I define a farm as a business enterprise that intends to sell its yields for a profit to others. So I have no interest in urban farming because my focus is only on providing yield to myself. However, in my view utilizing previously unproductive outdoor space (ie building rooftop) is still allowed under the constraints of an urban environment. I guess this somewhat depends on how committed you are to feeling that your individual solution should be scalable to everyone in the world, which in my opinion has merits but at a certain point is moot because we know not everyone in the world is going to do it. To me, turning a previously unproductive space into a productive one is a net win provided it is designed intelligently to factor in all energy inputs.
However, if we factor in your constraints and say the boundary is the indoor area of your urban apartment, then here's what I've come up with;
-low square footage
-production space and living space are the same
-low natural lighting
-inability to modify infrastructure
First off, you are limited by the most abundant natural energy source (the sun), which is what is the energy source for virtually all biomass production on Earth (whether directly with plants or indirectly with a food chain that at lower levels eats plants). In an ideal situation you would have some large south facing windows, which would also help with passive solar heating. But that's not a scalable solution since not everyone can have their apartment facing south in conventional building design. You are also limited by square footage because any production space is also being shared as living space.
-low square footage - make use of vertical space with shelf systems, walls, etc
-production space and living space are the same - multi-use and space-efficient furniture are essential to allow for decreased living space and increased production space (see boat, RV, tiny house designs), make use of barriers or intelligent design to mitigate effects of odors, humidity, etc on comfort/safety of living space
-low natural lighting - utilize viable window space for efficient production, utilize artificial lighting when natural lighting isn't possible, position artificial lighting such that it is multi-fuctional (ie a grow light can be used to light an entire room, or as a reading light, etc), maximize production that doesn't require lighting (insects, mushrooms, etc)
That means if you want to grow plants you are going to have to rely on artificial lighting. Whether using electricity to grow food makes sense from an energy perspective depends a lot on how the electricity is sourced and how efficiently you can turn the electricity into food. I would say microgreens qualify as an efficient use of electricity because of their exceptionally fast growth rates. Conventional vegetables grown hydroponically may not.
So now that we have some principles/big ideas laid out, we can get into specific solutions. The master list we've come up with so far is the following:
-worm composting bin
I would propose another possible candidate for inclusion is duckweed (Wolffia microscopica seems to be most suited for human consumption). Duckweed grows extremely fast in water with some nutrients and is 30-35% protein by dry weight. I've had some difficult finding accurate numbers for what kind of production one can expect per sqft, but I have seen one academic source say you can produce 20 grams dry weight (7 grams of protein) per square meter per day (1.85 gram protein per sqft).** I'm unsure if that figure is utilizing only a square meter of grow space or if it is from a multi-tiered system with a foot print of only one meter. The only design pictures I could find of something similar are here: https://www.planetduckweed.com/post/hyd ... ng-modules
More online research is needed to find out whether that 7 grams of protein per square meter per day is an accurate number. I've seen another source say 800 grams wet weight density per sq meter is possible, which would come out to 1.56 gram protein per sqft (potentially half that if we assume a one day doubling time so you can only harvest half of the biomass daily). Even with those conservative assumptions, if I were to convert my existing shelf system which has 4.16 sqft grow space per level and install clear trays every 4 inches, that would give me 75 sqft of grow space in a foot print of 5 sqft. If we assume .78 grams of protein per sqft per day harvested, that would yield 58.5 grams of protein per a day. That would be more than enough to supplement a full meal's worth of protein for me. Long term storage is possible if you dry out the duckweed into a powder. Some design work would be needed to be able to have the trays slide out for harvesting and to figure out how to rig up effective light systems (I'm thinking rope LEDs like that prototype linked).
*= BSF do show potential as human feed and there are some research reviews that dive deeper into the topic. However, a bin is going to require outdoor space (rooftop or balcony) to complete the BSF lifecycle, so maybe we keep it on the list with an asterisk. Further reading: https://onlinelibrary.wiley.com/doi/ful ... 4337.12609
** = https://www.rutgers.edu/news/could-duckweed-feed-world