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Silicone sealants for potable water

November 21, 2019 — BarryK

I have posted before about silicone sealants that are suitable for use in construction of the solar water distiller.

I have previously used Selleys 401, translucent, purchased from Bunnings, AU$19.63 for a 310g tube:

...rather expensive.

Besides, I am now wanting black sealant, and Bunnings only sell the Selleys 401 in translucent. Also, I am now favouring neutral-cure for applications other than glass.

I recently discovered Bostik Industrial Grade silicone, at the Total Tools website, for AU$6.95 per tube -- except that no stores anywhere in Australia actually stock it, and it is a "special order" with high postal charge. Anyway, here is the manufacturer's specs page:

...Bostik is an international company, and in many places, such as the USA, you could probably pick it up from your local hardware store.

The Fuller company, also international, do a "plumbers silicone", rated for potable water use, but not available in black.

About a week ago, I discovered Silastic 732, acetic-cure, available in black and white, and stocked at Autobarn, a car parts and accessories chain store in Australia. This is also rated for use in potable water. Price is AU$18.99 per tube:

...I bought two tubes.

There was something nagging at my memory though, another brand, locally available. Yes, found it: Prosil 10, neutral-cure, available in various colours including black. And cheap, AU$8.95:


Here is the manufacturer's page: 

There was another that I discovered some time ago... German company... forget the name, but they only sell to the trade here in Australia, No retail outlets, not even online ordering. I did phone them, and they gave me the number of a salesman. That's their marketing model, sales reps who go to tradie sites ...not interested. 

Tags: nomad

First coating silicone on aluminium mesh

November 20, 2019 — BarryK

I posted recently about the idea of using an inner liner for the basin-type solar still, constructed with aluminium insect-screen mesh and coated with silicone sealant:

The mesh is not raw aluminium, it has some kind of dark coating, I presume that is an anodized surface. Don't know anything about the properties of that surface, but the silicone seems to stick to it OK.

Now for the next step, to coat a large piece of mesh. The intention is that this will be inserted inside the distiller frame, and will completely isolate the frame from moisture. There will also be some insulation injected between the inner liner and the frame. Still planning on using pine for the frame.

Want one large coated surface, which will be folded afterward, so a minimum of cutting -- basically, the only cuts will be for inlet and outlet holes, and to trim the edges where the liner is sandwiched under the glass -- anyway, those details will be clarified as the project progresses.

Here is a photo of the mesh stretched out on a table:


...the g-clamp has two purposes, to prevent the mesh from folding up when I lift it off the table, and to hang it up on a hook. The other end has a length of small-section wood attached with wire.

Underneath the mesh is plastic sheet. That plastic just arrived in my backyard one day, a year or so ago. Thought it might be useful, so put it in the garage -- so, finally it has a use!

Wanted an area about 950x750mm. Used cheap Parfix kitchen-and-bathroom acetic-cure silicone sealant from Bunnings -- this has anti-mold additive, which I don't want, but it shouldn't matter in the final product, as it will be coated inside with black Silastic 732 sealant.

The end result was hung up:


...the white spots and streaks are holes. I have learned from experience not to try and patch the holes. Instead, let it set and tomorrow will give a second coat. Note, that large white area at the bottom is direct sunlight. One more detail: I wiped the plastic with a cloth, to remove silicone bumps, as the plastic will be used again tomorrow.  

Tags: nomad

Planning simple basin still prototype 2

November 16, 2019 — BarryK

I posted a test of the simple basin prototype #1 yesterday:

The result was promising, so decided to go ahead and design prototype #2. A major requirement is that it be as flat as possible, so as to take up little space when slid into the back of a hatchback car or 4wd vehicle. In fact, that was one of the main reasons, that and promised improved efficiency, that I went for the tilted wicking-cloth type of still. However, I have now learnt that the glass can be very close to the water, water depth can be mere millimetres, and the glass angle can be as little a 5 degrees, maybe even lower.

It may be possible to design the still to be only about 100mm (4 inches) thick at the widest end, which would be great. However, might not quite get that thin. This reference recommends a minimum glass angle of 10 degrees (scroll to page 445): is unclear whether that 10 degrees figure is for plastic panes. Water droplets stick to glass better, and perhaps it would still work for angles less than 10 degrees.

