I recently had a discussion about ACs and how they heat up cities.
Then I found an article about theoretical increase of efficiency of acs by using the heat pulled from a room to run a thermoelectric device and getting some of the energy back that was used in the ac.
I‘ve had this downstream thought many times already: since hot air is basically just energy stored. Could we theoretically pull (all?) the energy from the air (depending on desired temp) to cool it and casually fuel our society’s energy needs?
Entropy says no.
Energy likes to be heat and the only way to get heat energy out of something is by having a temperature differential. ACs spit out air that’s hotter than the environment so you could theoretically turn some of that back into useful energy, but the cost of doing that outweighs the benefits.
What’s the cost if we pipe the hot air through a steam engine?
Heat pumps (like AC units, fridges, etc) become less efficient the greater temperature difference they have to pump the heat. So pumping heat from a 25°C room to a >100°C steam engine would become terribly inefficient. It would need more energy, which creates more environmental damage and climate crisis to source, and that energy heats the cities even more.
The only sane way to cool cities is to get rid of as much concrete and asphalt as possible (especially the vast amounts of ground that is covered for cars), and keep only narrower sealed paths for small individial transport like bikes. Plaster everything with trees and grass and other greens. They cool down the city dramatically and are able to take up the water that comes down at extreme weather events.
Escaping the urban hellscape cannot be achieved by building more stuff and throwing more energy at it. Just visit a park in your city and observe how the temperature changes, it is that simple. Mobility cannot seal all surface area, it has the be minimal, i.e. narrow paths and trains with rails that can also run on open ground / green areas. This implies of course not building secluded areas for living, shopping, working etc… It has to be a mix, where commutes are short (i.e. like european cities, not american ones).
https://en.m.wikipedia.org/wiki/Organic_Rankine_cycle
That’s basically what this engine cycles doing. It’s taking heat from some source and trying to save as much energy as it can. Looking at efficiencies around 40 to 60%.
The greater the thermal gradient the easier it is to produce useful energy from it.
Sad but thanks for letting me know. Makes sense that there is some reason for it dissipating as heat most of the time.
But there are actually studies done to find out if energy can be harvested from the temp differential in acs https://www.sciencedirect.com/science/article/pii/S2352484722019023
So I suppose there might be some use to it.
There is a lot of bogus “science” out there, and this is part of it.
You need a temperature differential to harvest electric energy. You also need a differential to get heat energy to flow (usually from inside your apartment to outside). If you have that differential, you do not need an AC, you just open the window. If you do not have a differential (or if it points the wrong way, i.e. outside is hotter than inside), you need an AC + energy to create that differential, that lets thermal energy flow from your room to outside. There’s no “free leftover differential” in this, the differential it creates is literally to transport heat energy = why you have turned on the AC. Every bit you use of this differential for harvesting energy, you could turn down the AC a notch and have it save more energy than you could possibly harvest.
This idea is as mute as mounting a wind turbine to your electric car to “harvest” the headwind from driving
I understand. Thanks for explaining.
What still eludes me is if I have 35c outside and want 25c inside, those 10c times cubic meters go outside in the form of 50 or 60c hot air. And I think thats why people like me think it is possible to get those 15-25c difference back in the form of electricity (or at least not spend them so that the excess heat is not pumped outside).
Those 15-25c difference exist to drive the heat energy from the radiator into the air. If one would want to not waste as much energy for this, the actual solution would be to use a bigger radiator that can dissipate the same heat energy per h while being lower temperature. That would need way less additional material and be way more efficient than building another harvesting machine.