Combined Cycles and Cogeneration
Comprehensive university textbook unit covering fundamental principles, governing relationships, and engineering applications of Combined Cycles and Cogeneration.
§4.1 Heat-recovery steam generators
**Core topic:** gas-to-water energy recovery. **Representative relation:** `Q_HRSG = m_dot_g*cp_g*(T_g,in-T_g,out)`. **Case context:** unfired exhaust heat exchanger.
§4.2 Combined-cycle net efficiency
**Core topic:** system electric performance. **Representative relation:** `eta_CC = (P_GT+P_ST-P_aux)/Q_fuel`. **Case context:** gas and steam generator gross meters.
§4.3 CHP total fuel utilization
**Core topic:** simultaneous heat and electric services. **Representative relation:** `U_CHP = (P_e+Q_useful)/Q_fuel`. **Case context:** industrial process steam demand.
§4.4 Pinch, approach and exhaust constraints
**Core topic:** thermal contact and exchanger sizing. **Representative relation:** `DeltaT_pinch = T_g,evap_out-T_sat`. **Case context:** single-pressure HRSG temperature sketch.
§4.5 Integrated part-load operation
**Core topic:** coupled equipment at variable demand. **Representative relation:** `P_net = P_GT(load)+P_ST(load)-P_aux(load)`. **Case context:** daily dispatch and ambient-temperature variation.
Step-by-Step Power Plant Engineering Solutions
Three original questions connect calculations, definitions and interpretation. Open each solution to follow the reasoning.
Problem Statement: An HRSG receives 220 kg/s of exhaust gas, with assumed cp = 1.1 kJ/(kg K), entering at 560 °C and leaving at 160 °C. Sensible heat lost by gas is 220 × 1.1 × 400 = **96.8 MW(th)**. Actual heat delivered to steam is lower if there are heat losses or bypass streams.
Worked Engineering Derivation:
**Verification and limitations.** Finite temperature approach, material limits and stack condensation rules constrain real recovery. The result is illustrative and does not replace equipment-specific property data, measurements or professional review.
Problem Statement: A gas turbine produces 180 MW(e), a downstream steam turbine 90 MW(e), and auxiliaries consume 12 MW(e), with 450 MW(th) primary fuel input. Net output is 258 MW(e) and overall net efficiency is 258/450 = **57.33%**. The fuel denominator already supplied the gas turbine; recovered exhaust is not counted as a second fuel source.
Worked Engineering Derivation:
**Verification and limitations.** This is an illustrative operating point; ambient conditions and part-load behavior change real performance. The result is illustrative and does not replace equipment-specific property data, measurements or professional review.
Problem Statement: A CHP plant produces 30 MW(e) and delivers 40 MW(th) of genuinely useful heat while consuming 100 MW(th) primary fuel. Total fuel utilization is (30 + 40)/100 = **70%**. Its electric-only ratio remains 30%; a different customer heat-demand profile could substantially reduce the annual useful-heat credit.
Worked Engineering Derivation:
**Verification and limitations.** Measure heat at the customer export boundary rather than counting internal steam circulation. The result is illustrative and does not replace equipment-specific property data, measurements or professional review.
Problem Statement: In a schematic HRSG, the gas temperature leaving the evaporator is 195 °C and steam saturation temperature is 175 °C at that section. Pinch temperature difference is **20 K**. Reducing this difference would generally require greater heat-transfer area or different design; it cannot be reduced to zero in a finite exchanger for finite heat transfer.
Worked Engineering Derivation:
**Verification and limitations.** A local pinch value does not describe the complete multi-pressure temperature profile or acid-dew-point limits. The result is illustrative and does not replace equipment-specific property data, measurements or professional review.
Problem Statement: At a particular dispatch point, gas and steam turbines produce 150 and 70 MW(e), and station service consumes 10 MW(e). Net export is **210 MW(e)**. A 10% gas-turbine reduction need not yield an immediate 10% steam-turbine reduction because exhaust temperature, steam storage, controls and time response all matter.
Worked Engineering Derivation:
**Verification and limitations.** Do not use a fixed rated steam-to-gas power ratio to predict part-load output. The result is illustrative and does not replace equipment-specific property data, measurements or professional review.