Reverse air-conditioning

[FriedChicken]

How about an air conditioning that through fine adjustments of pressure can reverse what the air-conditioner does, so that cold air goes outside and warm air comes inside. This could be some form of alternative to burning fossil fuels. The only problem is running the pump which consumes a lot of energy.

[insane_alien]

your better with a heater and no air movement(apart from internal convection), insulation isn't a bad idea either.

[Klaynos]

How about an air conditioning that through fine adjustments of pressure can reverse what the air-conditioner does, so that cold air goes outside and warm air comes inside. This could be some form of alternative to burning fossil fuels. The only problem is running the pump which consumes a lot of energy.

 

And where do you get the energy from? Oh yeah that's right, fossil fuels!!!

[5614]

Imagine you have your air conditioner cooling your room. You could think of it as heating the outside. The opposite of air conditioning is easily made, although heaters are a lot more common as they consume less energy.

 

Having said that heaters, and reverse air conditioning, all run on electricity, which is currently generated, mainly, by burning fossil fuels. So quite how this is a solution to the 'fossil fuel issue' I can't see.

[swansont]

You've described a heat pump.

 

Ground source heat pump

http://www.est.org.uk/myhome/generating/types/groundsource/

[DaveC426913]

So, I should put my air conditioner in backwards and heat my house in the winter?

[Sisyphus]

Not only would running an AC backwards use more energy than a plain heater, but it would barely work. The heat energy has to be pumped from outside, which means the AC has to be colder than the outside, which means it wouldn't work when it was needed most!

 

Now, what you really want is Maxwell's Demon.

[J.C.MacSwell]

Not only would running an AC backwards use more energy than a plain heater, but it would barely work. The heat energy has to be pumped from outside, which means the AC has to be colder than the outside, which means it wouldn't work when it was needed most!

 

Now, what you really want is Maxwell's Demon.

 

Even if plain heaters are 100% efficient, which they basically are, heat pumps can deliver many times more energy than they use (pay for). Entropy increases as the high grade electrical energy is used to concentrate the much lower grade heat energy outdoors to not much higher grade room temperature. It is much more efficient (and less heating required anyway) the warmer the outside (or geothermal, lake water etc.) temperature.

 

Note that heat pumps can claim to be 300% and greater efficient, which is true, but that is not thermodynamic efficciency, just that it delivers more heat than the energy that it uses directly.

[5614]

Yep Sisyphus, Maxwell's Demon would work, in theory. I think there might be some technical issues with the delivery and installation of it though .

 

Just a quick summar for those who don't know, Maxwell's Demon is a thought experiment. In the original there is a demon which can see things on a molecular level, and it sits between two boxes (A and B) which are linked. The demon lets fast moving particles into A, but doesn't let them out. At the same time he lets slow moving particles into B, but doesn't let them out.

 

The overall effect is that box A increases in temperature whilst box B lowers in temperature, all of this without the demon adding any energy to the system. This defies the second law of thermodynamics.

 

There are various responses to this. Some include taking note of the fact that to observer the molecules energy, say photons, would be needed. Also the Demon itself would need to use energy to stop some particles, and to move out the way of others.

[swansont]

Note that heat pumps can claim to be 300% and greater efficient, which is true, but that is not thermodynamic efficciency, just that it delivers more heat than the energy that it uses directly.

 

Basically because you are using the reservoir for free, and only paying to run the pump.

[J.C.MacSwell]

Basically because you are using the reservoir for free, and only paying to run the pump.

 

Yes, and entropy increases but not by as much as if you had used a conventional electric heater at "100%" efficiency.

[Jacques]

Hi

I have a heat pump for my house. The efficienty depend on the difference of temperature between inside and outside. When the outside temperature is

-12 C the ratio of extracted energy to invested energy is 1. Over -12 C I extract more energy than what I invest. For 1 watt of energy used to run the heat pump, I extract more than 1 watt of heat. Below -12 C the oil heating kicks in and the heat pump turned off. During summer, the same heat pump is used in reverse to cool down my house

[DaveC426913]

The overall effect is that box A increases in temperature whilst box B lowers in temperature, all of this without the demon adding any energy to the system. This defies the second law of thermodynamics.

On a more pedantic level, would it not also reuslt in an imbalance of pressure? At some point, won't the demon be having to hold back a whole pile of molecules all trying to push in one direction?

[5614]

On a more pedantic level, would it not also reuslt in an imbalance of pressure? At some point, won't the demon be having to hold back a whole pile of molecules all trying to push in one direction?

Yes.

 

The hotter box will have a higher pressure, the molecules will be moving faster and consequently he'll have to block at a faster rate.

[J.C.MacSwell]

On a more pedantic level, would it not also reuslt in an imbalance of pressure? At some point, won't the demon be having to hold back a whole pile of molecules all trying to push in one direction?

 

At the size of the Demon it's not pressure, just momentary impacts. Also will not work according to Quantum physics.

 

The thought experiment demonstrates the statistical nature of the second law.

[5614]

Also will not work according to Quantum physics.

I know generally physics prevents the demon existing, or says that when you analyse the whole system the entropy does increase, but what specifically does QM have against it? Sure it would make the demon's job harder, what with quantum tunneling, but why does QM actually forbid it?

[J.C.MacSwell]

I know generally physics prevents the demon existing, or says that when you analyse the whole system the entropy does increase, but what specifically does QM have against it? Sure it would make the demon's job harder, what with quantum tunneling, but why does QM actually forbid it?

 

IIRC it is based on the uncertainty principle. On the scale the demon needs to operate he can't have the information he requires.

[5614]

That sounds reasonable.

 

He needs to know the momentum of the particle to calculate whether to let it through or not. He also needs to know the position so he knows when to block/unblock the connection between the boxes.

 

He can't know both of these parameters (in the detail he requires) due to the uncertainity principle, therefore he can't do his job, or at least he can't do it properly.

[Rocket Man]

the next generation of peltier effect materials sounds promising, (unlikely to rival compression pumps on efficiency though)

if you really wanted to save money on air cond, route the radiator of your fridge outside.

 

with maxwells demon, he can probably work on a scale big enough to avoid the uncertaincy principal, the radium(?) atoms had to be cooled extremely close to 0 K for the effect to become apparant. im sure he could take an approximate enough estimate of speed to open a hole in the right spot, hell, give him a fudge factor, a band of momentum where nothing gets through either way.

[J.C.MacSwell]

the next generation of peltier effect materials sounds promising, (unlikely to rival compression pumps on efficiency though)

if you really wanted to save money on air cond, route the radiator of your fridge outside.

 

with maxwells demon, he can probably work on a scale big enough to avoid the uncertaincy principal, the radium(?) atoms had to be cooled extremely close to 0 K for the effect to become apparant. im sure he could take an approximate enough estimate of speed to open a hole in the right spot, hell, give him a fudge factor, a band of momentum where nothing gets through either way.

 

I'm pretty sure if you include the uncertainty of the demon himself he cannot get an edge over random statistical variances. (someone summed up the laws of thermodynamics as "You can't win, you can't break even, but you must play the game")

[5614]

with maxwells demon, he can probably work on a scale big enough to avoid the uncertaincy principal, the radium(?) atoms had to be cooled extremely close to 0 K for the effect to become apparant.

Remembering that wave-particle duality is a consequence of the uncertainty principle; the double slit experiment has been performed with buckminster fullerenes. These massive C60 molecules showed interference, despite their size.

 

When the demon is working with molecules he cannot avoid the uncertainty principle. Furthermore atoms do not need to be cooled for the effect to become apparent.