Recent Innovations in Advanced Thermal Energy Systems towards Better Utilization of Energy Resources and Cleaner Environment: Issues and Challenges

The special collection on Recent Innovations in Advanced Thermal Energy Systems towards Better Utilization of Energy Resources and Cleaner Environment is available in the ASCE Library at http://ascelibrary.org/page/jleed9/energy_resources_cleaner_environment.

The two societal objectives referred to in the title of this special collection, i.e., efficient and innovative use of energy resources and a cleaner environment, have dominated the discourse on energy policy so much over the last four decades that the temptation emerges for them to be perceived as self-evident truths and their technological and social implications be underestimated. Upon careful examination, however, several exciting questions emerge whereby scientific and social issues are interwoven. For example, large populations in both the developed and developing world reside over low-quality coal reservoirs that can provide fuel at a price on the order of $\$10/\mathrm{t}$. Is the need for a cleaner environment supposed to be an impediment to the development of these populations? The diesel engine still remains the most thermally efficient power-generation device; still, electrification of transportation seems to be gaining ground on environmental concerns. Does the need for a cleaner environment contradict the pursuit of higher efficiency? The recent boom in renewable energy has also raised all sorts of technoeconomic questions. How can we go around the intermittency of solar power generation that has recently progressed so much? Are photovoltaics the only way forward or do thermal-solar technologies also deserve attention, especially when involved in schemes of multifaceted cogeneration (power, heat, potable water, refrigeration, air conditioning, etc.)?

The inventiveness with which the scientific community has responded to these and other similar concerns is nothing short of an impressive testament to human ingenuity. In this special collection, we would like to offer to the readers of the Journal of Energy Engineering a vista of these technologies, some of which were either unheard of only a decade ago or constitute evolutions of already existing technologies that nobody could reasonably foresee. A variety of topics is covered that spans from the use of ammonia and butanol as automotive (bio)fuels to the effect of CuO and ${\mathrm{Al}}_2{\mathrm{O}}_3$ on heat transfer for thermal energy storage. Particular emphasis has been placed on coal-fired power generation and reciprocating internal combustion engines because these two technologies seem to be the focus of most intense discussions on sustainability.

The advantages of second-generation biofuels such as biodiesel, biobutanol, and dimethyl ether are analyzed and a series of technologies are presented that can make coal not only cheap but also cleaner, such as oxy-fuel and ash use for $\mathrm{S}{\mathrm{O}}_2$ capture. In parallel, technologies are presented that may have not reached extensive industrial practice yet but do seem to show promise, such as solar flash desalination of water, combustion electrostatics, and phase-change materials for solar/thermal energy storage. Several papers underline scientific fundamentals of energy conversion such as combustion stability, flame stretch, and transient flow. In all of these studies, state-of-the-art computational and experimental methodologies were used that are shared with the scientific community for future use.

It is, of course, realized that the sample provided here cannot be expected to exhaust fully the scientific and technological issues that underlie efficient and clean thermal energy conversion. Our hope is that the special collection will act as reference material for practitioners of related research, and that it will provide a solid contribution to the scientific discourse on energy and the environment.

We would like to thank all the authors for their valuable contributions and the referees for conducting thorough and detailed reviews that have raised even further the level of the contributed papers. We express our gratitude to the Editor-in Chief of the Journal, Dr. Chung-Li Tseng, for the opportunity and his excellent cooperation during the production of this special collection.