Agri-tech & precision
Controlled-environment agriculture
Why the cost of photons governs the economics of indoor growing: the daily light integral, fixture efficacy in micromoles per joule, and the energy gap between leafy greens and staple calories.
Controlled-environment agriculture means managing every growth factor at once: temperature, humidity, CO2 concentration, nutrient solution and light. The first four cost money but do not set a ceiling. Light sets the ceiling, and this is the page in the cluster where its price is worked out; the urban farming and food security pages lean on this arithmetic rather than repeating it.
The photon as a purchased input
A plant needs a daily light integral. For lettuce and leafy greens the working target is 12–17 mol of photosynthetically active photons per square metre per day; greenhouse fruiting crops want two to three times that. In a field the dose is free. Under a lamp it is bought, and the exchange rate is the fixture’s photosynthetic photon efficacy — micromoles of photons per joule of electricity. The best production LED modules reach roughly 3–3.5 µmol/J; since a mole of red or blue photons carries on the order of 200 kilojoules, that corresponds to converting about two-thirds of the electricity into light. The headroom is physically small: even a perfect fixture would improve the picture by less than a factor of two.
The same balance has a second consequence, which is heat. Essentially all of the electrical power ends up as heat inside the building, so a closed farm’s cooling load is roughly equal to its lighting load, and that is paid for too.
Why greens work and calories do not
Put the numbers together. Lettuce at about 15 mol/m²/day for roughly a month absorbs on the order of 500 mol per square metre and yields a few kilograms of saleable mass; at 3 µmol/J that is around 45 kWh per square metre per cycle. A product retailing at a few euros a kilogram can carry that.
Now wheat. At a typical light-use efficiency of about a gram of dry biomass per mole of absorbed photons and a harvest index near 0.45, a kilogram of grain requires on the order of two thousand moles of photons — roughly 200 kWh of electricity at the fixtures. The grain itself contains about 15 megajoules, a little over four kilowatt-hours. The gap is about a factor of fifty, and it does not close with cheaper electricity, better automation or more stacked layers, because it follows from the quantum efficiency of photosynthesis rather than from the quality of the engineering.
That is where the industry’s crop list comes from — it looks like fashion and is arithmetic: leafy greens, herbs, microgreens, transplants, berries, medicinal crops. High price per kilogram, short cycle, and an above-ground mass that is either fully edible or highly valued. Anything sold as a source of calories does not add up under artificial light.
A greenhouse sits in between: its light is mostly solar, with lamps supplementing through the dark season. That is exactly why greenhouse tomato and cucumber are economically sound where the same tomato grown under lamps alone is not.