Gas Turbine Performance Explained: Heat Rate, Output and Site Conditions

A gas turbine’s rated output is only a starting point for evaluating a project. The more useful comparison is how much net electricity the plant can deliver at the required site conditions, how much fuel it consumes to do so, and whether it can maintain that performance across the expected operating range. A machine with a higher nameplate rating may deliver less usable power than expected on a hot day or consume more fuel when operating below full load.

Heat rate, electrical output and site conditions must therefore be assessed together. Each figure depends on a defined set of boundaries and assumptions, and those assumptions must match before competing proposals can be compared.

What heat rate tells you about fuel consumption

Heat rate expresses the fuel energy supplied to a plant for each unit of electrical energy produced. It is commonly stated in kJ/kWh or Btu/kWh. A lower heat rate means less fuel energy is required to generate the same electrical output under the stated conditions.

The relationship between heat rate and efficiency is direct when both use the same energy basis and plant boundary. On a kJ/kWh basis, electrical efficiency equals 3,600 divided by heat rate. This calculation is useful only if the figures describe the same fuel input and the same electrical output measurement point.

Two distinctions frequently change the comparison:

• Gross versus net output: Gross output is measured at the generator terminals. Net output subtracts the electrical power consumed by auxiliaries within the agreed plant boundary. A net heat rate is normally higher than a gross heat rate because it uses the smaller net output as its denominator.
• Lower versus higher heating value: Lower heating value (LHV) excludes the recoverable heat associated with condensing water in the combustion products; higher heating value (HHV) includes it. For the same fuel flow and electrical output, an HHV-based heat rate will be higher. Comparing an LHV figure with an HHV figure without conversion can make one proposal appear more efficient than it is.

Also check whether the quoted figure covers the gas turbine alone or the whole plant. A combined-cycle plant uses turbine exhaust heat to produce additional power, so its plant heat rate is not directly comparable with the heat rate of a simple-cycle unit.

Rated output is not necessarily site output

Manufacturers commonly publish performance at specified reference conditions. These figures are useful for initial screening, but they are not a substitute for a site-specific performance calculation. Inlet air conditions, pressure losses, fuel properties and operating limits all affect the power available at the project location.

For a power supply obligation, define the required output at the point where power is delivered, not just at the generator terminals. The difference can include plant auxiliaries, transformer losses and other loads within the agreed boundary. A proposal should identify which of these are included and which remain outside the calculation.

The relevant operating point matters as well. A plant designed around an annual average temperature may fall short when demand peaks during hotter weather. Conversely, sizing only for the most severe condition can result in equipment that spends much of its life at an inefficient part-load point. The decision depends on the required capacity, the consequences of a shortfall and the expected hours at each load.

How site conditions change gas turbine performance

Ambient temperature and elevation

As inlet air temperature rises, air density generally falls. The compressor then takes in less air mass for a given volumetric flow, which generally reduces available turbine output. Heat rate may also worsen, although the magnitude depends on the machine and its controls. High elevation has a similar effect through lower atmospheric pressure: less dense inlet air generally means less mass flow and less available power.

Evaluate output against the site’s temperature profile rather than one nominal ambient value. The important cases are often the conditions at which capacity is contractually required and the conditions under which most fuel will be consumed. These may be different.

Inlet and exhaust pressure losses

Air filters, silencers, ductwork and inlet cooling equipment create pressure losses before air reaches the compressor. On the exhaust side, silencers, ductwork and heat-recovery equipment add backpressure. Both can reduce output and influence heat rate.

Losses should be assessed at realistic operating conditions, including the effect of filter fouling and the proposed exhaust arrangement. A performance calculation based on a clean, low-loss inlet may not represent operation near the filter replacement limit. Equally, a turbine evaluated with a simple exhaust stack cannot be assumed to retain the same output when connected to heat-recovery equipment.

Humidity, fuel and cooling water

Humidity can affect inlet air properties and the performance of evaporative cooling systems. Fuel composition and heating value affect fuel flow, combustion behavior and emissions controls; variations may also influence operating limits. The fuel specification used in the performance calculation should represent the expected supply, including the permitted range rather than only a nominal composition.

For combined-cycle projects, cooling water temperature and availability can affect the steam cycle and therefore total plant output. Assessing the gas turbine alone will not capture that effect.

Full-load efficiency is only part of the operating cost

A full-load heat rate is valuable when a unit runs near its rated capacity for long periods. For plants that follow demand, however, part-load performance may be more influential. As output changes, control settings and the energy required by auxiliaries can cause net heat rate to change. A unit with the better full-load figure will not necessarily use less fuel over a year.

Request a performance curve across the intended load range and combine it with the expected operating hours at each point. Include starts, shutdowns and periods of minimum load where they are material to fuel use and availability. This produces a more useful fuel estimate than multiplying a single heat rate by annual generation.

Inlet cooling offers a related trade-off. It can recover some output during hot conditions by lowering compressor inlet temperature, but equipment cost, auxiliary consumption, pressure loss and, for some systems, water use all matter. Its value depends on how often high-temperature capacity is needed and whether the additional net output justifies those costs.

Set the performance and test boundaries before comparing bids

A sound technical evaluation starts with a common set of conditions for all bidders. Ask for guaranteed output and heat rate at the defined site conditions, alongside curves showing how performance changes with temperature and load. The submission should make the following points explicit:

• The electrical measurement point, included auxiliary loads and gross or net basis.
• The fuel composition, heating-value basis and expected fuel supply conditions.
• Ambient temperature, pressure, humidity and the assumed inlet and exhaust losses.
• The plant configuration, including any heat recovery, inlet cooling or emissions-control equipment.
• The operating condition for each figure, such as load, equipment configuration and relevant cooling conditions.
• The performance-test method, instrumentation, correction procedures, measurement uncertainty and treatment of deviations from test conditions.

Applicable test standards and contractual requirements should be specified consistently in the enquiry and agreed before guarantees are finalized. Correction curves are particularly important when a test cannot be conducted at the exact guarantee conditions: they define how measured results are translated to the agreed basis.

For a complete power plant, these boundaries also affect the interfaces between equipment suppliers and the parties responsible for design, construction and commissioning. SINO-QNP offers EPC services for power plant projects, including engineering, auxiliary systems and commissioning. For such a scope, the useful next step is to align the turbine performance assumptions with the actual plant layout and define who is responsible for each loss and auxiliary load.

The strongest proposal is not simply the one with the lowest headline heat rate or largest rated output. It is the one that meets the required net capacity at the relevant site conditions, provides a credible fuel-use profile across the intended duty, and defines performance in terms that can be tested.