Reading: Diagram Label Completion: Question 7

A basin-type solar still is one of the simplest technologies available for turning contaminated or saline water into drinking water, and it needs no power source beyond sunlight itself. Trials of a small still at a coastal research site in Tarrant Bay, conducted between 2018 and 2020, showed that a single square metre of still can reliably produce between two and four litres of clean water per day in sunny conditions.

At the base of the still sits a shallow basin, lined with a black liner that maximises the amount of solar radiation converted into heat rather than reflected away. Untreated water, whether drawn from the sea or from a brackish well, is poured into the basin to a depth of only a few centimetres, since a shallow layer heats and evaporates far more quickly than a deep one. As the sun warms the black liner, water evaporates from the surface of the basin, leaving behind salts, minerals and other dissolved contaminants, which are not carried upward in vapour form.

The rising water vapour condenses on the underside of a sloped glazing cover, a sheet of glass or clear plastic mounted at an angle above the basin. The slope of the glazing cover is not simply for appearance: it ensures that condensed droplets run downhill under gravity rather than dripping straight back into the basin and re-mixing with the untreated water. At the lower edge of the glazing cover, a narrow collection trough catches the runoff, and an outlet pipe then carries this clean water out of the still into a storage container standing beside it.

Because the still must retain heat rather than lose it to the surrounding air, the basin sits within an insulated frame, which reduces heat loss through the sides and base and keeps the internal temperature high enough for continuous evaporation. Left inside the basin, the increasingly concentrated leftover liquid, known as the brine, must eventually be drained away through a brine outlet at the base of the still and replaced with a fresh batch of untreated water.

The diagram below shows a cross-section of the solar still, from the base upward. Label the diagram below. Choose NO MORE THAN TWO WORDS from the passage for each answer.

1 ____________________. The shallow container at the base of the still that holds the untreated water 2 ____________________. The dark layer inside the basin that absorbs sunlight and converts it into heat 3 ____________________. The sloped, transparent surface above the basin on which water vapour condenses 4 ____________________. The narrow fitting at the low edge of the sloped surface that collects the condensed runoff 5 ____________________. The fitting that carries the collected clean water out to a storage container 6 ____________________. The layer surrounding the basin that reduces heat loss through the sides and base 7 ____________________. The fitting at the base of the still used to drain away the concentrated leftover liquid