1. CHAPTER 4
ND
2 LAW OF THERMODYNAMICS
4.1.1 The Concept Of The Second Law Of Thermodynamics
Satisfying the first law alone does not ensure that the process will actually take
place. For Example, a hot coffee left in a cooler room eventually cools off. But a reverse
process, hot coffee getting even hotter in a cooler room as a result of heat transfer from
room air will never take place. Yet doing so would not violate the first law as long as the
amount of energy lost by the air is equal to the amount gained by the coffee. Therefore,
processes proceed in a certain direction and in not the reverse direction.
The first law sets no limit on the percentage of heat supplied, which can be
converted into work. Nor does it indicate whether the energy conversion process is
physically possible or impossible. We shall see, though, that a limit is imposed by the
Second Law of Thermodynamics, and that the possibility or otherwise of a process can be
determined through a property of the working fluid called entropy.
The use of 2nd Law:
1. Identifying the direction of processes.
2. Provides the necessary means to determine the quality as well as the
degradation of energy during a process.
3. Determining the theoretical limits.
Thermal energy reservoir: 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.
Example: Atmosphere does not warm up as a result of heat losses from residential
building in winter.
* any physical body whose thermal energy capacity is large relative to the amount of
energy it supplies or absorbs can be modeled as reservoir.
Heat reservoirs: thermal energy reservoirs.
Source: A reservoir that supplies energy in the form of heat.
Sink: A reservoir that absorbs energy in the form of heat.
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%, or as for a power plant to
operate, the working fluid must exchange heat with the environment as well as the
furnace. The impossibility of having a 100% efficient heat engine is not due to friction or
other dissipative effects.
*2nd Law limitation: no heat engine can convert all heat it receives to useful work.

2. Clausius Statement:
It is impossible to construct a device that operates in a cycle and produces no effect other
than the transfer from a lower-temperature body to a higher temperature body.
This statement does not imply that a cyclic device that transfers heat from a cold
medium to a warmer one is impossible to construct but it simply states that a refrigerator
cannot operates unless its compressor is driven by an external power source, such as an
electric motor.
4.1.2 The Principles Of A Heat Engine And A Reverse Heat Engine.
Heat Engine
We know from experience that work can be converted to heat directly and
completely, but converting heat to work requires the use of some special devices. These
devices are called heat engines.
A heat engine is a system operating in a complete cycle and developing net work
from a supply of heat. The second law implies that a source of heat supply (or hot
reservoir) and a sink (or cold reservoir) for the rejection of heat are both necessary, since
some heat must always be rejected by the system.
Heat engines can be characterized by the following:
 They receive heat from a high-temperature source (for example solar energy, oil
furnace, nuclear reactor, steam boiler, etc.)
 They convert part of this heat to work (usually in the form of a rotating shaft, for
example gas turbine, steam turbine, etc.)
 They reject the remaining waste heat to a low-temperature sink (for example the
atmosphere, rivers, condenser, etc.)
 They operate on a cycle.

3. High-temperature
HOT RESERVOIR
at TH
QH
WORK OUTPUT
HEAT
W = QH – QL
Note:
QH = The heat supplied from the
ENGINE
source.
QL
QL = The heat rejected.
Low-temperature
COLD RESERVOIR W = The net work done.
at TL
Part of the heat received by the heat engine is converted to work,while the rest is
rejected to cold reservoir.
Heat engines and other cyclic devices usually involve a fluid that moves to and
fro from which heat is transferred while undergoing a cycle. This fluid is called the
working fluid.
The work-producing device that best fits into the definition of a heat engine are:
 The steam power plant
 The close cycle gas turbine
Reverse Heat Engine
The first and second laws apply equally well to cycles working in the reverse
direction to those of heat engine. In general, heat only flows from a high-temperature
source to a low-temperature sink. However, a reversed heat engine can be utilized to
pump the heat from a low-temperature region to a high-temperature region. The reversed
heat engine is called heat pump.
In the case of a reversed cycle, the net work done on the system is equal to the net
heat rejected by the system.
The work-producing device that best fits into the definition of a heat pump are:
 The refrigerator.
 The air-conditioner.

