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Mini Hot Air Balloon Altitude Control, Or...

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By Tyler Lucas October 18, 2016 - 1:26pm Comments (3)
Mini Hot Air Balloon Altitude Control, Or...

I'm pretty excited about this course. Who wouldn't be? We get to make robots! I have some experience with electronics, engineering, and mechanical projects, so I figured jumping into this would be a snap. How hard could choosing a project be? I've got dozens of ideas - I'll just pick one, right? Hah!

Controlling the altitude of a miniature hot air balloon with an Arduino directing a flame's heat seemed, to me, to spell out the perfect fit: a simple mechanical design, a straight-forward goal, and the possibility of advanced control algorithms using only a single sensor input (sonar) and a single actuator output. (Take a look at the sketch, below.) A simple electro-mechanical system with which one can demonstrate the full scope (and beyond the full scope) of the course's teachings is ideal, with little possibility of non-pertinent details getting in the way of success.

sketch

So, I expanded on the idea. Take a peek at my first draft, if you like. Spoiler: it didn't work out. Thankfully the project was approached from the top-down, allowing me to reach this conclusion after only a few hours of figuring as opposed to a few months of experimenting. Science works! Here's how this project was approached.

It starts with the above: state the entire scope of the project. Get rid of "feature creep" right from the start. Nothing is worse than having a project grow beyond your abilities or schedule before fruitation. (Well, some things are worse, like drowning, but let's not go there.) It may take a few iterations and sketches to define on paper the idea you have in your head, and may seem unnecessary, but it truely helps the creative process take form into a realizeable goal. My statement ended up as the following: Arduino directs heat from candle either into or outside of a hot air balloon to control its altitude.

The next step is to figure out the math and physics behind the limiting and most important factors in the project. What do I mean by "limiting factors"? These are assumptions that will tell us whether or not this project can get off the ground, so to speak. Will it work? What's the worst case scenario? Best case? Do these cases still fit what I'm trying to do? If so, we can proceed. If not, then it's either back to redefining the scope of the project or scrapping it entirely. This is similar to budgeting time, money, and work to a project in business: how long will it take, how much will it cost, and how much work will it be? In our cases, however, there are more constraints, specifically the Laws of Physics and course constraints.

This hot air balloon gizmo is lifted by the differences in density inside and outside the balloon, the same way a boat floats. One immediate constraint I could think of was the amount of lift I could get with the proposed system. Figuring this out proved more difficult than I originally thought, requiring a deep dive into thermodynamics. I started with basic thermo and physics equations like conservation of energy, the Ideal Gas Law, and heating and losses budgeting, but hit the limit of my experience pretty quickly (how to you account for heat losses by this open balloon system?). Thankfully we have the internet and someone has done this before, giving me a table of air densities for various temperatures. Plugging these numbers into Excel, I was able to figure out how much lift I could get from a sphere'ish shape of heated air (take a look via this finely crafted link). I didn't want to make the thing wider than 70 cm or taller than 150 cm so it could go through indoor doorways, which limited the maximum size for this thing. It was pretty clear that there wouldn't be enough lift, especially if it was going to be a candle or two heating it. The most I could expect was around 60 grams, and it would take a lot more than a candle to get it going. I did some calculations where a lift gas like hydrogen (kaboom!?) or helium was used in addition to the heated air to get a bit more lift, but it still wasn't enough, as the lift gas volume would have probably have had to be larger than the hot air portion (again, you can take a peek at the Excel file - it's all there).

The next step would be thoroughly defining each part of the system, but, in this case, the next step is to go back to the drawing board! A mini hot air balloon just doesn't work with hot air and a candle. It needs to be bigger, and/or use a heat source with more oomph. While it's certainly possible to go deeper down this rabbit hole by redefining the project scope, it would no longer fit the course constraints. This project is supposed to demonstrate your knowledge in the course syllabus, and diving into a larger balloon and flame system certainly doesn't fit.

Thus the hot air balloon altitude control system was scrapped after a few hours of math. This time was certainly not wasted, as I can now better and more quickly evaluate thermodynamic systems, and have a better idea as to how to choose the 'perfect' project.

Back to work...

