Open Steady Systems - TEST Tutorial

Tutorial (TOC) > Daemons > Exergy Analysis > Manual

a. Evaluation of stored and flow exergy

The specific stored exergy is represented by the symbol phi and the specific flow exergy by the symbol psi in TEST. As part of a state, they appear in both system and flow states. However, since they depend on the ambient conditions (or the dead state), the dead state must be calculated first. By convention, State-0 is the designated dead state.

Launch any state daemon and select the working substance. Now calculate the dead state, State-0, from the atmospheric pressure, p0, and temperature, T0. Thereafter, whatever state you evaluate, phi and psi are automatically calculated as part of the state. To obtain the total stored exergy of a system, you can evaluate expressions such as '=m1*phi1' or 'm2*phi2' in the I/O Panel.

b. Single-Flow Exergy Analysis: Single-flow open steady daemons have four tabbed panels: (i) State Panel (ii) Device Panel, (iii) Exergy Panel, and (iv) I/O Panel. All these panels except the Exergy Panel have been discussed in Tutorial>Daemons>Open Steady Systems.

b.1. Exergy Panel Once a steady open device has been analyzed, an availability or exergy analysis can be carried out in Exergy Panel, provided a designated dead-State, State-0, is evaluated first. Atmospheric temperature and pressure are all that is necessary to calculate the dead state. Remember, the working substance at the dead state must be the working substance in the system, which may not be air. The variables displayed in the exergy panel are essentially different terms of the exergy balance equation for an open steady device exchanging heat with up to two TER's (thermal energy reservoir), one of which is the outside atmosphere. Qdot_0 and Qdot_1 are heat transfer to the system from TER-0, the atmospheric reservoir at temperature T_0, and TER-1, another reservoir at temperature T_1. Note that you can set Qdot_1, but not Qdot_0; this is because Qdot (from Device Panel) must be equal to Qdot_0 plus Qdot_1. The default state of the exergy panel assumes that TER-1 does not exist (Qdot_1=0 so that Qdot=Qdot_0). To overwrite this, simply enter Qdot_1 and T_1, and the daemon will calculate Qdot_0=Qdot-Qdot_1. For a system that exchanges heat only with the atmosphere, simply click Calculate to evaluate all the exergy variables for the device selected in Device Panel, provided the device has been already analyzed.

Most variables on the exergy panel are for output purpose only as indicated by the absence of checkbox in the variable widgets. With all the terms of the exergy balance equation evaluated, a device-specific exergetic efficiency can be easily calculated. For instance, from the exergy terms displayed in Fig. 2, the Second Law or exergetic efficiency can be evaluated from Wdot_u/Psidot_net = 81/89.8. Note that the calculator in I/O Panel recognizes only the state properties and exergy related variables such as Wdot_u cannot be used in legal expressions.

c. Uniform Process Daemons: Uniform process daemons also have four tabbed panels: (i) State Panel (ii) Process Panel, (iii) Exergy Panel, and (iv) I/O Panel. All these panels except the Exergy Panel have been discussed in Tutorial>Daemons>Unsteady Closed Process.

Fig. 1 Image of Exergy Panel showing various exergy terms evaluated for a polytropic compression problem.

c.1. Exergy Panel: Once a steady open device has been analyzed, an availability or exergy analysis can be carried out in Exergy Panel, provided a designated dead-State, State-0, is evaluated first. Atmospheric temperature and pressure are all that is necessary to calculate the dead state. Remember, the working substance at the dead state must be the working substance in the system, which may not be air. The variables displayed in the exergy panel are essentially different terms of the exergy balance equation for a process executed by a closed uniform system while exchanging heat with up to two TER's (thermal energy reservoir), one of which is the atmosphere. Q_0 and Q_1 are heat transfer to the system from TER-0, the atmospheric reservoir at temperature T_0, and TER-1, another reservoir at temperature T_1. Note that you can set Q_1, but not Q_0; this is because Q (from Process Panel) must be equal to Q_0 plus Q_1. The default state of the exergy panel assumes that TER-1 does not exist (Q_1=0 so that Q=Q_0). To overwrite this, simply enter Q_1 and T_1, and the daemon will adjust Q_0 from Q_0=Q-Q_1. For a system that exchanges heat only with the atmosphere, simply click Calculate to evaluate all the exergy variables for the process selected in Process Panel, provided the process has been already analyzed.

Most variables on the exergy panel are for output purpose only as indicated by the absence of checkbox in the variable widgets. With all the terms of the exergy balance equation evaluated, a device-specific exergetic efficiency can be easily calculated. For instance, from the exergy terms displayed in Fig. 1 for a polytropic compression process, the Second Law or exergetic efficiency can be evaluated from W_rev/W_u = Delta_Phi/W_u =0.219/0.24=91.5%. Note that the calculator in I/O Panel recognizes only the state properties, so exergy related variables such as W_u cannot be used in legal expressions.