Advanced Exergy Research Articles

An Ocean Thermal Energy Conversion power plant: Advanced exergy analysis and experimental validation

An ocean thermal energy conversion power plant was simulated from the point of view of the first, the second law of thermodynamics, and advanced exergy analysis with the aim of increasing the efficiency of the cycle, quantifying and knowing the nature of the exergy destruction. First, the net power output and efficiency of the cycle were successfully calculated as a function of the surface temperature and a certain depth of the ocean, given a pinch temperature differential and the mass flow of seawater in the evaporator and condenser. The numerical simulation are compared with experimental evidence, obtaining better results than thermodynamic models previously reported. Then, the second law of thermodynamics analysis was made using appropriate correlations for the estimation of thermo-physical properties of seawater, revealing the imprecision of assuming properties of pure water in the calculation of the total exergy destruction of the system. Finally, advanced exergy analysis is developed, it provides the nature of the exergy destruction and the best strategy to increase the efficiency of the cycle. The evaporator and turbine are presented as the most promising pieces of equipment to be developed in new future research due to their avoidable endogenous nature of exergy destruction.

Advanced Exergy Analysis of Cascade Rankine Cycle‐Driven Reverse Osmosis System

A novel solar‐assisted cascade Rankine cycle‐driven reverse osmosis (RO) system is proposed and evaluated using conventional and advanced exergy analysis. The system comprises a solar collector as a heat input, steam Rankine cycle (SRC), and organic Rankine cycle (ORC) to generate turbine work and RO units for brackish water treatment. Water and R1233zd(E) are used as the working fluid in the SRC and ORC, respectively. The RO system is proposed to produce 1 m3 h−1 of permeate water from each RO unit. Conventional exergy analysis shows that the solar collector exhibits the highest exergy destruction of 0.798 kW, followed by the RO unit. However, advanced exergy analysis reveals that only 16.16% of the avoidable exergy in solar collectors can be reduced due to technical constraints. Advanced exergy analysis identified that endogenous avoidable exergy destruction contributes 24% of overall exergy destruction, which can be reduced by optimizing operating parameters and making technical improvements. The RO unit has a higher exogenous avoidable exergy, potentially suggesting improvements in other system component parameters. The advanced exergy analysis also prioritized the components for improving system performance in a sequence of RO high‐pressure pumps, RO modules, solar collector, ORC condenser, evaporative condenser, ORC expander, and SRC turbine.

Using advanced exergy analysis to select an optimal working fluid pair in an absorption refrigeration cycle

Using advanced exergy analysis to select an optimal working fluid pair in an absorption refrigeration cycle

Thermodynamic evaluation of shell and tube heat exchanger through advanced exergy analysis

Shell and tube heat exchanger is a pivotal equipment used in industries for heat transfer. Any effort to minimize the irreversibility in the heat exchanger will enhance the performance and leads to energy optimization and cost savings. In the current study, a water to water, segmental baffled shell and tube heat exchanger was considered for an investigation and designed using the Kern method. Exergy analysis and advanced exergy analysis was carried out to understand the performance of the heat exchanger and to determine the possibility of reducing irreversibilities. The results of the exergy analysis showed that the system has 684.6 kW of exergy destruction. Advanced exergy analysis was carried out through endogenous and exogenous modes and subsequently performed for avoidable and unavoidable components. Majority of the exergy destruction in the heat exchanger is avoidable. The results showed that 97.5 % of the total exergy destruction is of endogenous avoidable type. The system can be improved by changing the system configuration, design variables, mass flow rates, materials, and many other parameters. Subsequently, the exergy destruction in the pumps is unavoidable and no further design improvements are required.

