Exergy Efficiency Of System Research Articles

Thermodynamic modeling and analysis of cascade heat pump enhanced by vapor injection for steam production

ABSTRACT The cascade heat pump (CHP) system shows significant potential in generating high-temperature steam by upgrading the ambient heat, whose energy efficiency is constrained by the large temperature difference between the air source and terminal output. This paper integrates vapor injection (VI) technology into both low-temperature and high-temperature stages of CHP system. A thermodynamic model of the vapor injection cascade heat pump (VI-CHP) system is constructed utilizing Aspen Hysys. Furthermore, the effects of intermediate temperature, VI separators’ temperatures, evaporator superheat and condenser subcooling on COP are analyzed. Simulation results demonstrated that the VI-CHP with dual-stage VI effectively enhances efficiency, reaching a nominal COP of 2.17, which indicates 8.6% improvement over the CHP system without VI. This paper analyzes the variation of system operation at intermediate temperatures from 45°C to 65°C, low-temperature flasher outlet steam temperatures from 33°C to 45°C, high-temperature flasher outlet steam temperatures from 74°C to 86°C, and superheat and subcooling degrees from 0°C to 10°C. Sensitivity analysis shows that maximum COP can attain 2.67 under the specified operational parameters. Finally, the impact of separators’ temperatures on the exergy efficiency is discussed. It is revealed that the exergy efficiency of the proposed system can reach 2.67.

Energy, exergy, exergoeconomic, and environmental (4E) analyses of a combined system comprising reformed methanol high-temperature proton exchange membrane fuel cells and absorption refrigeration cycle

ABSTRACT 4E (energy, exergy, exergoeconomic, and environmental) analyses of a reformed methanol high-temperature proton exchange membrane fuel cell (RM HT-PEMFC) system identify inefficiencies, assess associated costs, and evaluate environmental impacts. However, such studies remain limited in the literature, particularly those exploring the impact of the operating parameters of the methanol steam reforming in the combined RM HT-PEMFC and single-effect absorption refrigeration cycle (ARC). This study addresses this gap by analyzing a tri-generation consisting of the RM HT-PEMFC and the single-effect ARC. Simulation results indicate a total exergy destruction of 21.65 kW, a total cost rate of 8.93 $/h, an exergoeconomic factor of 43.22%, and an exergy efficiency of 33.06% in the baseline case. Notably, the stack and burner account for the highest irreversibility, contributing 67.75% of total exergy destruction and 55.71% of the total cost rate. Parametric studies on four key variables – current density, stack temperature, reformer temperature, and steam-to-carbon ratio – reveal that higher system exergy efficiency is generally associated with lower carbon dioxide emissions. Uncertainty analysis shows that extending the HT-PEMFC’s lifespan to 40,000 h can reduce exergy cost per unit product by 16.23%, while decreasing the price of green methanol to 11.00 $/h can lower costs by 26.97%.

4E evaluation and multi-objective optimization of low concentration flue gas CO2 capture and multi-product conversion process

In order to address the current global warming, this work proposes CO2 capture from flue gas and CO2 hydrogenation to produce methane, methanol, and CO2 to produce ammonium bicarbonate. Aspen Plus process simulation model is developed for the three processes. Based on the thermodynamic model, sensitivity analysis of main operating parameters is conducted on the energy, economic, environment, and exergy (4E) indicator. Non-dominated sorting genetic algorithm is used to achieve multi-objective optimization. The optimum operating parameters of the three processes are obtained under the 4E indicators. It is concluded that the CO2 to ammonium bicarbonate is the optimal process route for CO2 conversion. The corresponding total annual cost is 4.071 M$/year, the CO2 equivalent emission per unit product is −0.0667 tCO2/t pro, the annual total utility consumption is 7062.75 kW/year, and the exergy efficiency of system is 53.42 %. This work shows significance for achieving resource utilization with capture CO2.

