# 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:

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.)

• 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.
– user2451
Oct 14, 2015 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. Oct 14, 2015 at 9:15
• @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. Oct 14, 2015 at 10:58
• 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, 2015 at 10:59
• 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. Oct 14, 2015 at 11:26