Co-gasification of biomass and plastic waste for green and blue hydrogen Production: Novel process development, economic, exergy, advanced exergy, and exergoeconomics analysis

A novel co-gasification process for biomass and plastic waste has been proposed to produce the blue and green hydrogen. For process feasibility, an Aspen Plus simulation model has been developed, and a sustainability analysis is being conducted, focusing on economic viability, exergy, advanced exergy considerations, and exergoeconomics evaluations. The current process has demonstrated economic sustainability, as evidenced by an internal rate of return (IRR) of 8 % at a process efficiency level of 70 %. The process with a waste capacity of 20 tons per hour has the potential to produce approximately 1079 kW-hours of electric power. The surplus electricity, exceeding the process requirements is utilized for green hydrogen production through an alkaline electrolysis cell (AEC). This surplus electricity has the potential to produce around 213.5 kg/day of hydrogen. The exergy analysis of this model highlights that the gasifier component exhibits the lowest exergy efficiency, resulting in the highest exergy loss, around 40 %. Furthermore, advanced exergy analysis identifies both the steam turbine and gasifier as primary sources of exergy destruction, with associated exergoeconomics costs of around $6,561.3 and $6,541.9 per hour, respectively. Consequently, improving the gasifier and steam turbine performance can enhance the overall sustainability of the process.

Advanced exergy analysis of a Reverse Brayton cycle using air as working fluid for cryogenic purposes

The global push to reduce greenhouse gas emissions has led to the ban of high global warming potential (GWP) refrigerants in the refrigeration sector, prompting the adoption of low-GWP alternatives. However, as regulations are expected to tighten, technologies like the Reverse Brayton cycles (RBC) are emerging. RBCs can operate with safe fluids and can reach very low temperatures through compressions, regeneration, and expansion, requiring only electricity. This research presents results from an RBC equipped with a radial turbine using natural air (R-729), a hazard-free fluid with zero ozone depletion potential (ODP) and emission potential. The cycle utilizes automotive components, offering a cost-effective, high-tech solution. The study evaluates the cycle COP at a design point and performs an exergetic analysis at 173K. Using a thermal and fluid dynamic model of the cycle, the critical components of the cycle influencing the COP are identified, and potential improvements are explored.

Advanced exergy and exergoeconomic analysis of a multi-stage Rankine cycle system combined with hydrate energy storage recovering LNG cold energy

In recent years, liquified natural gas (LNG) cold energy utilization technology has been widely investigated due to the increasing global use of LNG. The LNG stored at −162 °C needs to be regasified. To efficiently recover the large amount of cold energy released during the regasification process of LNG with lower investment, a multi-stage Rankine cycle system combined with hydrate energy storage and seawater ice-making cycle was carried out, and the advanced exergy and exergoeconomic analysis was conducted in this paper. The Aspen HYSYS V11 was selected to establish the LNG cold energy recovery process. The result of exergy analysis shows that the exergy destruction rate and exergy efficiency of the total system are 1662.655 kW and 41.149 %, respectively. For conventional analysis results, the burner gives the highest exergy destruction and total cost rate, while considering the result of advanced exergy and exergoeconomic analysis, the condenser 2 gives the highest avoidable exergy destruction, and the turbine 1 gives the highest avoidable total cost rate. In addition, 26.49 % of exergy destruction, 31.6 % of exergy destruction cost rate and 6.59 % of investment cost rate are avoidable, which indicated the optimization potential of the proposed system.

Performance modeling and advanced exergy analysis for low-energy consumption data center with waste heat recovery, flexible cooling and hydrogen energy

Performance modeling and advanced exergy analysis for low-energy consumption data center with waste heat recovery, flexible cooling and hydrogen energy

A comprehensive study of a green hybrid multi-generation compressed air energy storage (CAES) system for sustainable cities: Energy, exergy, economic, exergoeconomic, and advanced exergy analysis

