How the Concept Works
A tunnel diode is hugely different from a normal rectifier diode and is characterized by a fast response time. Also referring to the graph we find that, as the applied current IF is increased, after a certain value, astonishingly its forward voltage UF dips, quite unlike other normal diodes. Thus it clearly shows a negative resistance characteristic (shown by the shaded area) and basically is exploited for the present purpose of generating electricity from heat. This characteristic is normally represented as – Rd.
Now, according to Ohm’s Law, in an electric circuit loop consisting of a battery and a load, I = U/R, where I is the current, U is the applied voltage and R is the resistance offered by the circuit load. Obviously here the positive sign associated with I and R indicates that the battery will start discharging through R. Therefore conversely, if I and R carry a negative sign or a minus sign, theoretically and logically, it should start charging the connected battery.
Going by the above explanations, consider a closed loop circuit consisting of a chargeable Ni-Cd battery and a group of tunnel diodes (see figure and click to enlarge). If we apply heat externally to these diodes so that their IF increases to the desired level (– Rd) , the resultant Ohm’s law can be now derived as:
– I = U/– R. Also, the dissipated power will look like:
P = – I2 – R (by substituting Ohms law in P = IU).
The above expression convincingly proves that the circuit is now in fact charging the battery.
Interestingly here, the load itself acts as a voltage source with a low internal resistance. However, the generated voltage should be greater than the battery voltage to initiate the charging current. The charging current IC is expressed by the following equation:
IC = δ[Ʃ(UF) – (Ubat)]/Ʃ(Rd)+Rbat
In the above equation Ʃ(Rd) notation indicates that all the diodes involved in the circuit need to be operated specifically at their – Rd region and if any diode does not satisfy this condition it would simply cancel the whole objective of the concept. So it becomes imperative to carefully segregate these diodes by testing each of them individually so that the circuit is able to produce the desired function of generating electricity from heat.