Passive and active performance of a multi-side-stepped square pyramid solar still; experimental and modeling

New design of a multi-side-stepped square pyramid solar still (MSSPSS) covered with four-side glass was investigated experimentally and theoretically in passive and active moods. In comparison with single slope solar still with the same basin surface area, total area of the cover glass and the absorber area of the basin collector per square meter of solar still surface increased by 65% and 79.4%, respectively. This new design decreases the heat loss from the bottom and provides the ability of natural heat recovery of the basin's waste energy. Purification yield per square meter solar still surface (PY) for passive mood was found 2.035 for sunny day and 1.8 liters in a semi-cloudy in autumn, and 11.93, 12.39 liters for two sunny days in summer. Under active circumstances the inlet brine preheated with a salt-gradient solar pond, indicated PY of 13.99 a summer day, and 1.13 in the night, up to 13% and 9% increase in the diurnal and nocturnal production. Correlation between inlet water's rate and production has a crucial effect on PY. A thermal model was proposed to seek the effect of important parameters on the performance of MSSPSS and annual yield estimation. During the autumn, the radiative heat loss plays an important role while during the summer, the evaporation mechanism is more important. Error of the proposed model is 13.5% on average for estimation of PY and maximum error of 8.2% for thermal behavior. The estimated annual yield is around 2538 liters. Water-glass temperature difference has important effect on the MSSPSS performance. The correlation between inlet water's rate and production has crucial effect on PY.