Unit Exergy Research Articles

Thermodynamic and exergo-economic evaluation of biomass-to-X system combined the chemical looping gasification and proton exchange membrane fuel cell

The resource utilization of biomass is of great significance to the adjustment of energy structure. The syngas produced by biomass gasification can be used for power generation and production of green chemical raw materials. In this paper, a biomass-to-X (BtX) system based on the chemical looping gasification and high temperature proton exchange membrane fuel cell (PEMFC) is proposed to produce methanol, electricity, heating and cooling. The heat produced by the BtX system is harnessed for the regeneration of the carbon capture solution, as well as to drive both the organic Rankine cycle and the double-effect absorption chiller, enabling cascade utilization of the energy. The thermodynamic and exergo-economic models are carried out to evaluate system performance. The relationship between the material, energy and cost flow from biomass to products is analyzed. Finally, the influence of key parameters on system performance is studied. The results show that the energy efficiency of the BtX system is 54.0 % and the exergy efficiency is 35.4 %. The yield rate of methanol with 99.2 wt% purity is 6.48 tons/h. The cost of equipment, propanol and biomass accounts for 34.9 %, 28.9 % and 28.1 % of the total investment cost, respectively, while the cost of the PEMFC stack represents 50.4 % of the equipment costs. The unit exergy cost of the electricity and methanol are 0.068 $/kWh and 0.049 $/kWh, respectively. The methanol, carbon capture and cooling costs are most sensitive to the change in the propanol cost, followed by the biomass cost. For every 0.05 increase in the hydrogen fraction to methanol production, the electricity decreases by 3.1 MW and the methanol yield increases by 0.23 kg/s.

Thermoeconomic modeling of new energy system based on biogas upgrading to multigenerational purpose

The following article outlines an integrated approach toward the valorization of biogas for energy, chilled water, hot water, freshwater, and liquid carbon dioxide production. The suggested system includes a biogas upgrading unit with a biomethane-fueled burner, a Kalian cycle, an LNG regasification process, and a multi-effect desalination unit that can provide CO2, electricity, cooling, and heating. The new ideas in the proposed system include a cryogenic biogas upgrading process that can be used in many industrial settings, the production of liquid carbon dioxide through biogas upgrading, and the use of biomethane in the integrated combustion process. Furthermore, the grey wolf optimization technique is applied in various multi-objective optimization contexts to achieve optimal operating conditions and preferred performance outcomes. In this simulation, the generation of electricity reaches a value of 755 kW. Attached to it is a cooling load of about 5102 kW, with a heating load of 4725 kW, freshwater capacity about 1.33 kg/s, production rate of liquid CO2 at 0.58 kg/s, and natural gas at 3.1 kg/s. Thermodynamic results reveal that the proposed multigeneration performance scheme providently enhances energy and exergy efficiencies up to 59.82 % and 5.74 %, respectively, compared to the condition of a single product. The performed exergy analysis shows that the overall irreversibility of the suggested configuration is equal to 18,047 kW while about 42.6 % of this value is contributed from the heat exchanger E-104. Moreover, the CO2 emission intensity of the process can be kept as 0.35 kg/kWh. Cryogenic separation can obtain about 88 % of CO2 in biogas and turn it into its liquid phase. The performed economic assessment has resulted in the following total cost rate as well as cost per unit exergy: 321.38 $/h and 83.06 $/GJ, respectively. Under optimal conditions, an exergy efficiency of 13.40 % and a net power output of 2034.39 kW are attained.

Enhancing efficiency and economy of hydrogen-based integrated energy system: A green dispatch approach based on exergy analysis

Hydrogen-based integrated energy system holds promise for leveraging the low carbon advantage of hydrogen, but inefficiency of electrolysis hydrogen production leads to significant exergy loss within its operation. Addressing this, this paper focuses on the energy-saving and economic operation of the hydrogen-based integrated energy system, proposes a novel green dispatch approach using loss cost as the objective function and establishes a refined loss cost accounting model based on exergy analysis theory. Firstly, derived from the analogy of exergy loss, the concept and accounting framework of system loss cost are proposed. Secondly, employing exergy analysis and consistent unit exergy cost principle, equipment operation loss cost is calculated through energy, exergy flows analysis and cost allocation. Thirdly, utilizing the total of equipment operation loss cost, wind curtailment penalty cost, and transmission line loss cost as the objective function, a green dispatch model is formulated to minimize overall loss cost while ensuring energy balance and other constraints. Finally, simulations analysis are conducted. The results show that compared to economic dispatch, minimized exergy loss dispatch and multi-objective dispatch methods, proposed approach increases exergy efficiency by 0.54 %, reduces cost by 6.13 % and increases the scale of hydrogen utilization by 3.45 %, respectively.

