**3. Conclusions**

This review editorial was initiated by the Special Issue for carefully selected papers from the 2018 Process Integration, Modelling and Optimisation for Energy Saving and Pollution Reduction (PRES 2018) conference. The conference, which attracted more than 550 leading researchers worldwide, boosted the interest in a number of highly appealing scientific issues: Design and numerical study for innovative energy-efficient technologies, Process Integration—Heat and Power, Process energy efficiency or emissions analysis, Optimisation of renewable energy resources supply chain. These issues are having a strong influence on sustainability and the circular economy. The authors endeavoured to overview this field by adding relevant and recent references and suggested some conclusions for future research directions. The feedback from the readers and especially researchers in the field is most welcome and appreciated, and it should provide a ground for the next PRES'19 panel discussion devoted to the future Special Issues of the journal Energies.

**Author Contributions:** All authors contributed to the overview. J.J.K. has managed the overall process of the article creation., supervising and directing the work of the other authors. He has also been responsible for writing the abstract and the conclusions section, as well as most of Section 2.3. H.H.C. has contributed with Sections 1

and 2.4 and been responsible for the overall arrangemen<sup>t</sup> and graphics. P.O. has contributed with Section 2.1., P.S.V.—for Section 2.2 and partly Section 2.3 and provided the overall advise.

**Funding:** The EU supported project Sustainable Process Integration Laboratory—SPIL funded as project No.CZ.02.1.01/0.0/0.0/15\_003/0000456, by Czech Republic Operational Programme Research and Development, Education, Priority 1: Strengthening capacity for quality research in collaboration with Cracow University of Technology, Poland.

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