Urban and vertical farming

The energy arithmetic of indoor growing is worked out on the controlled-environment page and taken as given here. Urban farming adds one variable to it — siting — and its economics are decided by that variable.

Transport distance is a weak argument

The stock argument is a shorter journey from field to shelf. It is weaker than it sounds. Transport is a small share of the food system’s footprint — in the well-known analysis by Weber and Matthews (2008), transport accounted for about 11% of US food-system emissions and delivery from producer to retail about 4%, with most of the total arising in production. The reason is arithmetic: a lorry carries twenty tonnes, so the transport energy per kilogram of lettuce is a fraction of a kilowatt-hour. Lighting that same kilogram costs orders of magnitude more. Saved freight does not pay for photons and cannot.

What proximity actually buys

The real gain is in time rather than distance. Leafy greens lose saleable quality in days; across a long chain, losses between harvest and shelf run to tens of percent, and every kilogram lost carries all the cost already spent on it. A farm delivering the same day cuts that shrink, extends remaining shelf life at the point of sale and earns a quality premium. Add independence from season and weather: the winter price of imported salad at northern latitudes is a multiple of the summer price, and an urban farm competes against the winter one.

Rent works against all of this. Yield per square metre of floor is high, but so is the cost of that square metre in a city, and the infrastructure required — grid connection, ventilation, drainage — is that of an industrial building rather than a retail one. Hence the pattern actually observed: not towers downtown but warehouses on the periphery, half an hour from the distribution centre, where land is cheap and power is available and almost all of the time advantage survives.

Heat and CO2 integration

The strongest case for an urban site is the use of somebody else’s flows. Enriching a greenhouse atmosphere to 800–1000 ppm of carbon dioxide raises the productivity of C3 crops, and an industrial source can be cheaper than delivered liquid CO2: the Dutch OCAP system pipes captured CO2 from Rotterdam industry to the greenhouse cluster. The requirement is strict — the gas must be cleaned, because ethylene and nitrogen oxides damage crops at concentrations that are negligible by industrial standards.

Waste heat is more awkward, and honesty is required. Greenhouse production in a cold climate genuinely consumes low-grade heat and pairs well with cogeneration or a data centre. A stacked farm under lamps is the opposite: nearly all of its electrical load becomes heat inside the building, and its problem is rejecting heat, not acquiring it. So the symbiosis is real for greenhouses and largely decorative for rack farms.