MULTI-OBJECTIVE OPTIMIZATION FOR COMPREHENSIVE WATER, ENERGY, FOOD NEXUS MODELING

Abstract

Deteriorating environmental conditions and depleting resource supplies have given rise to a critical need for optimal resource allocation that maximizes resource efficiency and minimizes resource waste. The water, energy, food nexus (WEFN) provides a conceptual framework within which resources are considered holistically to account for their interactions across sectors in a manner that prevents improvement in one resource at the expense of another. In the first part of this work, a WEFN tool selection approach is developed to characterize available nexus tools and determine the role a user plays in determining the suitability of the tool. The second part of this work utilizes the lessons learned from the development of the tool selection approach to develop a multi-objective optimization model that concurrently simulates the water, energy, food nexus to yield optimal resource allocation (via three objective functions for each nexus sector). The model incorporates constraints that account for a healthy and sustainable diet, nutrient flows (carbon and nitrogen), economic considerations (trade and costs), planetary boundaries (nitrogen, carbon emission, and blue water), and the Sustainable Development Goals. The model is developed using the Python language and solved with the GEKKO package solver. A hypothetical case study is applied to showcase the model. Four scenarios are simulated, yielding different diet compositions and significantly different resource consumption values. Energy and water consumption values ranged from 184-428 MWh and 7-64 km3 for scenarios of 20% and 100% self-sufficiency respectively. In line with these ranges, differing values of emissions (17-101 tons CO2-eq), total land utilization (5-15 m2), and overall production costs (17-93 thousand USD). A Pareto analysis is applied to one of the case study scenarios, showcasing the trade-offs between maximizing food production (25-86 tons) and minimizing water (8-69 km3) and energy (323-428 MWh) consumption. Sensitivity analyses addressing population growth and increasing food self-sufficiency are evaluated. A constraint analysis is also conducted where different constraints are included/excluded to determine interactions between nexus components. Recommendations for future work include the incorporation of more segmented nexus sub-sectors and interconnections, novel/synthetic resources and technologies, dynamic phenomena, the capacity to shift between multiple spatial and temporal scales, political and social factors, an enhanced graphical user interface, and alternative research approaches (such as multivariate analysis).