First the background: OK - this is a personal project that I've always been interested in trying but never got around to it - a remote controlled hot air balloon.

Sadly I am not technically-inclined and I'd appreciate your guidance. :)

I am aware that the lift generated to make such a balloon airborne is that the air inside the balloon is warmer and therefore less dense than the air outside the balloon.

I have seen oil or petrol burners - even a candle being used to this effect - but would really like to be able to regulate the lift (hopefully direction also) in a responsive fashion.

This is why I am interested in a small electric heater of sorts to heat the surrounding air and generate lift - and also a small fan to quickly draw heat out of the balloon to decrease lift. Not too quickly as I don't want to see it go into an uncontrollable descent. :)

The questions:

  • Could one or more Lithium Ion batteries produce enough energy to generate sufficient heat for this purpose?
  • What kind of heat generating device would probably efficiently convert energy into heat?
  • Any foreseen complications?
  • 1
    \$\begingroup\$ As VC says - chemical fuels contain far more energy per mass. An 18650 LiIon battery is rated at about 2.2Ah and 3.6V mean = about 8 Watt.hours of energy. Hydrocarbon fuels contain typically about 10 kWh of energy per kg. If you can get 25% of this = 2500 Wh /kg then to get the 8 Wh of the LiIon cell you'd need 8/2500 * 1000 =~ 3.5 grams. That's probably 5%-10% the mass of the LiIon battery of equivalent energy. HOWEVER, you idea of using a fan is good. And you could control a valve from a Butane burner. Sounds like fun. \$\endgroup\$
    – Russell McMahon
    May 29 '14 at 13:42
  • \$\begingroup\$ @RussellMcMahon Thank you kindly for confirming VC's verdict on the matter. I am certainly looking at alternatives at this point - a burner 'is' tried and tested. However I must admit that part of me still is looking at ways to make electrical heating feasible - its just on "the backburner" under wishful thinking. :P \$\endgroup\$
    – Avestron
    May 29 '14 at 19:06
  • \$\begingroup\$ Under fault conditions Li batteries can generate quite a bit of heat, but I guess that's not what you want, as it's irreversible (as well as being uncontrolled). \$\endgroup\$ May 29 '14 at 21:55
  • \$\begingroup\$ Like bubbles in water, balloons behave like part of the air-mass they are suspended in; they cannot move through it. Seen from the ground, a balloon goes downwind. Period. The only way to control its direction of travel is to ascend or descend to a level where the air is moving more-or-less in the direction you want to go in. That will mean being able to rise to much higher altitudes than you (and your aviation authorities!) probably want your model to go to. \$\endgroup\$
    – JRobert
    May 29 '14 at 22:23
  • \$\begingroup\$ Don't worry - I am planning on tethered indoor testing initially - and looking into one of those kill-switches in case I get it too far out of range (not planning on it - but never know where or how the wind will blow) \$\endgroup\$
    – Avestron
    May 30 '14 at 5:10

I did not do any calculation at all, but I am pretty sure that a Lixx battery would not last long enough to heat.
The point is this: batteries stores electric energy, that is a quite high level energy. You can use it to run computers, power engines, lights, and to heat things. But batteries have a quite low specific energy.
Rechargeable lithium batteries are under 1 MJ/kg while oil is as high as 46 MJ/kg. This means that oil can provide a lot of energy more.
If you'd want to power an electronic device a battery would probably be a good idea, with oil you'd need a generator, but you want to generate heat. Heat is the lowest quality kind of energy, converting anything in heat is very very simple and above all is very efficient. For oil, you burn it. For electrons, you squeeze them through a resistor.

To answer your questions:

  • not at all
  • any, a properly sized resistor would do
  • see above

You might want to search for propane burners, I bet there's plenty of electronically adjustable burners. To change directions you might want to install some small propellers on your hot air balloon basket, and yes, these might be powered by a battery.

