The sustainability of commercial indoor urban farms

Supplying urban populations with locally grown food is widely viewed as a more resource-efficient alternative to the conventional supply chain using food grown in peri-urban or remote rural locations. Indoor, soilless cultivation in urban areas is portrayed as a particularly sustainable solution, by reducing food miles, minimizing land use and water consumption, and improving yields. However, to ensure optimal growing conditions for the crops, controlled-environment farms all rely on the artificial control of light, temperature, humidity and water cycles, and can therefore be highly energy intensive, depending on local climatic conditions and the specific characteristics of the host building. The carbon emissions of urban farms should therefore be carefully weighed against potentially reduced emissions, such as those from transporting food from rural and peri-urban farms. The elevated economic costs of urban farms, both in terms of infrastructure and operational costs, also need to be carefully assessed before undertaking such a venture.

Environmental sustainability

Located within the city and therefore closer to the consumer, high yield urban agriculture is often claimed to have a lower carbon footprint than rural food production, by cutting transportation distances (‘food miles’). However, depending on local climate conditions and urban farm typology, crop production in controlled environments can also be highly energy-intensive, which can considerably exacerbate its environmental impacts. The net carbon footprint depends on emissions caused by energy use for farm operation versus avoided emissions related to the existing supply chain, including the operational energy of the farms supplying the produce, and the energy used in transporting it. This can be illustrated by two examples from very different climatic zones in Europe. When the Global Warming Potential (GWP) related to the water, transportation and operational energy of three hi-tech urban farming scenarios in Portugal – a polycarbonate rooftop greenhouse, a vertical farm with windows and skylights on the top floor of a building, and a completely opaque vertical farm with no penetration of natural light on the ground floor of a building – were compared with the GWP of the current supply chain for tomatoes, and with a hypothetical low-tech unconditioned rooftop urban farm, the top floor vertical farm and the rooftop greenhouse had the best overall environmental performance, respectively cutting greenhouse gas emissions by half and by a third in comparison with the existing supply chain for tomatoes (Benis et al. 2017). These findings corroborate the results of a life cycle assessment of a rooftop greenhouse in Barcelona (Sanyé-Mengual et al. 2013; Sanyé-Mengual et al. 2015a). In contrast, Theurl et al. 2013 found that the production of tomatoes in heated greenhouses in Austria generated double the greenhouse gas emissions compared with the supply chain of tomatoes imported from Spain and Italy. Therefore, it is essential to keep in mind that while urban agriculture is claimed to be sustainable for cutting transportation distances, such energy intensive facilities may not be appropriate to every location, as the former does not consistently offset the latter.

However, the environmental performance of Building-Integrated Agriculture can potentially be enhanced by coupling flows of the agricultural practices – heat, water, CO₂ – with flows of the host building, and by optimizing the efficiency of the system through the implementation of passive conditioning methods, such as thermal insulation, natural ventilation, evaporative cooling, and the use of highly energy-efficient technologies, such as LED lighting.

Economic sustainability

The economic feasibility of commercial farms in urban contexts has to be evaluated taking into account the higher capital expenditures – in comparison with conventional rural farms – that are intrinsically related to their urban location. In a context of rapid urbanization, urban space is scarce and highly coveted, and the primary need that is generally sought to be fulfilled by municipalities is housing rather than food production, which is instead pushed further and further away from urban centres. While rooftop-integrated farming systems have to compete with other rooftop-integrated technologies such as solar photovoltaics or solar thermal, indoor systems compete with other urban uses which are usually more economically attractive than agriculture, like residential or commercial functions. Such a high competition for urban plots and buildings makes real estate ever more expensive (Benis & Ferrão 2018).

Around the world, the price of land is generally high in urban areas. Besides the elevated rents, high- tech commercial urban farming is a capital-intensive industry, as it involves the adaptation of the host building for cultivation, in accordance with local municipal regulations and building codes. This urban constraint was identified as one of the major barriers to the large-scale implementation of BIA (Cerón-Palma et al. 2012). The cost-effectiveness of the urban farm will depend on its typology. Plant factories need only 10% of the land area compared with greenhouses for obtaining the same productivity/m², and can easily be built in any disused building. While capital costs are high¹ – about 15% greater than that of a greenhouse – annual productivity is about 3000 lettuce heads/m²/year, which is 15 times that of a greenhouse (about 200 lettuce heads/m²/year). Thus the initial cost per unit production capacity of a plant factory is more or less the same as that of a greenhouse, although this estimation is rough and varies with many factors (Kozai et al. 2016).

In addition to involving high investment costs, high-tech commercial farming systems often lead to substantial operating costs due to their elevated energy needs (Thomaier et al. 2015). Moreover, whereas rural farms usually benefit from subsidized water and energy for agriculture, farms located in urban areas have to pay the urban costs of water supply and energy, applicable in accordance to the zoning. If the farm is located in a residential zone, then the costs will be higher than if it is located in a commercial zone (Benis & Ferrão 2018).

