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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============================================================
FRIAM Applied Complexity Group listserv
Meets Fridays 9a-11:30 at cafe at St. John's College
lectures, archives, unsubscribe, maps at http://www.friam.org
