Solar Water Distillation: several designs compared

As I discussed in a previous post, water is one of the more difficult prepping issues. Suppose that one disaster or another knocks out the power. You won’t have town/city water on which to rely. There are many different water purification products available: filters, tablets, bleach, etc. But each is fairly limited in the amount of water that can be purified. And the higher-volume options, such as the Lifesaver Jerrycan, are a little on the expensive side.

For inexpensive, high-volume water purification, one of the best options is solar water distillation. Essentially, this is a solar oven, operating at lower-than-oven temperatures, used to evaporate dirty water. The vapor is then cooled and condensed into pure water. Solar distillation, at temperatures above 165 degrees Fahrenheit, will kill bacteria, viruses, and parasites. The process of evaporation and condensation leaves behind heavy metals, debris, and other solid and particulate contaminates. Distillation can even desalinate sea water to produce drinking water. Solar distillation does it all. And if you build your own solar oven/distiller, the cost is relatively low.

This prepping and survival blog post will review a few different designs for DIY solar water distillation.

1. The simplest, but also least efficient method, uses a glass, a larger container, and some plastic wrap. See this YouTube video of one such device. There are many variations on this technique. The outer “container” can even be a hole in the ground.

Advantages: a quick and cheap way to obtain clean water from dirty water
Disadvantages: inefficient and low volume; also, the glass sits in the dirty water, allowing for the possibility that the clean water will become contaminated

Even so, it is one method of water purification that every prepper should know. In a pinch, you can make one of these devices in less than half an hour, and have drinking water within a few more hours.

2. Another approach is to build a solar oven, which evaporates the water and then condenses it on the glass at the top of the oven. One such design is explained here: Renewable Energy: Solar Water Distillers . The water collects at the bottom of the oven (which is tilted to cause the water to roll off of the glass), and is then piped into a container for collection.

Advantages: a larger, higher-volume method of water purification
Disadvantages: time, effort, and expense to build is greater; efficiency is still limited, due to a lack of insulation; again, water condensation and collection takes place in the same container as the dirty water, allowing for possible contamination.

3. The most efficient and effective method would be to build a large solar oven, solely for evaporation, and then cool and condense the water vapor in a separate container. I could not find any videos or instructions on this type of design. But the basic design is an insulated solar oven with large pipe or opening to a cooling container. The insulation for the oven should be about R-value 8 to 12 on the sides and bottom. The top should be a double layer of glass or plastic, also for insulation. The insulation increases the amount of water that can be evaporated per hour. A reflector will also increase the efficiency of the system.

Advantages: higher-volume and greater efficiency; water condensed in separate container to avoid cross-contamination
Disadvantages: time, effort, and expense to build is higher than other approaches; needs further testing and development.

Comparison to commercial products

There are commercial solar water stills, but — in my opinion — they are over-priced and they under-produce. For example, the Rainmaker 550 Solar Water Distiller costs $489.00 (as of this writing) and produces only 0.8 liters per hour of full sun. At 5 hours of full sun, that’s 4.0 liters, or a little more than one gallon per day. In my view, it’s too high a price to pay for too little water.

– Thoreau

4 Responses to Solar Water Distillation: several designs compared

  1. Google “watercone” for a really neat solar still.

  2. The problem with solar distillation is the yield per square meter of ground. From the Watercone site:
    Based on evaporation levels of 8.8 liters per square meter (average solar irradiation in Casablanca, Morocco), the WATERCONE┬« (with a base diameter of roughly 31.5″, 80 cm) yields between 1.0 to 1.5 liters of condensed water per day
    So one Watercone only produces a maximum of 1.5 liters per day, which means that you will need three or four Watercones per person to get the recommended minimum of four liters per person per day.
    At a theoretical maximum output of 8.8 L/m^2/day, it looks like solar distillation is not practical as a water source on land. Maybe at sea, where you could tow a large array. On land it could be a useful additional source, but not a sole source.
    I hope I’m wrong – I really like the idea of a solar still, but it’s starting to look like it just doesn’t have the capacity. Somebody please correct me if I’m mistaken.