Largest Exergy Loss Research Articles

Thermodynamic comparison of small liquid nitrogen generators driven by mixed-refrigerant J-T refrigerators and gas expansion cycles

The thermodynamic comparison of small mixed-refrigerant J-T (MRJT) and gas expansion (Kapitza cycle) LN2 generators are conducted by the exergy method in this paper. Both of two types are employed for low pressure N2 liquefaction and air separation (< 9.0 bar). Pure N2, PSA units or cryogenic air separation columns are used for LN2 production. The MRJT type is driven by single-stage compressors. It is indicated that the efficiency of MRJT type is higher than Kapitza type. With pure N2 at 8.0 bar, the figures of merit (FOM) of MRJT and Kapitza types are 26.86% and 12.79%, respectively. With mini air separation columns, the FOM of MRJT and Kapitza types are 21.42% and 11.34%, respectively. The large exergy losses in the compressor unit and non-isentropic expansion are the main reasons of the inferior efficiency of the Kapitza type. In addition, the Kapitza type requires larger total compressor displacement than the MRJT type. The MRJT type is recommended to be used in small LN2 generators.

Thermo-economic analysis of composite district heating substation with absorption heat pump

A novel composite district heating substation (CDHS) with absorption heat pump was proposed and analyzed in this paper. The CDHS was composed of a water-LiBr absorption heat pump and two plate heat exchanges, which could improve the utilization efficiency of geothermal water and the primary supply water of the primary district heating network. The effect of geothermal water temperature and mass flow rate, and primary supply water mass flow rate variation on system performance were investigated and analyzed by case study. The objective was to maximize net profit and minimize payback period by the cascade utilization of the geothermal water and primary supply water. In addition, economic analysis and multi-objective optimization were conducted to find the optimal mass flow rate of the primary supply water based on the TOPSIS decision making method. Exergy loss analysis was applied under the optimal condition to discover which components had the largest exergy loss. Results indicated that, the proposed system had a net profit of 16.22 M$ in the life time and the minimum payback period was 2.2 years at the optimal primary supply water mass flow rate of 46.16 kg/s where the COP and exergy efficiency of the system were 1.85 and 59.81%, respectively.

Optimization of light hydrocarbon recovery system in condensate gas field

During the middle and later stages of condensate gas field development, the flow rate and pressure of the light hydrocarbon recovery system gradually decrease, and there is a certain deviation between the design parameters and the actual operation parameters of the equipment, resulting in a decrease in the recovery rates of propane (C3) and higher hydrocarbons (C3+). Therefore, it is necessary to optimize the system to obtain greater economic value. In order to solve this problem, the paper analyzes the sensitivity of the equipment operating parameters based on the sequential-modular method. The black box model is used to analyze the equipment and the whole system, and the subsystem with large exergy loss is found. Moreover, this paper optimizes the light hydrocarbon recovery system using parameter optimization and process optimization, and compares the results. According to the analysis, the main conclusions are drawn: (1) The outlet pressure of expander and the temperature of deethanizer bottom have great influence on the light hydrocarbon recovery system, the reduction of the expander outlet pressure from 2.3 MPa to 1.8 MPa can increase the recovery of C3 by 5%. Decreasing the bottom temperature of the deethanizer from 80 ∘C by 20 ∘C can increase the recovery of C3 by 9.2%. (2) Optimized process using direct heat exchange technology has a better optimization effect than the parameter optimization process, the C3 recovery of the optimized system increased by 19.1% to 93%, the recovery of C3+ increased by 14.4% to 95%.

Energy, exergy, economic(3E) analysis and multi-objective optimization of a novel dual functional integration system

