A concentrated solar still is a system that uses the same quantity of solar heat input (same solar collection area) as a simple

solar still A solar still distills water with substances dissolved in it by using the heat of the Sun to evaporate water so that it may be cooled and collected, thereby purifying it. They are used in areas where drinking water is unavailable, so that clea ...

but can produce a volume of freshwater that is many times greater. While a simple

is a way of

distilling Distillation, or classical distillation, is the process of separating the components or substances from a liquid mixture by using selective boiling and condensation, usually inside an apparatus known as a still. Dry distillation is the heati ...

water by using the heat of the sun to drive evaporation from a water source and ambient air to cool a condenser film, a concentrated solar still uses a

concentrated solar thermal Concentrated solar power (CSP, also known as concentrating solar power, concentrated solar thermal) systems generate solar power by using mirrors or lenses to concentrate a large area of sunlight into a receiver. Electricity is generated when ...

collector to concentrate solar heat and deliver it to a multi-effect evaporation process for distillation, thus increasing the natural rate of evaporation. The concentrated solar still is capable of large-scale water production in areas with plentiful solar energy.

Performance

The concentrated solar still can produce as much as twenty times more water than the theoretical maximum of a standard solar still and in practice, can produce as much as 30x the volume. A typically 25% efficiency standard solar still (not allowing for any recovery of rejected latent heat), as the latent heat of vaporization of water is 2.26 MJ per kilogram, should evaporate kg (or liters) of water per m² per day in a region with an average daily

solar irradiation Solar irradiance is the power per unit area (surface power density) received from the Sun in the form of electromagnetic radiation in the wavelength range of the measuring instrument. Solar irradiance is measured in watts per square metre (W/m ...

of 21.6 MJ/m² ( watts/m²), or liters per year (like a

precipitation In meteorology, precipitation is any product of the condensation of atmospheric water vapor that falls under gravitational pull from clouds. The main forms of precipitation include drizzle, rain, sleet, snow, ice pellets, graupel and hai ...

height of ). A twenty times more productive still would have a daily output of or yearly.

Heat integration

Multiple stage evaporation

The concentrated solar still implements a method for recovering the latent heat of the distillate vapor not captured and reused by a standard solar still. This is done by using multiple stages of evaporation in series (see

multiple-effect evaporator A multiple-effect evaporator, as defined in chemical engineering, is an apparatus for efficiently using the heat from steam to evaporate water. Water is boiled in a sequence of vessels, each held at a lower pressure than the last. Because the b ...

). The latent heat of the distillate vapor produced in the n-1 stage (or effect) is recovered in the nth stage by boiling the leftover concentrated brine from the n-1 stage which produces distillate vapor whose latent heat will be recovered in the n+1 stage by boiling the leftover concentrated brine from the nth stage. Since brine is continuously concentrated in each stage, its boiling point will continue to rise under standard conditions. To overcome the boiling point elevation of the brine, each evaporator stage operates at a lower pressure than the previous stage, which effectively reduces the boiling point, allowing for sufficient heat transfer to take place in each stage. This process can be repeated until the distillate conditions are sufficiently degraded (i.e., pressure and temperature are very low and the distillate vapor volume is very large).

Heat pump

The final evaporation stage produces distillate vapor that is considered to be at very poor state conditions. This vapor can either be condensed in a final condenser, in which case its latent heat will be shed as waste, or it can be condensed by using a

heat pump A heat pump is a device that can heat a building (or part of a building) by transferring thermal energy from the outside using a refrigeration cycle. Many heat pumps can also operate in the opposite direction, cooling the building by removing ...

, in which case its latent heat (or a portion of it) can be recovered. In the latter case, the heat pump effectively "upgrades" the state conditions of the latent heat to more usable conditions (higher temperature and pressure) by performing work (e.g., compression). The conditions can be sufficiently upgraded such that the recovered heat can be used to provide additional heat for evaporation in the first effect.

References

{{reflist Solar power Water treatment Water technology