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Energy lost due to expansion

  1. Jun 24, 2011 #1
    Please will someone consider this with me

    When a simple water heater burns (a propane/air mixture, for example) is energy lost when the resulting gases expand into the atmosphere? Up the exhaust flue of a domestic boiler for example.

    Is this how steam turbine powered ships lose out efficiency wise (nautical miles per tonne of fuel) in comparison with large piston engine powered ships? That the energy resulting from the combustion reaction has it's expansion work utilized?

    We are thinking about efficent use of fuels for heat and motive power, simple combustion for water heating clearly appears too crude for the majority of minds here.
    We have looked at condensing boilers also but these only operate at the lower temperatures.

    thanks in advance
  2. jcsd
  3. Jun 24, 2011 #2


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    Yes, some energy is lost in the hot exhaust gasses. It is uneconomical to try to recapture this heat in small applications such as a water heater, although in larger cases it can be (see http://en.wikipedia.org/wiki/Combined_cycle" [Broken]).

    I think the tradeoffs between marine diesel vs. turbine are more for economic reasons. Either engine type can use some form of waste heat recapture in principle though. Steam turbines use reheaters to increase their efficiency, while diesel engines could use a turbocharger.
    Last edited by a moderator: May 5, 2017
  4. Jun 24, 2011 #3


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    This partly reduces to the problem of the thermodynamic efficiency of a heat engine. Basically, the maximum efficiency of an 'ideal' heat engine depends upon the difference between the temperature of the hot reservoir and the cold sink. (Google Carnot Engine)
    Real heat engines all fall a long way short of this because of, as is suggested, expelled hot gases etc..

    The most efficient engines use as high a temperature difference as possible - it's not always easy to establish exactly what those temperatures, effectively, are but you can imagine how a condenser at the output of a steam turbine or piston engine can help (that was a big step forward with early steam engines). Steam gas turbines can be very efficient but they need to be operated at very high revs. Marine turbines do not reach 50% (because water screws need to operate at slow revs) whereas large marine diesels often achieve more than that.

    Where Ships are concerned, one of the biggest factors in overall efficiency is the hull design and reducing the energy which is put into the bow wave. Having been hit frequently by the bow waves from large ships whilst sailing in the Solent I am always aware of all that wasted energy - and of the advantages of using that bulbous 'nose' at the bow of most large ships these days. It must save thousands of quid on a long sea journey.
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