site  contact  subhomenews

Further analysis of basin-type distiller test

December 16, 2019 — BarryK

I posted a couple of days ago, a showdown between three solar water distillers:

That was a very hot day, the ambient temperature peaking at 41 degrees C (106 Fahrenheit), with a slight breeze (11-23km/h, as reported by the Bureau of Meteorology for my suburb), and the distillers gave their best. My simple basin-type was measured to be 87% as efficient as the F-Cubed c1000 distiller.

So what is the absolute efficiency of my basin-type? Well, taking the claim by F-Cubed that the C1000 is 55%, that works out that the basin-type is 0.87*55, which is 47.8%. But, can I calculate it myself, from basic principles?

Interesting question. I looked at some research papers, and the formulae given are complicated, or confusing (for me anyway). However, I did find some web pages with basic explanations...

From basic principles, for a day test, we want the energy of the sun coming in, the water output, and a conversion constant which is the energy required to convert the water to distilled water. That constant is called "the latent heat of vaporization".

This is not my field. I was a solar energy researcher for awhile, in 1979-80, but that was in photovoltaics. Besides, I was an electronic engineer. Anyway, from online reading, this seems to be the appropriate formula:

Efficiency (%)

Water-out (kg) x latent-heat-of-vaporization (kJ/kg)

Energy-input (kJ)

x 100

Latent-heat-of-vaporization is a constant, 2260.

Distiller water output is shown as kilograms, but 1 kilogram equals 1 litre. My basin-type gave 2.06kg.

That leaves energy input, in kilojoules. This is the awkward one to calculate. The sun was pouring energy into the distiller, and I took hourly measurements of the instantaneous power input, in watts per metre-squared.

1 joule is 1 watt times 1 second. Joule is an energy figure, so if I break the day into 1 hour chunks, I can just multiply the watts hitting the distiller times 3600 seconds (1 hour), to calculate the energy.

The problem though, is that the sun is hitting the distiller at an angle, quite an acute angle early in the morning and late afternoon. I need to calculate the distiller surface area that is perpendicular to the sun, at each hour of the day. To help with that, there is a graph here:

This is mid-summer here in Perth, Australia, and the glass angle is 10 degrees, so at midday the sun is directly perpendicular to the glass -- so that one is easy, the area is just the area of the glass -- well, less the thickness of the wood frame it is sitting on.

There needs to be a start-time and end-time for the analysis. I took 8.00am as the start-time, as the distiller was mostly in shade earlier than that, and hadn't even started to warm up at 8.00am. I set the end-time as 5.00pm, as virtually no water was coming out after that.

With the help of the above link, I constructed this table:

Time (am,pm)
Irradiance (W/m2)
Perpendicular area (m2)
Power into distiller (W)
Energy into distiller (kJ)

...the energy is the power times seconds, divided by 1000, giving kilojoules.

The total energy input is the sum of the entries in the bottom row, which is 9867.6 kilojoules. Putting it into the formula:


2.06 x 2260


x 100


The basin-type distiller has an absolute efficiency of 47%

This agrees closely with the earlier estimate of 47.8% ...I kid you not, I did not fiddle with those figures. There is some approximation involved calculating the energy input, but seems to be pretty close to the actual energy input.

This is very pleasing. Various researchers have given different estimates of the maximum efficiency obtainable from a simple basin type, from 30% to 45% (and higher for a "non simple" design, such as ribbed-floor in basin, or a stepped-basin, up to around 60%), so I am at the top for the simple basin-type.

There were a couple papers that I read, where I could not agree with the logic of the efficiency calculations. For example, one paper claimed a seasonal efficiency variation from 14.8% to 31.1%, but it did not consider the angle of the sun, that is, the actual energy hitting the distiller. I thought that the meaning that they had given to the word "efficiency" was wrong, but I won't post links to those papers, as, as I have stated, this is not my field.

Anyway, this is my analysis, let me know if something is wrong! 

Further thoughts on tilted wick-type versus simple basin

I have already indicated that I am now favouring the simple basin-type, due to simple construction and easy-of-use.

There is another point to consider: the survival of pathogens, even the possibility of them breeding inside the distiller.

The simple basin-type exposes almost every part of the interior to UV radiation -- the exception would be just behind the distilled-water runoff at the front of the distiller -- however, I think that the white-painted walls would be reflecting UV into every shaded area. On the other hand, the tilted wick-type has a lot of places where pathogens can hide from UV light. In particular the F-Cubed design, where air rotates around the underside, but even a simpler conventional design would allow pathogens to hide in the wicking material.

Then there is internal temperature, the higher the better. Again, the basin-type wins. An internal temperature of 80 degC might not immediately kill some pathogens, but they are bound to be killed eventually. 

My recent tests highlighted the need to have sufficient water flow through the tilted wick-type, to avoid dry patches. This can result in a lot of waste water. With the C1000, There was about 15 litres in the container, and I had to refill it early in the afternoon. So, mid-summer, rough estimate is to have at least 25 litres in the container, and also mounted high enough, about 1.4 metres above the ground.

With the simple basin-type, there is no worry about water delivery rate. You just put in enough at start of day -- which could be governed by an overflow outlet inside the distiller. In my test, I put in 5 litres, and got 2 litres of distilled water. The left-over water then needs to be flushed out at end of day, to avoid build-up of sediments. Well, probably more importantly, left-over water is required so that there are no dry-spots on the basin floor, which would cause sediments to become baked-on to the floor.

Perhaps running some extra water through the distiller at end of day would help with flushing. The design could quite easily cater for this.

So perhaps comparing the two types of distiller from the water-maintenance side of things is a 50-50 situation. 

Well, no, I think that the basin-type wins, as in a campsite situation I don't really want to have to provide something 1.4 metres high on which to place the water container. The alternative is a small water pump, but that is getting too complicated. 

Tags: nomad