252,000 joules per hour, which is the preferred unit for thermochemical energy. However since there are 4184 joules in a kilocalorie, this calculates to 60.22 kilocalories per hour for the human pedaling the generator and lighting up the 70-watt bulb. [Note: I will use kilocalories for the calculations in this post rather than the usual shorthand of calories.] For those of you following closely, you may already see that FEASTA is focusing solely on the work done (the light bulb) and NOT the energies used by the sweaty human to get the work done. This is a major theoretical problem, as it confuses input with output. 

In one of my classes in high school, the instructor had the class tough guy come up and push against his desk, which was attached to the floor. Of course the desk didn’t move. The teacher then flicked his pencil a couple of times and said that he was doing more work than the tough guy grunting and straining against an immovable object. Of course he was right, as work is generally accepted as getting something done, like moving a block of stone or digging up dirt with a shovel. However, the work in this case is a measure of output, not input. In addition to the work done to pedal the generator or move the block of stone or dig the soil, we have to take into account the energy used to keep the engine going, like the energy to contract the muscles or make the wheel go around in the first place. If electric power plants operate at a thermal efficiency of 30% and a car operates at a thermal efficiency of 21%, we know we have a 70% and 79% waste of energy in the form of heat when we run these engines. We get the same result in a human that is reflected in a higher heart rate and the sweat running down the face of the human who is pedaling the generator to light up the 70 watt bulb. In other words, we cannot discount the energy that goes into powering the light bulb that is “wasted” as heat. This is the same kind of fiction that economists use to baldly state that industrial agriculture is a more efficient way to grow food, all the while blithely ignoring the much greater quantities of energy used to power the big tractors to grow the food and the inputs of energy to make the fertilizers. If we use a full accounting of how much energy is used to power the engine, i.e. the input rather than the output, we will have a much higher number of calories or joules or kilowatts or horsepower or the like. In short, the 60.22 kilocalories used per hour to power the 70 watt bulb are much lower than the actual kilocalories of energy used to power the human engine. 

Thermochemical energy is a term used in this very interesting website “Thermal Efficiency of a Human Being: http://mb-soft.com/public2/humaneff.html. A better term is internal energy (U), which is explained very well here: http://en.wikipedia.org/wiki/Internal_energy. Internal energy is the sum of potential and kinetic energy and the standard measure is the joule, which is the energy needed to move one newton one meter, or pass one ampere of current through one ohm for one second, or to produce one watt for one second. Even though the joule is the SI unit (from the French Système international d'unités), it can be translated into kilocalories or kilowatt hours or horsepower or BTU’s. Here is a very nice calculator so you don’t have to do the math: http://www.convertunits.com/from/joules/to/kilocalorie. There are 4184 joules in a kilocalorie. For another look at this calculation of energy input, albeit even more torturous than mine, check out this post from the Oil Drum here: http://europe.theoildrum.com/node/4315

The bottom line is that energy must be accounted for, and conflating energy input and output is stinky cheese. The problem arose from uncritical acceptance of a source (the FEASTA webpage) without checking the numbers. That is why I always try to lay out my methodology and sources when I make a statement based on number crunching. If you bear with me, I will go through my methodology one more time. 

As I have written many times, I calculate human energy in kilocalories (or just calories or Calories in the usual manner accepted by nutritionists and most popular writers). I know how many hours I put in over a year’s time to grow food - 3000 hours - and I know that I don’t sweat and strain for 16 hours a day in the summertime when I put in my long days. I also take time for lunch and a nap most days. After surveying multiple websites where the number of calories burned during sleep generally ranges from 400-600 and depends on weight, I arrived at the reasonable assumption that the average human uses about 500 calories in 8 hours of sleep. I use a 2500 calorie a day diet, even though the standard is 2000 for women and 2500 for men, as it makes my calculations on how many people I can feed more conservative. Using this higher number also has a conservative effect when calculating calories per hour used in manual labor. Since 2500 minus 500 is 2000, I have 2000 calories to burn up in a typical day, when I work anywhere from 0 hours during 2 months of the winter to 16 hours a day during 3-4 months of summer. Apportioning 2000 calories over 16 hours is reasonable when I observe that I spend as much energy walking out to the mailbox as I do weeding carrots. I have worked a lot of jobs in my life, from archaeologist to circus ringmaster to word processor to college professor to migrant worker to punch-press operator to construction worker to farmer. I don’t sweat and strain very much and most of my coworkers didn’t either. Even working construction allows an average fit human to go out dancing at night if he/she wants to. Certainly my tiredness at the age of 61 has more to do with my age than with the “hard manual labor” I do every day. All in all, 125 calories per hour for manual human labor (it really isn’t that hard!) is reasonable. This is more than twice as much as that calculated on the FEASTA site, but well below that calculated by fitness trainers for bicycling in general (500-600 per hour). 

The real benefit of my method of calculating internal energy (U) is that it is a metric that crosses all platforms. I can use it to calculate how many calories in that pound of tomatoes you bought at the farmers market versus the ones in the supermarket, I can compare horse-drawn plowing to a human with a shovel or a tiller or a tractor. I can argue for the positive feedback loop of eating part of what we grow and using those calories to grow more food as a very efficient method of growing food. Finally, I can make the claim that the human engine is more efficient than the internal combustion engine in the amount of work it gets done. Even though there are occasional flubs in general conceptions of “work” and “energy” and “efficiency,” I can actually run the numbers because my method is based on the laws of physics. Under the general rubric of “input/output analysis,” I have constructed a model of sustainable agriculture that can feed the world right now with very low fossil fuel inputs, as well as put people to work in a very efficient manner. In point of fact, input/output analysis provides a method to actually measure sustainability. I can show anyone the numbers of what I produced in the last several years at my farm and in 2010 at the food bank farm I manage. I can state with certainty that I produced 3.5 calories of food energy for every input calorie I used to grow that food in 2009 on my farm, measured in gasoline and human labor. I can state with conviction that we could get by quite nicely with only 20% of our adult population growing food for the other 80%. I know these things because I have done the work and I have the numbers. They are impressive enough. We don’t have to exaggerate the internal energy of a litre of petrol. As I said before, Rob Hopkins’ statement of “1 litre of petrol being equivalent to 35 days of hard human manual labor” is an exaggeration. Rob has the right idea but the wrong numbers.

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