The Vapour Compression Cycle is rather effecient, over 300% in some cases. Why couldn’t we use that to create a near limitless amount of energy by drawing heat from the atmosphere and using it to boil water to power a turbine?
​
In: Engineering
Without a constant source of energy, heat pumps cease to work.
Say you start with two boxes with equal temperatures, the heat pump moves heat energy from one side to the other so one side gets colder and the other warmer. After a short while, the pump stops working because it can no long move heat energy – one side is too hot and the other too cold. If you use the hot side to “power” something like boiling water to make steam and then run a turbine, the warm side just cools down quickly again. So there is no solution here. This is assuming 100% efficiency which is impossible. Energy is not created – it is simply moved around.
This is like trying to run an infinite turbine by letting a ball roll down a hill, then pushing the ball back up the hill again. Clearly you’d have to expend more energy pushing the ball back up the hill than would ever be generated from the ball rolling down.
Short answer: because the perpetuum mobile is not possible.
Long answer: the COP of the heat pumps is that high because there’s not much difference between the outside and inside temperatures and it’s easy to pump the energy across. But at the same time, utilising the same temperature difference is extremely inefficient because it’s small. It’s not possible to boil the water when your heating element is only 30C maximum.
The amount of energy you would get out of your turbine would be the same (or realistically) the same than you have required to power the air conditioner. So overall you would not earn any energy.
A heat pump, can bring something from Temperature 1 to temperature 2. This requires some amount of work (energy) (e.g. electricity). However you still get more heat energy from it than you have put work in (as additional energy comes from the temperature 1 reservoir), so you get efficiency over 100%.
If you build a machine which draws power from cooling something down, the efficency is limited (by the carnot efficiency limit). You will always get efficency below 100% and it is exactly the inverse of your heat pump. This is because the second law of thermodynamics (or the fact that entropy can never decrease globally)
There are examples of this that do work. Specifically for geo thermal power when the fluid is not very hot. Basically you take a large amount of warm water and use a heat pump to make a small amount of very hot water.
This kind of explains it.
https://www.planete-energies.com/en/media/article/very-low-temperature-geothermal-energy
>The Vapour Compression Cycle is rather effecient, over 300% in some cases.
Because that’s bullshit. Energy can’t be created, only transformed, so an efficiency of 300% is physically impossible and would break one of the most basics laws of nature. It may be 300% more efficient than an alternative solution, but that doesn’t mean it has a positive efficiency overall
From the point of view of my lounge room, I put in 1000W of electricity and get 3000W of heat, but that is because it’s moving heat that already exists outside to inside. From the point of view of the planet, energy is conserved and that 1000W of electricity becomes 1000W of heat (and some heat moves from outside to inside).
The “300% efficiency” is only from a narrow point of view.
The second law of thermodynamics forbids doing this kind of thing.
There’s a few equivalent ways of stating the second law. One is that “entropy always increases”. Another is known as the “Clausius statement”, which says:
>”It is impossible to construct a device which operates on a cycle and whose sole effect is the transfer of heat from a cooler body to a hotter body”
You can use a device to transfer heat from a cooler body to a hotter body, but you have to put in additional energy to do so, you can’t get it for free. The amount you’d have to put in would, in the best case, totally cancel out the amount you’d get out from the turbine.
Basically, there’s two ways that thermodynamics gets you. The first is that you can’t create energy out of nothing, and the second is that energy always gets converted into useless unrecoverable waste heat.
very shorty: we can and we do
(not actually air conditioning, but the same principles in reverse and not air, but ground)
Imagine you have a pump that sucks 3 gallons of gas from the ground by using 1 gallon of gas as fuel. The result is that you burn 1 gallon of gas and gain 3 gallons of gas, so you can call it 300% efficient.
However, it doesn’t mean that you created matter from nowhere. You just moved it from a useless (for you) spot to a useful (for you) spot. Obviously you can’t move more than the ground has.
In the case of the air conditioner, the 300% efficiency also does not mean that you generate energy from nowhere.
The key is that heat is not a source of energy. In order to get energy out, your have to have a place where it is hot, and a place where it is cold. Then there is energy to be had as your allow that hear to move through your mechanism.
Similarly, if you have energy, your can use it to move heat from a cold place to a hot place. That is what an air conditioner is doing.
Latest Answers