A university student (Jamie Winterton) and I were the first to
demonstrate that the BYU Solar Funnel Cooker can be used--at
night --as a refrigerator. Here is how this is done.
The Solar Funnel Cooker is set-up just as you would during
sun-light hours, with two exceptions:
1. The funnel is directed at the dark night sky. It should not
"see" any buildings or even trees. (The thermal radiation from
walls, trees, or even clouds will diminish the cooling effect.).
2. It helps to place 2 (two) bags around the jar instead of just
one, with air spaces between the bags and between the inner bag
and the jar. HDPE and ordinary polyethylene bags work well,
since polyethylene is nearly transparent to infrared radiation,
allowing it to escape into the "heat sink" of the dark sky.
During the day, the sun's rays are reflected onto the cooking
vessel which becomes hot quickly. At night, heat from the
vessel is radiated outward, towards empty space, which is very
cold indeed (a "heat sink").
As a result, the cooking vessel now becomes a small refrigerator.
We routinely achieve cooling of about 29 degrees Fahrenheit
(10 degrees Celsius) below ambient air temperature using this
remarkably simple scheme.
In September 1999, we placed two funnels out in the evening,
with double-bagged jars inside. One jar was on a block of wood
and the other was suspended in the funnel using fishing line.
The temperature that evening (in Provo, Utah) was 78 degrees
Fahrenheit. Using a Radio Shack indoor/outdoor thermometer, a
BYU student (Colter Paulson) measured the temperature inside the
funnel and outside in the open air. He found that the temperature
of the air inside the funnel dropped quickly by about 15 degrees,
as its heat was radiated upwards in the clear sky. That night,
the minimum outdoor air temperature measured was 47.5 degrees--but
the water in both jars had ICE. I invite others to try this,
and please let me know if you get ice at 55 or even 60 degrees
outside air temperature (minimum at night). A black PVC container
may work even better than a black-painted jar, since PVC is a
good infrared radiator--these matters are still being studied.
I would like to see the "Funnel Refrigerator" tried in desert
climates, especially where freezing temperatures are rarely
reached. It should be possible in this way to cheaply make ice
for Hutus in Rwanda and for aborigines in Australia, without
using any electricity or other modern "tricks." We are in effect
bringing some of the cold of space to a little corner on earth.
Please let me know how this works for you.