Coastal cities can generate electricity via wind and solar farms. However, the variable nature of these renewable sources makes power output fluctuate. Compressed air energy storage (CAES) is the best solution to address this issue. On the other hand, the challenge of providing potable water persists, even in coastal cities, despite the proximity to the sea. Therefore, reverse osmosis (RO) technology has been used to address this issue due to its reduced energy consumption and environmental impact. The waste heat from the CAES is recommended to increase the system's overall efficiency. In this regard, an innovative green hybrid multi-generation system based on a CAES is proposed. The organic Rankine cycle ORC and RO desalination system have been used in this system to recover waste heat and supply potable and domestic hot water. A comprehensive analysis of the proposed system includes energy, exergy, economic, exergoeconomic, and advanced exergy analyses. The findings indicate this system can produce 37.48 MWh of electricity, 19.42 kg/s of potable water, and 2.67 kg/s of domestic hot water. Furthermore, with a total profit of $ 24.54 M and a payback period of 3.3 years, this system's energy and exergy round trip efficiencies are 73.54 % and 55.01 %, respectively. According to the advanced exergy analysis of the system, 31.5 % of the total exergy destruction is avoidable.

Conventional and advanced exergy-exergoeconomic-exergoenvironmental analyses of an organic Rankine cycle integrated with solar and biomass energy sources

Considering the huge consumption of traditional energy and the rising demand for electricity, the development of renewable energy is very necessary. In this paper, an energy system integrating biomass energy, solar and two-stage organic Rankine cycle (ORC) is proposed, which uses the stable energy output of biomass energy to compensate for the volatility of solar modules. The proposed system comprises a biomass boiler, photovoltaic thermal panels (PV/T), evaporators, condensers, working medium pumps, turbines, a preheater and an air preheater. In addition, conventional and advanced exergy, exergoeconomic and exergoenvironmental (3E) analyses are carried out. Conventional 3E analyses reveal two components that require priority improvement. They are respectively evaporator 1 with the largest exergy destruction (708.2 kW) and exergy destruction environmental impact rate (775.3 mPt/h) and evaporator 2 with the largest exergy destruction cost rate (19.15$/h). The results of advanced 3E analyses show that the largest avoidable endogenous exergy destruction is condenser 1 (136.6 kW), the largest avoidable endogenous exergy destruction cost rate is condenser 2 (3.377$/h), and the largest avoidable endogenous exergy destruction environmental impact rate is condenser 1 (196.1mPt/h). These mean that these components have great potential for improvement in reducing exergy destruction, saving cost and protecting the environment. In addition, the avoidable endogenous exergy destruction/cost/environmental impact rate of evaporator 2 are negative, so evaporator 2 is not suitable as a priority component for improvement, which is contrary to the conclusions of conventional 3E analyses. It is found that conventional 3E analyses can only point out the biggest exergy destruction point, but cannot indicate whether the components with the greatest exergy destruction have the greatest potential for improvement. However, advanced 3E analyses can show the improvement potential of each component by improving its own performance and the external conditions. Therefore, it is necessary to conduct advanced 3E analyses.

Advanced exergy analysis of a double absorption heat transformer recovering industrial waste heat for pure water production

Conventional and advanced exergy analyses are applied for a double absorption heat transformer system utilizing low-grade industrial waste heat for water purification. The conventional method provides information about the location and magnitude of inefficiencies and does not consider any interaction among components regarding the irreversibility occurring in them. The advanced exergy technique however, exposes the real origins of irreversibilities and latent potential for enhancement of each system component, along with reflecting mutual interaction between components. The simulation of the cycle is carried out with engineering equation solver software. The results show that the overall improvement potential of the cycle is only 16%. Also, the mutual interdependency between components is found to be weak as 97% of the total exergy destruction appeared as endogenous. Furthermore, considering endogenous avoidable exergy destruction, the improvement strategy of components obtained by advanced exergy method is disparate from conventional one. Results of the advanced exergy suggest this order: condenser, absorber, evaporator, absorber/evaporator assembly, generator assembly, economizer 1, and economizer 2. Additionally, a parametric investigation is performed to assess the sensitivity of splitting exergy destruction to the variation of heat source temperature for highlighting improvement opportunities. The results indicate that the condenser yields the highest amount of avoidable endogenous exergy destruction in the entire studied temperature range.

