Some Thoughts on Air Conditioning

[FragmentedCurve]

I am not a chemist nor a chemistry major. My background is in mathematics and CS. However, when I was majoring in mathematics, I enjoyed chemistry very much and spent a lot of time studying it on the side. Years later I find economics pulling me far away from anything that gave me meaning. Because of this, I've recently been returning to things that gave me a reason to wake up in the morning. The point of me saying all this is to warn you my rhetoric and understanding may be crude when it comes to chemistry.

Back in college, when I was taking an inorganic chemistry course, I was daydreaming about gases and air conditioners and wondered why we spend energy to remove energy from the air. Couldn't we convert the kinetic energy in the air to cool down a room? I discussed this with the professor but she didn't have a lot to say about it. Then, I was thinking about chlorophyll and how specific parts of the light spectrum will hit it causing the electron (from the Mg atom I think) to become excited, converting that energy into chemical energy. Why can't a molecule do this for the kinetic energy in the air? From a user point of view, there would be a material on the wall, and the more material that is uncovered and exposed to the air, the more heat in the air would be converted into electrical energy. So the temperature of a room would be controlled by how much of the material is exposed.

Extending the crux of the idea a little, maybe the material would consist of layers. So the top layer would be an amplifier, to hit the chlorophyll-like molecule with enough force to excite the electrical energy. So, if 3 gaseous particles hit an amplifying molecule on the top layer, it redirects the impact from all three into a single point. The end goal of the material is to convert the heat in the air to electrical energy and store it somewhere else.

Besides the material not existing, is this absurd? Is there any merit in this line of thought? What are your thoughts?

[John Cuthber]

I'm fairly sure that what you propose is a breach of the laws of thermodynamics.

[FragmentedCurve]

19 minutes ago, John Cuthber said:

I'm fairly sure that what you propose is a breach of the laws of thermodynamics.

Can you narrow down which part is a violation for me or go into more detail?

[studiot]

10 minutes ago, FragmentedCurve said:

Can you narrow down which part is a violation for me or go into more detail? 

John is quite right what you propose would breach the Laws of Thermodynamics.

Additionally light energy is not the same as heat energy, which is why there is not an equivalent heat to chemical energy converter, as there are light to heat converters and light to chemical (potential) energy convertors.

Thermodynamically energy comes in various grades or a heirarchy with heat energy the bottom of the heap.

The use of energy requires converting energy from a higher form to a lower form and all forms eventually end up as heat.

These facts are embodied in the zeroth and second laws of thermo.

 

 

[FragmentedCurve]

1 hour ago, studiot said:

John is quite right what you propose would breach the Laws of Thermodynamics.

Additionally light energy is not the same as heat energy, which is why there is not an equivalent heat to chemical energy converter, as there are light to heat converters and light to chemical (potential) energy convertors.

Thermodynamically energy comes in various grades or a heirarchy with heat energy the bottom of the heap.

The use of energy requires converting energy from a higher form to a lower form and all forms eventually end up as heat.

These facts are embodied in the zeroth and second laws of thermo.

 

 

Okay, thank you.

[studiot]

1 hour ago, FragmentedCurve said:

Okay, thank you.

The zeroth laws says that to transfer heat you must have a temperature difference.

The second law says that  heat will not flow of its own accord from a colder body to a warmer one.

And, of course, your walls are warmer than the air you are trying to cool.

Edited May 18, 2019 by studiot

[Carrock]

1 hour ago, studiot said:

The zeroth laws says that to transfer heat you must have a temperature difference.

The second law says that  heat will not flow of its own accord from a colder body to a warmer one.

And, of course, your walls are warmer than the air you are trying to cool.

Maybe oversimplified, therefor misleading? Particularly "of its own accord" - rather anthropomorphic.

 

alternative:

Zero^th law: If two systems are isolated except for a heat permeable wall then heat will flow from hot system to cold system. (If the two systems do not change over time and there is no net heat flow they are at the same temperature.)

2^nd law: In an isolated system entropy will increase or remain the same.

Heat can flow from cold to hot provided total entropy does not decrease. An example is water evaporating into air at the same initial temperature. The higher energy water molecules leave preferentially, lowering the temperature of the remaining water and warming the air.

 

(Intended to complement earlier posts.)

[studiot]

52 minutes ago, Carrock said:

Maybe oversimplified, therefor misleading? Particularly "of its own accord" - rather anthropomorphic.

By all means provide a better explanation of your own if you think mine was inadequate.

(Note I threw the original post together in a hurry whilst the OP was still online.)

