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Author Topic: Can a heat pump system generate electricity from temperatures over 0 Kelvin?  (Read 13057 times)

Offline McKay

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I am wondering - can a heat pump system be used to generate electricity from T>0K (temperature larger then 0 kelvin)?
That is - if a heat pump system consumes 1 energy unit and gives back 3 units (that includes the input unit), where the excess energy is taken from somewhere, decreasing that places temperature - using a thermoelectric converter with efficiency of 40%, then 1 unit can be pushed back to repeat the process, 1,8 is lost (non-converted) heat and 0.2 can be harnessed as electrical energy. With the mechanical parts and the compression chamber in the same place, surrounded by the thermoelectric converter system not to loose heat energy at mechanical inefficiency. It almost seems as if a system could be built that just keeps on cooling its surroundings and generating electricity as long as there is T>0K in the surroundings. Cant it?
« Last Edit: 28/02/2016 21:26:48 by chris »


 
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Offline chiralSPO

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Re: heat pump system to generate electricity from T>0K?
« Reply #1 on: 20/10/2014 22:46:56 »
Only if there is a 0 K heat sink to connect the heat pump to. The efficiency of a heat pump depends on the *difference* in temperature between the two ends. No useful energy can be extracted from a heat pump that is in equilibrium (same temperature on both ends), and it would require input of energy to move heat from a cool body to a warmer one.
 
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Online alancalverd

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Re: heat pump system to generate electricity from T>0K?
« Reply #2 on: 20/10/2014 22:55:30 »
A heat pump doesn't create energy, it moves it from place to place. Given a heat source and sink, there are many ways to extract useful energy from the source, the Stirling engine being one of the most entertaining and more efficient than a heat pump since it does not require any additional input.
 

Offline McKay

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Re: heat pump system to generate electricity from T>0K?
« Reply #3 on: 21/10/2014 18:06:29 »
what I mean by a heat pump is the things used in building for heating - they can pull out, as far as I understand, heat energy from outside air, or ground that is colder then the inside, making the house warmer and the outside, ground even colder..
« Last Edit: 21/10/2014 18:14:47 by McKay »
 

Offline evan_au

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Re: heat pump system to generate electricity from T>0K?
« Reply #4 on: 21/10/2014 21:06:29 »
An air-conditioner used in winter moves heat energy from the cold outdoors (which must remain at > 0K) into the warm indoors, making the indoors warmer.

But it consumes electrical energy to move the heat energy from outdoors.

For a "perfect" heat pump, it takes just as much energy as you could obtain from a perfect heat engine working between the same two temperatures.

But real heat pumps & heat engines have electrical losses, mechanical losses, energy losses due to turbulence in their fluids, etc. So I'm sorry, but you can't make a perpetual motion machine with an air-conditioner coupled to a Stirling engine...  :(
 

Offline syhprum

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Re: heat pump system to generate electricity from T>0K?
« Reply #5 on: 21/10/2014 21:20:57 »
Evan_au

Such talk is going to depress the value of the shares in the company I intend to set up to sell heat pump-sterling engine systems.
 

Offline McKay

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Re: heat pump system to generate electricity from T>0K?
« Reply #6 on: 21/10/2014 22:21:59 »
It is not perpetual motion I am trying to suggest, dont get me wrong.
 

Offline McKay

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Re: heat pump system to generate electricity from T>0K?
« Reply #7 on: 21/10/2014 22:31:06 »
please, explain to me - if there the pump and the compression chamber is in a room, 1 unit of energy is put into the pump and 3 units of energy are released as heat in the compression chamber, energy being taken from outside, how is that not more energy then the room initially had? Then part of that energy could be used to power the pump (1 unit) and collect whatever is left.
If heat pumps dont work by "stealing" heat energy from outside/ ground, then how do they work, if at all?
« Last Edit: 21/10/2014 22:33:41 by McKay »
 

Offline chiralSPO

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Re: heat pump system to generate electricity from T>0K?
« Reply #8 on: 23/10/2014 19:21:16 »
There is no "stealing" going on. Heat will naturally move from a warm body to a cold body, if provided a path between the two. A heat engine can extract useful energy when it is the path between two bodies at different temperatures, but there is no energy created, and there is no energy "stolen." The hot body will give up its energy anyway, it's just an issue of putting that energy to use. Does that make sense?