4. High-temperature
HOT RESERVOIR at TH
QH
WORK INPUT
HEAT W
PUMP QH = W + QL
QL
Low-temperature
COLD RESERVOIR at TL
Reverse heat engine
QH = magnitude of heat transfer between cyclic device and high-temperature medium at
temperature TH.
QL = magnitude of heat transfer between cyclic device and low-temperature medium at
temperature TL.
4.1.3 The Essentials Of Heat Engine According To Working Substance, Heat
Source, Mechanical Arrangement And Working Cycle.
The steam power plant is an external-combustion engine that is, combustion takes
place outside the engine, and thermal energy released during this process is transferred to
the steam as heat. The schematic of a basic steam power plant is shown below. This is a
rather simplified diagram, and the discussion of actual steam power plant is given later.
The various quantities shown on this figure are as follows:
Qin = amount of heat supplied to steam in boiler from a high-temperature
source(furnace)
Qout = amount of heat rejected from steam in condenser to a low-temperature sink (the
atmephere, a ricer, etc.)
W out = amount work delivered by steam as it expands in turbine.
W in = amount work required to compress water to boiler pressure.

5. 4.1.4 Energy Balance For A Heat Engine (As A Black Box) And Efficiency.
Energy Balance
The net work output of this power plant is simply the difference between the total
work output of the plant and the total work input:
W net out  W out  W in
The net work can also be determined from the heat transfer data alone. The figure
above can be analyzed as a closed system. Recall that for a closed system undergoing a
cycle, the change in internal energy is zero, therefore:
W net out  Q in  Q out
Thermal Efficiency
Note that Qout is never zero. Thus, the net work output of a heat engine is always
less than the amount of heat input. The fraction of heat input that is converted to net work
output is a measure of the performance of a heat engine and is called thermal efficiency,
TH.
Thermal efficiency = Net work output
Total heat input

6. TH = W net, out
QH
 TH = 1 - QL
QH
It can be seen that the second law implies that the thermal efficiency of a heat
engine must always be less than 100% (Q1 > W ). Thermal efficiency is a measure of
how efficiently a heat engine converts the heat that it receives to work. The increase in
efficiency means the less fuel consumption.
Example 1
Heat is transferred to a heat engine from a furnace at a rate of 80 MW. If the rate of
waste heat rejection to a nearby river is 45 MW, determine the net work done and the
thermal efficiency for this heat engine.
FURNACE
QH = 80 MW
HEAT W=?
ENGINE
QL = 45 MW
RIVER
A schematic of the heat engine is given in the diagram above. The furnace serves as the
high-temperature reservoir for this heat engine and the river as the low-temperature
reservoir.
Assumption: Heat lost through the pipes and other components are negligible.
Analysis: The given quantities can be expressed in rate form as;
QH = 80 MW
QL = 45 MW
Wnet, out = QH – QL
= (80 – 45) MW
= 35 MW

7. W 35 MW
TH    0.4375 (or 43.75%)
Q1 80 MW
That is, the heat engine converts 43.75 percent of the heat it receives to work.
4.2.1 Explain the maximum possible efficiency
The efficiency of a heat engine cycle greatly depends on how the individual
processes that make up the cycle are executed. The net work, thus the cycle efficiency,
can maximized by using processes that require the least amount of work and deliver the
most, that is, by using reversible processes. Therefore the most efficient cycles are
reversible cycles, that is, cycles that consist entirely of reversible processes.
4.2.2 Explain the reversible and irreversible processes
Reversible Processes
Reversible Process is a process that can be reversed without leaving any trace on
the surroundings. That is allowing system and surroundings to be restored to their initial
states.
– no heat transfer
– no net work
– e.g., adiabatic compression/expansion of a gas in a frictionless piston
device:
• Reversible processes are considered ideal processes – no energy is “wasted”, i.e.,
all energy can be recovered or restored
– they can produce the maximum amount of work (e.g., in a turbine)
– they can consume the least amount of work (e.g., in a compressor or
pump)
– they can produce the maximum KE increase (e.g., in a nozzle)
– when configured as a cycle, they produce the maximum performance (i.e.,
the highest th or COP)
Irreversible Processes
• Irreversible Process - process that does not allow system and surroundings to be
restored to initial state
– such a process contains “irreversibilities”
– all real processes have irreversibilities
– examples:
• heat transfer through a temperature difference
• unrestrained expansion of a fluid
• spontaneous chemical reaction
• spontaneous mixing of different fluids
• sliding friction or viscous fluid flow

