ME 400 Energy Conversion Systems Topic 2 Presentation 5 Power Cycles

1. ME 400Energy Conversion SystemsTopic 2 Presentation 5Power 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. Rankine Power Cycle Example:continued - reheat • Examine a steam cycle with TMAX = 600°C. • A minimum temperature of 23°C is available for heat rejection • The turbine cannot go into saturation and the condenser outlet is quality = 0 • Assume adiabatic, reversible pump & turbine • Would reheat improve cycle efficiency?

3. 3 2’ 2 4 1 Rankine Power Cycle Example - no reheat • Reheat adds another turbine (or turbine stage) • The steam is returned to the boiler after the first stage for reheating • Ideally, no pressure change occurs in the reheater

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

5. 3 3” 2’ 3’ 2 4 1 Rankine Power Cycle Example - Reheat • Points 1 and 4 are the same as before • Point 2 is found as before, but it is at a higher pressure • Point 3” is the same as the previous point 3 • Point 3 is chosen such that 3’ is at the saturated vapor state • Constant pressure heating occurs between 3’ and 3”

6. Rankine Power Cycle Example - Reheat State 1: unchanged... State 4: unchanged... State 3”: (same as 3 before)

7. Rankine Power Cycle Example - Reheat State 3’: State 3: State 2:

8. Rankine Power Cycle Example - Reheat Pump Work: Boiler Heat Transfer: Reheater Heat Transfer: Turbine #1 Work: Turbine #2 Work:

9. Cycle Efficiency Rankine Cycle with reheat: Basic Cycle (no reheat): Ideal (reversible) cycle: In this case, reheat shows significant improvement to the cycle performance.

10. Rankine Cycle with Reheat • Will reheat always improve cycle efficiency? • Consider steam cycles that can run in saturation (ie, saturated vapor exits the boiler) • Consider other fluids

11. 3 2’ 3” 3’ 2 4 1 Water: Saturated Vapor Boiler Exit This heating has temperature difference causing inefficiency More efficient to continue to condenser directly (red arrow)

12. Reheat for Butane; T-s Plot • For a fluid such as butane (R600), the “inverted” shape of the saturated vapor line results in a superheated turbine exit • Reheat would reduce efficiency if the vapor is heated at an intermediate level due to temperature difference of heating in both reheater and condenser (desuperheating) Path of single turbine Reheat path

13. Summary • Reheating is often a benefit for power cycle fluids that have saturation domes (on T-s plots) shaped similar to water and turbines must stay out of the saturation region • Economics are also a factor in deciding the benefits of reheat (especially when combined with other cycle enhancements) • Abbott PP does not have reheat • We will consider “feedwater heating” as an alternative Rankine cycle enhancement