Introduction: Disinfecting Water With the Ft2 Solar Cooker

By


(This is an addition to my post on the square foot solar cooker which was about using it for bread making.  Plans on how to build the Ft2 are found here: https://www.instructables.com/id/Square-foot-solar-cooker-for-bread-and-as-an-assis/ )

The cooker will pasteurize a quart of water in a bit over 2 hours of good sunlight on a moderately warm day.
Below are some tests I conducted, the first was on April 6, 2010, and the second set of tests were done sometime this year (2012), on a warmer day.


April 2010 test.  The starting temperature outside was 44 degrees.  The initial water temperature was 48 degrees in all bottles.
As shown are 5 quart brown beer bottles.  From left to right are:
1. A benchmark, uninsulated bottle on a wooden base to keep it from losing heat to the cement.
2. A bottle inside a PET plastic gallon jar.  No reflector.
3. A bottle on the Ft2 cooker reflector, without an outer cover.
4. A bottle with a PET gallon jar outer cover.
5. A bottle with a gallon PP (polypropylene) translucent plastic outer cover.
Note, in this test I did not use a glass gallon outer cover.  I forgot to....

Test results after 20 minutes-
Jar 1-56 deg Jar 2-56 deg Jar 3-76 deg Jar 4-75 deg Jar 5-65 deg
Test results after 60 minutes-
Jar 1-68 deg Jar 2-70 deg Jar 3-111 deg Jar 4-111 deg Jar 5-107 deg
Test results after 120 minutes-
Jar 1-77 deg Jar 2-82 deg Jar 3-133 deg Jar 4-142 deg Jar 5-137 deg
Test results after 180 minutes-
Jar 1-85 deg Jar 2-92 deg Jar 3-148 deg Jar 4-166 deg Jar 5-157 deg

After 20 minutes, not too impressive, but a max gain of 28 degrees optimally, which is over one degree a minute. on the jar with no cover on the reflector.
After 60 minutes, not enough for disinfecting purposes, but two jars gained 63 degrees, which is still averaging one degree per minute.
After 120 minutes, not quite enough in my opinion to consider the water disinfected, however the max heat gain was on the #4 jar, which averaged quite a bit less than one degree per minute.
After 180 minutes, I would consider jars 3, 4 and 5 to be safe enough to drink. A search on the internet will show sources which suggest water is safe to drink when exposed to 149 degrees for a short period of time.  180 minutes is more than a short period of time.  149 degrees it is reported will kill E Coli, Rotaviruses, Giardia, and Hep A viruses.  A 'safer' range is considered 160 degrees, at which milk and foods may be considered pasteurized   Of course, if we wait until the water boils, then, most assuredly most 'germs' are dead.

At this fairly cold day, after 320 minutes, the jars of note were:
Jar 3   178 degrees Jar 4   207 degrees (boiling at this altitude of 4500 feet) Jar 5   202 degrees. It was breezy at some times during the test.


The second test I conducted was this year, and probably sometime in June or July.  The 4 samples were with:
1. A bottle on wood in the sun (to insulate it from losing heat to the concrete.
2. A bottle on the 3 reflective mirrors of the cooker.
3. A bottle on the mirrors, inside a larger glass jar cover.
4. A bottle as above, inside a plastic PET gallon jar.

The water in all bottles started at about 78 degrees, and the test started at 1:50 PM on a mostly sunny day. The outside temperature ranged from 84 to mid 90's. There was occasional cloudiness and loss of sun during the test.  Bottle # 1 will be considered as inconsequential, since it only reached a high temperature of 115 degrees, well below the 'safe' level of disinfecting. It was used as a benchmark.

To shorten the length of this post, I can tell you that after 120 minutes, jars 2, 3, and 4 were approaching the magic temp of 149 degrees.
After 190 minutes, jar 2 was 177 degrees, jar 3 was 196 degrees, and jar 4 was 198 degrees.
At 200 minutes, jar 2 was 183 degrees, and jars 3 and 4 were boiling.
At 210 minutes, jars 3 and 4 were both boiling with great vigor.

Here are my conclusions and observations about the second test:
A plain brown bottle with no reflectors will not give you safe drinking water in a day in the sun.  It will give you, however, some reasonably hot water to wash with.
Regarding the jars with reflectors (the square foot solar cooker is simply 3 mirrors fastened together as shown in photo), they will all reach pasteurization range within 120 minutes.
The uninsulated jar (#2) did not reach boiling, however it did get to a max of 192 degrees. In my mind that's certainly safe enough to drink.
The two insulated jars performed very well, and one could consider them safe from anywhere between 140 minutes and beyond.
After removing the jars from the sun, you can screw down the caps, and then store the jars away somewhere out of the sun for when you need a quart of water, which is probably safe to drink.  At least, probably safer than water from an unknown source.  After the caps are secured, the water will cool and condense, and hopefully create a vacuum.  You should in theory hear the vacuum when you open your bottle. But I would feel safe drinking it regardless.