Science to the Grower: The cold, hard facts about plant factories with artificial light

by Richard Evans

Some scientists are infatuated with technological toys, and they’ll leap at an opportunity to put those toys to work in their research. For example, scientists have invested a lot of time and taxpayers’ money to apply biotechnological techniques to modify corn in pursuit of efficient ethanol production, yet corn ethanol production remains a heavily subsidized industry. I think the energetics involved have always made biofuel production a questionable goal, but the quest provides plenty of research opportunities.

These days there is much buzz about “plant factories,” which, in the extreme case, are buildings for growing plants under artificial light. I suppose a source of inspiration for these plant factories with artificial light (PFALs) is production of the one crop for which they are widely used: cannabis. Of course the cannabis grower’s incentive is security; windowless buildings are more secure than greenhouses. Another source of inspiration is the knowledge that the world’s human population is becoming increasingly urban, coupled with mounting concern about global climate change. The world’s urban population now represents more than half of the total. Cities account for over 60% of human water use and 80% of human-produced carbon emissions (United Nations, 2012). It’s not surprising, in the face of these numbers, that many people express a desire for local food self-sufficiency and crop production systems that allow for extraordinary environmental control and production efficiency. 

The desire for local production has stimulated a search for innovative ways to produce crops in urban settings. Numerous schemes for rooftop gardens and vertical greenhouses appear now in scientific journals, including one design for a vertical greenhouse that would have 37 floors and a growing area of 57 acres (Banerjee and Adenaeuer 2014). However, skyscraper plant factories won't meet consumer demand. It has been estimated that it would take 30 times the area of New York City to feed its residents (van Iersel 2013). And cities occupy less than 4% of the world’s land, so there is plenty of surrounding land available for crop production. With affordable land that can support less expensive production methods in fields or standard greenhouses beyond urban borders, does it make sense to build PFALs?

Surprisingly, most research publications about PFALs rely on rosy assumptions and invalidated models rather than hard data. Even a recently-published academic book on PFALs (Kozai and others 2016) is sparsely populated with hard numbers about production costs. We need an accounting of all energy, water and raw materials that go into and out of the production system, as well as transportation costs associated with shipping of raw materials and finished crops.

Sometimes local production seems sensible. Say you live at the South Pole and want a fresh salad to go with your penguin tacos. You’re in luck! The South Pole Food Growth Chamber produces lettuce, herbs, tomato, pepper, cucumber, cantaloupe, edible flowers, aromatic plants and other greens at the Asmundsen-Scott South Pole Station. This PFAL facility has been studied by Patterson and others (2012). The chamber features nutrient film and deep trough hydroponic systems, metal halide lamps and CO₂ injection. Sensor measurements of relative humidity, light, CO₂, temperature, and pH and electrical conductivity (EC) of the nutrient solution are used to monitor and control the chamber environment. It is a semi-closed system in which air is recirculated (except for what leaks out) and the nutrient system is tweaked as needed to maintain constant pH and EC, except for complete replacement every few months. Plant growth was near the theoretical maximum for the amount of light provided, but the energy required to grow it was equivalent to about 7.5 kWh per head of lettuce, which is substantial. The authors of the study don’t go into this, but according to my rough calculations (available if you buy me a beer from a local craft brewer), the carbon footprint of producing the food locally is about three times the carbon footprint of growing it outdoors in Salinas and shipping it to the Costco at the South Pole.

The South Pole may seem an extreme example, and it certainly hasn’t been urbanized yet, but Albright (2013) points out that the cost of artificial lighting — even supplemental lighting —almost never balances the cost (in dollars or carbon) of growing the crop in a more favorable climate and then shipping it long distances to consumers.

This topic reminds me of the many times academic visitors have asked me to arrange tours of the high-tech greenhouses they assume we use in California to grow our high-quality crops. They usually are disappointed when I show them the irrigated fields and standard greenhouses that most growers use. The truth is that a grower’s annual profit on flowers or produce probably wouldn’t cover the architect’s fees for a PFAL design, and there is no good evidence that productivity would increase enough to pay for a plant factory’s capital and operating costs.

Richard Evans is UC Cooperative Extension Environmental Horticulturist, Department of Plant Sciences, UC Davis.

References

Albright L. 2013. Peri-urban horizontal greenhouses. Resource Magazine 20(2): 6.

Banerjee C, Adenaeuer L. 2014. Up, up and away! The economics of vertical farming. Journal of Agricultural Studies 2: 40-60.

Kozai T, Niu G, Takagaki M. Plant Factory: An Indoor Vertical Farming System for Efficient Quality Food Production. Academic Press Elsevier, Boston.

Patterson RL, Giacomelli GA, Kacira M, Sadler PD, Wheeler RM. 2012. Description, operation and production of the South Pole Food Growth Chamber. Acta Horticulturae 952: 589-596.

United Nations. 2013. World population prospects: the 2012 revision. http://esa.un.org/unpd/wpp/index.htm.

van Iersel M. 2013. The potential — and limitations — of urban farming. Resource Magazine 20(2): 27.