Combined Heat & Power (CHP) Overall Efficiency
Combined Heat & Power (CHP) Overall Efficiency
Combined heat and power (CHP, or cogeneration) plants capture and sell the heat that a conventional power plant would otherwise reject to a cooling tower or river, and because both the electricity and the useful heat displace something the customer would otherwise have to buy separately, CHP overall efficiency is defined as the sum of both useful outputs divided by the fuel energy input. This is why CHP plants routinely report overall efficiencies of 70–85%, far above the 35–45% electrical-only efficiency of a conventional power plant — the headline number is not a thermodynamic trick but genuinely reflects that far less of the fuel energy goes to waste when a nearby industrial process or district heating network can use the "waste" heat.
The overall CHP efficiency is eta = (Welec + Quseful)/Qfuel, the sum of net electrical output and useful heat recovered divided by the fuel heat input. where Welec is net electrical power generated, Quseful is the useful heat recovered and delivered to a process or heating network, and Qfuel is the fuel energy input rate.
Inputs
W_elec := 300 kW=300000 W
Q_useful := 450 kW=450000 W
Q_fuel := 1000 kW=1e+06 W
Adding the electrical output to the useful recovered heat and dividing by the fuel input credits the plant for both products it sells, rather than treating the recovered heat as a loss the way a power-only plant would.
eta_chp := (W_elec + Q_useful)/Q_fuel=0.75
Results
An overall efficiency of 75% is squarely in the range expected of a well-designed CHP system and is the main economic case for installing one over separate power-only generation plus a separate boiler. It is worth noting this number blends two different-quality energy forms — electricity is generally more valuable per unit of energy than low-grade heat — so some plants report a "power-to-heat ratio" alongside overall efficiency to capture that distinction. If the heat off-taker's demand drops (say, a factory shuts down for maintenance), Quseful falls and overall efficiency drops toward the plant's power-only figure, showing why matched, steady heat demand is essential to CHP economics.
Assumptions & limitations
- Assumes a matched, steady heat load able to use all recovered heat.
- Electrical and thermal energy summed on an equal energy basis, ignoring exergy quality differences.
- Steady-state operation at the stated loads.
- No credit taken for any additional heat recovery beyond Q_useful.