Second Low of Thermodynamics

1
Second Low of Thermodynamics
2
Example 1

• Heat always flows from high temperature to low
  temperature.
• So, a cup of hot coffee does not get hotter in a
  cooler room.
• Yet, doing so does not violate the first low as
  long as the energy lost by air is the same as the
  energy gained by the coffee.

3
Example 2

• The amount of EE is equal to the amount of energy
  transferred to the room.

4
It is clear from the previous examples that..

• Processes proceed in certain direction and not in
  the reverse direction.
• The first law places no restriction on the
  direction of a process.
• Therefore we need another law (the second law of
  thermodynamics) to determine the direction of a
  process.

5
Thermal Energy Reservoir

• It is defined as a body to which and from which
  heat can be transferred without a change in its
  temperature.

• If it supplies heat then it is called a source.

• If it absorbs heat then it is called a sink.

6

• Some obvious examples are solar energy, oil
  furnace, atmosphere, lakes, and oceans

• Another example is two-phase systems,

• and even the air in a room if the heat added or
  absorbed is small compared to the air thermal
  capacity (e.g. TV heat in a room).

7
Heat Engines

• We all know that doing work on the water will
  generate heat.
• However transferring heat to the liquid will not
  generate work.
• Yet, doing so does not violate the first low as
  long as the heat added to the water is the same
  as the work gained by the shaft.

8

• Previous example leads to the concept of Heat
  Engine!.
• We have seen that work always converts directly
  and completely to heat, but converting heat to
  work requires the use of some special devices.
• These devices are called Heat Engines and
• can be characterized by the following

9
Characteristics of Heat Engines..
High-temperature Reservoir at TH

• They receive heat from high-temperature source.
• They convert part of this heat to work.
• They reject the remaining waste heat to a
  low-temperature sink.
• They operate on (a thermodynamic) cycle.

QH
HE
W
QL
Low-temperature Reservoir at TL
10
Piston cylinder arrangement is an example of a
heat engine..
11
Difference between Thermodynamic and Mechanical
cycles

• A heat engine is a device that operates in a
  thermodynamic cycle and does a certain amount of
  net positive work through the transfer of heat
  from a high-temperature body to a low-temperature
  body.
• A thermodynamic cycle involves a fluid to and
  from which heat is transferred while undergoing a
  cycle. This fluid is called the working fluid.
• Internal combustion engines operate on a
  mechanical cycle (the piston returns to its
  starting position at the end of each revolution)
  but not on a thermodynamic cycle.
• However, they are still called heat engines

12
Steam power plant is another example of a heat
engine..
13
Thermal efficiency
Thermal Efficiency
lt 100
14
Thermal efficiency
QH magnitude of heat transfer between the
cycle device and the H-T medium at temperature TH
QL magnitude of heat transfer between the
cycle device and the L-T medium at temperature TL
Thermal Efficiency
lt 100
15
thermal efficiency can not reach 100
Even the Most Efficient Heat Engines Reject Most
Heat as Waste Heat
Automobile Engine 20 Diesel Engine
30 Gas Turbine 30 Steam Power
Plant 40
16
Can we save Qout?

• Heat the gas (QH100 kJ)
• Load is raisedgt W15 kJ
• How can you go back to get more weights (i.e.
  complete the cycle)?
• By rejecting 85 kJ
• Can you reject it to the Hot reservoir? NO
• What do you need?
• I need cold reservoir to reject 85 kJ

A heat- engine cycle cannot be completed without
rejecting some heat to a low temperature sink.
17
Example 5-1 Net Power Production of a Heat Engine
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. ltAnswers
30 MW, 0.375gt
18
The Second Law of Thermodynamics Kelvin-Plank
Statement (The first)

• The Kelvin-Plank 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.

19

• It can also be expressed as
• 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.
• Note that the impossibility of having a 100
  efficient heat engine is not due to friction or
  other dissipative effects.
• It is a limitation that applies to both idealized
  and the actual heat engines.

20

• Example 1 at the beginning of the notes leads to
  the concept of Refrigerator and Heat Pump..
• Heat can not be transferred from low temperature
  body to high temperature one except with special
  devices.
• These devices are called Refrigerators and Heat
  Pumps
• Heat pumps and refrigerators differ in their
  intended use. They work the same.
• They are characterized by the following

21
Refrigerators
High-temperature Reservoir at TH
QH
W
QL QH - W
Ref
QL
Low-temperature Reservoir at TL
Objective
22
An example of a Refrigerator and a Heat pump ..
23
Coefficient of Performance of a Refrigerator
The efficiency of a refrigerator is expressed in
term of the coefficient of performance (COPR).
24
Heat Pumps
Objective
High-temperature Reservoir at TH
QH
QH W QL
Read to parts of pp 259 and 260
W
HP
QL
Low-temperature Reservoir at TL
25
Heat Pump
26
Coefficient of Performance of a Heat Pump
The efficiency of a heat pump is expressed in
term of the coefficient of performance (COPHP).
27
Relationship between Coefficient of Performance
of a Refrigerator (COPR) and a Heat Pump (COPHP).
28
The second Law of Thermodynamics Clausius
Statement
The Clausius statement is expressed as
follows 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.
Both statements are negative statements! Read pp
262
29
Equivalence of the Two Statements
High-temperature Reservoir at TH
Net QOUT QL
QH QL
QH
Ref
HE
HE Ref
W QH
QL
Net QIN QL
Low-temperature Reservoir at TL
30
Example (5-5) Heating a House by a Heat Pump
A heat pump is used to meet the heating
requirements of a house and maintain it at 20oC.
On a day when the outdoor air temperature drops
to -2oC, the house is estimated to lose heat at
rate of 80,000 kJ/h. If the heat pump under these
conditions has a COP of 2.5, determine (a) the
power consumed by the heat pump and (b) the rate
at which heat is absorbed from the cold outdoor
air. Sol
31
Perpetual Motion Machines

• Any device that violates the first or second law
  is called a perpetual motion machine
• If it violates the first law, it is a perpetual
  motion machine of the first type (PMM1)
• If it violates the second law, it is a perpetual
  motion machine of the second type (PMM2)
• Perpetual Motion Machines are not possible

32

• The second law of thermodynamics state that no
  heat engine can have an efficiency of 100.
• Then one may ask, what is the highest efficiency
  that a heat engine can possibly have.
• Before we answer this question, we need to define
  an idealized process first, which is called the
  reversible process.

33
(No Transcript)
34
(No Transcript)