Heat consumption curves: where generation costs actually come from.

In steady-state operation a 980 MW combined-cycle plant is, broadly, a known quantity. Heat rate falls in a narrow band. The dispatcher knows what to expect.

The cost offer that earns the plant dispatch revenue is not a commercial judgment — it is a physical measurement. Specifically, it is derived from the plant's heat consumption curve, the same artifact in every wholesale market that uses merit-order dispatch.

That relationship is not assumed from OEM data. It is measured under field conditions, validated against ASME PTC test protocols, and submitted as part of the market-entry dossier — the MEM migration dossier in Mexico, equivalent submissions elsewhere.

What the curve is, in physical terms

The raw curve plots GJ/h of fuel input against net MW output. For most thermal plants, the curve is approximately linear above minimum stable load. The specific heat rate curve plots kJ/kWh against net MW — the U-shape that reveals efficiency. The piecewise-linear curve is the regulator-facing artifact: 3 to 6 linear segments across the operating range, with constant incremental heat rate per segment.

Why the regulator measures, not estimates

OEM guarantees are at design conditions. The published heat rate assumes a specific ambient temperature, specific fuel composition, specific cooling-water temperature, specific equipment-as-new condition. None of those conditions match the plant's actual operating environment over the life of the asset.

OEM guarantees are for performance acceptance, not dispatch. The OEM contract specifies a heat rate at one operating point. Dispatch economics need the curve across the full operating range.

OEM guarantees are not independent. Regulated wholesale markets require evidence produced under independent test, with measurement traceability and methodology that the regulator can audit.

How the test is structured

The heat consumption test is typically run as a series of steady-state test points distributed across the plant's operating range. Each test point holds the plant at a defined output for a measurement window of typically two to four hours, during which net electrical output, fuel flow, fuel calorific value, ambient conditions, cooling-water temperature, auxiliary load consumption, and steam conditions are all measured.

A simple-cycle gas turbine plant might be tested at 5 to 8 points. A combined-cycle plant typically needs 8 to 15 — the part-load behavior of the steam bottoming cycle changes the overall heat rate in ways that simple-cycle curves do not capture.

The relationship between fuel input and electrical output is physics. The curve is the measurement of that physics. The dispatch is the consequence. The revenue is the dispatch.

The plant's revenue model lives or dies by what gets measured during a few hundred hours of test campaign, conducted under specific protocols, with specific instrumentation, and documented to a specific standard. Treating that campaign as a checkbox is among the most expensive habits in plant operations.

Heat consumption curves are one output of a broader plant performance acceptance campaign — alongside capacity, heat rate at base load and start/shut economics.