10.3 Combined gas-vapor power cycles

Combined Gas-Vapor Cycles

Principles and Components

A combined gas-vapor power cycle pairs a gas turbine (Brayton cycle) with a steam turbine (Rankine cycle) so that waste heat from the gas turbine becomes the energy source for the steam cycle. This "topping-bottoming" arrangement extracts useful work at two different temperature ranges from a single fuel input, pushing thermal efficiencies well beyond what either cycle achieves alone.

The main components are:

• Gas turbine operating on the Brayton cycle: compresses air, combusts it with fuel, and expands the high-temperature gases through a turbine.
• Heat recovery steam generator (HRSG): captures exhaust heat from the gas turbine and uses it to produce steam.
• Steam turbine operating on the Rankine cycle: expands the steam to produce additional work.
• Condenser: rejects heat from the steam cycle to the surroundings.
• Electric generators: convert shaft work from both turbines into electricity.

The HRSG can be configured at one, two, or three pressure levels. A single-pressure HRSG is simpler but recovers less exhaust energy. Dual-pressure and triple-pressure designs add evaporator sections at lower pressures, capturing more of the exhaust enthalpy and raising overall plant efficiency.

Configurations and Design Considerations

• Single-pressure systems use one steam pressure level. They're straightforward to design and operate but leave more recoverable energy in the exhaust.
• Dual-pressure and triple-pressure systems generate steam at multiple pressures, matching the temperature profile of the exhaust gas more closely. This reduces the exergy destruction in the HRSG and improves overall efficiency.
• Selecting the right gas turbine, steam turbine, and HRSG design depends on power output requirements, fuel type, ambient temperature, and humidity. Higher ambient temperatures, for example, reduce air density and cut gas turbine output.
• Proper integration and coordinated control between the gas and steam sides are critical for reliable, efficient operation.

Thermodynamics of Combined Cycles

Gas Turbine Cycle (Brayton Cycle)

The ideal Brayton cycle has four processes:

1. Isentropic compression in the compressor ($1 \to 2$)
2. Isobaric heat addition in the combustion chamber ($2 \to 3$)
3. Isentropic expansion in the turbine ($3 \to 4$)
4. Isobaric heat rejection in the exhaust ($4 \to 1$)

The ideal thermal efficiency depends on the pressure ratio $r_p$:

$\eta_{Brayton} = 1 - \frac{1}{r_p^{(k-1)/k}}$

where $k$ is the specific heat ratio of the working gas. Higher pressure ratios and higher turbine inlet temperatures both improve efficiency. In practice, compressor and turbine isentropic efficiencies (typically 85–92%) reduce the actual cycle performance below the ideal.

Steam Turbine Cycle (Rankine Cycle)

The ideal Rankine cycle also has four processes, but note the order typically starts with the turbine:

1. Isentropic expansion in the steam turbine
2. Isobaric heat rejection in the condenser
3. Isentropic compression in the feedwater pump
4. Isobaric heat addition in the HRSG (instead of a conventional boiler)

Rankine cycle efficiency improves when you raise the steam turbine inlet temperature and pressure, or lower the condenser pressure. Regenerative feedwater heating (extracting steam from the turbine to preheat feedwater) also helps by raising the average temperature of heat addition.

Heat Recovery Steam Generator (HRSG)

The HRSG is the thermodynamic bridge between the two cycles. It transfers energy from the gas turbine exhaust (typically 450–650 °C) to the water/steam loop.

A key design parameter is the pinch point temperature difference: the minimum temperature gap between the hot exhaust gas and the boiling water/steam inside the HRSG. A smaller pinch point means more heat recovery but requires a larger (and more expensive) heat transfer surface. Typical pinch points range from 8–25 °C.

The HRSG design must also minimize the pressure drop on the gas side, since any back-pressure on the gas turbine exhaust reduces its net output. Supplementary firing (duct burners in the HRSG) can boost steam production when more power is needed, though it lowers the combined cycle's overall efficiency because that extra fuel input bypasses the gas turbine.

