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Registered Member #27
Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
Posts: 2058
I assume you talk about exctracting heat energy from the water.
The second law of thermodynamics puts limitations on what is possible with a heat engine and what efficiency is possible. Have a look here:
In this case a law means that it is thought to be a property of the universe we live in but giving a 100% proof is very difficult. No one has been able to make a complete proof and it is possible that a complete proof is impossible to make. On the other hand no one has been able to fault the law either and supporting experimental evidence is extremely strong.
You can use a thought experiment called Maxwell's demon to think about the implications. We had a discussion about it here:
If you know the temperature difference you can use this to calculate the efficiency in a simple way.
If thee air temperature is very cold and you can transfer heat energy from the water to the air then it will work and the water will come out cooler than it went in. If you don't use the air as a heat sink then nothing will happen.
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
My tetbook says that it is physically imposible to extract enegery from water to power a boat and then returning the water colder or as ice.
Second law of thermodynamics is based upon the fact that heat always flows from hotter to colder system, and nothing in universe can make this process reverse unless energy is used up and turned into heat to do so.
Considering that, you must allow heat to flow in order to extract energy at all; if there is no temperature difference, heat can't flow and you can get no work from it. As much as it is important to provide heat it is important to provide cold.
Any heat engine needs temperature gradient to operate, at maximum efficiency derived directly from temperature gradient it is powered by: n = 1-(Tcold/Thot) And the rest of energy *MUST* be wasted in the colder system by heating it up.
This also defines the maximum possible efficiency of a heat pump. In both cases, the real efficiency is always lower, and that is what prevents the possibility of a heat pump and heat engine being ran interconnected and violating the second law of thermodynamics.
If you wanted to make the water behind your boat colder, you would have to use heat pump and actually use energy to power it, which would, together with the pumped heat, have to be moved into another body of water or air as heat.
You will not be able to cool your kitchen by opening the fridge because it actually heats air.
If you wanted to go deeper, and ask why the second law of thermodynamics exists at all, I think the simplest explanation can be derived from classical laws of conservation of momentum and energy:
In an elastic collision, a faster moving object will *always* transfer some of it's energy to a slower moving object. The object that moved faster before collision will lose kinetic energy and one that was slower will gain kinetic energy, making their energies more similar to each other - the energy gradient between the objects has decreased!
If two gases of different temperatures are mixed, the particles of hotter gas will always transfer kinetic energy to slower moving particles of colder gas, in order to reduce the energy gradient between them, and ultimately their temperatures will equalize.
The slower moving particles of colder gas will never transfer energy to particles of hotter gas to make it even hotter.
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Why is this physically impossible? (heat engine)
