8
$\begingroup$

I'm building an artificial growing environment for a heavily overpopulated world (because you can always build up, right?). When I started thinking about this, I thought it might be as simple as "calculate the energy output of the sun for the growing season of wheat...." But as I thought it through, I needed:

  • Sunlight for photosynthesis
  • Sunlight (or something else) for warmth
  • Water distribution
  • Air distribution and purification/recomposition (right % of chemicals, etc.)
  • Fertilization and soil control
  • All the equipment...

And probably a bunch of other things. That might make this question too ambiguous, but I'm giving it a shot. Asuming I have a big, future-tech fusion generator, how much energy does it take to grow an acre of wheat?

  • I have the seeds and an acre of soil that needs no other treatment to plant the seeds - but they're not yet planted.

  • Don't include the energy needs of the human staff (how much energy is needed to run a human for the season is outside the scope of this question). However, you can't use this limitation to justify not using machinery. In other words, you can't assume an army of scythe-wielding humans to replace the combine they'd actually use and the energy it needs, which is part of this question.

  • I'm asking for energy because I'm assuming (for the sake of simplification) that I can reasonably convert everything to electric motors with better-than-average batteries and just recharge them off my fusion generator.

  • The question does include harvest. The end result is a field full of wheat stubble.

  • Processing the wheat after harvest isn't considered. In other words, once the kernels are in the hopper that part is over. Removing the bailed straw is part of the process, but once it's in off the field, that part is over, too.

  • Edit: Hydroponics are not part of this question. I'm working on a much more complex agri-industrial complex than just growing wheat and I'm looking for the energy number to help design the infrastructure supporting it. I'm not interested in alternatives to an acre of soil — just the energy needed to grow the wheat.

I recognize that I'm going to get a best-estimate because there are a thousand little details that would require a couple books to answer.

$\endgroup$
14
  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$
    – L.Dutch
    Aug 28, 2020 at 19:53
  • 1
    $\begingroup$ Since photosynthesis is used to primarily generate sugar for chemical energy, what if you modified the wheat to skip that process completely and instead absorb chemical energy from sugar dissolved in water, then generate that sugar on an industrial scale deeper underground where waste heat from the mantle can be used to power bacterial vats to produce glucose from Bacillus subtilis. Now you dont need to worry about piping in sunlight or generating artificial light $\endgroup$ Aug 28, 2020 at 23:20
  • $\begingroup$ @RichieFrame those are clever ideas! But they're not the kind of information I need. I need the number and those only modifies the number. $\endgroup$
    – JBH
    Aug 28, 2020 at 23:44
  • $\begingroup$ My best guess is it will reduce energy expenditure by 90 to 99% relative to grow lighting regular wheat as per Nosajimiki's answer, it may be even more efficient due to other factors, and also reduce the need for carbon dioxide intake in the grow area substantially, reducing the cost to pump in fresh air. With genomic optimization I would assume 60 Mwh/acre or better $\endgroup$ Aug 29, 2020 at 0:28
  • $\begingroup$ CO2 can be probably resolved by interspercing human habitats with grow rooms, that way the buildings can passively interchange gases. $\endgroup$
    – Nosajimiki
    Aug 29, 2020 at 0:46

1 Answer 1

10
$\begingroup$

Even before doing any research, I can tell you that all other power expenditures are pretty negligible compared to the the amount of energy required to produce the light that feeds the plants... but here are all the numbers anyway:

Water distribution

Water distribution is pretty cheap in terms of power because of how hydraulic systems work. If you have water to store in an elevated place, then every pipe attached to it will have water pressure no matter how far laterally it travels. If you build a dam that raises a reservoir above the level of your farms, then that means your water is absolutely free (in terms of energy to distribute.) Even if you need to pump the water https://www.engineeringtoolbox.com/pumping-water-horsepower-d_753.html show that a 1kw pump can life over 100 liters 15 meters per second. Wheat needs 100-125mm of rainfall a year. That is about 455 liters per year per acre; so, unless your farm is at the top of a highrise building, whole system could generally have it's entire annual water pumping needs meet off of the power stored by a AA battery.

Robotics (aka Fertilization/soil control/all the equipment)

Farming indoors requires a lot less of things like weeding, pest control, etc. While this is hard to find exact specs on, I'd imagine you probably are not using much more power than a laptop consumes simply because there is not really that much to do 99% of the time but have systems on standby waiting to scan something or controlling a few nozzle servos to start/stop watering. This is just a ballpark guess, but I would be surprised if you actually needed more than 4-5 kWh/day.

