A Note on the Significance of Entropy Generation and Work Fluxes on Humid Air Inside a Solar Still Due to Double-Diffusive Natural Convection

The motion of the humid air inside a solar still is due to double-diffusive natural convection, where convective heat and mass transfer phenomena are encountered. The humid air, an intermediate medium between the evaporating and the condensing surfaces of the solar still, is regarded as a heat engine that absorbs heat from the free surface of the warmed brackish or saline water and converts it into work to carry up the water vapor molecules to the inner surface of the glass cover. Like all heat engines, the humid air work is accompanied by a degraded energy due to the presence of irreversibilities. The aim of the present study is to give answers about the internal irreversibility origins and the associated work fluxes of the humid air inside the solar still due to double-diffusive natural convection. It is for the first time that the internal entropy generation and the work fluxes formulations are developed and investigated for the humid air inside a solar still. It is found that the irreversibility rate due to fluid friction is the major contributor of the total entropy generation for N > 0.3. The increase in the temperature and the concentration gradients enhances the augmentation of all irreversibility rates. The second law efficiency is an important parameter that should be evaluated to assess the system performance. The mean value of the irreversibility percentage is found equal to 6.83%. The reversible and the lost work fluxes increase with the increase in the humid air temperature.

Automated summary

The study investigates the motion of humid air inside a solar still, explaining its role as a heat engine and the impact of irreversibilities on system performance. It is found that fluid friction is the major contributor to entropy generation, and that temperature and concentration gradients enhance irreversibility rates. System efficiency should be evaluated based on reversible and lost work fluxes.