Exergy and thermoeconomic analysis of a novel polygeneration system based on gasification and power-to-x strategy

This study introduces a novel polygeneration system that integrates biomass gasification, anaerobic digestion, photovoltaic (PV) energy, and electrolysis to enhance system flexibility and efficiency. The system includes an allothermal gasification unit that processes lignocellulosic biomass sourced from a botanical garden. The gasifying agent, either steam or oxygen, is supplied by an alkaline electrolyzer, which operates under a Power-to-X strategy, utilizing surplus energy from a PV field. The resulting syngas and hydrogen are blended with biogas from an anaerobic digester, which treats municipal waste, to power a cogenerator. This cogenerator meets the electricity, heating, and cooling demands of a hospital, while the PV field powers the botanical garden. The systems components are modeled using various tools and integrated into the TRNSYS environment for dynamic simulation. An exergy analysis identifies the main sources of exergy destruction, while a thermo-economic analysis evaluates the energy, environmental, and economic impacts of a demonstration plant located in Bogotá’s Botanical Garden. Simulation results show that the system achieves an overall exergy efficiency of around 35 %, with a 96 % reduction in primary energy consumption compared to a reference system, avoiding nearly 6000 tons of CO2 emissions annually. From an economic perspective, the system is profitable, with a payback period of 3.02 years and a Net Present Value of $15.23 million, almost double the capital cost. The gasification unit's exergy efficiency is significantly higher when using oxygen (0.50) compared to steam (0.25), underscoring the importance of integrating electrolysis for improved biomass conversion. The alkaline electrolyzer operates efficiently within its optimal range, with an energy efficiency close to 0.70 and an exergy efficiency around 0.60, effectively utilizing 25 % of the total PV production.

Thermodynamic analysis of a modified two-stage transcritical CO2 refrigeration cycle with an ejector and a subcooler

In the application scope of large-scale supermarkets, the practicality of transcritical CO2 refrigeration device has been confirmed to be quite satisfying. A modified two-stage transcritical CO2 refrigeration cycle with an ejector and a subcooler (MTC) is proposed in this paper. In the modified cycle, the ejector recovers expansion works and reduces irreversible losses in the throttling process. The subcooler provides subcooling degree for the CO2 entering the low-temperature (LT) evaporator, thus increasing the refrigeration capacity and improving the COP of the modified cycle. Thermodynamic analysis has shown that the exergy efficiency (ηex) and COP of MTC under given operating condition has been enhanced by 13.7 % and 14.2 % compared to BTC. The discharge temperature at the outlet of high-pressure compressor in MTC is decreased by 8.3℃. Under typical operating condition, the optimal discharge pressure of MTC is 9.07 MPa, which is lower than that of BTC. The correlation to calculate optimal discharge pressure for single-stage CO2 refrigeration cycle is also suitable for MTC under given operating conditions. The MTC has also shown better performance under variable operating conditions. For MTC, when the temperature at the outlet of gas cooler increases from 35 – 45℃, the COP and ηex are enhanced by 14.1 % - 16.1 % and 12.8 % - 14.2 % compared to BTC, respectively. When gas cooler outlet pressure decreases from 11.0 to 7.5 MPa, the COP and ηex are enhanced by 14.0 % - 22.8 % and 13.4 % - 18.7 %. As the evaporating temperature at the cold side of subcooler increases from -25 to -15℃, the COP and ηex increase from 1.82 to 1.98 and 27.4 % to 29.7 %. With the ratio of refrigeration capacity (Rec) between medium-temperature evaporator and low-temperature evaporator varies from 0.7 to 1.2, the COP and ηex are improved by 10.7 % - 16.3 % and 10.7 % - 15.4 % compared to BTC. There is the maximum exergy loss at gas cooler in the MTC, whereas that of BTC is located in the expansion valve before LT evaporator. The economic analysis shows the cost per unit of exergy of MTC is decreased by 11.5 % under typical operation condition. According to simulation results, the modified cycle has better performance in severe working conditions such as high gas cooler outlet temperature and low gas cooler outlet pressure in the given range of working conditions compared to BTC.

