>The biggest drawback of vertical farming is the high cost of the electricity required to run the large number of LEDS...One way of saving electricity is to use LED s that generate only the colours that plants require, instead of the full spectrum of plain white light.
This is a big issue and I've been unable to get good data on it. Opponents of vertical farms say that the high electrical consumption doesn't justify the reduction in energy related to travel. Proponents say that it's possible to optimize LEDs and have continuous fake "seasons" which justify the cost of transportation & refrigeration for certain species of food.
Does anyone have any good references that go into the details and provides numbers? Most of the pro-vertical farming information I've found is from companies who are investing in it so they consider the energy numbers to be a trade secret. But there must be some research group or open source team willing to publish all their data so that people can draw their own independent conclusions.
There isn’t much to be secret about really. When you ask any of the big LED suppliers, like Philips or Hortilux, for a quotation, they ask what you are planning to grow and they give you a lighting plan. The energy usage is relatively predictable, if you know how the building you are in is insulated and you know your ventilation requirements. It isn’t trivial, but it is possible to do the numbers.
We will be publishing some data from our pilot installation next year (approx 250 m2 building, 100 m2 active growing area, 30 m3 water). It isn’t a vertical installation, but a Deep Water Culture (DWC) aquaponics installation. We are in the final stages of building it right now. Spent all day working on the decking around the fish tanks.
Not sure what numbers you’re looking for. Lettuce with state of the art horticultural leds probably can be grown optimally with as little as 10 watts per square foot. Flowering plants need much more and more full spectrum.
In non-medieval units, that's 110 W/m², 9 times less than direct sunlight. Couldn't you get those low-light conditions more cheaply by stacking lettuce trays in a sunlit greenhouse, with lightpipes to shade the upper levels and get the requisite amount of weak illumination to the lower levels?
Weather would then affect the production, making schedules unpredictable.
Plus, I am no expert but I guess you can't make plants actually use much more than 100W/m2 for photosynthesis, the rest of sunlight consists of unsuitable wavelengths. It ends up as waste heat that you'd have to cool away. Perhaps if you could efficiently convert green and infrared parts of sunlight to red/blue, that could help.
You mean because sometimes it's cloudy? Couldn't you solve that problem by turning on LEDs just when it's cloudy?
Plants are pretty inefficient, it's true, but I don't think they're very efficient even with optimal wavelengths. But, yes, solar cells and LEDs can indeed convert green and infrared sunlight to red and blue, with about 3% efficiency under normal circumstances.
If your plan is "light pipes, but fall back to LEDs" that means you have to build two complete systems. You get double the installation costs, incur the combined complexity of both systems (plus overhead), and box yourself into the limitations of both systems.
That might be true, but I'm not sure it is. It seems to me that you escape the limitations of both systems. Also, I think lightpipe daylighting generally costs orders of magnitude less than sunlight-comparable LED illumination, although I don't have firm numbers on lightpipe costs; so I don't think the installation costs double, just increase slightly. If we're talking about a greenhouse, where a leaky roof is a minor annoyance comparable to a leaky faucet, rather than a life-destroying disaster, lightpipe costs should go down quite a bit compared to their use in office and residential daylighting.
The big question for me is, if you're going to build a hothouse, why would you build it with an opaque roof? That just seems perverse, like writing an HTTP server in bash or assembly language or something: http://canonical.org/~kragen/sw/dev3/server.s
Efficient light pipes require solar concentration and thus tracking. That will drive costs up. Current residential/office light pipes are using only small fraction of sunlight, so they aren't relevant to this problem at all.
Also you haven't said anything about cooling, which the translucent roof inevitably requires in summer.
These are good points. Lightpipes can use non-imaging optics in many cases and thus avoid the need for tracking, but maybe not in this case, and I hadn't thought about that.
Cooling, though, is an issue whether your light comes from LEDs or the sun—either way it comes with a lot of extra heat. With the LEDs I'm familiar with, this is a worse problem than with sunlight, but in another thread https://news.ycombinator.com/item?id=20908476 I see a report that there are now illumination LEDs that reach an astounding 40–50% efficiency level. If that's true, it would make the LED cooling problem a little smaller than the sunlight cooling problem, instead of much bigger; and of course spectral matching helps.
> Couldn't you solve that problem by turning on LEDs just when it's cloudy?
Alternatively, every time when it’s not sunny... meaning all night every night. Of course, that would require experimenting if it’s actually more efficient, but AFAIK that’s what a MIT experiment found regarding growing basil (though they were optimising for flavour, not energy efficiency).
I bet you don't know off the top of your head how many watts per square foot sunlight is, though --- and that's the most relevant point of comparison. (Or horsepower, or Btus per hour, or whatever crazy unit.)
Thank you for the correction! My calculation error (I had written "11 W/m², 90 times less than direct sunlight"), though unintentional, is an excellent demonstration of why using medieval units is a bad idea.
Yeah, and I've certainly made that mistake more than once. (I try to avoid using centimeters or centi-anything for that reason, as well as deci-, deka-, and hecto-.) In this case, though, the calculation I biffed was a unit conversion from medieval units to modern units. It's harder to get calculations wrong when you don't have to do them at all.
This is a big issue and I've been unable to get good data on it. Opponents of vertical farms say that the high electrical consumption doesn't justify the reduction in energy related to travel. Proponents say that it's possible to optimize LEDs and have continuous fake "seasons" which justify the cost of transportation & refrigeration for certain species of food.
Does anyone have any good references that go into the details and provides numbers? Most of the pro-vertical farming information I've found is from companies who are investing in it so they consider the energy numbers to be a trade secret. But there must be some research group or open source team willing to publish all their data so that people can draw their own independent conclusions.