No Smoke and All Mirrors

How to build a miniature Concentrated Solar Thermal (CST) model.

TabletopCST model

Talking to people about renewable energy, I’ve found that most people haven’t heard of Concentrated Solar Thermal power plants like the Gemasolar one in Spain – so models and photos are really useful in explaining the concept.

Table-top models (like this one) that have been made are proving to be real attractions. But they’re big and heavy. Wonderful for larger venues, but I wondered if something smaller would work too. Something I could carry under my arm.

If you’re here looking for an explanation of how CST works, you might find this link useful. Otherwise,  read on . . . .

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What I bought . . .

what I boughtI found packs of 20 mirrors, one inch square, for $2.00 in a craft shop. Previously I’d been planning a slightly larger model using clamshell make-up mirrors, but I couldn’t resist the bargain here, and so the ultra-miniature CST model was conceived.

To mount the mirrors at an angle, onto a baseboard, I bought a pack of ¾” paper fasteners, also for $2.00, and a tube of Tarzan’s grip.

The whole thing was going to be not much bigger than a sheet of A4 paper!

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Gluing the fasteners to the mirrors

mirrors glued onto clipsI bent each paper fastener to about 30 degrees before gluing it to the back of a mirror. Tweezers came in handy to push the brass fastener down to the mirror and make sure it was as square-on as possible.

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The baseboard

The baseboardThe baseboard was just a 30cm square of particle board I had. It needs to be something that will stay flat and not warp in the hot sun.

For my model I decided to have the tower in one corner, about 7cm from the edges.

I had found a fairly long bolt in my box of stuff, with a matching nut. I glued the nut onto the baseboard. That way I’d be able to screw and unscrew the tower to pack the model away for transport.

I drew a series of arcs centred on the tower, at 25mm intervals, and radiuses at 22½ degree intervals. This was going to give me enough room to fiddle with each mirror as I glued it on the board. Each row of mirrors also had to be far enough back from the ones in front to get a clear straight line throw to the tower top.

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Placing the mirrors – first step

I glued the front row in place – 9 mirrors, all at a 75mm radius from the tower. At this stage I hadn’t built the tower, but in any event, I needed a larger screen to focus the reflected light.

I folded a sheet of white card and sellotaped it to the baseboard, with cross-hairs pencilled in about 18cm from the baseboard.

The important thing was to get each mirror aligned correctly in the L-R plane before the glue set.

Vertical adjustment would be easier later, by slightly bending the brass fastener. It helped to get each one right before gluing the next one into place.

( I actually did one row at a time – so this picture was taken at a later stage)

As I got more and more mirrors fixed, it got harder to see which reflects spot was the one I was trying to adjust. Placing a card to mask off the ones already fixed helped, so that only the newest mirrors actually produced a reflection, but then there was a danger of knocking them out of alignment before the glue set.

(I was concentrating on this bit so hard I forgot to take a photo of how to use a card to mask off unwanted mirrors. Here’s the second row in place, but not quite lined up yet.

Fast work needed, before the glue set too much.After the second row of mirrors, I found I needed sunglasses to do the job.

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Let the glue dry

let the glue dryIt was important to let the glue dry before trying to adjust the mirrors vertically.

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The tower – “your message here”

Now the tower. I was going to make a long and narrow box from white card (postcard weight): about 20cm high, and each side was 18mm. It would have five sides so that sides one and five would overlap for gluing. The box would fit over the bolt mentioned earlier.
To decorate the box, I drew up a pattern on the computer using a graphics program. Paintshop Pro is my favourite, but you could also do it with Word, making a table with five columns 18mm wide, and putting centred text (about 48-point Arial) into each column, and shading the background.

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Assembling the tower

I discovered that a matching length of garden hose would exactly fit over the head of the bolt, and fit snugly into the card box. That made it easy to assemble the whole thing.

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Aligning the mirrors – stage 2

With just the narrow tower, you can’t see the spots that are out of line – with a wide card you can. So I kept the wider card in place for the time being, to make sure all the mirrors were aligned.

In theory all the mirrors were aligned L-R, so the reflections should have fallen on the vertical centre line. In practice, a few had drifted a little as the glue set. The job was to tease each reflection into alignment – vertically by bending the brass fastener, with a slight horizontal twist if needed. I found the best I could do was to line them up to an area about twice the size of the mirrors (about 25mm square). After getting two lines of mirror settled I made up a third row and glued them in place in the same way.

You need to do this vertical alignment at the time of day you want the model to work best. Once set, the mirrors can’t be adjusted any more, and as the sun changes its elevation, the reflections will move.

I chose 1pm (solar midday in daylight saving). As the sun goes down, the reflections rise higher, and don’t converge so well. You can chase them a little by tilting the baseboard.

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Other structures

The tower in actionI fiddled about with some scraps of things I had around the place, to represent holding tanks, pipes etc. I might add a few more blocks for pumps, turbines etc – though of course they won’t be functional in any way.

Here’s the finished result, in the midday sun.

I might add some more tanks and pipes and other structures if I get around to it.

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The working model

No smoke and all mirrors

Last thing needed was a box to carry the thing in. I had planned to use a pizza box, but in the end I made one out of a cut-down wine carton which was exactly the right size. I put a little partition near one edge so the tower could be unscrewed and prevented from rolling about and banging the delicately adjusted mirrors.

The project needed a name. Since it’s all about replacing coal-fired power stations with solar ones, I called it  . . .

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NO SMOKE AND ALL MIRRORS.

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2 responses to this post.

  1. Posted by Steveanaki on March 21, 2012 at 9:07 PM

    Absolutely awesome, Dominic! I love it! And I want one. Now all I need is the time to build the little sucker…

    Steve

    Reply

  2. Great stuff

    Reply

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