Techno-economic assessment and multi-objective optimization of a hybrid methanol-reforming proton exchange membrane fuel cell system with cascading energy utilization

This article proposes an innovative tri-generation system aimed at improving the efficiency of proton exchange membrane fuel cell (PEMFC) combined methanol steam reforming (MSR) setups. The proposed system merges MSR-PEMFC with an organic Rankine cycle (ORC) and Latent-thermal energy storage (LTES) to produce heat, electricity, and purified water concurrently. The MSR process, using methanol and water, acts as a hydrogen source for the PEMFC. Waste heat from the reforming reaction and PEMFC exhaust gases is efficiently recuperated through ORC and LTES. Moreover, a composite phase change material comprising 8 % expanded graphite and 92% industrial paraffin wax is formulated for LTES, exhibiting a thermal conductivity of 4.65 Wm−1K−1. Parametric analysis of the methanol-reforming fuel cell system suggests a methanol to water mass ratio of 1.5 and a pressure swing absorption shunt fraction of 0.77. Multi-objective optimization of the recovery system using the Non-dominated Sorting Genetic Algorithm II algorithm is conducted, with objective functions including tri-generation system exergy efficiency, CO2 emission reduction, and total cost to evaluate system thermodynamic, environmental, and economic performance, respectively. The ORC system employing R245fa and R600 yields 27 % more electricity than the R152a-based system, with exergy efficiencies approximately twice as high. Compared to conventional systems, the proposed tri-generation system achieves an energy efficiency of 81 % and exhibits a 22 % enhancement in energy efficiency.

Experimental investigation on thermodynamic and environmental performance of a novel ocean thermal energy conversion (OTEC)-Air conditioning (AC) system

In the field of isolated island energy supply, the OTEC system is considered promising due to its large storage capacity, and pollution-free characteristics. To improve energy efficiency, polygeneration systems have gradually become a research hotspot for their low cost and high return features. In this context, a novel Ocean Thermal Energy Conversion system integrated with an air conditioning unit (OTEC-AC) is introduced, demonstrating capabilities in electricity, cooling capacity, and fresh water production by experiments. The effects of various flow rates and temperatures of the heat and cold sources, along with the air-conditioning system water flow rate on the system performance is explored. Besides, the overall performance of the OTEC-AC is compared with four representative OTEC models. The results indicate that there is a threshold for the influence of cold and heat source flow rate on the performance of power generation system. The OTEC-AC system shows a highest net power of 132.6 W, cooling capacity of 2.14 kW, condensate production of 3.24 kg/h, and achieves a system exergy efficiency and CO2 reduction of 34.7 % and 5801 kg/year, respectively. The annual CO2 reduction per kilowatt of installed capacity of the system is increased by 135.6 %, compared with the conventional OTEC system.

Exergy efficiency improvement by compression heat recovery for an integrated natural gas liquefaction-CO2 capture-NGL recovery process

For liquefied natual gas production, cryogenic distillation offers lower thermal energy consumption and a more compact footprint for carbon capture unit than the widely used chemical absorption. However, the reduction in thermal energy consumption comes at the cost of increased power consumption. To reduce the power consumption of cryogenic distillation based natural gas lqiuefaction plants, an integrated process for liquefied natual gas production, cryogenic CO2 capture and natural gas liquids recovery is proposed, which is called the base process. This base process is further integrated with a heat recovery unit (Organic Rankine cycle or Kalina cycle) to investigate the optimal utilization of compression heat in the system. The proposed processes are modeled in Aspen HYSYS and optimized using a genetic algorithm installed in Matlab. The results show that specific power consumption of the base case is 0.385 kWh/Nm3 NG. Due to the lack of compression heat recovery, significant exergy loss occurs at the water coolers, accounting for approximately 37 % of the total system exergy destruction. Among the evaluated methods, the supercritical Organic Rankine cycle, using R1234ze as the working fluid, recovers approximately 60 % of the exergy from compression heat with an 8.7 % improvement in system exergy efficiency. The subcritical Organic Rankine cycle, using R600a as the working fluid, achieves a slightly lower exergy efficiency improvement of 7.1 %. The Kalina cycle, while less efficient, still improves the system’s exergy efficiency by 6.3 %. Economic analysis reveals that the total annualized cost of the base process is $8,441,416, with a levelized cost of $0.1411/kg. The subcritical Organic Rankine Cycle provides the most cost-effective solution, with the lowest total annualized cost of $8,305,154 and a levelized cost of $0.1402/kg. The Kalina cycle ranks second in cost-effectiveness, with a total annualized cost of $8,345,914 and a levelized cost of $0.1405/kg NG. Although the supercritical ORC requires the highest capital investment, it proves more cost-effective than the base process due to its reduced annual operating costs.

