If a pipe that is 1km long can collect low grade heat that may raise the collecting liquid by 5 degrees. Is there a device that will concentrate that into a heat exchanger that is 1m long at say 50-90 degrees so the distance and change in temperature balances. This is to increase the low grade heat from a ground or air source to medium grade heat suitable to warm a home (which has far less volume than the ground that the heat was extracted from) to a comfortable level (many heat pumps have a COP of 4 or above but the heat generated gets lost too fast to warm a home properly). Note: how does a heat pipe freeze the ground or keep a permafrost without the use of electricity? And how does a room have a higher temperature at the ceiling than the floor?
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1Maybe the laws of thermodynamics help a bit. Zeroth law: you must play the game. First law: you cannot win. Second law: you cannot tie. Third law: you always lose. The rest is details. – Ed V Nov 03 '21 at 16:46
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1I've heard a similar quip regarding the laws of thermodynamics. You can't win, you can't break even, and you can't get out of the game. – David White Nov 03 '21 at 18:10
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If you have a really hot system a long way away and a parabolic mirror, you get to cheat. – g s Nov 03 '21 at 19:25
2 Answers
Heat moves spontaneously from hot systems to cold systems. This is the definition of temperature.
A device which moves heat from a cold system to a hot system is called a “refrigerator.” (If the point of the refrigerator is to increase the temperature of the hot system, it is sometimes called a “heat pump.”) A refrigerator, even an ideal refrigerator, requires external work input which depends on the temperature difference between the two systems.
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IMO, you're reversing the roles of those two terms. "Heat pump" is the generic term, and "refrigerator" names a specific application of a heat pump. – Solomon Slow Nov 03 '21 at 16:56
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3You say tomato, I say technically a berry. “Refrigerator” has the pedagogical advantage that people who have not yet studied thermodynamics already know exactly what the device does. The phrase “heat pump” tends to startle the novice: surely it can’t mean what it sounds like it means, because what would it mean to pump heat, like some kind of a fluid? That’s an hour’s worth of introductory thermo that I’d rather not clutter this brief answer. For you, @SolomonSlow, that hour happens to be in your past. – rob Nov 03 '21 at 17:25
It doesn't matter how you try, unfortunately there is no way to make heat flow from an area of lower temperature to an area of higher temperature without energy input. One of the side effects of this is that you can't collect heat energy from the ground at say 60 degrees F and have it flow into a home at 70 degrees F. Doing so must require energy. This is the same reason that if you focus light from the sun using a huge magnifying glass you can never achieve a higher temperature than the sun.
Because of the 2nd law of thermodynamics, energy always wants to spread out. There is no way to make the energy from the 1km long pipe concentrate itself together in a 1m long pipe without using energy (like a heat pump) to do so.
You are right that in both cases the total energy might be the same (5 degree 1km long pipe vs. 90 degree in 1m long pipe), but the useful energy isn't the same in both cases.
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How does a heat pipe move heat from a hot processor to a cooling fin then? With no moving parts or fans! And no use of electricity? – Lonewolf Nov 04 '21 at 16:16
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@Lonewolf Heat pipes use a liquid inside: it evaporates at the hot end and condenses at the cooler end. Like those drinking bird toys. – Ed V Nov 04 '21 at 17:09
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