What are pro/cons of steam as lifting gas for airships?

Question

I was reading Wikipedia page about hot air airship and another Wikipedia page about the first powered and steerable airship which was powered by a steam engine. So I ask myself "why didn't they use the exhaust steam from the steam engine as lifting gas?"

After quick research, I found this home made experiment of steam used as the lifting gas. This website even gives a comparison with other lifting gasses:

+-------------+----------+------------+-----------------+--------------------+--------+-----------+-------------------+------------------+
|     GAS     |   M.W.   | Temp. (°C) | Density (kg/m3) | Lift (N/m3) in ISA | Safety |   Cost    | Ease of provision | Buoyancy control |
+-------------+----------+------------+-----------------+--------------------+--------+-----------+-------------------+------------------+
| H2          | 2        | 15         | 0.084           | 11.19              | bad    | fair      | fair              | no               |
| He          | 4        | 15         | 0.169           | 10.36              | good   | very high | very bad          | no               |
| CH4         | 16       | 15         | 0.676           | 5.39               | bad    | low       | fair              | no               |
| NH3         | 17       | 15         | 0.718           | 4.97               | fair   | low       | fair              | no               |
| hot air     | 29 (avg) | 110  (avg) | 0.921 (avg)     | 2.98 (avg)         | good   | very low  | good              | yes              |
| steam (H2O) | 18       | 100        | 0.587           | 6.26               | good   | very low  | good              | yes              |
+-------------+----------+------------+-----------------+--------------------+--------+-----------+-------------------+------------------+

(source: http://flyingkettle.com/jbfa.htm)

Given the conclusion of this website, steam should now be more widely used as a lifting gas:

• it is available since the first lighter-than-air (plenty of time to develop the technology)
• it has a good lifting power
• it is cheap and safe

Yet, on 2020, almost all airships and balloons use helium or hot air, and no other lifting gas. So there must be drawbacks I fail to imagine. I hope finding those drawback will orient my research for my first question ("why didn't they use exhaust steam as lifting gas on airship fitted with steam engines?"). Moreover, there may be advantages I also fail to imagine.

Thus, what are pro and cons of using steam as a lifting gas?

Answer 1

The main disadvantage is the energy required just to get off the ground. Let's do a back-of-the-envelope calculation, comparing this to a hot air balloon.

• Let's heat 1m³ of air. To get equivalent lift, we need about 0.5m³ of steam.
• The density of air is about twice the density of steam. So, for a given amount of lift, we need to heat approximately the same mass of lifting medium.

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• To heat 1kg of air to 100°C above ambient, we need about 100kJ (specific heat of air is about 1kg/(kJ K)).

• To heat 1kg of water to 100° above ambient, we need about 420kJ just to heat the water. To vaporize the water, we need an additional 2250kJ, for a grand total of 2670kJ.

The most used hot air balloon size is 2800m³. For that we need to heat 1400kg water. Multiplying gives about 4GJ of energy or 75kg of propane just to get off the ground. Since we established steam can lift twice as much per unit of volume, we can maybe get away with 2GJ or 35kg of propane.

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Some more negative thinking... Condensation on the outer shell seems like a very efficient mechanism to transport heat energy to the environment, only partially offset by the reduction in surface area compared to a hot air balloon due to the cube-square law. This may require more energy in flight than a hot air balloon. Closed-cycle operation (only heat condensed water, not 'new' water) is preferred due to the high energy requirement, so descending means you have to wait for enough water to condense or be willing to pay the price in fuel. Superheated steam is not safe (despite what your table says); the high heat capacity and enthalpy of condensation can result in serious scalding burns compared to hot air. All of these effects are of course much more pronounced when comparing to a helium-filled blimp.

Still, it's a really neat idea, and sometimes you don't need a practical reason just to build something neat.

Answer 2

In short, helium (or hydrogen) remains in the gaseous state at much, much lower temperatures than water does. In particular, helium or hydrogen will remain gaseous in pretty much any part of Earth's atmosphere. Water, on the other hand, will not naturally remain gaseous in any part of the Earth's atmosphere. On the contrary, it will freeze in most of the Earth's atmosphere.

For steam to remain steam, it must be constantly heated to make up for the energy it's losing by heat transfer to its surroundings. You can aid this by insulating the airship really well, but you can't bring the heat transfer down to zero. And this will also add a significant amount of weight, reducing efficiency further.

Additionally, as Sanchises's answer mentions, it takes a large amount of energy to turn that water into steam in the first place. Water has a very high specific heat and also a high enthalpy of vaporization. That is, water takes much more energy per unit mass than most substances to heat up and also more energy per unit mass to convert from a liquid to a gas.

So, you're requiring a very large amount of energy to get the water into the gaseous state in the first place and you then need a large and continuous energy source to keep it there during flight.

Answer 3

Lighter than air balloon works by replacing air with some gas that is lighter than air. Your steam airship would work if we can keep steam at 100 C or higher or else you risk condensation, your balloon collapse into some saggy bag and you fall out of the sky. Wikipedia mentions that when water turns into steam it expand 1700 times so imagine, in reverse, your 1700 cu.m. hot steam balloon when cool down and becomes 1 cu.m. bag of water and you lose all the lift. That does not sound safe at all. Consider low temperature up there you need a lot of energy to combat heat loss through conduction over a large surface area of the gas bag.

To sum it up you need lots of energy to keep the balloon in positive buoyancy and a big safety issue to solve when that energy source fail.

Answer 4

In addition to the tremendous energy requirements, you'd need a highly specialized design that is not at all bothered by boiling hot water dripping back out of the balloon. Even if it was highly insolated, you'd need to be resupplying steam to keep the balloon inflated because even small amounts of local condensation would pull a lot of water out of the air because water expands so much when it becomes steam.

Even like, a cup of water running back down the balloon over the course of the entire voyage would be over 100 gallons of steam. This in general would be hard to build, fuel inefficient and difficult to control while being wildly dangerous.

They didn't use exhuast steam because the efficiency of a balloon isn't acheived by min/maxing the displacement of air by any means possible. It is acheived by realiability, cost and control. Blasting a second gas as a by product of the steam engine, doesn't improve that in the slightest, and likely would create problems.

Answer 5

I think if you would use a doble type flash boiler wich is a light steam generator (it was even used to power a plane) and you use the envelope to condence the steam and then have it gather at the bottom to be pumped into the generator again then you can reuse the water and the envelope doesnt need to be insulated. Also the steam that comes out of a flash boiler is dry superheated steam wich is super hot and is generated instantly so as soon as it comes into the open space of the envelope it is able to expand reducing presure and temperature and so creafing lift. Much like a hot air balloon but the cold air wich in this case is water is reused again. Maybe a metal clad envelope would be better suited than a balloon because it cant deflate.