Second Law of Thermodynamics

major uses of second law

1. Identify the direction of processes

2. asserts that energy has quality as well as quantity

3. used in determining theoretical limits for the performance of commonly used engineering systems

heat engine

Devices that convert heat to work

1. receive heat from a high-temp source

2. convert part of this heat to work

3. reject the remaining waste heat to a low-temp sink

4. operate in a cycle

Can heat rejection be reduced in a heat engine

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

Kelvin-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. No heat engine can have a thermal efficiency of 100 percent, meaning that for a power plant to operate, the working fluid must exchange heat with both the environment and the furnace.

Refrigerator

The transfer of heat from a low-temperature medium to a high-temperature one requires devices called refrigerators. They most commonly operate on vapor-compression refrigeration cycles

Coefficient of Performance

The efficiency of a refrigerator or heat pump

energy efficiency rating

the amount of heat removed from the cooled space in Btu’s for 1 watthour of electricity consumed

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. This means that a refrigerator cannot operate unless its compressor is driven by an external power source, such as an electric motor.

reversible process

a process that can be reversed without leaving any trace on the surroundings (no net heat transfer and no net work on surroundings

irreversibilities

the factors that cause a process to be irreversible

ex. friction, unrestrained expansion, mixing of two fluids, heat transfer across a finite temperature difference, electrical resistance, inelastic deformation of solids, chemical reactions

internally reversible process

if no irreversibilities occur within the boundaries of the system during the process

externally reversible

no irreversibilities within the system or its surroundings

totally reversible process

involves no irreversibilities within the system or its surroundings

involves no heat transfer through a finite temperature difference, no nonquasi-equilibirum changes, and no friction or other dissipative effects

carnot cycle

Reversible Isothermal Expansion (process 1-2, TH = constant)
Reversible Adiabatic Expansioncar (process 2-3, temperature drops from TH to TL)
Reversible Isothermal Compression (process 3-4, TL = constant)
Reversible Adiabatic Compression (process 4-1, temperature rises from TL to TH)

carnot heat engine

totally reversible cycle, therefore all the processes that comprise it can be reversed to become a carnot refrigerator

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.

   1. The efficiencies of all reversible heat engines operating between the same two reservoirs are the same

temperature scale to use for heat engines

kelvin

coefficient of performance relationships