fal08_p54-56

fal08_p54-56 - Small Thermoelectric Generators by G....

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54 The Electrochemical Society Interface • Fall 2008 T hermoelectric generators are all solid-state devices that convert heat into electricity. Unlike traditional dynamic heat engines, thermoelectric generators contain no moving parts and are completely silent. Such generators have been used reliably for over 30 years of maintenance-free operation in deep space probes such as the Voyager missions of NASA. 1 Compared to large, traditional heat engines, thermoelectric generators have lower efficiency. But for small applications, thermoelectrics can become competitive because they are compact, simple (inexpensive) and scaleable. Thermoelectric systems can be easily designed to operate with small heat sources and small temperature differences. Such small generators could be mass produced for use in automotive waste heat recovery or home co-generation of heat and electricity. Thermoelectrics have even been miniaturized to harvest body heat for powering a wristwatch. Thermoelectric Power A thermoelectric produces electrical power from heat flow across a temperature gradient. 2 As the heat flows from hot to cold, free charge carriers (electrons or holes) in the material are also driven to the cold end (Fig. 1). The resulting voltage ( V ) is proportional to the temperature difference ( T ) via the Seebeck coefficient, α , ( V = α∆ T ). By connecting an electron conducting (n-type) and hole conducting (p-type) material in series, a net voltage is produced that can be driven through a load. A good thermoelectric material has a Seebeck coefficient between 100 µV/K and 300 µV/K; thus, in order to achieve a few volts at the load, many thermoelectric couples need to be connected in series to make the thermoelectric device (Fig. 1). A thermoelectric generator converts heat ( Q ) into electrical power ( P ) with efficiency η . (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 by G. Jeffrey Snyder 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. 3 The efficiency of a thermoelectric converter depends heavily on the temperature difference T = T h T c 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 / T h ). 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 F IG . 1. Schematic of a thermoelectric generator. Many thermoelectric couples (top) of
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fal08_p54-56 - Small Thermoelectric Generators by G....

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