Performance analysis and optimization of a combined cooling, heating and power system based on active regulation of thermal energy storage

The energy storage unit can significantly address the issue of mismatch between the energy supply and demand of the combined cooling, heating and power (CCHP) system. Therefore, this article proposes a micro-gas turbine coupled with low-concentrating photovoltaic/thermal CCHP system with thermal energy storage active regulation, which can change the thermoelectric output ratio of the micro-gas turbine. The impact of the volume of the heat storage tank and the changes in ambient temperature on the system's performance is analyzed. This system is optimized with the objective functions of primary energy consumption saving rate, carbon dioxide emission reduction rate, total annual cost saving rate and exergy efficiency maximization. The results indicate that increasing the volume of the thermal energy storage tank enhances the exergy efficiency. Furthermore, an increase in ambient temperature can improve the objective functions. The optimum comprehensive performance of this system is achieved when the thermal energy storage tank volume is 40 m³ and the gas boiler capacity is 241.16 kW. With the thermal energy storage active regulation, the primary energy consumption, carbon dioxide emissions and annual total cost of the system decline by 2.24 %, 2.12 % and 1.48 %, and the system exergy efficiency improves by 1.72 %.

Thermodynamic analysis of a solar powered ejector cooling system

A steady-state model of an ejector cooling system activated by solar heat is presented and applied in response to the increasing demand of cooling in hot-climatic regions. The system comprises three loops connected by heat exchangers. The first loop, or collection sub-system, includes the solar collectors (both cylindrical-parabolic and flat plate collectors are modeled) and the heat transfer fluid (a thermal oil). The second loop also includes an ejector which uses polytropic efficiencies and provides the power necessary to circulate a second low-pressure stream of the refrigerant through the evaporator and an intermediate-pressure condenser. The third loop connects the load to the evaporator using ethylene glycol. In this study, the model is particularly based on the laws of classical and finite size thermodynamics and incorporates experimentally derived relations for the efficiency of the cylindrical-parabolic collectors as well as an ejector model which is based on physical laws rather than on refrigerant specific correlations used in previous studies. The results include a parametric study analysing the effect of the generator superheating and the calorific flow of the heat transfer fluid on the ejector dimensions and the overall system performance. The impact of the solar collector type on the collector surface and the system performance by a fixed incident radiation was investigated. The results illustrate that using parabolic trough collectors, the exergy efficiency of the entire system and the required collector's area are almost constant with the solar radiation intensity. They are approximately 35 % and 35 m2 respectively. It was also shown that further superheating the refrigerant by an additional 10 °C at the generator exit reduces the ejector's total length by 8.1 %, leading to positive economic impacts on raw material acquisition for the solar-powered ejector refrigeration system. Furthermore, doubling the calorific flow reduced the COP by half and increased the ejector length. The economic analysis of the system reveals two key findings: The largest portion of the initial costs is attributed to the solar collector, which is essential for providing the thermal energy required for the system's operation. The main hourly expense of the system is the electricity cost associated with powering the pumps.

Exergy Analysis of Hydrogen-Rich Syngas Production System Utilizing Palm Oil Empty Fruit Bunch using Pyrolysis Connected with Steam Methane Reformer

Fossil fuel consumption on a large scale continues to increase yearly, causing a shortage of energy sources. Hydrogen can help to overcome the shortage of energy sources. It can be produced from renewable energy sources, such as oil palm Empty Fruit Bunch (EFB) as biomass using pyrolysis processes equipped with steam methane reformer and water gas shift reactions. This research was conducted to determine the amount of hydrogen that can be produced and to analyze the exergy of the existing systems in the hydrogen production process. Aspen Plus has been used to design and analyze pyrolysis processes, steam methane reform, and water gas shift reactions. The results showed that when 6.54 kg Empty Fruit Bunch was used as raw material in a pyrolysis reactor, hydrogen could be produced in the amounts of 0.435 kg. The exergy values of raw materials and fuels are relatively high because the chemical exergy values of the compounds contained therein are also high. Based on the calculations, it is found that the combustion process is the process that experiences the most significant exergy destruction. The exergy efficiency of this hydrogen production process system is 22.77%.

