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Challenge: Distillation of Seawater Using Solar Power

Updated Monday, 28th January 2008

Our Rough Scientists took a challenge to distill water using just the sun - here's how they did it...

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On our tropical island close to the equator, both seawater and sunlight were in abundance but how could we capture the maximum amount of energy from the sun while allowing for the sun’s movement across the sky?

Firstly, what is solar power?

Apart from the tiny lights from twinkling stars, all the energy we have on earth ultimately has come from the sun. Even the wood and coal we burn to give us heat was made originally made by organic processes that depended on light and heat from the sun.

The atmosphere of the earth absorbs some of the energy and the more air the sun’s rays have to travel through to get to the earth’s surface, the weaker they become. When the sun is high in summer and travels through less air it appears stronger and heats more, while in winter the sun is low and much of the energy is absorbed.

How can we harness the sun’s energy to distil seawater?

One way to achieve this is by making a parabolic mirror. Parabolic mirrors are used in many instruments including astronomical telescopes, lasers, spotlights and even solar ovens. The purpose of the parabolic mirror is to collect the sun’s energy over as wide an area as possible and focus it onto a smaller area.

What is so special about the parabola shape?

The parabola has a very well defined mathematical formula. If we draw a graph with the y axis going up the page, the x axis across the page and ‘a’ being the distance of the focus from the mirror, the shape of the parabola is plotted out by the formula y2=4ax and forms a ‘C’ shape.

How can we use this formula to make a parabolic mirror?

To convert this into actual dimensions used to make the wooden frame to carry the mirror we can draw up a table. (We will turn the equation around in terms of x rather than y i.e. x = y2 / 4a so by putting in various values of y we can get corresponding values for x). Y is the distance along from the centre of the mirror and x is the height above the lowest part of the mirror.

For a mirror with a focus point (a) of a=0.5m above the bottom of the mirror we get the following table:

 y (metres) y2 y2 / (4 x a) height of mirror 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 0.01 0.04 0.09 0.16 0.25 0.36 0.47 0.64 0.81 1.00 0.005 0.020 0.045 0.080 0.125 0.185 0.235 0.320 0.405 0.500 5mm 2cm 4.5cm 8cm 12.5cm 18.5cm 23.5cm 32cm 40.5cm 50cm

Using these figures we can make a wooden frame made up of vertical wooden supports fixed to a flat board. The height of each vertical support is given in the right hand column of the table for various distances out from the centre (y). A thin wooden sheet was then placed on top to form the parabola on which to fix the mirror.

How was the mirror attached?

Once the framework for the parabola had been built, broken mirror pieces were placed on in a single layer and glued into position with cement. The bottom of the mirror was painted with black carbon paint so that it would absorb rather than reflect the heat.

A kettle filled with the seawater to be distilled was then fixed in place at the focus (0.5m) above the bottom or lowest point of the mirror. A metal support was put at the centre of the parabola to hold the kettle over the mirror and wire was used to keep this support rigid.

How much energy would it take to boil the kettle?

A standard kitchen kettle takes about 2000 Watts of electrical power. The energy available from the sun depends on how much air the light has to pass through. So obviously the sun is hotter on the equator than at latitudes such as the UK, whatever the time of year.

At the equator when the sun is straight overhead the power available from the sun is about 1000 Watts per square metre (1kW/m2). So if you have a parabolic mirror that has an area of 2m2 and you use it to focus the energy on to the bottom of a kettle you will achieve the same sort of power as you would from the electricity socket with an electric kettle.

Our mirror only had an area of 1m2 but unlike the kitchen kettle whose job is to boil water we don’t need to actually get the water boiling to produce a vapour that we can condense to form steam.

In what direction was the parabolic mirror positioned?

Once the mirror was made and the kettle fixed in place, we needed a device to point it into the sun. As the sun travels across the sky it takes different angles from east to west and also different angles above the horizon. We made a mount for the mirror that had one axis pointing parallel to the axis of the rotation of the earth and the other set once to the position of the sun.

All we had to do was set up the mirror for the particular day and just turn the apparatus around the one axis for the contraption to follow and scan with the movement of the sun ­ this is called an equatorial mount. This allowed us to follow the sun and get the most energy and efficiency from the system.

Did the parabolic mirror distil the seawater?

A tube was fixed to the spout of the kettle and this tube went into a small glass bottle immersed in cool water. As the seawater turned to steam, any water vapour went down the tube and into the bottle where it condensed into drops of pure, distilled water. A solar powered success!

Ellen, Jonathan and their parabolic mirror

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Solar Ovens made from Pizza Boxes - from the University of Michigan site

The Solar Funnel Cooker - from the Solar Cooking Archive site

Why are Satellite Dishes Parabolic? - from the Math Central, Canada site

Books

The Timeless Energy from the Sun by Madanjeet Singh, pub Thames & Hudson