The second was added later by way of further explanation.

 

 

Is the average room an isolated system?
Since the example you give is a non equilibrium system any heat transfer will soon cease, and is not applicable to the OP electrical generation reuqirments.
Note the word I missed out was continuously so "heat will not flow continuously from a colder body to a warmer one" is more accurate.
Why bring entropy into it at all?

 "of its own accord" Anthropomorphic? or just jolly and colloquial?

Well more formally (an including the continuously)  "without external effect"

But your system is isolated so there are no external effects.

 

It is very easy to get tripped up by all the ifs and buts in thermo.

 

 

Edited May 18, 2019 by studiot

[Ken Fabian]

Perhaps 0ptical Rectennas (aka nantennas) could work as an alternative to air conditioning -  in theory these should be able to convert IR to electricity and remove heat from the surrounds in the process. Like old style 'crystal' radios, they should convert EMR to electricity, independent of the temperatures of the radiant source and collector- the cooling would be from energy absorbed not becoming heat in the collector and not being re-radiated in turn. These currently don't work because diodes are not fast enough and are losing too much energy through heat loss - but that is not an intrinsic property of diodes and better ones remain a possibility .

Of the many possible technologies we have not yet succeeded at, I rate Optical Rectennas as one with a lot of potential;  they should be able to utilise bands like IR as well as visible light and be able to make use of ground heat that radiates up as well as atmospheric down radiation, ie will make power day and night. It may also have uses for energy recovery from low grade heat  and make new kinds of thermal energy storage possible.

Edited May 18, 2019 by Ken Fabian

[swansont]

2 hours ago, Carrock said:

Maybe oversimplified, therefor misleading? Particularly "of its own accord" - rather anthropomorphic.

Heat doesn’t spontaneously flow from cold to hot.

[Carrock]

1 hour ago, swansont said:

Heat doesn’t spontaneously flow from cold to hot.

I don't regard "of its own accord" as very scientific...

From my water example, does the heat associated with warm water molecules not accompany those molecules spontaneously/of its own accord when they become vapour? I would regard your statement as entirely accurate for systems referred to in the zero^th law.

I was basically objecting as politely as I could to studiot's second law, which is a duplicate of part of the zero^th law.

6 hours ago, studiot said:

The zeroth laws says that to transfer heat you must have a temperature difference.

The second law says that  heat will not flow of its own accord from a colder body to a warmer one.

The zero^th law involves thermal equilibrium and is needed to define temperature.

It's impossible (I think) to have stable thermal equilibrium without the explicit/implicit assumption or law that heat flows from hot to cold in the systems relevant to the zeroth law.

 

3 hours ago, studiot said:

Note the word I missed out was continuously so "heat will not flow continuously from a colder body to a warmer one" is more accurate.

Why bring entropy into it at all?

"heat will not flow continuously from a warmer body to a colder one" is also accurate.

Both are non equilibrium systems. I am assuming they are finite.

I brought entropy into it because because I thought you gave a hasty, inaccurate version of the second law.

It is very easy to get tripped up by all the ifs and buts in thermo.

Edited May 19, 2019 by Carrock
Added yet another clarification.

[Klaynos]

7 hours ago, Ken Fabian said:

Perhaps 0ptical Rectennas (aka nantennas) could work as an alternative to air conditioning -  in theory these should be able to convert IR to electricity and remove heat from the surrounds in the process. Like old style 'crystal' radios, they should convert EMR to electricity, independent of the temperatures of the radiant source and collector- the cooling would be from energy absorbed not becoming heat in the collector and not being re-radiated in turn. These currently don't work because diodes are not fast enough and are losing too much energy through heat loss - but that is not an intrinsic property of diodes and better ones remain a possibility .

Of the many possible technologies we have not yet succeeded at, I rate Optical Rectennas as one with a lot of potential;  they should be able to utilise bands like IR as well as visible light and be able to make use of ground heat that radiates up as well as atmospheric down radiation, ie will make power day and night. It may also have uses for energy recovery from low grade heat  and make new kinds of thermal energy storage possible.

The problem with trying to absorb IR is that the thing you're trying to absorb it with is normally about the same temperature and is therefore also radiating IR. It makes things tricky and not very efficient. Bolometers for IR, for example, are normally actively cooled. 

[studiot]

10 hours ago, Carrock said:

I don't regard "of its own accord" as very scientific...

 

I was basically objecting as politely as I could to studiot's second law, which is a duplicate of part of the zero^th law.

Yes you were very polite, but how is your German ?

Here is the original statement of the second law.

