Why Planting Farms in Skyscrapers Won't Solve Our Food Problems
Continued from previous page
Nevertheless, modern agriculture has managed to make food production an energy-losing proposition. Its emphasis on increasing yield per unit of land and per unit of human labor has meant a sharp increase in the input of fossil energy-with farms often using more energy to produce the food than is contained in the food. Some of the most notorious features of factory farming are dark, dank hog and poultry confinement operations; now, Despommier's plan would create plant-confinement operations as well.
Most of the attention that vertical farming has received in the media has been embedded in the context of rooftop gardens, greenhouses, and "green" high-rise facades ( pdf). But those methods for growing modest amounts of relatively expensive food (usually vegetables) differ from Despommier's plan to "farm" the interiors of buildings in one important respect: they are at least capable of capturing solar energy efficiently.
For obvious reasons, no one has ever proposed stacking solar photovoltaic panels one above the other. For the same reasons, crop fields cannot be layered one above the other without providing a substitute for the sunlight that has been cut off. Even with all-glass walls, the amount of light reaching plants on all but the top story of a high-rise would fall far, far short of what is needed. (On a sunny day, a room with plenty of windows may look well-lit to our eyes' wide-open pupils, but that light intensity is a tiny fraction of what is needed for crop production.) A significant portion of the light hitting the building would be turned back by the glass, and direct sunlight would penetrate into the interior of a vertical farm only when the sun is low in the sky (especially if, as Despommier recommends, two layers of plants are stuffed into each story.) Even then, it would reach the crop plants at a low angle, so that each square inch of leaf would receive much less light than if the light were hitting the leaf from above.
As a result, the lion's share of a vertical farm's lighting would have to be supplied artificially, consuming resource-intensive electricity rather than free sunlight. This led us to wonder, "What would be the consequences of a vertical-farming effort large enough to allow us to remove from the landscape, say, the United States' 53 million acres of wheat?" That's not an unreasonable question. In fact, it follows from Despommier's own reasons for promoting the practice. He argues, correctly, that soil is currently being abused on a massive scale; therefore, to address the problem, vertical farming would need to displace agriculture from a large proportion of the currently cropped landscape.
Our calculations, based on the efficiency of converting sunlight to plant matter, show that just to meet a year's U.S. wheat production with vertical farming would, for lighting alone, require eight times as much electricity as all U.S. utilities generate in an entire year [see calculations here]. And even if it were energetically possible, growing the national wheat crop under lights could substitute for only about 15 percent of US cropland. Were it to succeed, that energy buildup of unprecedented scale would still leave 85 percent of cropland in place.
Despommier suggests using renewable sources to supply the power needed for vertical farming but fails to consider the scale-up that would be required. Wind, solar, biomass, geothermal, and other renewable electricity sources combined account for about 2% of U.S. generation. So to grow the national wheat crop vertically using renewable sources would mean scaling up the nation's renewable sector by 400-fold just to run the lights! (His proposals for doubling plant density, using round-the-clock light, or pushing year-round production, even if they could be made to work, would increase production per unit of area but would not decrease the energy needed for lighting per unit of food produced.)