Energy Conversion Processes

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Energy Conversion Processes
Converted energy form
Initial energy form
Radiant
Heat
Chemical
Mechanical
Electrical
Nuclear
Reactor
Fuel cell, Battery discharging
Burner,Boiler
Chemical
Radiant
Photosynthesis
Absorber
Photovoltaic cell
Electrolysis, Batterycharging
Lamp,Laser
Electric motor
Resistance, Heat pump
Electrical
Electric generator, Magnetohydrodynamic(MHD)
generator
Friction, Churning
Mechanical
Turbines
Thermionic thermoelectricgenerators
Convector, Radiator, Heat pipe
Thermodynamicengines
Heat
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Thermodynamic engines
A thermodynamic engine is a mechanism which
utilizes a working fluid to absorb heat from a
high-temperature reservoir, perform work, and
reject waste heat to a low-temperature
reservoir. It then returns the working fluid to
its initial state so that the process may be
repeated.
Assume that the working fluid is a gas The
thermodynamic state of a gas is described by
its Pressure p p N /m2 Volume
V V m3 Temperature T T K
Work done by a gas
Constant Volume No work
Area A
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Heat
Heat
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The ideal gas
In an ideal gas the relationship between
pressure, volume and temperature is
Where n is the number of moles, R the universal
gas constant, N the number of molecules and k
the Boltzmanns constant. 1 mole the amount of
atoms in 12g of C12 (NA 6.0221415 10²³
Avogadros number) Universal gas constant R
8.31451 J / moleK Boltzmanns constant k
1.38065810 -23 J / K
The energy content of an ideal gas is only a
function of its temperature
On a microscopic level the heat content of a gas
is the kinetic energy of its molecules
Average kinetic energy of the molecules
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It is convenient to graph thermodynamic processes
on pressure - volume (p-V) or temperature -
entropy (T-S) diagrams. At any point on the p-V
diagram, e.g. T can be calculated from
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Thermodynamic processes

• There are 4 fundamental thermodynamic processes
  for a gas
• Isobaric (pressure remains constant)
• Isochoric (volume remains constant no work
  is done)
• Isothermal (temperature remains constant)
• Adiabatic (no heat exchange with the
  environment)

Thermodynamic processes are subject to the 1st
law of TD
Isochoric temperature increase
Isobaric temperature increase
In ideal gases the internal energy E is only
dependent on T, therefore
In isobaric processes
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Adiabatic processes
Thermodynamic processes are subject to the 1st
law of TD
Ideal gas equation
Adiabatic process
Adiabatic process
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The Carnot engine

• The Carnot cycle consists of 4 thermodynamic
  processes
• Isothermal expansion (heat Q1 is absorbed
  from a high-temperature reservoir)
• Adiabatic expansion (no heat exchange
  with the environment)
• Isothermal compression (heat Q2 is emitted to
  a low-temperature reservoir)
• Adiabatic compression (no heat exchange
  with the environment)
• After one cycle the internal energy of the
  working gas is back to its initial state

The efficiency of an energy conversion process is
defined as
Useful energy output
Total energy input
Therefore the efficiency of the Carnot engine is
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The Carnot engine
(1) Isothermal expansion
(2) Adiabatic expansion
(3) Isothermal compression
(4) Adiabatic expansion
The Carnot efficiency is the maximum efficiency
for ALL heat engines
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The Otto engine

• The Otto cycle consists of 4 thermodynamic
  processes
• Adiabatic compression (no heat exchange
  with the environment)
• (Compression stroke)
• Isochoric heating (heat Qin is
  generated through combustion)
• (Combustion process)
• Adiabatic expansion (no heat exchange
  with the environment)
• (Power stroke)
• Isochoric cooling (heat Qout is
  emitted to the environment)
• (Exhaust stroke)
• (Intake stroke)
• After one cycle the internal energy of the Otto
  engine is back to its initial state

The efficiency of the Otto engine is
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The Otto engine
(1) Adiabatic compression
(2) Isochoric heating
(3) Adiabatic expansion
(4) Isochoric cooling
is called compression ratio
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The Otto engine
Theoretical conversion efficiency of an Otto
engine
is called compression ratio
Typical values
Conversion efficiencies of real Otto engines are
much lower 20 - 30

• This is due to conversion losses like
• Friction of the moving parts (piston etc.)
• Heat loss to the cylinder walls
• Incomplete combustion of the air-fuel mixture

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Reading for Monday, 24 AprilChapter 4 from L R
Radovic Energy and Fuels in SocietyEfficiency
of Energy Conversionpdf available on course
website http//www.bren.ucsb.edu/academics/course
.asp?number288