Guest editorial

ENERGY Systems Integration (ESI) is the process of coordinating the operation and planning of energy systems across multiple pathways and/or geographical scales to deliver reliable, cost-effective energy services with minimal impact on the environment. Energy systems have evolved from individual systems with little or no dependencies into a complex set of integrated systems at scales that include customers, cities, and regions. This evolution has been driven by political, economic, and environmental objectives. As we try to meet the globally recognized imperative to reduce carbon emissions through the deployment of large renewable energy capacities while also maintaining reliability and competitiveness, flexible energy systems are required. This flexibility can be attained through integrating various systems: by physically linking energy vectors, namely electricity, heat, and fuels; by coordinating these vectors across other infrastructures, namely water, data, and transport; by institutionally coordinating energy markets; and, spatially, by increasing market footprint with granularity all the way down to the customer level. Smart grids and ESI are related, in particular, the coupling between electricity, consumers, data and transport. This special section focuses on the aspects of ESI where electricity is coupled to water, heat and fuels and where this coupling brings challenges and/or opportunities. The seven papers in this special section originate from Europe, Asia, North America and Australasia and several of the papers had multiple authors spread across the globe illustrating the international importance of ESI and the collaborations that have developed. The papers cover nearly the full range of ESI sub themes including the coupling between electricity with fuels, heat, gas; the coupling across scales and coupling with other infrastructures (e.g., water). Coupling between electricity and transport is a missing theme but not surprising considering the attention this topic has had in the past decade in the literature. Optimization at a planning and/or an operational time frame is central to nearly all the papers. The energy hub concept introduced over a decade ago by Geidl et al. at ETH Zurich features strongly in the literature survey sections of many of the papers, highlighting the importance of the concept. Renewables integration is an important driver of ESI highlighted in nearly all the papers. The flexibility benefits of integrated energy systems are highlighted throughout by a set of case studies that range from small district heating systems to national scale systems. Enhanced reliability, economic benefits and reductions in emissions all feature strongly across the seven papers and all stem from the ESI approach.