Quick and Easy Solar Hot Air Balloon

[Becky Stern] likes to harness the power of the Sun. Most of us will immediately think of solar cells and other exotic solar energy techniques. But [Becky] shows how to make a hot air balloon using nothing but tape and garbage bags.

The idea is quite simple. You form a large envelope from black trash bags and fill it with air. Becky does that by just running with it, tying it off, and topping off with a little manual blowing. Once the sun heats the black bag, it floats.

We were hoping she’d show us if there was any margin for payload, but she didn’t. However, electronics are getting smaller and lighter all the time. Sure, batteries weigh a lot, but maybe there would be an opportunity to mount some solar-powered device on the balloon to further harness the sun’s energy.

If you want to have a go at that, [Becky] has a free solar class that covers using the sun to engrave wood and using solar panels to charge a battery or power an Arduino.

We doubt this solar zeppelin will perform like a propane-powered balloon but it is still fun and educational. It doesn’t seem likely that you are going to reach space with it either.

62 thoughts on “Quick and Easy Solar Hot Air Balloon

        1. Or fresnel lens shielded parabolic troughs circulating through the super insulated isobaric closed loop isobaric heat exchanger storage tank of liquid sodium or NaK or supersaturated LiCl or H2-H3 mix or liquid NH3 then the hot end of an external combustion engine if not another heat exchanger to desalinate sea water through a close loop turbine geothermal distillation unit.

      1. You’re my hero! :-|) I made a comment on a youtube video regarding polarizing filters last week that some of us would be cool with “pun intended” cryocoolers, super-insulated liquid air tanks for the super cooling of the liquid helium-hydrogen (I wish) tanks for use in our electronic devices and what ever other experiments or systems. It’s like… man, I’ve observed submerged distilled water cooled and other forms of heat exchangers for cooling computer components which now the radiators are main stream consumer grade and not a cars radiator… though never a liquefied gas super cooled overclocked system. Man, righteous thought on the translation and transfer of energy!

      1. Other than thermal capacity issues though density is key… I do wonder what would be a really lightweight material like maybe carbon fiber where like maybe smallest feasible like wire rings and top and bottom connecting really small rods at least (for like the side view profile) would make rigid enough to pull a vacuum to lower the density if just air and if that would even make a significant improvement along with a coating to make rigid where the arc of the balloon would help re-enforce the shape under lower pressure.

  1. About the weight thing, you’d have to be pretty conservative of resources and be pretty clever. One that comes off of the top of my head would be to make one from reflective space-blanket stuff and use it as a kind of mirror to radio broadcasts.

      1. Reminds me of making something novel that I am not finding with the projects such as the GATR Technologies Satcom inflatable antenna systems and the Piasecki PA97 and the Lockheed-Martin P-791. I’ve always thought the later two were great ideas with bad control systems and project management as well as implementation planning. You almost require metrologists and meteorologists to use.

        Basically, you can have the bottom of the balloon clear or at least to the frequencies desired to communicate with if ground based or skywave system. Have this internal balloon antenna on a gimble like a gyro with a drone to better control or can even use a perimeter balloon where the central balloon is directable for optimal signal performance. I think a control drone can be best for stability.

        Then the top of the balloon can stay infrared and what ever best frequencies transfer the heat as well as filled with best thermal capacity material with like a one way mirror coating so to not let out heat, let in heat and store best though the back of the dish or trough should be black or whatever material absorbs heat best.

        Then for the inside of the balloon dish or trough design a clear membrane for the balloon bottom. The air or gas mixture will have to take into consideration absorption characteristics as well so not to adversely impact the signal frequency and strength.

        I was wondering about using window tint materials potentially as they may be main stream market products that can be used. Space blanket Mylar is a great idea also. I am also wondering about custom thinned mixtures of materials mixed in the coatings for optimization critically weighing all the materials to inductive design best.

        1. I think I invented another World’s first also… a drone controlled balloon rocket launch. The balloon doesn’t even have to be solar I’m guessing. You could also have a gimbal balloon stabilizer with perimeter balloons rocket launch and be a first too maybe.

    1. They used, or at least experimented with, atmospheric balloons as a pre-spaceflight form of communication satellite. Just using metallised plastic for purely passive signal reflection.

  2. Not very useful, just a rehash of a very old idea that is only good enough to demonstrate the concept. However if you look around you will find examples of much better designs that have significant buoyancy, such as an inverted tetrahedron (vertex down, face up) with the upper side made of clear material, this minimises heat losses compared to the “sausage” form. If teaching STEM is the point of the exercise it helps to make two balloons that are identical except that one is white, this also teaches the point of having a “control” in your experiments.

    1. Speaking of rehash, if you dig into other examples of these you’ll find plenty of trans-oceanic attempts (and successes). The part I always found exceedingly clever was the use of sugar cubes as ballast; the ballast is required to keep the altitude of the balloon low enough to prevent it from popping, but at night time when the balloon descends over the ocean the sugar cubes dissolve and allow the balloon more travel.

          1. Interesting. Thanks for sharing the link. I was thinking something like this for the solar design: http://jpaerospace.com/blog/?m=200712 (scroll down and look for Ascender 90 and 100). I wonder if a payload of a rocket can make a new “Worlds First” Rocketoon I like to say, though looks like they’re called “Rockoon’s” now days or I am remembering wrong. So a “Solar Hot Air Ballon Rockoon.” i haven’t found one launched yet.