Quote

Clausius

Die Warme can nicht von selbst aus einem kalteren in einem warmeren Korper ubergehen.

I think that of its own accord is a very reasonable translation of von selbst, don't you?

However this is really off topic here, as is your version of the Second Law.

I did (politely) suggest that you offered your own answer to the topic.

 

Since discussion of the issues surrounding the Four Laws is worthwhile in its own right I have started a separate topic for this, in the Engineering section, where I believe discussion of air conditioning systems also belongs.

 

Edited May 19, 2019 by studiot

[swansont]

11 hours ago, Carrock said:

I don't regard "of its own accord" as very scientific...

You made that clear. I gave a more rigorous description 

Quote

From my water example, does the heat associated with warm water molecules not accompany those molecules spontaneously/of its own accord when they become vapour? I would regard your statement as entirely accurate for systems referred to in the zero^th law.

Heat associated with warm water? Heat is energy transfer.

Quote

I was basically objecting as politely as I could to studiot's second law, which is a duplicate of part of the zero^th law.

No, it’s not. There are several ways to present the second law, and one is that spontaneous heat transfer from cold to warm is not possible (i.e. without work being done, or colloquially "of its own accord") is one of them.

http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/seclaw.html

[Carrock]

44 minutes ago, swansont said:

You made that clear. I gave a more rigorous description 

Heat associated with warm water? Heat is energy transfer.

I agree... I did not expect to find this definition of heat.

Quote

For the precise definition of heat, it is necessary that it occur by a path that does not include transfer of matter.^[12]

So heat can't flow from cold to hot. Got it.

 

12 hours ago, Carrock said:

I was basically objecting as politely as I could to studiot's second law, which is a duplicate of part of the zero^th law.

 

1 hour ago, swansont said:

No, it’s not.

 

According to all the references I could find I'm wrong here as well, but I can't see why.

e.g. from https://en.wikipedia.org/wiki/Zeroth_law_of_thermodynamics

Quote

The zeroth law of thermodynamics states that if two thermodynamic systems are each in thermal equilibrium with a third one, then they are in thermal equilibrium with each other. Accordingly, thermal equilibrium between systems is a transitive relation.

How can any system be in thermal equilibrium, or be measured to be in thermal equilibrium with another system unless 'heat flows from hot to cold' is part of the zero^th law? Alternatively, 'heat flows from hot to cold' could be part of both laws, but that is messy at best.

If (plausibly) the second law cannot be formulated without some version of the zero^th law then the zero^th law should surely not rely on part of the second law.

As my thermodynamics is (obviously) rusty, I expect I've overlooked something obvious.

[swansont]

3 minutes ago, Carrock said:

How can any system be in thermal equilibrium, or be measured to be in thermal equilibrium with another system unless 'heat flows from hot to cold' is part of the zero^th law? Alternatively, 'heat flows from hot to cold' could be part of both laws, but that is messy at best.

If (plausibly) the second law cannot be formulated without some version of the zero^th law then the zero^th law should surely not rely on part of the second law.

As my thermodynamics is (obviously) rusty, I expect I've overlooked something obvious.

The zeroth law doesn’t address how you get to equilibrium. Doesn’t address heat transfer at all. 

[Carrock]

2 minutes ago, swansont said:

The zeroth law doesn’t address how you get to equilibrium. Doesn’t address heat transfer at all. 

No, it assumes (parts of) the second law etc. The idea of the zeroth law being more fundamental than the second law while relying on concepts like equilibrium from the second law is what I find problematic.

[studiot]

For what it's worth, my understanding of the Zeroth law is that it introduces the three important equivalence relations into thermodynamics thus:

Consider 3 separate thermodynamic systems, Z1, Z2,and Z3 (we use Z to avoid confucion with entropy)

1) Then if Z1is in equilibrium with Z2 and Z3 is also in equilibrium with Z3 then Z1 is in equilibrium with Z3  ( This is the transitive property).

2) Z1 is in equilibrium with itself  (reflexive property).

2) Z1 in equilibrium with Z2 implies Z2 is in equilibrium with Z1 (symmetric property).

These assertions can be used to show that thermodynamic temperature is a property of a system and was originally proposed by Maxwell under the title the Law of Equal temperatures (1872)

[Ken Fabian]

16 hours ago, Klaynos said:

The problem with trying to absorb IR is that the thing you're trying to absorb it with is normally about the same temperature and is therefore also radiating IR. It makes things tricky and not very efficient.