Optimizing trigeneration energy systems: Biogas-centric methanol production via direct CO2 hydrogenation with advanced integration of PEM electrolyzer and LNG cold technology

Biogas, a sustainable alternative to fossil fuels, addresses issues of non-renewability and accessibility. Its structural similarity to fossil fuels makes it a potent option for energy systems. With this in mind, this paper discusses a novel trigeneration system that utilizes biogas and Liquefied natural gas cooling to produce methanol, electricity, cold water, hot water, oxygen, and natural gas. The system integrates various components such as a biogas burner, Kalina cycle, organic Rankine cycle, liquefied natural gas liquid gasification cycle, proton exchange membrane electrolyzer, and methanol synthesis unit. Simulation via Aspen HYSYS software includes an analysis of energy, exergy, economic, and environmental aspects. Efficiency assessment in single generation, cogeneration, trigeneration, and chemical trigeneration modes concludes chemical trigeneration as most efficient, with the proton exchange membrane electrolyzer being the most efficient subsystem. Key variables like Kalina cycle evaporator temperature, gas flow rate to the methanol reactor, and organic Rankine cycle working fluid pressure are assessed. Predictions on thermodynamic, environmental, and economic behaviors, along with their fluctuations, are made. Using a thermoeconomic approach, the economic analysis determines an exergy unit cost of 59.79 $/GJ and a total cost rate of 2764 $/h. Overall, this work presents a novel and efficient chemical trigeneration system that utilizes biogas and LNG cooling to produce multiple products.

Optimization and exergoeconomic analyses of water-energy-carbon nexus in steel production: Integrating solar-biogas energy, wastewater treatment, and carbon capture

The steel industry is one of the hard-to-abate sectors for decarbonization, and direct electrification is not possible or economically infeasible. This study investigates the application of the Water-Energy-Carbon nexus in the steel industry through a multi-generation plant, addressing the industry's demands for water, energy, and alternative fuel. The proposed multi-generation plant consists of parabolic trough solar collectors, an organic Rankine cycle, urban wastewater treatment, carbon capture, anaerobic digestion combined with heat and power, a proton exchange membrane electrolyzer, and a steelmaking plant. Thermodynamic and exergoeconomic analyses are conducted to investigate the system's efficiency and economic performance. A sensitivity analysis is conducted to identify the optimal links between nexus resources, followed by a multi-dimensional evaluation and multi-objective optimization. The results showed that under base conditions, the plant can annually produce 900 ktons of steel, 105 GWh of net power, 430.1 tons of water, and 517 tons of hydrogen, while preventing 34.7 ktons of CO2 emissions. The exergy efficiency and unit exergy cost of the products are 46.1% and 57.8 $/GJ, respectively. Under optimized conditions, the plant achieves a maximum net annual power output of 160 GWh, an exergy efficiency of 46.9%, and a minimum unit exergy cost of 51.4 $/GJ.

Thermodynamic analysis of trigeneration system with controlled thermal-electric ratio by coupling liquefied natural gas cold energy and biomass partial gasification

Efficient utilization of biomass energy is crucial for addressing environmental challenges. However, the low calorific value of syngas from biomass gasification poses subsequent utilization issues. This study proposes a trigeneration system integrating biomass partial gasification and LNG cold energy to address surge problems using syngas combined cycle systems. The design includes air separation units, combined cycle, and biomass gasifier, achieving an adjustable thermal-electric ratio. An air separation unit and two-stage Organic Rankine Cycle fully utilize LNG cold energy. A thermodynamic model was established in Aspen Plus, and energy, exergy, and economic analyses were conducted to assess feasibility, followed by a sensitivity analysis. Results show that with a mix burning ratio and carbon conversion rate of 0.6 and 0.7, the system's exergy, energy, and electrical efficiencies are 36.94 %, 60.14 %, and 33.32 % in cooling mode. The unit exergy cost and CO2 emissions are 0.0748$ and 95.29 kg/s, respectively. Analysis reveals variations in efficiency and outputs with changes in mix burning ratio and carbon conversion rate. Adjusting operational parameters enhances the system's ability to regulate the thermal-electric ratio, demonstrating its potential for efficient biomass energy utilization and environmental sustainability.

