3 ms·

Water is not even remotely an unlimited resource: https://www.businessinsider.com/adam-savage-scientific-fact-blows-my-mind-buckminster-fuller-oceans-water-201

• jvanderbot 4y ago
  Water vapor condenses and falls as rain. It does not linger in the atmo like CO2 does.
• credit_guy 4y ago
  For hydrogen production, water is for all practical purposes an unlimited source. To get a sense for that it's cute to use a unit of volume of 1 cubic kilometer. The world consumes about 36 billion barrels of oil per year, which comes to about 5.7 cubic kilometers. As a very strange coincidence, we consume about 3800 billion cubic meters of natural gas; if we were to liquefy it, it would come to virtually the same volume like the oil, more precisely 5.9 cubic kilometers. All in all, let's say we consume about 10 cubic kilometers of fossil fuels in liquid form. Water contains about the same amount of hydrogen per unit volume as oil or LNG. So, let's say (in an absurd extreme) that all of the world's crude oil and natural gas use goes towards hydrogen production, and we need to replace that with water, and for some reason that water is gone after 1 use (it's not), then we need to use 10 cubic kilometers of water per year. The world oceans have about 1.3 billion cubic kilometers of water.
• ncmncm 4y 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 4y 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 4y ago
      I am corrected. That said, condensing exhaust water vapor in a heat exchanger can benefit operating efficiency.