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Thermodynamic principles of energy conversion
Md. Mizanur Rahman School of Mechanical Engineering Universiti Teknologi Malaysia
Introduction
• Mechanical and electrical power developed from the combustion of fossil fuels or the fission of nuclear fuel or renewable sources
• This released energy is never lost but is transformed into other forms
• This conservation of energy is explicitly expressed by the first law of thermodynamics
• The work of an engine cannot be the same as the energy available in the fuel sources
• Second law of thermodynamics is to meet to happen the process
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Forms of energy
• Energy can exist in numerous forms such as thermal, mechanical, kinetic, potential, electric, magnetic, chemical, and nuclear, and their sum constitutes the total energy, E of a system.
• Mechanical Energy
• Kinetic energy, KE: The energy that a system possesses as
a result of its motion relative to some reference frame,
KE=1/2mV2.
• Potential energy, PE: The energy that a system possesses
as a result of its elevation in a gravitational field, PE=mgh
• For a moving body, E=PE+KE remains constant
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Internal energy, U: The sum of all the
microscopic forms of energy.
• For gases, molecules are so widely
separated in space that they may be
considered to be moving
independently of each other, each
possessing a distinct total energy.
• In the case of liquids or solids, each
molecule is under the influence of
forces exerted by nearby molecules
• Changes in internal energy are
measurable by changes in
temperature, pressure, and density.
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• Chemical energy: The internal energy associated with the atomic bonds in a molecule.
• Nuclear energy: The tremendous amount of energy associated with the strong bonds within the nucleus of the atom itself.
Internal = Sensible + Latent + Chemical + Nuclear
Thermal = Sensible + Latent
Energy form examples
• In a gasoline engine, the combustion of the fuel–air mixture involves U, PdV and Echem
• In a steam and gas turbine, only H and KE change
• In a nuclear power plant fuel rod, U and Enuc are involved
• In a magnetic cryogenic refrigerator, U and Emag are important.
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Total energy
per unit mass
The various forms of energy that can be possessed by a material body can
be added together to define a total energy, to which we give the symbol E,
Work and Heat interactions
• Work Interaction
W = pΔV
Heat Interaction
Q = mCpΔT
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THE FIRST LAW OF THERMODYNAMICS
• Energy can be neither created nor destroyed during a process; it can only change forms.
• The First Law: For all adiabatic processes between two specified states of a closed system, the net work done is the same regardless of the nature of the closed system and the details of the process.
• The change of energy in the system dE equals the heat dQ transferred to the system minus the work dW done by the system
In a cyclic process, Ei=Ef
I.e. Net heat and work are equal
Processes proceed in a certain direction and not in the reverse direction
A process must satisfy the first law to occur, however, satisfying the first law alone does not ensure that the process will actually take place.
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In a process for which dQ = 0, which is called an adiabatic process, the entropy may
remain the same or increase but may never decrease. An adiabatic process for which
the entropy increases is an irreversible process.
The Carnot cycle is composed of four reversible
processes—two reversible isothermal and two
reversible adiabatic process
In a P-V diagram the area under the
process curve represents the boundary
work for quasi-equilibrium (internally
reversible) processes,
The area under curve 1-2-3 is the work
done by the gas during the expansion
The area under curve 3-4-1 is the work
done on the gas during the compression.
The area enclosed by the path of the cycle
(area 1-2-3-4-1) is the difference between
these two and represents the net work done
during the cycle. 12
Net work done during Carnot cycle
REFRIGERATORS AND HEAT PUMPS
Heat is transferred in the direction of decreasing temperature, that is, from
high-temperature mediums to low temperature ones.
The reverse process, however, cannot occur by itself.
The transfer of heat from a low-temperature medium to a high-temperature one
requires special devices called refrigerators and heat pump .
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Coefficient of Performance
The COP of a refrigerator can be expressed as
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Rankine Cycle: The Ideal Cycle for Vapour 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
1-2 Isentropic expansion in a turbine
2-3 Constant pressure heat rejection in a condenser
3-4 Isentropic compression in a pump
4-5-1 Constant pressure heat addition in a boiler
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The simple ideal Rankine cycle
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Energy Analysis of Basic Rankine Cycle (ideal)
The steam flows round the cycle and each process is analyzed using steady
flow energy equation. Using energy balance for a steady flow system
For single stream (one-inlet-one-exit) systems, mass flow rate remains
constant.
