Is an electrically heated balloon lift support for AWES possible?

Engineering Lifters / Pilot kites
1

Is an electrically heated balloon support for AWES possible?
How much power do we need for enough hot air?
There’s LTA expertise on this forum, so I’m naively hoping this topic can be quickly resolved.

Lets say we wanted a lifting device for just 3000N of constant pulling force, to lift an AWES top end.

We choose an LTA kiteoon arangement… Such as this by Lockheed Martin, forecast to be in use servicing 20 ton payloads by 2019. Ahh so … ohh! Anyway…

Let’s also say we don’t use helium, instead, we use actively heated air.
Let’s also say we can supply the kiteoon with electrical power from our lifted turbine or grid power through anchoring lines…

Question
What would the likely electrical energy requirement be to heat air & maintain this 3000N line pull?

Is this feasible? Industrial space heaters >30kW mostly change from being electrical to LPG or Diesel… Is there an energy density, electrical to air heat transfer law which can’t be resolved or is this a design problem?

From https://en.wikipedia.org/wiki/Hot_air_balloon
The typical burners- produce 2 to 3 MW of heat
That’s for a standard open-bottomed person lifting balloon.
For buoyancy
The total lift for a balloon of 100,000 ft³ heated to (99 °C, 210 °F) would be 1595 lb, 723.5 kg. Double our requirement.
For typical atmospheric conditions (20 °C, 68 °F), a hot air balloon heated to (99 °C, 210 °F) requires about 3.91 m³ of envelope volume to lift 1 kilogram (62.5 ft³/lb).
3.91 * 300 =1173m3 for our balloon maintained at 99 °C with only hot air lift.

So we need to know the very basics…
How efficient and lightweight could we make the internal air heating element with electrical heating?
How insulating can we make the skin - skins?
How much control of lift does a kiteoon offer? vs safety, vs reliability, vs complexity…

Would heat being lost to the atmosphere be considered counterproductive?.. likely yes. Although if the energy is initially extracted from the air… fair’s fair.

Standard balloon envelopes lose a lot of heat… There are improvements.


https://www.festo.com/net/SupportPortal/Files/42096/Iso_Ballon_en.pdf (See relevant expert… Alexander Bormann)

Balloontype: M - 105
Volume: 3000 m3
Envelope weight: 158 kg
Passengers: 4 – 5
Fabric insulation: aerofabríx® [Iso] 45 | 4.5
Thickness: 6 mm
Specifi c weight: 45 g / m2
Thermal conductivity: 0,0265 W / mK
U value (6 mm): 4,5 W / m2
K Max. operating temperature: 140° C
Brand designation: aerofabríx® is a brand of Dr. - Ing. Alexander Bormann, Berlin, Germany
(Yes the same Alexander Bormann)

According to
UNIVERSITY OF CALIFORNIA,IRVINE
Heat Transfer Model for Hot Air Balloons
THESIS submitted in partial satisfaction of the requirements for the degree of
MASTER OF SCIENCE in Mechanical and Aerospace Engineering
by Adriana Lladó-Gambín


70% of the heat losses are due to the emitted
radiation from the balloon envelope and that convection losses represent around 20% of the
total.
Burner Duty cycle varies from 10 to 28%
The fabric weight is approximately 65 𝑔𝑟/𝑚2, its thickness 0.1 𝑚𝑚, and its thermal
conductivity can vary from 0.003 to 0.04 𝑊/𝑚𝐾 depending on the fabric color and the
temperature [8].
[8] W. Hallmann and U. Herrmann, “Energy Losses, Porosity, Strength and Stretch
Behavior of Hot Air Balloon Fabrics Subject to Temperature Loads and Ultraviolet
Radiation,” in International Symposium on Hot Air Aerostatic Vehicle Technology,
1991.

I guess a larger volume means lower surface area ratio… So, relatively less heat loss. So going big might be attractive but it means a harder to control lift kite component… Kites lift helps to compliment the drag down of the balloon in high wind.

Your thoughts please…
Is an electrically heated balloon support for AWES possible?

