How Do You Get Drinking Water From the Sea With No Power? Let the Sun Do It

The device is a shallow basin of water covered by a transparent sloping sheet. Sunlight passes through the sheet and warms the water, which evaporates, and the vapour rises and meets the underside of the cover, which is cooler because it is exposed to the air above. The vapour condenses there into droplets that run down the slope under gravity into a channel at the low edge, where they are collected. Everything dissolved in the original water stays behind, including salt, minerals and heavy metals, and so does everything living, since nothing biological survives the transition, so the output is essentially distilled water regardless of how foul the input was.

The quantity produced depends on a handful of factors:

• •The area exposed to sunlight, which is the dominant factor
• •The intensity and duration of the sunlight available
• •The temperature difference between the water and the cover
• •The depth of water, since shallow water heats faster
• •A dark basin lining, which absorbs more of the incoming heat
• •Wind over the cover, which cools it and improves condensation

A simple device of this kind produces roughly a few litres per square metre on a good day, which is the number that determines whether it is useful. A person needs two to three litres daily in moderate conditions and considerably more in heat, so supplying one person requires around a square metre of well-built equipment operating in strong sunshine. Survival manuals describe pits dug in the ground with a sheet over them, and field testing has repeatedly found these produce far less than the effort of digging costs in sweat, which is why several survival authorities now advise against them outright except where materials and time are abundant. The physics is sound and the scale is the difficulty.

A related family of devices extracts water from the atmosphere rather than from a contaminated source, and the distinction matters because the limits differ. Fog collection uses large vertical mesh screens that intercept droplets in moving fog, which coalesce and run down into a gutter, and installations in coastal Chile, Morocco and Peru produce useful quantities where the fog is reliable, requiring no energy at all. Dew collection uses surfaces engineered to cool below the dew point by radiating heat to the night sky, and yields are small. Condensation from air by refrigeration works anywhere with enough humidity and consumes substantial power, which is why devices advertised on that principle are rarely economic. All of them depend entirely on local humidity in a way a basin still does not.

The approach works where area is cheap, power is absent and the need is modest. Remote communities have used basin arrangements covering substantial areas for village water supply, with installations in Chile dating to the nineteenth century and modern versions in several arid regions. Household units sized for drinking water alone are deployed where the groundwater is contaminated with arsenic or fluoride, which conventional filtering does not remove and distillation does. Emergency kits include collapsible versions. Larger commercial desalination uses the same physical principle in far more efficient multi-stage arrangements driven by waste heat, which recycle the heat released by condensation rather than losing it, and those achieve many times the output per unit of energy.

Sunlight warms water under a transparent cover, the vapour condenses on the cooler underside and runs into a collecting channel, leaving salt, minerals and everything living behind. Output runs to a few litres per square metre on a good day, so supplying one person needs about a square metre in strong sun. Survival pits generally cost more in sweat than they return, and the approach suits arsenic-contaminated groundwater and arid village supply.