Actually, maybe not math geekery. Possibly simply somone who is properly numerate. Whenever I try and do these things I get lost in a welter of units and orders of magnitude and end up proving that Alpha Centauri is just past Basingstoke, on the right. And pink.
Now, what I want to do is work out how much energy it takes to grow 1 kg of wheat.
Leave aside all the oil and fertiliser that goes in. Just the sunlight that we use. This comes in two forms.
1) Insolation on the actual plants themselves. From here some numbers for the US. In the wheat growing areas looks like an annual average of 4 or 5 kilowatt hours per square metre per day. We want the annual number because wheat fields give one crop a year, right? We are devoting that much incoming energy to growing our wheat.
Wheat yields per hectare vary widely but according to Lester Brown something like 3 tonnes would be a useful average.
A hectare is 10,000 m sq, so our 1,642.6 kilowatt hours gives us 300 grammes of wheat, yes?
OK.
2) Water. It’s said that 1,000 tonnes of water are required to grow 1 tonne of wheat. (We’re talking not irrigated, this is rainfall, so that’s also why we’re happy to use the low yield number above). So we need 300 kg of water to grow our 300 grammes of wheat.
1 calorie is the energy required to elevate the temperature of 1 g of water by 1 oC, yes? Assuming average ocean temperature of 4 oC the water must therefore have been heated by 96 oC to become fresh water in the form of rain. 300,000 x 96 gives us 28,800,000 calories of energy in the production of the wheat purely from evaporation of the water that allowed it to grow.
Right. Kilowatt hours to calories. From here, kilowatt hour to calories is 1:820,000 odd.
So, we have energy going in to our wheat.
820,000 x 1,642 + 28,800,000 calories, yes?
1,346,440,000 + 28,800,000 or 1,375,240,000 or 1.4 billion calories as near as damn it.
Hhhm.
Whats the caloric energy we get from eating the wheat? From here, for flour (which I’ll assume is the same as wheat) 88 calories per ounce. Those are kilocalories (which is how we usually measure food) so in fact 88,000 calories per ounce, there’s roughly 30 g to an ounce so our 300 g is 880,000 calories. Or a million say.
So, just to be very roundy offy about this, we use a billion calories of energy to get 1 million (ie a thousand times less) in the form that we can actually use.
The point of all this? Not a lot really. Just think about this the next time someone tells you that obviously we’d never put more energy into a system than we get out of it. I mean obviously.
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