Q1
Figure P7.132 shows a cogeneration system producingtwo useful products: net power andprocess steam. Theaccompanying table provides steady-state mass flow rate,temperature,pressure, and flow exergy data at the ten numberedstates on the figure. Stray heat transfer andthe effects ofmotion and gravity can be ignored. Let T0 =298.15 K, p0=1.013 bar. Determine,in MW,
(a) thenet rate exergy is carried out with the process steam,(Ef9-Ef8).
(b) thenet rate exergy is carried out with the combustionproducts, (Ef7-Ef1).
(c) the rates of exergy destruction in the air preheater, heatrecoverysteam generator, and the combustionchamber.
Devise and evaluate an exergetic efficiency for the overallcogeneration system.
State / Substance / Mass Flow Rate (kg/s) / Temperature (K) / Pressure (bar) / Flow Exergy Rate, Ef(MW)1 / Air / 91.28 / 298.15 / 1.013 / 0
2 / Air / 91.28 / 603.74 / 10.13 / 27.54
3 / Air / 91.28 / 850 / 9.623 / 41.94
4 / Combustion
products / 92.92 / 1520 / 9.142 / 101.45
5 / Combustion
products / 92.92 / 1006.16 / 1.099 / 38.78
6 / Combustion
products / 92.92 / 779.78 / 1.066 / 21.75
7 / Combustion
products / 92.92 / 426.9 / 1.013 / 2.77
8 / Water / 14 / 298.15 / 20 / 0.06
9 / Water / 14 / 485.57 / 20 / 12.81
10 / Methane / 1.64 / 298.15 / 12 / 84.99
Q2
For the compressed-air energy storage system of Problem4.114, determine the amount of exergy destructionassociated with filling the cavern, in GJ. Devise andevaluate the accompanying exergetic efficiency. Comment.Let T0=290 K, p0=1 bar.
Problem 4.114
Figure P4.114 provides operating data for a compressedairenergy storage system using off-peak electricity to powera compressor that fills a cavern with pressurized air (see Sec.4.8.3). The cavern shown in the figure has a volume of105 m3and initially holds air at 290 K, 1 bar, which corresponds toambient air. After filling, the air in the cavern is at790 K,21 bar.
Q3
Figure P7.133 shows a combined gas turbine-vapor powerplant operating at steady state. Thegas turbine is numbered1–5. The vapor power plant is numbered 6–9. The accompanyingtablegives data at these numbered states. The total net poweroutput is 45 MW and the mass flow rateof the water flowingthrough the vapor power plant is 15.6 kg/s. Air flows throughthe gasturbine power plant, and the ideal gas model applies tothe air. Stray heat transfer and the effectsof motion and gravitycan be ignored. Let T0=300 K, p0=100 kPa. Determine
(a) the mass flow rate of the air flowing through the gasturbine, in kg/s.
(b) thenet rate exergy is carried out with the exhaust airstream, (Ef5=Ef1)in MW.
(c) the rate of exergy destruction in the compressor andpump, each in MW.
(d) thenet rate of exergy increase of the air flowing throughthe combustor,(Ef3=Ef2), inMW.
Devise and evaluate an exergetic efficiency for the overallcombined power plant.