Power a radio by the heat generated by a lantern seems like an unlikely proposition. But this is exactly what a product from UK-based Navitron does. The unlikely product is unique in the marketplace. Originally invented by the Russians during the 1960s, and used extensively throughout the Cold War, these lanterns have never really seen the light of day in the Western World. The unit had disappeared from production until a few years ago, when it reappeared on the market in small volumes.
The lantern-powered radio consists of a Hurricane lantern that has been modified with a TEG (Thermoelectric Generator), which produces a small amount of power which can be used to power a compact FM/AM radio built into the base of the lantern.
Figure 1: The lantern with accessories.
Figure 2: Close up of the lantern showing the tuning control/headphone connector and the volume/waveband selector.
The TEG incorporated into the lantern runs from the waste heat produced by the burning of fuel to generate light. Most TEGs designs are unable to cope with temperatures over 225C. This unit is designed for long term operation at much higher temperatures. The manufacturer claims a power output of 3W and a nominal voltage of 3.5v.
TEGs (also known as Seebeck devices) produce power by a phenomenon utilized by the common thermocouple - where joints between dissimilar metals produce tiny amounts of electricity when heated. In fact, in order to get a useful amount of power, these lanterns employ a large bank of hundreds of thermocouples to produce enough electricity to run the radio. The radio can be unplugged, allowing you to use the electricity for other low power low voltage requirements such as charging low power portable equipment such as phones.
These devices provide an excellent and useful demonstration of a novel method of converting waste heat into a more immediately useful form of energy. The TEG can quickly and easily be removed from the lantern by removing two clips, allowing it to be used for other applications.
Figure 3: TEG (removed from the lamp, and stripped of some of its insulation so that you can see the thermocouple array) utilizing the heat from a stove to produce electricity. The LED is powered from the TEG.
The disadvantage of the thermocouple technology is that it is much less compact than its semiconductor equivalents, so it is not suitable for all types of application. The advantage of this method is that it is much more heat resistant - it can easily cope with the heat of the lantern flame. Modern TEGs mostly fail at around 150C although some high temperature units can cope with temperatures of 220C.
The TEG Technology
Despite nearly two hundred years of progress, Thermoelectric Generators (TEG) systems today work on low single digit efficiencies. Twin phenomena – one generating electricity and one concentrating heat were discovered by early 19th century scientists, Thomas Seebeck and Jean Peltier. Seebeck found that a temperature gradient placed across the junctions of two dissimilar conductors caused electrical current to flow. Peltier, learned that passing current through two dissimilar electrical conductors caused heat to be emitted or absorbed at the junction of the materials. Apart from the brute force TEG used in the lantern powered radio, it took advances in semiconductor materials to make practical applications for thermoelectric devices feasible. Modern semiconductor techniques, enable thermoelectric modules that can deliver efficient solid state heat-pumping for both cooling and heating. When used in the reverse configuration, many of these modules can also be used to generate DC power.
Thermoelectric modules often consist of an array of Bismuth Telluride semiconductor pellets. These pellets are doped so that one type of charge carrier, either positive or negative, carries the majority of current. Pairs of P/N pellets are connected in series electrically, but in parallel thermally. The pellets are placed on a metalized ceramic substrate or copper disc with conductive tabs to connect the tops. The pellets, tabs and substrates form a layered configuration. Thermoelectric modules can function singularly or in groups with series, parallel, series/parallel electrical connections or stacked multi-stage modules.
Heat will be moved ('pumped') in the direction of charge carrier movement. The charge carriers transfer heat as a natural part of their movement. Consider the case in which N-type semiconductor material is used to fabricate a pellet. Then electrons will be the charge carrier that creates the Peltier effect. Heat is absorbed at the bottom junction and actively transferred to the top junction when electrons flow through the N-type material from bottom to top.
'P-type' semiconductor pellets are also employed in thermoelectric devices. P-type pellets employ charge carriers in the material that are positive. These carriers are ‘holes.’ Holes enhance the electrical conductivity of the P-type crystaline structure. This permits electrons to flow more freely through the material. Hole current flows in a direction opposite to that of electron flow. Use of the P-type material results in heat being drawn toward the negative and away from the positive source.
The use of two different semiconductor materials is key to the development of practical Peltier or Seebeck devices. A single pellet of either N or P type draws or generates a sub-volt voltage while consuming relatively large amounts of current. By combining N and P pellets together it is possible to create a series-parallel structure that operates or generates useful voltage and current levels.
The Lantern Radio is both a historic and currently useful device that relies on the Seebeck effect. Need when combined with ingenuity often creates some surprising solutions. Few products are as surprising and unusual, yet practical, than the cold war inspired radio.