Is an electrically heated balloon lift support for AWES possible?

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.

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.

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.

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

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.

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.

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?

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.

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.

1 Like

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)?

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.

1 Like

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.

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

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.

https://forum.awesystems.info/t/an-alternative-method-to-tap-high-altitude-winds/949/14?u=pierreb

See the “Table 5 Reference photovoltaic plant considered for hydrogen production”.
As the photovoltaic plant is onboard, some dangerous handling can be avoided.

All things considered Long permanence high altitude airships: the opportunity of hydrogen is an interesting track for all AWES.

“KIS” rather than “KISS”, politely removes the final “S” for “Stupid”. Its more professional that way.

Let those who imagine balloons to be relevant to AWE test them. Kytoons date back a century or more. KiteShip and KiteLab Portland developed several rather nice Kytoons in 2007, but they were not found worth LTA operational burdens. When I traveled West to join KiteShip, a helium regulator was in my single bag of luggage.

image

1 Like

Windy Skies: “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.”

This approach to electric heating adds a lot of wiring mass. The solar panels would likely be rationed on one side to save mass and cost. Internal baffles need not change the external shape.

I am not recommending any fixes here, just pointing out known barriers. Internal CFD cannot give a feel for how marginal the whole architecture is, just distract.

This sort of fringe AWES concept comes down to the lack of available talent to prove how poor it is, by at least making a compelling attempt.

Passive solar is still marginal, but at least saves the cost and mass of the PV-heater cycle.

Though LTA is an interesting thought, I don’t see much practicality in it. There are a few more practical options imho:

  • power a lifter by electric/energy from the ground
  • power the kite mechanically from the ground by tugging the tethers
  • have a lifter so high that 24/7 wind is guaranteed

The first two options will require all losses to be continually replenished. For a wing, the glide number (lift to drag ratio) describes the amount of losses per generated lift. High glide numbers will account for less power used during lulls. If the tether is very long and moving through the air, the drag is quite large and the power used to stay airborne during lulls will probably offset the power produced otherwise.

For electrical power transmission, the conductive tether adds substantial weight further increasing no-wind power consumption.

If you want a permanent lifter to stay at an high altitude, «dancing kites» with really high efficiency seems the only option worth pursuing to me.

If the lifter must stay airborne during the worst storms, I would argue that most huge superlightweight structures (such as LTA) are just not feasible. Unless you accept blowdown, such structures must have a certain glide number in high winds that will determine the elevation angle of the tether. The problem is reducing the drag of such structures, and once drag is minimized, how to sustain the lift that must be somewhat larger than the drag, and increasing with windspeed to the power of two.

I just dont see it happening for energy producing kites at scale.

For now at least, I am content pursuing AWE that is landed for low winds and high winds. It’s the pragmatic approach. Automatic landing and launch is an unsexy option, but compared to these other options, it is doable without inventing a new technology.

The pragmatic solution: either a simple tower or motorized drone to initiate launch, perhaps as a taxi that detached from the energy producing rig or built into the rig itself. Russian doll «Matrushka» increase in size of lifter kites may be a path to huge scale without weight and cost penalties.

Now if we still want to pursue LTA or ground based energized kites, perhaps they could be feasible at lowest possible altitude to keep the drag and weight to a minimum. For LTA gas in the form og hydrogen could be supplied through a tube and the gas could be produced at the ground. Once the wind blows, it could turn into a kite using ram-air construction and able to supply considerable utility lift.

All this being said, a simple tower/mast or a taxi drone seem the most viable options right now.

2 Likes

That’s an artificial and unnecessary requirement. Without that requirement current technology is already capable, even if too expensive for the application.

+1

No mention I think in this thread of airborne PV panels + hot air balloons.

If you want to make a hot air balloon that doesn’t lose so much heat, you’ll have to rely on stagnant gas. There’s different approaches you could try for that, and CFD analyses of your designs is the quickest way to weed out the non-starters, and refine your thinking. I also invoke rule #17.

WIndy Skies: “No mention I think in this thread of airborne PV panels + hot air balloons.”

“PV-heater cycle” is a definite mention-

Let Active Solar be understood as a hot air balloon heated by PV panels. CFD is not “the quickest way to weed out non-starters, and refine…thinking”. LTA/AWE heuristics have already defined this architectural space as unpromising. Let CFD naturally continue to languish as a poor predictive tool in AWES design.