This special issue of the Journal of Energy Resources Technology (JERT) is dedicated to the 29th International Conference on Efficiency, Cost, Optimization, Simulation and Environmental Impact of Energy Systems (ECOS), which was held in Portoroz, Slovenia, from June 19 to 23, 2016. The ECOS series of annual conferences was established in 1987 by the ASME Advanced Energy Systems Division. These conferences offer a platform to engineers and scientists worldwide to meet annually and discuss developments in the fields of conversion, storage, and transportation of energy, particularly the related thermodynamic, environmental, economic, and social issues.

This special issue contains a selection of ten papers presented at the ECOS 2016. These papers have passed the conference review process, were presented at the conference, and further improved and extended before submission to the journal. A further international peer-review process was conducted by JERT after the conference, which resulted in the selection of the papers included here.

This special issue is a valuable presentation of the state-of-the-art and of advances in the combined heat and power plants and heating technologies; district heating networks; nuclear power plants; thermal, thermochemical, and liquid air energy storage plants, as well as design of gas turbines and impact of air quality to their performance. Energy, exergy, and economic analyses have been applied to evaluate and optimize these systems.

Deep gratitude goes to the authors for their scientific contributions and to the reviewers for their valuable comments, which resulted in the improvement of the original papers. The publication of these ECOS 2016 papers in JERT would not have been possible without the significant technical assistance and support of the editorial office and the leadership of the JERT Editor-in-Chief, Professor Hameed Metghalchi. We want to thank all these persons for their contributions to the quality of the special issue.