If kinetic and potential energy are negligible, the energy equation becomes
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Types of Gas Turbine Cycles
There are two types of gas turbine cycle; Brayton/Joule
cycle and Atkinson cycle
Brayton Cycle
Heat added and rejected is at constant pressure
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Brayton Cycle: Ideal Cycle for Gas Turbine Cycle
Gas turbines usually operate on an open cycle.
Air at ambient conditions is drawn into the compressor, where its temperature
and pressure are raised. The high pressure air proceeds into the combustion
chamber, where the fuel is burned at constant pressure.
The high-temperature gases then
enter the turbine where they expand
to atmospheric pressure while
producing power output.
Some of the output power is used
to drive the compressor.
The exhaust gases leaving the
turbine are thrown out (not re-
circulated), causing the cycle to be
classified as an open cycle.
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The open gas-turbine cycle can
be modelled as a closed cycle,
using the air-standard
assumptions
The compression and expansion
processes remain the same, but
the combustion process is
replaced by a constant-pressure
heat addition process from an
external source.
The exhaust process is replaced
by a constant-pressure heat
rejection process to the ambient
air.
Brayton Cycle - Closed Cycle Model
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The ideal cycle that the working fluid
undergoes in the closed loop is the Brayton
cycle. It is made up of four internally reversible
processes:
1-2 Isentropic compression;
2-3 Constant-pressure heat addition;
3-4 Isentropic expansion;
4-1 Constant-pressure heat rejection.
The T-s and P-v diagrams of an ideal Brayton
cycle are shown beside.
Note: All four processes of the Brayton cycle
are executed in steady-flow devices thus, they
should be analyzed as steady-flow
processes.
The Brayton Cycle Process
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Otto and Diesel Cycle
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The ideal Otto and Diesel cycles are not totally reversible because they involve heat
transfer through a finite temperature difference
The irreversibility renders the thermal efficiency of these cycles less than that of
a Carnot engine operating within the same limits of temperature.
The Stirling cycle has two isentropic processes featured in the Carnot cycle
1-2 Isothermal heat addition (expansion)
2-3 Isochoric heat removal (constant volume)
3-4 Isothermal heat removal (compression)
4-1 Isochoric heat addition (constant volume)
Stirling cycle
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1-2 Isothermal heat addition (expansion)
2-3 Isobaric heat removal
3-4 Isothermal heat removal (compression)
4-1 Isobaric heat addition
The Ericsson cycle is often compared with the Stirling cycle, since the engine
designs based on these respective cycles are both external combustion
engines with regenerators.
The most well-known ideal cycle is the Carnot cycle, although a useful Carnot
engine is not known to have been invented. The theoretical efficiencies for both,
Ericsson and Stirling cycles acting in the same limits are equal to the Carnot
Efficiency for same limits.
Ericsson Cycle
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FUEL CELLS
• We see several different systems for converting the energy
of fuel to mechanical energy by utilizing direct combustion of
the fuel with air, each based upon an equivalent
thermodynamic cycle.
• In these systems, a steady flow of fuel and air is supplied to
the “heat engine,” within which the fuel is burned, giving rise
to a stream of combustion products that are vented to the
atmosphere.
• The thermal efficiency of these cycles, which is the ratio of
the mechanical work produced to the heating value of the
fuel, is usually in the range of 25% to 50%.
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FUEL CELLS
• This efficiency is limited by the combustion
properties of the fuel and mechanical limitations of
the various engines.
• Is there a more efficient way to convert fuel energy
to work? The second law of thermodynamics
places an upper limit on the amount of work that
can be generated in an exothermic chemical
reaction, such as that involved in oxidizing a fuel in
air.
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• A fuel cell is an electrochemical device that converts
chemical energy from a fuel into electrical energy without
any moving parts
• Fuel cells are operationally equivalent to a battery, but the
reactants or fuel in a fuel cell can be replaced unlike a
standard disposable or rechargeable battery
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Group discussion
Discuss with your friends about the following issues (5 minutes)
1. An modern engine can have more efficiency than a Carnot engine operating between same temperature limits?
2. Entropy generation can be avoided?
3. A fuel cell can have more efficiency than a Carnot efficiency?
4. How can we achieve close to 100% efficiency from fossil fuels to final energy consumption?
5. Heat and work, which is more valuable to you?
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