A novel mechanical vapor compression vacuum belt drying system: Model development, experimental verification and performance prediction
Huafu ZhangZhentao ZhangLige TongTiejian YuanJunling YangLi WangPeng XuYoudong WangZe YuJunhao Zhang
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Vapor-compression refrigeration
Vacuum drying
Exergy efficiency
The exergy efficiency model of rotary wheel is presented,and the exergy and exergy efficiency of channel are analyzed using a one-dimensional coupled heat and mass transfer mathematical model and experimental setup based on adsorption channel.The factors of influence on the performance and exergy efficiency,the number of transfer units(NTU),the rotary speed and the regenerative temperature,are investigated using exergy efficiency model,and results show the exergy efficiency is lower because the proportion of chemical exergy component,which is interest,is low,and recovery thermal exergy of regenerative process can rise its exergy efficiency.When the NTU is in 0～2.5,both the performance and exergy efficiency increase rapidly with an increasing NTU,however,when the NTU is bigger than 2.5,a further increasing of NTU will have little merit.Both the performance and exergy efficiency are the biggest under the optimal rotary speed.The power of dehumidification increases and exergy efficiency decreases when regenerative temperature is rising.
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Exergy analysis is application of the second law thermodynamics which provides information about large exergy, exergy efficiency, destruction, and destruction efficiency in each component of PLTU so can be reference for improvement and optimization in an effort to reduce losses and increase efficiency. The exergy value obtained from calculating mass flowrate, enthalpy, ambient temperature, and entropy. The destruction value is obtained from difference between input exergy value and exergy output. The destruction exergy value from comparison between output exergy value to input exergy value, and destruction efficiency value from comparison of destruction value to total destruction value of PLTU components. The results showed that the largest exergy occurred in boilers, namely 778.225 MW in 2018, 788.824 MW in 2019, and 796.824 MW in 2020, lowest exergy value in CP was 0.160 MW in 2018, 0.176 MW in 2019, and 0.160 MW in 2020. The largest destruction occurred in boilers, namely 163.970 MW with destruction efficiency 79.242% in 2018, 179.450 MW with destruction efficiency 82.111% in 2019, and 199.637 MW with destruction efficiency 83.448% in 2020, lowest exergy destruction value at CP, namely 0.056 MW with destruction efficiency 0.027% in 2018, 0.059 MW with destruction efficiency 0.027% in 2019, and 0.056 MW with destruction efficiency 0.023% in 2020. The exergy efficiency occurred in HPH 2, amounting to 94.750% in 2018, 95.187 % in 2019, and 94.728% in 2020, while lowest of exergy efficiency was in LPH 1, namely 43.637 MW in 2018, 33.512 MW in 2019, and 38.764 MW in 2020.
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In this article, a comparative study is carried out between two equations for the exergy efficiency of photovoltaic thermal (PV/T) air collectors; the first equation is based on net output exergy and the second equation is in terms of exergy losses. The exergy efficiency equation parametrically is dependent on thermal and electrical parameters of PV/T air collector; therefore, improved thermal and electrical models are used to calculate them. Developing an exergy balance for PV/T air collector system, the various exergy components in PV/T system are introduced and two equations for the exergy efficiency of PV/T air collector are derived. A computer simulation program is also developed which is based on the used improved thermal and electrical models. In order to validate the simulation results, a typical PV/T air collector has been built and some experiments have been carried out on it. The results of numerical simulation are in good agreement with the experimental results. Finally, parametric studies have been carried out and the effect of design and climatic parameters on two exergy efficiency equations has been investigated. It is observed that the improved exergy efficiency obtained in this paper is in good agreement with the one given by the previous literature and it is better because it shows the portion of each of exergy losses in the exergy efficiency equation, directly.
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Exergy analysis has been used to assess the intrinsic exergy efficiency of a spray drying system modeled to produce 1.25 kg s−1 of skim milk powder. From an exergy perspective, the dryer has a low exergy efficiency of 38% (on an evaporation basis), while the efficiencies associated with the mass transfer and heat transfer are 94% (thermomechanical efficiency) and 30% (transiting exergy efficiency), respectively. The improvement potential of 575 kW, of the 722 kW energy flow in the feed, also shows that the exergy efficiencies of spray dryers are intrinsically small. Reviewing exergy efficiency factors, there appears to be no universal efficiency factor for an exergy analysis. The inevitable (INE) exergy loss method is a potential shortcut technique based on the Carnot efficiency and first law analysis. There are some limitations on using the INE method for processes that are not exclusively thermal; in those cases, an entropy balance (second law property) is more appropriate. The INE method still shows potential as a starting basis of comparison because it shows the scale and the efficiency together, which is important for targeting areas for process improvement without doing a full exergy analysis. This work is a short review of the work on dryer exergy efficiency, mainly focusing on the various factors which are used, followed by a discussion and case study testing each factor to find a potential optimization method and a discussion on each factors merits.