A novel dual functional heat pump and power generation integration system (DFHPPGIS) using a scroll machine as compressor and expander is proposed in this paper. It can be operated in both reverse Carnot cycle (RCC) heat pump mode and organic Rankine cycle (ORC) power generation mode. Compared with single system, this system can improve the utilization efficiency of geothermal water and generate more economic benefits. Three kinds of ORC and RCC working fluids are compared to select refrigerant R245fa as the most suitable fluid for the system. Then the model of DFHPPGIS is built to analyze its energy, exergy and economic performance as the variation of geothermal water inlet temperature. Results of theoretical calculation show that there is a conflict between total cost and payback period of the system. The system with lower total cost shows high payback period and vice versa. Thus, a multi-objective optimization for the system is conducted to determine the optimal geothermal water inlet temperature based on the ideal point decision making method. Exergy loss of the system under the optimal condition is analyzed to reveal which component has the largest exergy loss. The results indicate that in this proposed system, if the total cost based optimized design is chosen, payback period is 136.4% higher than its minimum value. If the method of payback period based optimized design is selected, total cost is 15.3% higher than its minimum value. The system has a heat capacity of 250.19 kW and a power output of 17.83 kW at the optimal geothermal water inlet temperature of 80℃ as well as the total cost and payback period of the system are 37.31 k$ and 4.87 years. In addition, the scroll machine has the largest exergy loss in both RCC and ORC mode, which is the component that needs to be mostly optimized in the further development.

The steady thermodynamics of a flash-binary geothermal power system based on correction models validated by static tests

The operation parameters and performance of a flash-binary power system are analyzed based on four scenarios of the experiment, optimum design, error correction Model 1 and error correction Model 2. The operation parameters of a flash-binary power system are tested to explain steady in the experiment with heat source temperature ranges from 100-150?C. The results show that the simulation data of error correction Model 2 have a good agreement with the experiment and most parameter values of relative error, ?, are less than 5%. The expander with low isotropic efficiency results in the largest exergy losses in the flash-binary power system. The flash-binary power system has better performance to adapt to the mid-high temperature geothermal resource in China. Some valuable data obtained from the research can be applied in the future power plant construction in China?s southern Tibet, western Yunnan and Sichuan.

Thermodynamics and LCA analysis of biomass supercritical water gasification system using external recycle of liquid residual

Biomass supercritical water gasification is clean and renewable, which can convert biomass into hydrogen rich gas. Previous studies indicated that external recycle of liquid residual could improve gas yield and gasification efficiency. However, the influence mechanism of external recycle on energy and exergy efficiency is complicated and theoretical model for system optimization is insufficient. Thermodynamic model of external recycle of liquid residual was built in this paper. Exergy efficiency of the main components and exergy loss distribution were specified and the result showed that exergy loss of reactor and preheater accounted for 26.06% and 35.88% of the total exergy loss, which were the main exergy loss sources. Effective ways to reduce exergy loss of components with large exergy loss and to improve energy and exergy efficiency of the system were proposed. Moreover, life cycle assessment of biomass gasification process was carried out. The results indicated that the increase of gasification temperature, pressure and external recycle flow rate of liquid residual and decrease of biomass concentration could improve energy and exergy efficiency of the system. Energy and exergy efficiency reached 63.67% and 48.29% respectively at the condition of gasification temperature of 560 °C, pressure of 25 MPa, recycle flow ratio of 32.43%, biomass concentration of 2.78%. Besides, the increase of gasification temperature and decrease of biomass slurry concentration and pressure could decrease GWP.

A thermodynamic performance analysis on influence parameters of COG-CCHP based on exergy

PurposeThe purpose of this paper is to investigate the performance of coke oven gas (COG)-combined cooling, heating and power (CCHP) system and to mainly focus on studying the influence of the environmental conditions, operating conditions and gas conditions on the performance of the system and on quantifying the distribution of useful energy loss and the saving potential of the integrated system changing with different parameters.Design/methodology/approachThe working process of COG-CCHP was simulated through the establishment of system flow and thermal analysis mathematical model. Using exergy analysis method, the COG-CCHP system’s energy consumption status and the performance changing rules were analyzed.FindingsThe results showed that the combustion chamber has the largest exergy loss among the thermal equipments. Reducing the environmental temperature and pressure can improve the entire system’s reasonable degree of energy. Higher temperature and pressure improved the system’s perfection degree of energy use. Relatively high level of hydrogen and low content of water in COG and an optimal range of CH4volume fraction between 35 per cent and 46 per cent are required to ensure high exergy efficiency of this integration system.Originality/valueThis paper proposed a CCHP system with the utilization of coke oven gas (COG) and quantified the distribution of useful energy loss and the saving potential of the integrated system under different environmental, operating and gas conditions. The weak links of energy consumption within the system were analyzed, and the characteristics of COG in this way of using were illustrated. This study can provide certain guiding basis for further research and development of the CCHP system performance.

