Thermoelectric materials and devices convert heat into electricity, or vice-versa, work as electrical heat pumps to produce heating or cooling using no moving parts. They achieve this chiefly by two reciprocal effects, the Seebeck effect and the Peltier effect.
Thermoelectric generation – the Seebeck effect.
Thermoelectric generation (TEG) ofter attracts a lot of excitement – the ability to produce clean electricity from wasted heat. It sounds like a big win, but TEG is inherently inefficient and extreme performance remains elusive. Having said that, much worldwide research effort is dedicated towards the discovery of new and improved TEG materials.
When one end (or side) of a thermoelectric material is heated, charge carriers – electrons and/or holes – receive energy and drift towards the cold side. This imbalance results in a potential difference, the Seebeck voltage, which when harnessed in an electrical circuit produces a small amount of power. A material where electrons are the dominant charge-carriers is termed n-type (for negative type). A materials where holes are the dominant charge carriers is termed p-type (for positive type). The key to traditional thermoelectric generation is the combination of p-type and n-type thermoelectric ‘legs’ in an arrangement which is electrically in series, but thermally in parallel. This means all legs are subject to the same thermal gradient, but their contributions add together. N-type and p-type materials produce Seebeck voltages with opposite directions (signs), and thus when connected ‘back to back’, the effects of both leg types add together resulting in a total Seebeck effect that is the sum total of that of all the legs.
Thermoelectric cooling – the Peltier effect.
The heat-pumping ability of thermoelectric materials is often discussed in the framing of thermoelectric cooling (TEC), also known as Peltier cooling. This is probably because electrical heating is so common, whilst cooling is more counter-intuitive.
In the Peltier effect it is easier to understand that heating and cooling are always taking place, albeit at opposite sides of a device. When an electric current is passed through a thermoelectric material from one end (or side) to another, the charge carriers take their thermal energy with them. Thus depending on the direction of the current, and the dominant charge carrier type, heat will either be released on a given face (resulting in heating) or absorbed at a given face (resulting in cooling). The very useful thing about the Peltier effect is its reversibility: one only needs to reverse the direction of the current and the effect is also reversed – for example, cooling a given face instead of heating it. This allows fine and versatile temperature control with the ability to heat or cool in any chosen direction with respect to room temperature. Since only the current needs to change direction this also makes for reasonably fast temperature changes, depending upon device power. The Peltier effect is also best used in the same arrangement of n- and p-type legs in the electrically in series, thermally in parallel arrangement. This ensures that a given current direction produces the Peltier effect in the same sense on a given device face.
Thermoelectric Future 