re: lift a weight (5kg) with the energy from a candel SOLAR.KnowledgePublications.com

re: lift a weight (5kg) with the energy from a candel
6 aug 1998
>>let's say we don't use levers, and the balloon material weighs 10 grams
>>per square meter, and it has a thermal conductance of 5 w/m^2c, and the
>>candle makes 50 watts and weighs 500 grams, and the 20 c air in the room
>>weighs 1.2 kg per cubic meter... 
>
>and how large does the balloon have to be if it has no candle, but the top
>hemisphere is transparent and it receives 1 kw/m^2 of vertical sun, and the
>bottom hemisphere is black inside and silvery outside, with a thermal 
>conductance of 1 w/m^2c?

the balloon collects 1kw pi r^2 of sun and loses (t-20)(2pir^2x5+2pir^2x1)
of heat, so t = 20 + 1 kw/12 = 103 f (maybe, without much reradiation.) the
lift is 1200x4/3pir^3(1-(273+20)/(273+103)) = 1110r^3 grams, and the weight
is 5000 + 10x4pir^2, so r = 1.7 m should work.

>and can we keep the balloon up overnight by storing some solar heat in some
>water inside, if the top receives 5 kwh/m^2 on an average january day, when
>the outdoor temperature is 0 c? 

i don't think so, without the bubbles. with bubbles at night, and a 6 hour
solar collection day, and r = 3.4 m, and 0.027 m of water, the balloon's
daytime thermal conductance is 2pi3.4^2x5+2pi3.4^2x1 = 436 w/c, the average
temperature at dusk is 0+5000pix3.4^2/6h/436 = 69.4 c, the water volume is
v = pi0.027^2(3x3.4-0.027)/3 = 0.0078 m^3, it has heat capacity c = 9 wh/c,
the balloon area under the water is 2pi3.4(0.027) = 0.58 m^2, and it has a
thermal resistance r = 1.73, so the night time constant rc = 15.6 hours,
tdawn = 69.4exp(-18/15.6) = 21.9 c, and the lift in 1287 g/m^2 0 c air is 
(4/3pi3.4^3-0.0078)1287(1-273/(273+21.9)) = 15,734 grams and the weight is
5000 + 10^6(0.0078) + 10(4pi3.4^2) = 14,253 grams. the extra 1.5 kg might
lift a 5 watt pv panel, battery, air pump and controls. tiny cold soap
bubbles can last up to a year, and work as well as fiberglass insulation,
with 200:1 air/water by volume.

nick

...bill sturm @ specialty cea structures inc. in calgary, alberta, canada
(403) 274-8800 has a 12,000 sf greenhouse operating, that uses soap bubbles
as the insulation/shading method. they are building a 120,000 sf greenhouse
using this technology now.