Unit 61: Engineering Thermodynamics

1. Unit 61: Engineering Thermodynamics Lesson 8: Second Law of Thermodynamics

2. Objective • The purpose of this lesson is to derive a statement for the Second Law of Thermodynamics.

3. Background • Water flows down hill does it not! • Heat flows from a hot body to a cold one; rubber bands unwind; fluid flows from a high-pressure region to a low-pressure region – and we get old! • Our experiences in life suggest that processes have a definite direction.

4. Background • The first law of thermodynamics relates several variables involved in a physical process but does not give any information as to the direction of the process. • It is the second law of thermodynamics which helps establish the direction of a particular process.

5. First Law of Thermodynamics Consider once again, a weight W attached to a pulley / paddle wheel W

6. Background • The first law states that the work done by a falling weight is converted into internal energy of the air contained in the fixed volume, provided the volume is insulated so that Q = 0. • It would not be a violation of the first law if we postulated that an internal energy decrease of air is used to turn the paddle and raise the weight! • This however would be a violation of the second law of thermodynamics and would thus be an impossibility.

7. Heat Engines, Heat Pumpsand Refrigerators • We refer to a device operating a cycle as a heat engine, a heat pump or a refrigerator depending on the objective of the particular device… • If the objective of the device is to perform work it is a heat engine. • If the objective is to supply energy to a body it is a heat pump. • If the objective is to extract energy from a body it is a refrigerator

8. A Simple Heat Engine TH = temperature of heat source TL = temperature of heat sink TH QH Heat Engine W QL TH

9. A Simple Heat Engine • The net work produced by the engine would be equal to the net heat transfer – a consequence of the first law… W = QH – QL • Where QH and QL are the heat transfer from High to Low temperature reservoirs respectively

10. A Simple Heat Pump ora Refrigerator TH = temperature of heat source TL = temperature of heat sink TH QH Heat Pump W QL TL

11. A Simple Heat Pump ora Refrigerator • Reversing the cycle a net work input would be required. • A heat pump would provide energy as heat QH to the warmer body (e.g. a house) • A refrigerator would extract heat energy QL from the cooler body (e.g. a freezer). - W = - QH– (- QL) i.e. W = QH – QL

12. A Simple Heat Pump ora Refrigerator • Note: the engine of refrigerator operates between two thermal reservoirs, entities which are capable of providing or accepting heat without changing temperatures • The atmosphere or a lake act as heat sinks; furnaces, solar collectors, or burners serve as heat sources.

13. Thermal Efficiency / Coefficients of Performance • The thermal efficiency of the heat engine and the coefficients of performance of the refrigerator and the heat pump Η = W/QH; COPrefrig = QL/W; COPh.p. = QH/W The second law of thermodynamics places limits on these measure of performance.

14. Thermal Efficiency / Coefficients of Performance • The first law would allow a maximum of unity for the thermal efficiencyand an infinite COP • The second law however establishes limits that cannot be exceeded regardless of the cleverness of proposed designs.

15. Thermal Efficiency / Coefficients of Performance • Note: there are devices that we refer to as heat engines which do not strictly meet the definition – they do not operate on a thermodynamic cycle but instead exhaust the working fluid and then intake new fluid e.g. the internal combustion engine.

16. Statements of the 2ndLaw of Thermodynamics • We cannot derive a basic law but we merely observe that such a law is never violated. • There are a variety of ways to state the 2nd law. • Two such statements are…

17. Clausius Statement • It is impossible to construct a device which operates on a cycle and whose sole effect is the transfer of heat from a cooler to a hotter body. • This statement relates to a refrigerator (or heat pump). • It states that it is impossible to construct a refrigerator that transfers energy from a cooler body to a hotter body without the input of work

18. A Violation asStated by Clausius QL = QL TH QH Device QL TL

19. Kelvin-PlanckStatement • It is impossible to construct a device which operates on a cycle and produces no other effect than the production of work and transfer of heat from a single body

20. A Violation of theKelvin-Planck Statement QH = W TH QH Device W

21. Kelvin-PlanckStatement • In other words it is impossible to construct a heat engine that extracts energy from a reservoir, does work and does not transfer heat to a low temperature reservoir. • This rules out any heat engine that is 100% efficient.

22. Comparison of theStatements • The two statements of the 2nd law are negative statements • Neither has been proved – they are expressions of experimental observation • No experimental evidence has ever been obtained that violates either statement. • The two statements are equivalent.

23. Reversibility • A reversible process is defined as a process which having taken place can be reversed and in so doing leaves no change in either the system or the surroundings. • The most efficient engine that can possibly be constructed, an engine that operates with reversible processes only is called a reversible engine. • Note: the definition of a reversible processes refers to both the system and the surroundings.

24. Reversibility • A reversible process has to be a quasi-equilibrium process. In addition… • No friction is involved in the process • Heat transfer occurs due to infinitesimal temperature difference only • Unrestrained expansion does not occur. • The mixing of different substances and combustion lead to irreversibility's.