A novel exergy-based cost and carbon footprint allocation method in the multi-energy complementary system

Energy efficiency, economic cost and carbon emission of multi-energy complementary systems are three key indicators to sustainable development in the low-carbon background. This paper analyzes the coupling relationships between energy, cost, and carbon emission flows in a combined cooling, heating, and power (CCHP) system. The CCHP system consists of biomass gasification, co-firing internal combustion engine by product gas and natural gas, concentrated photovoltaic thermal solar collector, and absorption heat pump. A novel exergo-carbon method with exergo-economic analyses is proposed to determine the exergy cost and carbon footprint rate of each stream in the CCHP system, and the unit cost and carbon footprint of multiple products of electricity, cold energy and heat energy are obtained by the proposed method. The impacts of co-firing ratio and photovoltaic coverage ratio on energy and exergy efficiencies and cost and carbon footprint of products are analyzed. The sensitivity of biomass carbon footprint is performed to show its influences. The total cost of products including economic and carbon costs, is discussed to guide the carbon market's pricing strategies. Under the design conditions, the unit exergy cost of electricity, chilled water and hot water are 0.1094, 0.7659 and 0.466 $ (kWh exergy)−1, and their unit exergy carbon footprints are 0.3651, 3.9050, and 2.3780 kg CO2-eq (kWh exergy)−1, respectively.

Thermodynamic and life cycle assessment analysis of polymer-containing oily sludge supercritical water gasification system combined with Organic Rankine Cycle

With the development of polymer flooding technology, polymer-containing oily sludge (PCOS) has become a major pollutant on offshore oil platforms. Supercritical water gasification system is an efficient and pollution-free technology compared to conventional PCOS treatment. However, the system model for PCOS has not yet been established and the theoretical model for system optimization is insufficient. In this work, a novel auto-thermal system is established based on supercritical water gasification technology for the treatment of PCOS. Through the thermodynamic analysis, an optimization plan is concluded and used for the establishment of a modified system with Organic Rankine Cycle. Results show that the two-stage reactor reduces the gasification reaction temperature from 652 °C to 502 °C, and the Organic Rankine Cycle system using Isopropanol recover 9.1 % chemical exergy of feedstock with the optimal parameters (superheat, evaporation temperature and circulating pressure). The energy and exergy efficiency of the modified system has increased by 52.4 % and 42.4 % compared to the original system. Besides, a life cycle assessment is adopted for the environment analysis. The results show that the increasing gasification temperature and PCOS slurry concentration can reduce system unit exergy loss. Global Warming Potential reaching the minimum value of 26.1 at 65 wt%.

Thermoeconomic analysis of the IWERS system for steam heat recovery from bakery ovens

AbstractThe integrated waste energy recovery system (IWERS) recovers heat from bakery ovens in supermarkets and other commercial facilities to heat domestic hot water, resulting in energy savings. This article presents a thermoeconomic analysis of the system's operation in a supermarket over the course of 1 year. The study shows that exergy destruction in IWERS is directly proportional to ambient temperature. During the hottest months of the year, IWERS experiences a 4% decrease in exergy performance, while the exergy unit cost of the product increases by up to 4.2%. Additionally, the steam condenser is responsible for the highest relative exergy destruction, reaching almost 45%. Moreover, the addition of equipment upstream of the process increases the exergy cost of production due to the destruction of exergy in the equipment. These results provide valuable information with important implications for energy efficiency, economic savings, and sustainability. Improving system efficiency would generate substantial benefits in energy savings, economic savings, and environmental impact for all commercial and industrial establishments that use bakery ovens.

Renewable exergy return on investment (RExROI) in energy systems. The case of silicon photovoltaic panels

The ongoing energy transition towards renewable sources faces reliance on fossil fuels throughout their life cycle, primarily due to mining and material production for infrastructure. This study proposes the Renewable Exergy Return on Investment (RExROI) metric, which quantifies the renewable exergy obtained from each unit of non-renewable exergy invested in energy systems. It can be seen as a Renewation Index, applicable to any energy production system, indicating the degree of renewability of such technologies. Focused on silicon photovoltaic panels, the study explores five material intensities, nine scenarios based on the capacity factor and lifespan, and two alternatives for electricity used in the manufacture. Results show that material intensity increases RExROI from −0.6 MJ/MJ (non-renewable exergy is higher than electricity produced) to 5.7 MJ/MJ, increasing until 19 MJ/MJ with the best location and lifespan, and reaching 34 MJ/MJ if renewable electricity is used in manufacturing. Thus, carbon intensity can range from 734 to 7 gCO2eq/kWh. Furthermore, some strategies to enhance RExROI are discussed based on the (i) energy sources, (ii) materials, and (iii) production stages of photovoltaic panels. Thus, this study demonstrates the usefulness of RExROI in evaluating the energy-material-emission nexus of energy systems through exergy in the context of energy transition.