Think of it as water that can be a different heights. Water will naturally flow down from a high reservoir to a lower one. This can happen with no useful energy extracted (a waterfall) or it can go through a hydro-electric power station, which will extract some of this energy for use. You cannot connect the hydro-electric power station to only one reservoir, no matter how high it is because the water has to go somewhere. It is only by making the connection between the high water and the low reservoir that energy can be extracted (for heat engine, you need both a hot body and a cold body with the engine in between). You can pump water up from a low reservoir to a higher one, with the input of energy, but this energy will always be greater than the amount of energy one will get back by letting the water flow back down. Similarly, one must expend energy to move heat from a cold body to a warmer one, and this will be more energy than can be taken back by allowing the heat to move from the hot one to the cold one.
« Last Edit: 23/10/2014 19:23:30 by chiralSPO »
 
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Offline McKay

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Re: heat pump system to generate electricity from T>0K?
« Reply #9 on: 24/10/2014 22:34:58 »
I do understand energy conservation and that one cant get energy out of nowhere, but that doesnt really answer my question - how is using a heat pump for, say, heating a house more efficient then just running the same amount of electric power straight trough an electric radiator? Or are you saying that heat pumps dont work at all?
You do know what kind of heat pumps I am talking about and there is no confusion here, dont you?
 

Offline chiralSPO

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Re: heat pump system to generate electricity from T>0K?
« Reply #10 on: 24/10/2014 22:42:48 »
I think I know what you mean, but perhaps you should explain these heat pumps a little more, there might be some confusion...

I know of ways to extract heat from the ground during winter to heat a building and/or put heat into the ground during the summer to cool the building, but this relies on the fact that the ground temperature (a few meters deep) changes much less than the atmospheric or surface temperatures in response to seasonal changes.
 

Offline McKay

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Re: heat pump system to generate electricity from T>0K?
« Reply #11 on: 24/10/2014 23:07:01 »
well, there are ground based and air bases heat pumps and perhaps even water (small lake near a house?) based, ground based being more efficient because the temperature in the ground is a bit higher then the subzero air outside, but air bases forks on the same principles.
These pumps are exactly what I am talking about.
They dont rely on temperature difference, do they? Its not like they are just heat-conductive pipes that make the house the same temperature as the sub-surface ground - the system makes the house warmer then the ground and even warmer then it would be if the electric power would be used directly for a electric radiator. And make the ground colder in the process, of course.
 

Offline evan_au

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Re: heat pump system to generate electricity from T>0K?
« Reply #12 on: 25/10/2014 09:50:31 »
Quote from: McKay
how is using a heat pump for, say, heating a house more efficient then just running the same amount of electric power straight trough an electric radiator?

An electric radiator generating 1kW of heat consumes 1kW of electricity (plus about 50W more in transmission losses from the power station to your home).

An electric airconditioner generating 1kW of heat consumes around 250W of electricity (plus about 15W more in transmission losses from the power station) plus 750W of heat which it draws from the outside air/ground/water.

The more extreme the temperature difference from outside to inside, the more electrical power the airconditioner needs to pump that heat from the outside. This tracks the increase in power you could generate by a Stirling engine working between the outside temperature and inside temperature as this difference becomes more extreme.

So overall, an airconditioner powered by a Stirling engine will quickly grind to a halt (assuming it is initially started with a source of external power).

But far more efficient (in winter) is to pipe "waste" heat from a nearby power station to heat houses. This has some heat loss in the pipes, but no electrical transmission losses. But nobody wants a power station in their back yard, these days...  ;)
 

Offline McKay

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Re: heat pump system to generate electricity from T>0K?
« Reply #13 on: 26/10/2014 11:54:51 »
So overall, an airconditioner powered by a Stirling engine will quickly grind to a halt (assuming it is initially started with a source of external power).
I am not exactly sure if I understand you - you basically just said that an air conditioner (heat pump?) can generate 1 unit of heat energy requiring only .25 units of electric energy (that can initially come from either the grid or a accumulator pack, btw), taking extra energy from the environment. Why would there be ever (and quickly) increasing temperature difference ? How to ordinary heat pumps dont grind to a halt?
What I am talking about is to basically put some thermo-electric generators around the compression chamber to get some electricity from that. And, if the efficiency is high enough (40% seems to be high enough and doable in the foreseeable future using vacuum gap thermo-electric generators, probably), make the process self sustaining and even have some spare energy left..  or, even better - thermoelectic generators around the compression chamber and the pumping motor(s) to  get back some of that otherwise lost heat.

And since I am talking about electric power generation, then actual heat might not be the priority and it doesnt even  have to work in very cold weather..
« Last Edit: 26/10/2014 11:56:46 by McKay »
 

Offline evan_au

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Re: heat pump system to generate electricity from T>0K?
« Reply #14 on: 26/10/2014 18:17:45 »
Quote from: McKay
How to ordinary heat pumps don't grind to a halt?
Ordinary heat pumps do grind to a halt within about a second of turning off the electrical power source (I have also seen some refrigerators that work by burning an organic fuel: getting cool by producing heat!).

Quote from: McKay
It is not perpetual motion I am trying to suggest, don't get me wrong.
Quote from: McKay
self sustaining and even have some spare energy left
This is the definition of a "perpetual motion machine of the first kind".
 

Online alancalverd

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Re: heat pump system to generate electricity from T>0K?
« Reply #15 on: 26/10/2014 20:01:21 »

I am not exactly sure if I understand you - you basically just said that an air conditioner (heat pump?) can generate 1 unit of heat energy requiring only .25 units of electric energy

No, the heat pump doesn't generate a unit of heat energy, it moves one unit of heat from A to B and uses 0.25 unit to do so. 
 

