# Is it possible for human powered hot air balloon?

1. Jun 8, 2013

### ThainaYu

Hello everybody. I am just a man who interest in physics. And with the question I concern above is the reason I join this forum. Nice to meet you all

To the main point
I want to know exactly that, Is energy of one person enough to generate hot air for lift balloon system and himself?

By any means, with the main part from pedal and gear. Convert mechanical energy to heat the air into balloon. So we can fly with our energy created buoyancy force

I want to create such system. But first I need the possibility of theory. The calculation is too complicate, it need to calculate the volume of air a person can heat and leakage of heat to atmosphere

And the most important part is, which way is most efficient to convert mechanical energy into heating air?
I have design mostly electrical, pedal dynamo to nichrome coil and copper net plus Peltier heat pump(to cool air outside the balloon)
But there may be more efficient way, like fluid friction? or anything else?

I have approximate that human able to generate average 200 watt from pedal system
And overall gadget, bike skeleton and dynamo also the propeller and heat producer maybe 120 kg
But it more complicate to calculate the weight of envelop need to filled the hot air
So I need to lend you all wisdom

Last edited: Jun 8, 2013
2. Jun 8, 2013

### Staff: Mentor

Google for the information on the typical hot gas balloon burner BTU. Or check how much gas (probably propane/butane) they use for each few hours flight and convert it to power. Then compare the number you get to 200W.

I guess I know the answer without checking.

3. Jun 8, 2013

### VantagePoint72

"Watt hours" are a unit of energy, not power.

4. Jun 8, 2013

### ThainaYu

Burner type hot air balloon has big and heavy burner system, include many heavy propane tanks and the basket. So it really need very hot air and too much BTU while we just need to lift 1 person

Alternatively, there are a pure solar hotair balloon made from just black sheet to heat air inside and with enough size it can lift a person

On the other hand. If I can make this system into real use. Maybe I could use house electric power to filled the initial air, just that It need to unplugged and able to harness rider power to keep flying a long trip

Also there are human powered flying vehicle in other kind. I just think balloon buoyancy is more safe and stable

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Thanks for the checking. Yes, I actually want to mean 200 watts, not watt hours (And fixed that already)

Last edited: Jun 8, 2013
5. Jun 8, 2013

### Staff: Mentor

So how much lighter will your system be than typical? 1/10th? 1/20th? You can still ballpark it like Borek suggested.

6. Jun 8, 2013

### ThainaYu

About 1/3 to 1/5 I guess

7. Jun 8, 2013

### Staff: Mentor

That's not enough lighter. The lack of other people in the balloon alone will be 1/5. Then you're going to make a smaller balloon and basket.

Lets look at this from the other direction: the weight of the heater. Now, you want to do a pedal power system. Ok, that's good. The exact nature of the system doesn't matter: all are exactly 100% efficient at making heat. What matters is making it light enough. I'm guessing the minimum weight you could get it to would be about 20 kg since it has to at least be heavy enough to support your own weight.

So here's a camp stove that weighs less than a kg and puts out 3,000 Watts: http://www.target.com/p/single-burn...948523&LID=PA&ci_src=17588969&ci_sku=10948523

So you see, you're off by several orders of magnitude in what you could do human powered vs fuel powered.

8. Jun 8, 2013

### ThainaYu

I'm actually not do human powered vs fuel powered
I just proposed that human powered could be enough for just create lifting power

While airship stove or just camp stove could do it, that's too much more than enough
I just think human powered could be just enough if we design proper engine and That's why

We all know that household electric could charge cellphone and laptop easily. But human powered generator is enough to do that too, that's what I want from my idea, is it "just enough" or not?

9. Jun 8, 2013

### VantagePoint72

Why do you suppose that a gas burner is "more than enough"? Given the burner and tank adds mass to the system, I would expect that something smaller would be used if it were overkill. It would be much more economical. Your analogy is invalid: we need the extra power available from the household mains because we also do things that require more power than charging your cellphone. However, the burner on a balloon doesn't do anything else. If something smaller could do the job, that's probably what they'd use.

So, I think it is a very reasonable assumption that the amount of heat required to keep a balloon in the air is comparable to the power supplied by the gas burners used in balloons. Otherwise it's pretty bad engineering. Hence, whatever your engine is, it will need to produce roughly the same amount of heat, which means operating at roughly the same amount of power. Since that power far exceeds what a human can produce, the proposal isn't feasible. No amount of work trying to "design proper engine" can overcome the limitations imposed by the laws of thermodynamics.

