Glossary

10.3 Combined gas-vapor power cycles

6 min read•july 30, 2024

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 , 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 cycles, utilizing the waste heat from the gas turbine to generate steam for the steam turbine
  • Main components include a gas turbine, (HRSG), steam turbine, , and electric generators
    • Gas turbine operates on the , 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
  • 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 (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

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 (, )
  • 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
  • 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
    • 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: ηcombined=Wnet,GT+Wnet,STQin\eta_{combined} = \frac{W_{net,GT} + W_{net,ST}}{Q_{in}} where Wnet,GTW_{net,GT} is the net work output of the gas turbine, Wnet,STW_{net,ST} is the net work output of the steam turbine, and QinQ_{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: Pcombined=Pnet,GT+Pnet,ST−PlossesP_{combined} = P_{net,GT} + P_{net,ST} - P_{losses} where Pnet,GTP_{net,GT} is the net power output of the gas turbine, Pnet,STP_{net,ST} is the net power output of the steam turbine, and PlossesP_{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 can be evaluated using the (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

Key Terms to Review (27)

Boiler: A boiler is a device used to generate steam or hot water by transferring heat from a fuel source to water. It plays a crucial role in various thermodynamic cycles, especially in converting thermal energy into mechanical energy. The efficiency and design of the boiler significantly affect the overall performance of systems like steam power plants and combined gas-vapor cycles.
Brayton Cycle: The Brayton Cycle is a thermodynamic cycle that describes the functioning of a gas turbine engine, where air is compressed, mixed with fuel, and then combusted to produce work. This cycle is significant for understanding how heat engines convert thermal energy into mechanical energy and highlights the importance of thermal efficiency in energy conversion processes.
Carnot Efficiency: Carnot efficiency is the maximum possible efficiency of a heat engine operating between two temperature reservoirs, defined by the equation $$ ext{Efficiency} = 1 - \frac{T_C}{T_H}$$, where $$T_C$$ is the absolute temperature of the cold reservoir and $$T_H$$ is the absolute temperature of the hot reservoir. This concept highlights the ideal performance of reversible processes and serves as a benchmark for real-world engines. It emphasizes that no real engine can exceed this efficiency, which is crucial when comparing different thermodynamic cycles and systems.
Combined Cycle Power Plant: A combined cycle power plant is a highly efficient power generation system that utilizes both gas and steam turbines to produce electricity. This setup captures the waste heat from the gas turbine's exhaust and uses it to generate steam, which then drives a steam turbine for additional power production, maximizing energy output from the same fuel source.
Compressor efficiency: Compressor efficiency is a measure of how effectively a compressor converts input energy into useful work, specifically in compressing a gas or vapor. It is a critical factor in evaluating the performance of compressors within combined gas-vapor power cycles, influencing both energy consumption and overall system effectiveness. Higher compressor efficiency means less energy is wasted, leading to improved cycle performance and lower operational costs.
Condenser: A condenser is a heat exchanger used to condense vapor into liquid by removing heat from the vapor, usually through contact with a cooler surface or fluid. This process is crucial in various thermodynamic cycles and systems, enabling efficient heat transfer and phase change essential for energy conversion, refrigeration, and air conditioning.
Effectiveness-NTU Method: The effectiveness-NTU method is a technique used to analyze heat exchangers by relating the heat transfer effectiveness to the number of transfer units (NTU). This method is particularly useful for determining how well a heat exchanger performs without requiring detailed information about its physical construction, making it a versatile tool in thermal system analysis.
Energy balance: Energy balance refers to the accounting of energy input and output in a system, ensuring that energy is conserved throughout processes. In this context, it involves analyzing how energy is transferred, converted, or transformed in various systems and processes while maintaining the law of conservation of energy, which states that energy cannot be created or destroyed. Understanding energy balance is crucial for evaluating the efficiency and performance of different processes, particularly in dynamic situations and combined systems.
Exergy analysis: Exergy analysis is a method used to evaluate the quality or usefulness of energy, focusing on its potential to do work in a specific environment. It provides insights into the efficiency of energy conversion processes and helps identify irreversibilities in systems. By analyzing exergy, one can understand how energy can be better utilized or wasted during operations, shedding light on areas for improvement in energy systems.
First Law of Thermodynamics: The First Law of Thermodynamics states that energy cannot be created or destroyed, only transformed from one form to another, which means the total energy of an isolated system remains constant. This principle underlies various processes, cycles, and energy interactions that involve heat, work, and mass transfer in different systems.
Geothermal power system: A geothermal power system is a technology that harnesses heat energy from beneath the Earth's surface to generate electricity or provide direct heating. This renewable energy source taps into the natural heat produced by the Earth, which can be found in various forms, such as steam or hot water, and is often used in combined gas-vapor power cycles to enhance overall efficiency and energy output.
Heat Exchanger: A heat exchanger is a device that facilitates the transfer of thermal energy between two or more fluids at different temperatures, without mixing them. They are crucial in many engineering applications, where they improve energy efficiency by recycling heat or regulating temperatures, making them essential components in various systems.
