2009 January 10-12© M. Kostic Prof. M. Kostic Mechanical Engineering NORTHERN ILLINOIS UNIVERSITY Heat Transfer, Thermal Energy and Entropy - Demystified : Challenges and Opportunities for Improving Heat Transfer Processes - The Quest and Nature of Energy, Heat and Entropy PLENARY LECTURE The 6th WSEAS International Conference on HEAT and MASS TRANSFER (WSEAS - HMT'09) Ningbo, China, January 10-12, 2009

2009 January 10-12© M. Kostic Focus and Goal : Focuses on philosophical and practical aspects of energy, heat and entropy, with emphasis on reversibility and irreversibility, and goal to establish the concept of “reversible heat transfer,” regardless that heat transfer is a typical irreversible process.

2009 January 10-12© M. Kostic Heat transfer is Unique and Universal:  Heat transfer is a spontaneous irreversible process where all organized (structural) energies are disorganized or dissipated as thermal energy with irreversible loss of energy potential (from high to low temperature) and overall entropy increase.  Thus, heat transfer and thermal energy are unique and universal manifestation of all natural and artificial (man-made) processes, … and thus … are vital for more efficient cooling and heating in new and critical applications, including energy production and utilization, environmental control and cleanup, and bio-medical applications.

2009 January 10-12© M. Kostic Objective : … to emphasize known, but not so well-recognized issues about energy, heat and entropy, irreversibility and reversibility, as well as to put certain physical and philosophical concepts in perspective, as well as to put certain physical and philosophical concepts in perspective, and initiate discussion and arguments about the paper theme.

2009 January 10-12© M. Kostic The Grand Law of Nature The universe consists of local material structures in forced equilibrium and their interactions via forced fields. The forces are balanced at any time (including inertial - process rate forces) thus conserving momentum, while charges/mass and energy are transferred and conserved during forced displacement in space all the times, but energy is degraded as it is redistributed (transferred) from higher to lower non-equilibrium potential towards equilibrium (equi-partition of energy). (by M. Kostic) M. KosticM. Kostic

2009 January 10-12© M. Kostic Energy and Entropy DEFINITION of ENERGY: "Energy is a fundamental property of a physical system and refers to its potential to maintain a material system identity or structure (forced field in space) and to influence changes (via forced-displacement interactions, i.e. systems' re-structuring) with other systems by imparting work (forced directional displacement) or heat (forced chaotic displacement/motion of a system molecular or related structures). Energy exists in many forms: electromagnetic (including light), electrical, magnetic, nuclear, chemical, thermal, and mechanical (including kinetic, elastic, gravitational, and sound); where, for example, electro-mechanical energy may be kinetic or potential, while thermal energy represents overall potential and chaotic motion energy of molecules and/or related micro structure. "... Energy is the ‘‘building block’’ and fundamental property of matter and space and, thus, the fundamental property of existence. Energy exchanges or transfers are associated with all processes (or changes) and, thus, are indivisible from time.“ DEFINITION of ENTROPY: "Entropy is an integral measure of (random) thermal energy redistribution (due to heat transfer or irreversible heat generation) within a system mass and/or space (during system expansion), per absolute temperature level. Entropy is increasing from orderly crystalline structure at zero absolute temperature (zero reference) during reversible heating (entropy transfer) and entropy generation during irreversible energy conversion, i.e. energy degradation or random equi-partition within system material structure and space." (by M. Kostic)

2009 January 10-12© M. Kostic Energy: Property vs. Transfer

2009 January 10-12© M. Kostic More Definitions …

2009 January 10-12© M. Kostic Energy: Property vs. Transfer

2009 January 10-12© M. Kostic Energy: Forms of Heat Transfer

2009 January 10-12© M. Kostic Energy Interactions: Coupled, Adiabatic, and Caloric

2009 January 10-12© M. Kostic Energy & Entropy: Control Volume

2009 January 10-12© M. Kostic Heat Transfer : Heat transfer like any other energy transfer, may be achieved from any-to-any temperature level, and in limit be reversible, if temperature of an intermediary cyclic substance is adjusted as needed, using isentropic compression and expansion

2009 January 10-12© M. Kostic This is practically demonstrated… This is practically demonstrated in refrigeration and heat pump devices, and enables further increase in energy efficiency. A dual power-and-heat-pump cycle is introduced and analyzed here, to provide for reversible heat transfer. It may be considered as a reversible heat-transfer transformer, from-any-to-any temperature levels.