10. Jun 8, 2013

### Staff: Mentor

Agreed. It is illogical to think that the burner in a hot air balloon is a hundred thousand times bigger than it needs to be. Heck, if it were, it would make the balloon too hard to control.

11. Jun 8, 2013

### ThainaYu

I think what the gas burner for balloon is designed by this standard model and BTU is because the speed to gain or change altitude, speed of air filling, and for other things that need to carried along include safety, main point is convenience not because it required

Info that I have gathered is actual balloon would just drop the fire to minimum level when the altitude is enough for rider. And it would need to scale up again when they want to move higher. The average temperature of air while lifting balloon is around 75 c and to keep stable altitude is just around 40 c

Just so you know, There are already helium based balloon pedal copter, just filled helium inside balloon and you can fly UP. The pedal of this system make you go forward with back propeller
And Just so you know, Solar power is enough to heat black colored balloon to the point that could lift person. Just lifting person with balloon not really need gas burner, just large enough black sheet can gather sun heat to the point that enough to do it

So we not really need "same amount of heat" to gas burner to just lift things with just 1 m/s^2 antigravity acceleration (which is enough in my thought). Buoyancy somehow difference from direct conversion from mechanical to lifting. We just need to keep overall density and let the atmosphere hold us

The key factor is thermal insulation of envelop fabric that keep the power converted from us to build up inside. We need to calculated that how fast this material will leak the heat out. And do human power could cover that leakage. So I think trying to compare with burner is miss the point

12. Jun 8, 2013

### VantagePoint72

You didn't ask about a human powered dirigible, or a solar powered hot air balloon. You asked about a human powered hot air balloon. If you want to build a giant helium balloon with a pedal powered propeller, go for it. That doesn't invalidate anything you've been told. I'm not sure what you're attempting to prove with the solar example: the sun's wattage per square meter alone far exceeds a person's output, so it's hardly surprising that a large enough black envelope can lift a person. Again, that's not what you asked. You asked if a human can produce the power necessary to generate buoyant lift like a hot air balloon's engine provides. The answer is 'probably not'.

Again, do you think hot air balloon makers just decided willy-nilly to use a certain material without bothering to check its thermal conductivity? It's not just the insulation ability that matters: the weight of it is extremely important. They use a fabric that has a good mix of being lightweight and capable of holding in its thermal energy. Calculating how fast this material leaks the heat out is exactly what they do when determining how powerful the burner needs to be to cover the leakage. You seem to believe all engineers are very stupid.

13. Jun 8, 2013

### ThainaYu

I just take the real case that there are many kind of alternative to "just lift a person" without gas burner. Helium is one, Solar is two, and My idea is another that I'm not sure is it actually possible or just near possible

If we include non buoyancy based, there are human powered plane and human powered copter that just direct the mechanical to lifting force and still able to use human power to lift whole system. So theoretically human could use their sole power to lift themselves to the air with proper engine created from light weight material. And I try to explain this info because it could support that my idea is not theoretically too far from possible and never break any thermodynamic law

The fabric of balloon using nowaday is what I thought it best possible material too. I just want to point out that the burner system is far too much from the need of lifting. It designed for convenience and speed as I already said. To just lift a person we could use smaller balloon. So the weight of whole and volume of air that need to heat is lesser

But the problem is human power is very low compare to burner, so even the little % of leakage is more important compare to power that human could produce. Human could make average 200 watt so if the heat would leak just 300 watt then my idea fail instantly. Compare to burner that can generate 15 million BTU then such leakage will never be a problem. In fact it can lift whole balloon with high speed easily and even shut it down for many minute it can opened and make lift in high speed again

I think I need to repeat again. I never think Engineer who make the balloon and burner is stupid. But they make it with many factor for convenience to use
Burner is over the need to lift but it need for speed up the lifting process
Just black sheet and sunlight is enough to lift but it much slower than burner. People who want to fly don't want to wait for a hour to let the balloon bath in sunlight before use it and that's why they use burner

If you need to compare I think at least you should compare my system with the black balloon bath in sunlight, not burner

I really don't understand why it too hard to communicate like this. Is my English is not correct? I'm so sorry I'm not native speaker but if you have some curious can you ask me first?

I see you already miss the point when you think I do man power vs fuel power. I never think that way. Why you underestimate me too much. Do you think I am stupid?

Last edited: Jun 8, 2013
14. Jun 8, 2013

### Staff: Mentor

Re Solar: you should Google the power density of sunlight to see how much black fabric you need to absorb 200W.

