Making a Solar Still from a Cardboard Box

In the Escuela Naciones Unidas (the school in El Ocotal, Yuscarán, El Paraíso, Honduras) we have a solar panel, a deep-cycle battery, and an inverter to power our computer, radio, and CD-player.  Maintenance of the battery requires distilled water, but locally distilled water is quite expensive.  To provide an inexpensive source of distilled water, we in the 6th-grade class constructed a solar still.

The Principle

 

The principle behind our solar still is to use the energy of the sun passing through a transparent window to heat water, which then evaporates and condenses on the under surface of the window.  The window is sloped so that the condensed water runs down to a barrier and drips off into a trough.  The trough is also sloped and directs the water through a hose into a container.

The Cardboard Box

We used a sturdy, corrugated cardboard box about 50 cm (20 in) long, 30 cm (12 in) wide, and 30 cm (12 in) high.  On one side of the box that we'll call the front, we drew a horizontal line 15 cm (6 in) above the bottom of the box.  From each end of that line, we then drew a line up to the top corner of the back of the box.  We then carefully cut along the line with a sharp utility knife to leave the front of the box lower than the back and each end sloped toward the front.

From the cut-off top portion of the box, we cut a band 8 cm (3 in) wide with tabs on the ends as shown in the diagram and folded it in the middle (the dotted line in the diagram) to form a V-shaped trough.  We glued the trough inside the front of the box with the bottom of the "V" at one end 5 cm (2 in) from the bottom of the box and the other end about 5 cm (2 in) higher.  At the bottom of the "V" at the lower end, we drilled a hole to fit a plastic hose elbow and glued the elbow in place.

 

This is what the box looked like when we were done.

We then covered the cardboard with polyester resin (the kind that is used with fiberglass.)  We prepared the resin in small batches, adding the hardener (catalyst) with thorough mixing just before it was applied.    It is important to use the kind of resin that is used for the finish coat (sometimes called the "gel coat") rather than the resin that is used for laminating, so that the finished surface is dry rather than sticky.  Since the hardened resin cannot be cleaned from clothes, it is best to wear old clothes.  We also used eye protection and plastic gloves, and we covered the table with a plastic sheet to protect it.

We painted every surface (inside and out) with a thick coat of the resin, making sure that no cardboard was left exposed.  We had to work quickly because the resin began to harden, and once hardened, it couldn't be removed with the solvent.  Between batches, we cleaned the paintbrush with acetone.  We also cleaned up any spills before they hardened with acetone and a  paper towel.

After the first coat of resin had hardened, we used autobody filler to fill all possible leaks.  The autobody filler, like the fiberglass resin, has two components--a putty-like resin and a hardener (catalyst).  We mixed the two components thoroughly before application, and since the autobody filler hardened very quickly, we mixed it in small batches.

We applied the autobody filler with plastic spatulas, forcing it into all the holes, cracks, and crevices where water might leak out or penetrate into the cardboad.  We worked quickly to use up a batch of filler before it hardened.

Once the autobody filler had hardened, we sanded it to remove all the rough spots.  We paid particular attention to make the the top edge flat because the window has to rest on a straight edge with no gaps.  We then applied 2 more coats of polyester resin to the entire box.

The Plastic Window

We made the window out of a sheet of acrylic plastic 3 mm (1/8 in) thick.  Rather than cut the plastic, we scored it and broke it much the same way you would with glass.  We first clamped the plastic between two angle irons at the edge of the table.  Then using the angle iron as a straight-edge guide and pushing down very hard with the point of the cutting tool, we scored a deep, straight groove.  It took about 10-20 repeated passes to make the scoring deep enough to get a nice, clean break.

The acrylic plastic is too hard to score with the same utility knife that we used to cut the cardboard.  The tool that we used was a broken hacksaw blade with tape wrapped around it to protect our hands.  Using the sharp corner of the broken end, we could carve out little chips of plastic as we pulled the blade along the edge of the angle iron.

We made several strips 2.5 cm (1 inch) wide to form the sides and drip strip of the window.  We broke the strips to the desired length by scoring and breaking as before.

We glued the strips to the top and side edges of the window with a multi-purpose nitro cellulose cement (Duco cement).
















We cut little pieces of cardboard at the same angle as that in the top corner of the side of the box and taped them in place temporarily to hold the drip strip at the proper angle while the glue dried.

The Wooden Base

We made the base out of wood, taking care to measure everything carefully and make the angles square.















We assembled the frame with screws, using a brace-and-bit with a Philips-head screwdriver bit to make the work easier.

Before placing the wooden frame on the 4 bricks, we carefully leveled them, digging out a little earth where necessary.

After placing the wooden frame on the bricks, we made sure that it didn't wobble and we checked the level again.

The reservoir for the distilled water is a clean plastic jug with a hole in the top through which the hose can fit very tightly with no gaps.  As a further measure to keep the distilled water clean, we passed the hose through an inverted funnel made from the top of another plasic bottle which also had a hole through which the hose fit very tightly.  Once the jug was in place, we slipped the funnel down to fit around the top of the jug.

This is the way it looked with the box in place.













We then added enough water to bring the level up to the bottom of the trough, taking great care not to splash any water into the trough for the distilled water.  We sprinkled a little powdered charcoal in the water to increase the absorption of the sun's energy.

We then put the window in place...

... and stepped back to admire our work!