Solar powered distillation of water can be defined as measures to separate and extract clean water by vaporization. This can be particularly useful to turn seawater, brackish or even contaminated water into clean water safe for drinking.
The history of solar powered water distillation started with Aristotle, already back in the 4th century BC, described a technique of solar powered water distillation. However, it was not before late 19th century, that the first large scale solar water distillation plant was built. This plant, located in Chile, supplied as much as 20 000 liters of safe water to nearby inhabitants.
What is the difference between solar distillation and solar desalination?
Before we get into how these processes work let’s take moment to clear any confusion around distillation and desalination. As mentioned above, solar distillation is separation of clean water by vaporization. Desalination refers to the process where salt and other minerals are removed from water. While heat is necessary for distillation of water, electrical or mechanical driven systems can use reverse osmosis to desalinate water.
How Solar Water Distillation Works
Distillation of water with solar energy is quite simple. Think about how nature creates rain: The Sun heats and evaporates water, which at the same time is separated from salt, dirt or anything else for that matter. When the temperature and pressure is right, the water molecules reforms and returns to liquid. Regardless of where in the World you are located, if you catch rain before it hits the ground, it is considered safe to drink.
Devices that are built around the concept of solar distillation all mimic the natural process of rain formation and can be described in two simple steps:
- Evaporation to remove impurities.
- Condensation to collect the water.
Different Still Designs
Single-Basin Solar Stills
A basin is covered by sloped glass or plastic where the evaporated water is condensed and lead into a rainwater channel. The darker the color of the basin, the more sunlight is turned into heat.
The clean water output is higher in the evening due to greater temperature differences between warm water on the inside of the device and the outside ambient air.
A stationary installation should be made out a glass basin to ensure longevity, but plastic would be better for a portable solar still.
An emergency solar distillation is essentially the same thing as the single-basin still described above except it is simpler.
A mixing bowl can be used as the basin and plastic foil is put on top. A rubber holds the two together. Then a small stone is placed in the middle on top of the plastic foil and a cone shape forms. A cup and saltwater can be placed in the mixing bowl you have an emergency still. In case you don’t have a mixing bowl you can dig a pit to use as your basin.
Multi-Stage Flash Distillation (MSF)
This is where it gets a little bit more advanced. Multi-stage flash distillation (as well as multiple-effect distillation below) is used in larger scales where distillation plants are made to supply clean water for hundreds if not thousands of people.
Multi-stage flash distillation is used to distill seawater by flashing water (lowering pressure to induce vaporization) in several stages. Distillation plants using multi-stage flash supplies 85% of all desalinated water worldwide.
Multiple-Effect Distillation (MED)
Another big one, as with multi-stage flash distillation above, multiple-effect distillation also consists of several stages (effects). Seawater is heated by steam in tubes, leading to partial evaporation and more steam in the next stage. The process is repeated several times. Every stage reuses some of the energy from the previous stages.
Uses of Solar Water Distillation
People Die from Unsafe Drinking Water
Humans can go weeks or even months without food, but just after a few days without water, chances of survival are getting slim. Water is absolutely essential. Most people take a supply of fresh and clean drinking water for granted, but in some places in the World or in emergency situations clean drinking water is not always readily available.
World Health Organization (WHO) states that almost 1 billion people don’t have adequate access to water that is safe for drinking and that 2 million deaths are attributable to unsafe water, sanitation and hygiene every year. Solar powered distillation of water can help to reduce these numbers.
Cooking, Bathing and Washing
These are also common uses of solar-distilled water. By using solar distillation of water, people are less dependent on rainwater to supply all their needs.
As mentioned with the solar emergency distillation stills above, solar distillation is a good way to provide water in emergency situations. In places where hurricanes are likely to strike, and electricity and water supplies can be cut, solar distillation can be lifesaving.
Lifeboats are often equipped with solar stills. During the second world war, the U.S. Navy produced more than 200 000 inflatable solar stills for their lifeboats.