Advanced Exergy-Based Analysis of an Organic Rankine Cycle (ORC) for Waste Heat Recovery.

In this study, advanced exergy and exergoeconomic analysis are applied to an Organic Rankine Cycle (ORC) for waste heat recovery to identify the potential for thermodynamic and economic improvement of the system (splitting the decision variables into avoidable/unavoidable parts) and the interdependencies between the components (endogenous and exogenous parts). For the first time, the advanced analysis has been applied under different conditions: constant heat rate supplied to the ORC or constant power generated by the ORC. The system simulation was performed in Matlab. The results show that the interactions among components of the ORC system are not strong; therefore, the approach of component-by-component optimization can be applied. The evaporator and condenser are important components to be improved from both thermodynamic and cost perspectives. The advanced exergoeconomic (graphical) optimization of these components indicates that the minimum temperature difference in the evaporator should be increased while the minimum temperature difference in the condenser should be decreased. The optimization results show that the exergetic efficiency of the ORC system can be improved from 27.1% to 27.7%, while the cost of generated electricity decreased from 18.14 USD/GJ to 18.09 USD/GJ.

Investigation of improvement potential of a double-efficient CO2 cooling and heating system using an ejector with advanced exergy analysis

This paper presents an improvement potential investigation of a double-efficient CO2 cooling and heating system using an ejector, which can operate in cooling or heating mode. For the first time, advanced exergy analysis is employed to offer improvement direction and guidance for the practical application of this novel system. Compared to the conventional exergy analysis, which is widely used in previous work, advanced exergy analysis can supply more information about the interaction among components and the avoidable exergy destruction under the technical constrains. Thus, more practical and accurate conclusions can be drawn. The results reveal that 74.56 % of the total exergy destruction is endogenous in cooling mode, demonstrating the weak interaction among components. The compressor should be given the highest priority for improvement, due to its largest sum of avoidable exergy destruction (17.71 W). The effect of ejector component efficiency is also discussed. As it varies from 0.5 to 0.9, the system exergy efficiency rises from 11.59 % to 14.18 %. Furthermore, the proposed system shows great advantage with 31.31 % less exergy destruction compared to ejector expansion refrigeration cycle. In heating mode, the compressor also deserves to be optimized first. But the expansion valve possesses no effort for improvement due to its negative sum of avoidable exergy destruction (−1.203 W).

Experimental investigation on a closed-cycle single-stage turbo-refrigerator for deep freezing

Turbo-refrigerator based on reverse Brayton cycle is an attractive solution for providing refrigeration at ultra-low temperatures below −60 °C for deep freezing. Employing a motor-driven turboexpander compressor (MTEC) not only allows the turbo-refrigerator to achieve high compactness, reliability and flexibility, but also leads to new operating characteristics especially in a closed cycle. This paper aims to investigate the operating characteristics of a closed-cycle single-stage turbo-refrigerator with a MTEC. A high-speed turbo-refrigerator system with the design refrigeration temperature of −86 °C was manufactured and then tested under variable rotating speeds and refrigeration conditions. Test results show that a refrigeration temperature below −96.4 °C was obtained in a pull-down progress. Compared with a fixed-speed operation, variable-speed operation enables the independent control of the refrigeration temperature and the cooling capacity. With the decrease in the refrigeration temperature (or in the cooling capacity), the rotating speed has a greater effect on the COP than that on the cooling capacity (or on the refrigeration temperature). The mass flow rate along with the compression and expansion work shows a decreasing trend as the refrigeration temperature decreases at a fixed speed of the MTEC. The advanced exergy analysis shows a total exergy efficiency of 0.42 (with a COP of 0.62 at refrigeration temperature of −86 °C) could be expected with component improvement in the near future, and it is of the priority to increase the efficiency of the turboexpander.

