10.3 Combined gas-vapor power cycles

Combined gas-vapor power cycles blend gas and steam turbines for maximum efficiency. By using waste heat from gas turbines to generate steam, these systems achieve higher power outputs and lower fuel consumption than standalone cycles.

These hybrid systems offer numerous benefits, including improved thermal efficiency, reduced emissions, and operational flexibility. Understanding the principles behind combined cycles is crucial for optimizing power generation and meeting evolving energy demands.

Combined Gas-Vapor Cycles

Principles and Components

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• Integrate both gas and steam turbine cycles, utilizing the waste heat from the gas turbine to generate steam for the steam turbine
• Main components include a gas turbine, heat recovery steam generator (HRSG), steam turbine, condenser, and electric generators
  • Gas turbine operates on the Brayton cycle, compressing air, mixing it with fuel, and combusting the mixture to generate high-temperature, high-pressure gases that expand through the turbine
  • HRSG captures the exhaust heat from the gas turbine to produce steam, which is then used to drive the steam turbine operating on the Rankine cycle
• Can be configured in single-pressure, dual-pressure, or triple-pressure steam systems, depending on the desired efficiency and complexity (single-pressure, dual-pressure, triple-pressure)

Configurations and Design Considerations

• Single-pressure combined cycle systems have one pressure level in the HRSG and steam turbine, offering simplicity but lower efficiency compared to multi-pressure systems
• Dual-pressure and triple-pressure systems incorporate multiple pressure levels in the HRSG and steam turbine, allowing for better heat recovery and higher overall efficiency (dual-pressure, triple-pressure)
• The selection of the gas turbine, steam turbine, and HRSG designs should be optimized based on factors such as power output requirements, fuel type, and ambient conditions
• Proper integration and control of the gas and steam cycles are crucial for optimal performance and reliability of the combined cycle power plant

Thermodynamics of Combined Cycles

Gas Turbine Cycle (Brayton Cycle)

• Involves four main processes: isentropic compression in the compressor, isobaric heat addition in the combustion chamber, isentropic expansion in the turbine, and isobaric heat rejection in the exhaust
• The efficiency of the Brayton cycle depends on factors such as the pressure ratio, turbine inlet temperature, and component efficiencies (compressor efficiency, turbine efficiency)
• Higher pressure ratios and turbine inlet temperatures generally lead to improved gas turbine cycle efficiency

Steam Turbine Cycle (Rankine Cycle)

• Consists of four processes: isentropic expansion in the steam turbine, isobaric heat rejection in the condenser, isentropic compression in the pump, and isobaric heat addition in the HRSG
• The efficiency of the Rankine cycle is influenced by factors such as the steam turbine inlet temperature and pressure, condenser pressure, and the presence of regenerative feedwater heating
• Increasing the steam turbine inlet temperature and pressure while lowering the condenser pressure can enhance the Rankine cycle efficiency

Heat Recovery Steam Generator (HRSG)

• Acts as the link between the gas and steam cycles, transferring the waste heat from the gas turbine exhaust to the water/steam in the Rankine cycle
  • Pinch point temperature difference is a critical parameter in HRSG design, representing the minimum temperature difference between the hot exhaust gases and the water/steam
• The HRSG design should optimize the heat transfer while minimizing the pressure drop on the gas side and the heat transfer surface area
• Supplementary firing can be employed in the HRSG to increase the steam production and power output, albeit at the expense of reduced efficiency

Advanced Cycle Modifications

• Regeneration, reheating, and intercooling can be incorporated into the combined cycle to enhance overall efficiency
• Regeneration involves preheating the feedwater using steam extracted from the steam turbine, reducing the heat input required in the HRSG (regenerative feedwater heating)
• Reheating involves expanding the steam in stages, with reheating of the steam between stages to increase the average temperature of heat addition and improve efficiency
• Intercooling in the gas turbine compressor can reduce the compressor work and increase the net output of the gas turbine cycle

