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Reflections on the basin-type proto-2 test

December 04, 2019 — BarryK

Yesterday was hot, ambient peaked at 41 degrees C (105.8 Fahrenheit), mostly clear blue sky, light breeze, mid-summer here in Perth Australia (Latitude 31 degrees), so the conditions were right to obtain maximum distilled water output that my basin-type prototype #2 is capable of. Here is the report of the test:

The panel produced 1.85 litres (0.49 gallons), with an effective glass area of 0.35 metre-squared, which calculates at about absolute efficiency of 48%, extraordinarily high for a simple basin-type distiller.

This blog post reflects on how that high efficiency was achieved.

There are three main factors:

  1. White painted inside walls
  2. Glass close to water surface
  3. Very low water depth

1. White painted inside walls
I posted about a research paper that obtained a 6.8% absolute efficiency gain by doing this:

In my case, I used white silicone sealant, Prosil 10.

2. Glass close to water surface
There are several research papers that have found the closer the glass to the water surface, the higher the efficiency. To achieve this, many designs have gone for a stepped basin-type still.

In my case, it is a compromise. The glass is about 15mm above the water at the front end, and about 115mm away at the back side. The distance of the glass at the back side is kept low by a very low glass angle of 10 degrees.

The downside of the 10 degrees is that for the latitude of Perth, 31 degrees, the efficiency of the distiller is going to drop right off in winter. The acute angle of the sun to the glass is going to mean most light will be reflected off the surface of the glass, instead of going into the distiller. I was thinking of using an external reflector in the winter.

3. Very low water depth
I only put 5 litres of water into the distiller, giving a water depth of 14mm average. However, my prototype #2 has a very uneven basin floor, due to misadventure experimenting with expanding foam, so the depth was variable, but there was water covering the entire surface.

There is one research paper that experimented taking the water depth down to 5mm and efficiency kept increasing as depth was reduced. This paper tested depths of 5, 10 and 20mm:

...they achieved output of 1141, 758, 305 millilitres in a day, from 5, 10 and 20mm water depth respectively. Quite an extraordinary variation.

So, if I had got the basin floor more flat, and setup the still to be very level, I could have put in less water and achieved higher efficiency.

Raising the efficiency further

Apart from lowering the water depth, is there anything else that can be done to further increase the efficiency of the simple basin-type distiller? Yes, there is...

I used 4mm thick window glass. Here are two more factors to increase efficiency:

  1. Thinner glass
  2. Low-iron glass

I have a collection of glass panes, that were cut for me by Casey at Perthglass, used in the various prototypes, and I just happened to have that 4mm piece available so used it. However, there is a research paper that has determined the efficiency improves as glass thickness is decreased.

This overview paper references another paper that shows efficiency improvement going from 4mm down to 3mm glass:

...that "other paper" is not free to download. Unfortunately, most academic research papers are seen as a business opportunity by companies such as Elsevier.

It was found that efficiency improved by 16.5% going from 6mm to 3mm thick glass.

I did locate another free-to-download paper that investigated glass thickness, can't locate it now.

Point-5 is low-iron glass. I have used ordinary window glass, that has some iron in it. This reduces the efficiency. However, low-iron glass is expensive. This paper reports that low-iron glass gave an increase of 6% efficiency compared with normal window glass:

I don't think that I will bother with obtaining that extra 6%. Will stay with normal cheap window glass, but certainly will go for 3mm, unless the panel is large and the extra strength of 4mm glass would be desirable. 

EDIT 2019-12-06:
I found some academic papers, free to download, that analyze glass thickness:

4mm, 4mm-two-layers, 4mm-two-layers-air-gap:

Comparing 4, 8, 12mm thickness:

Comparing 4, 5, 6mm thick:

3, 4, 5, 6mm low-iron:

...interesting, that last one determined that 4mm glass gave the highest output, not 3mm. They all obtained significant efficiency improvements going down to thinner glass, except for that last paper.

There is another consideration to improve efficiency: insulation thickness. My prototype has 35mm expanding foam under the basin floor, but tapering to 12mm at the back, and the wood side-walls are thin only 12mm. So, pretty obvious that this could be improved.

So, add this as item number 6:

  1. Thicker insulation

But how thick? This academic paper analyzes various thicknesses and different materials:
pdf d/l: depends on the material, but the ball-park figure they have got is 60mm thick is optimum, beyond which efficiency does not improve much. So, my prototype can be improved with thicker insulation. 

Tags: nomad