Chapter 8 Production of Power from Heat

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

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

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

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

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

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.

Issues in Carnot cycle

2 1 QH Pump( Ws) Ws 4 3

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

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

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

Rankine Cycle

Practical power cycle - Rankine Rankine Cycle Carnot Cycle

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

Mass flow rate

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

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