CHEE2325 – Lecture 7
100% efficient Carnot cycle:
- To achieve 100% efficiency in the Carnot cycle, a 0 K temperature cold
reservoir is required.
- The 3rd law of thermodynamics states:
‘The entropy of all substances in the perfect crystalline state is zero at the
absolute zero temperature.’
- As reaching absolute zero is almost impossible (as no system can reach zero
entropy), a 100% thermal efficiency in a cyclic process is impossible.
Kelvin-Planck expression:
- Every heat engine must waste some energy by transferring heat to a cold
reservoir, even under ideal conditions.
- The Kelvin-Planck statement is another statement for the 2 nd law of
thermodynamics:
‘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.’
- Hence, a heat engine must exchange heat with a high-temp. source, and low
temp. sink to keep operating.
Refrigerators:
- To transfer heat from a low temperature reservoir to a high temperature one, a
work input is required.
- A refrigerator is a device that transfers heat from a low temperature medium
to a high temperature one.
- The efficiency of a refrigerator is expressed in terms of the coefficient of
performance (COP).
QC
COP R=
W¿
100% efficient Carnot cycle:
- To achieve 100% efficiency in the Carnot cycle, a 0 K temperature cold
reservoir is required.
- The 3rd law of thermodynamics states:
‘The entropy of all substances in the perfect crystalline state is zero at the
absolute zero temperature.’
- As reaching absolute zero is almost impossible (as no system can reach zero
entropy), a 100% thermal efficiency in a cyclic process is impossible.
Kelvin-Planck expression:
- Every heat engine must waste some energy by transferring heat to a cold
reservoir, even under ideal conditions.
- The Kelvin-Planck statement is another statement for the 2 nd law of
thermodynamics:
‘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.’
- Hence, a heat engine must exchange heat with a high-temp. source, and low
temp. sink to keep operating.
Refrigerators:
- To transfer heat from a low temperature reservoir to a high temperature one, a
work input is required.
- A refrigerator is a device that transfers heat from a low temperature medium
to a high temperature one.
- The efficiency of a refrigerator is expressed in terms of the coefficient of
performance (COP).
QC
COP R=
W¿
