Freshwater production by combination of solar still, earth-air heat exchanger and solar chimney for natural air draft

Abstract

A theoretical study of humidification-dehumidification (HDH) processes inside a system combining a solar still with an earth-air heat exchanger and a solar chimney was introduced. Energy and mass balances in a transient mode for the solar still and the earth-air heat exchanger in addition to the solar chimney were formulated and numerically simulated. The solar radiation heats water in the solar still basin, which in turn warms and humidifies dry air entering the solar still due to the heat and mass transfer into the airstream. When the glass/or the EAHE pipe wall temperatures are lower than or equal to the dew point of the humid air, the vapour condenses and runs down. The results show that the water, the air, and the glass temperatures increase with time to reach their maximum values (66.9 oC,61.8 oC, and 61.24 °C respectively) in June from 15.00 to 16.00. The humidity ratio along with the solar still length increases to reach a uniform value whenever the moist air is saturated. At the same time, the humidity ratio along EAHE decreases due to the condensation of the moisture inside the EAHE pipe. The condensation rate in the EAHE decreases gradually along a pipe length of 70 m until it completely vanishes at the pipe outlet. The hourly condensation rate in the EAHE attaining its maximum value of 49.27 kg/hr. at 14:32 (Basra local time) in June where the solar radiation is at maximum value. Also, the results indicated that the productivity of freshwater in the solar still is strongly affected by the water, air, and glass temperatures. The maximum productivity in the solar still achieved in March was 157 kg/day while it was 369 kg/day in July for the EAHE. The increase of the air velocity increases the productivity in EAHE and decreases it in the solar still. The daily average freshwater production for the system (solar still and the earth-air heat exchanger) throughout the year was found to be 207.44 kg. The thermal efficiency of the system over the interval from 6:00 to 19:00 was found to be 0.23–0.55. The economic evaluation showed that the cost of freshwater production was 0.0282 $/kg. A comparison of the current analysis with other works showed a good agreement.

Keywords

• earth-air heat exchanger
• open-type solar still
• solar humidification-dehumidification