Performance comparison of single-stage mixed-refrigerant Joule–Thomson cycle and reverse Brayton cycle for cooling 80 to 120 K temperature-distributed heat loads

Thermodynamic performance comparison of single-stage mixed-refrigerant Joule–Thomson cycle (MJTR) and pure refrigerant reverse Brayton cycle (RBC) for cooling 80 to 120 K temperature-distributed heat loads was conducted in this paper. Nitrogen under various liquefaction pressures was employed as the heat load. The research was conducted under nonideal conditions by exergy analysis methods. Exergy efficiency and volumetric cooling capacity are two main evaluation parameters. Exergy loss distribution in each process of refrigeration cycle was also investigated. The exergy efficiency and volumetric cooling capacity of MJTR were obviously superior to RBC in 90 to 120 K temperature zone, but still inferior to RBC at 80 K. The performance degradation of MJTR was caused by two main reasons: The high fraction of neon resulted in large entropy generation and exergy loss in throttling process. Larger duty and WLMTD lead to larger exergy losses in recuperator.

Combined pinch and exergy analysis of an ethylene oxide production process to boost energy efficiency toward environmental sustainability

Ethylene oxide production process is one of the highest energy consumers in chemical industry, and therefore even a slight improvement in its overall efficiency can have a significant impact on the sustainability of the process. Efficiency improvement can be carried out using the exergy-aided pinch analysis outlined in this paper. The overall exergy loss distribution in different unit operations of an ethylene oxide process was first evaluated and mapped out in the form of “visualized exergetic process flowsheet”. An initial analysis of the four main functional blocks of the process showed that the exothermic reaction block contained the largest exergy loss (6043 and 428 kJ/kg of internal and external losses, respectively) which can be reduced by isothermal mixing, as well as increasing reaction temperature and reduction in pressure drop. The absorption block was also estimated to have the second highest contribution with total exergy losses of 3640 kJ/kg which were mainly due to the cooling column. These losses were then recommended to be reduced by improvements in the concentration and temperature gradients along the tower. Following the block-wise analysis, exergy analysis was then carried out for individual unit operations in each block to pinpoint the main sources of thermal exergetic inefficiency. Thermal solutions to reduce losses were also proposed in accordance with the identified sources of inefficiency, leading to a comprehensive list of cold and hot process streams that could be introduced to reduce losses. Finally, pinch analysis was brought into action to estimate the minimum energy requirements, to select utilities, and to design heat exchanger network. Thus, the methodology used in this work took advantage of both exergy and pinch analyses. The combined thermal-exergy-based pinch approach helped to set energy targets so that all the thermal possible solutions supported by exergy analysis were considered, preventing exclusion of any hot or cold process stream with high potential for heat integration during pinch analysis. Results indicated that the minimum cold utility requirement could be reduced from 601.64 MW (obtained via conventional pinch analysis) to 577.82 MW through screening of streams by the combined methodology.

Techno-economic analysis of polymer electrolyte membrane fuel cell system configurations

The major constraints restricting wide-spread commercialization of fuel cell systems are both technical and economical. Many researchers have designed polymer electrolyte membrane (PEM) fuel cell system configurations, which are different in terms of energy utilization and cost. Therefore, it is imperative to determine the best system configuration in order to produce efficient and economically viable PEM fuel cell systems. This paper reports an exergy and economic analyses conducted on five different PEM fuel cell system configurations with a view to assess their performance. Thermolib – a tool box for MATLAB/Simulink designed for modelling and simulation of energy systems was used to model and simulate the operation of each of the systems (5 kW stack power, 40 cells and 0.25m2 active membrane area) and data obtained was used for the exergy analysis. It was found that largest exergy loss occurred in the fuel cell stack (over 90%). The overall exergy and energy efficiencies of the studied systems were between 24.23% to 30.18% and 47.77% to 59.48%, respectively. A hybrid PEM fuel cell system configuration was proposed and analyzed. It was found to have an overall exergy and energy efficiencies of 31.95% and 62.97%, respectively. The return-on-investment evaluated for each of the five studied PEM fuel cell system configurations as well as the proposed system configuration were 1–27% while the payback periods were 3–13 yrs. The proposed system configuration was found to have the best performance in terms of energy utilization and had the lowest cost per kilowatt net power.

