Entropy generation rate minimization for hydrocarbon synthesis reactor from carbon dioxide and hydrogen

Entropy Generation Minimization (EGM) in High Temperature Superconducting (HTS) cables for optimization of thermohydraulic performance

Based on the finite time thermodynamics theory, the entransy theory and the entropy theory, the Stirling cycles under different conditions are analyzed and optimized with the maximum output power as the target in this paper. The applicability of entransy loss (EL), entransy dissipation (ED), entropy generation (EG), entropy generation number (EGN) and modified entropy generation number (MEGN) to the system optimization is investigated. The results show that the maximum EL rate corresponds to the maximum power output of the cycle working under the infinite heat reservoirs whose temperatures are prescribed, while the minimum EG rate and the extremum ED rate do not. For the Stirling cycle working under the finite heat reservoirs provided by the hot and cold streams whose inlet temperatures and the heat capacity flow rates are prescribed, the maximum EL rate, the minimum EG rate, the minimum EGN and the minimum MEGN all correspond to the maximum power output, but the extremum ED rate does not. When the heat capacity flow rate of the hot stream increases, the power output, the EL rate, the EG rate and the ED rate increase monotonously, while the EGN and the MEGN decrease first and then increase. The EL has best consistency in the power output optimizations of the Stirling cycles discussed in this paper.

Constructal entropy generation rate minimization for asymmetric vascular networks in a disc-shaped body

Based on the theory of finite-time thermodynamics (FTT), the effects of three design parameters, that is, inlet temperature, inlet pressure, and inlet total mole flow rate, of a tubular plug-flow sulfuric acid decomposition reactor on the total entropy generation rate (EGR) and SO2 yield are analyzed firstly. One can find that when the three design parameters are taken as optimization variables, the minimum total EGR and the maximum SO2 yield of the reference reactor restrict each other, i.e., the two different performance objectives cannot achieve the corresponding extremum values at the same time. Then, the second-generation non-dominated solution sequencing genetic algorithm (NSGA-II) is further used to pursue the minimum total EGR and the maximum SO2 yield of the reference reactor by taking the three parameters as optimization design variables. After the multi-objective optimization, the reference reactor can be Pareto improved, and the total EGR can be reduced by 9% and the SO2 yield can be increased by 14% compared to those of the reference reactor. The obtained results could provide certain theoretical guidance for the optimal design of actual sulfuric acid decomposition reactors.

Entropy generation is often used as a figure of merit in thermodynamic cycle optimizations. In this paper, it is shown that the applicability of the minimum entropy generation method to optimizing output power is conditional. The minimum entropy generation rate and the minimum entropy generation number do not correspond to the maximum output power when the total heat into the system of interest is not prescribed. For the cycles whose working medium is heated or cooled by streams with prescribed inlet temperatures and prescribed heat capacity flow rates, it is theoretically proved that both the minimum entropy generation rate and the minimum entropy generation number correspond to the maximum output power when the virtual entropy generation induced by dumping the used streams into the environment is considered. However, the minimum principle of entropy generation is not tenable in the case that the virtual entropy generation is not included, because the total heat into the system of interest is not fixed. An irreversible Carnot cycle and an irreversible Brayton cycle are analysed. The minimum entropy generation rate and the minimum entropy generation number do not correspond to the maximum output power if the heat into the system of interest is not prescribed.

Investigation on the thermohydraulic performance of high temperature superconducting (HTS) cables with heat loads using Entropy Generation Minimization (EGM) approach

On the equivalence between the minimum entropy generation rate and the maximum conversion rate for a reactive system

Constructal entropy generation rate minimization for cylindrical pin-fin heat sinks

Optimization of a separator assisted two-phase thermosyphon loop by using entropy generation analysis

The concepts of entransy and entropy are applied to the analyses of the irreversible Carnot engines based on the finite time thermodynamics. Taking the maximum output power and the maximum heat-work conversion efficiency (HWCE) as objectives, the applicability of the entransy theory and the entropy generation minimization method to the optimizations is investigated. For the entransy theory, the results show that the maximum entransy loss rate always relates to the maximum output power, while the maximum entransy loss coefficient always leads to the maximum HWCE for all the cases discussed in this paper. For the concept of entropy generation, the maximum entropy generation rate corresponds to the maximum output power when the Carnot engine works between infinite heat reservoirs, while the entropy generation number cannot be defined in this case. When the Carnot engine works between the finite heat reservoirs provided by streams, the minimum entropy generation rate corresponds to the maximum output power with prescribed heat flow capacity rates and inlet temperatures of the streams, while the minimum entropy generation number corresponds to the maximum HWCE. When the heat capacity flow rate of the hot stream is not prescribed, the entropy generation rate increases with increasing output power, while the entropy generation number decreases with increasing HWCE. When the inlet temperature of the hot stream is not prescribed, the entropy generation rate increases with increasing output power, and the entropy generation number also increases with increasing HWCE.

