How it Works - Biolite

So to answer the question of how it works, I'm posting this diagram I pulled from Biolite's web site.  Essentially it generates electricity using a thermoelectric generator (TEG).    TEG convert heat energy into electrical energy.  TEGs are made from thermoelectric modules which are solid-state integrated circuits that employ three established thermoelectric effects known as the Peltier, Seebeck and Thomson effects. It is the Seebeck effect that is responsible for electrical power generation. Their construction consists of pairs of p-type and n-type semiconductor materials forming a thermocouple. These thermocouples are then connected electrically forming an array of multiple thermocouples (thermopile). They are then sandwiched between two thin ceramic wafers.

Thermoelectric generators have been in use for many years by NASA to power spacecraft and the oil and gas industry to power remote monitoring stations around the globe.

So for you engineers (By Jeffrey Snyder);

A thermoelectric produces electrical power from heat flow across a temperature gradient.    As the heat flows from hot to cold, free charge carriers (electrons or holes) in the material are also driven to the cold end.  The resulting voltage (V) is proportional  to the temperature difference (∆T) via the Seebeck coefficient,  α, (V =  α∆T).

A thermoelectric generator converts heat (Q) into electrical power (P) with efficiency η.  

 P = ηQ  (1)

The amount of heat,  Q, that can be directed though the thermoelectric materials frequently depends on the size of the heat exchangers used to harvest the heat on the hot side and reject it on the cold side. As the heat exchangers are typically much larger than the thermoelectric generators themselves, when size is a constraint (or high P/V is desired) the design for maximum power P = ηQ Small Thermoelectric Generators may take precedence over maximum efficiency. In this case the temperature difference (and therefore thermoelectric efficiency as described below) may be only half that between the heat source and sink.   The efficiency of a thermoelectric converter depends heavily on the temperature difference  ∆T =  Th –  Tc across the device. This is because the thermoelectric generator, like all heat engines, cannot have an efficiency greater than that of a Carnot cycle (∆T/Th). The efficiency of a thermoelectric generator is typically defined as

(2)

Where the first term is the Carnot efficiency and ZT is the figure of merit for the device. While the calculation of a thermoelectric generator. Many thermoelectric couples (top) of n-type and p-type thermoelectric semiconductors are connected electrically in series and thermally in parallel to make a thermoelectric generator. The flow of heat drives the free electrons (e-) and holes (h+) producing electrical power from heat.

While the calculation of η thermoelectric generator efficiency can be complex, use of the average

material figure of merit, zT, can provide an approximation for ZT.

(3)

Here, Seebeck coefficient (α), electrical resistivity (ρ), and thermal conductivity (κ) are temperature (T) dependent materials properties.    Recently, the field of thermoelectric materials is rapidly growing with the

discovery of complex, high-efficiency materials. A diverse array of new approaches, from complexity within

the unit cell to nanostructured bulk, nanowire and thin film materials, have all lead to high efficiency materials.