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Exergy destruction associated with the operation of a solar heating system is evaluated numerically via an exergy cascade. As expected, exergy destruction is dominated by heat transfer across temperature differences. An energy analysis is also given for comparison of exergy cascade to energy cascade. Efficiencies based on both the first law and second law of thermodynamics are calculated for a number of components and for the system. The results show that high first-law efficiency does not mean high second-law efficiency. Therefore, the second-law analysis has been proven to be a more powerful tool in identifying the site losses. The procedure used to determine total exergy destruction and second law efficiency can be used in a conceptual design and parametric study to evaluate the performance of other solar heating systems and other thermal systems.
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Abstract The present study aims to conduct a comprehensive evaluation of the energetic and exergetic performance of a dehumidification system utilized in the processing of raw pistachios. The assessment involved the application of the first and second laws of thermodynamics to calculate the exergy aspects of each component of the system, including input and output exergy rates, output/input exergy efficiency, product/fuel exergy efficiency, exergy destruction rate, exergy loss rate, exergy improvement potential rate, and specific exergy consumption. Furthermore, the effect of variations in reference state temperature on the exergy parameters was also investigated. The results indicated that the pre‐dryer chamber had the highest input exergy rate among all the components of the dehumidification system. The product/fuel exergy efficiency is specified to be 35.10%, 9.47%, and 60.43%, for the electro‐fan, heater, and pre‐dryer chamber, respectively, while their output/input exergy efficiency are 87.87%, 22.10%, and 56.28%, respectively. The values of the exergy destruction rate of these components are 0.83, 147.14, and 1.12 kW whereas the exergy loss rate values are found to be 0.03, 4.12, and 9.24 kW, respectively. The improvement potential rate values of these components are obtained to be 0.10, 117.83, and 4.53 kW, while the amount of specific exergy consumption for the dehumidification system is determined as 481.85 kJ/kg. The study also reveals that the exergy parameters vary with changes in reference state temperature and that the exergy efficiencies decrease linearly as reference state temperature rises. Therefore, the findings of this investigation demonstrate the potential for using exergy analysis as an effective tool to improve the performance of dehumidification systems in industrial settings, specifically in the production of pistachios. Practical applications Pistachio nut known as green gold because of its high economical value, is one of the popular nuts over the world. Dehumidification system based on drying technology could be used in food industry with many distinct advantages for processing of particulate crops like pistachios. For a comprehensive assessment of this system, analysis of the first and second laws of thermodynamics is applied. Exergy aspect based on the thermodynamic analysis the is an important stage for designing, modeling, optimizing, and performance assessment of the dehumidification system to produce processed pistachio. The results of this study show that the performance of dehumidification system could be improved by incorporating a steam compressor, a secondary heat exchanger, self‐heat recuperation technology, and a pinch method. It is believed that such a study would contribute to the industrial exploitation of pistachio which can provide insight into decreasing energy consumption and reducing capital costs for engineers and factory owners.
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In the current study, a comprehensive thermodynamic investigation through energy and exergy analyses is performed to study and assess the performance of a three-stage tea drying system which has vibro-fluid bed dryer. Also, energy and exergy efficiencies are, in this regard, investigated with the thermodynamic data obtained from the literature. The parametric studies are carried out to investigate the effects of varying the state properties and operating conditions on the performance of the drying system. The overall energy and exergy efficiencies of the system are found as 42% and 7.2%, respectively. The main reason for the low exergy efficiency is the high exergy destruction rate for the components of the system. The total exergy destruction ratio of drying chambers is calculated as 58% of overall exergy destruction rate of the system.
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In this paper, a comparative study on the different exergetic performance of a solar photovoltaic (PV) module is presented. The exergetic efficiency of the PV module was obtained as functions of environmental, operational and design parameters, and calculated in four cases. In Case I, electrical exergy, thermal exergy and exergy destructions were considered. In Case II, a novel expression on the calculation of solar exergy was proposed. In Case III, chemical and physical exergy components including enthalpy and entropy concepts were discussed. In Case IV, an empirical expression depending on the power conversion efficiency of the system was used. As a result, the exergy efficiency varied in the range 9.25% to 18.32% in Case I, whereas it varied between 9.41% and 18.34% in Case II. In addition, the exergy efficiency varied from 6.58% to 19.99% in Case III, whereas it varied in the range of 9.30% to 18.89% in Case IV during November 2015.
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