Thermodynamics 2nd Law

Thermodynamics 2nd Law

1. Introduction to the law

   • A cup of coffe does not get hotter in a cooler room
   • Process occur in a certain direction and not in the reverse durection
   • A process must satisfy both laws of thermodynamics

2. Uses

   • Ensure quality as well as quantity
   • determining the theoretical limits for the performance of commonly used engineering systems
   • The second law may be used to identify the direction of processes

3. Thermal Energy Reservoirs

   • A hypothetical body with a relatively large thermal energy capacity (mass x specific heat) that can supply or absorb finite amounts of heat without undergoing any change in temperature
   • large bodies of water such as oceans, lakes, and rivers as well as the atmospheric air can be modeled accurately as thermal energy reservoirs because of their large thermal energy storage capabilities or thermal masses

4. Heat engines

   • They receive heat from a high temperature source (solar energy, oil furnace, nuclear reactor, etc.).
   • They convert part of this heat to work (usually in the form of a rotating shaft.)
   • They reject the remaining waste heat to a low-temperature sink (the atmosphere, rivers, etc.).
   • They operate on a cycle

5. Thermal Efficiency

   • Some heat engines perform better than others (convert more of the heat they receive to work).
   • Even the most efficient heat engines reject almost onehalf of the energy they receive as waste heat.

6. Saving Qout

   • Every heat engine must waste some energy by transferring it to a low-temperature reservoir in order to complete the cycle, even under idealized conditions.
   • A heat-engine cycle cannot be completed without rejecting some heat to a lowtemperature sink.

7. Kevin-Planck Statement

   • It is impossible for any device that operates on a cycle to receive heat from a single reservoir and produce a net amount of work.
   • The impossibility of having a 100% efficient heat engine is not due to friction or other dissipative effects. It is a limitation that applies to both the idealized and the actual heat engines.

8. Coefficient of performance

   • The efficiency of a refrigerator is expressed in terms of the coefficient of performance (COP).
   • The objective of a refrigerator is to remove heat (QL ) from the refrigerated space.
   • The objective of a refrigerator is to remove QL from the cooled space.

9. Heat Pumps

   • The objective of a heat pump is to supply heat QH into the warmer space.
   • The work supplied to a heat pump is used to extract energy from the cold outdoors and carry it into the warm indoors.
   • Most heat pumps in operation today have a seasonally averaged COP of 2 to 3. Most existing heat pumps use the cold outside air as the heat source in winter (air-source HP).
   • Air conditioners are basically refrigerators whose refrigerated space is a room or a building instead of the food compartment.The COP of a refrigerator decreases with decreasing refrigeration temperature. Therefore, it is not economical to refrigerate to a lower temperature than needed.

10. Clasius Statement

    • It is impossible to construct a device that operates in a cycle and produces no effect other than the transfer of heat from a lower-temperature body to a higher temperature body.
    • It states that a refrigerator cannot operate unless its compressor is driven by an external power source, such as an electric motor.
    • This way, the net effect on the surroundings involves the consumption of some energy in the form of work, in addition to the transfer of heat from a colder body to a warmer one.
    • To date, no experiment has been conducted that contradicts the second law, and this should be taken as sufficient proof of its validity

11. Reversible and Irreversible

    • Reversible process: A process that can be reversed without leaving any trace on the surroundings. Irreversible process: A process that is not reversible.
    • All the processes occurring in nature are irreversible.
    • The factors that cause a process to be irreversible are called irreversibilities. • They include friction, unrestrained expansion, mixing of two fluids, heat transfer across a finite temperature difference, electric resistance, inelastic deformation of solids, and chemical reactions.
    • Internally reversible process: If no irreversibilities occur within the boundaries of the system during the process. Externally reversible: If no irreversibilities occur outside the system boundaries. Totally reversible process: It involves no irreversibilities within the system or its surroundings. A totally reversible
    • The Carnot heat-engine cycle is a totally reversible cycle. Therefore, all the processes that comprise it can be reversed, in which case it becomes the Carnot refrigeration cycle.

12. The Carnot Principles

    • 1. The efficiency of an irreversible heat engine is always less than the efficiency of a reversible one operating between the same two reservoirs. 2. The efficiencies of all reversible heat engines operating between the same two reservoirs are the same.
    • The Carnot heat engine is the most efficient of all heat engines operating between the same high- and low temperature reservoirs.
    • No heat engine can have a higher efficiency than a reversible heat engine operating between the same high- and low-temperature reservoirs.