Nature of Heat and Thermal Energy: From Caloric to Carnot's Reflections, to Entropy, Exergy, Entransy, and Beyond
Abstract:. Nature of thermal phenomena is still elusive and sometimes misconstrued. Starting from Lavoisier, who presumed that caloric as weightless substance is conserved, to Sadi Carnot who erroneously assumed that work is extracted while caloric is conserved, to modern day researchers who argue that thermal energy is indistinguishable part of internal energy, to the generalization of entropy and challengers of the Second Law of thermodynamics, the relevant thermal concepts are critically discussed here. Original reflections about nature of thermo-mechanical energy transfer, classical and generalized entropy, exergy, and new entransy concept are reasoned and put in historical and contemporary perspective with an objective to promote further constructive debates and hopefully resolve some critical issues within the subtle thermal landscape.
Figure 2 [Ref.]. Thermal and mechanical internal energies are distinguishable parts of the thermodynamic internal energy, the former increasing the thermal and the latter increasing the mechanical part of the internal energy, resulting in different states, regardless that the internal energies are the same, as illustrated by 1 kJ heating or 1 kJ compressing of ideal gas (A), or a spring (B).
Likewise, the most critical misconception is that “during isothermal expansion the input heat is 'directly' converted to work output”: The expansion work W, at T isothermal process, is not due to input heat Q regardless that incidentally W=Q (isothermal is also iso-energy for ideal gas). The expansion work is solely due to mechanical-energy potential within gas and only possible if gas initial pressure Pi is bigger than surrounding pressure (Psurr). During such mechanical-expansion the gas temperature tends to lower below the thermal reservoir temperature, driving heat transfer to equalize temperature and internal energy of ideal gas on its own, not the other way around. The work-energy potential is compensated by heat (due to isothermal heat transfer), however, the energies of the two states (i & f), although equal magnitude, are not the same qualities, so the final state “f” is not the same as initial “i” but with bigger entropy and volume, thus lower energy quality, see [Ref.]. Gas expansion-work cannot be perpetual like heat-engine cyclic repetitions, but is transient, due to initial gas pressure and possible only until the compressed gas expands to the surrounding pressure. Mechanical pressure expansion tends to lower temperature and drives heat transfer, not the other way around -- critically important to comprehend “cause-and-effect” and duality of mechanical energy (integral volumetric boundary kinetics) and thermal energy (random particle kinetics) within the ideal gas internal interaction, see [Ref.]. Furthermore, direct transfer of molecular (thermal-particle) kinetic energy to work (piston-movement) is limited by boundary surface pressure (depending of system volume), i.e., mechanical energy being part of total internal energy. Note that for steady or cyclic processes, Qin-Qout=Qnet=Wnet, the reversible-process efficiency is (Qin-Qout)/Qin=(Tin-Tout)/Tin, but not for a transient processes when the system energy quantity and/or quality is changed.
... CHALLENGES AND CONCLUDING REMARKS
Starting from Clausius till nowadays, the obvious but in general not quantified thermal energy, is ‘lumped’ into the internal energy, the latter well-quantified and tabulated in Thermodynamic reference books. Some (or many) argue that thermal energy is not definable, but internal thermal energy is manifested as heat transfer due to temperature difference. It is argued here and elsewhere (related manuscript being finalized by this author) that heat (and anything else for that matter) could be transferred only if it exists as stored quantity in kind, in the first place. Therefore, thermal energy is stored heat (directly related to the system heat capacity), UThº QStored, and heat is the thermal energy transfer, Qº UTh,transfer. It is obvious and self-evident in caloric processes and quantified by the caloric quantities, i.e., system heat capacity and related properties .
The Second law is not about disorder and probability per se (or any other math or physics 'tools' per se used to describe it), but about spontaneous, forced-tendency (natural process-forcing displacement) of mass-energy redistribution in certain, irreversible direction (process driving force), from higher to lower energy-potential (mass-energy density in space). Spontaneity implies forced-directionality and in turn irreversibility. No spontaneous, irreversible process could ever be completely reversed or undone. For example, the driving force for life on Earth is the irreversible dissipation of energy from the Sun.
