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The tri-reforming process was employed for syngas production from biogas at elevated pressures in this study. In the tri-reforming process, air and water were added simultaneously as reactants in addition to the main biogas components. The effects of various operating parameters such as pressure, temperature and reactant composition on the reaction performance were studied numerically. From the simulated results, it was found that methane and carbon dioxide conversions can be enhanced and a higher hydrogen/carbon monoxide ratio can be obtained by increasing the amount of air. However, a decreased hydrogen yield could result due to the reverse water–gas shift reaction. A higher level of methane conversion and hydrogen/carbon monoxide ratio can be obtained with increased water addition. However, negative carbon dioxide conversion could result due to the water–gas shift and reverse carbon dioxide methanation reactions. The dry reforming reaction resulting in positive carbon dioxide conversion can only be found at a high reaction temperature. For all cases studied, low or negative carbon dioxide conversion was found because of carbon dioxide production from methane oxidation, water–gas shift, and reverse carbon dioxide methanation reactions. It was found that carbon dioxide conversion can be enhanced in the tri-reforming process by a small amount of added water. It was also found that first-law efficiency increased with increased reaction temperature because of higher hydrogen and carbon monoxide yields. Second-law efficiency was found to decrease with increased temperature because of higher exergy destruction due to a more complete chemical reaction at high temperatures.

Increasing global demand for fresh water is driving the development and implementation of a wide variety of seawater desalination technologies. Entropy generation analysis, and specifically, Second Law efficiency, is an important tool for illustrating the influence of irreversibilities within a system on the required energy input. When defining Second Law efficiency, the useful exergy output of the system must be properly defined. For desalination systems, this is the minimum least work of separation required to extract a unit of water from a feed stream of a given salinity. In order to evaluate the Second Law efficiency, entropy generation mechanisms present in a wide range of desalination processes are analyzed. In particular, entropy generated in the run down to equilibrium of discharge streams must be considered. Physical models are applied to estimate the magnitude of entropy generation by component and individual processes. These formulations are applied to calculate the total entropy generation in several desalination systems including multiple effect distillation, multistage flash, membrane distillation, mechanical vapor compression, reverse osmosis, and humidification-dehumidification. Within each technology, the relative importance of each source of entropy generation is discussed in order to determine which should be the target of entropy generation minimization. As given here, the correct application of Second Law efficiency shows which systems operate closest to the reversible limit and helps to indicate which systems have the greatest potential for improvement.

Exergy efficiency is viewed as the degree of approaching reversible operation, with a value of 100 percent for a reversible process characterized by zero entropy generation or equivalently zero exergy destruction since Xdestroyed = T0Sgen. As such, exergy efficiency becomes a measure of thermodynamic perfection. There are different conceptual definitions of exergy efficiency, the most common ones being (1) the ratio of exergy output to exergy input ηex = Xoutput/Xinput = 1 − (Xdestroyed + Xloss)/Xinput, (2) the ratio of the product exergy to fuel exergy ηex = Xproduct/Xfuel = 1 − (Xdestroyed + Xloss)/Xfuel, and (3) the ratio of exergy recovered to exergy expended ηex = Xrecovered/Xexpended = 1 − Xdestroyed/Xexpended. Most exergy efficiency definitions are formulated with steady-flow systems in mind, and they are generally applied to systems in steady operation such as power plants and refrigeration systems whose exergy content remains constant. If these definitions are to be used for closed and unsteady-flow systems, the terms need to be interpreted broadly to account for the exergy change of the systems as exergy input or output, as appropriate. In this paper, general exergy efficiency relations are developed for closed and unsteady-flow systems and their use is demonstrated with applications. Also, the practicality of the use of the term exergy loss Xloss is questioned, and limitations on the definition ηex = Wact,out/Wrev,out are discussed.

