Exergy-based Evaluation Research Articles

Energy- and exergy-based economical and environmental (4E) evaluation of the influence of natural pollutants on PV array performance

In the present investigation, thermodynamic/eco/environmental analysis of the relationship between time-dependent particle deposition and thermal-based losses and incident solar radiation intensity on the PV front cover glass during the months with the least precipitation in the city of Samsun (Turkey) was performed. To evaluate the effect of dust accumulation, controlled experiments were carried out where the surface of one set of PV panels was periodically cleaned with water and the other one was left to natural contamination. The results showed that over three months, the difference in energy loss rates between cleaned and uncleaned PV arrays ranged from 2.53% to 8.1%, with the exergy efficiency difference measured at 1.3%–2.44%. According to the energy-based analyzes, August was the most effective month, with cleaned PV arrays reducing CO2 emissions by 401.5 kg and saving $6.02, compared to uncleaned PV arrays which reduced emissions by 362.21 kg and cost $5.43, respectively. Similarly, in the exergy-based evaluation, August was the most efficient month, with cleaned PV arrays decreasing CO2 emissions by 401.5 kg and saving $6.02, compared to uncleaned PV arrays, which dropped CO2 emissions by 362.21 kg and costing $5.43, respectively. While the cleaned surfaces were initially positive, a decrease was observed over time. However, the efficiency of PV decreased with increasing panel temperature. The study concludes that PV surfaces need to be cleaned at certain intervals, depending on the location’s weather conditions.

Exergy based assessment for decarbonization of CO2 foam flooding enhanced oil recovery process and energy transition with hydrogen

Water Flooding (WF) and CO2 Foam Flooding EOR (CFF) processes are exergy intensive and CO2 emissive in nature. Hence, the objective of this work is to investigate the ways to attain decarbonization and to evaluate hydrogen requirement for attaining energy transition during WF and CFF processes under different operational conditions. This was performed by using exergy-based evaluation metrics (i.e., Exergy-Return on Exergy-Investment (ERoEI), carbon emissions and hydrogen intensity). It was found that CFF process was 1.69 times more exergy intensive than WF process. Further, for every barrel of oil produced, WF process had net-CO2 emission of 5.18 kg, while CFF process had net-CO2 sequestration of 814.5 kg. Moreover, during CFF process, reducing the surfactant concentration from 2.5 wt% to 0.5 wt% had correspondingly decreased CO2 emissions from 70% to 31%. Further, to attain energy transition during production operations, it was evaluated that WF process requires 1.7 times more hydrogen than CFF process.

Evaluation of the intermittent performance of heating terminals based on exergy analysis: Discriminate the impacts of heat and electricity input

Intermittent space heating is widely recommended for building energy conservation. Although various heating terminals have been developed, the performance improvement for intermittent heating is still worth evaluating to maximize the energy-saving potential. Currently, energy-based and entransy-based methods are commonly applied for performance analysis, but these methods cannot reflect the comprehensive utilization efficiency of various energy forms. Therefore, this work conducts a discrimination study on the intermittent energy utilization of different heating terminals based on an exergy-based evaluation method. Specifically, this study analyzes the intermittent exergy flow performance of the “heating source-terminal-indoor environment” process. Moreover, we compare how various energy forms impact performance improvement aiming to recognize the potential of different optimization strategies from the aspect of heat and electricity exergy. Furthermore, the holistic performance of heating terminals under different heating source plants is evaluated to support practical applications. The experimental results suggest that the convective terminals attained the best intermittent performance and that the exergy efficiency of the fan coil is the highest (32.9%), with a greater improvement achieved by raising the fan power and connecting the fan coil to the heat pump system. This study provides a novel aspect of analyzing heating performance and optimization path of heating systems.

