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From thermoelectric materials to generators

Since its inception, the field of thermoelectric energy conversion has held great promise to convert heat to electrical power, using a technology that is free of moving parts and thereby silent and extremely durable. It is particularly promising for the recovery of waste heat that is all too abundant in modern society, as thermoelectric generators (TEG) can be made in very compact sizes, with areas and thicknesses even below 1 cm2 and 1 mm, respectively, allowing their use in domains where other energy recovery technologies like Rankine engines are inapplicable. Unfortunately, there are several reasons why, so far, TEGs have not managed to become commercially relevant beyond nicheapplications.

From a scientific perspective, the two critical numbers determining the heat-to-electrical power conversion efficiency are the Carnot efficiency ηC = (ThotTcold) ⁄ Thot that sets thethermodynamic upper limit to the power conversion efficiency of an ideal enginerunning between heat reservoirs at different temperatures T, and thethermoelectric figure of merit zT = σS2T/к that determines which fraction of the Carnot efficiency is actually reached. A TEG based on active materials with zT → ∞ can operate at the Carnot efficiency, while zT ≈ 1 is often used as a threshold for commercial relevance.

Increasing zT is a far from trivial task as the constituting quantities, the electronic conductivity σ, the Seebeck coefficient (or thermo power) S and the thermal conductivity к are typically counter-indicated, meaning that improving one leads to a deterioration of the others. For inorganic thermoelectrics, this has in part been mitigated by expensive nano-structuring to suppress к. From a practical perspective, the most efficient inorganic thermoelectric materials, including the field’s standard Bi2Ti3 (zT ≈ 1 at room temperature), are often based on rare and/or toxic elements, are brittle and require energy-intensive fabrication processes.

The ongoing development of the internetof things (IoT) leads to completely new opportunities for thermoelectric generators based on organic materials (OTEG), in which the eminent and unique strengths of organic (semi)conductors are exploited, while their relative weaknesses are of minor or no importance. The revolutionary aspect of the IoT is the distributed nature of an otherwise highly interconnected network of autonomous devices. In many situations it is therefore unpractical or outright impossible to connect the nodes to the power grid – think of an adhesive biosensor that monitors a mobile patient or arrays of humidity sensors thatmonitor the soil in a vineyard.

Likewise, batteries are unpractical (need for changing) and undesired (environmentally unfriendly). In many IoT applications, sunlight-based powering by solar cells will be most practical. However, there are many applications where solar cells will not work due to intermittent (day/night) or complete absence of illumination, but where some form of heat is available. It is these applications we target. With the current market volume of ultra-low powered TEGs soon to surpass 100 million USD already, a small improvement in TEG operation may open the door to a billion-dollar market.

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