Powering desalination by waste heat is often proposed to mitigate energy consumption and environmental impact; however, thorough technology comparisons are lacking in the literature. This work numerically models the efficiency of six representative desalination technologies powered by waste heat at 50, 70, 90, and 120 °C, where applicable. Entropy generation and Second Law efficiency analysis are applied for the systems and their components. The technologies considered are thermal desalination by multistage flash (MSF), multiple effect distillation (MED), multistage vacuum membrane distillation (MSVMD), humidification-dehumidification (HDH), and organic Rankine cycles (ORCs) paired with mechanical technologies of reverse osmosis (RO) and mechanical vapor compression (MVC). The most efficient technology was RO, followed by MED. Performances among MSF, MSVMD, and MVC were similar but the relative performance varied with waste heat temperature or system size. Entropy generation in thermal technologies increases at lower waste heat temperatures largely in the feed or brine portions of the various heat exchangers used. This occurs largely because lower temperatures reduce recovery, increasing the relative flow rates of feed and brine. However, HDH (without extractions) had the reverse trend, only being competitive at lower temperatures. For the mechanical technologies, the energy efficiency only varies with temperature because of the significant losses from the ORC.

Increasing global demand for fresh water is driving the development and implementation of a wide variety of seawater desalination technologies driven by different combinations of heat, work, and chemical energy. This paper develops a consistent basis for comparing the energy consumption of such technologies using Second Law efficiency. The Second Law efficiency for a chemical separation process is defined in terms of the useful exergy output, which is the minimum least work of separation required to extract a unit of product from a feed stream of a given composition. For a desalination process, this is the minimum least work of separation for producing one kilogram of product water from feed of a given salinity. While definitions in terms of work and heat input have been proposed before, this work generalizes the Second Law efficiency to allow for systems that operate on a combination of energy inputs, including fuel. The generalized equation is then evaluated through a parametric study considering work input, heat inputs at various temperatures, and various chemical fuel inputs. Further, since most modern, large-scale desalination plants operate in cogeneration schemes, a methodology for correctly evaluating Second Law efficiency for the desalination plant based on primary energy inputs is demonstrated. It is shown that, from a strictly energetic point of view and based on currently available technology, cogeneration using electricity to power a reverse osmosis system is energetically superior to thermal systems such as multiple effect distillation and multistage flash distillation, despite the very low grade heat input normally applied in those systems.

In the study, the focus was on evaluating the second law efficiency of a heat exchanger tube operating under continuous heat flux and turbulent flow conditions. The evaluation involved the use of a hybrid GnP and Fe 3 O 4 and modified coiled wire as passive heat transfer enhancement techniques. The primary objective was to investigate the impact of these combined techniques on thermal and hydraulic performance, entropy generation number, Bejan number and second law efficiency. To achieve this, different mass fractions of GnP and Fe 3 O 4 nanoparticles were used in the hybrid nanofluid, along with two forms of modified coiled wire: barrel type and hourglass type. The experimental results indicated that the utilization of hybrid nanofluids and modified helical inserts led to a noticeable improvement in the second law efficiency of the heat exchanger tube. However, it was observed that the differences in entropy generation number and Bejan number between the barrel and hourglass types were not significant, mainly due to higher frictional losses associated with the latter. The highest recorded second law efficiency was 0.416, while the lowest entropy generation number was 0.118. These values were achieved through the combined use of GnP and Fe 3 O 4 with a mass fraction of 0.4% and a barrel-type coiled wire insert with a pitch ratio of 0.5.

Exergy efficiency is a measure of thermodynamic perfection. A device that operates reversibly has an exergy efficiency of 100 percent and is said to be thermodynamically perfect. A reversible process involves zero entropy generation and thus zero exergy destruction since Xdestroyed = T0Sgen. Exergy efficiency is generally defined as the ratio of exergy output to exergy input ηex = Xoutput/Xinput = 1 − (Xdestroyed + Xloss)/Xinput or the ratio of exergy recovered to exergy expended ηex = Xrecovered/Xexpended = 1 − Xdestroyed/Xexpended. In this paper, exergy efficiency relations are obtained first for a general steady-flow system using both approaches. Then, explicit general relations are obtained for common steady-flow devices, such as turbines, compressors, pumps, nozzles, diffusers, valves and heat exchangers, as well as heat engines, refrigerators, and heat pumps. For power and refrigeration cycles, five different forms of exergy efficiency relations are developed, and their equivalence is demonstrated. With the unified approach presented here and the insights provided, the controversy and confusion associated with different exergy efficiency definitions are largely alleviated.

