Solar Thermal Water Pasteurization

Methods for treatment of supply water which are appropriate for developing countries include chlorination, filtration, unltraviolet irradiance, and pasteurization. This study discusses the fundamentals of flow-through solar thermal water pasteurization and illustrates system optimization. We derive an analytical expression for the flow rate using a linear collector model and assuming pureconduction heat transfer in the heat exchanger. Flow initiates near that irradiance in which the corresponding stagnation temperature equals the pasteurization temperature. Flow increases monotonically with increasing irradiance, decreasing pasteurization temperature, and increasing heat exchanger area. Predicted clear-day production is 900, 1210, 2700 liters/d/m2col (238,320, and 712gal/day/m2col) for non-selective, selective, and evacuated tube systems, respectively, assuming optimally-sized heat exchangers. Optimum heat exchanger size minimizes water treatment cost, and depends upon the ratio of hear exchanger cost to collector cost. It is observed that the optimum hear exchanger size corresponds to roughly equal investments in the heat exchanger and the collector,independent of the relative costs.