Light from the sun is broad spectrum. Photosynthesis uses only part of the spectrum. In a charmed world, you could power LEDs using energy from the unused spectrum, and be more efficient than sunlight.

Edit:

Turns out, there is a Y-Combinator company making agricultural LEDs:

https://blog.ycombinator.com/transcend-yc-s15-makes-extra-ef...

Growing in the middle of the city might just make up for the power used for the LEDs. Less fuel to transport to the customers. Hopefully less pesticides too.

I am usually the first one to come on here and say if you dig deep you will find that the numbers do not work. Replacing the sun is inefficient.

But this time it may be different because of the extremely low cycle times for the micro greens. If they got a very good deal on leasing the tunnels the economics may very well work.

Two problems, one is that they probably can rather quickly outgrow their market and second what if the micro greens fad ends?

Seems like the target issue here is water scarcity. Indoor farms require considerably less water.