Exergy Analysis of Alternative Configurations of Biomass-Based Light Olefin Production System with a Combined-Cycle Scheme via Methanol Intermediate

Thermodynamic performance of three conceptual systems for biomass-derived olefin production with electricity cogeneration was studied and compared via exergy analysis at the levels of system, subsystem and operation unit. The base case was composed of the subsystems of gasification, raw fuel gas adjustment, methanol/light olefin synthesis and steam & power generation, etc. The power case and fuel case were designed as the combustion of a fraction of gasification gas to increase power generation and the recycle of a fraction of synthesis tail gas to increase olefin production, respectively. It was found that the subsystems of gasification and steam & power generation contribute ca. 80% of overall exergy destruction for each case, of which gasifier and combustor are the main exergy destruction sources, due to the corresponding chemical exergy degrading of biomass and fuel gas. The low efficiency of 33.1% for the power case could be attributed to the significant irreversibility of the combustor, economizer, and condenser in the combined-cycle subsystem. The effect of the tail gas recycle ratio, moisture content of feedstock, and biomass type was also investigated to enhance system exergy performance, which could be achieved by high recycle ratio, using dry biomass and the feedstock with high carbon content. High system efficiency of 38.9% was obtained when oil palm shell was used, which was 31.7% for rice husk due to its low carbon content.

Energy, Conventional and Advanced Exergy Analysis of Low Temperature Geothermal Binary-flashing Cycle Using Zeotropic Mixtures

Due to deep utilization of geobrine and high net power output, binary flashing cycle (BFC) is deemed to be the future geothermal energy power generation technology. The BFC using R245/R600a zeotropic mixtures is presented in this paper. The thermodynamic model of the system is built, and energy, conventional and advanced exergy analysis are carried out, to reveal the real optimization potential. It is demonstrated that the optimal composition mass fraction of R245fa and dryness of working fluid at the evaporator outlet ranges are 0.30~0.50 and 0.40~0.60, considering the thermodynamic performance and the flammability of the mixtures, simultaneously. Conventional exergy analysis indicates that the maximum exergy destruction occurs in condenser, followed by expander, evaporator, flashing tank, preheater, high-pressure pump and low-pressure pump. While the advanced exergy analysis reveals that the expander should be given the first priority for optimization, followed by condenser and evaporator. The BFC has a large potential for improvement due to higher avoidable exergy destruction, about 48.6% of the total system exergy destruction can be reduced. And the interconnections among system components are not very strong, owing to small exogenous exergy destructions. It also demonstrates the effectiveness of advanced exergy analysis, and the approach can be extended to other energy conversion systems to maximize the energy and exergy savings for sustainable development.

Exergetic Performance Coefficient Analysis and Optimization of a High-Temperature Proton Exchange Membrane Fuel Cell

Performance of a high-temperature proton exchange membrane fuel cell (HT-PEMFC) and the influence of different parameters on HT-PEMFC is analyzed in this study. Firstly, mathematical expression for energy efficiency, power density, exergy destruction and exergetic performance coefficient (EPC) are derived. Then, the relationship between the dimensionless power density, exergy destruction rate, exergetic performance coefficient (EPC) and energy efficiency is compared. Furthermore, the effect of flow rate, doping level, inlet pressure and film thickness are considered to evaluate the performance of HT-PEMFC. Results show that EPC not only considers exergetic loss rate to minimize exergetic loss, but also considers the power density of HT-PEMFC to maximize its power density and improve its efficiency, so EPC represents a better performance criterion. In addition, increasing inlet pressure and doping level can improve EPC and energy efficiency, respectively.

Assessment of a Geothermal Combined System with an Organic Rankine Cycle and Multi-effect Distillation Desalination

An analysis is reported of a geothermal-based electricity-freshwater system in which an organic Rankine cycle is integrated with a multi-effect distillation desalination unit. The system is driven by geothermal hot water extracted from the production well. Mass, energy, entropy, and exergy rate balances are written for all system components, as are energy and exergy efficiency expressions for each subsystem. The exergy destruction rate associated with the temperature and chemical disequilibrium of the freshwater and brine with the reference environment are taken into account to reveal accurate results for irreversibility sources within the desalination process. The developed thermodynamic model is simulated using thermodynamic properties of the working fluids (i.e., ammonia, seawater, distillate, and brine) at each state point. A sustainability analysis is performed that connects exergy and environmental impact concepts. That assessment expresses the extent of the contribution of the system to sustainable development and reduced environmental impact, using exergy methods. Results of the sustainability analysis indicate that, with an increase in the reference environment temperature from 20 to 35 $$^\circ{\rm C}$$ ∘ C , the exergy destruction rate decreases for the multi-effect distillation and organic Rankine cycle systems respectively from 6474 to 4217 kW and from 16,270 to 13,459 kW. Also, the corresponding sustainability index for the multi-effect distillation and organic Rankine cycle systems increases from 1.16 to 1.2 and 1.5–1.6, respectively, for the same increase in reference environment temperature.