2009 January 10-12© M. Kostic Limits and Practical Potentials : The reversible heat transfer limits are the most efficient and demonstrate limiting potentials for practical heat transfer processes.

2009 January 10-12© M. Kostic REVERSIBILITY AND IRREVERSIBILITY: ENERGY TRANSFER AND DISORGANIZATION, RATE AND TIME, AND ENTROPY GENERATION Net-energy transfer is in one direction only, from higher to lower energy-potential, and the process cannot be reversed. Thus all real processes are irreversible in the direction of decreasing energy-potential (like pressure and temperature)

2009 January 10-12© M. Kostic Quasi-equilibrium Process : in limit, energy transfer process with infinitesimal potential difference (still from higher to infinitesimally lower potential, P). Then, if infinitesimal change of potential difference direction is reversed P+dP → P-dP with infinitesimally small external energy, since dP → 0, the process will be reversed too, which is characterized with infinitesimal entropy generation, and in limit, without energy degradation (no further energy disorganization) and no entropy generation thus achieving a limiting reversible process.

2009 January 10-12© M. Kostic REVERSIBILITY –Relativity of Time: Therefore, the changes are ‘fully reversible,’ and along with their rate of change and time, totally irrelevant, as if nothing is effectively changing (no permanent-effect to the surroundings or universe) The time is irrelevant as if it does not exist, since it could be reversed or forwarded at will and at no ‘cost’ (no permanent change and, thus, relativity of time).

2009 January 10-12© M. Kostic REVERSIBILITY –Relativity of Time (2): Real time cannot be reversed, it is a measure of permanent changes, like irreversibility, which is in turn measured by entropy generation. In this regard the time and entropy generation of the universe have to be related.

2009 January 10-12© M. Kostic  It is possible to obtain work from the equal amount of disorganized thermal energy or heat, if such process is reversible.  For example:  reversible expansion at constant internal energy, e.g. isothermal ideal-gas expansion, (dW=dQ), see Fig. 1a, and  reversible adiabatic expansion (dW=-dU).  Work potential is lost during unrestricted expansion (Fig. 1b)

2009 January 10-12© M. Kostic Heat Transfer and Irreversibility: ENTROPY TRANSFER and GENERATION

2009 January 10-12© M. Kostic Entropy … … entropy of a system for a given state is the same, regardless whether it is reached by reversible heat transfer or irreversible heat or irreversible work transfer. However, the source entropy will decrease to a smaller extent over higher potential, thus resulting in overall entropy generation for the two interacting systems.

2009 January 10-12© M. Kostic Entropy … We could consider a system internal thermal energy and entropy, as being accumulated from absolute zero level, by disorganization of organized or higher level energy potential with the corresponding entropy generation. Thus entropy as system property is associated with its thermal energy (but also space). Thus entropy as system property is associated with its thermal energy (but also space).

2009 January 10-12© M. Kostic Entropy Primer: entropy could be transferred in reversible processes along with heat transfer, and additionally generated if work or thermal energy are disorganized at the lower thermal potential during irreversible processes. Once a process completes, any generated entropy due to irreversibility becomes (permanent) system property and cannot be reversed by itself (thus, a permanent change).

2009 January 10-12© M. Kostic Entropy Primer (2): Thus, entropy transfer is due to reversible heat transfer and could be ether positive or negative, while entropy generation is always positive and always due to irreversibility.