Innovative multi-generation system producing liquid hydrogen and oxygen: Thermo-economic analysis and optimization using machine learning optimization technique

This research investigates the multi-objective optimization of a geothermal-driven multi-generation system designed to produce liquid hydrogen and oxygen as its primary outputs. The system incorporates a Proton Exchange Membrane (PEM) Electrolyzer alongside a modified ejector-based Organic Rankine Cycle to generate the required Energy (heating bar, cooling bar, electricity and hydrogen). Case studies were conducted in three distinct locations: Zanjan in Iran, Aarhus in Denmark, and San Bernardino in the United States, utilizing actual geological and meteorological data for accuracy. Simulations were executed applied Engineering Equation Solver & optimization of the system was carried out to enhance performance by focusing on the objective functions of system exergy efficiency and cost reduction. This was accomplished by utilizing response surface methodology (RSM) along with Minitab software (MS).To optimize the system, six decision variables were identified as critical parameters influencing performance. The optimization results indicated that the system can achieve an exergy efficiency of 53.74 % while maintaining a cost rate of $31.56 per hour. The overall cost rate for the geothermal system was determined to be $54.50 per hour, with $10.10 per hour allocated to hydrogen production and $44.40 per hour to electricity generation. Site-specific analyses highlighted San Bernardino as the most favorable location for implementing this system, projecting an annual production of 90.8 tons of liquid hydrogen and 208.3 kg of oxygen. In August, the geothermal system in San Bernardino can produce up to 7691.6 kg/h of liquid hydrogen, while in December, production decreases to 7402.8 kg/h, marking the lowest output for the year. In contrast, Zanjan and Aarhus demonstrated lower production capacities, underscoring the system's feasibility as dependent on site-specific conditions.

Exergy based yearly performance analysis of a poly-generation system for a small-town application

The current study conducts a comprehensive year-round exergy analysis of an integrated system centered around a reversible solid oxide cell (RSOC), aimed at fulfilling the power and fresh water requirements of a small town, equivalent to 2.7 MW and 2000 m³/day, respectively. The integrated system encompasses four subsystems: the solar field, RSOC, organic Rankine cycle coupled with ejector cooling (ORC), and zero liquid discharge multi-effect desalination (MED-ZLD). The proposed system is analyzed using MATLAB. The annual exergy efficiency for each subsystem is examined, yielding average values of 11.9 %, 48.11 %, and 56.22 % for the solar field, ORC cycle, and MED-ZLD, respectively. Notably, the exergy efficiency of the Rankine cycle is higher in colder months compared to warmer ones. This phenomenon is attributed to the cycle's greater heat generation relative to the cooling it produces during the cold season. The RSOC exhibits noteworthy average exergy efficiencies of 93 % in electrolysis mode, 69.87 % in fuel cell mode, and an overall average exergy efficiency of 79.62 % throughout the year. The average exergy efficiency of the overall system is evaluated throughout the day, nighst, and the entire operational period. The results are values of 10.74 %, 34.31 %, and 23.34 %, respectively. This discrepancy is due to the primary energy supply source, which is solar energy during the day and input hydrogen during the night. The highest exergy efficiency is observed in December, while the lowest is in June. Furthermore, the study examines the distribution of exergy destruction and identifies cycles with significant impacts by analyzing the exergy destruction ratio across various months. The solar field is the primary contributor to daytime exergy destruction, peaking at 94.9 %. At night, the MED-SET cycle leads in exergy destruction, followed closely by the RSOC cycle operating as a fuel cell. The research also delves into exploring key contributors to exergy destruction in various cycles, shedding light on crucial insights into the intricate dynamics of the integrated system's performance.

