Except that you can transport the electricity much cheaper and with less pollution than you can the produce, and optimize the produce for things other than overcoming transport effects/concerns. So the PV installations can be far away on cheap land while the produce is grown right in the city where it's consumed.
Not saying it's a good idea, just that it seems there are some offsetting concerns.
Except what you say is not exactly true. Transportation losses for electricity are roundabout 25% - 30%. For a good comparison it makes sense to compare area efficiency. Plants in green houses can use 95% - 97% of direct sunlight depending on glazing type. Artificial lighting has the following calculation:
* 25% PV efficiency best case
* 75% transport efficiency
* 50% LED light efficiency best case
This amounts to a total area efficiency of 9.5%. So artificial light has a ten times worse area efficiency compared to using direct sunlight.
Transport energy consumption for the product on the other hand is often over estimated. Transporting amounts to something between 1% - 5% of the total energy cost of produce, depending on type and distance of course.
The vertical farm fallacy lies in the assumption that area is the scarce resource. This is simply not true if you move outside the city borders. Area within the city is very valuable and incredibly expensive. Even if you only pay a comparably low rent, lets say 2.5$/sqm (25ct/sqft), that amounts to ~30.000$ rent per year (for a 1000sqm indoor farm). You can buy farmland for this amount of money and forward from that the cost for area is not there any more.
In my (our faculty) opinion the sweet spot for urban farming ist in the peri urban part around the city. Area is still cheap, you can use direct sunlight with little or no artificial light and transportation cost and energy can be slashed by 90%.
Depending on the industry in that area there may even be some synergy potentials with residual heat and other materials (eg. compostable stuff from near by production).
Dutch Prof. P. Smeets proposes argoparks as a sustainable urban agriculture concept. We are in favour of that.
Transport efficiency for electricity is way higher than 75%. Total transmission losses are around 7% in the US. Plant efficiency is worse than 95%, because chlorophyll only responds to certain wavelengths well. LED lighting for plant growth doesn't use white LEDs.
Ok, I was not aware that there has been a lot of progress on reducing transport losses in the electricity grid over the last decades. Thanks for the heads up! But even taking this into account area efficiency of direct sunlight compared to artificial light still differs significantly (25% * 95% * 50% = 12%).
Plants actually _do_ use the green light of the spectrum, just not as much, thus leaves appear in green color. How high would you estimate the efficiency gain by using a better spectrum? My guess would be that this gain would be below 50%. So area efficiency of artificial light would still be below 20%. And the calculation has been made "best case". 25% photovoltaic efficiency are lab figures for the very best cells possible. Installed base is lower still, monocrystallin cells 20 - 22 %, polycrystallin 15 - 20 %.
Not saying it's a good idea, just that it seems there are some offsetting concerns.