Cogen, Regen Combined Cycle

Rankine Cycle Tavg(2->2’) < Tavg(2’->3) T at which heat is added is lower than Carnot leads to lower efficiency. Use Feedwater Heaters to Raise TH Ideal Regenerative Cycle: Circulate the fluid leaving the pump around the turbine casing

Ideal Regenerative Cycle Fluid Leaving the pump circulates around the turbine casing counterflow to the direction of vapor flow. Heat is transferred from the vapor in the turbine to the liquid. QH = QH Carnot QL = QL Carnot Therefore Nth = Nth,carnot! However, the ideal regenerative cycle is not practical. Can’t possibly transfer the required heat from turbine. Moisture content is too high.

Practical Regenerative Rankine Cycle: Use Feedwater Heaters Extract Vapor from the turbine and use feedwater heaters. Steam enters turbine @ 5 expands to 6 where some steam is extracted and sent to feedwater heater. Rest of steam is expanded to 7 and then condensed. Condensate at 1 is pumped to the feedwater heater and mixed with the steam. Proportions are set such that x3 = 0 (saturated liquid) TH, avg is increased! Careful  Flow splits: Mdot = Mdot63 + Mdot67

Cogeneration In many industrial settings there is a need for energy in the form of heat (process heat) Process heat is usually supplied by steam @ 5 to 7 atm, 150-200 oC. Called Cogeneration – can save $$

Combined Cycle Quest for higher thermal efficiency has led to the combined cycle which uses a gas power cycle topping a vapor power cycle. Gas turbines operate at higher temperatures which leads to higher thermal efficiencies. However high temperature exhaust limits thermal efficiency – use the high temp exhaust to heat a steam