Power-to-liquid hydrogen: Exergy-based evaluation of a large-scale system

Hydrogen as an energy vector is seen as a key for the energy transition. Recently, more than 30 countries have launched their hydrogen strategies and roadmaps. Hydrogen storage and transportation are challenging steps of the hydrogen economy since all available options have significant drawbacks. This paper evaluates a power-to-liquid hydrogen process; the system is “charged” with electricity from renewable sources to produce hydrogen via water electrolysis; the produced hydrogen gas is liquefied and stored at ambient pressure and cryogenic temperature. The purpose of this paper is to report the first evaluation results of a system including a polymer electrolyte membrane electrolyser and a hydrogen liquefier. The evaluation was conducted using exergy-based methods, i.e. exergetic, exergoeconomic and exergoenvironmental analyses. The process of hydrogen liquefaction was simulated with the aid of the Aspen Plus software. The exergetic efficiencies for the liquefaction process and for the electrolyser are 42% and 47%, respectively. While the total exergetic efficiency of the power-to-liquid hydrogen system amounts to 44%. The total exergy destruction for the liquefier amounts to 9.3 MW and for the polymer electrolyser membrane electrolyser amounts to 19.3 MW. The electrolyser followed by the hydrogen compressors were identified as the components with the highest exergy destruction values and investment costs, while the compressors and the recuperators account for the highest exergoenvironmental impact. The sensitivity analysis shows that the specific liquefaction cost of hydrogen strongly varies with the electricity price and the cost of green hydrogen.

Exergy-based evaluation of a waste heat driven polygeneration system with CO<SUB align="right">2 as the working fluid

A waste heat driven polygeneration system that couples a supercritical power cycle with a transcritical refrigeration cycle is proposed and evaluated in this study. The stand-alone system using carbon dioxide as the working fluid produces power, refrigeration, and heating capacities simultaneously. Three scenarios of the system with different ambient temperatures (25-35°C) are optimised using exergoeconomic approach to obtain the lowest average cost of the products. The obtained results show that the average product cost is the highest ($113/GJ) for the scenario with the ambient temperature of 30°C, while the product cost is reduced to $82/GJ when the ambient temperature is 35°C. Moreover, the pressure for merging the sub-systems influences the performance of the system significantly, and the variations of the operating conditions differ among the scenarios by increasing the merging pressure. Besides, a multi-objective optimisation is implemented for supporting decision-makers to find the optimal solutions.

Exergy-based evaluation of a waste heat driven polygeneration system with CO<SUB align="right">2 as the working fluid

A waste heat driven polygeneration system that couples a supercritical power cycle with a transcritical refrigeration cycle is proposed and evaluated in this study. The stand-alone system using carbon dioxide as the working fluid produces power, refrigeration, and heating capacities simultaneously. Three scenarios of the system with different ambient temperatures (25-35°C) are optimised using exergoeconomic approach to obtain the lowest average cost of the products. The obtained results show that the average product cost is the highest ($113/GJ) for the scenario with the ambient temperature of 30°C, while the product cost is reduced to $82/GJ when the ambient temperature is 35°C. Moreover, the pressure for merging the sub-systems influences the performance of the system significantly, and the variations of the operating conditions differ among the scenarios by increasing the merging pressure. Besides, a multi-objective optimisation is implemented for supporting decision-makers to find the optimal solutions.

Energy and Exergy Evaluations of a Combined Heat and Power System with a High Back-Pressure Turbine under Full Operating Conditions

High back-pressure technology is a promising method for the waste heat recovery of exhaust steams in combined heat and power systems. In this research, a 300 MW coal-fired subcritical combined heat and power system was selected as the reference system, and modeled in EBSILON professional. Then, energy-based and exergy-based performances of the high back-pressure system and traditional combined heat and power system were compared under full operating conditions. Moreover, a novel exergy-based evaluation method, which considers the energy level of the heating supply, was proposed and applied to evaluate the two systems. Results show that: In design conditions, both the heating capacity and power output of the high back-pressure system were higher than those of the extraction condensing system, which led to 17.67% and 33.21% increments of the gross thermal efficiency and generation efficiency, respectively. Compared with the extraction condensing system, the exergy efficiencies of the high back-pressure system were 7.04–8.21% higher. According to the novel exergy-based evaluation, the exergy efficiencies for the generation of the high back-pressure system and extraction condensing system were 46.48% and 41.22%, respectively. This paper provides references for the thermodynamic performance evaluation of the combined heat and power system.