It is hard to believe that a serious scientist, who truly comprehend the Second law and its essence, would challenge it based on incomplete and elusive facts. Sometimes, highly respected scientists in their fields, do not fully comprehend the essence of the Second law of thermodynamics. The Second law ‘challengers’ need to demonstrate and quantify destruction of entropy to challenge the universal validity of the Second law. It has been reasoned and thus proven that destruction of entropy, i.e., violation of the Second law, is against the forced tendency of natural processes and thus impossible, leaving 'No Hope' for the challengers . After all, the 'Wishful Maxwell's demon' could not be realized since 1867. After all, before ‘the Second law violation’ claims are stated, the reliable criteria for the Second law violation, including proper definition and evaluation of entropy, should be established based on full comprehension of the fundamental Laws.
A critical perspective within elusive thermal landscape of ‘Entransy concept and controversies’ is given recently by this author . Regardless of entransy redundancy, being derived from other physical quantities, it does not diminish its uniqueness and usefulness in thermal analysis and optimization. Actually, it is argued in  that the entransy, due to its unique nature, may contribute to better comprehension of often obscured thermal phenomena. Despite the need for further development and clarifications of the new concept, it would be premature and unjust to discredit entransy, based on limited and subjective claims, as if the ‘already established’ concepts and methodologies are perfect, and do not need alternatives and innovations, as if further progress is not needed.
As the fundamental laws of nature and thermodynamics are expended from simple systems in physics and chemistry, to different space and time scales and to much more complex systems in biology, life and intelligent processes, there are more challenges to be comprehended and understood.
There are many puzzling issues surrounding thermodynamics and the nature of heat, including subtle definitions and ambiguous meanings of very fundamental concepts. In modern times, there is a tendency by some authors to unduly discredit thermal energy as being indistinguishable from other internal energy types. Heat and thermal energy are more subtle and elusive than many other forms of energies. Nature is, and so is heat, what it is, no more and no less. In fact, all other forms of energies are ultimately dissipating in thermal heat, the omnipresent and universal phenomena, quantified with perpetual and irreversible generation of thermal displacement, i.e., entropy.
There is a need to "shed more light onto dissipative heat." It is the goal of this treatise to contribute to that aspiration. A critical analysis of selected issues is presented here. Richard Feynman once stated, “It is important to realize that in physics today, we have no knowledge what energy is”. This Feynman's statement has a deeper connotation, since we tend to simplify, pre-judge, and proclaim definite meanings of the fundamental concepts, or to discredit new ones.
The science and technology have evolved over time on many scales and levels, so that we now have advantage to look at related historical developments more comprehensively than the pioneers. The fundamental laws of thermodynamics, and especially the issues related to thermal energy and entropy, including the Second law challenges, have been primary interest and topic of this author’s past presentations and recent writings. Long-contemplated reflections on some critical issues of thermoscience concepts, from unpublished presentations and selected citations with updates, are presented here to hopefully contribute to further discussions and encourage due debate.
Heat, as transfer of thermal energy, is the unique and universal manifestation of all existence and changes in nature. The thermal energy, as stored heat, represents chaotic motion and interactions of a system structure. It is transferred from higher to lower temperature, and thereby, on-its-own conserved (as in original caloric and heat exchangers). In addition, heat is also generated due to spontaneous and inevitable dissipation of all other energy forms to heat. Entropy, as dynamic, thermal energy space, is associated with thermal energy only, and transferred with heat only, therefore, always irreversibly generated with heat generation. Entropy is conserved during reversible processes, including limiting reversible heat transfer, as in power and refrigeration Carnot cycles. There is no way to destroy entropy, since it will imply energy and heat transfer from lower to higher energy potential, i.e., spontaneous generation of non-equilibrium from within an equilibrium, against the natural forces. Exergy is the work potential of all energy forms, including thermal, and thus function of non-equilibrium state with regard to a reference ‘dead-state’. New entransy property represents quantity and quality of thermal energy since it also includes its temperature potential. The entransy represents a state property, while exergy, as work potential is function of non-equilibrium states. Entropy, exergy and entransy are directly correlated since all three quantify the thermal irreversibility in different ways, and have their inherited advantages and disadvantages. In modern times, there is a tendency by some scientists to unduly discredit the ‘thermal energy’ as being indistinguishable from internal energy. Denying existence of thermal energy is the same as denying existence of its (heat) transfer. A critical analysis of selected issues is presented here.