Solar Oven Design — part 1

My goal for this project is to design an inexpensive easy-to-build low temperature solar oven that can be used for a few different purposes:

1. to pasteurize water by heat
2. to produce distilled water
3. to cook food at low temperatures

And all this would be done at temperatures well below the boiling point.

1) Pasteurize

For pasteurization, the water needs to be hot enough to kill viruses, bacteria, and parasites. If you are pasteurizing water on your stovetop, you would simply boil the water. One minute at a full rolling boil is recommended by the EPA and FEMA. But if you wanted to use a solar oven to pasteurize water, a lower temperature is sufficient. The CDC says:

“Pasteurization is not a sterilization process; its purpose is to destroy all pathogenic microorganisms. However, pasteurization does not destroy bacterial spores. The time-temperature relation for hot-water pasteurization is generally ~70ºC (158ºF) for 30 minutes.” [CDC]

The amount of time needed to pasteurize water safely depends on the temperature and which pathogens you are trying to kill. The above stated 30 minutes at 158ºF, which is exactly 70ºC, is a good guide. This temperature, or anything higher, will kill viruses, parasites, and all pathogenic bacteria. But there are a couple of catches.

“Microbes are killed by heat. If food is heated to an internal temperature above 160ºF, or 78ºC [sic, this should say 70, not 78], for even a few seconds this sufficient to kill parasites, viruses or bacteria, except for the Clostridium bacteria, which produce a heat-resistant form called a spore. Clostridium spores are killed only at temperatures above boiling. This is why canned foods must be cooked to a high temperature under pressure as part of the canning process.

“The toxins produced by bacteria vary in their sensitivity to heat. The staphylococcal toxin which causes vomiting is not inactivated even if it is boiled. Fortunately, the potent toxin that causes botulism is completely inactivated by boiling.” [CDC]

First, some bacterial spores (a dormant state bacteria use to survive difficult conditions) will survive 30 minutes at 158ºF, and will even survive boiling. Second, some bacteria produce a toxin that causes sickness, even if all the bacteria are dead. But if the toxin (a type of protein) is heated to sufficient temperatures, it is inactivated. Pasteurizing water at 158ºF times 30 minutes will not address either danger. However, most other water purification methods that you might use, instead of heat, also will not kill all pathogens, nor inactivate all toxins.

Water purification by pasteurization is highly effective, killing viruses, parasites, and most bacteria. This level of water purification exceeds that of iodine tablets and low-levels of chlorine bleach, both of which are ineffective against cryptosporidium parasites. Pasteurization is effective against all viruses, whereas the vast majority of water filters do not filter out viruses, due to their small size. So pasteurization is more effective than many other options, but it does not guarantee absolutely pure water.

2) Distill

And that is why the second goal of this solar oven project is to use the oven as the main component in a solar water distillation design. The oven would contain water, which, even if heated to well-below boiling temperature, will produce water vapor. The oven would then be connected to a second closed container by pipes (plastic, or aluminum, or maybe copper) so as to condense the water vapor. This approach of using a full-fledged solar oven, connected to a separate cooling chamber, should be much more efficient than many of the designs I’ve seen on YouTube and various websites.

The typical solar water distillation unit has several problems. First, the interior of the distiller should be black, so as to turn as much of the sunlight into heat as possible. In many solar distiller designs, this is not the case. Second, the heating chamber should have insulation, so as to retain as much heat as possible in the water. Without insulation, a large portion of the solar energy that enters the distiller will be lost.

Third, almost any solar oven will be much more efficient if it has large reflectors, directing more solar energy into the oven. If you are making a solar oven for cooking, this is essential. Yet most solar distillation designs have no reflectors at all. Fourth, the typical design only allows for condensation to occur on the inside top of the heating chamber. But the more efficiently you heat the chamber, the less condensation you will get. A separate chamber is needed for cooling, which can easily be kept at a much lower temperature than the solar oven part of the distiller.

Fifth and finally, the water will sit in the solar distiller for a number of hours at temperatures that are far too low to kill bacteria. This is especially true at night and in the early and late hours of the daytime. Even a well-made solar oven will not retain its high temperatures through this time. The problem, then, is that bacteria can grow in the warm water, and can then possibly contaminate the inside top of the distiller or the area where the distilled water is collected. Having a separate cooling chamber for condensation and collection of the water lessens this possibility. But another approach would be to add copious amounts of salt to the heated water. The distilled water would still be salt-free. And the salt would prevent the growth of bacteria in the heating chamber when temperatures are lower. The bacteria are killed by heat during the day, and the salt prevents bacterial contamination and growth at other times.

3) Sous Vide

This type of cooking is used in some fine restaurants (but none that I’ve patronized) to cook meat in a way that is particularly tender, while still pasteurizing the food. The usual cooking temperatures for this method are between 140 and 150 degrees Fahrenheit. Here is a good overview of ‘sous vide’ cooking: A Practical Guide to Sous Vide Cooking, which includes information about pasteurization and food safety. And here’s a site with a good time vs. temperature chart for low temp cooking:

I’ll leave an extensive treatment of this subject for later posts. But for now, I’ll just point out that a solar oven should be able to raise food to a high enough temperature to cook the food safely.

– Thoreau

2 Responses to Solar Oven Design — part 1

  1. If you want to purify water with the sun, use a Fresnel lens and boil it. A Fresnel lens can be obtained for free (you may have to buy wood for a frame) from any rear projection TV. One of these lenses will boil water within minutes. Be careful, as they can easily start fires. A board placed at the focal point will burst into flames within seconds. Look for this on You Tube.
    Fresnel lenses were used in lighthouses.

  2. If you want to go to the trouble, you can make a solar oven that will get as hot as 3-400F. You can find videos on the net.