Transcript Slide 1

Application of Thermodynamics
POWER CYCLES
Thermodynamics
Cycle
Refrigeration
Cycles
Refrigerator &
AirConditioner
Power Cycles
Heat Pump
Engines
POWER CYCLES
The analysis of many
complex processes can be
reduced to a manageable
level by utilizing some
idealizations.
Most power-producing devices operate on
cycles.
Ideal cycle: A cycle that resembles the actual
cycle closely but is made up totally of
internally reversible processes.
Reversible cycles such as Carnot cycle
have the highest thermal efficiency of all heat
engines operating between the same
temperature levels. Unlike ideal cycles, they
are totally reversible, and unsuitable as a
realistic model.
POWER CYCLES
The idealizations and
simplifications in the analysis of
power cycles:
1. The cycle does not involve any
friction. Therefore, the working fluid
does not experience any pressure
drop as it flows in pipes or devices
such as heat exchangers.
2. All expansion and compression
processes take place in a quasiequilibrium manner.
3. The pipes connecting the various
components of a system are well
insulated, and heat transfer through
them is negligible.
POWER CYCLES
POWER CYCLES
On both P-v and T-s diagrams, the area enclosed by the
process curve represents the net work of the cycle.
On a T-s diagram,
the ratio of the area
enclosed by the
cyclic curve to the
area under the heataddition process
curve represents the
thermal efficiency of
the cycle. Any
modification that
increases the ratio of
these two areas will
also increase the
thermal efficiency of
the cycle.
POWER CYCLES
For both ideal and actual cycles: Thermal efficiency increases with an increase
in the average temperature at which heat is supplied to the system or with a
decrease in the average temperature at which heat is rejected from the system.
POWER CYCLES
Air-standard assumptions:
1. The working fluid is air, which continuously
circulates in a closed loop and always behaves
as an ideal gas.
2. All the processes that make up the cycle are
internally reversible.
3. The combustion process is replaced by a heataddition process from an external source.
4. The exhaust process is replaced by a heatrejection process that restores the working fluid
to its initial state.
The combustion process is
replaced by a heat-addition
process in ideal cycles.
Cold-air-standard assumptions: When the
working fluid is considered to be air with constant
specific heats at room temperature (25°C).
Air-standard cycle: A cycle for which the airstandard assumptions are applicable.
POWER CYCLES
Reciprocating Engine
•
•
Spark-ignition (SI) engines
Compression-ignition (CI) engines
Nomenclature for reciprocating engines.
POWER CYCLES
RECIPROCATING ENGINE
Compression ratio
Mean effective pressure
POWER CYCLES
POWER CYCLES
POWER CYCLES
• The two-stroke engines
are generally less
efficient than their fourstroke counterparts but
they are relatively
simple and inexpensive,
and they have high
power-to-weight and
power-to-volume ratios.
POWER CYCLES
Otto Cycle
Ideal SI cycle
with application
of air standard
assumption.
POWER CYCLES
POWER CYCLES
In SI engines,
the
compression
ratio is limited
by
autoignition
or engine
knock.
Thermal efficiency of the ideal
Otto cycle as a function of
compression ratio (k = 1.4).
The thermal efficiency of the
Otto cycle increases with the
specific heat ratio k of the
working fluid.
POWER CYCLES
DIESEL CYCLE
In diesel engines, only air is
compressed during the
compression stroke,
eliminating the possibility of
autoignition (engine knock).
Therefore, diesel engines can
be designed to operate at
much higher compression
ratios than SI engines,
typically between 12 and 24.
In diesel engines, the spark plug is
replaced by a fuel injector, and only air
is compressed during the compression
process.
POWER CYCLES
1-2 isentropic
compression
2-3 constant-volume
heat addition
3-4 isentropic
expansion
4-1 constant-volume
heat rejection.
POWER CYCLES
Cutoff ratio
Thermal
efficiency of the
ideal Diesel cycle
as a function of
compression and
cutoff ratios
(k=1.4).
for the same
compression ratio
Application Of Thermodynamics
Vapor & Combined Power Cycle
RANKINE CYCLE: THE IDEAL CYCLE
FOR VAPOR POWER CYCLES
Many of the impracticalities associated with
the Carnot cycle can be eliminated by
superheating the steam in the boiler and
condensing it completely in the condenser.
The cycle that results is the Rankine cycle,
which is the ideal cycle for vapor power
plants. The ideal Rankine cycle does not
involve any internal irreversibilities.
The simple ideal
Rankine cycle.
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Energy Analysis of the Ideal Rankine Cycle
Steady-flow energy equation
The efficiency of power plants in
the U.S. is often expressed in
terms of heat rate, which is the
amount of heat supplied, in Btu’s,
to generate 1 kWh of electricity.
The thermal efficiency can be interpreted
as the ratio of the area enclosed by the
cycle on a T-s diagram to the area under
the heat-addition process.
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DEVIATION OF ACTUAL VAPOR POWER
CYCLES FROM IDEALIZED ONES
The actual vapor power cycle differs from the ideal Rankine cycle as a
result of irreversibilities in various components.
Fluid friction and heat loss to the surroundings are the two common
sources of irreversibilities.
Isentropic efficiencies
(a) Deviation of actual vapor power cycle from the ideal Rankine cycle.
