Some Thoughts on Air Conditioning

[FragmentedCurve]

On 5/20/2019 at 6:12 AM, studiot said:

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).

No apologies are needed. These are exactly the kind of discussions I was hoping to stimulate. Given that I am more rusty than Carrock, I feel like I don't have a lot to contribute. I'm reviewing material and trying to answer my own questions before coming back here and naively pushing them onto you.

 

On 5/20/2019 at 6:12 AM, studiot said:

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.

I know it wouldn't produce a lot of electrical energy. Also,  electrical energy isn't a requirement either. The reason I mentioned electrical energy is because its probably the most useful form of energy for practical purposes. But the end result doesn't matter much as long as its somewhat more useful than the input (the hot air). The point behind my post was about structuring a material that would be able to excite an electron on a chlorophyll-like molecule by only the kinetic energy of gaseous particles alone.

I just realized why I'm having trouble communicating my response. There's really two parts to my inquiry. The first part is about constructing such a material. The second part is what are the effects of such a material on the temperature of a gaseous body. The thoughts about air conditioning were just the catalyst for thinking about the chemistry and physics of this situation.

I attached a poorly drawn conceptual diagram. So the green particles hit A at the same time causing enough energy to be transferred to B and excite B's electron. Which laws of nature stop A from successfully exciting B's electron?

On 5/20/2019 at 6:12 AM, studiot said:

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

I hope I did not come across a someone looking to convert heat with 100% efficiency or worse (someone looking for free energy).

[Endy0816]

5 hours ago, FragmentedCurve said:

No apologies are needed. These are exactly the kind of discussions I was hoping to stimulate. Given that I am more rusty than Carrock, I feel like I don't have a lot to contribute. I'm reviewing material and trying to answer my own questions before coming back here and naively pushing them onto you.

 

I know it wouldn't produce a lot of electrical energy. Also,  electrical energy isn't a requirement either. The reason I mentioned electrical energy is because its probably the most useful form of energy for practical purposes. But the end result doesn't matter much as long as its somewhat more useful than the input (the hot air). The point behind my post was about structuring a material that would be able to excite an electron on a chlorophyll-like molecule by only the kinetic energy of gaseous particles alone.

I just realized why I'm having trouble communicating my response. There's really two parts to my inquiry. The first part is about constructing such a material. The second part is what are the effects of such a material on the temperature of a gaseous body. The thoughts about air conditioning were just the catalyst for thinking about the chemistry and physics of this situation.

I attached a poorly drawn conceptual diagram. So the green particles hit A at the same time causing enough energy to be transferred to B and excite B's electron. Which laws of nature stop A from successfully exciting B's electron?

I hope I did not come across a someone looking to convert heat with 100% efficiency or worse (someone looking for free energy).

The closest to what you are describing is thermoelectric cooling.

https://en.m.wikipedia.org/wiki/Thermoelectric_cooling

 

It is possible to use heat flow(hot and cold side) across one to generate power. Then turn around and use the resulting electricity to cool down somewhere else.

Due to losses the cooling would be less than if you used the cold side directly however.

 I would say the first and second laws of thermodynamics would be the most applicable ones here.

Cooling is often described as pumping heat uphill. Effectively reversing the normal flow of hot to cold. This takes some work as a result.

Edited May 24, 2019 by Endy0816

[swansont]

6 hours ago, FragmentedCurve said:

 

I attached a poorly drawn conceptual diagram. So the green particles hit A at the same time causing enough energy to be transferred to B and excite B's electron. Which laws of nature stop A from successfully exciting B's electron?

I hope I did not come across a someone looking to convert heat with 100% efficiency or worse (someone looking for free energy).

At room temperature, the average KE of a particle is about 25 meV, so you would be able to excite only very low-energy transitions the vast majority of the time.

[studiot]

8 hours ago, swansont said:

At room temperature, the average KE of a particle is about 25 meV, so you would be able to excite only very low-energy transitions the vast majority of the time.

Translating this to Joules the average kinetic energy of one single air molecule at room temperatures is of the order of 10^-21 Joules.

The lowest ionisation energies are around 10^-18 Joules per electron.

So you can see that even if the air molecule lost all its energy colliding with the wall this would be 1000 times too small to liberate one electron.

 

There are phenomena that convert mechanical energy directly to electricity but the amout available is very small for similar reasons.

Triboelectricity (generated by friction)

Piezoelectricity (generated by stress/strain)

are often offered as generators in fanciful green energy schemes.

 

Any form of work or work equivalent generation also suffers from the thermal efficiency formula

[math]{\rm{Efficiency = }}\frac{{{{\rm{T}}_{{\rm{hot}}}}{\rm{ - }}{{\rm{T}}_{{\rm{cold}}}}}}{{{{\rm{T}}_{{\rm{hot}}}}}}[/math]

As you can see there will be a very small temperature difference between the wall and the air ( perhaps less than one degree) which leads to a hoplessly inefficient generator.

 

A final comment

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

to hit the chlorophyll-like molecule with enough force to excite the electrical energy.

 

Chlorophyll extracts energy from photons not air molecules.
Photoelectricity is an entirely different ball game.