A combined-cycle plant brings together two thermodynamic cycles: a gas turbine, which generates electricity, and a steam cycle, which converts part of the gas turbine's exhaust heat into steam to create additional power in a Rankine process.
Simulating the two systems together helps engineers understand how equipment choices, ambient conditions, and operating pressures affect the performance of the whole plant.
This article explains how to simulate such a power plant in IPSEpro and IPSE GO, using the APP_Lib_GT library. We will follow a single-pressure heat recovery steam generator (HRSG) example with a steam drum, deaerator, and simple condensing turbine. As an alternative the gas turbine can run via an exhaust-gas bypass. The aim is to explain the model structure and the main engineering decisions, providing a starting point for a steady-state design study.
Advantages and options of combined cycle plants
In the gas-turbine cycle, air is compressed, mixed with fuel, and burned. The hot gases expand through a turbine, driving the compressor and producing useful power. The exhaust still contains substantial thermal energy.
A heat recovery steam generator transfers some of that energy to water and steam. The steam expands through a turbine, then condenses and returns to the HRSG through the feedwater system. The gas-turbine cycle is commonly described as the Brayton cycle; the water-steam cycle is the Rankine cycle.
The principal advantage is greater electrical output from the same fuel input. Without supplementary firing, the steam cycle uses heat already present in the gas-turbine exhaust. This improves electrical efficiency and reduces fuel consumption, and therefore fuel-related CO₂ emissions, per unit of electricity compared with the same gas turbine operating alone.
Combined cycles are used for electricity generation and can also supply process steam or district heat in cogeneration applications. For engineers, simulation supports decisions such as selecting a gas turbine, choosing steam conditions, assessing heat recovery, and comparing cooling arrangements. It also reveals the interactions between these choices: additional HRSG heat recovery can increase steam production, while exhaust-side pressure loss can reduce gas-turbine performance.
The example plant and its main components
Figure 1. The example plant. Exhaust gas travels upward through the HRSG; the blue connections form the water-steam circuit. The pale branch routes exhaust around the HRSG.
Read the flowsheet as two connected energy paths. At the lower left, air and fuel enter the gas turbine. Its exhaust passes through successive heat-recovery sections before reaching the stack. On the water-steam side, feedwater is heated, evaporated, and superheated, then expanded through the steam turbine on the right. The condenser and pumps close the loop.
| Plant section | Main library components | What the model establishes |
|---|---|---|
| Gas turbine and boundaries | Gas-turbine performance model, ambient air source, fuel source, inlet pressure-loss element | Electrical output, fuel demand, exhaust flow, temperature, and composition |
| HRSG | Counter-current heat exchangers, drum, circulation pump | Feedwater heating, evaporation, superheating, and gas-side cooling |
| Steam power train | Steam turbine, shaft connection, generator | Expansion, shaft power, and electrical output |
| Condensate and feedwater system | Condenser, condensate pump, deaerator, feedwater pump | Heat rejection, water return, and pressure recovery |
| Exhaust bypass and stack | Splitter, bypass pipe, mixer, ambient sink | Exhaust routing and pressure boundary |
IPSEpro assembles the equations of these components into a coupled process model. Stream connections carry thermodynamic states and mass flow; shaft connections carry power. Shared global objects provide compositions and ambient conditions. This lets the model calculate the consequences of a specification across the entire plant.
Start with the gas turbine
Build the process model in stages, calculating each stage successfully before adding the next.
Begin with the gas-turbine side alone: the air supply, fuel supply, gas-turbine performance model, and an exhaust boundary. Select a turbine model that matches the available equipment data, then define the ambient conditions, fuel properties, and operating load. Keep the working-fluid definitions consistent: the library uses mass fractions for compositions, and the exhaust composition is calculated from combustion.
Solve this section and check the electrical output, fuel consumption, exhaust mass flow, and exhaust temperature. These results establish how much heat is available to the steam cycle. A working gas-turbine model also gives you a clear starting point for identifying the effect of each addition.
Add a simple steam loop
Next, build the simplest complete water-steam circuit: a heat source, a steam turbine coupled to a generator, a condenser, and a pump returning the water to the heat source. A simple boiler model can temporarily supply the heat while you establish a working loop; the exhaust-fed HRSG will take over that role when the two cycles are connected. Use the library's pure WATER formulation for the water-steam streams so that evaporation and condensation are represented correctly.
