13.3 Combined cycles and cogeneration

Combined cycles and cogeneration are game-changers in power generation. They boost efficiency by integrating multiple thermodynamic cycles and capturing waste heat. This clever approach squeezes more energy out of fuel, reducing costs and environmental impact.

These systems aren't just about making electricity. They also produce useful heat for various applications. By maximizing energy use, combined cycles and cogeneration outperform traditional power plants in efficiency, economics, and environmental friendliness.

Combined Cycles and Cogeneration

Advantages of combined cycles

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  Biomass Combined Heat and Power Generation for Anticosti Island: A Case Study View original

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  Biomass Combined Heat and Power Generation for Anticosti Island: A Case Study View original

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• Integrate two or more thermodynamic cycles (Brayton and Rankine) to improve overall efficiency and power output
• Waste heat from gas turbine generates steam for steam turbine
• Higher thermal efficiency up to 60% or more compared to single-cycle power plants
• Increased power output per unit of fuel consumed
• Reduced greenhouse gas emissions per unit of electricity generated
• Flexibility in fuel use (natural gas, biogas)

Principles of cogeneration

• Cogeneration or Combined Heat and Power (CHP) simultaneously produces electricity and useful heat from a single fuel source
• Captures and uses waste heat from electricity generation for heating, cooling, or industrial processes
• Utilizes waste heat that would otherwise be released to the environment
• Increases overall energy efficiency by reducing the need for separate heat and power generation
• Applications in industrial settings (process heating, steam generation, cooling), commercial settings (space heating, water heating, absorption cooling), and residential settings (district heating and cooling systems)

Performance analysis of power systems

• Uses thermodynamic principles like the First Law of Thermodynamics for energy balance and efficiency calculations
  • $\eta = \frac{W_{net}}{Q_{in}}$, where $\eta$ is thermal efficiency, $W_{net}$ is net work output, and $Q_{in}$ is heat input
• Applies the Second Law of Thermodynamics for exergy analysis and irreversibility assessment
  • $\psi = \frac{E_{out}}{E_{in}}$, where $\psi$ is exergetic efficiency, $E_{out}$ is exergy output, and $E_{in}$ is exergy input
• Factors affecting performance include:
  1. Turbine inlet temperature and pressure
  2. Heat recovery steam generator (HRSG) design and effectiveness
  3. Condenser pressure and cooling system efficiency
  4. Fuel composition and quality
  5. Ambient conditions (temperature, humidity, pressure)

Benefits vs traditional generation

• Economic benefits include lower fuel consumption per unit of electricity and heat generated, reduced operating and maintenance costs, and potential for revenue generation through the sale of excess electricity and heat
• Environmental benefits encompass lower greenhouse gas emissions per unit of energy produced, reduced water consumption compared to separate electricity and heat generation, and decreased reliance on fossil fuels when using renewable or low-carbon fuels (biomass, hydrogen)
• Compared to traditional power generation methods, combined cycles and cogeneration offer higher overall efficiency, lower carbon footprint than single-cycle power plants, more cost-effective than separate electricity and heat generation, and increased energy security and reliability through decentralized generation