2 Likes
2

It was this picture got me thinking …


I imagined those disks on the bottom as heat exchangers…
I’m supposing a sealed heater inside the envelope is better?

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Always been impressed by @PierreB 's ballon jumping … but a solar balloon is only so reliable in Scotland.
The mix of solar to heat and inflate the balloon, then a driven Magnus effect addition is interesting and covered on this forum.
I’m hoping this thread can cover the direct heating of air inside the envelope by electrical equipment.

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As Wubbo insisted, we can design AWES in any form our engineering talent allows. Peter Lynn’s giant kites tend to float by solar gain. Its just a very marginal capability in high wind, when kite lift is strongest; or at night, sunless.

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It’s an interesting thought.

I see some possibilities:

  • Keeping a power plant aloft during zero wind, powered from the ground
  • Increasing lift in low wind situations, powered by onboard power generation

A single hot air balloon could perhaps keep a few more traditional kites airborne during lulls. Eg. one hot air lifter for a larger canopy. Excess heat could be used as an energy storage.

I am not in a position to say anything about feasibility of this idea unfortunately.

Not to start with the problems presented, but it seems maintaining a large inflated structure airborne during high wind seems a major one. One option would be to ground the hot air lifter during medium to high wind situations, perhaps even deflated. If it was powered from the ground, the tether weight would probably be a significant factor, but the altitude would not have to be higher than necessary in order for no lifter kites to touch the ground.

I guess the Helikite (Alsopp) could be an inspiration, combining lighter than air with aerodynamic lift when there is wind. Windy situations will surely increase energy loss (increased cooling), and these situations might be solved by foregoing lighter than air lift for aerodynamic lift.

Wikipedia states that 4 m3 is necessary to lift 1 kg.

This article states that one hour of operation of a hot air ballon of 77.000 ft3 will spend 30 gallons of liquid propane. In SI units the balloon is 2.200 m3 (2-3 person?, 550 kg lift) and average energy expenditure 930 kW. That should be a good starting point for making some assumptions. This page states the properties of liquid propane.

E_{burner} \approx 30 \, \mathrm{gal} \cdot 4.54609 \frac{\mathrm{gal} }{\mathrm{l}} \cdot 0.493 \cdot \frac{\mathrm{kg}}{\mathrm{l}} / \left( 44.097 \frac{\mathrm{kg} }{1000 \cdot \mathrm{mol}} \right) \cdot 2.2 \frac{\mathrm{MJ} }{\mathrm{mol}} \cdot 0.2777 \frac{\mathrm{kWh} }{\mathrm{MJ}} \approx 930 \mathrm{kWh}

Here is a claim to insulate a hot air baloon to dramatically decrease energy losses (also using steam rather than hot air).

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There is little value to keeping a WECS kite up in calm, with no production, if kite landing and relaunching is effective. LTA operation is expensive. Winch step-towing (reverse-pumping) is a competitive alternative for keeping kites up in calm.

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You’d really need to do a CFD analyis using ANSYS or the like to get a feel for it, because the (humid) air inside the balloon will not be stagnant because of convection. The less well your envelope insulates or the larger the temperature differences, the more the air moves, and the less well you’ll be able to make use of the (stagnant) air inside the balloon as an insulator. If you’re using humid air, you’d like to keep the temperature of the material that envelops that above the dew point to avoid condensation.

You could try some back of the napkin calculations. Pick a constant inside and outside air temperature, and thickness and thermal conductivity of your envelope, then play with this equation: https://www.khanacademy.org/science/physics/thermodynamics/specific-heat-and-heat-transfer/a/what-is-thermal-conductivity

What is lost must be supplied by your heat source. Excluding extra heat loss from the influence of wind, I think this is a worst case scenario because in some places the air that is closer to the envelope inside of the balloon may be colder than that elsewhere (near the top of the balloon or close to the heat source) and it doesn’t take into account the extra insulation stagnant air inside the balloon may give, if you manage to get stagnant air inside the balloon.

I personally think this is a dead end. I think using a hydrogen balloon would be less of a dead end.

