3 ms·

I'm trying to reconcile how a consumer-grade compressor fits into this. Clearly such a device can compress a few gallons of gas, which could be allowed to cool

I'm trying to reconcile how a consumer-grade compressor fits into this. Clearly such a device can compress a few gallons of gas, which could be allowed to cool to room temperature, and then used to spin a small dynamo. This action clearly converts some amount of energy, but where does it come from? Perhaps the problem is we're not looking at a closed system at that point.

lazide 2y ago
If you empty that compressed gas, it cools everything it comes in contact with. Room temperature gas still has a lot of thermal energy. The effect the prior poster was discussing is most apparent at large scales when someone is doing actual efficiency analysis - compressed air is not a very efficient way of storing or transporting energy in most contexts. No one would use compressed air for a power grid, because it would quickly be apparent how terrible it is. It is however a great way to transfer and use energy in many industrial contexts, as turbines and pistons using high pressure compressed air can be very compact while also being very powerful, and have built in cooling. At the level of typical household use, the efficiency effects aren’t notable. No one is going to care or even notice if it cost 1/2 cent of electricity (at the compressor) to use that air impact wrench vs 1/10th of a cent to use an electric one. Especially when the electric one is heavier and has less torque.
gparke 2y ago
I think you are right about the closed system. When the compressed air spins the dynamo, it will cool, and then absorb heat from the environment. In an ideal, lossless scenario, that heat is equal to the heat given off when it cooled. So maybe you can view it as a battery that stores the energy in the environment?
pfdietz 2y ago
The room temperature compressed air still has some internal energy, and some of that energy can be converted to work by expanding the air through a turbine. The air at the outlet of the turbine will be really cold. Expanding compressed air through expanders like this is part of how air liquefaction plants operate.

lazide 2y ago

If you empty that compressed gas, it cools everything it comes in contact with. Room temperature gas still has a lot of thermal energy. The effect the prior poster was discussing is most apparent at large scales when someone is doing actual efficiency analysis - compressed air is not a very efficient way of storing or transporting energy in most contexts. No one would use compressed air for a power grid, because it would quickly be apparent how terrible it is. It is however a great way to transfer and use energy in many industrial contexts, as turbines and pistons using high pressure compressed air can be very compact while also being very powerful, and have built in cooling. At the level of typical household use, the efficiency effects aren’t notable. No one is going to care or even notice if it cost 1/2 cent of electricity (at the compressor) to use that air impact wrench vs 1/10th of a cent to use an electric one. Especially when the electric one is heavier and has less torque.

gparke 2y ago

I think you are right about the closed system. When the compressed air spins the dynamo, it will cool, and then absorb heat from the environment. In an ideal, lossless scenario, that heat is equal to the heat given off when it cooled. So maybe you can view it as a battery that stores the energy in the environment?

pfdietz 2y ago

The room temperature compressed air still has some internal energy, and some of that energy can be converted to work by expanding the air through a turbine. The air at the outlet of the turbine will be really cold. Expanding compressed air through expanders like this is part of how air liquefaction plants operate.