Air Conditioning

Air conditioning and filtration will vary a lot relative to your setup. In general, wheat prefers 70-80°F (21-27°C). If you are building in an archaeology located in a desert, this could take a lot more A/C to keep cool, but if you are in a cooler region where you design your vertical farm buildings to have a natural heat-sink shape, your AC could be practically passive. If you assume a home needs a 20 BTU/sqft AC running an average of 3 hours a day to maintain these temperatures during growing seasons where wheat is normally grown to offset the heat of the sun, then an acre of enclosed farmland should be fine with an 871,200 BTU HVAC system. That will make your HVAC system require about 261.36 kWh/day.

Grow lights

Cannabis and wheat have similar light requirements; so, your indoor farm will probably need about 40-50 watts worth of LED lighting per square foot to grow. That means your wheat will need a total of 1960.2 kW worth of lighting which will need to be on for an average of 14hrs a day to simulate grow season daylight time. That is a total of 31,363.2kWh/day.

Harvesting

My personal experience with my battery powered weed wacker is that a 2Ah 20v battery can take out about 100sqft of tall thick grass which would be comparable to the process of harvesting wheat. 2Ah @ 20v = 40Wh so a similar device used to harvest your whole field would consume about 17.4kWh per acre making this another negligible energy cost.

Conclusion

By maintaining ideal grow conditions, you should be able to grow about 37 bushels of wheat in about 6 months, which will yield a total of about 1550 pounds of flour after processing. This gives you a cost of about 5,756,600 kWh or 3814 kWh/lb of flour. Your civilization better have some pretty cheap power though or this flour is going to cost you hundreds of dollars a pound to grow.

Since you mentioned in comments that this is an archaeology the size of Connecticut, you have a total area per story of 3,548,427 acres which will meet the annual wheat consumption at current demand for about 30.5 million people per floor at a cost of 2.04e13 kWh per floor per 6 month growing cycle. This is about 10 times the United State's entire energy budget in 2019.

As a final suggestion: vertical farming is already horribly inefficient, and wheat is a particularly bad crop to do it with. If you need more room to grow your wheat it might be better to do it on barges taking advantage of all that unused flat space we call oceans, plus it makes use of an otherwise wasted part of our planet's thermal budget so that you aren't causing massive global warming by generating thermal energy at levels that are proportionally significantly compared to direct sunlight. There might also be some other creative scifi solutions like growing grains without the plants (sort of the same concept as growing meat without the animal) which could give you a wheat like GMO that could be 10s of times as efficient both per acre and per kWh.

$\endgroup$
10
  • $\begingroup$ I assume this isn't in a desert because you could simply glass in the roof and not need artificial lights. In fact, the main use case I could see would be something like a deep-space colony where you have minimal energy from the sun, and you'd probably need to supply base heating. $\endgroup$
    – Cadence
    Aug 28, 2020 at 18:16
  • 2
    $\begingroup$ @JBH One last piece of food for thought (and this is why stacked farms this size don't make sense) is that each floor of farmland will need to generate as much heat as the sun does beaming down on the Earth; so, if you say stack 60 of these in a contiguous structure the size of Connecticut then the thermal output of all those lights will be like taking a magnifying glass the size of Texas and focusing all that sunlight onto Connecticut... the results would not be pleasant. $\endgroup$
    – Nosajimiki
    Aug 28, 2020 at 19:34
  • 1
    $\begingroup$ @Nosajimiki Stacked farms make sense on my world because there's no other choice. Remember... fiction.... I need a number to move forward with my suspension-of-disbelief story so that I can make the design "real enough." The purpose of this site isn't always to make the solution viable to today's physics (in fact, it's rarely for that purpose). Please don't over complicate the question. $\endgroup$
    – JBH
    Aug 28, 2020 at 19:47
  • 1
    $\begingroup$ @JBH LOL! fair enough. That said, bringing up the real world concerns can lead to extra little details that make your story more unique, like maybe your world has a deadzone over the farm area because it's beaming a giant death ray off into space to bleed off the heat or something like that. Did not mean to make it sound like I was shooting down the idea of stacked farms entirely $\endgroup$
    – Nosajimiki
    Aug 28, 2020 at 20:03
  • 1
    $\begingroup$ What do you mean by an archaeology in this answer? None of the definitions actually fit... $\endgroup$
    – Brian
    Aug 29, 2020 at 9:02

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .