Efficiency booster for candles
Before electric lighting became common in the early 20th century, candles and oil lamps were commonly used for illumination. From ancient times until the mid-1850s, candles were made from various forms of natural fat, tallow, and wax. Today, most candles are made from paraffin wax, a product of petroleum refining.
A modern candle produces approximately 80 W of heat energy with a luminous efficacy of about 0.16 lumens per watt. This is almost a hundred times lower than the luminous efficacy of an incandescent light bulb. Tea lights have smaller wick and a smaller flame compared to candles whose main purpose is illumination. A standard tea light has a power output of around 32 watts, depending on the wax used.
The peltier lamp converts a small fraction of the heat energy released by a tea light into electricity in order to drive a highly efficient LED USB reading light. As such we get a lot more light from the tea light boosting the total luminous efficacy.
A peltier element is a solid state active heat pump which transfers heat from one side of the device to the other, with consumption of electrical energy. It can be used either for heating or for cooling, although in practice the main application is cooling. This effect can also be reversed if one side of the device is heated to a temperature greater than the other side, and as a result, a difference in voltage will build up between the two sides. This effect is also known as the Seebeck effect and a device like this is called a thermoelectric generator (TEG).
The typical efficiency of a thermoelectric generator is around 5-8 %. So, in theory we should be able to get 1.6 - 2.56 W of electrical power out of a tea light. In practice, we only get around 250 mW with our peltier lamp. This is due to the fact that not all of the heat energy of the tea light is captured and the use of peltier elements which are not optimized for use as thermoelectric generators. However, the generated energy is sufficient to run a small fan and drive a USB reading light in a satisfactory manner.
The thermoelectric generator in this design is built around two 40 x 40 mm TEC1-12706 peltier elements, connected in series, mounted between two heatsinks. The smaller bottom heatsink spreads the heat of the tea light in order to heat the peltier elements evenly. The larger top heatsink, which is equipped with a small fan, maximizes the temperature difference between the bottom and top side of the peltier elements.
It is a bit counter intuitive but despite the fact that the fan draws also energy from the peltier elements, the overall efficiency is improved to the point there is more energy available for the USB light than without using the fan. The fan also prevents the peltier elements from overheating. These are soldered internally with a bismuth alloy solder and they should never operate above 138°C, the melting point of the solder.
A laser cut acrylic enclosure acts as a wind shield for the tea light and allows for adjusting the distance between the thermoelectric generator and the tea light.
As the thermoelectric generator only generates about 2.2 V without load, some kind of boost converter is necessary to convert the output of the peltier elements to a more usable voltage in order to drive the USB light. As a thermoelectric generator is a high impedance voltage source, we used a LT3105 energy harvesting IC. This IC has maximum power point control which allows us to get the most out of the peltier elements. Such an energy source typically delivers the most power when the output voltage is kept to half the value under no load conditions. In this case, the maximum power point voltage is around 1.1 V. We've set the maximum power point control of the LT3105 slightly lower to 1 V to make the start up of the system somewhat easier.
As the USB light draws too much current at 5V, we've set the output voltage of the boost converter to 3.3 V. At this voltage, the USB light puts out an acceptable amount of light without consuming too much power.
The fan is a 12 V fan. We use it here out of spec and it doesn't start automatically at 3-3.3 V. A slight push is all it needs to start spinning. We intentionally didn't use a lower voltage fan as this would draw too much current and the air flow might interfere with the tea light flame.
We designed two different enclosures for this project. Which one do you prefer ? Please let us know in the comments below.
- on Home & Garden
- Labs project
March 26, 2017
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