Vertical farms and controlled environments
Grow leafy greens, herbs and transplants on stacked tiers under electric light, with the electricity per kilogram worked out before building, and the room held safe for the people working in it.
Everything for this step
- Field guide
- Greenhouses → 15 sections
- Builds
- None yet.
- Reference sheets
- None.
- Sciences it rests on
- Agricultural science · Physics · Engineering · Biology
- Track
- Protected growing, level 3 of 3
Why this matters
A building that replaces the sun with lamps uses far less land and water but far more electricity. Producing 1 kg of lettuce dry weight took 247 kWh of electricity in a plant factory, against 70, 111, 182 and 211 kWh in greenhouses in the Netherlands, the United Arab Emirates and Sweden, while the plant factory did better on water, CO₂ and land (Graamans et al., 2018).
Staple calories do not pay. Wheat on 10 stacked layers could yield 700–1,940 t/ha a year, 220–600 times the world average of 3.2 t/ha, but "it is unlikely that indoor wheat farming will be economically competitive with current market prices in the near future" (Asseng et al., 2020). Vertical farms are for high-value, fast, perishable crops grown near their buyers; grain stays in fields.
The shape of it
- Grow only what pays under lamps: leafy greens, herbs, seedlings and transplants; never staple grains.
- Price the electricity before building: about 247 kWh per kg of lettuce dry weight, more than 3 times a Dutch greenhouse's 70.
- Buy LEDs of at least 2.30 µmol/J, the DesignLights Consortium's current minimum.
- Set light by the daily light integral: 17 mol/m² a day for lettuce is 295 µmol/m²/s over 16 hours.
- Remove the heat and recover the water: nearly all lamp power ends as heat, and the crop's transpired water is condensed and reused.
- Enrich CO₂ only with alarms: people may work in no more than 5,000 ppm over 8 hours; 40,000 ppm is immediately dangerous to life.
- Run the nutrient solution as in soilless growing, with every reading logged.
Doing it, step by step
The summary above says what has to happen. This says how, with the numbers that decide whether it works.
1 · Choose crops that pay for their light
Lamps cost the same per photon whatever the crop, so grow what sells for most per kilogram and turns over fastest:
- Leafy greens and herbs: lettuce, spinach, basil, coriander, microgreens.
- Seedlings and transplants for field and greenhouse growers, raised fast and uniform.
- Not staples: wheat on 10 layers reached 700 t/ha a year measured and 1,940 t/ha at the modelled maximum, 220–600 times the field average, yet the authors judged it unlikely to compete with market prices (Asseng et al., 2020).
Grow near the buyers: the gain is freshness, year-round supply and short transport, not cheaper calories.
2 · Price the electricity before you build
Start from the measured figure: 247 kWh per kg of lettuce dry weight in a plant factory, against 70 in a Dutch greenhouse, 111 in the UAE, and 182–211 in Sweden (Graamans et al., 2018).
Worked example: at an electricity price of 0.15 per kWh, that is about 37 per kg of lettuce dry weight for electricity alone, before labour, capital, seed and packaging. Put your own tariff, your expected yield per m² and your selling price into the same sum, and plan for the tariff rising.
The business record: AeroFarms, one of the largest vertical farms, filed for Chapter 11 bankruptcy protection in June 2023, citing "significant headwinds faced by the vertical farming industry"; the report noted dwindling investor interest and the sector's "sky-high initial costs" (Food Dive).
3 · Light: efficient lamps, set by the day's total
Buy LED fixtures of at least 2.30 µmol/J: the photosynthetic photon efficacy every product on the DesignLights Consortium's qualified list must reach (Horticultural Technical Requirements V3.0, 2024). Higher efficacy is cheaper to run.
Set the light by the daily light integral (DLI): Cornell grows lettuce at 17 mol/m² a day (Cornell CEA). Spread over 16 hours, that is 17,000,000 µmol ÷ 57,600 s = 295 µmol/m²/s at the leaves.
The power it takes: 295 µmol/m²/s ÷ 2.30 µmol/J = 128 W per m² of growing area, 2.05 kWh per m² per day on every tier.
4 · Take out the heat, recover the water
Nearly all the electricity ends as heat in the room — from the lamps, pumps and fans — and the crop transpires most of the water it takes up as vapour. Size the cooling for both the heat and the vapour:
- Cool and dehumidify with coils that condense the vapour, and return the condensate to the nutrient tanks: this is where the water saving comes from.
- Move air across every tier, so the leaves at the back of a rack see the same temperature and humidity as those at the front; Cornell keeps lettuce at 24 °C by day and 18 °C at night, 50–70 % relative humidity.
- Keep a standby supply for pumps, fans and controls: a power cut stops everything at once.
5 · Enrich CO₂ only behind alarms
CO₂ enrichment raises growth where light is plentiful: Cornell uses 1,500 ppm when the lights are on, against about 390 ppm outside. People share that air. The NIOSH limits for carbon dioxide:
- 5,000 ppm averaged over an 8-hour shift.
- 30,000 ppm for a short exposure.
- 40,000 ppm is immediately dangerous to life or health.
Fit fixed CO₂ sensors at working height in every growing room, an alarm that sounds and shuts the CO₂ supply above 5,000 ppm, and ventilation that runs before anyone enters a closed room.
6 · Run the roots and keep the record
The crop grows hydroponically, so the nutrient solution is run exactly as in soilless growing: EC, pH and dissolved oxygen read daily, the returning solution filtered or disinfected, and every pump on residual-current protection.
Log everything against the plan: kWh per day, kilograms harvested, light, temperature, humidity and CO₂. The farm survives only if the measured kilowatt-hours per kilogram match the business plan.