Solar water pasteurizer

High Output Solar Water Pasteurizer

There are more than a billion people who do not have reliable access to safe drinking water. These people are often disabled by waterborne diseases and become dependent on their community thus reducing the overall viability of the entire community.

The device described below is an improvement on most current devices used for solar water pasteurization because of the greatly increased quantity of water that can be made safe with a low input of solar heat and because its operation is automatic. The only attention required is keeping water in the input tank. The water is not boiled but merely heated to a temperature of at least 65 degrees C that will quickly kill any pathogenic organisms in it. The water is then cooled as its heat is transferred to the incoming water. The transfer of heat from the outgoing water to the incoming water is what permits the large increase in the amount of water that can be made safe.

This solar water pasteurizer can be made inexpensively using a thermostat, a metal cooking pot, copper tubing, insulation, and other low cost hardware. The thermostat can be the type that is used in automotive cooling systems. These are available in a choice of operating temperatures. The one that is used should be made for an operating temperature of at least 65 degrees C. if a higher temperature is used, the water will be more sterile, although it is generally agreed that 65 degrees C is sufficient for making the water safe. Be sure that the thermostat that is used closes completely at temperatures below its set temperature. Some thermostats have a small hole to permit a low flow at low temperatures. If there is such a hole, close it off. To connect the thermostat to the water feed tube, it is necessary to use an adapter to connect the small tube to the large flange that separates the two sides of the thermostat. A small funnel could be used or the top of an old spray can. The adapter can be soldered to the flange so that the incoming water must pass through the thermostat valve in order to get into the pot.

The water feed tank must be elevated above the level of the pot so that water will flow into the pot whenever the thermostat valve is open. The tank should be connected to the cooking pot by inexpensive 1/4 inch copper tubing. Inside the cooking pot the feed tube connects to the thermostat valve near the bottom of the pot that will open to admit water only when the temperature is high enough to pasteurize the water. The hot water output is taken from a tube that passes through the side of the pot near the top. This output water passes through another copper tube that is in close thermal contact with the tube carrying the water from the feed tank. The outgoing water must flow in the opposite direction from the incoming water. Binding the two tubes together with copper wire ensures that heat is easily transferred between the two tubes The incoming water is therefore heated by the hot water coming from the cooking pot.. This should be done for a distance that is sufficient for complete thermal transfer (the water in the two tubes should have reached very close to the same temperature). The pair of tubes must be insulated from the surrounding air to conserve the solar heat. Since the solar heat is recycled into the incoming water, the system can process much more water. The water enters the pot at almost the temperature that is necessary for pasteurization and needs very little additional heat. Thus, the output of safe water will be sufficient for many people. The feed tank can be made from any container that will hold enough water for an extended period of operation, To increase the minimum time that the water is kept heated, a series of baffles inside the cooking pot can control the path that the water takes from input to outlet. The cooking pot would be heated by sunlight in a solar stove. There are many very good designs for solar stoves to heat the cooking pot. Some require nothing more than a piece of glass, some aluminum foil, some black paint, and some corrugated cardboard from old boxes for insulation.

All of the materials needed for construction are inexpensive, readily available, and easily assembled with local labor. The major components of this system could be sold as a low cost kit for local assembly. To prevent the output water from becoming re-contaminated by airborne organisms care should be taken to keep contamination out. Use of a pressure cooker for the heated pot will keep contaminated air out, and is convenient because the cover will already have holes to connect the input and output tubes, and the cover will help transmit heat to the pot. Because heating the water will release dissolved air, be sure that the air can be vented without causing problems with the flow of output water. Eliminating any bends in the output tube that could trap air should be enough to avoid this problem. The reason that the thermostat is placed in the input feed line is because the thermostat can then compensate for changing water levels in the feed tank. This permits long periods of unattended operation.

This pasteurizer can be used indefinitely as there are no consumables, and no fuel is used. The flow rate is self-regulating. It automatically shuts down at night and adding mirrors to direct more sunlight to the solar stove can increase the output.
If the contaminated water source in not clear, then filter it through a piece of fabric to remove the particulates before putting it in the feed tank.