Concept of optimum basin thickness in heat exchanger-assisted solar stills

It is suggested by many works on solar stills that water thickness should be kept as low as possible to reduce its thermal inertia and obtain best results. However, when external heat addition is employed by an internal exchanger, the amount of heat addition is also affected by the still water thickness, and so, the above-mentioned finding could be proved incorrect. In this context, here, the performance of a solitary basin solar still aided by external heat addition has been revisited. At first, an expression for the maximum possible effectiveness of the above-mentioned exchanger is derived. Based on this, improved closed form solutions for the still water temperature and still productivity have been obtained using integrating factor method. It is shown that the closed form solutions presented here are mathematically superior to other analytical solutions available in the literature on the topic, in the sense that the present ones are strictly analytical, while all others use an iterative algorithm to solve the pertinent differential equation. Contrary to the available studies on solar stills which suggest keeping the water thickness to its minimum possible value, the present results reveal an optimum thickness of the same to maximize the system productivity. Using the optimized still water thickness, about 35 and 41% enhancement in the daily and annual productivity respectively can be acquired with respect to the thickness suggested by the available literature. The optimum value stems from a trade-off between the efficiency of external heat addition and the thermal inertia of the water mass. This work therefore suggests the thickness of water mass in heat exchanger-based solar stills to be designed at an optimum value for best results, and also provides useful improved expressions for performance assessment of such stills.