The above reference also recommends water depth of 20-30mm and side and bottom insulation thickness of 20-30mm.

In all of my reading, I keep coming back to silicone as the superior product. Inert, extreme resistance to corrosion and UV light, and retaining its flexibility for 10-20 years.

Rather than build the still out of wood, as I have done so far, I am thinking of using aluminium insect screen mesh, coated with silicone sealant. This would give me a flat surface that I can bend into shape, to form the basin and support for the glass. There could be an outer frame, into which insulation is poured -- maybe polyurethane expanded foam -- or even silicone mixed with foam bubbles.

Thinking along these lines, I have started testing coating of insect screen mesh. I bought a roll of aluminium mesh, 1220x2050mm, AU$23:

Also some cheap Parfix brand acetic-cure white silicone sealant, AU$6.68:

I have a plastic spreader, that was used for the early wicking-cloth type prototypes:


I tested spreading on a mesh offcut, with newspaper backing, plastic backing, and hanging vertically in the air:


I then pulled the backings off and hung it up to set.

From this initial test, it looks like two or three coats will be required. For the final construction, I will probably do two coats of cheap white silicone, then one coat of black Silastic 732 silicone.  In earlier experiments I found that silicone-on-silicone adheres extremely well, so there will be no adverse issues with doing multiple coats. 

Tags: nomad

First test basin type proto 1

November 15, 2019 — BarryK

Yesterday I posted some design notes, and construction details of a first prototype of a simple basin-type solar water distiller, using parts found in my garage:

After knocking it up, further reading revealed some design shortcomings. However, this first prototype will probably be useful for "ball park" measurements, to see if this type has potential. As already stated, I was very surprised at the claimed output of the Rainmaker 550, and if I can also achieve this level of efficiency, then this simple design looks like the best way for me to go.

Here in Perth, Western Australia, we are having our first summer heatwave. Prediction today was 40 degC (104 Fahrenheit), superb day for testing!

I set it up early in the morning. The previous evening, I had stuck some cardboard on the sides, a rather feeble attempt to improve the insulation, don't know if it will make much difference. Here it is, photo taken at 8.20am, the sun has just hit the top corner:


...I only put in 3 litres of water, as efficiency is said to be improved with less water depth. Glass is 600x900, as is outside dimensions of the base, so water depth is only a few millimetres.

Oriented the panel for peak output at about 11am, and waited for the sun to march across the panel. About 9.00am had sun on all of the panel, and a tiny bit of distilled water appeared, so started measuring from 9.00am.

9.00am. Some gusty wind. Sunny sky with some fluffy clouds.

10.30am. Sun: 870 W/m2, ambient: 31 degC, temperature about 3/4 up the panel, IR sensor about 2 inches away from glass: 53 degC, 1/4 up: 52 degC, back side of distiller, about 2 inches away: 39 degC

12.00 noon. Only very slight breeze. Fluffy clouds mostly gone. Sun: 890 W/m2, ambient: 34 degC, 3/4: 64 degC, 1/4: 63 degC, back: 45 degC

1.30pm. Sun: 960 W/m2, ambient: 38 degC, 3/4: 64.7 degC, 1/4: 64 degC, back: 48 degC

3.15pm. Stop.

Collected distilled water: 1.525 litres

Collection duration was 6 hours and 15 minutes, so collected 244 millilitres/hour. Collection area is 0.568x0.868 metres2 which is 0.493 m2.

Now for the exciting part, to compare with the F-Cubed panel. Different weather conditions of course, so this comparison is a very rough guide only. Refer to prototype #4 results:

Quoting, result for prototype #4:

the collection: 1.1 litres, in 7 hours. That is 157 millilitres/hour. The cloth has a surface area of 0.3327 m2, so to compare efficiency with the F-Cubed panel which is 1 m2, need to scale up: 157*(1/0.3327) which is 471.9 ml/hr. The F-Cubed panel gave 563.6 ml/hr, so the test has a comparable efficiency of 83.7%.