Advanced exergy analysis of a novel air source heat pump system coupled with the liquid receiver and gas-liquid separator

In previous studies, an air source heat pump system coupled with the liquid receiver and the gas-liquid separator was proposed, which improved the defrosting efficiency according to the improvement of the refrigerant mass flow during defrosting and shortened the defrosting time. In this paper, the exergy efficiencies and exergy loss (including endogenous and exogenous exergy loss) of the new system are studied. They are compared with the original air source heat pump system by constructing the advanced exergy study model of the system. Results show that the exergy efficiencies of the compressor, evaporator, and throttle valve of the new system are increased by 0.2%, 2.3%, and 5.9%, respectively. The endogenous exergy loss of the new system accounts for 65.16% of the total exergy loss of the system, and most of the exergy loss in the new system is caused by the components themselves.

Exergy destruction analysis of a power generation system utilizing the cold energy of LNG

The purpose of this research is in-depth understanding of the internal causes of exergy destruction in various parts of the system and to identify potential improvements for the components. The focus is on a combined cycle power generation system that utilizes the organic Rankine cycle (ORC) and direct expansion cycle (DEC). To investigate the primary sources of exergy destruction in each component, advanced exergy analysis (AEA) is utilized. The result demonstrates that the net out power of the proposed system can reach 106.64 kW with energy efficiency of 11.22%, and exergy efficiency of 21.40%. The heat exchanger is identified as the primary contributor to exergy destruction, constituting 81.70% of the total ratio. Specifically, the condenser exhibits the highest exergy destruction ratio at 59.82%, indicating a need for prioritized optimization efforts. The findings of AEA reveal that the primary source of component irreversibility stems from the endogenous part. This shows that, while most exergy destruction is unavoidable, there remains room for system improvement. Regarding the turbine, its exergy destruction is primarily attributed to inefficiencies, leading to irreversibility. Nevertheless, there is exergy destruction that may be avoidable and can be reduced by 25.93 kW, which is 2.5 times greater than that of the heat exchanger. This finding underscores the high potential for improvement in ORC and DEC turbines, making them a priority for optimization efforts.

Exergy-based techno-economic and environmental assessments of a proposed integrated solar powered electricity generation system along with novel prioritization method and performance indices

This study focused on the two important gaps in the literature. The first is solar energy- powered electricity generation in a more economical way via the integration of flat plate solar collector (FPSC), an Organic Rankine Cycle (ORC), and an absorptional heat transformer (AHT) system. Another gap is advanced exergy analysis of the AHT cycle/ORC process based on renewable energy integration to reveal clues for improving the system. To close these gaps, a novel system including a lithium bromide AHT cycle-ORC with a FPSC system application was proposed in this study. In this proposed system, the temperature of the heat source for the ORC system was upgraded via an integration of the AHT and FPSC cycles. The main components of the AHT cycle are the condenser (ABScon), refrigerant cycle pump (P1), evaporator (EV), absorber (ABS), solution heat exchanger (SHX), absorbent cycle pump (P2), expansion valve (V), generator (Gen), ORC turbine (ORCT) and ORC condenser (ORCcon). To demonstrate the electricity production from solar energy in a more economical way thanks to the proposed system, a comparison was made with similar-scaled existing solar power plants. The results supported the main purpose of this study. The annual electricity production with the proposed system was calculated as 2601 MWh, with initial investment cost and payback period values of US$3.924 million and 4.531 years, respectively. The conventional and advanced exergy, exergoeconomic, environmental impact, and sustainability analyzes were also performed. Based on these, the novel performance parameters and prioritization method were proposed to assess the improvement potential of the system. The results indicated that SHX and FPSC had the highest exergy destruction rates (EDRs) of 23.711% and 21.849% over 5853.89 kW due to the stronger thermal and chemical reactions. Similarly, Gen, FPSC, and SHX had the highest ED cost rates (CRs) of 67.59%, 59.09%, and, 47.98%, respectively. Gen, V, and ORCcon were higher contributors to the exergy destruction rates of almost all the components. However, these showed an adverse manner for irreversibility activities. So, the temperatures of Gen and ORCcon should be optimized carefully. ABScon, P2, P1, ABS, Gen, ORCT, and ORCcon had high development priority to improve the whole system.

