4.4 Modifications and Improvements to Gas Power Cycles

Gas power cycles are the backbone of many energy systems. Modifications like intercooling, reheating, and regeneration can boost their efficiency and power output. These tweaks reduce losses and make the most of the energy in the fuel.

Advanced configurations take things further. Combined cycles, cogeneration, and supercritical CO2 systems push the limits of what's possible. They mix and match different cycles or use unique properties to squeeze out even more power and efficiency.

Gas power cycle enhancements

Modifications to improve performance

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• Modifications to gas power cycles aim to improve thermal efficiency, specific power output, or both by reducing irreversibilities and losses in the system
• Common modifications include intercooling, reheating, and regeneration, which can be implemented individually or in combination to optimize cycle performance
• The selection and implementation of modifications depend on specific application requirements, such as power output, fuel type, operating conditions, and economic considerations

Advanced cycle configurations

• Other potential modifications involve advanced cycle configurations, such as combined cycles, cogeneration systems, and supercritical CO2 cycles
  • These configurations leverage unique thermodynamic properties or integrate multiple cycles for enhanced efficiency
  • Examples include combining gas turbine and steam turbine cycles (combined cycles), utilizing waste heat for industrial or domestic heating (cogeneration), and using supercritical CO2 as the working fluid (sCO2 cycles)

Impact of modifications on efficiency

Intercooling

• Intercooling involves cooling the working fluid between compression stages, reducing compressor work input and increasing net work output, thereby improving cycle efficiency
  • Particularly beneficial for gas turbine cycles with high pressure ratios, as it reduces the average temperature during compression and minimizes compressor work
  • Effectiveness depends on factors such as the number of intercooling stages, the intercooling temperature, and the pressure ratio distribution between stages
  • Example: In a gas turbine with a pressure ratio of 30, introducing a single stage of intercooling can improve cycle efficiency by 2-3 percentage points

Reheating

• Reheating involves reheating the working fluid between expansion stages, increasing the average temperature during expansion and improving cycle efficiency
  • Commonly used in steam power cycles, where steam is partially expanded in a high-pressure turbine, reheated, and then expanded further in lower-pressure turbines
  • Optimal number of reheating stages and reheating temperature depend on the cycle operating conditions and the trade-off between efficiency gains and increased complexity
  • Example: In a steam power plant with a main steam temperature of 600°C, introducing a single stage of reheating at 600°C can improve cycle efficiency by 4-5 percentage points

Regeneration

• Regeneration utilizes heat exchangers to transfer heat from the hot exhaust gases to the cooler compressed air, preheating the air before it enters the combustion chamber and reducing the required heat input
  • Particularly effective in gas turbine cycles with high turbine outlet temperatures, as it recovers waste heat that would otherwise be lost to the environment
  • Effectiveness depends on factors such as the heat exchanger effectiveness, pressure losses, and the temperature difference between the exhaust gases and the compressed air
  • Example: In a simple gas turbine cycle with a turbine outlet temperature of 600°C, introducing a regenerator with an effectiveness of 80% can improve cycle efficiency by 10-15 percentage points

Efficiency vs complexity trade-offs

Balancing efficiency gains and complexity

• Implementing modifications to gas power cycles often involves trade-offs between improved efficiency and increased system complexity, cost, and maintenance requirements
• The optimal level of modification depends on a thorough analysis of the specific application, considering factors such as the expected efficiency gains, capital and operating costs, reliability, and operational flexibility

Intercooling and reheating complexity

• Intercooling adds complexity to the compression process, requiring additional heat exchangers, piping, and control systems, which can increase capital costs and maintenance needs
• Reheating introduces additional turbine stages, piping, and control valves, increasing the complexity and cost of the expansion process

Regeneration and advanced cycle challenges

• Regeneration requires the integration of heat exchangers, which can introduce additional pressure losses, increase the system's size and weight, and complicate maintenance procedures
• Advanced cycle configurations, such as combined cycles and supercritical CO2 cycles, offer significant efficiency improvements but also involve higher complexity, specialized components, and unique operational challenges

Advanced gas power cycle configurations

Combined cycle power plants

• Combined cycle power plants integrate gas turbine and steam turbine cycles, utilizing the gas turbine exhaust heat to generate steam for the steam turbine, achieving high overall efficiencies (up to 60% or more)
  • The gas turbine operates on the Brayton cycle, while the steam turbine operates on the Rankine cycle, with a heat recovery steam generator (HRSG) serving as the link between the two cycles
  • Offer flexibility in terms of fuel selection, load-following capabilities, and cogeneration opportunities (combined heat and power)
  • Example: A combined cycle plant with a gas turbine inlet temperature of 1400°C and a triple-pressure HRSG can achieve a net efficiency of around 62%

Cogeneration (CHP) systems

• Cogeneration systems, also known as combined heat and power (CHP) systems, utilize the waste heat from gas power cycles for various industrial or domestic heating applications, increasing the overall energy utilization efficiency
  • Can be designed as topping or bottoming cycles, depending on whether the power generation or the heat supply is the primary purpose
  • Applications include district heating, industrial process heat, and desalination, among others
  • Example: A gas turbine-based CHP system providing electricity and district heating can achieve an overall energy utilization efficiency of 80-90%

Supercritical CO2 (sCO2) cycles

• Supercritical CO2 (sCO2) cycles operate with carbon dioxide as the working fluid under supercritical conditions, offering the potential for high efficiency, compact turbomachinery, and enhanced heat transfer properties
  • Can be implemented in various configurations, such as the simple regenerative cycle, the recompression cycle, and the partial cooling cycle, each with unique performance characteristics
  • Challenges include high operating pressures, material compatibility, and the need for specialized components (compact heat exchangers and high-pressure turbomachinery)
  • Example: A closed-loop sCO2 Brayton cycle with a turbine inlet temperature of 700°C and a pressure of 30 MPa can potentially achieve cycle efficiencies of 50% or higher