Today's test (simple basin type, prototype 1) also needs scaling: 244*(1/0.493) which is 495 ml/hr. Efficiency relative to the F-Cubed panel is 87.8%

Wow! And that's with a design that probably has room for improvement.

I must re-state that these comparison percentages may be way off. The F-Cubed was tested mid-winter, full sun but much lower ambient temperature. I really do need to set up my panels alongside each other for a proper comparison test.

The most obvious improvement that can be done, as seen from today's test, is to improve thermal insulation on sides and bottom. At 1.30pm, the back side of the panel measured 48 degC, 10 degC above ambient.

Another area of improvement will be to decrease glass angle, and bring glass very close to the water surface. There is however, a tradeoff, as if sun angle to the glass is more than about 70 degrees off perpendicular then it just reflects off the glass. A balance needs to be found, to suit summer and winter sun angles.  

Tags: nomad

Simple basin solar distiller design notes

November 14, 2019 — BarryK

A few days ago, I posted about the Rainmaker 550 single-slope simple-basin type of solar water distiller:

...for which the manufacturer claimed remarkably high distilled water output. I want to find out for myself, so decided to build one, no, at least two.

I re-purposed prototype #2, which was the trickle-down type, just one sheet of glass, and the frame was MDF. The interior was coated with silicone sealant. That was built back in June:

...took those legs off, and it then became the water basin. Cut some more pieces of MDF, and coated the inside with black silicone sealant, then a bit more patching with some leftover grey sealant:


...also stuck some foam silicone strips on top, and the glass (600x900mm) will just sit on top. Used a length of 10x10mm aluminium channel as the distilled water runoff.

I used cheap Parfix brand black sealant, two 310 gram tubes, used it all, hence resorted to the grey sealant for final patching.

For the record, black silicone sealant rated as "food grade" and "potable water" safe, is Silastic 732, available at a very reasonable price from Autobarn (Australia), AU$18.99:

The build so far is good enough for basic ball-park testing I suppose, however, there are two things "wrong" with this design -- let's call it prototype #1 of the simple-basin design.

Firstly, there is going to be too much heat loss. This design needs insulation, and I might just stick cardboard onto the sides and bottom of the MDF.

Secondly, the glass angle is 35 degrees, however, after some more reading, I found a recommendation that the optimum angle is 15 degrees. This information comes from here:

...which has a link to this PDF which is where the 15 degrees figure came from:

Another recommendation is that the water depth in the basin be kept low, like about 10mm. Can do.

I could cut a slice out of it to lower the angle, but might just leave it as-is, the after some testing, move onto prototype-2.

For basin-prototype-2, might investigate a better insulating material. I am thinking of black corflute, such as Tunnelcore, from Bunnings:

...corflute is made with polypropylene, and Tunnelcore has UV resistance. Polypropylene is one of those slippery kind of plastics that is very difficult to glue. I am planning to experiment with heat-gluing. 

Tags: nomad

Prototype 4 warped wood has unwarped

November 09, 2019 — BarryK

Fascinating! I posted about first test of solar water distiller protototype #4 two days ago:

And the wood warped and split:


Most curiously, there was no noticeable warping or splitting when testing prototype #3, which was essentially the same design. I think what happened in the last test, it was longer duration, higher ambient temperature, and when the wood started to split, that allowed water ingress which accelerated the warping.

It was the top piece of wood that had warped the worst. This would be the hottest part of the distiller.

After the test, I pulled it apart to dry out, and the big surprise this morning:


...the wood has returned to normal shape, and the crack closed.