Advantages of Combined Cycles

High Efficiency

• Achieve higher thermal efficiencies compared to standalone gas turbine or steam turbine plants by utilizing the waste heat from the gas turbine exhaust
• The integration of the gas and steam cycles allows for a more efficient use of the input fuel energy, reducing the overall fuel consumption and associated costs
• Combined cycle power plants can reach thermal efficiencies of over 60%, surpassing the efficiencies of single-cycle plants (50-60% efficiency)

Environmental Benefits

• Have lower greenhouse gas emissions per unit of electricity generated compared to single-cycle plants, contributing to a reduced environmental impact
• The higher efficiency of combined cycle plants results in less fuel consumption and, consequently, lower carbon dioxide (CO2) emissions per megawatt-hour of electricity produced
• The use of natural gas as the primary fuel in combined cycle plants further reduces emissions compared to coal-fired power plants (natural gas, coal)

Operational Flexibility

• The flexibility of combined cycle power plants allows for rapid startup and load-following capabilities, making them suitable for meeting variable power demands
• Gas turbines can quickly ramp up or down their power output in response to changes in electricity demand, providing valuable grid support
• Combined cycle plants can also operate in a variety of modes, such as base load, intermediate load, or peaking service, depending on the market requirements (base load, intermediate load, peaking)

Compact Design

• The compact design of combined cycle power plants results in a smaller footprint compared to separate gas and steam turbine facilities
• The integration of the gas turbine, HRSG, and steam turbine into a single plant reduces the land area required for the power generation facility
• The smaller footprint of combined cycle plants can be advantageous in areas with limited land availability or where minimizing the environmental impact is a priority

Efficiency and Power Output of Combined Cycles

Overall Efficiency Calculation

• The overall efficiency of a combined cycle power plant is determined by the sum of the net work outputs from the gas and steam turbines divided by the total heat input from the fuel
  • Net work output of the gas turbine is the difference between the turbine work and the compressor work
  • Net work output of the steam turbine is the difference between the turbine work and the pump work
• The efficiency can be expressed as: $\eta_{combined} = \frac{W_{net,GT} + W_{net,ST}}{Q_{in}}$ where $W_{net,GT}$ is the net work output of the gas turbine, $W_{net,ST}$ is the net work output of the steam turbine, and $Q_{in}$ is the total heat input from the fuel

Power Output Determination

• The power output of a combined cycle power plant is the sum of the net power outputs from the gas and steam turbines, accounting for any mechanical and electrical losses
• The power output can be calculated as: $P_{combined} = P_{net,GT} + P_{net,ST} - P_{losses}$ where $P_{net,GT}$ is the net power output of the gas turbine, $P_{net,ST}$ is the net power output of the steam turbine, and $P_{losses}$ are the mechanical and electrical losses in the system
• The net power outputs of the gas and steam turbines are determined by multiplying their respective net work outputs by the mass flow rates of the working fluids (air for the gas turbine and steam for the steam turbine)

Component Efficiencies

• The efficiency of the gas turbine cycle can be calculated using the Brayton cycle efficiency equation, considering the pressure ratio, specific heat ratio, and turbine inlet temperature
• The efficiency of the steam turbine cycle can be determined using the Rankine cycle efficiency equation, taking into account the steam properties at the turbine inlet and condenser outlet
• The effectiveness of the HRSG in transferring heat from the gas turbine exhaust to the steam cycle affects the overall efficiency of the combined cycle power plant
  • The HRSG effectiveness can be evaluated using the number of transfer units (NTU) method or the effectiveness-NTU (ε-NTU) method, which relate the actual heat transfer to the maximum possible heat transfer
• Improving the efficiencies of individual components, such as the gas turbine, steam turbine, compressor, and HRSG, can lead to higher overall combined cycle efficiency

Optimization and Performance Enhancement

• Parametric studies and optimization techniques can be employed to determine the optimal design parameters and operating conditions for a combined cycle power plant
• Factors such as the gas turbine pressure ratio, turbine inlet temperature, steam turbine inlet temperature and pressure, and HRSG configuration can be varied to maximize the overall efficiency and power output
• Advanced materials, cooling technologies, and control strategies can be implemented to enhance the performance and reliability of combined cycle power plants
• Regular maintenance, performance monitoring, and upgrades are essential to ensure the long-term efficiency and availability of combined cycle power plants