Experimental study and process evaluation of coproduction graphene, acetylene and hydrogen from methane pyrolysis by thermal plasma

This paper developed a process for the coproduction of graphene, acetylene and hydrogen from methane pyrolysis by thermal plasma. Bench-scale tests under three specific energy input (SEI) were carried out to investigate the morphology of carbon nanomaterials and the product selectivity. The process was evaluated by exergy and exergoeconomic analyses. Results indicated that a higher SEI improved the selectivity of graphene nanoflakes (GNFs) and reduced their defects. The product selectivity of GNFs played a critical role in the decrease of total fuel exergy per unit of GNFs as the SEI increased. The plasma reactor exhibited the highest exergy loss ratio in the entire process due to heat dissipation. The quenching device exhibited the second-highest exergy loss ratio in the process, stemming from irreversible entropy increase causing exergy loss. Improving the design of the plasma reactor to enhance thermal efficiency and implementing a feasible waste heat recovery system were identified as effective strategies to enhance exergy efficiency. Excluding consideration of co-products, GNFs with high SEI had the lowest cost of 199.41 CNY/kg, when co-products were considered, the lowest cost decreases by 30%, and the low SEI had the lowest GNFs cost at 140.71 CNY/kg. This indicated that there was enough investment potential to tailor the properties of GNFs produced by such processes for application scenarios. The unit exergy cost of GNFs decreased from 0.89 CNY/MJ to 0.80 CNY/MJ as the SEI of the plasma reactor increased. Subsequently, it rose to 1.16 CNY/MJ. Prioritizing the increase in investment costs for the plasma reactor to reduce exergy loss costs should be considered first, and optimizing the waste heat recovery process was also a key direction to minimize exergy loss costs.

Exergoeconomic analysis and optimization of wind power hybrid energy storage system

The hybrid energy storage system of wind power involves the deep coupling of heterogeneous energy such as electricity and heat. Exergy as a dual physical quantity that takes into account both 'quantity' and 'quality, plays an important guiding role in the unification of heterogeneous energy. In this paper, the operation characteristics of the system are related to the energy quality, and the operation strategy of the wind power hybrid energy storage system is proposed based on the exergoeconomics. First, the mathematical model of wind power hybrid energy storage system is established based on exergoeconomics. Then, wind power experiments of three forms of thermal-electric hybrid energy storage are carried out, and RSM is used to analyze the power quality and exergoeconomic characteristics of the system, and the optimal working conditions of the experiment are obtained. Finally, an optimization strategy is proposed by combining experiment and simulation. The system efficiency, unit exergy cost and current harmonic distortion rate are multi-objective optimization functions. The three algorithms evaluate the optimal solution based on standard deviation. The results show that the exergoeconomics can effectively judge the production-storage-use characteristics of the new system of ' wind power + energy storage'. The thermal-electric hybrid energy storage system can absorb the internal exergy loss of the battery, increase the exergy efficiency by 10%, reduce the unit exergy cost by 0.03 yuan/KJ, and reduce the current harmonic distortion rate by 8%. It provides guidance for improving the power quality of wind power system, improving the exergy efficiency of thermal-electric hybrid energy storage wind power system and reducing the unit cost.

Exergy, exergoeconomic, and exergoenvironmental analyses of novel solar- and biomass-driven trigeneration system integrated with organic Rankine cycle

This study proposes a combined cooling, heating, and power (CCHP) system driven by biomass and solar energy integrated with an organic Rankine cycle (ORC). Its exergy, exergoeconomic, and exergoenvironmental performances are investigated. First, the thermodynamic parameters of each material and energy flow for the proposed CCHP system are simulated using Aspen Plus. Second, the exergy and exergoeconomic performances of the system are investigated, and an exergoenvironmental analysis of the system is performed. The results show that the unit exergy costs (UEC) of domestic hot water, electricity generated by an internal combustion engine (ICE) and the ORC, and chilled/heated water under summer/winter conditions are 2.742/2.742, 6.713/6.629, 12.930/12.930, and 27.100/12.530 MW/MW, respectively, with corresponding unit exergoeconomic costs (UEEC) of 41.11/41.11, 124.40/139.20, 181.80/181.80, and 507.10/302.60 USD/MWh, respectively. Meanwhile, the corresponding environmental impact rates per exergy unit are 0.616/0.616, 2.988/3.023, 0.030/0.030, and 6.567/3.072 mPts/MJ, respectively. The results highlight the urgency to prioritize improvements to the solar collector performance to reduce the UEC of electricity generated by the ORC. The absorption chiller/heater, heat exchanger, and ICE exhibit significant potential for enhancing their exergoeconomic performance. Additionally, the gasifier, ICE, and ORC can be optimized to mitigate their environmental impact with relatively minimal exertion.