Performance comparison of single-stage mixed-refrigerant Joule–Thomson cycle and pure-gas reverse Brayton cycle at fixed-temperatures from 80 to 180 K

Performance comparison of single-stage mixed-refrigerant Joule–Thomson refrigeration cycle (MJTR) and pure-gas reverse Brayton cycle (RBC) at fixed-temperatures from 80 to 180 K was made in this paper. The simulation was mainly conducted under nonideal conditions with extrinsic irreversibilities. Exergy efficiency and volumetric cooling capacity are two main evaluation parameters. Exergy loss distributions along the cycles were analyzed. Under ideal conditions, RBC achieved the highest exergy efficiency at all temperatures, but lower volumetric cooling capacity than MJTR at middle-high temperatures. Under nonideal conditions, both the exergy efficiency and volumetric cooling capacity of MJTR were obviously superior to RBC from 100 to 180 K, but inferior to RBC at 80 K. Two reasons account for the sharp performance degradation of MJTR: The high fraction of neon resulted in large entropy generation and exergy loss in throttling process. Larger recuperator duty and WLMTD lead to larger losses in the recuperator.

Proposal and research on a combined heating and power system integrating biomass partial gasification with ground source heat pump

Biomass and geothermal energy are becoming promising traditional energy alternatives. For biomass utilization with thermal-chemical gasification technologies, adopting total gasification scheme always leads to large exergy destruction and low thermal efficiency. Besides that, in typical biomass based combined heating and power systems, the flue gas at middle temperature is usually utilized to produce domestic hot water directly, which also leads to large exergy loss for relative large energy grade difference. In the light of geothermal energy utilization, i.e. ground source heat pump systems, for the relative high temperature required of output stream, a relative high compression ratio is required accordingly, which leads to a low coefficient of performance of ground source heat pump. In order to solve the above referred problems, a novel heating and power co-generation system coupling biomass partial gasification and ground source heat pump is proposed and analyzed in this study. The proposed system consists of four subsystems: biomass partial gasification subsystem; gas turbine power generation subsystem; steam turbine power generation subsystems; and ground source heat pump subsystem. The features of this novel system focus on: only part of the biomass is gasified not all (partial gasification), the syngas and un-reacted char are used as fuels for gas turbine (Brayton cycle) and steam turbine (Rankine cycle) power generation respectively; in the ground source heat pump subsystem, the temperature of required water is reduced to 39°C instead of 55°C, and will be further reheated to 55°C by flue gas at middle temperature from biomass based combined heating and power subsystem. The thermodynamic performance of the proposed system is numerically investigated. The results indicate that the proposed system can make a better use of biomass and geothermal energy, which can achieve an overall energy efficiency of 72.12% with a heat to power ratio 3.93, and the coefficient of performance of ground source heat pump subsystem can be improved to 5.6 with the compression ratio 3.2. What's more, the system integrating mechanism in this study is very helpful for other advanced biomass based and multi-energy coupling systems.

Exergy analysis: An optimization tool for the performance evaluation of an organosolv process

This work provides an assessment of the exergy performance of the fractionation of olive tree by organosolv process. The process combines a mixer, a heater, a reactor and, finally, a filtration step for the components separation. Flowsheet modeling and exergy analysis were carried out using Aspen Plus® program, after which, an optimization of the process operational conditions for a better energetic behavior was performed. Thus, the effects of the most influential operational variables in the reactor (delignification temperature, ethanol concentration and solid/liquid ratio) were assessed, which enabled the identification of the most favorable operational conditions. Results indicated that the largest exergy losses were related to the heater with a contribution of 28.8%. Moreover, the delignification temperature was found to be a decisive parameter in the process performance. Under the optimum operating conditions (T=185°C, ethanol concentration=0.7 and biomass/ethanol=0.6), a decrease of about 20% of the total exergy losses was achieved.