Economic and technical optimization of a tri-generation setup integrating a partial evaporation Rankine cycle with an ejector-based refrigeration system using different solar collectors

Three collector types, namely parabolic trough solar collector (PTSC), linear Fresnel collector (LFC), and dish collector, are employed to drive a tri-generation configuration composed of a partial evaporation Rankine cycle (PERC), a hot water production unit (HWPU), and a cascade ejector-based compression refrigeration system (ECRS) for electricity, cooling, and heating production. The ECRS is integrated with the topping PERC through an absorption refrigeration system (ARS). Moreover, the collector is indirectly connected to the system by a storage tank to increase the operating hours of the system. The performance of the system is evaluated by thermodynamic and economic analysis. The three-objective optimization result is a testament to the fact that utilizing PTSC brings about the best performance of the system. The system exergy efficiency (ηex) in the case of PTSC is 8.1 % and 24.9 % higher than in the case of LFC and dish collector, respectively. The economic performance of the system in the case of PTSC is superior to that of the dish collector. Furthermore, the PTSC case results in a relatively similar economic performance to the LFC case. The system produces power, cooling, and heating at rates of 200.8 kW, 564.9 kW, and 795.7 kW in the optimum case of utilizing PTSC, respectively. Moreover, ηex, total cost rate (C˙tot), and specific cost of tri-generation (ctri) are obtained as 11.48 %, 204.7 $h−1, and 44.91 $GJ−1, correspondingly. The results indicate that ηex of the proposed layout in the case of PTSC is superior to a comparable tri-generation configuration introduced in the field by 2.78 % points.

Thermodynamic Comparative Analysis of Cascade Refrigeration System Pairing R744 with R404A, R448A and R449A with Internal Heat Exchanger: Part 2—Exergy Characteristics

The cascade refrigeration systems (CRS) used in hypermarkets and supermarkets, which are used by many people, have been employing R744 for the low-temperature cycle (LTC) and R404A for the high-temperature cycle (HTC) due to environmental and public safety issues. However, the use of R404A is limited due to its high GWP, and therefore research on alternative refrigerants is necessary. Nevertheless, there is no detailed study in the literature that compares and analyzes the three refrigerants for practical design by applying R744 for LTC and R404A, R448A, and R449A for HTC in CRS. Therefore, this study aims to provide data for the practical detailed design of an alternative system to R744/R404A CRS. Under standard conditions, we analyzed how the exergy destruction rate (EDR) and exergy efficiency (EE) of the system and the EDR of each component change when the important factors affecting CRS (degree of superheating (DSH), degree of subcooling (DSC), and internal heat exchanger (IHX) efficiency of HTC, DSH of LTC, condensation temperature (CT), evaporation temperature (ET), cascade evaporation temperature (CET), and temperature difference of CHX) are varied over a wide range. The main conclusions are as follows. (1) Under the given constant conditions, the smallest change in system EDR based on R448A is DSH of HTC (decreased by 0.07–0.1 kW), followed by IHX of HTC (decreased by 0.12–0.3 kW), DSH of LTC (increased by 0.19–0.25 kW), DSC of HTC (decreased by 0.59–0.69 kW), temperature difference of CHX (increased by 1.57–1.83 kW), CET (decreased and then increased by 0.67–4.43 kW), CT (increased by 1.49–3.9 kW), ET (decreased by 2.39–4.61 kW). (2) The highest change rate of system EE based on R448A is CET (increased and then decreased by 1.38–8.28%), followed by temperature difference of CHX (decreased by 2.96–3.16%), ET (increased and then decreased by 0.63–2.75%), DSC of HTC (increased by 1.26–1.34%), CT (increased and then decreased by 0.24–1.12%), IHX of HTC (increased by 0.11–1.02%), DSH of LTC (decreased by 0.35–0.49%), and DSH of HTC (increased by 0.14–0.19%).

Performance analysis of a medical waste gasification-based power generation system integrated with a coal-fired power unit considering dispatching optimization

This paper collects into a medical waste plasma gasification, a cleaning system, a gas turbine, a syngas storage tank and a coal-fired power unit to establish the medical waste plasma hybrid peak shaving system. In this system, medical waste is transported into a plasma gasification furnace and prepared as syngas by reaction. Then, the syngas is delivered to the gas turbine after cooling and deacidification. Finally, the flue gas from the combustion of the gas turbine is used to heat the condensate of a coal-fired power unit. Compared to the coal-fired power unit, the system has a larger range of peak load capability. Establish the integrated regional power grid dispatching model. The paper selects five typical days, which combine the actual data to comprehensively consider many factors. Then analyze the optimal economy of the regional power grid and propose the optimal dispatch scheme for the regional power grid. After modification, the system energy efficiency is 37.38 %, and system exergy efficiency is 36.19 %. After the dispatching optimization, the average daily profit is 140.78 k$, the profitability can reach initial cost in 6.20 years, and the net present value generated by the medical waste plasma hybrid peak shaving system is 178,410.43 k$.

