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| Buoyancy and Energy Conservation |
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| msuffidy:
I have seen some persons attempt to extract power from the constant force of gravity. This however usually fails because power output seems to require a one time motion, and a potential change in a falling object. So I came up with a situation that potentially breaks conservation, or it is more subtle where the inputs are. So I was thinking that say you have a hydrogen balloon. So it goes up to as high as it can go and then releases the hydrogen. But say you have propellers on the sides that charge a battery as the object falls. Is it possible you would have more charged energy than is required to make more hydrogen from water? Or is this somehow fundamentally flawed? Could buoyancy be a potential changing displacement that breaks conservation? Also there may be a vacuum balloon version of this that requires pumping, but that may be too heavy to actually do. |
| Zero999:
The answer is no. The balloon can only lift an object equal to the mass of the air inside it. Sucking the gas out and replacing it with a vacuum would require the same amount of energy as lifting the same mass of air to outer space. This is easier to calculate for a tank under water, because water doesn't compress like air does. |
| nctnico:
Well... if you want to make use of gas rising up... you could make a system where you let water evaporate at a low place, make it condense again at a high place and make the liqiuid water stream drive a turbine + generator. The sun would be an ideal heat source for such a system. |
| Domagoj T:
--- Quote from: Zero999 on May 19, 2020, 08:42:18 pm ---The answer is no. The balloon can only lift an object equal to the mass of the air inside it. Sucking the gas out and replacing it with a vacuum would require the same amount of energy as lifting the same mass of air to outer space. This is easier to calculate for a tank under water, because water doesn't compress like air does. --- End quote --- OP is suggesting to generate hydrogen, not vacuum. Anyway, I started doing the math, and either I'm doing something wrong, or there might be something to it. Somebody double check my math? The current balloon altitude record holder (BU60-1) reached 50 km, and had a volume of 60 000 m^3. The balloon in question had a mass of ~34,5 kg and the scientific payload + parachute of another ~5,5 kg. If we can be generous and replace that science payload and parachute with wind turbine + battery of equal mass (total mass of 40kg) and keep the 50km altitude. To lift 40 kg you need 36 m^3 of hydrogen. Potential energy of a 40kg object at 50km is 19.600 kJ. That's absolute maximum of energy available to extract. On the other side, to generate 3,2 kg of hydrogen, at 50 kWh/kg, we need 512 kJ (160 kWh). Sure, back of the envelope calculation, but that's quite a margin. |
| Gregg:
--- Quote from: nctnico on May 19, 2020, 08:56:46 pm ---Well... if you want to make use of gas rising up... you could make a system where you let water evaporate at a low place, make it condense again at a high place and make the liqiuid water stream drive a turbine + generator. The sun would be an ideal heat source for such a system. --- End quote --- That has already been done and examples are really cheap. |
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