Under the Choko Tree By Nevin Sweeney

Refurbing the Solar Oven

Our solar oven has been in regular use for over 15 years and has been starting to look a bit ……. worn. The reflectors are not flat anymore and are not so reflective anymore either. The reflective surface becomes dusty and oxidised over time and just doesn’t work as well, some of the hinges holding the reflectors in place have come off also. The main box of the oven is still in good nick, although the felt edge seal is a bit worse for wear too. All up it is time for a refurb.

The oven as it was

The original set of reflectors were made from 6mm MDF (Medium Density Fibreboard) with a melamine coating on one side. They worked pretty well and were light enough but over time they have warped so that not all of the light falling on them is reflected into the oven box, reducing the oven’s efficiency. This time I am having a go with 6mm 3 ply plywood which is a bit heavier but is also more rigid and resistant to warping because of the laminations. The plywood is also a bit more expensive. I was able to pick up 3 600mm x 1200mm sheets which is more than enough to make the four square main reflectors and four triangular corner reflectors.

To start I removed the originals from the oven box and recovered as much of the hardware (screws and hinges) as possible. I then marked out the new reflectors based on the dimensions of the originals with a one metre stainless steel rule and pencil, then made the required cuts with my hand circular saw. There was a bit of damage to the sharp points of the triangular corner sections but generally the approach worked fairly well.

The carcase of the oven with reflector attachments in place

After some consideration I decided it was easier to apply the reflecting material to the reflectors before installing them on the oven. The reflecting material which I used was the same as last time ie metallised plastic film gift wrap, with the unprinted reflective side facing out. It must be going out of fashion because I found it much more difficult to locate this time, so if you are going to make one of these ovens, start looking in newsagents and “el cheapo” shops now. If you can’t find the metallised plastic film then use aluminium foil with the shiny side out.

My original idea was to use some double sided tape to secure the film to the reflectors – bad idea! First off if I pulled it off the reel too fast it left most of the adhesive of the other surface and my double sided tape became single sided. Even when I was able to secure it to the reflector with an adhesive side out, id did not stick very well to the plywood so as I tried to apply the film the tape came away from the plywood and of course stuck to itself and to the reflecting film making one big mess!

The plywood reflectors cut out and ready to coat

I ended up by going back to the way I made the first one, painting each of the boards with a mix of PVE glue and water (50:50). The PVA is not an instant stick to you can remove and re-fix the film if there is a problem and by rubbing the surface over with a cloth you can move air bubbles to the edge of the plastic film and then out, making the surface flatter. To make the job easier I also cut the film oversized, then once the glue was set and the film stuck to the plywood well and truly I trimmed off the edges with a VERY sharp knife.

With the reflectors now in good shape I had a look at the bent steel brackets which connected the main square reflector panels to the plywood carcase of the oven. There a two of these on each side, each one connected by a single screw fixed into the side of the oven. They had become a bit deformed and bent out after being well used for 15 years or more so I unscrewed them and panel beat them back into shape with my trusty ball pein hammer and using the flat spot on the back of one of my engineers’ vises as an anvil. The ones on each side of the oven had a tendency to slip back and forward if I tugged on the reflectors because they were only secured by a single screw, so I installed a second screw on each one to prevent this happening again.

Installing the second screw to stop the attachment from rotating

To fix the triangular corner reflectors to the main square side ones I used hinges at the top and bottom of each reflector. The easiest way to put the oven back together is to screw the square reflectors back into place on the oven carcase, affix the hinges to the triangular sections and then screw the triangular sections onto the square sections, thereby tying everything together (I hope!).

The freshly coated reflectors

The one downside of reassembling the oven with the film in place on the reflectors is that I do it by placing each hinge on the outside (uncoated) surface of the reflectors and then drilling through from the outside to the inside (coated surface) of the reflectors and then putting  bolt through each of the holes. Why is this a problem I hear you ask? Well, drilling the hole in from the uncoated side means that, more often than not, the drill dislodges and mangles the film rather than cutting through it cleanly. The only way I have found to do it without screwing it up is to hold a piece of wood firmly over the film and then drill through the reflector and film into the wood block. If you can hold the wood in place the hole is very neat and no other damage is done. (if you can’t it still screws up!)

I worked my way around, first securing the square reflector which stands vertically at the back of the oven, and then the one on the right hand side. I then then secured the corner reflector between the two squared ones. I then fitted the next square reflector in the series and the next corner one until they were all fitted.

The Finished Article

With the reflectors replaced the oven looks more like its old self, and is working better than ever. It has been well worth the effort and was long overdue!

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