The process optimization and exergy efficiency analysis for biogas to renewable hydrogen by chemical looping technology

The study on biogas-Chemical Looping Hydrogen Production (biogas-CLHP) highlights its potential to produce high-purity hydrogen with recycling waste bioenergy. It delves into the gas-solid reaction characteristics and optimizes crucial process parameters for the fixed-bed CLHP system. Movement rate equations for the reaction fronts of Fe2O3-Fe3O4 (Rf1), Fe3O4-FeO (Rf2), and FeO-Fe (Rf3) are formulated, aiding in clarifying CO breakthrough curves. The identified convergence temperatures (Tm) under CO and H2 atmospheres are 916 °C and 907 °C, respectively. Rf1 and Rf2 converged above Tm, which leads to the disappearance of the Fe3O4 region in oxygen carrier (OC) beds. A theoretical model for calculating the reduction solid conversion (Xaver) is established, accurately characterizing Xaver under varying reaction times, H2/CO ratios, and temperatures. By using the Particle Swarm Optimization (PSO) algorithm and thermodynamic analysis, the study determines the optimal length-radius ratio (L/r), reduction temperature (T), and H2/CO ratio (α) for the OC bed as 12.5, 916 °C, and 0.72, respectively, at which a maximum conversion ratio of 43.55 % is achieved. Based on optimal parameters, the results of Aspen simulation reveal energy and exergy efficiencies of 74.9 % and 70.8 %. Generally, this research comprehensively outlines the process dynamics, optimizes key parameters, and evaluates exergy efficiency of the biogas-CLHP system.

Design and performance estimation of a novel solar multiplate mirror concentration PVT system based on nanofluid spectral filter

This paper presents the design and performance evaluation of a novel solar multiplate mirror concentration photovoltaic and thermal (MPVT) system using ethylene glycol/indium tin oxide liquid spectral filter (LSF). The LSF is prepared, and the test results show that its average spectral transmissivity and average spectral absorptivity are 69.2 % and 30.8 %. Optical characteristics and operation performance of the MPVT system are evaluated, and parametric interaction analysis is also carried out to reveal the relationships of optical and structural parameters of the MPVT system. The results indicate that the MPVT system is technically feasible and has acceptable adaptability on solar tracking error. The decrease in LSF flow channel width and increase in LSF flow channel installation height can increase the concentration ratio. The electric power and photoelectric efficiency of the photovoltaic module are 818.2 W and 29.07 %. The thermal and exergy efficiencies of the MPVT system are 23.88 % and 32.81 %. By increasing the inlet LSF flow velocity and ambient temperature or decreasing the inlet LSF temperature properly, the thermal performance of the MPVT system can be improved. But the effects of the three parameters on the exergy efficiency of the MPVT system are all relatively small.

Exploiting the waste heat in a proton exchange membrane fuel cell with a capacitive salinity/heat engine

For fully utilizing the low-grade waste heat (LGWH) of the proton exchange membrane fuel cell (PEMFC), a novel coupled system primarily involving a PEMFC and a capacitive salinity/heat engine (CSHE) is proposed. Based on thermodynamic and electrochemical theories, mathematical formulas of the power output, efficiency, and exergy efficiency are deduced accounting for the significant irreversible losses within the coupled system. The superiority is expounded by contrasting the generic performance characteristics of the sub-systems and the coupled system. In addition, numerical results demonstrate that the maximal power output density and its corresponding efficiency and exergy efficiency of the coupled system enhanced by 11.43%, 12.75%, and 9.96%, respectively, over the PEMFC alone under the same operating conditions. Furthermore, the impacts of some parameters, for instance, proton exchange membrane (PEM) thickness, the PEMFC working temperature and working pressure, electric charge density ratio, charging cut-off voltage, and Stern layer distance, on the coupled system performance are meticulously discussed. The results of the study may supply a reference for the design and operation of such a salinity gradient energy recovery system and create a new pathway for recycling LGWH from PEMFC.

