Document Type

Restricted Campus Only

Publication Date

4-26-2007

Abstract

The "Solar Water Heater" (SWH) group designed a portable solar water pasteurizer (SWP) for U.S. backpackers in central Texas-that is, a device that heats clear, colorless, non-turbid surface water to al least 65°C for at least six milrntes to make it potable using solar power alone and at a rate of nine liters per day. To avoid "re-inventing the wheel," the group researched existing designs of solar heaters and found many helpful ideas which it could leverage. Findings included heaters operating in either a batch or a continuous fashion, active heaters which use valves or pumps to control water temperature and flow or passive heaters which work on differences in water density, and features such as collapsible solar collectors to enhance the device's portability. From this research, the group decided that to harness solar energy, solar collectors rather than solar panels in conjunction with electric heaters would fit its needs, where solar collectors convert solar to thermal energy while solar panels are photovoltaic cells which convert solar to electrical energy. This choice was made because solar collectors are much more efficient and are less costly than solar panels. Also, for the initial design which the group developed-called the prototype design-the group decided to use a continuous mode of operation because such a mode generally produces more water per unit time and requires much less monitoring of water temperature than a batch mode.

The prototype design which the group then developed involved a density-driven convection loop. It consisted of an input tank to hold the un-pasteurized water, an output tank to collect the treated water, and the pasteurizing unit itself in the middle which contained the circulation loop and two solar collectors connected to the loop. After some mathematical analysis, a setpoint temperature of 80°C was chosen, which is high enough for the water to have been above 65°C for at least six minutes. After modeling the device to determine the collector size needed to heat the water, the prototype was built using readily available and relatively inexpensive materials when possible to save time and money. This led to selecting wood for the housing, or outer "shell," of the collector, sheet metal for the absorber plate which "collects" the heat and transfers it to the copper piping chosen to place within the collector, CPVC tubing for the loop, and other auxiliary items.

To test the device in a controlled laboratory setting, the group used an array of 250-W heat lamps to simulate sunlight. Thermocouples were used to measure the temperature of the water throughout the loop. After the initial round of testing, it was discovered that the collectors functioned properly, heating the water to the necessary temperatures; however, once the water had reached its setpoint, it was traveling back into the input tank instead of exiting the circulation loop into the output tank. To remedy this, a check valve was implemented, but it was found that the valve required high water pressure to function properly in conjunction with the loop. The entrance height to the loop was also lowered, creating more resistance for the hot water to flow back to the input tank, but this ultimately did not solve the backflow problem either.

Because of these shortcomings, the group moved to a final design which abandoned the circulation loop in favor of a thermostatic valve commonly found in car radiators. The thermostatic valve used actuates at a temperature of 82°C. Thus, when the valve is put in series with the copper pipe, it opens to allow hot water to leave the collector and closes to retain cooler water until it heats up again. In addition, to improve the portability of the device, the collector area was cut in half, the steel was replaced with aluminum for the absorber plate, and the housing of the device was made of fiberglass-reinforced material. These modifications decreased the weight of the device from 55 to 23.5 pounds and reduced the collector volume such that it could fit in a typical, larger size hiking backpack.

When the final design was tested, it worked as designed. The collectors heated the water enough to cause the thermostatic valve to open, sending the hot water out of the collector and into the output tank. The valve then closed as cold water replaced the hot water. However, the thermostatic valve was observed to allow m1-pasteurized water to leak through to the output tank while in the closed position. This meant that much of the water in the output tank was not pasteurized, and therefore the exact amount of water pasteurized was undetermined. However, based on the observed rate of the leak compared to that of pasteurized water, a volumetric flow rate of 1.1 L/hr was roughly estimated, meaning that 9 L would be pasteurized in about 8.4 hours. Also, while filling the collector with water, air pockets formed in the piping, slowing the hot water in reaching the valve and thereby increasing pasteurization time. Without air pockets and a leaking valve, the SWP is expected to meet the original throughput criterion.

The group's recommendations for improving the design are: I) to either seal the thermostatic valve or replace it with one that has a good seal, and 2) to install an air release valve in the collector pipe near the thermostatic valve to alleviate air pockets at start-up.

Comments

Group Advisor: Dr. Diana Glawe

Course Administrator: Dr. J. P. Giolma

ENGR 4381-4382: Engineering Design VII-VIII

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