I received some advise from David ('wdt' in the forum), on a method to preserve wood:

Wood that is well impregnated with wax, is waterproof,
will last almost forever, very stable
Melt wax, no water in bottom (normal for protection,<100)
Wax to 115-125C, do not leave unattended with heat on
Wood in for at least 10 min or longer
Problem is pot, something large enough
Large truck exhaust pipe? up to 3.5", flatten slightly
Sand or file one end 1/4", flatten, crimp in vise, solder with acid flux
OR split with slitting cut-off wheel, solder ends on both
Will not be able to glue ever
More convenient if you can do 2 pieces at once
Careful not to burn, maybe small nail 1/2way in, string

Obviously as you add wood it will cool wax as it boils water out of wood, will also carry the air entrained in wood.
For best impregnation, let it cool in wax until start of solidification, then heat for adequate drainage
Let drip over WET newspaper for easiest clean-up

...looks like an excellent method! However, I have already painted the wood with mineral oil. Mineral oil is food-safe, can be ingested as a laxative, or used as a skin softener. It is describe in Wikipedia:

The problem is to find it without additives. Pharmacies in Australia sell it as liquid paraffin, though one has to be careful about that word "paraffin" -- there may be other products in hardware stores with that name, that are not mineral oil.

I bought a bottle of Gold Cross liquid paraffin from a local pharmacy. This is 100% pure mineral oil. The 200ml bottle cost AU$6.95. I am sure that it could be obtained cheaper, but was not sure about the products on offer at Bunnings. Anyway, don't need much.


Unlike wdt's solution, I think that the mineral oil is not a permanent fix. The wood will need periodic recoating.

There is also the problem of the cracks in the wood. After two coats of mineral oil, I might smear the surface where the cracks are, with silicone sealant.

Another thing to bare in mind when considering how to treat the wood, is that the internal temperature of the distiller can get up to about 65 degC (149 degF). This can melt and leach out some treatments. I have read this 65 degC figure in online reports, for inclined wicking-type distillers, but maybe could get even higher -- I have yet to test in ambient 40+ degC heatwave!

Changing the subject, I have started to build a basic basin-type distiller, using parts from prototype #2. It is going to be extremely interesting to setup three distillers alongside each other, the prototype #4, F-Cubed Carocell 1000, and the simple basin-type. 

Tags: nomad

Rainmaker 550 solar water distiller

November 08, 2019 — BarryK

Taking some time out to think about the design, after yesterday's test of prototype #4:

I got to thinking that I should build a simple basin type of solar water distiller, for comparison. It could be run alongside my own design, and could also run the F-Cubed panel alongside, to get a true comparison. I did this awhile back with photovoltaic solar panels, and it was very meaningful to see one particular photovoltaic was shown to definitely have overstated its claimed power output.

While browsing on the Internet last night, I looked up a commercially available simple basin type distiller, the Rainmaker 550, just wanted to see it's specs. The simple basin type is supposed to be the least efficient, I say "least" because that is what I have read, however the published figures for the Rainmaker have taken me by surprise.

Here is a photo:


Here is a webpage with specs:

They are claiming:

In temperate climates, the Rainmaker 550 produces up to 1.5 gallons (six liters) per day in the summer, and approximately half that in the winter months. Thermal efficiency (60%) is equivalent to about 550 Watts when operated in full sunlight. Production is about 0.8 liters per sun hour (kWh/m2).

The surface area is given as 0.93 m2. So, 800 millilitres/hour, and adjust to compare with the F-Cubed panel, 800*(1/0.93) which is 860 ml/hr -- surely not, that is far higher output than I got with the F-Cubed Carocell 1000 panel!

Are the Solaqua people telling porkies?

I only got 564 litres/hour when testing the F-Cubed panel, but that was mid-winter, though a sunny day (ambient was about 17 degC). So, to be fair here, need to compare with the claimed output. See here, for the Carocell 1000 (the model I own):

...they claim 7 litres/day in summer with ambient temperature 30 degC. OK, a tad better than the 6 litres claimed for the Rainmaker 550, but gosh, the Rainmaker is a far simpler design, easier to setup and use in the field also.

Hmmm, I have to find out for myself. I can easily knock one up from bits and pieces lying around in the garage, will hardly cost anything. Already have glass from the earlier prototypes. I could even build it with cardboard, as the inside will be lined with black silicone sealant and should be able to prevent any moisture to reach the cardboard -- good enough for doing the comparison testing anyway. 

Just one extra note: one reason that I went for the sloping wick-type distiller, is want something fairly flat that will slide into the back of a car and take up minimal space. That, and improved efficiency. So if I do decide to go for a simple basin type, it will have to be a collapsible design. 

Tags: nomad