MEC2405 - Thermodynamics Study Notes

MEC2405 - Thermodynamics - S2 2024 Study Notes

Green Thermodynamics Overview

  • Topics Covered:

    • Supercritical Power Plants

    • Cogeneration

    • Combined Cycles

    • Non-Fossil-Fuel Heat Sources

    • Waste Heat Recovery

Supercritical Power Plants

  • Efficiency:

    • Efficiency of a power cycle increases with the temperature of heat input.

    • Modern steam power plants utilize supercritical pressures ($P > 22.06 \, ext{MPa}$).

  • Phase Change:

    • In supercritical plants, water does not undergo a boiling phase change; instead, it transitions smoothly from liquid to gas.

  • Technological Advancements:

    • Improvements in boiler design and advancements in metal alloys accommodate high pressures.

    • Historical context: 1922 boilers operated at approximately $2.7 \, ext{MPa}$; current supercritical plants operate around $30 \, ext{MPa}$.

    • Capable of generating net power output of $1000 \, ext{MW}$ or more in large power plants.

  • Combined Features:

    • Supercritical plants enhance efficiencies nearing $50 ext{%}$ through methods like reheating and regeneration.

    • The T-s diagram for the basic supercritical Rankine cycle illustrates these processes intricately.

  • Diagram Observations:

    • The reheat section and multiple regeneration streams with closed feedwater heaters minimize external steam heating needs, optimizing boiler efficiency from State 36 to State 1 instead of State 18 to State 1.

Cogeneration

  • Definition:

    • Cogeneration systems simultaneously produce electricity and process heating, typically used in chemical industries.

  • Process Heat:

    • Steam is drawn from turbines to supply heating demands, enhancing energy utility.

    • Cogeneration plants utilize the exhaust heat effectively, achieving a utilization factor of $100 ext{%}$ even when electrical efficiency is below $100 ext{%}$.

  • Efficiency Metrics:

    • Actual cogeneration plants can have utilization factors exceeding $80 ext{%}$.

    • Designed for flexible operations to redirect steam based on current heating or power generation demands.

Combined Power Plants

  • Combined Cycle Concept:

    • Combines gas power cycle (topping) and vapor power cycle (bottoming) to surpass $50 ext{%}$ efficiencies.

    • Uses high-temperature gas turbine exhaust as the energy source for the steam cycle.

  • Design Dynamics:

    • The gas turbine exhaust (State 8) heats the steam boiler.

    • Typically requires multiple gas turbines to provide adequate heat to the steam cycle.

    • Incorporates regeneration and reheating, with potential additional fuel combustion in gas exhaust for enhanced steam heating.

  • Efficiency Achievement:

    • Example: General Electric's Bouchain plant reported $62.2 ext{%}$ efficiency for its combined cycle in 2016, with an output of $594 \, ext{MW}$.

    • Future goals for thermal efficiency are towards $65 ext{%}$, nearing the Carnot limit.

Using Non-Fossil-Fuel Heat Sources

  • Versatility of the Rankine Cycle:

    • The Rankine Cycle can operate independently of heat source types, making it adaptable for various applications like solar, geothermal, and molten salt reactors.

  • Concentrated Solar Power:

    • A solar tower uses mirrors to concentrate solar energy, storing heat in molten salt for steam generation, allowing operational reliability even during night or cloudy conditions.

  • Molten Salt Reactor Plants:

    • Utilizes molten radioactive salt as the primary heat source.

    • Achieves operational temperatures around $700 \, ^ ext{C}$, permitting integration with gas power plants.

    • Can employ alternative working fluids other than air, enhancing cooling options for the gas power cycle.

  • Geothermal Applications:

    • Geothermal fluids exist as hot salt water (brine) due to sub-surface geothermal activity.

    • Involves flashing where high-pressure brine vaporizes upon throttling, driving steam turbines.

    • Reinjection of condensed steam and brine back into the well facilitates sustainable geothermal energy extraction.

    • For lower-temperature geothermal fluids ($< 100 \, ^ ext{C}$), the Organic Rankine Cycle (ORC) is utilized, employing refrigerants rather than direct steam.

Waste Heat Recovery

  • Industry Focus:

    • Significant interest in recovering electric power from hot waste streams.

  • Organic Rankine Cycle (ORCs):

    • ORCs are applied to waste heat recovery, allowing conversion of waste heat into electrical energy.

    • Example: Geothermal power plants using flash vaporization as heat sources can utilize condenser waste heat to operate an ORC for additional power recovery.

    • MicroORCs:

    • Small units designed for limited output (less than $5 \, ext{kW}$), comparable in size to refrigerators, can serve small offices or communities effectively.

  • Scroll Expander Technology:

    • Utilizes scroll expanders instead of turbines for vapor expansion, enabling mechanical power generation in compact setups.

Practical Applications

  • Applied Classes and Calculations:

    • Continuous reinforcement through real-time applied examples in sessions.

    • Necessary to grasp calculation techniques for future practical applications.