The closed loop needs a connector element, marked C just before the steam turbine in our flowsheet. Each ordinary component contributes a mass-balance equation. Once the water circuit is closed, one of these equations repeats a condition already imposed by the others, making the equation system redundant. The connector removes that redundant mass-balance equation; mass continuity is still enforced by the rest of the loop. This mathematical role is explained in the manual's simple steam-cycle example.
The turbine also needs engineering assumptions to define its expansion. In our example, the inlet pressure is 40 bar and the isentropic efficiency is 0.87. These represent a selected steam condition and an assumed turbine performance; they should be based on the equipment and design case being studied.
The outlet pressure is set to 3e-2 bar, or 0.03 bar absolute (3 kPa). Here, this is a design assumption for the low-pressure end of the steam cycle. The appropriate value varies from case to case with the cooling conditions, condenser performance, and pressure losses. Lower outlet pressure generally allows more expansion work, but the assumed cooling system must be able to support it. The simulation calculates the consequences of this choice; the pressure setting still needs to be justified by experience, measurements, or equipment data.
For the first calculation, the condenser can provide heat rejection without a detailed cooling-water circuit. Solve the simple loop and check the turbine expansion, condensation, and pressure recovery before adding the feedwater-heating stages.
Develop the heat recovery and preheating stages
With both parts working, connect the steam cycle to the gas-turbine exhaust and replace the temporary boiler with the HRSG sections. APP_Lib_GT represents the HRSG through individual heat exchangers, allowing you to develop the arrangement gradually.
On the water-steam side, the economizer preheats the feedwater, the evaporator supplies the heat for steam generation, and the superheater raises the steam temperature before expansion. The drum separates steam from the water circulating through the evaporator. On the gas side, the hottest exhaust reaches the superheater first, then the evaporator and economizer. This arrangement uses the remaining, lower-temperature exhaust heat for preheating.
Add the feedwater-heating stages progressively and calculate the model after each addition. In our example, a further heat-recovery section supplies a separate heating branch to the deaerator. The deaerator combines this stream with returning condensate and supplies warmed feedwater to the main feedwater pump. Together with the economizer, this extends heat recovery toward the cold end of the HRSG.
At this stage, the main assumptions are the pressure levels, pressure losses and the temperature differences needed to transfer heat. They influence the steam production, steam temperature, stack temperature, and gas-turbine backpressure. As the sections are connected, remove temporary boundary settings that are now determined by the combined model, and use the previous solution as a starting estimate. The exhaust bypass can be added once the main heat-recovery path works.
The completed example
The image below shows the final configuration and component settings used in our example. The values inside the floating boxes indicate the prescriptions for each unit. These values describe this particular design case and should be adapted to the equipment and operating conditions of another plant.
The displayed results give approximately 150.2 MW from the gas turbine and 80.8 MW from the steam turbine. Their sum, about 231 MW, shows the additional output obtained from the recovered exhaust heat. Net plant output also needs to account for pumps, cooling equipment, and other auxiliary consumption.
Figure 2. Final configuration and settings for the example combined-cycle model.
Creating the HRSG diagram
The HRSG diagram brings the heat-recovery calculation into one view by plotting temperature against transferred heat.
Figure 3. The HRSG diagram generated with IPSE GO for the completed example.
The red curve shows the exhaust gas cooling through the HRSG, from about 614 °C to a stack temperature of about 131 °C. The blue lines show the corresponding water-steam temperatures. The sloping sections represent preheating and superheating; the nearly horizontal section around 250 °C represents evaporation, where heat is absorbed with little change in temperature. The additional blue segment at the cold end corresponds to the separate deaerator-heating branch.
The gap between the curves is the temperature difference driving heat transfer. The close approach near the end of the evaporating section is the pinch point, which limits how much heat can be recovered for the chosen steam conditions. A smaller pinch can improve heat recovery, but generally requires more heat-transfer surface. Checking the whole profile also reveals temperature crossings that would make the proposed heat exchange physically inconsistent.
The horizontal span of each section indicates its share of the transferred heat. In this example, evaporation occupies the largest span, showing how much of the recovered exhaust heat goes into changing water into steam. Reading these spans alongside the temperature gaps helps identify both the largest heat duties and the constraints on further recovery.