Posted by
Owen Densmore on
Apr 11, 2009; 11:03pm
URL: http://friam.383.s1.nabble.com/A-leetle-thermodynamics-tp2618412p2622238.html
As usual, Wikipedia has some info:
http://en.wikipedia.org/wiki/Solar_cooker#Using_a_solar_cookerLooks like Peter is right about the time it takes. Pretty slow
process, but that may not be as big an issue for the users compared to
gathering fuel etc.
-- Owen
On Apr 10, 2009, at 8:40 PM, Victoria Hughes wrote:
> Hey, that is a pretty cool bunch of information.
> Egyptian Ice, eh - a new delicacy, only for heirophants.
> How did they know to do that? Arabs?
> Sounds Tom Robbins-y. Love it.
> Tory
>
> On Apr 10, 2009, at 3:25 PM, Peter Lissaman wrote:
>
>> Solar cookers can break your heart, but not the laws of
>> thermodynamics. Consider this elementary fact, my dear Dr. Watson.
>> The insolation on earth near the equator is about 800 W/m2, it is
>> less at the end of the day, and much less after sunset. For an
>> aperture of 0.1 m2, you getting about 80 W black body, ignoring
>> losses. Concentrators have nuttin to do with it! This amounts to
>> about 270 BTU/hr from which you could boil a bit less than 2 pints
>> of water in an hour, assuming no losses.
>> BTW, you can, with care and ceremony, make ice in the Egyptian
>> deserts every cloudless night, by exploiting radiation to the stars
>> from shallow water trays, and careful control of nucleation,
>> convection and vaporization. In fact, the temple priests used to do
>> it on the flat roofs of the temples to impress the unwashed on the
>> bounty of whatever God they were scamming that week. Much hoopla,
>> involving sanctified water brought up from the basement (where it
>> had got pretty cool, mixed with yesterday's ice), throwing holy
>> dust on the surface (to provide nucleation particles) and wafting
>> the surface at just the right time and rate with magic ostrich
>> featherwands to actually control heat transfer due to convection
>> and vapors. It's just thermodynamics, Nefertiti! And if sometimes
>> the ice didn't form, it was because someone's mother-in-law was a
>> witch! It's amazing what them religious guys know!!
>> I usedta teach elementary courses in thermo in CA and the
>> conversion constants are from memory and only roughly correct.
>>
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