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So, I know that battery powered planes aren't practical, because there just isn't enough power to weight. However, What about an airship? Airships don't have to move to stay aloft.

How would this work for a realistic airship with a big baloon filled with hydrogen or helium gas?

I specified a liquid-metal battery because I just learned about them, and I've been told they have a higher power output than lithium-ion batteries and without degredation over time (here's more on the subject: https://news.utexas.edu/2020/07/06/new-room-temperature-liquid-metal-battery-could-be-the-path-to-powering-the-future/), but if there's a better means of power storage available, I'd like to hear about that, too.

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    $\begingroup$ "I want to know if the energy storage to weight ratio is comparable to a diesel-electric setup": Being generous, a liquid–metal battery may have an energy density of 3 MJ/kg. Diesel fuel has an energy density of 45 MJ/kg. Let's the diesel engine has an efficiency of about 40%; overall, the energy density of the diesel power plant will be about 6 times higher than that of the battery. (And why diesel-electric? Actual airships used diesel engines directly, without the electric part.) $\endgroup$
    – AlexP
    Commented Jan 18, 2021 at 0:02
  • $\begingroup$ @Tantalus’ touch I was asking about multiple scenarios... in either case, the main question is wether the batteries provide enough power (mostly for propulsion) to justify their weight on an airship... but going back to the question, that second bit doesn’t really add anything useful, does it? I will modify the question. $\endgroup$
    – Globin347
    Commented Jan 18, 2021 at 0:02

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There is a legitimate case for electrically-powered airships:

  • Unlike heavier-than-air aircraft, airship range does not significantly increase as fuel mass decreases. Yes, a fuel-powered airship that's almost empty would be slightly more agile and faster compared to its full state, but due to the inevitably poor aerodynamics of such a craft, it wouldn't be that much of a boost in range.

  • Electrical power enables on-the-go recharging and potentially infinite range. With solar cells getting thinner and lighter every day, it's only logical to coat the top of the aircraft in solar cells. Not only can the airship then fly above clouds and weather to get full-day sun, but solar intensity is also higher at higher altitudes because there's less atmosphere between the sun and the panel. So long as you can keep lifting gas in the bag, your airship can fly forever with solar power.

  • Electrical motors have advantages over combustion engines at airship altitudes:

    • Electrical motors work better in cold conditions and don't require warmers, chokes, or fluids specially adapted for cold conditions

    • A brushless electric motor is mechanically simpler, has less moving parts, and is more reliable than a combustion motor

    • Electric motors are generally lighter than their combustion counterparts especially considering that they don't need gearboxes or clutches and are capable of exerting maximum torque even at a standstill

    • Combustion engines can simply die at high altitudes because there isn't enough air pressure to feed them enough oxygen to successfully perform combustion. Especially since an airship is slow and ram-air intakes aren't possible, collecting enough air would be enegy-intensive. A fueled, high-altitude airship would potentially need to bring oxidizer along with fuel

EDIT: Should be noted that "Liquid metal" is not the right battery technology to use on an airship--its primary benefit is how (potentially) cheap it is and thus well-suited for large scale grid energy storage. They run very hot and are not built to be lightweight or extremely energy dense: li-ion or li-po batteries would be better for an airship.

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  • $\begingroup$ Airships and high altitudes do not mix. There numerous problems with trying to fly an airship at high altitude. (For example, what do you do with the lifting gas? Allow it to expand? Vent it? Do you provide a pressurized cabin to avoid passengers dying? How do you manage the loss of lifting force?) $\endgroup$
    – AlexP
    Commented Jan 18, 2021 at 14:14
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    $\begingroup$ @AlexP During WWI, zeppelins frequently operated at altitudes over 6000 meters to perform bombing runs, and that was over 100 years ago. Sure, it's not jetliner-cruising altitude, but it's still high enough to fly in permanent sun and we could do better with today's technology compared to in early 1900s. Similarly, building a pressurized cabin isn't difficult by modern standards and something you're going to need to do anyways. As for the lifting gas, you can just compress it into storage to maintain gas envelope volume. Lifting force will be lost, but that can be compensated for in other ways $\endgroup$
    – Dragongeek
    Commented Jan 18, 2021 at 14:34
  • $\begingroup$ @AlexP 1.Pump it into tanks 2.Yes 3.Stop gaining altitude. Consider the parameters of all of the above when designing the ship. $\endgroup$
    – clyf
    Commented Jan 20, 2021 at 0:23
  • $\begingroup$ Aircraft piston engines don't have clutches and most don't have gearboxes either, but have the propeller bolted directly to the crank shaft. In contrast I expect any aviation electric motor to have a reduction gearbox. It will still be much simpler, but lack gearbox and clutch are not the reason. $\endgroup$
    – Jan Hudec
    Commented Nov 14, 2021 at 12:39
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Since your airship isn't relying on power to remain aloft the energy requirements are not only greatly reduced but the energy density of your storage becomes essentially irrelevant. You could stick a ton (or several tons) of lead acid batteries on your airship and still get it in the air if the envelope volume is large enough.

Of course bigger is slower, and this is where you'll get some gains from liquid metal batteries. Higher energy density means less overall mass, smaller envelopes, lighter frames, less drag... which adds up to more speed and agilty. Small, agile airships zipping around sounds kind of interesting, no?

The higher the energy density of the storage the better. Which is why hybrid ultracapacitor/fuel cell storage might be an even better option.

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    $\begingroup$ "Small, agile airships zipping around sounds kind of interesting, no?" - nup, no - reason is mass aka payload it can carry is physically limited by air density. the density of batteries indeed is irrelevant, energy to mass ratio however is not. in the answer, there is good thinking on which u may improve it $\endgroup$
    – MolbOrg
    Commented Jan 30, 2021 at 20:38

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