(b) The effect of pump and turbine irreversibilities on the ideal Rankine cycle.
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HOW CAN WE INCREASE THE
EFFICIENCY OF THE RANKINE CYCLE?
The basic idea behind all the modifications to increase the thermal efficiency
of a power cycle is the same: Increase the average temperature at which heat is
transferred to the working fluid in the boiler, or decrease the average
temperature at which heat is rejected from the working fluid in the condenser.
Lowering the Condenser Pressure (Lowers Tlow,avg)
To take advantage of the increased
efficiencies at low pressures, the condensers
of steam power plants usually operate well
below the atmospheric pressure. There is a
lower limit to this pressure depending on the
temperature of the cooling medium
Side effect: Lowering the condenser
pressure increases the moisture content of
the steam at the final stages of the turbine.
The effect of lowering the
condenser pressure on the
ideal Rankine cycle.
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Superheating the Steam to High Temperatures
(Increases Thigh,avg)
Both the net work and heat input
increase as a result of
superheating the steam to a higher
temperature. The overall effect is
an increase in thermal efficiency
since the average temperature at
which heat is added increases.
Superheating to higher
temperatures decreases the
moisture content of the steam at
the turbine exit, which is desirable.
The effect of superheating the
steam to higher temperatures
on the ideal Rankine cycle.
The temperature is limited by
metallurgical considerations.
Presently the highest steam
temperature allowed at the turbine
inlet is about 620°C.
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Increasing the Boiler Pressure (Increases Thigh,avg)
For a fixed turbine inlet temperature,
the cycle shifts to the left and the
moisture content of steam at the
turbine exit increases. This side
effect can be corrected by reheating
the steam.
The effect of increasing the boiler
pressure on the ideal Rankine cycle.
Today many modern steam power
plants operate at supercritical
pressures (P > 22.06 MPa) and
have thermal efficiencies of about
40% for fossil-fuel plants and 34%
for nuclear plants.
A supercritical Rankine cycle.
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A steam power plant operates on the cycle shown in the
figure below. If the isentropic efficiency of the turbine is 87
percent and the isentropic efficiency of the pump is 85
percent, determine
(a) the thermal efficiency of the cycle
(b) the net power output of the plant for a mass flow rate of
15 kg/s.
Application of Thermodynamics
Refrigeration Cycle
REFRIGERATORS AND
HEAT PUMPS
The transfer of heat from a low-temperature
region to a high-temperature one requires
special devices called refrigerators.
Another device that transfers heat from a
low-temperature medium to a hightemperature one is the heat pump.
Refrigerators and heat pumps are essentially
the same devices; they differ in their
objectives only.
The objective of a refrigerator is to remove heat
(QL) from the cold medium; the objective of a heat
pump is to supply heat (QH) to a warm medium.
for fixed values of
QL and QH
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THE REVERSED CARNOT CYCLE
The reversed Carnot cycle is the most efficient refrig. cycle operating between TL and TH.
It is not a suitable model for refrigeration cycles since processes 2-3 and 4-1 are not practical
because Process 2-3 involves the compression of a liquid–vapor mixture, which requires a
compressor that will handle two phases, and process 4-1 involves the expansion of highmoisture-content refrigerant in a turbine.
Both COPs increase as
the difference between the
two temperatures
decreases, that is, as TL
rises or TH falls.
Schematic of a
Carnot refrigerator
and T-s diagram
of the reversed
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Carnot cycle.
THE IDEAL VAPOR-COMPRESSION
REFRIGERATION CYCLE
The vapor-compression refrigeration cycle is the ideal model for refrigeration
systems. Unlike the reversed Carnot cycle, the refrigerant is vaporized completely
before it is compressed and the turbine is replaced with a throttling device.
This is the most
widely used cycle
for refrigerators,
A-C systems, and
heat pumps.
Schematic and T-s
diagram for the ideal
vapor-compression
refrigeration cycle.33
The ideal vapor-compression refrigeration cycle involves an irreversible (throttling)
process to make it a more realistic model for the actual systems.
Replacing the expansion valve by a turbine is not practical since the added
benefits cannot justify the added cost and complexity.
Steady-flow
energy balance
An ordinary
household
refrigerator.
The P-h diagram of an ideal vaporcompression refrigeration cycle. 34
ACTUAL VAPOR-COMPRESSION
REFRIGERATION CYCLE
An actual vapor-compression refrigeration cycle differs from the ideal one owing
mostly to the irreversibilities that occur in various components, mainly due to fluid
friction (causes pressure drops) and heat transfer to or from the surroundings.
The COP decreases as a result
of irreversibilities.
DIFFERENCES
Non-isentropic compression
Superheated vapor at evaporator exit
Subcooled liquid at condenser exit
Pressure drops in condenser and evaporator
Schematic and
T-s diagram for
the actual
vaporcompression
refrigeration35
cycle.
A refrigerator uses refrigerant-134a as the working fluid and
operates on an ideal vapor-compression refrigeration cycle
between 0.14 and 0.8 MPa. If the mass flow rate of the
refrigerant is 0.05 kg/s, determine
(a) the rate of heat removal from the refrigerated space and
the power input to the compressor,
(b) the rate of heat rejection to the environment,
(c) the COP of the refrigerator.