A comprehensive exergy-based evaluation on cascade absorption-compression refrigeration system for low temperature applications - exergy, exergoeconomic, and exergoenvironmental assessments

Abstract The main target of present work is to assess a new cascade absorption-compression refrigeration system from the exergy, exergoeconomic, and exergoenvironmental standpoints. The combined system is simulated by Aspen HYSYS V10, and Matlab software. This hybrid system is able to supply a cooling duty of 60.65 kW with a coefficient of performance of 0.226. Also, the evaporation temperature of this system is calculated to be −54.62 °C. In order to analysis the energy quality and performance of this process, an exergy assessment is conducted. Based on the gained outcomes of exergy evaluation, the total exergy efficiency of process is computed to be 69%. As well as, the overall exergy destruction rate is obtained by 83.4 kW, and the greatest value of irreversibility is found to be in the rectifier (16.05 kW). In continue, to evaluate the process economically, the exergoeconomic evaluation is carried out. It was found that the rectifier has the largest amount of investment cost rate (2.817 $/h). As well as, the highest values of relative cost difference (r) and exergoeconomic factor (f) are related to the compressor and gas heat exchanger, respectively. The environmental impacts of hybrid system are determined by combining life cycle assessment and exergy evaluation (exergoenvironmental method). According to the achieved outcomes, it can be noticed that rectifier and low - pressure pump have the largest and least values of environmental impact. At the final stage of this research, to evaluate the interaction between environmental, technical, and economical aspects of process, the 3D parametric assessment is conducted on devices of the auto - cascade refrigeration system.

Hydrogen Economy Model for Nearly Net-Zero Cities with Exergy Rationale and Energy-Water Nexus

The energy base of urban settlements requires greater integration of renewable energy sources. This study presents a “hydrogen city” model with two cycles at the district and building levels. The main cycle comprises of hydrogen gas production, hydrogen storage, and a hydrogen distribution network. The electrolysis of water is based on surplus power from wind turbines and third-generation solar photovoltaic thermal panels. Hydrogen is then used in central fuel cells to meet the power demand of urban infrastructure. Hydrogen-enriched biogas that is generated from city wastes supplements this approach. The second cycle is the hydrogen flow in each low-exergy building that is connected to the hydrogen distribution network to supply domestic fuel cells. Make-up water for fuel cells includes treated wastewater to complete an energy-water nexus. The analyses are supported by exergy-based evaluation metrics. The Rational Exergy Management Efficiency of the hydrogen city model can reach 0.80, which is above the value of conventional district energy systems, and represents related advantages for CO2 emission reductions. The option of incorporating low-enthalpy geothermal energy resources at about 80 °C to support the model is evaluated. The hydrogen city model is applied to a new settlement area with an expected 200,000 inhabitants to find that the proposed model can enable a nearly net-zero exergy district status. The results have implications for settlements using hydrogen energy towards meeting net-zero targets.

Exergy-based evaluation of methanol production from natural gas with CO2 utilization

Energy and exergy analyses were carried out for a medium-capacity methanol plant based on a low-pressure synthesis process for natural gas. The process comprises a pretreatment of natural gas, a steam-methane reforming unit for generation of synthesis gas, a methanol synthesis, a distillation of crude methanol, and an integrated steam cycle for waste heat recovery. Carbon dioxide from carbon capture is used for gas conditioning by adjusting the syngas module for methanol synthesis through counterbalancing of the excess hydrogen. A sensitivity analysis was performed to identify favorable operation parameters for the tubular steam reformer. The energetic and exergetic efficiencies for the overall system were found to be 35.9% and 37.7%, respectively. The specific energy requirements (energy intensities) are 19.6 GJth/tCH3OH and 0.8 GJel/tCH3OH, while the specific methane consumption was calculated to be 0.54 ton CH4 per ton CH3OH. Compared to a stand-alone plant, the utilization of carbon dioxide increases the methanol yield by 22%. The exergy analysis shows that the highest inefficiencies occur in the reforming unit, the steam cycle, and the synthesis unit. In particular, the steam reformer, the synthesis reactor, and several heat exchangers show a high potential for thermodynamic improvement.