Heat Recovery Steam Generator: A heat recovery steam generator (HRSG) is a type of heat exchanger that captures exhaust heat from gas turbines or other sources and uses it to produce steam. This steam can then be utilized for power generation or other processes, significantly improving the overall efficiency of combined gas-vapor power cycles by reusing waste heat.
Heat transfer fluid: A heat transfer fluid is a substance used to transfer heat from one location to another within a system, facilitating thermal energy exchange between components. These fluids can be liquids or gases, and their properties, such as specific heat capacity and thermal conductivity, play a crucial role in the efficiency of heat transfer processes. They are essential in systems like combined gas-vapor power cycles, where they help maximize energy conversion efficiency.
HRSG Effectiveness: HRSG effectiveness refers to the efficiency with which a Heat Recovery Steam Generator (HRSG) captures and utilizes waste heat from gas turbines to produce steam for additional power generation. This term highlights how well the HRSG converts exhaust gases into usable energy, influencing the overall performance and efficiency of combined gas-vapor power cycles.
Isentropic process: An isentropic process is a thermodynamic process that occurs at constant entropy, meaning there is no heat transfer into or out of the system, and it is reversible. This concept plays a crucial role in analyzing various cycles, where it simplifies the calculations of efficiency and performance by assuming idealized conditions without entropy changes. Isentropic processes are often used to represent idealized transformations in real-world systems, linking them to key principles in energy conversion and thermodynamic efficiency.
Isobaric Process: An isobaric process is a thermodynamic process in which the pressure remains constant while the volume and temperature may change. This type of process is significant as it helps to understand various physical phenomena, such as phase changes and energy transfer in systems like engines and refrigeration cycles.
Net work output: Net work output is the total amount of work produced by a thermodynamic cycle after accounting for the work input required to operate the system. In the context of combined gas-vapor power cycles, it reflects the efficiency and effectiveness of the cycle in converting energy into useful work, showcasing the balance between the energy provided and the energy consumed. This term is crucial for evaluating the performance of power cycles that integrate both gas and vapor systems.
Number of transfer units: The number of transfer units (NTU) is a dimensionless measure used in heat and mass transfer operations to quantify the efficiency of a process, particularly in heat exchangers and absorption systems. It represents the number of times a unit of energy or mass is transferred from one phase to another and is essential for evaluating the performance of combined gas-vapor power cycles where heat recovery is critical.
Overall efficiency: Overall efficiency refers to the ratio of the useful output of a system to the total input energy supplied to that system. It provides a measure of how effectively a power cycle converts input energy into useful work, taking into account various factors like heat losses and irreversibilities that can affect performance. Understanding overall efficiency is crucial when evaluating and comparing different power generation systems, particularly when analyzing processes that involve multiple cycles or stages.
Rankine cycle: The Rankine cycle is a thermodynamic cycle that converts heat into work through a series of processes involving a working fluid, typically water or steam. It consists of four main processes: isentropic compression, isobaric heat addition, isentropic expansion, and isobaric heat rejection, making it a foundational concept in the study of heat engines and energy conversion systems.
Regenerative feedwater heating: Regenerative feedwater heating is a process used in power plants to improve efficiency by using steam from the turbine to preheat the water before it enters the boiler. This method reduces the energy required to convert the feedwater into steam, thereby enhancing the overall performance of the combined gas-vapor power cycle. By recovering waste heat, it minimizes thermal losses and optimizes energy use.
Second Law of Thermodynamics: The Second Law of Thermodynamics states that the total entropy of an isolated system can never decrease over time, and it tends to increase, leading to the concept that energy transformations are not 100% efficient. This law establishes the directionality of processes, implying that certain processes are irreversible and energy has a quality that degrades over time, connecting tightly to concepts of heat transfer, work, and system analysis.
Specific work output: Specific work output refers to the amount of useful work produced by a system per unit of mass or energy input. This concept is particularly significant in analyzing the efficiency and performance of power cycles, especially in combined gas-vapor systems where both gas and vapor phases are utilized for energy conversion. Understanding specific work output helps evaluate how effectively different components of a power cycle contribute to overall energy production.
Thermal efficiency: Thermal efficiency is a measure of how well an energy conversion system, such as a heat engine, converts heat energy into useful work. It is defined as the ratio of the useful work output to the heat input, typically expressed as a percentage. This concept is crucial for evaluating and optimizing the performance of various thermodynamic cycles and systems.
Turbine: A turbine is a mechanical device that converts fluid energy into mechanical work, typically by rotating blades driven by a flowing fluid such as water, steam, or gas. This conversion is crucial for various applications, particularly in energy generation and propulsion systems, where turbines play a significant role in harnessing energy from different sources.
Turbine efficiency: Turbine efficiency is a measure of how effectively a turbine converts the energy in a fluid into mechanical energy, typically expressed as a percentage. It indicates the ratio of useful work output to the energy input from the working fluid. High turbine efficiency is essential for maximizing the performance of energy systems, particularly in applications where energy conversion processes are critical.
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