Heliostat hot air lantern

This is a simple project where i tried to combine hot air lantern and heliostat mirrors. I tried with 8 mirrors but the bag was 2 heavy.

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Removed chimney, closed bottom, painted side black.heliostats produced enough temperature, wind blowing hard.

windy. tested weight of paper & plastic. Paper= 5 X plastic, Heated plastic bag with 8 mirrors 15x15cm.weighed "balloon" after 15 min.Weighed same.ill try&get black plastic.

Windy. No black trashbags. Tomorrow buy.

Trashbag too heavy. Idea: inflate black baloon and measure time difference heating up with or without heliostats. This should work.

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Dan Maloney wrote 04/08/2020 at 17:17 point

This is clever, I'd love to see it work. I tried a sun-heated "bag balloon" with the kids once - tape a bunch of the thinnest black plastic bags you can find together and let the sun heat up the air inside. It was supposed to get rigid and fly away, but no such luck.

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Robert Petrušić wrote 04/08/2020 at 17:49 point

Hope you will come again in a few days to hear the news. I have the temperature needed to make the ballon lift but the wind stopped me couple of times. It's much slower process than with gas, and i hope my waxpaper is light enough to rise. Anyway, thanks for your comment, really.

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Robert Petrušić wrote 04/08/2020 at 08:55 point

nice. Thanks a lot.

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Simon Merrett wrote 04/07/2020 at 23:12 point

You asked about this on the stack. If the sun's energy is 1000 W/m2 in perfect conditions, your mirror area was 0.09m2, so if you were managing to somehow get all the reflected light to fall on to your heat receiving surface (which if it was narrower in any dimension than 15cm you are unlikely to have achieved with flat mirrors) then you were receiving 0.09 x 1000W = 90W. 

The problem I think you have is that because that was radiant heat and you decided to make your "chimney" or heat receiver from a material with very low emissivity ( = reflects radiant heat, rather than absorbing it). Why don't you try with a matt black material as your "chimney" before worrying about adding more mirrors. Perhaps black plastic bin liner bag would be lighter than aluminium too.

If you want to work out how much power is needed, you could look at taking the 90W mentioned about earlier and calculating how long it would take to heat the volume of air contained in your balloon by enough Kelvin to balance the weight of your balloon ( by changing it's density at higher temperatures). You would need to make an allowance for the rate of cooling for air within the balloon too. 

If you decide to try the black "chimney", I would suggest also trying a transparent skirt dangling down from the balloon to the same level as the base of the chimney, as otherwise it won't really reach a high temperature. You could try making the whole balloon black and heating the balloon directly with your mirrors. This way, you'd benefit from direct solar heating at the same time.

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Robert Petrušić wrote 04/07/2020 at 23:20 point

thanks for the vast reply. I understand everything, and i am wondering if i would to close the balloon, would it heat the fastest possible? Maybe just to leave a small pressure vent at the bottom? The idea with trash bag isnt bad. Its much lighter than aluminium. I will probably try everything. But what do you think about the idea?

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Simon Merrett wrote 04/08/2020 at 06:35 point

Is a similar idea and it seems that you need very light skin material and large volume to overcome the mass of the balloon. I like the idea but until you do some calculations or try different designs, we won't know if physics "likes" your concept! Good luck! 

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Robert Petrušić wrote 04/08/2020 at 10:02 point

i saw those balloons. Seems like it could work great. Now we need to test.

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Robert Petrušić wrote 04/08/2020 at 11:03 point

Hey Simon, listen i was looking and all i found was watt to celsius per minute. ( i am In celsius). So it says that 350 watts is 12 celsius per minute. So how does that work for me? I really dont know these equations, or where to find them.

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