Combined cycle power plants merge gas and steam turbine cycles, boosting efficiency to 60%. This integration captures waste heat from gas turbines to power steam turbines, maximizing energy use and reducing emissions.

These plants offer operational flexibility and baseload power, making them ideal for varying electricity demands. By combining Brayton and Rankine cycles, they achieve higher efficiency than standalone gas or steam turbine plants.

Combined cycle power plants

Integration of gas and steam turbine cycles

• Combined cycle power plants integrate gas turbine and steam turbine cycles to generate electricity more efficiently than either cycle alone
• The gas turbine operates on the Brayton cycle, compressing air, combusting fuel, and expanding the hot gases through the turbine to generate power
• The steam turbine utilizes the Rankine cycle, using the heat from the gas turbine exhaust to generate steam and drive the turbine

Higher efficiency and lower emissions

• Combined cycle plants can achieve thermal efficiencies up to 60%, significantly higher than the efficiencies of standalone gas turbine (30-40%) or steam turbine (30-45%) plants
• The higher efficiency results in lower fuel consumption and reduced greenhouse gas emissions per unit of electricity generated compared to single-cycle plants
• The reduced fuel consumption leads to lower levels of other pollutants, such as nitrogen oxides (NOx) and sulfur oxides (SOx), contributing to improved air quality

Operational flexibility and baseload power

• Combined cycle plants offer operational flexibility, as the gas turbine can be started up quickly to meet peak demand
• The steam turbine provides stable baseload power, ensuring a consistent supply of electricity
• The combination of flexibility and baseload capability makes combined cycle plants well-suited for meeting varying electricity demand

Gas turbine and steam turbine integration

Gas turbine Brayton cycle

• In a combined cycle plant, the gas turbine operates on the Brayton cycle
• The Brayton cycle involves compressing air, combusting fuel, and expanding the hot gases through the turbine to generate power
• The hot exhaust gases from the gas turbine, typically at temperatures around 500-600°C, are directed to the heat recovery steam generator (HRSG)

Heat recovery steam generator (HRSG)

• The HRSG uses the heat from the gas turbine exhaust to generate high-pressure steam for the steam turbine
• The HRSG consists of three main sections: the economizer, evaporator, and superheater
  • The economizer preheats the feedwater before it enters the evaporator
  • The evaporator generates saturated steam from the preheated feedwater
  • The superheater raises the steam temperature to the desired level for the steam turbine

Steam turbine Rankine cycle

• The high-pressure steam from the HRSG is expanded through the steam turbine, which operates on the Rankine cycle, to generate additional electricity
• The steam turbine exhaust is condensed in a condenser, and the condensate is pumped back to the HRSG to complete the steam cycle
• The integration of the two cycles allows for efficient heat transfer and minimizes energy losses, resulting in higher overall plant efficiency

Efficiency and power output

Calculating individual cycle efficiencies

• To calculate the overall efficiency of a combined cycle plant, first determine the gas turbine efficiency (ηGT) and steam turbine efficiency (ηST)
• Gas turbine efficiency: ηGT = WGT / QGT, where WGT is the gas turbine work output and QGT is the heat input to the gas turbine
• Steam turbine efficiency: ηST = WST / QST, where WST is the steam turbine work output and QST is the heat input to the steam turbine

Determining heat input to the steam turbine

• The heat input to the steam turbine (QST) is the heat recovered by the HRSG from the gas turbine exhaust
• QST can be calculated using the exhaust gas flow rate, specific heat, and temperature difference across the HRSG
• Example: If the exhaust gas flow rate is 100 kg/s, the specific heat is 1.1 kJ/kg·K, and the temperature difference across the HRSG is 400°C, then QST = 100 kg/s × 1.1 kJ/kg·K × 400 K = 44,000 kW

Overall combined cycle efficiency and power output

• The overall combined cycle efficiency (ηCC) is calculated as: ηCC = (WGT + WST) / QGT, where WGT and WST are the work outputs of the gas and steam turbines, respectively, and QGT is the heat input to the gas turbine
• Example: If a combined cycle plant has a gas turbine output of 100 MW, a steam turbine output of 50 MW, and a gas turbine heat input of 250 MW, the overall efficiency would be: ηCC = (100 MW + 50 MW) / 250 MW = 0.6 or 60%
• The total power output of the combined cycle plant (PCC) is the sum of the gas turbine and steam turbine power outputs: PCC = PGT + PST, where PGT is the gas turbine power output and PST is the steam turbine power output

Environmental and economic benefits

Reduced greenhouse gas emissions

• Combined cycle power plants offer significant environmental benefits compared to single-cycle plants due to their higher efficiency and lower fuel consumption per unit of electricity generated
• The reduced fuel consumption results in lower greenhouse gas emissions, particularly carbon dioxide (CO2), helping to mitigate the impact of power generation on climate change
• Example: A combined cycle plant with 60% efficiency will emit approximately 33% less CO2 per unit of electricity generated compared to a single-cycle gas turbine plant with 40% efficiency

Economic advantages

• The higher efficiency of combined cycle plants leads to lower fuel costs per unit of electricity generated, making them economically attractive for power producers
• The operational flexibility of combined cycle plants allows them to respond quickly to changes in electricity demand, reducing the need for less efficient peaking plants and potentially lowering overall system costs
• The compact design and smaller footprint of combined cycle plants compared to separate gas and steam turbine facilities can result in lower capital costs and reduced land use requirements

Cleaner alternative to coal-fired generation

• The use of natural gas as the primary fuel in combined cycle plants provides a cleaner alternative to coal-fired generation
• Natural gas combustion produces lower levels of pollutants, such as sulfur dioxide (SO2), nitrogen oxides (NOx), and particulate matter, compared to coal combustion
• In regions with abundant natural gas resources, combined cycle plants can offer a more economically stable and environmentally friendly option for electricity generation