Optimization of water-energy nexus: A network representation-based graphical approach

We present a scalable and systematic method for the design and optimization of complex water-energy nexus using graph theory-based network representation and a novel water-energy nexus (WEN) diagram. The graph theoretic approach defines a nexus as a directed bipartite graph with water and energy flows. The network representation allows the decomposition of a complex nexus into its essential and redundant components. We show that for specified external grid demands, the optimal nexus configuration with minimum water and energy generation is the one without any redundant subgraphs. We then propose a systematic method to identify and eliminate redundant cycles, flows and entities within a nexus leading to (i) minimum generation/extraction of water and energy resources from the environment, or (ii) maximum yield of water and energy to meet external demands. Our approach is simple to implement and results in optimal nexus configurations that also satisfy operational constraints, restrictions and water quality specifications. The approach is demonstrated using case studies on water-energy nexus systems focusing on power generation, seawater desalination, groundwater and surface water at regional and national scales.