4E assessment of ejector-enhanced R290 heat pump cycle with a sub-cooler for cold region applications

Currently, the worldwide implementation of the Montreal Protocol has accelerated the development of low GWP refrigerants. R290 as an alternative refrigerant with low GWP has great advantages over R134a for heat pump applications. However, the use of expansion valve throttling in the heat pump cycle has a large irreversible loss. Therefore, an ejector-enhanced sub-cooler vapor injection heat pump cycle (ESVIC) with R290 for water heating is proposed in this paper to improve the energy efficiency by applying ejector. The thermodynamic models are established based on the 4E (energy, exergy, economic and environmental) analysis to compare its performance with the basic ejector-enhanced heat pump cycle (BEEC) and the sub-cooler vapor injection heat pump cycle (SVIC). The energy analysis results show that compared with the BEEC and SVIC, the volumetric heating capacity of the ESVIC is improved by 42.0 % and 21.2 %, and the heating coefficient of performance (COPh) of the ESVIC is improved by 27.6 % and 5.3 %, respectively. The exergy analysis results indicate that in BEEC, SVIC and ESVIC the exergy destruction of the expansion valves is 0.3 %, 13.5 % and 0.8 %, respectively. It is indicated that the adoption of ejector can significantly decrease the exergy destruction of expansion valves. Moreover, the environmental analysis results reveal that the carbon emissions of ESVIC is 21.5 % and 5.0 % lower than those of BEEC and SVIC, respectively. Besides, for the ESVIC system, the carbon emissions of the life cycle climate performance when using R290 are 9.9 % lower than that of using R134a, indicating that the R290 is a potential substitute for R134a in heat pump applications. The economic analysis results demonstrate that the unit exergy production costs of ESVIC is 21.6 % and 5.1 % lower than BEEC and SVIC, respectively.

Modeling and exergy-economy analysis of residential building energy supply systems combining torrefied biomass gasification and solar energy

A novel hybrid energy system driven by torrefied biomass gasification and solar energy is proposed for the first time, and their complementarity to enhance the system’s energy efficiency is analyzed and shown. The system mainly combines biomass torrefaction, biomass gasification subsystem, solar heat collection and absorption refrigeration technology. The effects of different operating parameters on the gasification products were studied. The economic analysis is carried out based on the theory of exergy economy. The results show that torrefaction pretreatment of biomass can significantly improve its fuel quality and gasification performance. With the increase of torrefied temperature, the volume fraction of CO2 and H2 decreased while the volume fraction of CH4 and CO increased. The economic analysis shows that the expected annual income is $72,735, and the investment recovery period is 2.89 years. Through the calculation of the energy-saving and environmental benefits, it can be concluded that the hybrid energy system can save 1.05 × 106 MJ of heat per year. This equates to about 358.26 tons of standard coal or a CO2 emission reduction of 550.59 tons, the participation of solar energy is conducive to the system emission reduction. By tuning the parameter sensitivity, the influence of biomass price change on the unit exergy cost of system products was more obvious.

Transcritical [formula omitted] precooled single mixed refrigerant natural gas liquefaction process: Exergy and Exergoeconomic optimization

