Solar Cooking
Last edited: 26 July 2017      

Bob Nepper and Bill Stevenson with their water pasteurizer

Fabricating the simple wax thermostat that controls the flow of potable water.

Bob Nepper and Bill Stevenson are retired engineers with 3M, and long time members of the Solar Oven Society in Minnesota, USA. Bob is a mechanical engineer and Bill is an electrical engineer. Bill was inspired by the plastic corrugated political signs he saw used in a recent election. He speculated that the porous corrugations would have potential as an excellent heat exchanger.

Nepper and Stevenson set to work with just a few components: two buckets, a pasteurizer and rays of sunshine. Water is poured into a bucket through a fine cloth, which sifts out any particles that may be present in the water. Water enters the corrugated pasteurizer through polyurethane tubing. The polypropylene plastic panel is painted black to absorb as much radiant energy as possible. A thin layer of clear mylar plastic goes over the panel, which Nepper and Stevenson said keeps wind from wicking away solar heat from the panel.

Radiant heat is transferred to a foam that maintains the temperature. Water heats up, until the thermostat hits 71 °C (160 °F), then pasteurized water flows into a separate bucket. The thermostat itself is custom-fabricated by Nepper. Originally, the two experimented using one similar to an automobile thermostat but the store-bought devices allowed some bad water to leak into the good. Nepper created one that was leak-proof and that fit into the pasteurizer easily.

The invention produces 15.5 liters (4 gal.) of pasteurized water per hour, and probably more with stronger sunlight.

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Bob Nepper and Bill Stevenson
Solar Oven Society
3225 Hennepin Avenue East
Minneapolis, Minnesota 55413

Tel.: +1 (612) 623-4700