Final Exam (Jan): Gas Turbine Cycle (with solution)
- Due No Due Date
- Points 12
- Submitting on paper
- Available after Jan 11, 2018 at 8:55am
Q2. Gas Turbine Cycle (12 points)
An externally-heated open-cycle gas turbine in a free power turbine arrangement is driven by a concentrating solar tower and operates with very low pressure ratio, relying on a large amount of regenerated (recuperated) heat. The layout sketch is shown below. The working fluid is pure air throughout the cycle. Assume dry technical air without any thermal losses. Calculate the net electrical efficiency of the cycle! The following parameters are given:
Ambient air conditions (hot day) 1 bar, 40 °C (p1=p7=1bar)
Air mass flow through the cycle 20 kg/s
Pressure ratio of the compressor (p2/p1) 4
Isentropic efficiency of the compressor (ηSC) 87 %
Specific heat ratio for compression (kC) 1.392
Isentropic efficiency of each expander (ηST) 91 %
Specific heat ratio for each expander (kT) 1.34
Pressure losses in the regenerator, both streams 10 %
Pressure losses in the solar air heater 7 %
Mechanical efficiency for each shaft (ηmech) 93 %
El. Generator efficiency (ηgen) 93 %
Thermal energy rate exchanged in the regenerator 6000 kW
Air temperature after the solar heater (T4 in °C) 800 °C
Hint: Work with air enthalpies as per the enthalpy table in the help file!
The list of partial answers below shows the necessary calculation steps:
|
|
Sub-task |
Units |
Points |
|
1) |
Temperature after the compressor (t2) |
°C |
1.5 |
|
2) |
Enthalpy after the compressor (h2) |
kJ/kg |
0.5 |
|
3) |
Main air enthalpy after regenerator (h3) |
kJ/kg |
1 |
|
4) |
Main air temperat. after regenerator (t3) |
°C |
0.5 |
|
5) |
Enthalpy after the solar heater (h4) |
kJ/kg |
0.25 |
|
6) |
Energy rate delivered by the solar heater |
kW |
1 |
|
7) |
Main air pressure after solar heater (p4) |
bar |
0.5 |
|
8) |
Power need for compression process |
kW |
0.5 |
|
9) |
Power that first turbine must deliver |
kW |
0.5 |
|
10) |
Enthalpy after the first turbine (h5) |
kJ/kg |
0.5 |
|
11) |
Temperature after the first turbine (t5) |
°C |
0.25 |
|
12) |
Pressure after the first turbine (p5) |
bar |
1.5 |
|
13) |
Pressure ratio across the second turbine (p5/p6) |
- |
0.75 |
|
14) |
Temperature after second turbine (t6) |
°C |
1 |
|
15) |
Enthalpy after second turbine (h6) |
kJ/kg |
0.25 |
|
16) |
Net el. power delivered by the cycle |
kW |
0.5 |
|
17) |
Net electrical efficiency of the cycle (in %) |
% |
1 |
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