Links:

Comments

  • I really enjoyed reading this thought process; it is (was) a very creative and clever project proposal. My initial thought was, "controlling a combustible on a small scale...this guy has gone mad in all the right ways!", but then I did see your renewed approach using a lightbulb, or any other heating source, which I thought was clever. I wonder though, ignoring the lift-constraints and specific heat conversions, if you used a lighbulb, or heating element other than a combustible, would you achieve the same lift? I don't know anything about thermodynamics, but I am making the assumption that a combustible produces a more directional heat (eg. up), while other heating sources would produce a more radial heat. Without the upward "drive" created by a combustible, would you need more heat? Or would it simply take longer to achieve the lift?

    Out of curiosity, did you weigh the Arduino and potential battery packs required? Do you think it is possible, even at large scale to produce a hot-air balloon using batteries as the power source? For instance, I'm guessing there is an equation for a lithium-ion battery that can provide available power output based on weight (roughly)? For example: (made up numbers here) a 500g lithium-ion batter is capable of producing 2000mAh of power, 2000mAh power can be converted into X-amount of heat using my heat source, which can achieve a maximum lift of 400g. I am mostly curious if the relationship is linear in that you would never be able to achieve enough lift regardless of heat source since the batteries output is never enough to overcome the weight of itself.

    Also, what were you going to use as the altitude sensor? Does such a thing exist, or were you going to program a sonar sensor to compute the distance and adjust accordingly?

    This is a great thought-provoking proposal. As you had mentioned, it is too bad you couldn't get it off the ground!

    Josh Weston January 26, 2017 - 8:05am

  • Hey Josh, good questions and points. You're right, there are differences in heating with an open flame and a mostly radiant source like a light bulb.

    The flame heats the air immediately surrounding it (and the CO2 released by its own chemical reaction) in a process called conduction or diffusion, which then flows up due to the very buouncy forces we are investigating in a process called advection, then swirls around a bit in a chaotic fashion. Together, this is called natural convection. The flame also radiates heat with photons streaming out and being absorbed partially by the air it encounters and primarily by the balloon wall, at which point the heat is diffused or reflected back into the air inside the balloon, spread through to the outside of the balloon wall, and outside of the balloon.

    The light bulb is a hot filament that does the same thing as the flame but in different proportions -- there is more radiation being emitted than convective. This means there is less "swirling" or mixing than with the flame.

    Convection heats the air in the balloon more effectively than radiation, just like a convection oven cooks more evenly than those without forced air. Radiation from a light bulb heats objects better than air as it emits mostly infrared, which is not well absorbed by air (see Wikipedia: Infrared Window). This means it heats the surface of the bulb and the balloon material first, then the glass bulb and the material heats the air. Reflective material helps reflect heat back into the balloon. In the end, both methods heat the air in the balloon, but convection is more efficient.

    The lifting effect of the initial upward draft from an open flame is cancelled out in a closed system (the rising air mass has an equal portion of lowering air), but it does allow placing the heating element below the heated chamber while maintaining high heat transfer ratios compared to the amount of heat that would escape with a radiative source in the same location.

    Another practical consideration you mention is that a light bulb would need a lot of electrical energy, namely batteries. The maximum energy density of available batteries is around 0.5 kWh/kg or 1.2 kWh/L. By comparison, that of paraffin wax is around 12 kWh/kg (42 kJ/g) or 6.1 kWh/L, at least five times more energy dense. Without taking into account system efficiencies, a battery operated heater would require at least 5 times more fuel mass (batteries) than a paraffin wax heater (candle). Batteries could still be used, however, with a sufficiently small time scale (batteries die) and a sufficiently large volume of air. Although fuel weight (and volume) to energy output is linear, including batteries or paraffin wax, air volume to material weights is not, so scaling up the size does work. The only limiter is fuel suitability -- can you release and spread the energy quickly enough to heat the air? I leave this up to the hot air balloon pilots... :)

    Cheers

    Tyler Lucas January 27, 2017 - 1:14pm

  • I forgot about your other questions, whoops.

    I did weigh the Arduino and estimated other componenets: SparkFun RedBoard (Arduino) with all SIK components 173g; balloon envelope 17.3g per square meter; and 50g for the frame, tea light, flame flue, and battery (I hadn't planned on using the battery to heat the air, only to power the microcontroller and flue). This was a high estimate for total weight.

    Altitude sensor would have indeed been a sonar sensor.

    Tyler Lucas January 28, 2017 - 5:29pm

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