A solar concentrator can purify water by boiling it and condensing the steam, but distillation costs 2,591 kJ per kilogram against 188 kJ for pasteurization, so it should be used only when the water is salty or chemically contaminated. Making 5 L of distilled water an hour needs about 3.6 kW of heat and 20 L an hour about 14.4 kW, far more than a small trough supplies, so expect only 1-2 L an hour from a small trough or dish.
Part of High-Temperature Applications
Solar water purification here means boiling or distilling water with the heat of a concentrator, to remove microbes, salt and many dissolved contaminants.
Pick The Treatment By What Is In The Water
Energy to treat 1 kg of water starting at 20°C (standard values: 4.18 kJ/kg·K; 2,257 kJ/kg latent heat):
| Treatment | Heat per kg | Per litre in kWh |
|---|---|---|
| Pasteurize to 65°C (rise 45 K) | 4.18 × 45 = 188 kJ | 0.052 |
| Heat to 100°C (rise 80 K) | 4.18 × 80 = 334 kJ | 0.093 |
| Evaporate (distil) | 334 + 2,257 = 2,591 kJ | 0.72 |
Distilling therefore costs 2,591 ÷ 188 = 13.8 times the heat of pasteurizing and 2,591 ÷ 334 = 7.8 times that of just heating to 100°C. If the problem is microbes, boiling or pasteurizing is cheaper (Boiling Purification, Water Pasteurization (SODIS & WAPI)). Distillation earns its cost when water is salty, or carries dissolved chemicals that boiling does not remove.
Distillation does not remove everything. Volatile contaminants such as alcohols and some solvents boil off with the water and can condense into the product, so do not distil water polluted by fuel or solvent and drink it. The first fraction can carry volatiles; discard it.
How Much Water From A Given Collector?
Heat for 1 kg of distillate from 20°C feed is 2,591 kJ, ignoring any recovery of the heat released in the condenser. The heat needed per hour for a given output:
- 5 L/h: 5 × 2,591 = 12,955 kJ/h, and 12,955 ÷ 3,600 = 3.6 kW of net heat.
- 20 L/h: 20 × 2,591 = 51,820 kJ/h, and 51,820 ÷ 3,600 = 14.4 kW of net heat.
At 50% collector efficiency and 1,000 W/m², those are 7.2 m² and 28.8 m² of aperture. Going the other way, a 2 m² trough nets about 1 kW, which distils 1 kW ÷ 0.72 kWh/L = 1.4 L an hour; a 1.8 m dish delivering 1.5 kW distils at best 1.5 ÷ 0.72 = 2.1 L an hour, and less in practice. The parent page’s 5-20 L/h from a trough of 2 m or more is therefore several times too high for troughs of the size it describes: plan on 1-2 L/h from a small trough or dish.
What You Need
- A concentrator and tracker (Parabolic Trough)
- A boiler vessel — dark, with an open path for steam
- A condenser — a cooled pipe or vessel, in the shade, draining to a clean container
- Clean tubing and containers (not ones that held fuel)
- A cover for the concentrator
Method: Run A Solar Still
Step 1: Fill the dark vessel with feed water, never to more than half, with a steam outlet that is never blocked.
Step 2: Run the steam line downhill into a condenser kept cool by shade or a wet cloth, ending in a clean container.
Step 3: Uncover and track. Discard the first cupful of distillate. Collect the rest.
Step 4: Stop before the vessel boils dry; salt or sludge left behind overheats the vessel.
Common Mistakes
- Closing the steam outlet. A sealed boiler under a concentrator can burst.
- Distilling to remove microbes. It wastes about 14 times the heat of pasteurization.
- A condenser in the sun. It stays too warm to condense.
Safety
Scalding steam, a sealed boiler, and concentrated sunlight
Steam from a solar still scalds severely: 1 kg of condensing steam releases 2,257 kJ. Never block the steam path; a sealed vessel under the focus is a pressure vessel with no relief. Concentrated sunlight ignites materials at once and blinds permanently and painlessly; keep the focus away from the operator and cover the concentrator when not in use. Distilled water from polluted feed can still carry volatile poisons.
What’s Next
- Learn next: Water Purification — choosing among boiling, SODIS and filtering before reaching for a still
- Combine with: Steam Generation — the same heat budget for power