1. Chapter 8
Production of
Power from Heat

10. Heat into work
Most present day methods based on the evolution of heat and subsequent conversion of part of the heat into useful work.
Fossil fuel steam power plants (Efficiency: 35%)
Combined cycle plants (Efficiency: 50%) – advanced technology gas turbines

11. Internal combustion energy
Conversion of chemical energy of fuel directly into internal energy
Eg: Otto engine, Diesel engine, gas turbines

12. Conversion of chemical energy directly into electrical energy
Eg.: Electrochemical cell (battery), fuel cell (Efficiency: 50% and more)

13. Power plants
Working fluid such as steam is separated from heat source and heat is transferred across a physical boundary

14. Simple steam power plant
Boiler – part of heat from fuel oil converts water to steam at high T and P QH
Turbine – shaft work by a turbine Ws
Condenser – condenses exhaust steam at low T QC
Pump – pumps water back to boiler. Ws

15. Steam Power Plant
Steam power plants operate in a cycle where the working fluid receives heat in a boiler, produces work in a turbine, discharges heat in a condenser, and receives work in a pump which returns the fluid to the boiler.

18. Issues in Carnot cycle

20. 2 1 QH
Pump( Ws) Ws 4 3

21. External source of thermal energy not important for our analysis (coal, nuclear, wood, solar, etc)

22. 10.1.1 Comparison to Carnot Cycle
Carnot Vapor Power Cycle
Maximum heat engine efficiency for cycle between TMAX and TMIN
Process 12:
2-Phase pressure increase
Mechanically difficult to do reliably

23. Comparison to Carnot Cycle
Simple Rankine Power Cycle
Modify Carnot Cycle to increase pressure of pure liquid (not liquid)
Mechanically more reliable
Thermodynamically easier to implement (wPUMP << wTURBINE)
Lower efficiency than Carnot cycle

24. Rankine Cycle

25. Practical power cycle - Rankine
Rankine Cycle
Carnot Cycle

28. Rankine efficiency
The efficiency of the Rankine cycle is not as high as Carnot cycle but the cycle has less practical difficulties

29. Mass flow rate

30. Woutturbine = m (h3-h2) (negative value)
Turbine and pump work
Work output of the cycle (Steam turbine), Wturbine and work input to the cycle (Pump), Wpump are:
Woutturbine = m (h3-h2) (negative value)
Winpump = V(P1-P4)
where m is the mass flow of the cycle. Heat supplied to the cycle (boiler) QH and heat rejected from the cycle (condenser), QC are:
Q in = QH = m (h2-h1)
h1 = Win,pump + H4
Qout = QC = m (h4-h3) (negative value)
The net work of the cycle is:
Wnet = -Wturbine + Wpump

33. Seat work
A power plant using a Rankine power generation cycle and steam operates at a temp of 800C in the condenser, a pressure of 2.5 MPa in the evaporator and a maximum evaporator temp of 7000C. Draw the two cycles described below on a temp-entropy diagram for steam and answer the following questions (a) What is the efficiency of this power plant, assuming the pump and turbine operates adiabatically and reversible ? What is the temp of the steam leaving the turbine ? (b) If the turbine is found to be only 85% eff, what is the overall eff of the cycle ? What is the temp of the steam leaving the turbine in this case