Advanced Cycle Modifications

Several modifications can push combined cycle efficiency higher:

• Regenerative feedwater heating: Steam extracted from the steam turbine preheats the feedwater before it enters the HRSG, reducing the heat duty required from the exhaust gases.
• Reheating: Steam expands through a high-pressure turbine stage, returns to the HRSG for reheating, then expands again through a low-pressure stage. This raises the average temperature of heat addition and keeps the steam quality high at the turbine exit.
• Intercooling: Cooling the air between compressor stages in the gas turbine reduces compressor work, increasing the net work output of the Brayton cycle.

Efficiency and Power Output of Combined Cycles

Overall Efficiency Calculation

The combined cycle thermal efficiency is:

$\eta_{combined} = \frac{\dot{W}_{net,GT} + \dot{W}_{net,ST}}{\dot{Q}_{in}}$

where:

• $\dot{W}_{net,GT}$ = gas turbine work output minus compressor work
• $\dot{W}_{net,ST}$ = steam turbine work output minus pump work
• $\dot{Q}_{in}$ = total rate of heat input from fuel combustion

There's a useful relationship that connects the individual cycle efficiencies to the combined efficiency. If $\eta_{GT}$ is the gas turbine cycle efficiency and $\eta_{ST}$ is the steam cycle efficiency (based on the heat it receives from the exhaust), then:

$\eta_{combined} = \eta_{GT} + \eta_{ST} - \eta_{GT} \cdot \eta_{ST}$

This formula shows why combined cycles are so effective. Even if each cycle is only moderately efficient on its own, the combined efficiency is significantly higher. For example, a gas turbine at 40% and a bottoming steam cycle at 33% yield a combined efficiency of roughly $0.40 + 0.33 - (0.40)(0.33) = 0.598$, or about 60%.

Modern combined cycle plants achieve thermal efficiencies above 60%, compared to roughly 35–45% for standalone gas or steam turbine plants.

Power Output Determination

Total plant power output is:

$\dot{W}_{combined} = \dot{W}_{net,GT} + \dot{W}_{net,ST} - \dot{W}_{losses}$

where $\dot{W}_{losses}$ accounts for mechanical friction, generator losses, and plant auxiliary power consumption. The net work from each turbine equals its specific work multiplied by the mass flow rate of its working fluid (air for the gas turbine, steam for the steam turbine).

Component Efficiencies

Each component's performance affects the overall result:

• Gas turbine and compressor isentropic efficiencies determine how much the actual Brayton cycle deviates from the ideal.
• Steam turbine isentropic efficiency governs how effectively the steam's enthalpy is converted to shaft work.
• HRSG effectiveness measures how much of the available exhaust energy actually transfers to the steam cycle. This can be evaluated using the effectiveness-NTU ($\varepsilon$-NTU) method, which compares actual heat transfer to the thermodynamic maximum.

Improving any one of these component efficiencies raises the overall plant performance.

Optimization and Performance Enhancement

To maximize combined cycle performance, engineers vary parameters like gas turbine pressure ratio, turbine inlet temperature, steam conditions, and HRSG configuration through parametric studies. Practical improvements include:

• Advanced blade materials and thermal barrier coatings that allow higher turbine inlet temperatures
• Improved cooling technologies for gas turbine blades
• Optimized HRSG designs with multiple pressure levels and reheat
• Sophisticated control strategies that coordinate the gas and steam sides during load changes

Advantages of Combined Cycles

High Efficiency

Combined cycles achieve over 60% thermal efficiency by extracting work from both the high-temperature gas turbine exhaust and the lower-temperature steam cycle. This means less fuel is burned per unit of electricity, directly reducing fuel costs.

Environmental Benefits

Higher efficiency translates to lower $CO_2$ emissions per MWh of electricity. Combined cycle plants burning natural gas produce roughly 50–60% less $CO_2$ per MWh than conventional coal plants, due to both the higher efficiency and the lower carbon content of natural gas.

Operational Flexibility

Gas turbines can start up in minutes and ramp quickly, giving combined cycle plants the ability to follow load changes and support grid stability. These plants can serve as base load, intermediate load, or peaking units depending on market conditions.

Compact Design

Integrating the gas turbine, HRSG, and steam turbine into a single facility requires less land than building separate gas and steam plants of equivalent total capacity. This smaller footprint is valuable in space-constrained locations.