Wind energy-driven medical waste treatment with polygeneration and carbon neutrality: Process design, advanced exergy analysis and process optimization

This study presents a novel and original rich-O2 steam gasification-based process integrated with carbon capture, utilization, and storage (CCUS) for carbon-neutral valorization of medical waste (MW) in Hong Kong. The process employs a calcium loop to capture CO2 from the gasifier-produced syngas, simultaneously facilitating H2 production. The captured CO2 is optionally utilized to generate high value-added products like ethylene, ethanol, and acetic acid through electroreduction. The process incorporates multiple steam Rankine cycles for converting waste heat to power, achieving a total energy efficiency of 71.38%. Advanced exergy analysis reveals over 70% endogenous exergy destruction in reactors and 79.05% avoidable exergy destruction in heat exchangers and condensers, which may be paid more attention. Driven by wind energy, the process has the potential to achieve zero CO2 emission. Economic optimization demonstrates potential profits up to 10410 HK$/h with sufficient wind energy supply and 2548.2 HK$/h with limited supply. This study highlights the novelty of a carbon-neutral MW valorization approach, CCUS utilization, advanced exergy analysis, optimization and renewable wind energy for sustainability and economic viability.

Advanced exergy analysis and optimization of a coal to ethylene glycol (CtEG) process

Coal to ethylene glycol (CtEG) process is criticized for its high energy consumption and poor thermodynamic performance. This process is detailed modeled by dividing it into coal gasification, water gas shift, acid gas removal, dimethyl oxalate synthesis, ethylene glycol synthesis, and ethylene glycol refining units. After comparing and verifying with literature and industrial data, this work conducts a detailed advanced exergy analysis and optimization of the CtEG process. Results show that the exergy efficiency of the CtEG process is only 33.56%, and the coal gasification, dimethyl oxalate synthesis, and ethylene glycol synthesis units have the largest exergy destruction. Furthermore, the exergy destruction of all components in the CtEG process is dominantly endogenous (76.03%) and avoidable (64.62%). The real improvement potential of the CtEG process is 64.62%. A parametric study was then performed to investigate the effect of key parameters on the thermodynamic performance of the CtEG process to reduce its avoidable endogenous and exogenous exergy destruction. After optimization, the exergy efficiency of the improved CtEG process is increased by 12.34%, and the avoidable exergy destruction is reduced by 160.48 MW compared with the reference system. Therefore, the improved CtEG process has better thermodynamic performance than the reference process.

Conventional and Advanced Exergy Analyses of Industrial Pneumatic Systems

Pneumatic systems are widely used in industrial manufacturing sectors. However, the energy efficiency of pneumatic systems is generally much lower than their hydraulic and electric counterparts. It is necessary to explore more elaborate theories and methods for achieving better energy performance in pneumatic systems. In this study, for investigating the interaction effects between pneumatic components and the accessible improvement potential of energy efficiency in a pre-existing pneumatic system, the advanced exergy analysis is conducted with a better understanding of exergy destruction. The conventional exergy analysis is also carried out for comparison. The results show that an exergy efficiency of 17.3% could be achieved under the real condition in the case of the investigated pneumatic system. However, under unavoidable conditions, the theoretical maximum exergy efficiency could reach 70.5%. This means there is a significant potential for improving the energy performance of the investigated system. Furthermore, both conventional and advanced exergy analyses indicate that the pneumatic cylinder has the greatest potential for improvement. The advanced exergy analysis reveals the complex and variable interactions between pneumatic components. It highlights that the exergy destruction of some components is caused by other components in the system, and thus, improving energy efficiency at the system level rather than at the component level is of great significance. Besides, a priority order of all pneumatic components is determined, thereby guiding the improvement of the energy efficiency of the pneumatic system.