As I understand it the IR collected by the rectenna and converted to electricity would not be raising the temperature of the collector, and the re-radiated heat will be only from that portion that was not converted, plus diode or other heat loss - these are not converting 'heat' to energy but converting EMR. That should be independent of the temperature of the collector.

[Carrock]

4 hours ago, studiot said:

For what it's worth, my understanding of the Zeroth law is that it introduces the three important equivalence relations into thermodynamics thus:

Consider 3 separate thermodynamic systems, Z1, Z2,and Z3 (we use Z to avoid confucion with entropy)

1) Then if Z1is in equilibrium with Z2 and Z3 is also in equilibrium with Z3 then Z1 is in equilibrium with Z3  ( This is the transitive property).

2) Z1 is in equilibrium with itself  (reflexive property).

2) Z1 in equilibrium with Z2 implies Z2 is in equilibrium with Z1 (symmetric property).

These assertions can be used to show that thermodynamic temperature is a property of a system and was originally proposed by Maxwell under the title the Law of Equal temperatures (1872)

I agree with your post with caveats - see below.

I've been looking through Wikipedia and it doesn't exactly help.

My problem is how do you tell if e.g. Z1 is in equilibrium with itself and with Z2 without assuming heat flows from hot to cold? An answer seems to be that if Z1 and Z2 are in contact for a long time without changing they are in equilibrium.

From https://en.wikipedia.org/wiki/Zeroth_law_of_thermodynamics

Quote

The zeroth law of thermodynamics states that if two thermodynamic systems are each in thermal equilibrium with a third one, then they are in thermal equilibrium with each other. Accordingly, thermal equilibrium between systems is a transitive relation.

Two systems are said to be in the relation of thermal equilibrium if they are linked by a wall permeable only to heat and they do not change over time.

But if heat doesn't flow from hot to cold (or from cold to hot) they presumably wouldn't change from whatever weird states they were in i.e. mutual equilibrium would be a meaningless concept. So heat flows from hot to cold seems to be a necessary postulate for the zero^th law....

 

And from https://en.wikipedia.org/wiki/Temperature#Zeroth_law_of_thermodynamics

 

 

Quote

 

Zeroth law of thermodynamics

Main article: Zeroth law of thermodynamics

When two otherwise isolated bodies are connected together by a rigid physical path impermeable to matter, there is spontaneous transfer of energy as heat from the hotter to the colder of them. Eventually, they reach a state of mutual thermal equilibrium, in which heat transfer has ceased, and the bodies' respective state variables have settled to become unchanging.

One statement of the zeroth law of thermodynamics is that if two systems are each in thermal equilibrium with a third system, then they are also in thermal equilibrium with each other.

Heat flows from hot to cold assumed.....

 

 

More Wikipedia : Heat flows from hot to cold is three laws down from the second law?!

From https://en.wikipedia.org/wiki/Thermal_equilibrium#Internal_thermal_equilibrium_of_an_isolated_body

Quote

One may imagine an isolated system, initially not in its own state of internal thermal equilibrium. It could be subjected to a fictive thermodynamic operation of partition into two subsystems separated by nothing, no wall. One could then consider the possibility of transfers of energy as heat between the two subsystems. A long time after the fictive partition operation, the two subsystems will reach a practically stationary state, and so be in the relation of thermal equilibrium with each other. Such an adventure could be conducted in indefinitely many ways, with different fictive partitions. All of them will result in subsystems that could be shown to be in thermal equilibrium with each other, testing subsystems from different partitions. For this reason, an isolated system, initially not its own state of internal thermal equilibrium, but left for a long time, practically always will reach a final state which may be regarded as one of internal thermal equilibrium. Such a final state is one of spatial uniformity or homogeneity of temperature.^[4] The existence of such states is a basic postulate of classical thermodynamics.^[5][6] This postulate is sometimes, but not often, called the minus first law of thermodynamics.

Wikipedia is usually reliable but all I can really say is that my knowledge of what I don't know about thermodynamics has significantly increased.

Edited May 19, 2019 by Carrock
Omitted part of a quote

[studiot]

9 minutes ago, Carrock said:

I agree with your post with caveats - see below.

I've been looking through Wikipedia and it doesn't exactly help.

My problem is how do you tell if e.g. Z1 is in equilibrium with itself and with Z2 without assuming heat flows from hot to cold? An answer seems to be that if Z1 and Z2 are in contact for a long time without changing they are in equilibrium.

My apologies I have spotted the glaring error.