Evaluation of an alternative chlorine production process for energy saving toward sustainability

Exergetic‐based solutions are proposed in this work to reduce the energy footprint of an alternative chlorine production process. By presenting exergy analysis results in the form of visualized exergetic process flowsheets using a solution panel, this study does not only find the low‐exergy‐efficient unit operations, but also proposes ways to improve the exergy efficiency based on the major sources of irreversibility of individual unit operations. Based on the amount of exergy losses, the alternative chlorine production process is divided into three main functional blocks. The total internal and external exergy losses for the entire process are calculated as 14,523.5 and 286.1 kJ/kg, respectively. The electrolysis block shows the highest potential for improvements with the largest exergy loss (responsible for more than 96.0% of total losses; with 14,093.2 kJ/kg of internal losses and 146.5 kJ/kg of external losses). Isothermal mixing, lower reaction temperature, and reduced pressure drop are suggested as avenues for improvement. Exergy losses in the gas treatment block with total exergy losses of 411.7 kJ/kg (equivalent to 2.8% of total losses) are mainly due to the cooling column (241.9 kJ/kg), where modifications in column might be helpful as concentration and temperature gradients along the tower are sources of exergy loss. The highest exergy loss in the feed pretreatment block with the total exergy losses of 158.2 kJ/kg (i.e., 1.1% of total losses) belongs to the mixers (106 kJ/kg) where isothermal and also isobaric mixing is required. © 2016 American Institute of Chemical Engineers Environ Prog, 35: 1512–1520, 2016

A resource efficiency assessment of the industrial mushroom production chain: the influence of data variability

We compare the exergetic performance of a conventional industrial mushroom production chain with a mushroom production chain where part of the compost waste is recycled and reused as raw material. The critical exergy loss points (CEPs) identified are the cooking-out process of the spent mushroom substrate, and the phase I composting process which are related to chemical and physical exergy losses, respectively. The total exergy input requirements for the conventional chain are higher (24 GJ per three flushes of mushrooms) than for the alternative chain (17 GJ per three flushes of mushrooms) since more raw materials are required. The largest exergy losses are due to unclosed material balances, i.e. chemical exergy losses, which represent 69% of the total exergy losses for the conventional chain, and 56% for the alternative production chain. Therefore, it only makes sense to reduce any avoidable physical exergy losses after utilizing all mass streams maximally that translate into chemical exergy flows. Further comparison of exergetic indicators (e.g. specific exergy losses, and exergetic cost) shows that recycling material streams would improve the resource efficiency of the industrial mushroom production chain considerably. The variations in the assumed electricity consumption values for the ventilation in phase I composting and for the ammonia scrubbing process affect greatly the exergetic indicators and the number of critical exergy loss points indicating that any further improvement on the exergetic performance of the mushroom production chain should focus on these two process variables. This study shows that variability in data can influence both quantitatively and qualitatively the outcome of exergetic analyses of food production chains since it can lead to the calculation of different values for the selected indicators as well as to the identification of completely different critical exergy loss points.

Thermodynamic analysis of absorption heat transformers using [mmim]DMP/H2O and [mmim]DMP/CH3OH as working fluids

The thermodynamic performances of absorption heat transformers (AHTs) using [mmim]DMP/H2O and [mmim]DMP/CH3OH as working fluids were numerically analyzed. Variations in coefficients of performance (COPs), exergy efficiencies, and circulation ratios of the [mmim]DMP/H2O and [mmim]DMP/CH3OH AHTs with varying operating temperatures were simulated and compared with those of the LiBr/H2O system. COPs and exergy efficiencies of the [mmim]DMP/H2O and [mmim]DMP/CH3OH AHTs were slightly lower than those of the LiBr/H2O system. But the properties of GTL for [mmim]DMP/H2O and [mmim]DMP/CH3OH systems are superior to the LiBr/H2O system. The exergy losses of each component and the entire system for the three working fluids were calculated and compared. Changes in exergy losses with varying absorption, generation, and evaporation temperatures were analyzed. For the [mmim]DMP/H2O and LiBr/H2O systems, the largest exergy losses were produced in generator and evaporator. For the [mmim]DMP/CH3OH system, the largest exergy losses were produced in generator and absorber.

