Sun Bowl       [Download a PDF Copy]

During one of my (unsuccessful) attempts to desalinate seawater using solar energy,
I managed to invent an effective and simple tool to pasteurize contaminated water. 
I call it the Sun Bowl and it consists of a  20 quart stainless steel mixing bowl  and
a  320 oz (10 quart) clear plastic  punch bowl.  Under ideal conditions, the Sun Bowl
has the capacity to pasteurize up to 12-quarts of contaminated water per day.  Hence,
one Sun Bowl can meet the water treatment needs of the average household almost
anywhere in the world. The basic Sun Bowl and its' components are shown in Figures
1 and 2. The Sun Bowl is a solar thermal, water pasteurization tool. To use the tool,
you  place the steel bowl in the sunlight, place your water pot in the center of  the
bowl and cover the pot with the clear plastic punch bowl. 

Since 1890, pasteurization has been used to kill harmful waterborne bacteria and virus.
To be pasteurized, water must be heated to at least 150° F (65° C)  for one hour  (1).
Always use a  thermometer,  WAPI or SPADE to measure water temperature and
avoid heating beyond  160° F  (71° C). Once temperature reaches 160° F, you only
need 15 seconds to kill all  harmful waterborne germs. This does NOT include
chemically contaminated water, seawater or brackish water. Also, the Sun Bowl is
NOT a filter or purification system. The water used should be reasonably clear and
free of obvious particles or have already passed some other pre-filter system (2).

The Sun Bowl works because the large mixing bowl acts like a deep parabolic reflector
that collects and focuses sunlight in the center of the bowl where you place your water
pot.  The black color of the pot acts like a light magnet absorbing all available light
striking its' surface.  The pots' metal molecules are excited by the photons that make up
the light resulting in heat which is transferred to the water (or food) in the pot.  Finally,
The clear  plastic bowl  helps to retain heat by promoting a greenhouse effect and
preventing heat loss due to wind and air temperature. Because of the round shape of
the bowl, no additional turning is required  to catch the available sunlight as the sun
moves across the sky during the day.  Some sunlight is always being focused on the
water pot no matter what the angle or position of the sun in the sky.

I have tested the Sun Bowl with a black 2-quart teapot;  4-quart and 6-quart black enamel
stock pots.  Given sunny skies and air temperatures of at least 75° F (24° C),  I found I
could reach pasteurization temperatures using a 2-quart teapot in three hours, a 4-quart
stock pot in four hours, and a 6-quart stock pot in six hours.  Whatever water container
you use, it needs to be black or very dark metal and have a dimension no greater than
10-inches wide x 6-inches tall.  I also found that I could heat the water faster in a pot by
replacing the metal lid with a clear glass plate as shown in  Figure 3. Allow about 1/4 inch
air space between the top on the water and the bottom of the plate. Using a glass lid allows
more available sunlight to strike the black metal surface area of the pot - both inside and
outside - which excites more metal molecules to produce heat.  You can reduce heating
time by at least 1/2 hour for 6-quart pots with a glass lid.

In addition to heating water, the Sun Bowl can be used  for heating and cooking food. 
This is a tool that can perform double duty.  I have also used the large steel bowl to light
tissue paper on fire! The main drawback is that a Sun Bowl will cost your about you
about $30 which does not include the cost of your pot(s).  However, the Sun Bowl is a
robust tool and with proper  maintenance, it should last for many years.  For proper
maintenance, keep the bowls clean and dry.  Keep the punch bowl covered and out of
the sun when not in use.  Periodically,  use a polishing cloth with metal polish to maintain
the shine of the mixing bowl and prevent discoloration.

Finally, I have amplified the basic Sun Bowl with an additional back reflector as showed 
in Figure 4.  This option further reduces heating time by up to one hour for 6-quart water
pots. The back reflector is made from self-adhesive HVAC duct insulation and 12-inch
heavy duty aluminum foil.  Details for making a flexible back reflector are described for
my PanCooker.   Light amplification may be used to reduce time during emergencies or
when the sun and air temperature don't want to cooperate.  However, under ideal
conditions, where the sky is mostly sunny and the air temperature is at least 75° F (24° C), 
a back reflector is not needed.


In Conclusion

The Sun Bowl is the 'lazy man's' solar cooker that can pasteurize 6 quarts of water in
about six hours.  With the exception of the optional back reflector, everything is
off-the-shelf and can be assembled in a matter of seconds. As seen in Figure 5, storage
is a snap with everything fitting easily into the large stainless steel bowl. Folks, it just
doesn't get any easier than this.





References

1.  Ciochetti, D. A., and Metcalf, R. H., Pasteurization of Naturally Contaminated Water
     with Solar Energy,      Applied and Environmental Microbiology, 47:223-228, 1984.

2.  Burch, J. D., and Thomas, K. E., Water Disinfection for Developing Countries and Potential
     for Solar Thermal Pasteurization,      Solar Energy Vol. 64, Nos 1–3, pp. 87–97, 1998