This work extends the steady flow Lenoir cycle within finite-time thermodynamics (FTT) by incorporating heat transfer irreversibilities through the ε−NTU formalism and a non-isentropic expansion modeled via the expander isentropic efficiency ηE. The total conductance UT (sum for the two heat exchangers) is partitioned between hot and cold units using uL=UL/UT, with UT=UH+UL. For each triplet (τ=TH/TL, UL, UT), we closed the cycle by determining T1, the working fluid temperature at the cooler outlet and heater inlet, T2, the heater outlet and expander inlet, and T3, the expander outlet and cooler inlet. Using these states, we compute the heat rates Q˙12, Q˙31 and the net power P. In addition to the thermal efficiency η, the following extended objective functions are evaluated: the efficient power EF, the ecological efficiency ϕ, and the second law efficiency ηII. Parametric sweeps on uL for τ ϵ 3.25,3.75 and UT ϵ 2.5,5.0,7.5,10 kW show unimodal curves for P(uL) and maxima. A robust result places the optima of P, η, EF, ϕ, and ηII in a distribution band at uL~0.6. This guideline offers clear design guidance for allocating exchange area in heat recovery and microgeneration, maximizing power, high η, and exergetic utilization with contained entropic penalty.

This work extends the steady flow Lenoir cycle within finite-time thermodynamics (FTT) by incorporating heat transfer irreversibilities through the ε−NTU formalism and a non-isentropic expansion modeled via the expander isentropic efficiency ηE . The total conductance UT (sum for the two heat exchangers) is partitioned between hot and cold units using uL=UL/UT , with UT=UH+UL . For each triplet ( τ=TH/TL , UL , UT ), we closed the cycle by determining T1 , the working fluid temperature at the cooler outlet and heater inlet, T2 , the heater outlet and expander inlet, and T3 , the expander outlet and cooler inlet. Using these states, we compute the heat rates Q˙ 12 , Q˙ 31 and the net power P . In addition to the thermal efficiency η , the following extended objective functions are evaluated: the efficient power EF , the ecological efficiency ϕ , and the second law efficiency ηII . Parametric sweeps on uL for τ ϵ  3.25,3.75 and UT ϵ  2.5,5.0,7.5,10   k W show unimodal curves for P( uL ) and maxima. A robust result places the optima of P , η , EF , ϕ , and ηII in a distribution band at uL 0.6 . This guideline offers clear design guidance for allocating exchange area in heat recovery and microgeneration, maximizing power, high η, and exergetic utilization with contained entropic penalty.

In this study, the energetic and exergetic performance of a household refrigerator with a unique compact heat exchanger composed of a mini-channel flat tube condenser and offset strip fins (OSF) is investigated experimentally by considering varying amounts of R600a (48–64 g) and capillary tube lengths (2800, 3300, and 3800 mm). The experiments have been performed for two refrigerators involving two different innovative condensers composed of mini-channel flat tube and offset strip fins which have varying orientations instead of conventional wire-on-tube condensers. The experiments were conducted in a climatic chamber at a temperature of 25 ± 0.5°C. According to the standard of IEC 62552:2015, the target temperatures of the fresh food and freezer compartments are set at 4°C and −18°C, respectively. Specific and relative exergy destructions of each component, total exergy destruction, the coefficient of performance, and second-law efficiency of the overall system with two different mini-channel condensers are reported for varying amounts of refrigerant and capillary tube lengths. The major concern of the investigation is to reveal the key components contributing to the degradation of the overall performance of the unique design. It is observed that the evaporator, which has a ratio of 52–69% in total exergy destruction within all investigated cases, is the most exergy destructive component and the exergy destruction of the evaporator decreases when the amount of R600a increases. The mini-channel condensers are the second exergy destructive components with a ratio of 19–22%. As an important outcome of this unique design’s investigation, exergy destruction of the condenser and compressor does not show a monotonic change with respect to the refrigerant amount and capillary length. Besides, the highest coefficient of performance and second-law efficiency are obtained when the amount of R600a is 48–50 g in all three capillary tube lengths.