cogeneration

name	cogeneration
description	Cogeneration (CHP) — topping/bottoming cycles, electrical efficiency, heat recovery, total efficiency, heat-to-power ratio, gas turbine CHP, reciprocating engine CHP, ASHRAE 90.1, EPA CHP catalog.
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Cogeneration (CHP) — Complete Skill

Concept and Benefits

Cogeneration: simultaneous production of electrical power and useful thermal energy from single fuel source CHP efficiency advantage:

• Separate systems: power plant η_e ≈ 30–40% + boiler η_t ≈ 85% → combined fuel for both = 100 + 85 = 185 units per 100 useful
• CHP: same fuel → 80–90% total efficiency → saves 30–50% fuel vs. separate generation

Energy utilization factor (EUF) or total efficiency: η_total = (W_electric + Q_thermal) / Q_fuel [dimensionless; typically 0.75–0.90] η_electric = W_electric / Q_fuel [0.25–0.45] η_thermal = Q_thermal / Q_fuel [0.35–0.65]

Heat-to-power ratio (HPR): HPR = Q_thermal / W_electric [typical: 1.0–3.0 for most CHP systems]

CHP Prime Mover Types

Reciprocating Engine CHP

Engine types: natural gas spark ignition (SI); diesel (CI); dual-fuel Electric efficiency: 25–45% (higher at larger sizes; modern NG engine ≈ 40–42%) Heat recovery:

• Jacket water (85–95°C): Q_jacket ≈ 30–35% of fuel input
• Exhaust (400–550°C): Q_exhaust ≈ 25–35% of fuel input; HRSG produces hot water/steam
• Oil cooler, intercooler: 5–10%

Total thermal recovery: Q_thermal = Q_jacket + Q_exhaust_usable ≈ 45–60% of fuel input Total CHP efficiency: η_total = 0.40 + 0.50 = 0.90 (typical for well-designed recip engine CHP)

Sizes: 20 kW to 10 MW (most common: 100 kW to 5 MW for industrial/commercial)

Gas Turbine CHP

Electric efficiency: 20–35% (simple cycle) — lower than recip at same size Exhaust temperature: 450–600°C (much hotter than recip) → better HRSG performance HRSG (Heat Recovery Steam Generator): Q_steam = m_exhaust × c_p_gas × (T_exhaust - T_stack) [kW; T_stack = 120–150°C minimum to avoid acid dew point] c_p_gas ≈ 1.05–1.10 kJ/(kg·K) for flue gas; m_exhaust from turbine specs

Total efficiency: η_total = 0.30 + 0.55 = 0.85 (typical gas turbine CHP; high T heat useful for industrial steam)

Sizes: 1 MW to 300+ MW (more scalable than recip; better for large industrial) Best fit: industrial processes requiring both electricity + high-pressure steam (150–700 kPa)

Steam Turbine (Bottoming Cycle)

Operation: high-pressure steam from boiler → turbine → extract at medium pressure for process heat "Backpressure turbine": extract steam at 3–15 bar for process; no condenser η_electric (backpressure) = 10–20% (low vs. condensing; most energy exits as useful steam) Total efficiency = boiler efficiency ≈ 80–90% (almost all fuel → useful energy)

Best fit: processes needing large steam quantities with opportunity for power recovery

Microturbine CHP

Size: 30–1000 kW η_electric: 20–30% Exhaust T: 250–320°C → lower quality heat than gas turbine HPR: 2.0–3.5 Advantages: compact; no cooling water needed; clean emissions

Fuel Cell CHP

η_electric: 40–55% (PEMFC, PAFC, SOFC) Heat quality: SOFC exhaust 700–800°C; PEMFC < 80°C (low quality) Advantage: very low emissions; silent; modular η_total: up to 85–90% (SOFC CHP)

Heat Recovery System Design

HRSG design (gas turbine exhaust): Q_HRSG = ε × m_exhaust × c_p_gas × (T_exhaust - T_water_in) [kW; ε = heat exchanger effectiveness]

Approach temperature: ΔT_pinch ≥ 15°C (minimum temperature difference at pinch point) T_exhaust - T_steam_sat ≥ 15°C; T_water_out = T_steam_sat + superheating desired

Stack temperature limit: T_stack > T_acid_dew [120–150°C for natural gas; 160°C for sulfur-containing fuels] Below dew point → H₂SO₄ or HCl condensation → corrosion

Economic Analysis

Payback period: Simple payback = Capital cost / Annual fuel savings Annual savings = Q_fuel_displaced × C_fuel - (C_elec_displaced - C_elec_generated)

Fuel savings: ΔF_annual = (Q_fuel_CHP / η_grid) + (Q_thermal_CHP / η_boiler) - Q_fuel_CHP [GJ/yr] η_grid = 0.33; η_boiler = 0.85

Electricity value: If selling back to grid: market rate; if avoiding purchase: utility rate (more valuable) Self-consumed electricity: typically saves $0.08–0.15/kWh

Typical payback (industrial CHP): 3–8 years for > 500 kW; shorter if high electricity + heat demand

Operating Strategy

Heat-following: size and operate based on thermal demand; sell excess electricity Electricity-following: size based on power demand; dump or sell excess heat Baseload: operate CHP at constant output (maximum utilization) Peak shaving: operate during peak electricity tariff hours only

Minimum load factor for CHP economics: LF = (hours_operated × rated_capacity) / (8760 × rated_capacity) LF ≥ 6000 hr/yr (68%) → good economics; LF < 4000 hr → poor economics

Standards and References

Standard	Scope
ASHRAE 90.1	Energy efficiency; CHP systems
EPA CHP Catalog	Technology profiles; performance data
IEEE 1547	Interconnection standards for distributed generation
IEC 62257	Micropower systems including CHP

Output

Provide: prime mover type and size [kW_e], electric efficiency η_e [%], thermal efficiency η_t [%], total CHP efficiency η_total [%], heat-to-power ratio HPR, recoverable heat Q_thermal [kW] and temperature [°C], HRSG design (if gas turbine): steam flow [kg/hr] and pressure [bar], stack temperature [°C], annual fuel input Q_fuel [GJ/yr], fuel savings vs. separate generation [GJ/yr and %], simple payback period [years], applicable standard (EPA CHP Catalog, ASHRAE 90.1).