Heat Rate and Thermal Efficiency of a Power Plant, Explained
Heat rate is the standard way to express how efficiently a thermal plant turns fuel into electricity. Lower is better. Understanding exactly what goes into the number is the first step to improving it.
Definition
Heat rate = heat energy supplied by the fuel Γ· electrical energy sent out, usually in kJ/kWh or, in India and some other markets, kcal/kWh. Because one kilowatt-hour is 3600 kJ (about 860 kcal), efficiency follows directly: efficiency = 3600 Γ· heat rate in kJ/kWh, or 860 Γ· heat rate in kcal/kWh. A plant with a heat rate of 2,450 kcal/kWh therefore runs at about 35 %; one at 10,000 kJ/kWh at 36 %.
Gross versus net
Gross heat rate divides by the energy produced at the generator terminals. Net (or station) heat rate divides by the energy exported after the plantβs own auxiliary consumption (fans, pumps, mills, cooling-water pumps). Net is always worse than gross. Always check which one a figure refers to, and whether the fuel energy is on a gross (higher) or net (lower) calorific-value basis β the difference can be several percent for coal and much more for hydrogen-rich fuels.
What drives heat rate
- Steam conditions. Higher main-steam pressure and temperature raise the average temperature of heat addition. This is why supercritical and ultra-supercritical units exist.
- Condenser pressure. Lower back-pressure increases the enthalpy drop across the turbine. It depends on cooling-water temperature and condenser cleanliness.
- Reheat and regeneration. Reheating improves efficiency and keeps exhaust steam dry; feedwater heaters raise the average heat-addition temperature.
- Boiler efficiency. Stack temperature, excess air, unburnt carbon and moisture in the fuel set how much of the fuel energy reaches the steam.
- Auxiliary power. Fans, mills and pumps cost net output.
- Part load. Most units run less efficiently away from design load.
Measuring it
The direct method divides fuel input by output, but it needs accurate coal flow and calorific value, which are difficult to measure on a continuous basis. The indirect (heat-loss) method computes boiler efficiency by summing the individual losses, and turbine cycle heat rate is computed from steam flow and enthalpy rise. The two are combined into a station figure. The Plant Performance Indicators and Thermal Plant Estimator calculators help estimate these quantities.
Using heat rate to find losses
A deviation from the expected heat rate at a given load is not a diagnosis, but it narrows the search. Elevated condenser pressure points to cooling-water or air-ingress problems; high stack temperature points to boiler fouling or air-heater issues; higher TTD on feedwater heaters points to heater problems; and excessive auxiliary power points to fans, mills or pump operating points. The feedwater heater and condenser pages on this site cover two of those items.
Related calculators
More guides
- How Orifice-Plate and Venturi Flow Measurement Works (ISO 5167)
- Steam Tables and IAPWS-IF97: What the Numbers Are and Why They Matter
- Feedwater Heaters: TTD, DCA and What They Tell You About Plant Health
- SAMA Diagrams Explained: How to Read Boiler and Turbine Control Logic
- Condenser Performance and Cooling-Water Flow in a Steam Power Plant
- All guides