A comprehensive classification of food-energy-water nexus optimization studies: State of the art

To tackle the globally increasing discrepancy between food, energy and water demands and resource availability sustainably, resource supply system models have to incorporate the inter-dependencies and -connectivities to other supply systems. This leads naturally to a food-energy-water nexus (FEWN) approach. The FEWN can be interpreted as the study of the connections between the food, energy and water resource systems, emphasizing how decision-making influences the synergies, conflicts and trade-offs among the various sectors. In recent years, modeling and optimization of FEWN systems has been receiving an increasing interest in the open literature, however, with limited emphasis on how decisions of the FEWN are derived. In this review, FEWN optimization studies are analyzed with focus on the employed objectives, optimization and solution strategies, as well as the selected sub-systems and their corresponding spatial and temporal scales to uncover in detail how decision-making is facilitated. More specifically, FEWN optimization studies are classified according to their modeling and solution strategies. Based on this classification it is uncovered that (i) the decision-making itself has not yet been investigated in detail in FEWN literature, (ii) the incorporation of all aspects of the FEWN is still a challenge, (iii) the interconnection between FEWN systems and society has to be further investigated, and (iv) the implications of uncertainty for the resiliency, robustness and security of process systems is not yet well defined. Additionally, a generic FEWN resource-task network formulation is introduced to illustrate the similarities across the various resource supply sectors. Special interest is placed on how synergies are identified and competition be avoided among resource systems. It is shown that the selected spatial scale as well as the utilized modeling and optimization strategies significantly influence the synergy level of obtained solutions. Furthermore, it is derived that the energy transition has to incorporate FEWN systems thinking for sustainable solution generation. Overall, this review summarizes the different applications and implications of process systems engineering concepts to FEWN systems.