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CFD is not required to just “get a feel” for this. For an ordinary balloon shape, the convection pattern will be a simple torus. Simple fabric baffles would interfere with convection losses. Soft-good principles are enough of an analytic “feel” for baffle design.

Heated air is low humidity, as cold air starts at dew-point. Both solar and lifting-gas LTA are dead ends in AWE, except in small niche roles, like kytoon design, where He is favored.

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Yes, you’d use a closed envelope and raise humidity by releasing steam, for example.

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Steam adds a further complication to electric or solar gain, as if KIS did not count.

What’s needed is for someone who is attracted to this marginal AWES concept space to develop prototypes, to settle speculative doubts.

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With electric heating you don’t need to rely on a single heat source. You can distribute the heat sources inside your balloon, and their output, and see how that effects convection. Also with the addition of baffles or the like we’ve already left the “normal balloon shape” behind.

If you’re going to rotate the balloon, that also likely will have some interesting consequences. Will you better be able to keep the hotter air in the center of the balloon then for example, just like in zero gravity?

There you go again making up words. It’s KISS.

While remembering rule #14, raising humidity level is relatively trivial compared to the other challenges.

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This may be a futile chase…
Say we had a 100kW air heater in our balloon…
How much more lift will we get from a 100kW drone/kite?

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Indeed (for example by using a Flettner rotor) an effect of the centrifugal force would be to push the heavier colder air against the walls of the balloon, adding some insulation.

As a favorable result.

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I would rather expect the rotation would lead to thorough mixing of the air, in turn increasing heat losses.

With the large amount of energy losses and also other technical challenges, I am just now afraid the more direct approach using propellers is more feasible (and still no lighter than air volumes/kites/balloons)

My previous calculations hinted at 1 kW per kg to remain airborne (for rather large balloons).

This page says less than 0.25 kW per kg lifted (for quadcopters). My guess is that a kite with thrust could require even a lot less than this.

The calculation is more complex due to the weight of the lifting gear vs what payload it could take. But it seems a uphill struggle for lighter than air.

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Maybe or probably. To me an interesting question is, is it possible to make a balloon so that isn’t the case?

First questions perhaps are, ignoring the temperature gradient, what happens to the air inside a balloon when you rotate the balloon (faster and faster) and how can you influence that behaviour? Could you think of similar questions or systems that might inform this?

That calculation used a conventional hot air balloon as a starting point. It would be foolish to try to use those to lift an AWE as they are so horribly inefficient.

For a hot air balloon, not for all LTA. That’s not the question though, the question is: is it possible (in theory)?

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The better the balloon is insulated, the smaller the temperature and density differences (and the lower the convection), so the smaller the effect this may have, if the effect is significant at all. @Rodread knows about spinning things so it might be easy for him to calculate the difference in centrifugal force over relevant gas densities, balloon diameters, and rotational speeds.

My initial thinking on rotation was that it might interfere with the hotter air rising.

Edit: another way you could try to make use of rotation is to try to make any insulating material (like soap bubbles or sacks filled with air, helium or nitrogen) stick to the sides instead of falling to the bottom.

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I rather thought about some solar balloons like
ballons solaires photo
The transparent envelope is not specially insulating but if the black envelope is external losses by convection would be huge due to the wind when the balloon is motionless.

Assuming the black wall (or another internal heat source) is not moving the effect of the rotation would be pushing the colder air against the transparent envelope, decreasing losses by convection. But some negative effects could also occur.

Beside it lifting support by electrically heated balloon or LTA is possible but obviously not cost effective.

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My practical opinion on this is that currently hot air balloons won’t work. Wait for someone to develop something and try that maybe. But even then it probably won’t be cost effective.

Hydrogen balloons will work and should be cost effective and an application like yours I think is an ideal use case for them, if you make them strong enough and small enough to withstand higher winds. I think they are also legal to use if you won’t be carrying people. https://sites.google.com/site/airshipwiki/the-team

1 Like
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Hydrogen balloons are not cost effective due to the high level of maintenance (to start by raising and lowering them according to the weather) and by requirement of heavy containers, and are dangerous.