**5. Conclusions**

This review targeted novel developments in a rapidly developing field and was helpfully supported by a special topical issue presented in 2019 at the PRES 19 conference (Process Integration, Modelling and Optimisation for Energy Saving and Pollution Reduction), from which we selected 20 important papers. This conference was held at a venue on the Greek Island of Crete, benefiting from a clean island environment and serving as an intensive cross-fertilisation meeting of leading scientists from more than 50 countries. As an important extension to reviewing the selected Special Issue papers, this review article also provided a wider overview of a number of the most recent developments in heat integration and heat transfer; integrated and newly developed heat exchangers; integration of renewables; and roles in economic and environmental sustainability. Topics relating to emerging tools for the design of energy systems to increase economic and environmental sustainability were considered in the selection of the contributions. A wide variety of system design tools have been proposed and targeted for different energy-related issues. This review suggested that the achievement of a sustainable energy system design relies on social aspects receiving greater research attention, thus placing them on a level equal to that of economic and environmental concerns. The consideration of social impacts, including behaviour analysis, could further enhance the practicality of the tools for system design.

**Author Contributions:** Conceptualisation, Y.V.F. and J.J.K.; methodology, J.J.K. and Y.V.F.; formal analysis, Z.N.P., Y.V.F., J.J.K.; resources, Y.V.F., J.J.K., Z.N.P.; writing—original draft preparation, Y.V.F., Z.N.P., J.J.K.; writing—review and editing, Y.V.F., J.J.K., Z.N.P.; visualisation, Y.V.F., J.J.K.; supervision, project administration, funding acquisition J.J.K. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the EC supported project Sustainable Process Integration Laboratory – SPIL, grant number CZ.02.1.01/0.0/0.0/15\_003/0000456 by Czech Republic Operational Programme Research, Development, and Education, Priority 1: Strengthening capacity for quality research under the collaboration agreement with the University of Maribor, Slovenia.

**Conflicts of Interest:** The authors declare no conflict of interest.

## **References**


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