Solar Water Plant - (Still & Pump)

 

 

This page describes a solar water still and pump, designed by Stephen Salter, PEng.  I am making no claim to these designs, and I hope anyone interested will take advantage of the ideas on this site.  At the same time, those who build a device based on the ideas described here must take precautions to ensure the final results are safe.

 

 

How it works

 

The first model

 

The second model

 

Alternative designs

 

Construction and design details

 

 

 

 

 

Stephen J. Salter, PEng

Victoria, BC, Canada

(250) 370-9664

 


 

How it works

When I read that a willow tree transpires hundreds of litres of water every day, I began to see trees as solar-powered water pumps. 

 

 

If we could copy nature's design and collect the evaporating water, we could have pure water from marshy or even contaminated soils.

This is a drawing of a model of the "solar water plant".  This cylindrical model was built to show that the principle works, but a flat design is more practical (please see the notes below).

This first model was built on April 6, 2003 in Victoria, Canada.

 


 

The first model

The model before being "planted".

Stephen Salter, PEng and Joanne Posthumus with the model they built in an hour on April 6, 2003.
 

 


 

The well shaft (the "trunk") could extend to a significant length, as it does in a tree.  This length could either be used to reach groundwater at some depth below the surface (as shown on the left), or to lift the reservoir of pure water above the ground, to allow gravity feed (as shown on the right).

 

 


The second model

 

The advantages of this design are that condensing water collects on the back of the still in stead of the front, and so does not cause solar energy to be lost by reflection. The back of the still should also be cooler, which will help the vapour condense.

1. The frame for the second model, built on
April 24, 2003.

 

In this design, the absorbent fibre is flat, and placed behind a black plastic sheet.

 

 

 

 

 

 

 

2. The wick in this model is fibreglass. 

 

In this design the reflectivity of the wick is not important, since solar energy is absorbed by the black plastic sheet.

 

 

3. The fibreglass wick/black plastic sheet after being enclosed in clear plastic, which is stapled to the wooden frame.

 

A trough to collect condensing water drops was formed by making a fold in the back side of the clear plastic.

 

4. The model installed:  the collecting jar for fresh water is in the ground at the lower left-hand corner of the frame.

 

 


 

Alternative designs

 

In this alternative, there is less risk that materials (e.g. salts) that will build up on the wick could fall into and contaminate the fresh water.

In this design, water vapour circulates by natural convection through condenser tubing and an underground water storage tank.

 


In this design, salt water siphons down the wick material, to limit the build-up of salt. As before, evaporating water  is condensed and collected.


Construction and design details

 

Where it can work

This design can extract water from water-logged soil, or from salt water, but does not need a body of water such as a stream or lake.

 

Advantages

·        Produces pure, distilled water from ground water, even if the source is contaminated with inorganic materials such as salt

·        Pumps water to the surface (through capillary action) and distils it at the same time

·        Can be made locally with recycled or inexpensive materials

·        Can be made in any size and shape

·        Has a low environmental impact

·        Has no moving parts

 

Current limitations

·        The still/pump needs strong sun and temperatures above freezing

·        The still/pump operates slowly:  at the rate of insolation in equatorial countries, and at 50% efficiency, the pump should provide 2-3 litres of distilled water per m2 per day

·        The evaporating section will need to be cleaned or replaced as mineral salts are deposited there by the evaporating water

 

Safety precautions

·        Since the wicking material contains water with impurities and possibly pathogens, the construction method must ensure that the wick material, or contaminants that may fall from the material, cannot come into contact with the condensed fresh water

·        This design will not purify water that is contaminated with volatile organic compounds, which can evaporate and condense in the fresh water

·        The materials that come into contact with the fresh water must be non-toxic

 

What makes the pump work well

The evaporating section (the “leaf”) should be as large as possible in proportion to the condensing section.  This helps by:

·        giving the largest possible area for evaporation

·        causing the highest possible temperature difference between the housing and the surrounding air;  the greater this temperature difference, the faster the water will condense on the inside of the housing

 

Materials

The evaporating surface and the “root” fibres can be made from any material that will wick and hold water.  Materials that work well include:

·        natural fibres such as cotton or burlap

·        man-made, hydrophilic fibres (having a low contact angle with water) such as bare fibre glass

 

Although I have not tested them, I think raw plant fibres such as shredded roots, shredded wood fibres or hemp could also work well as wicking materials.

 

Water-repelling fibres such as nylon would not work well.

 

 

 

 

If you would like to talk about this idea or have comments or suggestions, please feel free to send an email message.

 

Stephen J. Salter, PEng

Victoria, BC, Canada

 

 

 

 

 

 

 

 

 

Keywords: solar water plant, solar water pump, solar distillation, water purification, solar still