Development of Chinese Exergy-Based Characterization Factors for Emissions in Life Cycle Impact Assessment

The development of eco-materials has become an important direction of materials science. Using appropriate methods to evaluate the environmental impacts caused by materials production is important for eco-materials research. For the purpose of obtaining more objective results to environment impacts, several exergy-based methods to indicate resource depletion have been established. However, no proper exergy-based evaluation methods for emissions in China have been reported. The objective of this study is to establish comprehensive exergy-based characterization model for life cycle assessment, and to apply this model to steel production.

An exergy-based evaluation model for the performance of the fossil fuel life cycle

This paper combines exergy analysis and life cycle theory in evaluating the performance of fossil fuels. The entire life cycle of fossil fuels, including production, utilisation, recycling and treatment, are taken into consideration, with a focus on exergy efficiency and environmental performance. In addition to the exergy depletion of fossil fuels themselves and the direct environmental impact, indirect exergy cost and indirect environmental impacts during the life cycle of fossil fuels are tracked, and the environmental impacts are quantified using exergy methods. The overall exergy efficiency and the exergy-based environmental impact index of the fossil fuel life cycle are proposed to evaluate the utilisation efficiency and environmental impacts of the fossil fuel. Moreover, a case study from Xi'an, China, regarding heating illustrates the application of this method.

Comprehensive exergy-based evaluation and parametric study of a coal-fired ultra-supercritical power plant

In this paper, both conventional and advanced exergy analyses were conducted to a large-scale ultra-supercritical coal-fired power plant. The objectives of the conventional one are to compare the exergetic performances of different components, to identify and quantify the sites with the largest exergy destruction and losses, and to find the fuel-savings potential by improving each component in isolation. The advanced exergetic analysis focuses on the thermodynamic interactions among components and the sources for energy-saving potential of each component. Moreover, comparisons with several subcritical units are conducted and a sensitivity analysis shows the dependencies of the overall exergetic efficiency on a number of key design parameters. The results display the spatial distribution of exergy destruction and losses in detail and three performance ranges for different types of heat exchangers involved in the system. The energy-saving potentials at both the system and the component levels by improving an individual component are not in accordance with the amount of its exergy destruction. Improvement strategies for different components differ significantly due to the varied contributions of endogenous/exogenous parts to their avoidable exergy destructions. With an increase in the steam conditions, the exergy destruction ratio of the boiler is significantly reduced, contributing mainly to the system improvement. The most effective and achievable measure for reducing the fuel consumption is still the reasonable utilization of available low-grade heat. This framework provides a basis for the quantifying proposals of exergy-driven strategies for improving the system.

Exergetic performance evaluation and parametric studies of solar air heater

The present study aims to establish the optimal performance parameters for the maximum exergy delivery during the collection of solar energy in a flat-plate solar air heater. The procedure to determine optimum aspect ratio (length to width ratio of the absorber plate) and optimum duct depth (the distance between the absorber and the bottom plates) for maximum exergy delivery has been developed. It is known that heat energy gain and blower work increase monotonically with mass flow rate, while the temperature of air decreases; therefore, it is desirable to incorporate the quality of heat energy collected and the blower work. First it is proved analytically that the optimum exergy output, neglecting blower work, and the corresponding mass flow rate depend on the inlet temperature of air. The energy and exergy output rates of the solar air heater were evaluated for various values of collector aspect ratio (AR) of the collector, mass flow rate per unit area of the collector plate (G) and solar air heater duct depth (H). Results have been presented to discuss the effects of G, AR and H on the energy and exergy output rates of the solar air heater. The energy output rate increases with G and AR, and decreases with H and the inlet temperature of air. The exergy-based evaluation criterion shows that performance is not a monotonically increasing function of G and AR, and a decreasing function of H and inlet temperature of air. Based on the exergy output rate, it is found that there must be an optimum inlet temperature of air and a corresponding optimum G for any value of AR and H. For values of G lesser than optimal corresponding to inlet temperature of air equals to ambient, higher exergy output rate is achieved for the low value of duct depth and high AR in the range of parameters investigated. If G is high, for an application requiring less temperature increase, then either low AR or high H would give higher exergy output rate.