The transitive sentence should read

"1) Then if Z1is in equilibrium with Z2 and Z3 is also in equilibrium with Z2 then Z1 is in equilibrium with Z3  ( This is the transitive property). "

 

But please note I didn't originally think it useful to put it in this manner.

The other simpler properties are actually deductions from it or consequences of it. 

Have you heard of the equipartition theorem?

http://vallance.chem.ox.ac.uk/pdfs/Equipartition.pdf

Edited May 19, 2019 by studiot

[swansont]

11 hours ago, Carrock said:

I agree with your post with caveats - see below.

I've been looking through Wikipedia and it doesn't exactly help.

My problem is how do you tell if e.g. Z1 is in equilibrium with itself and with Z2 without assuming heat flows from hot to cold? An answer seems to be that if Z1 and Z2 are in contact for a long time without changing they are in equilibrium.

Bodies are in equilibrium with other bodies, not with themselves.

You don't have to assume that heat flows in a particular way. The only requirement is that no heat is flowing.

 

11 hours ago, Carrock said:

From https://en.wikipedia.org/wiki/Zeroth_law_of_thermodynamics

But if heat doesn't flow from hot to cold (or from cold to hot) they presumably wouldn't change from whatever weird states they were in i.e. mutual equilibrium would be a meaningless concept. So heat flows from hot to cold seems to be a necessary postulate for the zero^th law....

Heat could flow from cold to hot and the law still works because the law is not about the details of heat flow. It is merely to define the transitive property of equilibrium (i.e. it's about temperature, not heat)

 

[studiot]

Fragmented curve we are geeting further and further away from your original intention.

My apologies, but that is the price you pay for posting in a fundamnetal science part of the forum rather than applied (engineering).

 

Perhaps I should have offered a more down to earth response.

On 5/18/2019 at 6:21 AM, FragmentedCurve said:

and wondered why we spend energy to remove energy from the air. Couldn't we convert the kinetic energy in the air to cool down a room? I

 

Yes we can cool the air, and since in removing some heat, that heat has to go somewhere some of it could be converted to electricity.

But whether your idea is viable is an engineering question, not a fundamental science question.

Fundamental science in the guise of thermodynamics tell us that we need to spend energy to remove some heat from the air.

I have a heat pump which does just this, but it is run by electricity. At this time of year the heat I get out is equivalent to a little over 3 times the electrical energy I use in the machinery.

But the heat I get out comes out in the form of hot water not electricity.

It is theoretically to use this hot water to drive an electrical generator, but I would get significantly less electrical energy that the heat energy I use.
And I want the hot water.

There is no such thing as a free lunch in this universe.

 

If you wish to continue the discussion in these less esoteric terms please come back to us.

[swansont]

On 5/18/2019 at 1:21 AM, FragmentedCurve said:

 From a user point of view, there would be a material on the wall, and the more material that is uncovered and exposed to the air, the more heat in the air would be converted into electrical energy. So the temperature of a room would be controlled by how much of the material is exposed.

You can do this, but only as long as there is a reservoir at a lower temperature. Then heat will flow, and you can make it do work, though not at 100% efficiency, since the rejected heat will be at some temperature.

As mentioned before, the issue is often that you want to cool the room down without that reservoir, and that requires that you do work, as it will not happen spontaneously.

[Carrock]

11 hours ago, studiot said:

Have you heard of the equipartition theorem?

http://vallance.chem.ox.ac.uk/pdfs/Equipartition.pdf

Yes. From a brief look the reference you gave is clear and accurate.

In it 'temperature' is frequently used but not derived. e.g.

Quote

The equipartition theorem can go further than simply predicting that the available energy will be shared evenly amongst the accessible modes of motion, and can make quantitative predictions about how much energy will appear in each degree of freedom. Specifically, it states that each quadratic degree of freedom will, on average, possess an energy 1⁄2kT.

So this version, probably edited/simplified for students, of the equipartition theorem can't be used in a definition of temperature. I hope this response is relevant.

 

1 hour ago, swansont said:

Bodies are in equilibrium with other bodies, not with themselves.

You don't have to assume that heat flows in a particular way. The only requirement is that no heat is flowing.

 

Heat could flow from cold to hot and the law still works because the law is not about the details of heat flow. It is merely to define the transitive property of equilibrium (i.e. it's about temperature, not heat)

 

I don't doubt that you're right and I'm wrong, not least from your work on clocks. Understanding how you're right is still an issue for me.

Thanks for your help and patience; I've now reached a point where I have to do some actual reading.

I'm now rather dubious about reader friendly Wikipedia, so I'm going to have to look for relevant peer reviewed papers, or at least preprints.