Efficient food production

[james_davis_nicoll]

I was noodling around on soc.history.what-if and made a calculation I'd never bothered with before: if a human needs enough food to produce N Watts, how many square meters are required to intercept that much sunlight? OF course I was too lazy to actually look up insolation for various latitudes but the BOTEC I committed seemed to show that it should be a few square meters.

Even Fairbanks, Alaska, gets from 90 to 350 watts/m^2. Say your mark 1 human needs at least 100 watts worth of food to keep functioning [1]: They'd need about one square meter dedicated to collecting solar powers, asssuming no losses. The entire population of North America should require a few hundred to a thousand square kilometers of converters to power themselves. Even a factor of ten losses should mean that we'd need about 300 square kilometers to feed all of Canada, assuming the lowest insolation in Alaska is what we have to work with, and about 3000 square kilometers to feed all of the USA. That's a square less than 20 kilometers on an edge for Canada and a bit over 50 kilometers on an edge for the USA. Feeding the entire planet should require about 60,000 square kilometers or a square about 250 km on an edge (or less, if we pick someplace sunnier than Fairbanks to grow food).

Clearly modern methods of coverting solar (and fossil) energy into human energy are criminally inefficient.


1: Googling says "at least 2500 kilocalories" per day so call it 4000 to be safe. That works out to about 50 watts, which I will double just because.

Comments

[liveavatar.livejournal.com]

At first I misread this as "how many square meters [of human] are required to intercept that much sunlight," and yes, I know that's not what you really meant.

But until I figured that out I was having a grand time imagining a lot of people who went around fat and naked as a statement of low energy consumption (or as a statement that they could only afford their Guaranteed Solar Income).

Yrs in the name of stfnal life through incorrect assumptions,
Liveavatar

[autopope.livejournal.com]

1. IIRC, an adult human metabolism dissipates on the order of 400 watts at rest -- the increase when working isn't that great. I'm not sure there isn't something missing here in the Kcal calculation ...

2. Again IIRC (ancient biology/biochem lessons surfacing here) photosynthesis is bloody inefficient -- less than 1% of the incident photoelectric energy landing on a chlorophyl molecule ends up being used for ADP->ATP synthesis. When you add the not-unreasonable requirement that plants spend the majority of their metabolic energy on processes not directly contributory to human nutrition (e.g. synthesis of indigestible lignified cell walls, required to stop them flopping around in the dirt), the available energy drops further.

So as long as we're using a plant-based biochemistry, I figure you need to add three orders of magnitude to your estimate. Advanced nanotech might shave two orders of magnitude off of this, but I'm not sure how.

[del-c.livejournal.com]

Humans dissipate between 100W and 200W.

Plants are about 1-3% efficient over their whole life cycle (saying that parts of the process are close to 100% efficient is sort of true, but irrelevant, as the 100% efficient parts are about 1-3% of the total, in ways that can't be increased)

The equator gets about 420W/m2 averaged over the year. You'd think the poles would get zero, but since they get zero in winter anyway, and >zero in summer, the average for our 23° tilt planet comes to a surprisingly high 160W m-2 or so, according to a quick average of one of my old spreadsheets. That seems wrong somehow, but I don't see where I might have screwed up.

latitude
0 	420
10 	414
20 	396
30 	368
40 	330
50 	283
60 	233
70 	186
80 	161
90 	158