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?
Heat difference is what you can get energy from, not heat itself. You need something cold to get energy from the heat.
Makes a ton of sense but how does that translate to burning coal for example? You just set it on fire and it churns out tons of energy. I suppose its stored „organized“ energy which then gets released and is allowed to increase its entropy.
If you only had access to the coal furnace you couldn’t make power. The coal furnace is hot and it’s surrounded by room temperature air. The furnace really wants to heat the air around it and the air wants to cool the furnace because nature generally doesn’t like large differentials. So what we do is we force that heat to turn an engine before it can get to the cool ambient air.
It’s like a putting a turbine in the way of a waterfall. The water wants to fall, so we force it to turn an engine before it can get to the ground.
So back to your initial question, an AC is a heat pump. It pumps heat from the cooler inside to the warmer outside. It’s just like if we pumped the the water from the bottom of the waterfall to the top. Yes you can than use that water to generate energy, but you’re the one who pumped it up there in the first place so it’s a bit counterproductive.
Thats an awesome explanation! Thank your very much!
So, from this and many other comments and some independent reading on my side, we‘re technically just walking batteries getting fed by the sun, being buried under ground after dying and becoming coal so to speak.
So, theoretically, we would need to build some way to exhaust the excess heat into space (and could also get work done in the form of electricity) if we wanted to use the current overheating earth to our advantage while cooling it off. Thinking of a giant ac at this point. :D
But jokes aside, this means that the average laypersons idea about „energy“ is false. We need „work“, not energy. Because the dissipated energy can not perform work anymore. Correct?
Sounds like you’re on the right track there. As far as energy goes, you’re right, when things are dissipated, or all the same, you can’t extract anything. You need a differential, like a hot place and a cold one, a high voltage and a low one, a fast object and a slow/stopped one, a high object and a low one. The higher the differential the more you’re going to be able to extract. If it’s too small you might not be able to get any useful work out at all.
Thanks! That’s good to hear.
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. :)
a specific amount of energy (watts?)
Energy is measured in joules. Watts are joules per second, and a measure of how quickly the energy is being used.
And from there to absolute zero (0K) would be “available energy” in my perception.
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.
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).
However you use heat energy to generate other energy, it will not solve the problem of heat being increased in cities. Heat is an end State energy form.
This is why you may have heard the phrase the heat death of the universe. Entropy can be thought of as all energy being converted to heat, evenly distributed in the universe. The best you can do is move the heat elsewhere.
Heat death of the Universe happens when the heat is evenly distributed and there’s no way to move it to produce other forms of energy, like electricity.
Theoretically, heat can be turned into matter per Einstein, but we haven’t figured that out yet.
That was quite an awesome read. Thank you very much! :)
You are welcome! You’re making me blush.
some AC/ heat pump models pull the heat in the air and put it into water. those have existed for quite some time. look for “air to water heat pump”.
of course, there is some residual heat from the operation of the machine itself. and once the water is hot enough it will continue to release the heat into the air, but they do release way less heat into the air than regular AC units.
Could we run a water line through it from the water heater and use it as a supplemental water heater? At least we would be using that energy for something then.
they already sell appliances that do that. Daikin Altherma line, for example, can combine A/C, sanitary hot water and home heating.
In summer it takes heat from the inside of your house an puts it in the water tank. In winter, it takes heat from the air in the atmosphere and puts it both inside your home and water tank.
