solar water distillation: energy exchanges

1: Introduction | 2: Energy Exchanges

The basin-type still is a relatively simple device to construct, but the energy transfer processes occuring within it are quite complex, as summarised in Fig 3:

Fig 3: Energy flow in a basin-type still (after Lof 1961).

Direct and diffuse radiation is absorbed by the black bottom of the basin. There are some reflection losses from the cover, the water surface, and the bottom of the basin itself, along with some absorption of heat by the cover. Energy is transferred from the basin bottom to the water largely by conduction and convection, with a small fraction of the absorbed heat being lost by conduction into the ground.

Energy is transferred from the water surface to the cover by three mechanisms. The main one is vapourisation of water from the surface. By diffusion and convection, the vapour is then transferred to the cover, where its latent heat is liberated by condensation. The second mechanism is the free convection of air inside the still. The third mechanism is the radiation of heat from the water surface to the cover. The cover may be partially transparent to thermal radiation, some of which may pass through into the atmosphere.

Heat conducted through the cover is transferred to the atmosphere by convection and radiation. Additional thermal flows may be present as sensible heat in water entering and leaving the system, while the leakage of water and vapour would also involve thermal losses.

Michael Hall
October 2007