Download - Energy Conversion[1].ppt
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Energy Conversion
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FORMS OF ENERGY
Chemical Thermal Mechanical Electrical
(nuclear, electromagnetic)
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ENERGY CONVERSION
• Chemical -> thermal combustor
• Thermal -> mechanical heat engine
• Mechanical -> electricalgenerator
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HEAT TRANSFER
• Thermal energy flows from high Temperature to low Temperature
• A HEAT EXCHANGER brings the hot and cold media in contact without mixing
• To move thermal energy from low temperature to high temperature we need to expend some energy (typically mechanical energy)
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ENERGY FROM FOSSIL FUELS
• HEAT
• POWER GENERATION
• CO-GENERATION
(Simultaneous generation of heat and power)
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1st law of thermodynamics
You can’t get something for nothing!
(Energy can be transformed from one form to another but cannot be created or destroyed)
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2nd law of thermo
You can’t even “break even”
(It is not possible convert ALL of the thermal energy derived from a high temperature source to mechanical energy; some thermal energy must be rejected to a low temperature sink)
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Efficiency of conversion
hot
cold
in
out
in
outin
inengineheat T
T
Q
Q
Q
Q
W
11
Definitions Carnot cycle (reversible)(maximum) efficiency
Therefore:
- add heat at as high a temperature as possible
- reject heat at as low a temperature as possible
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Practical considerations:
• Temperature of heat source: Combustion: Adiabatic flame temperature? (Less in practice because AFT is for Q = 0)
• Large heat sinks:Atmosphere, oceans. Lakes. ~ ambient temperatures, Tcold = 10 C = 283 K
• Limitations of materials of construction, T, P of working fluids, Thot = 600 C = 873 K
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For Tcold = 10 C = 283 KThot = 600 C = 873 K
68.0873
2831maximum
We get less in real cycles (irreversible)
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Power Plant with Simple Vapour Cycle
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The efficiency of a power plant:
~ 35% for modern plants
inenergychemical
outenergyelectricalplantpower
producedenergyelectricalnet
requiredenergyChemicalRATEHEAT
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i.e., the inverse of efficiency, usually expressed as kJ/kWh or Btu/kWh
1 kWh = 1 kJ/s * 3600 s = 3600 kJ
So,
a HEAT RATE of 10,500 kJ/kWh means 34.3% efficiency
η= 3600/10,500 = 0.343
producedenergyelectricalnet
requiredenergyChemicalRATEHEAT