Wednesday, 3 December 2014

Free heat - with provisios

Drink can solar heater 
Last year I got the chance to make a solar air heater for a BBC Scotland series called The Great Big Energy Saving Challenge. It was a fun show that for reasons I was not party to, was not picked up by the national BBC network. The premise of the series was fairly self evident. A bunch of families in Stonehaven, near Aberdeen were given the challenge of reducing their energy consumption by 30%. Or maybe it was to 30%. I'm a bit hazy about that bit, but it is not relevant to this blog. What is important is that I learnt two things. Firstly Stonehaven is a really pretty town, especially when the sun shines, and secondly that a solar air heater made from recycled fizzy drink cans can be incredibly efficient.

The solar air heater is very simple. Take a huge pile of fizzy drink cans, knock the bottoms out and stick them together to make a series of long aluminium tubes. Spray the tubes thus made black, then mount them inside a big box with a clear lid and shove the box outside on a sunny day. Now arrange, by cunning contrivance for air from inside your house to be pushed by a small fan into the base of the box, letting it flow up through the black tubes, out the top and back to the house duly warmed up to a toasty 30 degrees.

Lots and lots and lots of cans
The science is simple, the sun light shines through the clear lid of your box, hitting the matt black tubes. Since they are black, the light is absorbed and the aluminium warms up. As the aluminium warms up, the heat is radiated back out, but at an infra-red wavelength that bounces off the clear box lid. Which means the heat stays inside the box and can't escape. You have made a greenhouse.

When we tested this on a cold, windy Scottish day the sun warmed the box until the temperature at the top of the box was in excess of 80 degrees. That's hot enough to scald you and cook things. What's more, the box immediately set up a convection current that started pulling cold air in the bottom of the box and pumping warm air out - all without the aid of a fan. At the time we were filming with two cameras, one inside the house with the owner sat by the outlet pipe and me outside with the heater and a thermometer. The convection current was so strong, and the air coming into the house so warm (30 degrees) that they naturally presumed the fan was on. But it wasn't.

For somebody with lots of sunshine it is clearly a brilliant source of free heat. It is not, however, a completely trivial thing to build in a way that would last. Since the series transmitted, I have had a continual trickle of people asking for more information about how to build one. So, here is a link to a PDF I produced with as much information as possible. If you are intrigued, have a sunny place to mount one and are not daunted by a bit of advanced DIY, why not give it a try.


  1. This comment has been removed by the author.

  2. Dude, seriously you're a genius. Thinking of using the empty frizzing can to make solar heater is an amazing idea. I'm so delighted to learn about this invention. Solar heating system is always good for energy saving.


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Science TV Presenter, live show performer, writer, strange prop builder and all round Science Bloke. All opinions expressed are mine alone. Not the BBC's, just mine.