Solar water purification system project

[searchingfortruth]

Basically my idea is to use solar panels to power rotating mirrors that follow the sun. The mirrors reflect the sunlight to a large "Still" filled with salt water. The still heats up and the water evaporates leaving the salt behind. The water is collected and distributed for drinking and the salt is cleaned out of the bottom of the "Still" through a hatch. The picture is very... Kindergartenish, but it was all I could do spur of the moment. I would appreciate opinions, ideas, and constructive criticism.

[Enthalpy]

It does work, as is known experimentally (...sorry for the disappointment - inventor's daily life).

 

One strong limit to desalination through evaporation is the amount of heat, hence the collector area, neeed to produce a bit of fresh water.

 

One (already operational) improvement is to reuse the heat many times. Condensation of vapour releases the heat invested in evaporating, and that released heat serves to evaporate more water. Of course, temperature drops at each heat exchange, so it can't be done at +100°C everywhere: it takes pumps to establish a pressure drop, hence a temperature drop, across all cells - but electricity is obtained from Solar cells, for the smaller pumping power needed. That way, Solar heat can be used 10-20 times.

 

Though, this is not the preferred way presently, because even if reusing the heat, vaporization uses so hugely more energy than the separation of water and salt really needs, which is very little. Meanwhile we have reverse osmosis (see Wiki) which takes just a few times more energy than the theoretical minimum and is convenient : affordable hardware, reasonable maintenance, compact... Reverse osmosis is so much better that even Solar cells to feed its pump are cheaper than light concentrators for evaporation. It's an industry meanwhile.

 

You can try to put figures on vaporization, and reuse heat many more times (how?) to try to match the economics of reverse osmosis, but that will be hard.

 

Improvements must be possible in the reverse osmosis apparatus: better membrane materials, processes to assemble the wide-and-thin membrane (it's a fibre) in a small volume and still allow water to flow easily, avoid dirt accumulation...

 

Fun: mixing sweet and salty water does produce a little bit of energy. Someone has found a way to exploit that, a tthe size of an estuary, and one demo plant exists (in Norway?).

 

I like your sketch, as it focusses on the essentials.

[Externet]

Hi.

Many years ago I was obsessed with the subject and dug deep into the solar methods to obtain freshwater for drip irrigation or human consumption, expandable at any size scale.

 

I found that can be made simple or complex, both tilting the scale of convenience one side or the other for best efficiency or lowest cost, which this latter is very important.

 

A wimpy solar panel operated electric vacuum pump has merit for destillation on modest partial vacuum, increasing productivity by reaching evaporation at lower temperatures, but means batch operation. The pump can also help to move the vapors into a condensing chamber, serving double duty.

 

A way I found important to consider was to avoid batch processing, eliminating salt precipitation and cleanup at the same time. Draining metered brine instead makes it viable, unattended.

 

Heat interchanging to optimize efficiency also plays a role to consider. The fresh water and the drained brine contain heat that should not be wasted : Simple concentric pipes one inside the other can transfer that heat into the cool seawater inlet. That cool seawater inlet also promotes condensation.

 

There is dozens of different devices that can be designed to do the solar distiller. The one I found with most merit and simplicity is ----> http://www3.telus.net/farallon/

I contacted the designer but could not clarify something I was never able to have it clear in my mind, was the need or not of air circulation on the vapors side to transport the moisture to the discharge condenser surface. In other words, the hermeticity or not of the chamber.

That design provides full sun strike on one side for evaporation and shade on the other for condensation. It is not perfect as drawn/built, but interesting to polish. The vapor migration to the shade side was a point to improve. Air currents to sweep vapor off the evaporating surface (which can be a black towel) and towards the shaded condensing surface was another item to re-think. This tilted panel also lacks heat exchangers.

I would ignore the plant-in-a-bottle scheme.

 

Simplicity is very important. Heliostats are very complex, and too expensive for the final product yield cost. Avoid them.

 

If you could come up with a contraption to distill using discarded materials -say 2 or 3 litres soda bottles ! by the thousands !, it would be a home run.

 

Finally, there is electrodialysis. I believe it was discussed here at SFN a while ago.

 

Please read slowly again as there is much in the above text that can help you.

Edited October 6, 2013 by Externet