This study proposes a two-stage transcritical CO2 pre-cooled single mixed refrigerant process to investigate the potential improvement in exergy efficiency and exergoeconomic performance. Two different objective functions, namely maximizing exergy efficiency and minimizing the unit cost exergy of LNG, were optimized using the particle swarm optimization algorithm to compare the performance of the CO2 pre-cooled single mixed refrigerant process with single mixed refrigerant process. The results of the first objective function indicate a higher exergy efficiency of the CO2 pre-cooled single mixed refrigerant process, with an increment of 7.56%. This improvement can be attributed to not only the lower total power consumption but also the better matching of the temperature-difference composite curve, particularly the location of the pinch temperature at the warm end of the heat exchanger. Additionally, the exergy destruction of the CO2 pre-cooled single mixed refrigerant process was 30.12% lower than that of the single mixed refrigerant process. The second objective function demonstrated that the CO2 pre-cooled single mixed refrigerant process had a unit exergy cost value of 7.46 $/GJ, which was 11.98% lower than the single mixed refrigerant process. In conclusion, the proposed process contributes to energy saving throughout the LNG chain. Previous studies on the transcritical CO2 precooling cycle were limited to only one-stage configuration paving the way for thermodynamic efficiency enhancement by modifying the design of the process. This study also considers the effects of two objective functions maximizing exergy efficiency and minimizing the unit exergy cost of LNG on the optimization of decision variables which has not been discussed yet in the literature. Moreover, innovative exergoeconomic analysis presents a comprehensive criterion for evaluating economic performance, providing guidance for the initial construction and sustainable development of LNG plants.

Assessment of a combined heating and power system based on compressed air energy storage and reversible solid oxide cell: Energy, exergy, and exergoeconomic evaluation

The electricity grid with high-penetration renewable energy sources has urged us to seek means to solve the mismatching between electricity supply and demand. Energy storage technology could accomplish the energy conversion process between different periods to achieve the efficient and stable utilization of renewable energy sources. In this paper, a hybrid energy storage system based on compressed air energy storage and reversible solid oxidation fuel cell (rSOC) is proposed. During the charging process, the rSOC operates in electrolysis cell (EC) mode to achieve the energy storage by converting the compression heat to chemical fuels. During the discharging process, the cell operates in fuel cell mode for electricity production and the gas turbine is conducted to recover the waste heat from cell. To evaluate the comprehensive performance of the proposed system, the energy, exergy, and exergoeconomic studies are conducted in this paper. Under the design condition, the results indicated that the proposed system is capable of generating 300.36 kW of electricity and 106.28 kW of heating; in the meantime, the energy efficiency, exergy efficiency, and total cost per unit exergy of product are 73.80%, 55.70%, and 216.78 $/MWh, respectively. The parametric analysis indicates that the increase in pressure ratio of air compressor, steam utilization factor of rSOC stack under EC mode and current density of the rSOC stack under EC mode reduce exergy efficiency and total cost per unit exergy of product simultaneously, while the increment of operating pressure of rSOC stack under FC mode enhances the exergy efficiency and decreases total cost per unit exergy of product. The multi-objective optimization is carried out to improve the comprehensive performance of the proposed system, and the results expressed that the best optimal solution has the exergy efficiency and total cost per unit exergy of product of 65.85% and 187.05 $/MWh, respectively. Compared to the basic operating condition, the improvement of the proposed system has led to the maximum enhancement of 20.32% in exergy efficiency and 18.60% in total cost per unit exergy of product.

Simulation and comprehensive 4E study of a high-efficient CCHP arrangement utilizing geothermal energy and eco-friendly heat recovery

In recent years, geothermal energy has received significant attention as it provides a sustainable and renewable energy resource for various applications. The primary objective of this study is to manage the relative irreversibility and enhance productivity through efficient utilization of geothermal energy. The present paper establishes a high-efficiency and environmentally sustainable heat integration model in a novel configuration to be a geothermal energy source having the ability to produce power, heat, and cooling. The proposed framework consists of a Kalina cycle, two organic Rankine cycles, an ammonia Rankine cycle, a single-flash power unit, and a combined cooling and power unit. The simulation of the proposed system is implemented within the Aspen HYSYS software, with an examination of its feasibility across energy, exergy, economic, and environmental considerations. The research findings indicate that the system can generate 23250.8 kW of power, 582.1 kg/s of hot water, and 2.812 kg/s of chilled water. Therefore, the total exergy destruction rate is 14104 kW, with the Kalina cycle accounting for the majority share at 40.47 % Moreover, the energy and energy efficiencies of the suggested structure are 61.81 % and 72.52 %, respectively. The environmental analysis indicates that the entire process lacks carbon dioxide emissions. Furthermore, the economic analysis reveals that the total unit exergy cost and levelized cost of energy are determined to be 5.13 $/GJ and 0.511 $/kWh, respectively. Also, the proposed system exhibits a favorable net present value of approximately 32.16 M$. Hence, the project demonstrates profitability.