Exergy Analysis Of Process Research Articles

Experimental subcritical CO2 continuous extraction of tiger nut oil and process exergy analysis

Experimental subcritical CO2 continuous extraction of tiger nut oil and process exergy analysis

A novel thermodynamic method and insight of heat transfer characteristics on economizer for supercritical thermal power plant

The technology of installing economizer has been proven to be an efficient way of recovering waste heat from flue gas of coal-fired power plants. In this study, a novel theoretical model of economizer involving heat transfer characteristics and regarding the variation of physical properties for working fluid is proposed to evaluate and analyze the thermodynamic performance of economizer for supercritical unit. With such a tool, a theoretical investigation on heat transfer characteristics and exergy analysis of process with regard to economizer installed in a 660 MW supercritical unit is performed. The performance parameters on heat transfer are graphically presented. Moreover, the heat transfer performance of economizer from novel theory is compared with that obtained by traditional method, and the drawbacks of traditional model are thoroughly revealed.Investigation results show that, the temperature difference of counter-flow heat transfer is not always greater than that of forward-flow heat transfer along heating surface length, but the boundary point appears at the dimensionless length of 0.2–0.3 of economizer area, and the lower boiler load, the closer boundary point is to 0.2. The deviation between average heat transfer coefficient and local heat transfer coefficient reaches the maximum at flue gas outlet of economizer, and which varies with load and layout. Furthermore, the heat transfer area required for each 1 °C rise of water temperature and exchanged heat, influenced by the change of physical properties of heat transfer medium, show a different trend with increment in water temperature for economizer under different arrangements. The exergy efficiency presents different variation laws in process heat transfer with different layout and different loads, and such a phenomenon is obviously different from that obtained on overall heating surface. This work provides a broadly applicable approach for economizer and even other heat exchangers to formulaically revealing the veritable microcosmic principles of heat transfer characteristics.

Quantification of irreversibilities in practical cyclic processes using exergy analysis and Gouy-Stodola theorem

The exergy analysis of a process to quantify the irreversibilities is advantageous over the entropy analysis in that it provides the definition of efficiency of a process, referred to as exergetic efficiency (defined as the ratio of exergy recovered to exergy supplied to the process). Unfortunately, the exergy analysis of practical multi-unit cyclic processes is rarely covered adequately in the undergraduate courses in engineering thermodynamics. In this article, the quantification of irreversibilities is illustrated in detail for a practical cyclic steam power plant using exergy analysis and Gouy-Stodola theorem. The efficiencies are determined for the various components and for the whole process. The theoretical background related to exergy, exergy analysis, and Gouy-Stodola theorem is also covered briefly for the benefit of the students. An assessment problem dealing with vapor-compression refrigeration cycle is included at the end in order to assess the intended learning outcomes of this article. The key solution steps along with answers are also provided for the benefit of the readers. As exergy analysis involves advanced level concepts in thermodynamics, the appropriate place for the introduction of the material presented in this article is the second, advanced level, course in thermodynamics.

A dual-benchmark based energy analysis method to evaluate control strategies for building HVAC systems

Optimal control strategies have been increasingly developed in heating, ventilation and air conditioning (HVAC) systems. The commonly used evaluation method, which compares the system’s energy consumption under proposed strategy with that under its original control, has several limitations. A dual-benchmark based CPI (control-perfect index) evaluation method is presented to assess the operation of an airport HVAC system under various control strategies. Firstly, the exergy loss models based on the HVAC productive structure are developed to analyze the exergy efficiency of component and system. Secondly, control-perfect index based on the process exergy analysis is presented as an evaluation factor for the operation of HVAC system. In addition, the original control of system is used as the 1st benchmark to estimate the energy saving capacity of proposed strategy. The ideal operation, which is obtained through the global optimization of exergy loss models, is used as the 2nd benchmark to estimate the improving potential of candidate strategy. Finally, twelve control strategies used in the airport HVAC system are evaluated through the CPI analysis of both component and system. Their energy saving ratios to Benchmark 1 and improving potentials to Benchmark 2 are compared, respectively.