Low cost thermoformed solar still water purifier for D&E countries

Abstract

Introduction Solar distillation mimics nature’s hydrologic water cycle by purifying water through evaporation (using solar energy) and condensation (rain). It is one of the most basic purification systems available today to obtain high quality drinking water and can remove non-volatile contamination from almost any water source. This low-tech technology should therefore be ideally suited for developing and emerging countries where sun shines in abundance. In the past century numerous designs have been realised with footprints ranging from 0.5 m 2 to thousands of square meters. Despite all efforts, this intriguing technology has not been applied widely yet. Among the challenges that remain are: (1) its low yield, (2) obtaining local commitment to operate/maintain large scale systems properly, and (3) relatively high initial investment costs. The objective of this study has been to address challenges 1 and 3 by using standard plastic thermoforming technology to realize a small scale single slope solar still for personal use (2–4 l per day) with adequate efficiency and at low production costs. Materials and methods The solar still consists of two parts: a basin that holds the dirty water and a transparent tilted cover onto which the clean water vapour can condense. The basin has a footprint of 1.34 m 2 and is made of a 3 mm thick sheet of black high-density polyethylene (HDPE) which is thermoformed using standard equipment for making fish-ponds. This allows for the incorporation of detailed features, like reinforcements and a clean-water collection gutter, at no extra cost. The transparent cover is made of UV stabilised low-density PE-foil which is under a slope of 10° to transport condensed water droplets to the lower located collection gutter. Throughput and purification performance were evaluated in duplicate at our Bangalore R&D facilities in India, over a short term (5 day) period. Solar radiation was measured using a Pyranometer. The system was loaded with 40 l of laundry rinse water. Results At an average solar radiation of 12.95 MJ/day/m 2 the average yield of purified water was 3 l/day. This resulted in a calculated overall system efficiency of 39%. Purification performance (contaminated versus purified water) of the solar still loaded with the most contaminated water source was: Total dissolved solids (TDS) from 2925 ppm to 40 ppm, pH from 9.6 to 5.5, conductivity from 6130 mS/cm to 26 mS/cm, turbidity from 394 NTU to 0.4 NTU, total viable count (TVC) from 314 million cfu/ml to <10 cfu/ml. Conclusion Thermoforming allowed for the realisation of a single slope solar still that can sustainably produce high quality drinking water at point of use from waste water with an above average efficiency and at a manufacturing price (in The Netherlands) of below €25-per system. Next step should focus on a long term evaluation (months, instead of days) to access the full potential of the solar still to produce safe drinking water at point of use in an economical and reliable way.