The early 20th century saw the end of the use of candles and oil lamps as electric lighting became more common. Earlier, candles were made from various items such as natural fat, wax, and tallow. However, most manufacturers make candles from paraffin wax, a substance obtained from refining petroleum.
Compared to an incandescent bulb, a candle produces nearly a hundred times lower luminous efficacy. The luminous efficacy of a modern candle is about 0.16 lumens per watt, and it produces nearly 80 W of heat energy. Another form of the candle, tea lights, come with a smaller wick and produce a smaller flame. However, a standard tea light produces about 32 W, depending on the wax it uses.
The Peltier cell makes it possible to convert a small fraction of the heat energy from tea light into electricity. This can be used to drive a highly efficient LED light. This arrangement helps to boost the total luminous efficacy of the tea light and we can get a larger amount of light.
The Peltier element is really a solid-state active heat pump. Electricity applied to the element causes it to transfer heat from one side of the device to the other. Therefore, a Peltier element can be used for heating or cooling. If one side of the Peltier element is heated to a temperature higher than that on the other side, the Peltier element works in reverse, generating a difference of voltage between the terminals. This reverse effect is known as the Seebeck effect and the device works as a thermoelectric generator.
As the efficiency of a typical thermoelectric generator is only around 5-8%, the heat from a tea light should be capable of generating about 1.6-2.56 W of electrical power from the Peltier element. In practice, the Peltier element gives only about 0.25 W with the heat from the tea lamp. The reason being the inability of the Peltier element to capture the entire heat produced by the tea lamp to generate electricity—some heat is lost in transmission, and some in heating up the Peltier element. However, the energy generated by the Peltier acting as a thermoelectric generator is capable of running a small fan and drive an LED lamp satisfactorily.
A thermoelectric generator can be built around two 40×40 mm TEC1-12706 Peltier elements, mounted between two heat sinks, and connected in series to boost the voltage output. The smaller heat sink at the bottom serves to spread the heat from the tea light to heat up the Peltier elements evenly. The larger heat sink at the top has a fan to cool it and maximize the temperature difference between the two sides of the Peltier elements.
Although the fan draws power from the Peltier elements, it also helps to improve the efficiency of the system and make more energy available for the LED light. The fan also helps to keep the Peltier elements from overheating. Peltier elements are internally soldered with a bismuth allow solder melting at 138°C. Therefore, no Peltier element should operate above this temperature.