Productions costs (labour, electricity, depreciation, and others) vary around the world. In Japan, for example, the component costs of plants factories are, on average, 25-30% for labour, 25-30% for electricity, 25-35% for depreciation, and 20% for other production costs (land rent, seeds, water, lamp replacement, office goods, packing materials, delivery costs, etc.). Labour costs are so high because most plant factories are small-scale, and handling operations therefore have to be conducted manually. It is estimated that a 15-tier plant factory with a floor area of 1 ha needs more than 300 full-time employees. In comparison, most handling operations in a greenhouse complex with a floor area of 10 ha or more are automated, and so need only a few employees per hectare (Kozai et al. 2016).

¹ about US$4000/m² in 2014 (Kozai et al. 2016)

Table 1 shows the energy conversion process in a culture room of an energy-efficient plant factory. The electrical energy fixed as chemical energy in the saleable part of the plants is 1-2%. The remaining electric energy is converted into heat energy in the culture room, so the heating cost of a thermally well-insulated plant factory is zero. In plant factory production cost management, the weight percentage of the edible or usable part of the plant to the total plant weight is an important index for improving cost performance. Since electric energy is consumed to produce the roots, if the roots are not saleable, the root mass must be minimised without compromising the growth of the aerial part of the plant.

Table 1: The energy conversion in a plant factory (from Kozai et al. 2016)

The electricity cost can be reduced by (1) using advanced LEDs to improve the conversion factor from electric to light energy; (2) improving the lighting system with well-designed reflectors to increase the ratio of light energy emitted by lamps to that absorbed by plant leaves; (3) improving the light quality to enhance growth and the quality of the plants; (4) optimally controlling temperature, CO₂ concentration, nutrient solution, humidity, and other factors; and (5) increasing the percentage of the saleable part of the plants by improving the culture method and selection of cultivars (Kozai et al. 2016).

Electricity costs can also be reduced by using solar panels. Urban plant factories in free-standing buildings, such as former warehouses and factories, have more possibilities for generating their own electricity than those located in buildings which are part of a dense urban matrix. The amount of energy required to power free-standing plant factories is contingent on the dimensions of the building. When a building occupies a larger area, the lighting and water requirements increase, but so does the amount of energy available via solar panels on the roof and, potentially, the facade. The amount of power that can be generated by solar panels obviously depends on the geographic location of the plant factory.

The net water consumption for irrigation in a plant factory is about 2% of that of a greenhouse, because about 95% of the transpired water vapour from the plant leaves is condensed at the cooling panel (evaporator) of the air conditioners as liquid water, which is collected and then returned to the nutrient solution tank after sterilization. Drained nutrient solution from the culture beds is also returned to the nutrient solution tank after sterilization. Thus the amount of water that needs to be added to the tank is equal to the amount of water kept by the harvested plants, and the amount that escapes outside as water vapour through air gaps. Similarly, the amount of nutrient that is added is equal to the amount of nutrients absorbed by the harvested plants. Thus the efficiency of water and nutrient use is more than 0.95 and 0.90 respectively (Kozai et al. 2016).

Urban farming and the circular economy

The circular economy is currently one of the most discussed terms among environmental economic scientists and is a focus of the European Union Horizon 2020 strategy. Its core defining element is the ‘restorative use’ of resources: instead of becoming discarded waste, raw materials are recycled and reused (Geisendorf & Pietrulla 2018). Urban agriculture offers various possibilities for embracing this approach, which is best exemplified by The Plant. In 2010, social enterprise Bubbly Dynamics LLC acquired a former meatpacking plant in Chicago and developed a plan to use the building as a space for incubating food and farming businesses, thereby bringing much-needed jobs back to a disinvested community in a ‘food desert’ lacking healthy food options. The 8686 m² facility currently houses over a dozen small businesses, including indoor and outdoor farms, kombucha and beer breweries, a bakery, a cheese distributor, a coffee roaster, and other food producers and distributors. As of early 2018, there were approximately 85 full-time equivalent employee positions based at the facility. The Plant is still under construction and is approximately 70% leased out; full occupancy is expected in 2019.

Founded on a model of closing waste, resource, and energy loops, The Plant is working to show what truly sustainable urban food production looks like. The planned anaerobic digester is a key feature, as it is designed to solve several critical issues by reusing what is conventionally considered ‘waste’ in order to create several valuable outputs. Waste from the building will be a fraction of the volume of waste processed by the digester, yet the digester will demonstrate that even food production businesses, which are typically waste and energy intensive, can operate sustainably by closing waste loops. Figure 8 is a conceptual diagram of the various processes anticipated at The Plant at full occupancy.

Figure 8: Waste (green) and energy/gas (orange) cycles at The Plant, Chicago

Copyright © Partners of the Aqu@teach Project. Aqu@teach is an Erasmus+ Strategic Partnership in Higher Education (2017-2020) led by the University of Greenwich, in collaboration with the Zurich University of Applied Sciences (Switzerland), the Technical University of Madrid (Spain), the University of Ljubljana and the Biotechnical Centre Naklo (Slovenia).