2009 January 10-12© M. Kostic Entropy Primer (3): If heat or work at higher potential (temperature or pressure) than necessary, is transferred to a system, the energy at excess potential will dissipate spontaneously to a lower potential (if left alone) before new equilibrium state is reached, with entropy generation, i.e. increase of entropy displacement over a lower potential. A system will ‘accept’ energy at minimum necessary (infinitesimally higher) or higher potential. Furthermore, the higher potential energy will dissipate and entropy increase will be the same as with minimum necessary potential, like in reversible heating process, i.e.: However, the source entropy will decrease to a smaller extent over higher potential, thus resulting in overall entropy generation for the two interacting systems,

2009 January 10-12© M. Kostic Reversible Heat Transfer and Practical Potentials: Dual Power-Heat Pump cycle

2009 January 10-12© M. Kostic Coefficients of Performance for Three Typical Cases of Reversible Heat Transfer the most efficient reversible heat transfer from system H at higher temperature T H to system L at lower temperature T L as presented on Fig. 3b may be obtained (as limiting case) by using a dual power-and-heat-pump cycle (PHP), which is governed by the following conditions (W PC = W HPC )

2009 January 10-12© M. Kostic Conclusion …   Energy is a fundamental concept indivisible from matter and space, and energy exchanges or transfers are associated with all processes (or changes), thus indivisible from time.   Energy is “the building block” and fundamental property of matter and space, thus fundamental property of existence. For a given matter (system) and space (volume) energy defines the system equilibrium state, and vice versa.   For a given system state (structure and phase) addition of energy will tend (spontaneously) to randomly distribute (disorganize) over the system microstructure and space it occupies, called internal thermal energy, increasing energy-potential (temperature) and/or energy-displacement (entropy), and vice versa.

2009 January 10-12© M. Kostic Conclusion (2):   Energy and mass are conserved within interacting systems (all of which may be considered as a combined isolated system not interacting with its surrounding systems), and energy transfer (in time) is irreversible (in one direction) from higher to lower potential only, which then results in continuous generation (increase) of energy-displacement, called entropy generation, which is fundamental measure of irreversibility, or permanent changes, the latter also measured with the passing time.   Reversible energy transfer is only possible as limiting case of irreversible energy transfer at infinitesimally small energy- potential differences, thus in quasiequilibrium processes, with conservation of entropy. Since such changes are reversible, they are not permanent (could be reversed without leaving any relevant or effect on the surroundings) and, along with time, irrelevant (NOT permanent).

2009 January 10-12© M. Kostic Conclusion (3):   Entropy may be transferred from system to system by reversible heat transfer and also generated due to irreversibility of heat and work transfer.   Heat transfer, like any other energy transfer, may be achieved from any-to-any temperature level (performed in real power and refrigeration cycles), and in limit be reversible, if temperature of an intermediary cyclic substance is adjusted as needed, using isentropic compression and expansion. The reversible heat transfer limits are the most efficient and demonstrate limiting potentials for practical heat transfer processes.

2009 January 10-12© M. Kostic Conclusion (4):   The “Dual Power-Heat Pump Cycle,” introduced here, may be considered as a reversible heat-transfer transformer, from-any-to-any temperature levels.   The simple analysis of this dual, combined cycle (Eq. 4. and Fig. 3b), to achieve reversible heat transfer only (from higher to lower temperature system) and without any net-work produced or utilized,   Presented emphasis (with analysis) of underlying physical phenomena, including several hypothesis, is intended contribution of this paper.

2009 January 10-12© M. Kostic Heat Transfer Potentials : Minimize Irreversibilities and Entropy generation Enhanced Heat-Transfer Transformer Power-Heat Pump cycle Key Words: Conservation with Optimization (to increase efficiency) Cogeneration (to minimize irreversibility) Insulation (to minimize losses) Regeneration (to recover losses) Enabled by Sophistication of NEW Knowledge and Technology

2009 January 10-12© M. Kostic For further Info you may contact Prof. Kostic at: or on the Web: Prof. M. Kostic Mechanical Engineering Mechanical Engineering NORTHERN ILLINOIS UNIVERSITY