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Background

In Professor Bugbee's presentation, he describes some drawbacks of vertical farming (VF) versus traditional horizontal agriculture. Most of Bugbee's arguments against VF hinge on grow lamp efficiency. In July of 2015, a step towards a 400lm/W monolithic white laser was announced. Laser lights are several times brighter than LEDs and use much less energy.

He illustrates a compelling argument as follows:

Solar and Agriculture

Arrows represent relative inputs and outputs. Sunlight, being large and free, dwarfs fossil fuels (black), water (blue), and crop yield (green).

Problem

Bugbee's calculations with respect to his assumptions about photon energy conversion into biomass appear to be correct. Therefore his conclusion also seems to be correct: vertical farms are far less efficient than traditional agriculture (for growing tomatoes). This doesn't necessarily mean they aren't viable.

Questions

Given the assumptions above:

  1. What is the lumens per watt efficiency necessary to allow vertical farming to compete with horizontal agriculture (in terms of energy usage and production)?
  2. How did you calculate that efficiency?

Note: While PAR is the preferred way to measure photon use by plants, it is onerous to convert PAR into the theoretical luminous efficiency of grow lamps.

Basically, barring compact fusion, can can we wholly replace (in time) traditional plant-based agriculture farming by growing indoors using technology? (For example, concentrated solar capture at 85%, batteries with 85% storage efficiency, 95% transmission efficiency, and highly-efficient grow lamps could be in the ballpark. I don't know, hence the question.)

Addendum

See also:

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  • Aren't you comparing apples and oranges here, i.e. artificial light versus natural light? As anyone taking photographs can tell you the amount (intensity) of natural light dwarfs anything we can produce artificially. What you should be comparing is horizontally versus vertically grown produce under artificial lightning. – Jan Doggen Oct 14 '15 at 7:04
  • Coming close to the Sun's capacity (horizontal parity) is a poor choice of comparison. I'm trying to understand whether we can manufacture lighting requirements at sufficient efficiencies to make vertical farms viable. There's much discussion around current technology, but little of future (forthcoming) technology. – Dave Jarvis Oct 14 '15 at 9:15
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    @JanDoggen we can and sometimes do produce artificial light at intensities equal to, or greater than, natural light. It's just very expensive, both in lamp cost and electricity consumption. – EnergyNumbers♦ Oct 14 '15 at 10:58
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    I suggest you draw a box around a vertical farm with all inputs and outputs. Same for a sunlit farm. Then you tell us wich of those inputs and outputs you want parity for. The way it stands the question is not ansqwerable. – mart Oct 14 '15 at 10:59
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    I think you're asking (slightly) the wrong question. I can't see that farming under artificial light is ever going to achieve equivalence to conventional agriculture in terms of energy use divided by yield, because sunlight is free. You'd need to introduce the other considerations that make vertical farming attractive in the first place - e.g. I'm guessing space limitations. – Flyto Oct 14 '15 at 11:26
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Vertical Farming does already compete with horizontal farming. I read an article about a lettuce farm in Japan that was producing 10,000 head of lettuce a day. The research showed that the vertical farm not only used less water but it also produced more lettuce with less labour. Because LEDs have been designed specifically for this purpose by GE, the farm also uses a fraction of the energy a traditional horizontal farm uses in terms of machinery. I don't have the original article as I read it in a magazine while waiting in the doctors office, but here is a link to a similar article on the same farm and here is another link. Considering that this farm is now the largest producer of lettuce for the European market, I would assume it is not only sustainable but maybe for this particular product more efficient.

So yes artificial lighting can compete with horizontal agriculture. This is probably because although the sun is much brighter than any artificial light we can produce in a sustainable manner, it is not the brightness that matters but the frequency range of light produced.

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