Economically viable solar stills

Goldie, I. (Ian) (2003-12)

Thesis (PhD)--Stellenbosch University, 2003.

Thesis

ENGLISH ABSTRACT: The use of solar distillation as a means of desalination to provide potable water to communities in remote and arid regions has often been discarded on economic grounds mainly because of the inherent low efficiency of relatively expensive solar distillation units (solar stills). Closer analysis of this constraint showed that by following a mainly Physical Chemistry approach (rather than the traditional engineering one), the technology could be made more economically attractive through (1) lowering the construction cost of the solar still and / or (2) increasing its operational life and / or (3) increasing its efficiency. The study into different solar still designs showed that, despite its limitations, the basin solar still is preferred due to its simplicity, ease of operation and low maintenance. Given the solar distillation process as formulated in this study, substitute durable components could be identified and optimised for this design. A basin solar still that costs about 80% less than a reference unit without an apparent drop in performance was consequently developed and successfully tested up to full plant scale. An in-house experimental facility that simulates solar still behaviour under controlled environmental conditions was developed to simulate conditions of lower radiation intensity and productivity, which enabled research into performance enhancement and microbiological water quality. It was found that (a) absorption of the radiation by the water plays an important part in productivity and that a productivity decrease of up to 33% can be expected as the black interior lining becomes contaminated, (b) the simulated winter productivity of the basin still was about 25% of the summer value due to the lesser amount of solar radiation hours, the lower angles of radiation incidence onto the cover and the decrease in water area receiving direct radiation, (c) an annual productivity increase of about 10% can be obtained when a particular reflector configuration is fixed behind the basin, and (d) the microbiological pasteurisation temperature is less than the distillation onset temperature. This study played an important role in making solar still technology affordable for use by poor, rural communities, as was demonstrated by the successful use of the research product (basin solar still) in a pilot drinking water plant at a typical target community. This was made possible through the use of evaluation models developed as part of this research, which addressed relevant construction and performance factors influencing the economic viability of the specific solar still. The following outcomes of this study can be regarded as new contributions to the field of solar distillation technology, namely (a) a test matrix that can be used to screen solar still construction materials at component level, (b) an evaluation model that can be used to determine the economic viability of solar stills at a given location, (c) a laboratory testing facility to study solar still behaviour under controlled conditions and (d) a solar still design that has been optimised and tested for local field application.

AFRIKAANSE OPSOMMING: Die gebruik van son-energie om brakwater deur middel van distillasie te ontsout en sodoende drinkwater aan afgeleë gemeenskappe te verskaf, is herhaaldelik in die verlede as ekonomies nie-lewensvatbaar aangetoon. Dit is hoofsaaklik toeskryfbaar aan die inherente lae effektiwiteit van sodanige stelsels, asook die hoë kapitaalkoste daaraan verbonde. Die hoofsaaklike faktore wat die tegnologie bekostigbaar kan maak, is egter hieruit afgelei en volgens 'n Fisiese Chemie benadering geoptimeer, naamlik (1) vermindering van die konstruksiekoste van die sondistilleerder-eenhede, en / of (2) verlenging van die operasionele leeftyd daarvan, en / of (3) verhoging van effektiwiteit van die stelsel. Die literatuurstudie het aangetoon dat die pan-tipe sondistilleerder-eenheid verkieslik as navorsingsbasis is omdat dit eenvoudig en maklik bedryf- en onderhoudbaar is. Vanuit die geformuleerde beskrywing van die sondistillasie-proses kon plaasvervangende komponente vir die sondistilleerder-eenheid geïdentifiseer en getoets word, wat daartoe gelei het dat 'n eenheid met dieselfde funksioneringseienskappe vir sowat 80% goedkoper ontwikkel kon word. 'n Laboratoriumopstelling waarmee lae bestralingskondisies gesimuleer kan word, is ontwikkel om navorsing op produktiwiteitsverbetering en mikrobiologiese produkwaterkwaliteit te doen. Dit is aangetoon dat (a) absorpsie van die inkomende bestraling deur die water belangrik is en dat 'n sondistilleerder-eenheid tot 33% minder produktief kan wees indien die swart voering binne die eenheid besoedel sou word, (b) winterproduktiwiteit daal tot 25% van somerproduktiwiteit as gevolg van die verminderde sonlig-ure, die laer invalshoek van die bestraling op die glasdeksel, en die gepaardgaande verkleining van die blootgestelde water-oppervlakte, (c) 'n gemiddelde produktiwiteitsverhoging van 10% deur weerkaatsers agter die sondistilleerder-eenheid teweeggebring kan word, en (d) die distillasie-temperatuur die water-ontsmettingstemperatuur oorskry. Hierdie navorsmg het 'n belangrike rol gespeelom sondistilleerder-tegnologie bekostigbaar vir behoeftge gemeenskappe in landelike gebiede te maak. Dit is gedemonstreer deur die suksesvolle toepassing van die navorsingsproduk (sondistilleerder-eenheid) in 'n drinkwatervoorsieningsprojek by 'n tipiese teikengemeenskap. Dit is vermag deur die gebruikmaking van evaluasie-modelle wat gedurende hierdie studie ontwikkel is, en wat die toepaslike konstruksie- en funksioneringsfaktore wat die ekonomiese lewensvatbaarheid van die eenheid beïnvloed, aanspreek. Die volgende navorsingsprodukte kan beskou word as nuut in die veld van sondistillasie-tegnologie, naamlik (a) 'n toetsmatriks wat gebruik kan word om komponente vir sondistilleeerders te evalueer, (b) 'n ekonomiese evalueringsmodel wat gebruik kan word om die bekostigbaarheid van sondistilleerders in 'n bepaalde gebied te bepaal, (c) 'n laboratorium-toetsfasiliteit om die werking van sondistilleerders onder beheerde kondisies te ondersoek en (d) 'n sondistilleerdereenheid wat doelgeskik vir plaaslike omstandighede ontwikkel en getoets is.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/53401
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