  • \$\begingroup\$ Well thank you for being blunt about the matter (better to get an idea now then after numerous failed tests). Looks like I may need to come up with an alternative plan - thank you for the suggestions. \$\endgroup\$
    – Avestron
    May 29 '14 at 8:39
  • \$\begingroup\$ I believe that some RC commercial models exist, search and see how they work. \$\endgroup\$ May 29 '14 at 8:41
  • \$\begingroup\$ Even this many years later, with improved lithium batteries, every commercially sold RC hot air balloon uses propane. \$\endgroup\$ Mar 13 '18 at 22:21

Since you seem interested specifically in the idea of electric heat giving you more "responsive" control over the lift, there is an issue, which is that fuel power might actually give you more control over lift, because you can control it in smaller time intervals. This is because even if you could magically have the same energy density between fuel and battery (giving the same flight time), the battery would probably have a much lower power density, or ability to generate heat quickly. The control loop would be slower or less stable and you'd be less, not more, able to control the altitude well. In practice this would look like an inability to get the air hot quickly enough when you realize you're in descent, leading to longer downward paths before returning to an upward path.

Think of it this way, you can burn all the fuel at once in a big fireball. A battery cannot give up all the stored energy so quickly even with nearly a dead short across it, due to internal resistance, or slowness of the chemical processes going on inside it, which are much slower to release energy than fuel burning is. Also you'd probably get a very hot heating element which would take a long time to transfer that heat into the air (even with a fan), compared to burning fuel having vastly higher "surface area" for heat transfer to air. In daily life you may notice that an electric stove burner typically heats up the frying pan much more slowly than a gas burner.

On the other hand, both rates are relative to how long it takes the balloon movement to react to warmer air and float higher, which might be very slow. In general the issue is that electric heat would need to be on more of the time. If fuel is, say, 5x faster than electric at raising air temperature, then any part of the flight where the fuel burner duty cycle is more than 1/5 or 20% on time, electric would not be putting out as much heat on a moving average basis, and the balloon might not get enough lift before hitting the ground. Could be that this is not a problem once in the air but that lifting off would seem to take forever and landing softly might be difficult.

  • \$\begingroup\$ Another solid nail into the idea that battery power can be used to feasibly generate lift for a hot air balloon :) Thank you for the confirmation! \$\endgroup\$
    – Avestron
    May 30 '14 at 5:02

The buoyant force in kg of a hot air balloon, at typical altitudes and atmospheric pressure, is

V * (1013 / 2.87) * (1/tempAmbient - 1/tempEnvelope)

where V is in cubic meters and the temperatures are in degrees K (i.e., C + 273).

For example, when V = 1, tempAmbient = 20 deg C, tempEnvelope = 60 deg C, then the buoyant force is 0.145 kg.

How much energy does it take to heat 1 cubic meter of air from 20 to 60 degrees? Its specific heat is about 1.004 kJ/kg.K, and 1 m^3 of air is 1.225 kg, so the energy is 1.004 * 1.225 * (60-20) = 49.2 kJ.

A battery stores about 460 kJ/kg, so the amount of battery needed to store 49.2 kJ is 49.2/460 = 0.11 kg. That's just less than 0.145 kg, so if the rest of the balloon is under 35 grams and if you're lucky, the balloon could just get airborne before the battery completely discharged.

Varying V doesn't change this analysis because both the buoyant force and the battery weight vary linearly with V. But let's change the difference in temperature from 60-20 to 25-20 degrees.

Buoyant force = 0.020 kg.

1.004 * 1.225 * (25-20) = 6.15 kJ energy, so the battery weighs 6.15/460 = 0.013 kg.

It'll float if the rest of the balloon is under 7 grams. Not likely.

OK, let's go the other way, to 90-20. (Ouch, that's hot!)

Buoyant force = 0.232 kg.

1.004 * 1.225 * (90-20) = 86.1 kJ. The battery weighs 86.1/460 = 0.187 kg.

It'll float if the rest of the balloon is under 45 grams.

So, the hotter the envelope, the more likely it'll float. But even a "30C" rated battery takes 2 minutes to discharge safely. That's slow enough for the envelope to lose heat to the outside. I don't know how fast heat is lost across the envelope. But even if you cheat by preheating before launch, the battery might discharge too slowly to overcome that cooling.

(Could we exploit the battery's own "waste heat" as it approaches 60 deg C? Its specific heat is about 0.85 kJ/kg.K. So for the 90 deg C scenario, this extra thermal energy is 0.85 * 0.187 * (60 - 20) = 6.4 kJ. Just 7% more than the original 86.1 kJ, not enough to overcome the weight increase due to the extra fan, fins, etc.)

Achieving liftoff for a few seconds would be barely possible. So you might as well leave the battery on the ground, tethered to the balloon.


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