What if we are talking theoretical limits instead of what is practical today?
Couldn't capturing the full spectrum of sunlight (e.g. through multi-junction cells) and then emitting an optimized spectrum for plants to absorb boost efficiency?
Photosynthesis efficiency is also affected by temperature. So at least in hot climates having the plants indoors while capturing the sunlight outside could yield increased efficiency when compared to fields exposed to direct sunlight.
Myself I am a chemical engineer working in agricultural research. We are working on aquaponics and also test vertical growbeds. Vertical growbeds underperform significantly unless you put expensive light on them.
Generating the electricity for light through PV uses about ten times the area compared to using direct sunlight. No way to counter that by using the full spectrum.
Not saying it's a good idea, just that it seems there are some offsetting concerns.
* 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.
Edit: Clarified on indoor farm area.
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 %.
Theoretically, an infinite-junction solar cell can achieve up to 68.7% efficiency under non-concentrated sunlight. The best demonstrated so far is about half of that: https://www.sciencedaily.com/releases/2016/05/160517121811.h...
Efficiency under concentrated illumination is higher but concentrating designs require direct illumination and have their performance fall off a cliff under even partially cloudy conditions.
Spectral tuning of the light sources can make illumination more photosynthetically efficient, and so can evening out the illumination intensity. If you could get lights and multi-junction solar cells that operated at the theoretical limit it might be more efficient than natural sunlight growing, but that's a tall order to achieve in practice. To achieve a net environmental benefit not only does it have to be more efficient than single-story natural illuminated structures, it has to be enough more efficient to pay back the environmental costs of manufacturing the extra equipment required up-front in the vertical case.
Or is cooling horizontal greenhouses easy enough, even in deserts?
Artificial illumination farming of course does make sense for crewed deep space missions because there's not any other option. That's actually how I first started thinking about this problem -- "what if you wanted to feed a crewed nuclear powered outpost in orbit around Neptune?" It turns out that it takes a lot of electricity to feed people that way because our food crops suck at turning light-energy into human-edible energy. Having a rough idea of the enormous electricity expenditures, I started doing double-takes when vertical farm enthusiasm started spreading a few years ago.
[1] http://www.abc.net.au/news/2016-10-01/sundrop-farms-opens-so...
No, that's not necessarily true.
Sunlight is actually fairly wasteful when landing on a leaf. It goes across a wide band of frequencies, most of which aren't used by the plant. And lots of it scatters on the earth near the plant, also unused. Finally, the solar irradiation is almost always larger than the total energy a plant could absorb, even if it were the perfect frequencies.
Solar panels turn a much higher percentage of that falling energy into electricity. By re-emitting only the frequencies best absorbed by the plants, and having a much denser planting than would be possible outdoors, and, most importantly, only providing each plant with the light it needs, you can take that huge volume of "wasted" solar energy and use it to grow many more plants.
So if you take all the sunlight that is falling on the roof of the farm, even after you account for all the conversion losses you can still come out ahead because you're using the remaining energy in a much more efficient manner for plant growth.
Plants only need in the order of 10-100 W/m^2 in order to grow. A solar panel, however, easily generates 200 W/m^2 and more. Therefore, a 1m^2 solar panel can support the needs of > 1m^2 of plants, even before taking into account the spectrum efficiencies.
Too much attention is given to classifying energy sources as "renewable" or "green", rather than the less restrictive criteria of sustainability and cost.
As an example, fission power with a closed fuel cycle, using either uranium or thorium and possibly incorporating seawater extraction could accomodate all of humanity's energy demands for hundreds of millenia at current consumption rates. Nuclear power is already fairly cheap, but I do not think it overly optimistic to expect that economies of scale and automation could make it much cheaper. Yet the United States still refuses to even reprocess spent fuel, and one of the most promising projects -- the Integral Fast Reactor -- was cancelled in 1992 by the Clinton presidency when it was almost complete.
AeroFarms supplies greens to the dining rooms at the Times, Goldman Sachs, and several other corporate accounts in New York. At the moment, the greens can be purchased retail only at two ShopRite supermarkets, one on Springfield Avenue in Newark and the other on Broad Street in Bloomfield. The AeroFarms clamshell package (clear plastic, No. 1 recyclable) appears to be the same size as its competition’s but it holds slightly less—4.5 ounces instead of five. It is priced at the highest end, at $3.99. The company plans to have its greens on the shelves soon at Whole Foods stores and Kings, also in the local area. Greens that come from California ride in trucks for days. The driving time from AeroFarms’ farm to the Newark ShopRite is about eleven minutes.
All of this makes sense. People will pay a premium for fresh produce and herbs, particularly if growing them close to consumption means that you can optimize for deliciousness rather than for transport-durability and long shelf life. I expect vertical farming to work well in this niche.
But the rest of it -- feeding all of New York City using just the space available in NYC, "Feeding the World in the 21st Century", "what might come of it when we’re nine billion humans on a baking, thirsting globe?" -- that's nonsense. For staple crops that provide most of the calories in a vegetarian diet, there is no net economic or environmental benefit to vertical farming. And there are good reasons to suspect that there will never be such benefits even as the technology evolves.
It's the vertical stacking of greenhouses that doesn't make sense outside of a high-priced freshness niche, and that's because of the requirement for artificial lighting. Artificial lighting is a very expensive way to drive photosynthesis compared to natural sunlight, and remains more expensive even with optimistic assumptions about future LED efficiency and falling costs for clean electricity. The energy requirements to grow e.g. soybeans in a vertical farm inside the NYC city limits wipe out all the environmental benefits of "locavorism," compared to just transporting them from the Midwest like usual. And New York has one of the cleaner electricity mixes in the US. It's worse if you use a fossil-heavy mix to power the lights.
I could see large scale use of greenhouses in the future, particularly things like this: http://www.sundropfarms.com/
Greenhouses and perhaps soil-free growing may be great ways to produce food in places that face extreme weather or lack soil or natural precipitation. But one level construction only, illuminated by natural sunlight. You can ship a ton of dried beans or wheat across oceans for significantly less energy than it takes to grow the same calories locally via artificial illumination.