6.4 Leafy greens and mushroom production integrated CEA system
By Murat Kacira, Director, Controlled Environment Agriculture Center, Professor, Department of Biosystems Engineering, University of Arizona
Barry Pryor, Professor, School of Plant Science/BIO 5 Institute, University of Arizona
Leafy greens and mushroom production in integrated controlled environment agriculture (CEA) system for circularity
Projections forecast that the global demand for food, energy, and freshwater will increase considerably over the next decades due to multiple factors, e.g., increases in population growth and mobility, economic development, urbanization, diversified diets, cultural and technological changes, and climate change (FAO, 2014). There is growing demand for local year-round production of safe, healthy, nutritious, and affordable fruits, vegetables, and protein sources. One of the reasons for this rising local demand relates to consumer perception that long distance transport of food from field production sites significantly deteriorates freshness, product quality and nutritional content upon arrival at distant markets. Another important aspect of the local-grown movement is the desire to move toward environmental sustainability with reduced carbon footprint.
When hydroponic cropping systems are stacked in vertical tiers within repurposed warehouses, with overhead LED lighting at each production level, this type of indoor agriculture is termed vertical or indoor farming (Figure 1). Compared with field production of the same crops per land-area-footprint and annual production basis, the indoor farming systems can be at least two orders of magnitude more productive compared to open-field agriculture (Kozai, 2013). The emerging indoor farming industry has attracted venture capital investors and entrepreneurs, and is rapidly growing with technological developments, but often without deep knowledge of crop needs, engineered and integrated system designs, and lack of resource conserving and innovative environmental control strategies. A major challenge to address for indoor vertical farming-based systems is to reduce use of electrical energy demanded by the LED lighting systems which accounts for about 40% of total energy use. This can be achieved in several ways, including innovative light-delivery designs for LED fixtures, engineered air conditioning system design and resource conserving environmental control strategies, optimized growth prescriptions including interactions between light intensity and CO2 levels for crops during each crop growth stages, and an integrated systems-based approach to co-produce and reuse/re-cycling of resources for smart, sustainable, and profitable indoor agriculture.
Consumption and demand for mushrooms as a protein or a health benefiting compounds source has been significantly increasing in Arizona and across the US, and US production has increased nearly 30% over the last 10 years (2018, NASS). Currently, most of the statewide demand for mushrooms is met by larger producers out of state, notably Pennsylvania. However, mushrooms are perhaps the most perishable product in the produce aisle and regional production provides a much higher quality product over that transported for many miles and many days. This difference in quality is immediately recognizable, particularly with specialty mushrooms. Thus, regional markets support developing local mushroom industries, particularly in the specialty mushroom category. Indeed, specialty mushroom production is increasing in many states to supply local markets, and this presents strong opportunity for new business development.
Because mushrooms are produced in indoor controlled environment facilities, new mushroom producers can enter the market at many economic levels, as either small or large business entities. Another important aspect of mushroom production is that significant amounts of CO2 are emitted into the production system which must be vented out (e.g. wasted) to enable and optimize mushroom production. However, this valuable wasted CO2 is in great demand and can be re-cycled as a resource for crops, such as leafy greens, in indoor CEA production systems. It can even be possible to reduce the light intensity, minimizing the demand for electrical energy, while slightly increasing the CO2 levels to achieve the desired crop yield and qualities. The cost of increased CO2 levels to enhance the crop yield is much more cost effective than increased light intensity for enhanced crop yields, thus lowered light intensity with slightly increased CO2 level can lead to significant energy savings and enhance profitability of the production. Some of the indoor crop growers also use natural gas burners in indoor growing spaces to generate CO2 for the crops which leads to increased heat and water in the growing space further demanding for energy use for cooling and dehumidification, which accounts about 50% of the total electrical energy use in indoor agriculture systems. Therefore, the use of CO2 ejected from mushroom production system can reduce the electrical energy demand for cooling and dehumidification and lead to enhancing environmental sustainability using a re-cycled resource. However, to our knowledge such integrated system with hydroponics based leafy greens and mushroom production in indoor vertical farming system has not been experimentally evaluated and implemented to date at scale. With experimental and modeling-based research conducted at UArizona-CEAC, Shasteen (2022) demonstrated 15% electrical energy savings can be achieved with 11.2 daily light integral (DLI) and 1000 ppm CO2 setpoints compared to 13. 5 DLI with 500 ppm CO2, with both control setpoints resulting in same fresh lettuce crop yield of 200 gr per head. Furthermore, Chung (2020) found that it is possible to grow four head lettuce per one mesquite/alfalfa substrate bag with Pleurotus ostreatus mushroom grown, or fourteen head lettuce per one mesquite/alfalfa substrate bag with Ganoderma lucidum mushroom grown (Figure 2).
