5.2 Brayton cycle and combined cycle applications

The Brayton cycle powers gas turbines for electricity and jet engines. It compresses air, adds fuel, burns it, and expands hot gases through a turbine. Combined with the Rankine cycle, it forms super-efficient power plants.

Gas turbines, compressors, combustors, and turbines work together in the Brayton cycle. Add a heat recovery steam generator, and you've got a combined cycle system. These setups squeeze more power from fuel, reaching efficiencies up to 60%.

Brayton Cycle Components

Gas Turbine Cycle Overview

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• Brayton cycle forms the basis of gas turbine engines used in power generation and aircraft propulsion
• Consists of four main processes: compression, combustion, expansion, and heat rejection
• Operates on the principle of compressing air, adding fuel, combusting the mixture, and expanding hot gases through a turbine
• Ideal cycle assumes isentropic compression and expansion, constant pressure heat addition and rejection
• Real cycle experiences losses due to friction, heat transfer, and component inefficiencies

Key Components and Their Functions

• Gas turbine serves as the primary power-generating unit in the Brayton cycle
• Compressor increases the pressure of incoming air, typically achieving pressure ratios between 10:1 and 30:1
• Combustion chamber (combustor) mixes compressed air with fuel and ignites the mixture, raising temperature to 1300-1500°C
• Turbine extracts energy from high-temperature, high-pressure gases, driving both the compressor and an external load (generator)
• Recuperator improves cycle efficiency by preheating compressed air using turbine exhaust heat
  • Can increase overall efficiency by 5-10% depending on operating conditions

Performance Optimization Techniques

• Increasing turbine inlet temperature improves cycle efficiency, limited by material constraints
• Higher pressure ratios generally increase efficiency but require more robust compressor designs
• Intercooling between compressor stages reduces work input and increases power output
• Reheat between turbine stages increases power output at the expense of slightly lower efficiency
• Closed Brayton cycles use inert gases (helium, nitrogen) as working fluids for specialized applications (nuclear power, space systems)

Combined Cycle Systems

Integration of Brayton and Rankine Cycles

• Combined cycle systems merge gas turbine (Brayton) and steam turbine (Rankine) cycles to maximize overall efficiency
• Utilizes high-temperature exhaust gases from the gas turbine to generate steam for the Rankine cycle
• Achieves thermal efficiencies of up to 60%, significantly higher than individual cycles (gas turbine ~35%, steam turbine ~40%)
• Heat recovery steam generator (HRSG) acts as the interface between the two cycles, recovering waste heat to produce steam
• Topping cycle refers to the gas turbine portion, operating at higher temperatures and producing the primary power output
• Bottoming cycle describes the steam turbine portion, utilizing recovered heat to generate additional electricity

Heat Recovery Steam Generator Design

• HRSG consists of economizer, evaporator, and superheater sections
• Economizer preheats feedwater using low-temperature exhaust gases
• Evaporator converts preheated water to saturated steam
• Superheater raises steam temperature above saturation point for improved turbine efficiency
• Multi-pressure HRSGs (typically triple-pressure) optimize heat recovery across different temperature ranges
• Duct burners can be added to increase steam production during peak demand periods

Efficiency Enhancement Strategies

• Intercooling in the gas turbine compressor reduces work input and increases overall plant efficiency
• Implementing steam injection into the gas turbine combustor increases mass flow and power output
• Using advanced materials allows higher turbine inlet temperatures, improving both Brayton and Rankine cycle efficiencies
• Optimizing HRSG design to minimize exhaust gas temperature and maximize steam production
• Employing reheat in the steam cycle to increase power output and overall plant efficiency