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?
You can’t output more energy than the one available in a system.
The real solution to cool off cities is trees and less pavement :)
I agree. It’s thermodynamics, right?
The reason I’m asking is that 100 m³ of 60 C air would have a specific amount of energy (watts?) in them, right? And from there to absolute zero (0K) would be “available energy” in my perception. Or is “available” something else?
Thanks for elaborating. :)
Energy is measured in joules. Watts are joules per second, and a measure of how quickly the energy is being used.
No, it’s not available. The only way to use heat energy is to find something that’s colder, to be able to transfer that heat to, and use that heat transfer to drive some other process that puts the energy in another form: in a chemical bond, in an electrical charge, in a moving object, into moving something heavy higher, etc.
Once everything in the universe completely evens out in heat, where none of the heat can go into anywhere else (because everything else is just as hot), that’s known as the heat death of the universe.
So if you’re starting with stuff that’s all the same temperature, and you want to make one part of that system colder by pumping heat out from the place to be cooled and dumping that heat into an already hot place, it’ll always cost more energy than you can capture again when you try to use that heat for other stuff. That’s because if you want to use that heat energy, the only way to do it would be to take advantage of the heat differential between the hot zone and the cold zone, by equalizing the temperature between two zones. Well, if you’re going to do that, then why did you spend energy cooling the cold zone in the first place? It’ll cost more energy to capture the heat as it returns to the cold zone than it cost to make the cold zone in the first place, so it would’ve been more efficient to just let the two zones remain equal temperature.
You would need an enormous amount of energy to achieve 0 K (-273 °C). See the system here is the atmosphere so you can think about the average outside temperature as the “state of least energy”. So you actually need to use a lot of energy to achieve that because you are going way further (although in the other direction- negative temps). Our system is earth, so 0K ain’t easy chief - check quantum computing (we need almost 0 K to work and those are huuuuge resource intensive machines). If you were in between the emptiness of galaxies, then that would be indeed the “normal default” which entropy would “go towards “. Basically, “our” entropy has a different temperature goal, because we are the system that is fed and bound to the sun. I can’t explain better because I also have limited knowledge, just the basics, sorry. Also the “” is to explain better, do not quote those as scientific.
Energy is only “available” when there is a region of higher energy density and a region of lower energy density, that you can extract work from by allowing that energy to flow from the former to the latter until they are equalized, at which point no further energy can be extracted from that system.
In the case of air conditioning, you can make heat flow “uphill”, so to speak, by applying additional energy from outside of the inside air / outside air system, usually in the form of electricity generated at a power plant. In the very large picture, though, it’s all just moving energy around from other regions of higher and lower densities, a losing usable energy with each transfer. That’s what entropy means.
Veritasium did a really good video on this idea a couple months ago, if you’re interested: https://youtu.be/DxL2HoqLbyA?si=bru50t1VYEKXKmKX