Could a system exist which doesn't need to vent heat to the outside, but converts it to electricity?

[ryan7585]

Sorry to invade the forum (I'm not an HVAC engineer, but a utility engineer)...... But I had a thought the other day and it's been on my mind, so I thought I'd ask.

Since there are ways to convert heat to electricity without steam, such as a thermionic converter, couldn't one create a sort of "space cooler" that doesn't need to vent heat to the outside, because it's converting that heat to electricity and using it to help power the other devices in your home?

If not, why? If so, why isn't it more popular?

 

[BrentEubanks]

No, this is not possible, for basic thermodynamics. All power extraction requires a hot side and a cold side. Even thermoelectric coolers/generators (i.e. Peltier junctions) have to have a "sink" for heat. Without a sink, there is no gradient, so there is no flow, so there is no energy to be extracted.

Another way to look at it: if what you wanted to do was possible, you would have a perpetually powered house, as you could use the waste heat from your refrigerator to power your computer, and use the waste heat from your computer to power your refrigerator, etc.

 

[ryan7585]

Well the energy conversion wouldn't be 100% efficient, so it wouldn't be "perpetually powered" in that sense. One of the main purposes of a thermionic converter is to convert waste heat to electricity in a variety of applications. In fact there IS a hot and cold side, in that the thermionic converter has a hot plate and a cool plate, with a vacuum in-between where electrons leave one plate toward the other in response to the heat. With modern technology this can be achieved with about 40% efficiency, based on some of the things I've read. Basically there IS a heat sink, but instead of air moving the heat away from it, it's conducted away from it, through some conductive material, to the hot plate of the thermionic converter.

The question, in my mind, isn't whether this is possible, really... It's whether this could actually cool a room to any degree that a human could perceive, or if it would be a negligible effect. Or maybe it's just that these newer, more efficient converters are too expensive to be used in consumer projects or something.I just feel like I must be missing something, since I don't know of any existing or developing projects which use thermionic converters in this way, and it seems to me like a pretty obvious application of it.

 

[ryan7585]

 

[IRstuff]

I just feel like I must be missing something

Yeah, well, hot room is hot side, so where is cold side going such that it's hotter than where it's going?

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[ryan7585]

That sentence was very confusing. I already explained where the heat traveled to. I also explained that thermionic converters are already used to convert waste heat to electricity.

 

[ryan7585]

attempted a basic diagram of a simple version of this.....

 

[MintJulep]

. .I just feel like I must be missing something,

The device does not create a thermal gradient.

It depends on an already existing gradient.

 

[ryan7585]

Care to elaborate? If not I understand... it's not your job to be my teacher. Especially since I don't plan to build this and it's mostly a thought experiment.

If thermionic converters are used to harvest waste heat in other scenarios, why can't it be used in an AC system? What's the difference between this, and, say, using them to harvest waste heat from a car engine or a computer, which is already being done?

Can you think of a way to change this diagram so it doesn't "depend on an already existing thermal gradient" as you say?

 

[MintJulep]

why can't it be used in an AC system? What's the difference between this, and, say, using them to harvest waste heat from a car engine or a computer, which is already being done?

Well, for starters the devices work at temperatures that are not within the range of typical comfort air conditioning.

And once again, the device does not create a thermal gradient. It requires that one exist.

Also, for the typical comfort cooling case the thermal gradient that does exist is:
1. Of too low quality for the device.
2. Too small.
3. In the wrong direction.

See

http://fti.neep.wisc.edu/neep602/SPRING00/lecture9.pdf

for more reading.

 

[lukaiENG]

According to Britannica:

https://www.britannica.com/technology/thermionic-power-converter[/URL]]They can provide efficiency in the range of 12 to 15 percent at temperatures of 900 to 1,500 K (about 600 to 1,200 °C, or 1,200 to 2,200 °F). Since these converters function best at high temperatures, they may eventually be developed for use as topping devices in conventional fossil fuel power plants.

At those temperatures, rejecting from the heat sink to atmosphere should be no problem, the household AC might be running a little hard though.

 

[ryan7585]

Ahhhhh ok..... The high operating temp definitely ruins it enough. However I was referring to a new type of converter which has no need for cesium atoms as a catalyst inside the vacuum, and therefore runs more efficiently (around 40% according to the article I read..... I'll find that later when I'm not working)

Maybe if, in the future, we get converters that can operate at any temperature, some engineer with better knowledge of heat engine physics could take a crack at it.

Thanks!

 

[BrentEubanks]

Feel free to share the article, but I'm dubious of the 40% claim. It might attain 40% of theoretical (Carnot) efficiency - which would be pretty good, for a thermoelectric device. But the basic efficiency is going to be a function of the magnitude of the temperature difference, and that's going to be small in the context of any building. There's no way you get 40% thermal-to-electricity conversion with a delta-T under 100 degrees.

 

[IRstuff]

My point is that you need to have a temperature DIFFERENCE, somewhere across the device, otherwise, there is no work that can be done by the device, regardless of its efficiency. That means that ANY device that uses heat, must have a cold side to provide the temperature difference, and to sink the 60% of the heat that it collects and doesn't convert to electricity

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[ryan7585]

the "hot plate" of the thermionic converter would feel cold to the touch, ideally, as it is removing the heat from your finger.... just like a heat sync. as it doesn't operate at room temperature this is irrelevant, but i would think it should work if it did

 

[ryan7585]

Also if it removed the other 60% of heat, the air coming out would be zero kelvin which is physically impossible.... right?

 

[IRstuff]

No, you are randomly correlating concepts. If something is 40% efficient, it means that 60% of the energy it "consumes" must show up somewhere in the universe as waste heat, which requires a heat sink of some sort. The "coldness" that your finger feels is a transient event, like charging up a capacitor, your finger's heat is going into warming up the heat capacity, which can only be maintained until the device's temperature matches that of the heat source.

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[chicopee]

I am not sure if any of the responders mentioned thermopiles--Check out the topic for your application