  3. This summer, I threw a heavy duty trash bag over some furniture I was keeping outside. It was only supposed to be a temporary thing, but it stayed out all summer, and almost every day the sun inflated it, even though it was open at the bottom. Then a few weeks ago it broke. I don’t know if it was uv radiation or high winds, but it was kind of our wacky waving inflatable tube armless blob. So I’m thinking about putting another on next summer. Who wants boring furniture anyway?

  4. >>We were hoping she’d show us if there was any margin for payload

    Easy enough to figure out. 4 trash bags gives around 120 gallons (look to be 30G each) since she cut off some and to make the math easy, we’ll call it 100 gallons (~400L, again for simplicity, I know that’s not an accurate conversion). Density at 20C is 1.2 kg/m^3, 1.18 kg/m^3 @ 25C, 1.15 kg/m^3 @ 35, So our 0.4 m^3 (400L) envelope worth of air weighs 472g / ~16oz @ 25C. The air around it weighs 480 grams. So assuming everything goes your way and you can get the envelope to 35C (460g) you’ll be able to lift 20g (just shy of an ounce).
    Obviously cooler outside air improves lift, but you won’t be able to keep the heat once it gets cold enough. Also don’t use a ton (or even worse, tonne!) of tape.
    And all of this ignores the weight of the envelope, but with so many weights of garbage bag out there, I didn’t feel like doing that math.

  5. Wow! I remember one summer probably 30 years ago buying a balloon like that at department store. Kept me entertained for a few days of the school holidays till it got torn on a tree :(

  6. It is very important to not let it go? I can not make any sense out of that admonition. Something about pollution? It is made out of oil, which the jet overhead is burning at 5000+ pounds per hour!

    1. If you let it go, you should have a plan for how you’re getting it back. There are plenty of examples of animals being harmed by plastic bags, and they are unsightly if scattered randomly across the landscape.


      People do release their balloon experiments rather often; they’ll attach small payloads to a balloon including a card which reads “IF FOUND, PLEASE CALL $NAME AT $PHONE”. Larger balloons will have radio transponders or cell phones for near-real-time geolocation.

      There are also tools online that let you predict where your balloon will fly and land, so you can drive out to the right county and pick it up. Some people/organizations have enough experience that they can drive upwind of their hometown and release their balloons at such a place and time so that they come down only a few miles away from home.

  7. The idea is not new, bin bag balloons and even bin bag kites were one of my childhood fascinations in the 70s and early 80s. A lot of ebay sellers punt them, try a “solar balloon 50′” ebay search and you will find a bunch of over priced 50′ bin bags from various sources. Spray adhesive was the way to go when attaching one bag to the next as I recall. Place a large bit of cardboard the width of the flattened bag in the neck of the first bag. place a second bit of card on top to mask off all but about 4″ of overlap. spray on adhesive, wait till tacky. Tack down neck of next bag, flip over, repeat with reverse side. Rinse, repeat with more bags. Fill with hot air from hair dryer once the glue has dried. Take outside, crash in to tree. Go back indoors and watch TV.

    Bin bag kites on the other hand could be huge, and somewhat scary in terms of lift.

    1. If you have a perfect seal and fill it completely before the air inside gets any hotter then surrounding air, it will not float at all even if it doesn’t rip.

      In fact, if it rips a little, so that pressure from inside the vessel is equalized to outer pressure, it may help it float.

      Do you understand why?

      1. So you’re saying that if you could make the pressure INSIDE way less than OUTSIDE it would float really well???

        Finding a material rigid and light enough would really suck balls.

        1. Hot air balloons don’t depend on the pressure being lower inside than out; they depend on the DENSITY being lower inside, and hot air is less dense than cooler air, for any given pressure. A tight seal is not helpful.

          1. Nope, a thight seal supposedly will prevent the mass inside the balloon from changing and assuming the volume didn’t change the density also cannot change. The volume would obviously change slightly because the bags have some elasticity but not enough for it to help.

            In any case it’s better to not fill 100%, or you can add a valve in the form of an open bottom for example (with a weight to keep the bottom at the bottom)

          2. Hot (or warmer than surrounding) and dryer preferably. Humidity will play into as a factor since a component of air. I guess air pollution too if you’re really being critical. I suppose if moisture content was equal inside and out… heat still will apply to creating a difference in buoyancy.

          3. jafinch78: you’ve got that backwards, though. Although it’s counterintuitive, water vapor is lighter than dry air. Look it up, or figure it out: H2O has a molecular weight of 2+16 = 18, O2 has a weight of 32, and N2 has a weight of 28. So to make air less dense, you want it to be HIGHER humidity. This works well for hot-air balloons, since there is a lot of water vapor produced by burning propane.

          4. Chipmunks or moles decided to move in around the basement on top of abandoned garden around the house for a few years and looks like with the chestnuts and oaks acorns… squirrels are digging up the yard.

      1. Nah, it doesn’t need any stretch; you just need to let it leak a little, preferably at the bottom. Maybe some styrofoam formers to keep it from collapsing, but pressure is both unnecessary and a hinderance to buoyancy.

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