466874 Managing Trade-Offs Between Food, Renewable Energy and Ecosystem Services

Tuesday, November 15, 2016: 8:30 AM
Union Square 15 & 16 (Hilton San Francisco Union Square)
Rebecca Hanes, Varsha Gopalakrishnan and Bhavik R. Bakshi, William G. Lowrie Department of Chemical and Biomolecular Engineering, The Ohio State University, Columbus, OH

Competition between renewable energy and food production, driven by increasing food and energy demands, creates incentives to increase agricultural productivity. In the short term, productivity can be increased through the use of intensive farming management practices that use extensive tillage and large quantities of chemical fertilizers. Such practices have been shown to contribute to ecological degradation of farmland via an increase in soil erosion and a decrease in soil carbon, among other impacts. The surrounding watershed is also affected negatively by the nitrate and phosphorus-contaminated runoff from fertilized fields, which degrades the water quality and contributes to the growth of harmful cyanobacteria and other microorganisms. In the long term, this ecological degradation results in reduced agricultural productivity, such that increasing amounts of chemical fertilizers and other technological inputs are required. Farming for maximum productivity without accounting for the ecological impacts thus creates a vicious cycle, in which productivity is maintained by inflicting ecological harm.

Experimental studies have demonstrated that conservation farming practices, with reduced tillage and in some cases reduced amounts of fertilizers, insecticides and herbicides, can mitigate the ecological damage of conventional agriculture. Farming with reduced or no tillage helps to increase and maintain the carbon content in soil, providing an offset for the emissions produced in the agricultural life cycle, and reduces the rate at which soil erodes. Reducing chemical fertilizers or partially replacing them with soil amendments such as agricultural residues reduces the concentration of nitrates and phosphates in contaminated runoff and cuts down on wasted fertilizer as more of the applied fertilizer is taken up by crops. Conservation practices offer myriad ecological benefits but do not necessarily offer similar productivity benefits: the rate at which biomass is produced can be decreased under conservation farming, creating trade-offs between ecological preservation, agricultural production, and farm economics. With demand for both food and energy likely to increase for the foreseeable future, the need for food and bioenergy crops must be balanced against the need for ecological preservation.

The objective of this work is to design a system for the co-production of food and renewable energy with respect to three conflicting criteria: food productivity, energy productivity and net ecosystem service (ESS) supply. Ecosystem services are the goods (food, fuel, materials) and services (climate regulation, water quality regulation, erosion control) provided by nature that are necessary to support human activities, including agriculture. Here we define net ESS supply as the difference between ESS supply, provided by nature, and ESS demand, required by human activity. Climate regulation is a simple example. The net climate regulation supply is the quantity of greenhouse gas emissions generated by human activities, less the quantity of emissions that can be sequestered or otherwise removed from the atmosphere by ecological processes such as biomass growth and sequestration in soil. Net ESS supply provides a quantitative way to measure ecological degradation: a negative net supply means that the demand for ESS is greater than can be supplied by nature, and ecological degradation is the likely consequence. In this work, we will identify options for food and renewable energy co-production that have a net ESS supply greater than or equal to zero, for key services.

The system we consider is comprised of two stages: land use and biomass-to-energy conversion. Land use includes several options for crops both grain and cellulosic, rotation systems and farming management practices, as well as the production of wind and/or solar electricity. Land can also be used to create additional ESS supply through reforestation; however, land used for ESS supply cannot also provide food or energy, thus there is competition between the land uses. Options for biomass-to-energy conversions cover a range of liquid fuels, both ethanol and hydrocarbon, as well as electricity. Grain crops can be sold directly as food products or converted to liquid fuels; the system thus has a variety of configurations in which both food and renewable energy is produced from the same farmland. The land use and biomass-to-energy conversions are modeled at the value chain and equipment scales, respectively, of the process to planet (P2P) multi-scale modeling framework. For this system, the value chain scale may be thought of as a regional scale. The third and largest scale of the P2P framework, the economy scale, is used to capture the life cycle of all inputs to the land use and biomass-to-energy conversion stages. Net supplies of two ESSs are quantified as decision criteria: climate regulation, quantified with carbon dioxide emissions and carbon sequestration, and air quality regulation, quantified with nitrogen oxide emissions and removal. Carbon dioxide emissions (climate regulation demand) is quantified at all scales, equipment, regional and life cycle. Carbon sequestration (climate regulation supply) is quantified at the regional scale only, as the land use options are the only components of the system that provide a variable supply of ESS. Air quality regulation is quantified only at the smaller two scales, with supply again provided only by the land use options and demand created by both the land use options and the biomass-to-energy conversion options. Air quality regulation is not considered at the life cycle scale because the spatial presence of criteria air pollutants and its effects, are either local or regional in nature depending on the pollutant molecule.

To analyze trade-offs between food, energy and ecosystem services, the co-production system is optimized under several constraints. Food and energy production are constrained relative to the maximum production possible, to ensure that all optimal system designs are truly co-production systems. Where possible, net ESS supply is also constrained to be greater than or equal to zero; this is not always achievable for air quality regulation, but net climate regulation supply can reach above zero for many system configurations. Within these constraints, the system is optimized for maximum food production, maximum energy production, and maximum net ESS supply for both services. With the design space thus defined, Pareto curves are found between food production and net ESS supply, and between energy production and net ESS supply, in order to identify sets of compromise system designs. Preliminary results indicate that maximizing either food or energy productivity leads to the worst possible performance for net ESS supply, as expected. However, by either sacrificing some amount of productivity or by using land for both food and energy production and ESS supply, designs can combine adequate net ESS supply with relatively high food and energy productivity.

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See more of this Session: The Food-Energy-Water Nexus
See more of this Group/Topical: Sustainable Engineering Forum