Reply To: turn heat into electricity

[dennisp]

I just ran some numbers on indoor farming. Average sunlight per square meter is 250 watts:
http://en.wikipedia.org/wiki/Insolation

The dutch plantlab doubles photosynthetic efficiency, so we can figure 125 watts/m^2.

Crops and pastures take up about half of U.S. land area:
http://www.ajcn.org/content/78/3/660S.full

U.S. total land area is about 9 million square kilometers. Let’s assume we get more efficient with our food production, perhaps going more vegetarian and/or converting to a lot of lab-grown meat by 2050 or so. Also we’re getting rid of ethanol production. For nice round numbers we’ll say we need one million square kilometers for food production. Maybe we can do with less, but probably not by an order of magnitude.

A million square kilometers is a trillion square meters, so we’d need 125 terawatts to convert entirely to indoor farming, in the U.S. alone, not counting desalination. Current world energy production is about 17 terawatts. Not to mention, we’d need to build a million square kilometers’ worth of indoor structures and light sources.

With focus fusion it might be a distant possibility, but even then it’d be tough. But it might work for high-value specialty crops that are normally shipped long distances.

But that’s not the theoretical limit. What if we could use some kind of nanotechnology to synthesize healthy, good-tasting food?

One watt-hour is about equal to one kCal (dietary calorie). Let’s say we need 2000 per day per person. At 25% thermodynamic efficiency for the synthesis process, that’s an input of about 8 kWh per day per person. Divide by 24 to get a third of a kilowatt per person.

By the time we have technology like this, we could easily have 9 billion people on the planet. Food synthesis would then require just 3 terawatts for the entire world’s food supply.

In the meantime, Zapkitty’s idea for fertilizer production sounds pretty good…