Performance Evaluation of a Full-Scale Fused Magnesia Furnace for MgO Production Based on Energy and Exergy Analysis

A three-electrode alternating current fused magnesia furnace (AFMF) with advanced control technology was evaluated by combined energy and exergy analysis. To gain insight into the mass flow, energy flow and exergy efficiency of the present fused magnesia furnace, the exergy destruction was analysed to study the energy irreversibility of the furnace. Two different production processes, the magnesite ore smelting process (MOP) and light-calcined magnesia process (LMP), are discussed separately. Two methods were carried out to improve LMP and MOP; one of which has been applied in factories. The equipment consists of an electric power supply system, a light-calcined system and a three-electrode fused magnesia furnace. All parameters were tested or calculated based on the data investigated in industrial factories. The calculation results showed that for LMP and MOP, the mass transport efficiencies were 16.6% and 38.3%, the energy efficiencies were 62.2% and 65.5%, and the exergy destructions were 70.5% and 48.4%, respectively. Additionally, the energy efficiency and exergy efficiency of the preparation process of LMP were 39.4% and 35.6%, respectively. After the production system was improved, the mass transport efficiency, energy efficiency and exergy destruction were determined.

Thermodynamic Evaluation of a Direct Expansion Ground-Sourced Heat Pump with Horizontal Ground Heat Exchangers Using Advanced Exergy Analysis

This paper deals with the advanced exergetic analysis of a horizontal direct-expansion ground sourced CO2 heat pump operating in a transcritical cycle. The cycle is thermodynamically modeled in Engineering Equation Solver (EES) considering the pressure drops in both high and low temperature heat exchangers, and the system is to provide a fixed heating load. Conventional exergy analysis orderly suggests a compressor, expansion valve, gas cooler and ground heat exchanger to be considered for system improvement, while tracing exergy destruction of all components in detail demonstrates true improvement potential of each and all components and the system as a whole and offers a different order. Advanced exergy analysis points out that the compressor is directly and indirectly responsible for 56% of the overall exergy destruction generated in the cycle, confirming the detrimental role of this component in the system. The second influential component is recognized to be a ground heat exchanger accounting for 20% exergy destruction of the compressor as well as submitting 89% avoidability in its own exergy destruction, and expansion valve proves to be the last option for system improvement according to this analysis.

Thermodynamic Limitations and Exergy Analysis of Brackish Water Reverse Osmosis Desalination Process

The reverse osmosis (RO) process is one of the most popular membrane technologies for the generation of freshwater from seawater and brackish water resources. An industrial scale RO desalination consumes a considerable amount of energy due to the exergy destruction in several units of the process. To mitigate these limitations, several colleagues focused on delivering feasible options to resolve these issues. Most importantly, the intention was to specify the most units responsible for dissipating energy. However, in the literature, no research has been done on the analysis of exergy losses and thermodynamic limitations of the RO system of the Arab Potash Company (APC). Specifically, the RO system of the APC is designed as a medium-sized, multistage, multi pass spiral wound brackish water RO desalination plant with a capacity of 1200 m3/day. Therefore, this paper intends to fill this gap and critically investigate the distribution of exergy destruction by incorporating both physical and chemical exergies of several units and compartments of the RO system. To carry out this study, a sub-model of exergy analysis was collected from the open literature and embedded into the original RO model developed by the authors of this study. The simulation results explored the most sections that cause the highest energy destruction. Specifically, it is confirmed that the major exergy destruction happens in the product stream with 95.8% of the total exergy input. However, the lowest exergy destruction happens in the mixing location of permeate of the first pass of RO desalination system with 62.28% of the total exergy input.

Optimization of the exergy efficiency, exergy destruction, and engine noise index in an engine with two direct injectors using NSGA-II and artificial neural network

Direct Dual Fuel Stratification (DDFS) strategy is a novel Low Temperature Combustion (LTC) strategy that has comparable thermal efficiency to the Reactivity Controlled Compression Ignition (RCCI) strategy, while it offers more control over the combustion process and the rate of heat release. The DDFS strategy uses two direct injectors for the low- and high-reactivity fuels (gasoline and diesel) to benefit from the RCCI concept. In this study, the injection strategy of the injectors of a gasoline/diesel DDFS engine was optimized from the thermodynamic perspective to maximize exergy efficiency and minimize exergy destruction and an engine noise index. An artificial neural network was developed with 576 samples from a CFD code to predict the DDFS mode behavior, and the non-dominated sorting genetic algorithm (NSGA-II) was used to obtain the Pareto Front and the optimal solutions. Compared to the base case, the exergy efficiency of the optimal cases increased by up to 2%, exergy destruction and Peak Pressure Rise Rate (PPRR) reduced by about 2.3%, and 2 bar/deg, respectively, in the optimal solutions. NOX and soot emissions were reduced by 40% and 35%, respectively, in the best-case scenarios.