Vapor and Combined Power Cycles

1. Vapor and Combined Power Cycles The steam cycle and more…

2. Carnot Cycle • The standard all others are measured against • Not realistic model for vapor cycles

3. 1-2 isentropic compression (pump) 2-3 constant pressure heat addition (boiler) 3-4 isentropic expansion (turbine) 4-1 constant pressure heat rejection (condenser) Rankine Cycle, Ideal

4. Rankine Cycle, Ideal

5. Rankine Cycle Energy Analysis • Energy balance, each process • For pump

6. Rankine Cycle Energy Analysis • For boiler • For turbine • For condenser

7. Rankine Cycle Energy Analysis • Thermal efficiency • Heat rate: amount of heat (Btu) to generate 1 kWh of electricity

14. Real vs. Ideal Cycle

15. Real vs. Ideal Cycle • Major difference is irreversibilities in pump and turbine

20. Increase Efficiency? • Lower condenser pressure • Increase superheat temperature

21. Increase Efficiency? • Increase boiler pressure

22. Reheat • Materials limit temperature of steam, but can we take advantage of higher steam pressures and not have quality of steam issues?

23. Reheat • Equations become: • Purposes of reheat: keep turbine inlet temps within limits, increase quality of steam in last stages of turbine

30. Regeneration: effective use of energy Open (direct contact) feedwater heaters (mixing chambers) Closed feedwater heaters (heat exchangers) Ideal Regenerative Rankine Cycle

31. Ideal Regenerative Rankine Cycle

32. Ideal Regenerative Rankine Cycle

33. Ideal Regenerative Rankine Cycle

34. 2nd Law Analysis • Ideal Rankine cycle is internally reversible • Analysis indicates where irreversibilities are • Again for steady-flow system:

35. 2nd Law Analysis • For a cycle:

40. Cogeneration

48. Combined Gas-Vapor Power Cycle • Use of two cycles to maximize efficiency • Gas power cycle topping a vapor power cycle • Combined cycles have higher efficiency than either independently • Works because: • Gas turbine needs high combustion temp to be efficient, vapor cycle can effectively use rejected energy