ENGR 260 Section 6'9 6'11

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ENGR 260Section 6.9 6.11
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The Thermodynamic Temperature Scale
Absolute Temperature Ratio
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The Carnot Heat Engine
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Carnot Efficiency

• This is the highest efficiency for a heat engine
  operating between two thermal energy reservoirs.

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Efficiency Limits

• For two heat engines operating between the same
  temperature limits.

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Comparison to Carnot Efficiencies

• Most work-producing heat engines in operation
  today have efficiencies under 40.
• What is the Carnot efficiency of a power plant
  operating between the temperatures of 1000K and
  300K?

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Steam Plant Efficiencies

• Compared to 70, 40 is a lot better.
• Improvements can still be made.
• How can you increase Carnot efficiency?

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Quality of Energy

• What are the carnot efficiencies of a cycle where
  the heat is supplied at 1000K versus 500K if the
  low temperature reservoir is at 300K?
• What does this tell us?

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Conversion of Heat to Work

• Can all work be converted to heat?
• Can all heat be converted to work?

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Solar Energy

• Solar energy can be stored in solar ponds at
  about 350 K. This energy is then supplied to a
  heat engine to create electricity.
• What is the carnot efficiency if the low
  temperature reservoir is at 300K?

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Quantity versus Quality

• We must consider both to see the entire picture.
  
• Tea doesnt get warm just sitting on the counter.

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Carnot Refrigerator and Heat Pump
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Efficiency Limits

• For two refrigerators operating between the same
  temperature limits.
• A similar statement can be made for heat pumps by
  replacing COPR with COPHP

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Problem 6-84

• An inventor claims to have devised a cyclical
  engine for use in space vehicles that operates
  with a nuclear fuel generated energy source whose
  temperature is 1000 R and a sink at 550 R that
  radiates waste heat to deep space. He also
  claims that this engine produces 5 hp while
  rejecting heat at a rate of 15,000 Btu/hr. Is
  this claim valid?

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Problem 6-110

• A Carnot heat pump is to be used to heat a house
  and maintain it at 20oC in winter. On a day when
  the average outdoor temperature remains at about
  2oC, the house is estimated to lose heat at a
  rate of 82,000 kJ/h. If the heat pump consumes 8
  kW of power while operating, determine (a) how
  long the heat pump ran on that day (b) the total
  heating costs, assuming an average price of
  .085/kWh for electricity and (c) the heating
  cost for the same day if resistance heating is
  used instead of a heat pump.