15.4 Carnot's Perfect Heat Engine: The Second Law of Thermodynamics Restated

15.4 Carnot's Perfect Heat Engine: The Second Law of Thermodynamics Restated

  • The net work output is related to the area inside the loop.
  • The drinking bird is an example of Carnot's engine.
    • The methylene chloride is mixed with a dye and boiled at a very low temperature.
    • One has to get the bird's head wet.
    • As the water goes down, fluid moves up into the bird's head, causing it to become heavy and dive back into the water.
    • This cools down the methylene chloride in the head, and it moves back into the abdomen, causing the bird to tip up.
    • Except for a small amount of energy, the bird becomes a motion machine.
  • The second law of thermodynamics states that a heat engine can't be 100% efficient since there must always be some heat transfer to the environment.
    • The answer to this question was given in 1824 by a young French engineer, Sadi Carnot, in his study of the heat engine technology crucial to the Industrial Revolution.
    • The Carnot cycle can be used to restate the second law of thermodynamics.
  • The Carnot cycle is defined by the fact that only reversible processes are used.
    • Dissipative factors include friction and turbulence.
    • This reduces the efficiency of the engine and increases heat transfer to the environment.
    • The processes that are reversed are superior.
  • When operating between the same temperatures, all engines have the same maximum efficiency.
  • The diagram in Figure 15.22 is for a Carnot cycle.
    • There are two isothermal and two adiabatic processes in the cycle.
    • Both isothermal and adiabatic processes are irreversible.
  • Carnot determined the efficiency of a perfect heat engine.
  • Carnot found that for a perfect heat engine the ratio is equal to the absolute temperatures of the heat reservoirs.
  • The Carnot efficiency is the best that a real heat engine can do, and an actual efficiency of about 0.7 of this maximum is usually the best that can be accomplished.
    • The drinking bird is a fascinating novelty, but the ideal Carnot engine has zero power.
    • It is unrealistic for any applications.
  • Carnot's result suggests that 100% efficiency would only be possible if the cold is at absolute zero.
    • The only way to have all heat transfer go into doing work is to remove all thermal energy.
  • When the ratio is small, the greatest efficiency can be obtained.
    • The Otto cycle shows that efficiency is greatest for the highest possible temperature of the hot and the lowest possible temperature of the cold.
    • The temperature of the environment and the type of heat source influence the actual temperatures of a heat engine.
    • Consider the following example.
  • During the isothermal pathAB, heat transfer occurs into the working substance.
    • The isothermal path CD takes place at constant temperature and is where heat transfer occurs.
  • The Carnot engine has the same maximum efficiency as any heat engine that operates between these two temperatures.
  • There is a schematic diagram of a pressurized water nuclear reactor.
  • In order to avoid radioactivity, heat exchange is used to generate steam.
    • The cost of operating a single generator that produces the same amount of electrical energy is more expensive than using two turbine generators.
    • To keep exit steam pressure low and aid the flow of steam through the turbine, steam is condensed to liquid before being returned to the heat exchanger.
    • There is no direct heat transfer from the local environment to the aquatic environment if a cooling tower is used.
  • The Carnot efficiency can be calculated using the temperatures for the hot and cold parts of the heat engine.
    • The temperatures must be converted tokelvins.
  • A nuclear power station's actual efficiency is about 35%, a little better than 0.7 times the maximum possible value, a tribute to superior engineering.
    • The boilers of electrical power stations can reach higher temperatures and pressures, making them more efficient.
    • The local environment limits the cold temperature in these power stations.
    • Both have cooling towers that allow water from the condenser to enter and be sprayed downward.
  • Both have cooling towers that evaporate water into the environment.
    • The nuclear reactor is located inside the containment buildings.
  • Even with the best heat engine, there are always dissipative processes in peripheral equipment.