Turbine Efficiency & Heat Rate

Why Heat Rate Is the Number That Matters

Heat rate measures fuel energy input required per unit of electrical output — the lower the heat rate, the more efficient the plant. It's the practical, day-to-day performance metric that everything in Modules 1.1–1.3 (expansion efficiency, staging design, reheat, regeneration) ultimately feeds into. Understanding what actually moves heat rate — and what doesn't — is what separates knowing turbine theory from being able to use it operationally.

The Efficiency Chain — From Steam to the Grid

Overall plant efficiency is the product of several distinct efficiency links, each with its own losses. Isentropic efficiency compares the turbine's actual enthalpy drop to the ideal, loss-free enthalpy drop for the same pressure conditions — this is the direct measure of how well the steam path itself (blading, seals, flow passages) is performing. Mechanical efficiency accounts for shaft losses to bearing friction and driving auxiliaries. Generator efficiency accounts for electrical conversion losses in the generator itself. Multiplying through these gives turbine cycle heat rate — fuel energy per unit of turbine-generator output specifically.

Net plant heat rate is broader still: it includes boiler efficiency losses and subtracts all plant auxiliary power consumption (pumps, fans, controls) from gross generation, landing on the actual net electrical output the plant delivers and gets paid for. Turbine cycle heat rate and net plant heat rate measure different scopes and are not interchangeable numbers.

Condenser Vacuum — The Most Watched Field Driver

Condenser vacuum sets the low-pressure boundary the entire turbine expands against. Degraded vacuum (rising back pressure) — from warm cooling water, fouled condenser tubes, or air in-leakage — reduces the total pressure drop available across every upstream stage, cutting output and worsening heat rate for the same steam flow. Because vacuum responds quickly and measurably to controllable conditions, it's one of the most closely tracked field performance indicators on any steam turbine, tied directly back to Module 1.1's point that condenser performance sets how much work the whole machine can extract.

Valve Points, Fouling, and Feedwater Heater Status

Several other real-world conditions move heat rate without triggering any alarm. On nozzle-governed turbines, operating at a valve point — where a governor valve sits partially open rather than fully open or shut — introduces throttling losses that show up as small, predictable heat rate variations across the load range. Blade fouling from deposits (carryover, silica, copper oxide transport) roughens blade surfaces and changes profiles, gradually reducing isentropic efficiency with no discrete failure event — it only shows up as a slow upward drift in heat rate over weeks or months of trending. And as covered in Module 1.3, taking a feedwater heater out of service removes that stage's regenerative preheating, forcing the boiler to add more heat to reach the same steam conditions.

Verifying Performance — ASME PTC 6 and Correction Curves

When plants need to formally verify turbine performance — after an overhaul, or to check against contract guarantees — they follow standardized test procedures like ASME PTC 6. Because field test conditions (steam pressure, temperature, condenser vacuum) rarely exactly match design reference conditions, raw test data is adjusted using correction curves that isolate true turbine condition from that day's specific operating conditions, allowing fair comparison to design values or previous test results.