Cogeneration

Cogeneration, or Combined Heat and Power (CHP), is the simultaneous production of electrical (or mechanical) power and useful thermal energy from a single primary energy source. In a conventional power plant, a large fraction of the fuel energy is rejected as low-grade heat through the condenser; a CHP plant recovers much of that heat for district heating, process steam, or industrial drying, raising overall fuel utilization to 70–90% compared with 30–50% for electricity-only generation. Common CHP configurations include the gas turbine with a heat recovery steam generator (HRSG), the reciprocating engine with exhaust heat recovery, the steam turbine with extraction or back-pressure, and combined-cycle systems. The key performance indicators are the electrical efficiency eta_e = W_net / Q_fuel, the thermal efficiency eta_th = Q_useful / Q_fuel, and the overall efficiency eta_overall = (W_net + Q_useful) / Q_fuel. The power-to-heat ratio (PHR) describes the balance between electrical and thermal output and must match the site's demand profile. Exergy analysis is often more revealing than first-law efficiency, because electricity is high-grade work while low-temperature heat has limited thermodynamic value. The main technical constraints are matching thermal and electrical loads, heat transport distance, and the temperature level required by the thermal user. Economic feasibility depends on the spark spread, fuel price, capital cost, operating hours, and any feed-in tariff or carbon credit.