Saturday, April 30, 2005

solar water purification on the cheap

Not too many weeks ago NPR carried a story from a refugee camp in the Democratic Repubic of Congo. The camp is next to a lake. The refugees were washing their clothes, themselves, and doing whatever else they needed to with the lake water. They were also drinking it. As a result, they were getting sick and dying from cholera. All that water, right there, and they can't drink it because it'll make them sick. That's where this project came from. The goal is to come up with a way to purify water that is cheap, reliable, and could be deployed by aid agencies very quickly.

This is a design for a solar distillation system. It doesn't work very well, yet, but perhaps it will eventually be useful with some changes to boost efficiency. Currently, the design calls for a 2 liter soft drink bottle and a plastic cup. I don't know how available that stuff is in D.R. Congo's refugee camps, but we produce huge numbers of bottles and cups here, at extremely low cost per unit.

I'm including directions here for those who want to try their own experiments and post the results.

Start by cutting the top off the bottle about an inch (or the length of the tip of your thumb -- the dimension isn't critical) from where the cylindrical part of the bottle begins.

Next, you'll need to cut a couple slits in the top piece so it can nestle inside the bottom piece. That way, as condensation forms on the top, it'll drip down to the bottom.

The construction work for the basic still design is now complete. The theory and operation are simple. You put a dark colored cup of contaminated water inside the still and set the thing in the sun. Sunlight warms the water in the cup, causing it to evaporate. The water then condenses on the cooler plastic of the bottle. The distilled water drips down and collects in the bottom. Here's a shot of the basic still in operation:

Finally, in this close-up, you can see the distilled water that's accumulated:

This particular test used simulated contaminated water: I let a cup of Earl Grey tea steep overnight. That way, it both had a chance to get good and strong, and it was at room temperature before starting the test. The color of the tea makes it immediately obvious whether the distillation process is leaving behind the heavier stuff, and the distinctive odor is a good marker for the aeromatics. In this case, I performed an el-cheapo chemical analysis by drinking the clear water condensate. There was no hint of tea flavor or color, and only a slight whiff of Earl Gray. That's a good sign that distillation is working as expected.

Unfortunately, all is not not perfect. In fact, right now the still is too inefficient to be useful. After about 5-6 hours of operation, what you see is all the water that accumulated. So, some improvements are in order.

First of all, you'll notice that I'm using a blue cup, not a black one. Black cups are proving hard to find. If you know of a national chain that sells black plastic cups, please feel free to post a comment pointing me there.

Secondly, notice from the shot of the still in operation that the condensate forms above the rim of the cup. That suggests moving the cup higher (or the top lower) will concentrate the condensate in the top of the bottle, leading to more drips forming and accumulating in the bottom.

A taller, skinnier cup would probably help the situation by giving a higher ratio of surface area to volume, increasing evaporation rate.

Also, I plan to try taping an aluminum foil reflector to the outside of the bottle. That may concentrate more sunshine on the cup, speeding the process. (The fact that the bottle is cylindrical conveniently puts the cup at the focal point.)

I'll post more updates as I have time to run more tests.

Later: There's a follow-up here.

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