3 ms·

While the original claim is silly, we will need to be careful about leaking hydrogen, because it produces, all told, something like 200x the greenhouse effect o

by ncmncm 5y ago

While the original claim is silly, we will need to be careful about leaking hydrogen, because it produces, all told, something like 200x the greenhouse effect of CO2, per unit mass. By itself, it is only 6x. But it also prolongs the lifetime of CH4 in the atmosphere, and produces tropospheric ozone, which combined have much larger effect. So, we will need to be careful to minimize H2 leaks (not easy) and burn it completely. It may mean that, instead of letting our tanks of atmospheric pressure LH2 slowly boil off and vent, we will need to carefully flare the boil-off, or actively refrigerate them. Water vapor is itself a pretty potent greenhouse gas, and we probably should be making an effort to condense it to water (e.g., by heat exchange with input, after a first compression stage) before exhausting it. That can actually make combustion more efficent by producing a partial vacuum at the exhaust port of the burner.

credit_guy 5y ago
You're overthinking this. Water has a cycle in nature. It evaporates, makes clouds, then condenses and creates precipitations. Over one year, the average precipitation level across the whole planet is about 1 meter (1000 millimeters). Since the Earth area is 500 million km2, then every single year about 500 million cubic kilometers of water evaporate and then rain down. We are not in any danger to contribute meaningfully to this. The theoretical minimum energy to electrolyze 1 liter of water is 16 MJ. Or 16 billion GJ per km3. This is the same as 4444 TWh per km3. The entire world electricity production per year is about 22 TWh, so if we use all of it to electrolyze water we would use up a grand total of 5 cubic kilometers of water. Which later would be released in the atmosphere as water vapor, when we burn up the Hydrogen. That means we would add 1 part in 100 million to the annual water cycle in nature.
- ncmncm 5y ago
  I am corrected. That said, condensing exhaust water vapor in a heat exchanger can benefit operating efficiency.

by ncmncm 5y ago

credit_guy 5y ago

You're overthinking this. Water has a cycle in nature. It evaporates, makes clouds, then condenses and creates precipitations. Over one year, the average precipitation level across the whole planet is about 1 meter (1000 millimeters). Since the Earth area is 500 million km2, then every single year about 500 million cubic kilometers of water evaporate and then rain down. We are not in any danger to contribute meaningfully to this. The theoretical minimum energy to electrolyze 1 liter of water is 16 MJ. Or 16 billion GJ per km3. This is the same as 4444 TWh per km3. The entire world electricity production per year is about 22 TWh, so if we use all of it to electrolyze water we would use up a grand total of 5 cubic kilometers of water. Which later would be released in the atmosphere as water vapor, when we burn up the Hydrogen. That means we would add 1 part in 100 million to the annual water cycle in nature.

ncmncm 5y ago
I am corrected. That said, condensing exhaust water vapor in a heat exchanger can benefit operating efficiency.