One December evening there was a power cut and all I had to hand was a big bag of tea lights for illumination. I was sat on a deck chair in the living room surrounded by 20 or so getting plenty of heat but barely enough light to see my hand. This got me wondering whether you could get more light from an LED than the tea light itself by harvesting the heat from the flame. To cut a long story short, in the process of answering this question, I ended up developing a fold-able Christmas lantern which is hopefully available to buy for Christmas 2024 on somewhere like Etsy. For full product spec please click the following link:

​

​

​

​

​

​

The key points to answering the question are:

​

1) A tea candle produces ~20W of power output by way of raw heat.

 

2) The candle will stay lit for 3-4 hours which equates to about 0.07kWh of energy. At the time of writing, you could buy 500 tea candles from Amazon for £35. This implies a cost of about £1/kWh which, even at today’s exorbitant energy prices, is about 3 times the cost of domestic gas. So no, the internet is wrong, you can’t heat your house cost effectively with tea candles and a flowerpot.

 

3) Tea candles are nonetheless quite impressive. Duracell’s best AA batteries can deliver 2,500mAh which is about 0.00375kWh (10 to 20 times less energy than a humble tea candle!).

 

4) However, Peltier modules are not very efficient. They melt at around 120 degrees C which means you can’t get much efficiency out of them. Assuming you’re in a 15 degrees C room, the best you could theoretically get is 27%. In practice, given the limited bandwidth of the semiconductor in the module and other losses, you might achieve ~5%-10%. This takes us from 0.07kWh to ~0.0035kWh, pretty much back to where we were with a single AA battery.

 

5) Factoring the above efficiencies, from our ~20W power input, we get ~1W to 2W of electrical power output from the Peltier module but at a pretty measly 0.3V to 0.8V.

 

6) We need to boost this up which we do with a little ‘boost converter’ under the LED (wrapped in black heat-shrink). This is a little inductor, transistor, diode and a small microcontroller which pulses power into the inductor to step up the voltage to ~3.5V.

 

7) The original objective with this lantern was to get more visible light out from the LED than from the flame out of the tea candle… Not sure this has been achieved in this version but should theoretically be possible with a bit of refinement! Again, please heed warning 1 above!