8. • electric current through a resistance
• magnetization with hysteresis
• inelastic deformation
Internally Reversible Processes
• A process is called internally reversible if no irreversibilities occur within the
boundary of the system
– the system can be restored to its initial state but not the surroundings
– comparable to concept of a point mass, frictionless pulley, rigid beam, etc.
– allows one to determine best theoretical performance of a system, then
apply efficiencies or correction factors to obtain actual performance
Externally Reversible Processes
• A process is called externally reversible if no irreversibilities occur outside the
boundary of the system
– heat transfer between a reservoir and a system is an externally reversible
process if the outer surface of the system is at the reservoir temperature
4.2.4 The Carnot cycle, Carnot principles, Carnot heat engines, Carnot refrigerator
heat pump.
The Carnot Cycle
• The Carnot cycle is the best-known reversible cycle, consisting of four reversible
processes:
– adiabatic compression from temperature TL to TH
– isothermal expansion with heat input QH from reservoir at TH
– adiabatic expansion from temperature TH to TL
– isothermal compression with heat rejection QL to reservoir at TL
• Note:
– the heat transfers (QH , QL) can only be reversible if no temperature
difference exists between the reservoir and system (working fluid)
– the processes described constitute a power cycle; it produces net work and
operates clockwise on a P-v diagram
– The Carnot heat engine can be reversed (operating counter-clockwise on a
P-v diagram) to become a Carnot refrigerator or heat pump
– the thermal efficiency and coefficients of performance of Carnot cycles
correspond to maximum performance
The Carnot Principles
• Several corollaries (the Carnot principles) can be deduced from the Kelvin-Planck
statement:
– the thermal efficiency of any heat engine must be less than 100%
Wnet,out QL
th   1
QH QH

9. – th of an irreversible heat engine is always less than that of a reversible
heat engine
– all reversible heat engines operating between the same two thermal
reservoirs must have the same th
Carnot Heat Engine
• Carnot Heat Engine:
TL
th ,rev  1 
TH
Carnot Refrigerator and Heat Pump
• Carnot Refrigerator:
• Carnot Heat Pump:

10. Tutorial
1. Study the statements in table below and decide if the statements are
TRUE (T) or FALSE (F).
STATEMENTS TRUE or FALSE
i. The Second Law of Thermodynamics is represented by
the equation Q1 – Q2 = W.
ii. The heat engine receives heat from a high-temperature
source.
iii. The heat engine convert part of the heat to internal
energy.
iv. The work-producing device of a heat engine are the
steam power plant and a close cycle gas turbine.
v. A reversed heat engine is called a heat pump.
vi. The work producing device for a heat pump is the
refrigerator.
vii. In heat engines, the net work done must be greater than
the gross heat supplied,
i.e W > Q1 .
2. The work done by heat engine is 20 kW. If the rate of heat that enters into the hot reservoir is
3000 kJ/min, determine the thermal efficiency and the rate of heat rejection to the cold reservoir.
3. Heat is transferred to a heat engine from a hot reservoir at a rate of 120 MW. If the net work done
is 45 MW, determine the rate of waste heat rejection to a cold reservoir and the thermal efficiency
of this heat engine.
4. Heat is transferred to a heat engine from a furnace at a rate of 80 MW. If the rate of waste heat
rejection to a nearby river is 50 MW, determine the net power output and the thermal efficiency
for this heat engine.
5. The food compartment of a refrigerator is maintained at 4°C by removing heat from it at a rate of
360 kJ/min. If the required power input to the refrigerator is 2 kW, determine:
a. The coefficient of performance of the refrigerator.
b. The rate of heat rejection to the room that houses the refrigerator.
6. A heat pump is used to meet the heating requirements of a house and maintain it at 20°C. On a day
when the outdoor air temperature drops to -2ºC, the house is estimated to lose heat at a rate of
20,000 kJ/h. If the heat pump under this conditions has a COP of 2.5, determine:
a. The power consumed by the heat pump
b. The rate at which heat is absorbed from the cold outdoor air