Thermal design and optimization for reverse water gas shift (RWGS) reactors is particularly important to fuel synthesis in naval or commercial scenarios. The RWGS reactor with irreversibilities of heat transfer, chemical reaction and viscous flow is studied based on finite time thermodynamics or entropy generation minimization theory in this paper. The total entropy generation rate (EGR) in the RWGS reactor with different boundary conditions is minimized subject to specific feed compositions and chemical conversion using optimal control theory, and the optimal configurations obtained are compared with three reference reactors with linear, constant reservoir temperature and constant heat flux operations, which are commonly used in engineering. The results show that a drastic EGR reduction of up to 23% can be achieved by optimizing the reservoir temperature profile, the inlet temperature of feed gas and the reactor length simultaneously, compared to that of the reference reactor with the linear reservoir temperature. These optimization efforts are mainly achieved by reducing the irreversibility of heat transfer. Optimal paths have subsections of relatively constant thermal force, chemical force and local EGR. A conceptual optimal design of sandwich structure for the compact modular reactor is proposed, without elaborate control tools or excessive interstage equipment. The results can provide guidelines for designing industrial RWGS reactors in naval or commercial scenarios.

In this paper, the performance of a concentrating photovoltaic/thermal solar system is numerically analyzed with a mathematical and physical model. The variations of the electrical efficiency and the thermal efficiency with the operation parameters are calculated. It is found that the electrical efficiency increases at first and then decreases with increasing concentration ratio of the sunlight, while the thermal efficiency acts in an opposite manner. When the velocity of the cooling water increases, the electrical efficiency increases. Considering the solar system, the surface of the sun, the atmosphere and the environment, we can get a coupled energy system, which is analyzed with the entropy generation minimization and the entransy theory. This is the first time that the entransy theory is used to analyze photovoltaic/thermal solar system. When the concentration ratio is fixed, it is found that both the minimum entropy generation rate and the maximum entransy loss rate lead to the maximum electrical output power, while both the minimum entropy generation numbers and the maximum entransy loss coefficient lead to the maximum electrical efficiency. When the concentrated sunlight is not fixed, it is shown that neither smaller entropy generation rate nor larger entransy loss rate corresponds to larger electrical output power. Smaller entropy generation numbers do not result in larger electrical efficiency, either. However, larger entransy loss coefficient still corresponds to larger electrical efficiency.

Entropy generation in condensation in the presence of high concentrations of noncondensable gases

The reverse water gas shift (RWGS) reaction can convert CO₂ into useful industrial raw materials, which meets the requirement and trend of carbon-neutral energy development. For the one-dimensional tubular plug flow RWGS reactor, the heat transfer process was assumed to obey the linear phenomenological heat transfer law &lang;<i>q</i>&prop;&Delta;(&Tau;^-1)&rang;. Under the conditions that all of the CO yield, inlet temperature, inlet pressure and inlet compositions were given and the temperature of the heat source outside the tube was fully controllable, the minimum total entropy generation rate (EGR) of the RWGS reactor and the corresponding optimal temperature distribution of the heat source outside the tube were solved by applying finite time thermodynamics and optimal control theory. The optimization results were further compared to the performances of two reference reactors with the constant and the linear heat source temperatures and those for the case with Newtonian heat transfer law &lang;<i>q</i>&prop;&Delta;(&Tau;)&rang;. The results show that optimizing the heat-source temperature distribution could reduce the total EGR of the RWGS reactor by more than 48% compared to those of the two reference reactors, and the main reduction is the EGR in heat transfer and chemical reaction processes; heat transfer laws have significant effects on the minimum total EGR of the RWGS reactor and the corresponding optimal temperature distribution of the heat source outside the tube. The obtained results in this paper have certain guiding significance for the design of RWGS reactors in actual engineering.

Constructal optimization for line-to-line vascular based on entropy generation minimization principle