ME 400 Energy Conversion Systems Topic 2 Presentation 3 Power Cycles

1. ME 400Energy Conversion SystemsTopic 2 Presentation 3Power Cycles Emad Jassim & Ty NewellDepartment of Mechanical Science and EngineeringUniversity of Illinois at Urbana-Champaign © 2011 University of Illinois Board of Trustees. All Rights Reserved.

2. Power and Refrigeration Cycles • We will examine 3 common power cycles • Rankine cycle (steam power plant) • Brayton cycle (gas turbine, jet engine) • Otto cycle (internal combustion engine) • Our goal is to develop some familiarity with these cycles, and more importantly, describe cycle analysis techniques • Each of these cycles may be “reversed” to form a refrigeration cycle….we will examine the reversed Rankine, or vapor refrigeration cycle.

3. Working Fluid Analysis Cycle Rankine Vapor SSSF Brayton Ideal Gas Closed Otto Power/Refrig Cycles

4. Power and Refrig Cycles • In addition to examining these cycles, we will investigate cycle enhancements • None of these cycles are inherently reversible • Finally, we will examine the characteristics of 3 cycles that are inherently reversible: • Carnot cycle • Ericsson cycle • Stirling cycle

5. Rankine Cycle • As heat moves into and out of a system, entropy is transported with the heat transfers • Reversible limit can only be reached if all heat transfers occur with no temperature differences • Must also eliminate free expansions (eg, valve pressure drops) and friction interactions…let me know if you have some ideas for accomplishing this • Common enhancements to cycles generally help reduce heat transfer temperature differences

6. Boiler 2’ Qboiler 3 2 High P Wturbine Pump Low P 1 Turbine Wpump 4 Condenser Qcondenser Rankine Power Cycle Recall the processes of our previous example problem, but now we will superheat the steam exiting the boiler.

7. Rankine Power Cycle T-s Diagram Tflames 3 DT for boiler heat transfer Superheat used to keep turbine out of saturation 2’ 2 4 1

8. Rankine Power Cycle Associated with every heat transfer is an entropy transfer. And, entropy generated by heat transfer with a temperature difference is directly related to the cycle’s inefficiency.

9. Alternative view of equipment process paths Qboiler 3 2 2’ Wpump Wturb 4 1 Qcond Rankine Power Cycle P-h Diagram

10. Rankine Power Cycle • 2 Common Rankine Cycle Enhancements • Reheating; Increasing the steam to a higher temperature and pressure through an initial turbine. To avoid saturation, steam exhaust from first turbine is returned to boiler for reheating (superheating). A second stage turbine then drops the steam exhaust to condenser pressure. • Feedwater Heating; boiler feedwater is passed through one or more heat exchangers that heat the water with an intermediate steam temperature (tapped from the turbine). Efficiency is increased because the steam is closer to boiler water temperature than the boiler combustion gases.

11. Boiler 2’ Qboiler 3 Turbine #1 2 Wturbine#1 High P 3” 3’ Pump 1 Qreheat Low P Wturbine#2 Wpump Turbine #2 4 Condenser Qcondenser Rankine Power Cycle - Reheat

12. Rankine Power Cycle - Reheat Tflames 3 3” DT for boiler heat transfer reduced 2’ 3’ 2 4 1

13. Rankine Power Cycle • In the next lecture, we will work through an example for the Rankine power cycle in order to demonstrate the effect of reheat on a superheated steam cycle.