Offline McKay

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Re: heat pump system to generate electricity from T>0K?
« Reply #16 on: 29/10/2014 13:29:04 »
exactly, it moves the energy that is already there. In the end of the day, the system provides a home (the compression chamber) with 1 unit of energy while  consuming 0.25 units from the electric grid (or whatever).
 

Offline McKay

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Re: heat pump system to generate electricity from T>0K?
« Reply #17 on: 29/10/2014 13:31:40 »
Quote from: McKay
How to ordinary heat pumps don't grind to a halt?
Ordinary heat pumps do grind to a halt within about a second of turning off the electrical power source (I have also seen some refrigerators that work by burning an organic fuel: getting cool by producing heat!).

Quote from: McKay
It is not perpetual motion I am trying to suggest, don't get me wrong.
Quote from: McKay
self sustaining and even have some spare energy left
This is the definition of a "perpetual motion machine of the first kind".

But its not what I mean - it is supposed to take (move) that extra energy from the outside world heat that already exists.
 

Offline McKay

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Re: heat pump system to generate electricity from T>0K?
« Reply #18 on: 02/11/2014 21:53:24 »
Answer me this, please - is the fluid of a heat pump, just after compression, hotter than it could be if the same amount of electric energy was put directly to heating that fluid?
Does a house require less electric energy to heat it up using heat pump than it would require using simple radiators? Because from what you guys are saying, it seems like heat pumps are a hoax and dont work as a heating element.
 

Offline syhprum

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Re: heat pump system to generate electricity from T>0K?
« Reply #19 on: 02/11/2014 23:02:49 »
They are certainly not a hoax they really work but the only snag is the capital cost rather like that other free lunch solar panels.
 

Offline McKay

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Re: heat pump system to generate electricity from T>0K?
« Reply #20 on: 08/11/2014 18:55:58 »
Ok, so they work - using them, the house gains more heat energy taking less electric energy from the grid (or wherever), right? How much more?
 

Online alancalverd

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Re: heat pump system to generate electricity from T>0K?
« Reply #21 on: 09/11/2014 00:30:17 »
Depends on the outside temperature, but I get about 3:1 from an airsource pump feeding a low temperature (30 degC) underfloor heating coil, and a bit less efficiency when heating the hot water tank (50 degC). Costwise it works out about the same as heating with gas or oil, but it's better than anything else for frost protection of a large open-plan area, where wallmounted radiators would just produce hotspots and electric underfloor heating would have required more mains supply capacity than was available - and cost 3 times as much to run!   

 

Offline evan_au

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Re: heat pump system to generate electricity from T>0K?
« Reply #22 on: 09/11/2014 02:48:50 »
Quote from: McKay
is the fluid of a heat pump, just after compression, hotter than it could be if the same amount of electric energy was put directly to heating that fluid?

Yes, the refrigerant would contain more heat from the outside world than if you obtained it from an electrical heating element.

However, depending on the boiling temperature of the refrigerant, and temperature/pressure operating conditions of the airconditioner, the temperature of the refrigerant may not change significantly, but it might change from gas to liquid.
 

Offline SisGhidir

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Re: heat pump system to generate electricity from T>0K?
« Reply #23 on: 29/06/2015 09:31:27 »
McKay,

Don't know why these guys don't grasp your concept. In theory your idea could work, but in practice there seem to be some complications.

You're right that a decent household heat pump will deliver heat at a ratio of about 1:4 to electrical input. This heat is transferred from outside, and is replenished by environment heat (no going to 0K).

A decent power plant will run at an efficiency of about 60%. So (in theory) you could use 42% of the electricity generated (=25% of the original energy input) by the power plant to resupply it with enough heat to keep it running. And all that with 58% of the electricity for other uses. This is neither magic nor a perpetual motion machine. It's a power plant that uses environment heat as a "fuel" source and would be a very nice way of combatting global warming.

The main problem however is that (i) heat pumps work at low output temperatures and their efficiency decreases sharply when requested output temperature goes up, whereas (ii) power plants require high temperature output to be able to generate electricity efficiently. The output temperature problem is less one of environmental heat decreasing (could easily be tapped from stable underground sources) but of the coolants used to operate heat pumps and extra efforts required to increase the temperature differential between input and output.

If you could design the right heat pump, this could work. But that won't be all that easy...

Hope this makes sense,
Sis
 

Offline SisGhidir

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Re: heat pump system to generate electricity from T>0K?
« Reply #24 on: 29/06/2015 10:11:04 »
Just to nuance myself after thinking this over for another 10 minutes: designing the right heat pump would not just be difficult, it would be impossible. It would be an impossible carnot cycle and violate the 2nd law of thermodynamics...

Too bad.
 

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Re: heat pump system to generate electricity from T>0K?
« Reply #24 on: 29/06/2015 10:11:04 »

 

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