Fig. 17. Schematic diagram of single slope single basin solar still with PCM .Figure optionsDownload full-size imageDownload as PowerPoint slide
The UMI-77 balance equations for basin liner and phase change material (charging mode) are given byequation(172)I(t)sτgτwαb=hb(Tb−Tw)+KbLb(Tb−Tpcm)equation(173)(Kb/Lb)(Tb−Tpcm)=(Mequ/Ab)(dTpcm/dt)+Ub(Tpcm−Ta)(Kb/Lb)(Tb−Tpcm)=(Mequ/Ab)(dTpcm/dt)+Ub(Tpcm−Ta)
For a selected time interval ?t, the energy balance equation for phase change material (discharging mode) is given byequation(174)ForTpcm=TmlMpcm(hfg)pcm/AbΔt=h′b(Tpcm−Tb)+Ub(Tpcm−Ta)equation(175)ForTpcm≠Tmlh′b(Tpcm−Tb)+Ub(Tpcm−Ta)=(Mequ/Ab)(dTpcm/dt)where h′b=Kpcm/Lpcm is the conductive heat transfer coefficient from the PCM to the basin liner.
The still performance with and without PCM was studied by computer simulation on typical summer and winter days. It was found that the daily productivity is found to decrease slightly with increasing the mass of PCM. But the overnight and daily productivities are significantly increasing with the increase in the mass of PCM. Comparisons between the results obtained for the still with and without the PCM showed that, using 3.3 cm of stearic acid under the basin liner, 9.005 (kg/m2/day) of fresh water can be obtained on a summer day with a daily efficiency of 84.3% compared to 4.998 (kg/m2/day) when the still is used without PCM. The PCM becomes more effective at lower masses of basin water during the winter.