Second Law - Quality of Energy

Second Law - Quality of Energy Part 1

Introduction

  • Objectives of the Lesson:
    1. Explain the need for the second law of thermodynamics in real processes.
    2. State the general and specific statements of the second law of thermodynamics.
    3. Define the meanings of reservoirs and working fluids.
    4. List the characteristics of heat engines.
    5. Describe the difference between thermodynamic heat engines and mechanical heat engines.
    6. Sketch an energy-flow diagram indicating the flow of energy and label all energies and reservoirs for a steam power plant.
    7. Sketch a schematic diagram for a steam power plant and label all energies, flow of energies, and reservoirs.
    8. State the desired output and required input for a steam power plant.
    9. Define engine performance and analyze the performance of a steam power plant in terms of heat exchange.
    10. State the Kelvin-Planck statement applicable to steam power plants.
    11. Sketch an energy-flow diagram for a refrigerator, labeling all energies and reservoirs.
    12. Sketch a schematic diagram for a refrigerator, labeling all energies, flow of energies, and reservoirs.
    13. State the desired output and required input for a refrigerator.
    14. Analyze the performance of a refrigerator in terms of heat exchange.
    15. Sketch an energy-flow diagram for a heat pump, labeling all energies and reservoirs.
    16. Sketch a schematic diagram for a heat pump, labeling all energies, flow of energies, and reservoirs.
    17. State the desired output and required input for a heat pump.
    18. Analyze the performance of a heat pump in terms of heat exchange.
    19. State the Clausius statement for refrigerators and heat pumps.
    20. Solve problems related to steam power plants, refrigerators, and heat pumps.

Review of the First Law of Thermodynamics

  • All processes must obey energy conservation.
  • Processes that do not obey energy conservation cannot occur.
  • Processes that do not obey mass conservation are similarly infeasible.
  • Key components discussed include:
    • Piston-cylinders
    • Rigid tanks
    • Turbines
    • Compressors
    • Nozzles
    • Heat exchangers

Energy and Mass Balances

  • To relate changes to causes, consider:
    • Dynamic Energies as change agents, transitioning from initial state (E1, P1, T1, V1) to final state (E2, P2, T2, V2).
    • Changes in properties indicate a change of state.
    • Mass entering ( ext{Mass in}) and leaving ( ext{Mass out}) the system affect the energy ( ext{Work in} and ext{Work out}) and heat exchanges ( ext{Heat in} and ext{Heat out}).

Energy Balance

  • The energy balance is expressed as: E_{in} - E_{out} = riangle E_{sys},
    • where energy can be measured in kJ or kJ/kg depending on the context.
  • The energy entering a system minus the energy leaving a system equals the change in the system’s energy.

Mass Balance

  • The mass balance is given as: m_{in} - m_{out} = riangle m_{sys},
    • where mass flow rates are expressed in kg/s.
  • Steady-Flow Systems:
    • Conditions where properties in the boundary remain constant over time:
    • riangle E_{sys} = 0, riangle m_{sys} = 0

Introduction to the Second Law of Thermodynamics

Definition and Implications

  • The second law introduces the concept that processes occur in a natural direction.
  • Heat flows from high-temperature media to low-temperature media:
    • Energy possesses quality; quality increases with temperature, allowing for more work potential.
    • Example: Coffee cooling in a room demonstrates natural energy flow from a higher to a lower temperature.

Major Uses of the Second Law

  1. Identifying Process Direction: The second law helps predict the natural direction of energy transfers.
  2. Energy Quality Assessment: It asserts that energy has quality, providing criteria to evaluate energy degradation during a process.
  3. Performance Limits: The second law establishes theoretical limits for the performance of engineering systems (heat engines, refrigerators), predicting the extent of chemical reactions.

Heat Engine Basics

Characteristics of Heat Engines

  • Heat engines receive heat from a high-temperature source (e.g., solar, fossil fuels).
  • They convert part of this energy into work (usually through rotating shafts).
  • Any excess heat is then rejected into a low-temperature sink (e.g., atmosphere, water bodies).
  • Heat engines operate cyclically.

Working Fluids

  • During operation, heat is transferred via a fluid called the working fluid, commonly water or refrigerants.)

Types of Heat Engines

  • Thermodynamic Heat Engines: External combustion, such as steam power plants.
  • Mechanical Heat Engines: Internal combustion, examples include jets, cars, and motorcycles.

Energy-Flow Diagram for a Steam Power Plant

  • High Temperature Reservoir (TH): Furnace: Receives heat, q_{in} = q_H, required for work generation.
  • Low Temperature Reservoir (TL): Water from a river: Receives waste heat, q_{out} = q_L.
  • Working Fluid: Water circulates through the system.
  • Net Work Output: w_{net,out} = q_{in} - q_{out}
    • This diagram demonstrates the energy transfer processes in the plant, with specific outputs and inputs detailed.

Efficiency of Heat Engines

  • Thermal Efficiency: ext{Efficiency} = rac{W_{net,out}}{Q_{in}}
    • Indicates how effectively the machine converts input heat into useful work.
  • Expression highlights that a significant amount of energy is typically lost as waste heat, often up to half of the input energy.

Refrigeration and Heat Pumps

Refrigerators

  • Functionality involves the transfer of heat from low-temperature areas to high-temperature areas, necessitating refrigeration cycles.
  • Commonly utilize refrigerants and vapor-compression processes.
  • Key Components: Evaporator, compressor, condenser, and throttle valve.
  • Objectives:
    • To extract heat from cold spaces (e.g., a refrigerator's interior).
    • Performance expressed via coefficient:
      COP_R = rac{Q_{L}}{W_{net,in}}

Heat Pumps

  • Similar mechanisms as refrigerators but designed to deliver heat into a space rather than remove it.
  • Performance expressed similarly with its coefficient of performance defined by:
    COP_{HP} = rac{Q_{H}}{W_{net,in}}

Clausius Statement for Refrigerators/Heat Pumps

  • Stipulates that heat transfer from low to high temperatures is not possible without external work being applied to the device.
  • More energy is required from surrounding sources to achieve this heat transfer.

Schematic Diagram for Refrigerators and Heat Pumps

  • Diagrams include components such as the compressor, evaporator, and condenser with labeling of energy flows in kW.

Exercise: Example Problem

  • Problem proposed involves drawing a schematic for a refrigerator, identifying all components, energy exchanges, and conducting a COP performance metric assessment based on given rates.
  • Case studies discussed exemplifying how to derive efficiencies from various thermodynamic systems, alongside practical applications within engineering contexts.