Exergy analysis of building thermal load and related energy flows in buildings

In this study, the exergy analysis method was extended to analyse the building thermal load and related energy flows, aiming to investigate the exergy loss as well as improvement potential for building design and analysis. Five office buildings in five major climate zones in China were taken as case studies for the analysis. The building thermal loads and related energy flows were calculated and analysed using the exergy analysis method. Results show that the building exergy load is relatively low compared to the building thermal energy load. However, it is always met by consuming energy of high exergy value (e.g. electricity), indicating the exergy mismatch between the energy demand and supply. The latent exergy load cannot be disregarded for building thermal load and energy flows analysis, especially for buildings in humid climate. Large exergy loss, which is dominated by solar gain, was found in all five reference buildings across different climate zones. This study provides a comprehensive understanding of building thermal load and related energy flows, and would benefit building evaluation and thermal design of building envelop.

Theoretical analysis of a combined power and ejector refrigeration cycle using zeotropic mixture

A theoretical study on a combined power and ejector refrigeration cycle using zeotropic mixture isobutane/pentane is carried out. The performances of different mixture compositions are compared. An exergy analysis is conducted for the cycle. The result reveals that most exergy destruction happens in the ejector, where more than 40% exergy is lost. The heat exchange in generator causes the second largest exergy loss, larger than 28%. As the mass fraction of isobutane changes ranges from 100% to 0%, the relative exergy destruction of each component is also changing. And mixture isobutane/pentane (50/50) has the maximum exergy efficiency of 7.83%. The parametric analysis of generator temperature, condenser temperature and evaporator temperature for all the mixtures shows that, all these three thermodynamic parameters have a strong effect on the cycle performance.

Coal gasification integration with solid oxide fuel cell and chemical looping combustion for high-efficiency power generation with inherent CO2 capture

Abstract Since solid oxide fuel cells (SOFC) produce electricity with high energy conversion efficiency, and chemical looping combustion (CLC) is a process for fuel conversion with inherent CO 2 separation, a novel combined cycle integrating coal gasification, solid oxide fuel cell, and chemical looping combustion was configured and analyzed. A thermodynamic analysis based on energy and exergy was performed to investigate the performance of the integrated system and its sensitivity to major operating parameters. The major findings include that (1) the plant net power efficiency reaches 49.8% with ∼100% CO 2 capture for SOFC at 900 °C, 15 bar, fuel utilization factor = 0.85, fuel reactor temperature = 900 °C and air reactor temperature = 950 °C, using NiO as the oxygen carrier in the CLC unit. (2) In this parameter neighborhood the fuel utilization factor, the SOFC temperature and SOFC pressure have small effects on the plant net power efficiency because changes in pressure and temperature that increase the power generation by the SOFC tend to decrease the power generation by the gas turbine and steam cycle, and v.v.; an advantage of this system characteristic is that it maintains a nearly constant power output even when the temperature and pressure vary. (3) The largest exergy loss is in the gasification process, followed by those in the CO 2 compression and the SOFC. (4) Compared with the CLC Fe 2 O 3 and CuO oxygen carriers, NiO results in higher plant net power efficiency. To the authors’ knowledge, this is the first analysis synergistically combining in a hybrid system: (1) coal gasification, (2) SOFC, and (3) CLC, which results in a system of high energy efficiency with full CO 2 capture, and advances the progress towards the world’s critically needed approach to “clean coal”.

Thermodynamic analysis of serogroup C antigen production by Neisseria meningitidis

Kinetic and thermodynamic models are developed to relate substrate consumption, serogroup C antigen production and growth rate of Neisseria meningitidis, a meningitides causing bacterium during the uncontrolled, pH controlled and dissolved oxygen controlled cultivation. The model shows that the microorganisms used their energy metabolism the most efficiently in the case of the guaranteed presence of the dissolved oxygen, e.g., the ultimate acceptor of the electrons in the electron transport chain. The largest heat loss per unit biomass production, the largest exergy loss per unit biomass production, and the largest entropy generation per unit biomass production are accounted in the pH controlled experiment, indicating that the growth is achieved under thermodynamically unfavourable conditions. The exergetic efficiency of the microbial production and the antigen production are not parallel, confirming the discussion accounted in the literature that the antigen is produced best under the stress conditions.