The solar pot skirt:
Greenhouse and pot stand for solar panel cooking
David Delaney, Ottawa
May 14, 2002
An inexpensive, easily fabricated, transparent enclosure--a greenhouse--is
needed to meet the requirements for solar panel cooking described in "Solar
cooking on the roof". The
solar pot skirt
meets those requirements.
A solar pot skirt is a simple cylinder of a special transparent plastic,
open at both ends. (Glass would be even better, but the focus here
is on construction from easily available materials. If you can find a glass
cylinder of suitable dimensions, by all means use it.) The pot skirt stands
on one of its ends on the reflective floor of a solar panel cooker. A black
cooking pot hangs in the top end of the solar pot skirt. The
lip of the pot traps a volume of air between the exterior of the pot and
the solar pot skirt. Air cannot escape through the top opening of
the solar pot skirt, or at least not very quickly, because of the
seal created by the pressure of the rim of the pot on the top edge of the
solar pot skirt. Hot air cannot escape through the open bottom of
the pot skirt, because the hot air is lighter than the outside
air, and can only rise. Sunlight passes through the transparent solar
pot skirt, striking the black pot hanging in the pot skirt and heating
it. The pot heats the air adjacent to the pot. This air is trapped as just
described, reducing heat losses, to the environment, and allowing the pot
to rise to a high temperature. Although transparent to sunlight, the pot
skirt is opaque to thermal wavelengths, and is warmed directly by thermal
radiation from the black pot. Heated both by the hot air in contact
with it, and by this thermal radiation from the black pot, the solar pot
skirt re-radiates energy to the black pot, thereby reducing its loss of
heat by radiation.
The plastic of the pot skirt is polycarbonate, a very tough,
very transparent, material that keeps its strength at temperatures as high
as 120C to 140C--just high enough to be safe for solar panel cooking. (But
not for some solar box cookers--don't try to glaze a large or well insulated
solar box cooker with polycarbonate.) I bought a 24" x 48"
x 0.030" sheet of polycarbonate on Feb. 1, 2002, for Cdn$16.75 (US$10.53
on the day of purchase.) This is enough polycarbonate for four or five
solar pot skirts. Polycarbonate was invented in the 1950s. Its inventor
and first manufacturer, General Electric, called it by the trademark "Lexan",
and still sells it under that name. Polycarbonate is manufactured in thin
sheets that are ideal for the fabrication of pot skirts. A thickness of
0.030 inches (referred to as "oh-three-oh", or "thirty thou", equal to
0.076 millimeters) is ideal for fabrication of solar pot skirts for pots
of diameters from six inches to 12 or 14 inches (15 to 35 centimeters).
Pots of these diameters range from one to six or eight liters of capacity.
You might think that a plastic material only 0.076 mm thick would be too
weak to hold an eight kilogram (16 lb pot). You would be wrong. The strength
of these thin polycarbonate sheets is amazing.
I intend to test this design in the summer of 2002.