Thermodynamics 2
MEC 151
Lecture (3) : Second Law of Thermodynamics
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Dr. Suzzan Abbas Hassan
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Limitation of the First Law of thermodynamics
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First law does not help to predict whether the certain process
is possible or not.
First law does not give information about direction.
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Limitation of the First Law of thermodynamics
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The mechanical work done by the shaft shown is first converted to
the internal energy of the water. This energy may then leave the
water as heat.
We know from the experience that any
attempt to reverse this process will fail (i.e
transferring heat to water doesn’t cause the
shaft to rotate.
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The Second Law of thermodynamic
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 The second law of thermodynamic is used also used in determining the
theoretical limits for the performance of commonly used engineering
systems, such as heat engines and refrigerators.
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“Heat Engine”
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Thus , the work can be converted to heat directly, but
converting heat to work requires to use special devices. These
devices are called heat engines.
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“Heat Engine”
Heat engines differ considerably from one
another, but all can be characterized by the
following:
1. They receive heat from a high-temperature source
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2. They convert part of this heat to work (usually in the
form of a rotating shaft).
3. They reject the remaining waste heat to a lowtemperature sink (the atmosphere, rivers, etc.).
4. They operate on a cycle.
Heat engines and other cyclic devices usually involve a fluid to and from
which heat is transferred while undergoing a cycle. This fluid is called the
working fluid.
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Second law of thermodynamics
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Heat Engine
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Second law of thermodynamics
Heat Engine
A measure of the performance that is called the Thermal efficiency.
The thermal efficiency of a heat engine can be expressed as:
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Or
Conservation of energy
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The net work output and thermal efficiency
relations for any Heat Engine (shown in Fig)
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can also be expressed as:
QH = magnitude of heat transfer between the cyclic device and the high
temperature medium at temperature TH .
QL = magnitude of heat transfer between the cyclic device and the low
temperature medium at temperature TL .
Schematic of a Heat Engine.
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Second law of thermodynamics
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Thermal Efficiencies of Heat Engines
The thermal efficiencies of work-producing devices are relatively
low.
Spark-ignition engines (η𝑡𝑡𝑡 =25 %). That is , an automobile engine
converts about 25% of chemical energy of gasoline to mechanical
work.
Diesel engines (η𝑡𝑡ℎ =40 %).
These meaning that , the one half of energy supplied ends up in
the atmosphere as waste or useless energy (𝑸𝑸𝒐𝒐𝒐𝒐𝒐𝒐 )
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Second law of thermodynamics
1-“Kelvin-Plank statement”
 The kelvin-Plank statement of the second law of thermodynamic
is states that
 It is impossible to for any device as heat engine
that operates on a cycle to receive heat from a
single reservoir and produce net amount of
work.(i.e no heat engine can have a thermal
efficiency of 100 %)
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 This statement means that only part of total heat absorbed by heat engine
from a high temperature is converted to work, the remaining heat must be
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rejected at low temperature.
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Second law of thermodynamics
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Heat engine VS. Heat Pumps
The objective of a
heat pump is to
supply heat ( 𝑄𝑄𝐻𝐻 )
into the warmer
space
𝑸𝑸𝑳𝑳
C.O.P=
𝑾𝑾𝒊𝒊𝒊𝒊
for
Refrigeration
𝑸𝑸𝑯𝑯
C.O.P=𝑾𝑾
𝒊𝒊𝒊𝒊
for Heat Pump
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2-“Heat Pumps and Refrigerators”
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 Heat Pump is a device that transfers heat from a low temperature
medium to a high temperature one
 The measure of performance of a heat pump is also expressed in terms of the
coefficient of performance 𝑪𝑪𝑪𝑪𝑪𝑪𝑯𝑯𝑯𝑯 , defined as
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 Refrigerators : Operates exactly like heat
pump expect that the desired output is the
amount of heat removed out of the system.
The working fluid used in the refrigeration cycle is called a refrigerant.
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The efficiency of a refrigerator is expressed in terms of the coefficient of
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performance
(COP), denoted by 𝑪𝑪𝑪𝑪𝑪𝑪𝑹𝑹 .
 The objective of a refrigerator is to remove heat 𝑸𝑸𝑳𝑳 from the refrigerated
space. To accomplish this objective, it requires a work input of
Wnet,in .Then the COP of a refrigerator can be expressed as:
The conservation of energy principle for a cyclic device requires that
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Then the COP relation becomes
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 Notice that the value of 𝑪𝑪𝑪𝑪𝑪𝑪𝑹𝑹 can be greater than unity. That is, the amount
of heat removed from the refrigerated space can be greater than the amount of
work input. This is in contrast to the thermal efficiency, which can never be
greater than 1.
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Second law of thermodynamics
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Example 1
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Example 2
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Example 3
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