A naturally circulated humidifying/dehumidifying solar still with a built-in passive condenser

A numerical study has been carried out to investigate the transient thermal performance of a naturally circulated humidifying/dehumidifiying solar still. A comparison of forced circulation performance and the influence of different environmental, design, and operational parameters on the still productivity and efficiency were investigated. The naturally circulated still shows very similar results to that of forced circulation. This finding is of significant technical and economic importance. Different attempts have been considered to investigate the effect of partial storage of basin energy and partial recovery of condensation energy. The results show insignificant changes on still performance. An economic assessment of water production cost was also highlighted and showed that solar stills can challenge other technologies for special applications.

Fig. 16. Manufactured HDD solar still. 1. The thermal performance of a naturally circulated HDD solar still was investigated. The still has a simple design with a tilted configuration. The results show still productivity and efficiency of about 5.1 kg/m2.d, similar to that of a forced circulation model [2]. In addition to its simplicity, natural circulation is more economical and technically less complex than a forced circulation still. 2. The influence of different environmental, design, and operational parameters shows that increasing solar intensity (high input energy) and ambient temperature (less energy loss) improves the still’s productivity. This is an advantage of solar desalination as water production increases under summer conditions, which are consistent with the water demand during this season. 3. Different attempts to partially store basin energy and/or recover condensation energy were also studied. These attempts cover (1) only partial storage of basin energy for overnight reuse, (2) only partial recovery of condensation energy at the condenser, and (3) a combination of energy storage and partial recovery. The results show an insignificant improvement in the still’s productivity. It is more economical, therefore, to store water rather than store energy. 4. An economical assessment of water production costs shows that 1.0 m3 of product water costs 45 EP ($9). However, if the still cost is reduced to 100 EP, then 1.0 m3 will cost only 25 EP ($5). In general, solar stills cannot challenge the lower cost of large units (MSF, RO, etc.). However, for special applications of (1) very small communities demands, (2) unavailability of fresh drinking water in remote areas, (3) unavailability of energy (fuel or electricity) and (4) unavailability of technical support within the communities, solar stills seem to be the only (technically and economically) competing alternative.