Experimental optimisation of a simple basin solar still; Improved heat loss management and evaporation rate

Abstract

Numerous modi cations were made to simple basin solar stills with the aim to improve the performance of the stills and develop an understanding of the operation of the system. The changes were focused on reducing energy losses from the system, and increasing the rate of both evaporation and condensation. The stills had a cover area of 0:5m2 and were coated with Duram c Durapond as the absorber surface and waterproo ng. A cost target of 0.08 ZAR per litre was set in order for the system to be competitive with existing small-scale desalination systems. To reduce energy losses the e ect of insulation thickness was tested. Using Arma exR

foamed nitrile rubber insulation, increasing the thickness such that the thermal resis- tance values were increased by 0:25K W􀀀1, 0:33K W􀀀1, and 0:58K W􀀀1 resulted in increases in yield of 9 %, 30 %, and 27% on average when compared to the reference still. It was observed that the back wall of the still reached exceptionally high temperatures, between 70 C and 80 C; to decrease losses through the back wall and better utilise that energy, aluminium panels were added to the inside of the still. This resulted in a higher rate of increase of water temperature and maximum water temperature in the still. The aluminium panels successfully redirected energy from the back wall of the basin still, reducing the temperature of the surface by around 10 C. This did not result in the desired increase in yield as it was observed that condensation occurred on the panels themselves, overnight, thus resulting in a loss of condensate that would otherwise have been collected. The evaporation rate was modi ed primarily by increasing the absorbance of solar irradi- ation, this was done by testing a polyvinyl chloride (PVC) coated textile as the absorber surface, adding a carbon black nano uid, adding activated charcoal, and adding a carbon felt. The PVC absorber improved the yield by 98% on average when compared to the reference still. The nano uid proved impractical as the uid degraded and the particles settled out with multiple heating-cooling cycles, additionally the increase in yield when compared to the reference still increased from 46% to 72% as the particles settled sug- gesting that the nano uid performed worse when the particles were in suspension. The activated charcoal resulted in an increase in yield of 98% on average, and the carbon felt gave a 110% increase. The carbon felt caused lower bulk water temperatures due to the tendency of the felt to oat just beneath the surface of the water and allowing for evaporation to occur from a thin lm of water which heated up signi cantly quicker due to its small thermal mass. All modi cations signi cantly reduced the time between start-up and the onset of condensate collection which was shown to increase the yield. Observation of the still during operation suggested condensation to be a limiting step in the process due to the speed at which droplets would re-form after running down the cover plate. Attempts at increasing the condensation rate included increasing the internal area by milling grooves into a portion of the plate, when compared to the reference still an improvement between 7% and 27% was observed in the yield. Other modi cations included the addition of a heat sink to the top of the cover plate on the outside of the still where the temperature was highest; visually it could be seen that condensate formed more quickly around the heat sink but no signi cant e ect on the yield or overall cover temperature was observed. Manually tapping on the cover improved the yield by forcing drop movement down the cover, this suggests drop movement to be a limiting step in the production of condensate. A nal still was designed and constructed using the information gained from the exper- iments performed. The still achieved water temperatures up to 11 C hotter than the reference still and resulted in a 180% increase in yield when compared to the reference still. Analysis of the energy balance for the solar still indicates that the majority of the losses are linked to the cover plate - re ective, radiative, and convective losses, as well as radiative losses from the base of the still. It is of course necessary for some heat to be removed from the cover in order for condensation to occur, but the high temperature of the cover results in unnecessary losses which greatly reduce the e ciency of the system. It is recommended that steps be taken to reduce the cover temperature and provide an additional surface on which condensation can occur.