Energy Conversion

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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 -gt thermal combustor
• Thermal -gt mechanical heat engine
• Mechanical -gt electrical generator

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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 cant 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 cant 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
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 K Thot 600 C 873 K
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

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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
• ? 3600/10,500 0.343