CO2-mediated oxidative dehydrogenation of propane to propylene and syngas: Reaction and energy performance matrices

The performance of CO2-assisted oxidative dehydrogenation of propane to propylene is investigated. The energy performance matrices are defined as energy efficiency and exergy efficiency at both the reactor level and the overall system level. Oxidants are the main factor that dictates the performance of the dehydrogenation of propane. Oxygen is a strong oxidant which helps in promoting the endothermic dehydrogenation of propane. CO2 is considered a mild oxidant that hinders the over-oxidation of propane. The addition of CO2 promotes propane conversion but sacrifices the other performance matrices including propylene selectivity and the H2/CO ratio of the syngas. Consequently, the energy and exergy efficiencies of the reactor as well as the DHP system decrease with increasing CO2 supply. The addition of O2 has positive impacts on the energy performance matrices at a certain temperature range (from 863 K to 1273 K). However, the addition of O2 leads to an adverse impact on the propylene selectivity and H2/CO ratio of the syngas. Under the absence of O2, the DHP exhibits a propane conversion of 91% with the propylene selectivity and system exergy efficiency of 90% and 86%, respectively when the CO2/C3H8 ratio of 0.25 at 973.15 K. This work provides strong evidence that the reaction performance matrices have a strong relation to the energy performance matrices and offers a baseline for future research and optimization in the presence of O2 and CO2.

Techno-economic analysis of a novel solar-based polygeneration system integrated with vanadium redox flow battery and thermal energy storage considering robust source-load response

In this study, a novel solar-based polygeneration system incorporated with a partially covered parabolic trough photovoltaic thermal (PCPVPVT) collector, vanadium redox flow battery (VRFB), thermal energy storage, and absorption chiller/heat pump is proposed. A robust energy management strategy is developed to alter the operating states of the system to respond to the inevitable intermittent source inputs and varying demands throughout the year. Furthermore, a comprehensive comparison of the techno-economic performance differences between the novel system and the Power to Hydrogen to Power (P2H2P) combined cooling, heating, and power (CCHP) system is carried out. In addition, the sensitivity analysis of the key technical and external economic parameters as well as a multi-objective optimization are investigated. The results illustrate that: the charging, discharging, and round-trip efficiencies of the VRFB unit reach 88.99%, 88.23%, and 78.52%, respectively, and the current of the VRFB has significant effects on the termination voltage, charge and discharge time, efficiencies, and stored and output energy of the VRFB. The PV coverage ratio of the PCPVPVT and the capacity of the VRFB unit considerably affect the system's thermodynamic and economic performance. The exergy efficiency of the proposed system is always greater than that of the P2H2P CCHP system because the efficiency of the VRFB with 78.52% is significantly higher than that of the P2H2P electricity storage with 41.73%, and the maximum difference reaches 4.25%. The levelized cost of energy of the present system is 0.0503 $/kWh, which is less than that of the P2H2P CCHP system by approximately 6.85%, indicating superior economic strengths.

Machine learning optimization of a zero energy building (ZEB) by waste heat recovery in a co-generation system

Waste heat recovery is known as one of the newest methods to increase the energy and exergy efficiencies of thermal systems. The concept has been also developed in co-generation systems where productions of electricity, cooling and heating are considered, simultaneously. This study investigates tri-generation system with a co-generation ammonia-water cycle (AWC) and an organic Rankine cycle (ORC) to supply cooling, heating, and power, simultaneously. As novelty, six different working fluids of Ammonia, R124, R134a, R152a, R245fa and R290 were investigated, as six scenarios for ORC. As result, the R124 was found as the best one and the next investigations were followed just with this fluid. Minimizing total cost and maximizing exergy efficiency at the same time are the main aim of this paper. Design Expert software was used to find out about this optimum point. Four cities in Australia were selected and the effect of annual ambient temperatures of them were studied on the outputs of the system, using TRNSYS software. Also, the optimum values of the effective parameters, including the isentropic efficiencies of pumps, compressor, and turbines, heat source temperature, ammonia concentration, utmost pressures of ORC and AWC, were obtained and reported by means of TOPSIS. As results, the monthly generations in electricity, heating and cooling were estimated. It was revealed that the lower ambient temperature, the higher electricity generation, according to the thermodynamic cycle of the proposed co-generation system. Finally, the results showed that application of the suggested co-generation system can cover the electricity demand of 294 apartments, heating for 208 apartments, and cooling load for 18,999 apartments.