Wednesday, 28 August 2013

Nanogeneration camping: A tealight-powered LED lantern

When we go camping we often meet Paul from Fireside. He imports wood-burning stoves and deals with a lot of outdoor stuff, and always has interesting camping tackle. People send him samples that he likes to try out in the field before unleashing them on his customers. A couple of years ago he had a twig burner with a thermocouple that could charge a phone. He seemed to be spending most of the time feeding twigs to the burner, and drinking a lot of tea from the kettle on top. I didn't notice him bringing this bit of tackle the next time we saw him.

This year he had a tea-light LED lamp. Somebody is raving about it here in the Chronicle Herald. That's a Canadian publication so it may be Chronic le Herald and I'm misreading the URL.

It uses a thermocouple to convert the heat from the tea light into electricity, which drives eight LEDs. It certainly is bright. The LEDs are something like 20 times brighter than the candle. The problems with a lot of LED camping lights are that they don't put enough light out, and the light is going in the wrong direction. The Tilly lamp makes a lot of sense if you have to carry around a light source with liquid fuel, but designing LED lights in that shape seems a little strange. On a campsite you don't really want the light shining out in all directions. You often want it pointing down at the table you're eating from, or the surface you're working on. This light is both bright and allows the light to point down towards the table. 

And it only uses candles. This is really good because candles are cheap and easy to carry. They don't go off, and it's easy to see how much of them is left, unlike batteries. And of course they are made from fossil fuels. That's not necessarily a good thing. Also, for the weight, size and cost of the lantern, you should be able to get a high-powered battery LED, several rechargeable batteries and a charger for them. You probably have a battery charger anyway. LED technology is still fairly young, and there are still plenty of models with excessively long battery life and inadequate brightness. The good news is that if you leave them on they'll still be shining a couple of days later, but that bad news is that you may not notice. This lantern goes out when the candle burns out.

It provides a wonderful lesson in energy efficiency. Thermocouples have an efficiency of something under 10%. Let's say 5% for easy calculation, so they're turning one twentieth of the heat they use into electricity. Then LEDs, while really efficient compared to other lighting sources, only turn about 10% of the electrical power that reaches them into light. So the combination of the thermocouple and LEDs is only only getting one two-hundredth of the power into light. And this electric light is twenty times brighter than the candle was, so the candle is turning less than a four-thousandth of the energy in the wax into light. A few times less, in fact, since a lot of the heat from the candle is not even going to get to the thermocouple. 

So how much better is this than if the fossil fuels, instead of being sent to the candle factory, were sent to a power station, then to your house, and into a rechargeable battery?