上游充通大￥ SHANGHAI JIAO TONG UNIVERSITY Engineering Thermodynamics I Lecture 37-38 Chapter 8 Exergy-A measure of work potential Spring,2017 Prof.,Dr.Yonghua HUANG 强 MAALLMMMMAR http://cc.sjtu.edu.cn/G2S/site/thermo.html 1日G
Engineering Thermodynamics I Lecture 37-38 Spring, 2017 Prof., Dr. Yonghua HUANG Chapter 8 Exergy – A measure of work potential http://cc.sjtu.edu.cn/G2S/site/thermo.html
Previous:energy analysis All considered the isentropic process as the goal to strive for. Maximize isentropic efficiency This approach is short sighted for three reasons: 1.It ignores processes where heat transfer is present.(The majority of all practical processes.) 2.It assumes that reversibility can be obtained. 3.It assumes that the exit state of a device can "float",i.e.,cases where the exit pressure is fixed,but the exit temperature is allowed to fall below the temperature of the surroundings. >Need different approach for thermodynamic analysis: Exergy (Availability)Analysis 上游气通大 Monday,April 24,2017 2 SHANGHAI JIAO TONG UNIVERSITY
Monday, April 24, 2017 2 Previous: energy analysis All considered the isentropic process as the goal to strive for. Maximize isentropic efficiency This approach is short sighted for three reasons: 1. It ignores processes where heat transfer is present.(The majority of all practical processes.) 2. It assumes that reversibility can be obtained. 3. It assumes that the exit state of a device can “float”, i.e., cases where the exit pressure is fixed, but the exit temperature is allowed to fall below the temperature of the surroundings. Need different approach for thermodynamic analysis: Exergy (Availability) Analysis
Deady state Hot ◆Heat coffee 10m/s Heat engine Wnet.out Wmax rig Low-temperature Sink Has a potential to do work Has a potential to do work Has a potential to do work Systems from State A>State B(equilibrium):work A system that is in equilibrium with its environment is said to Air 25C 101 kPa T0=25C be at the dead state. V=0 Po=101 kPa z=0 Not able to do any work Monday,April 24,2017 3 SHANGHAI JIAO TONG UNIVERSITY
Monday, April 24, 2017 3 Deady state A system that is in equilibrium with its environment is said to be at the dead state. Has a potential to do work Has a potential to do work Has a potential to do work Not able to do any work Systems from State A State B (equilibrium): work
Defining exergy Environment To:Po Notes: CM2 CM1 》Wuse=net useful work of combined I Closed I system(CM1)! 1 System use 》The goal is to maximize Wuse! Exergy Availability Maximum theoretical work output that could be done by a system if it was to come into equilibrium with its environment! 上游充通大 Monday,April 24,2017 4 SHANGHAI JIAO TONG UNIVERSITY
Monday, April 24, 2017 4 Defining exergy Exergy/Availability = Maximum theoretical work output that could be done by a system if it was to come into equilibrium with its environment! Notes: » Wuse = net useful work of combined system (CM1)! » The goal is to maximize Wuse! Closed System Environment @ To ,po Qi CM2 CM1 Wi Wuse
Exergy Analysis for Closed Systems © 1st law for CM1 (finite time process):---- Environment To.Po AECMI =QCMI -Wuse Closed System Wuse Assumptions Maximize Wuse>final state of CM1 is the dead state e Sole effect is work out>QcM1=0 → Wuse.max =-AECMI=-(AUCMI +AKECMI +APECMI) =-(AUcM2+AKECMa2+APECM22+AU。+AKE。+PEe) 上游气通大粤 Monday,April 24,2017 5 SHANGHAI JLAO TONG UNIVERSITY
Monday, April 24, 2017 5 Exergy Analysis for Closed Systems 1st law for CM1 (finite time process): Assumptions • Maximize Wuse final state of CM1 is the dead state • Sole effect is work out QCM1 = 0 E Q W CM1 CM1 use use,max CM1 CM1 CM1 CM1 CM2 CM2 CM2 e e W E ( U KE PE ) ( U KE PE U KE PEe ) Closed System Environment @ To ,po Qi CM2 CM1 Wi Wuse