A study on the upper limit efficiency of solar still by optimizing the mass transfer

Abstract

Solar still is an green energy, low-cost and easy-to-maintain desalination system. Enhancing the mass transfer is a very important strategy for improving solar still. However, the analysis of the upper limit performance of the solar still related to the optimized mass transfer is lacking. In this work, by assuming the most ideal heat and mass transfer condition in solar still, the theoretical upper limit performance of solar still is given. The theoretical results reveal that the mass transfer in solar still is sensitive to the inner air circulation. The mass transfer in the solar still reaches up to the upper limit quickly when the air circulation is enhanced. By enhancing the mass transfer process, the theoretical upper limit of the energy efficiency in solar still is around 87%, 91.5%, and 94.5%, respectively, for the input power density at 300 W/m2, 500 W/m2, and 700 W/m2. Experimental results show that in a solar still with the basin size at 25 cm × 25 cm, the theoretical upper limits can be approached by only using 0.2 W ∼ 0.6 W of fan power. Compared to the efficiency of conventional solar still without modification, the upper limit energy efficiency is 48%, 28%, and 20% higher, respectively, under 300 W/m2, 500 W/m2, and 700 W/m2 input power density. Meanwhile, to reach the upper limit, the total extra cost of the modification can be as low as 3 $ by using a fan in solar still. This work offers a new understanding of the mass transfer process and provides an effective and economical optimizing way not only for the solar still but other themal systems with heat and mass transfer.