15. Jun 8, 2013

### rcgldr

The human powered planes were powered by elite bicyclists that could generate about 185 to 245 watts for an hour or more. No human powered helicopter has been able to achieve more then a short period of ground effect lift.

http://en.wikipedia.org/wiki/Gossamer_Albatross

16. Jun 8, 2013

### ThainaYu

Yes I know. But the actual problem is material and structure that still too heavy for lifting unless using power from professional cyclist. There are still room for development in both material and design. I just proposing alternative way to store man power into heat for buoyancy

17. Jun 8, 2013

### Staff: Mentor

No, the limitation is not the weight of the aircraft it is the weight if the pilot: the pilot is significantly heavier than the craft. Since the pilot weighs twice as much as the plane, even if the plane weighed nothing it would still take a professional cyclist to power it.

18. Jun 8, 2013

### Staff: Mentor

Commercial (sight-seeing) balloons are not optimized for the best lift to power ratio. They don't have to, fuel is cheap relative to the crew and the balloon. With more expensive materials and an optimized design, ...

If we can use a small heat engine, it is possible to get several hundred Watts from an average human.

At least here in Germany, burners have a large safety margin in their maximal power - they are not operated continuously during the flight.

I don't know if that can work. There is no clear border between "human-powered and some color" and "black and maybe some human power".

19. Jun 8, 2013

### HallsofIvy

Ah, c'mon, any one who has attended a faculty senate meeting know how much hot air a human can produce!

20. Jun 9, 2013

### ThainaYu

It's about equilibrium. As you can see, normal human power is 250 watt which not enough to use human powered plane. But pro cyclist power 350 watt is enough
That's point out their weight plus the plane weight need power between 250 to 350 watt to lift. But if the plane is lighter, make overall weight lighter, the power needed would be lower

In this aspect. While any kind of energy would produce waste heat anyway. I think if we just convert all energy to heat air. And collect it to make buoyancy. It might work

Last edited: Jun 9, 2013
21. Jun 10, 2013

### CWatters

If you could reduce the heat loss from the balloon to below that which a human can replace (eg <200W) without adding too much weight then it would be possible. You could preheat the balloon on the ground and then the pilot would only have to maintain that temperature. The problem is that insulation is heavy....

A small "hopper balloon" has a volume of 500-1000 cubic meters so the surface area is around 400 square meters. That means we would need to reduce the heat loss to around 200/400 = 0.5W per square meter.

Google says that temperatures of around 100-120C inside the balloon are typical. Lets assume the outside air is 20C giving around 100C across the insulation.

The thermal resistance required Rth would be..

Rth =ΔT/power
=100/0.5
= 200 C/W

One of the best and lightest insulators is Silica Aerogel. If I have done my sums right you would need a layer about 1 meter thick of the very best aerogel to achieve that.

Wikipedia says the lowest density Aerogel weighs about 1kg/cubic meter and you need about 400 cubic meters. So I estimate that the insulation would add around 400kg to a small hopper balloon even without support structure to stop the fragile insulation falling off.

This would obviously be impractical. A larger balloon would be needed to carry the weight of the insulation and that in turn would mean a bigger balloon is required needing even more insulation to limit the heat loss. However since lift is proportional to volume and volume scales faster than surface area there might well be a size at which the numbers do add up. I'll leave that as an exercise.

22. Jun 10, 2013

### ThainaYu

Thank you very much. This is very useful and relevant explanation

23. Jun 10, 2013

### Staff: Mentor

Hmm, let's look at the scaling:

For any given balloon size, if we scale it with a factor of r:

Lift goes up by r^3 (approximately)
Weight goes up by r^2 (assuming the balloon weight is dominant)
Therefore, the temperature difference can go down by 1/r.
Thermal power loss scales with surface area times temperature difference, that leaves r.
Bigger balloons are harder to keep up, if we can neglect the weight of the pilot.
Of course, the balloon cannot be too small as the weight of the pilot is there, so there is some optimal size.

The insulation has some ideal thickness, too, due to nonlinear effects in the lift<->temperature difference relation and the weight of the pilot.

24. Jun 10, 2013

### Integral

Staff Emeritus
This is very optimistic, the actual air temp at flight time must be well below 20C. That is why you do not see many Hot Air balloons flying in the afternoon. Here they usually launch in the early morning while the air is cool.

25. Jun 11, 2013

### CWatters

mfb and Integral...

I agree. I suspect the numbers for the performance and weight of the aerogel were also on the optimistic side. Still it's nice to ball park things once in a while.