A critical challenge in using thermoelectric generators (TEGs) for charging the portable or wearable electronics has been their limited outputs, as available temperature differential on human body (∆Text) is typically less than 10 K. Furthermore, the thermal resistance (Rth) at the TEG–air interface often overwhelms Rth of TEG itself, which makes the temperature differential within the TEG merely a small fraction of ∆Text. Here, the designs of TEG systems for wearable applications based both on theory and systematic experiments are studied. First, this study fabricates the TEGs having different fill factors (equivalently, varied internal Rth of the TEGs) and finds an optimum fill factor that is determined by both thermal matching condition and the electrical contact resistance. Next, to investigate the effects of heat sink and external air flow, this study combines plate fin heat sinks with the TEGs and evaluates their performance under three different convection conditions: natural convection, and convection with either parallel or impinging flow. Lastly the effect of Rth at the skin–TEG interface is studied. Although the TEG system produces an output power of 126 µW cm−2 (∆Text = 7 K) on a smooth heat source (Cu heater), it generates reduced power of 20 µW cm−2 (∆Text = 6 K) on wrist (uneven heat source).

Although the Harman method evaluates the thermoelectric figure-of-merit in a rapid and simple fashion, the accuracy of this method is affected by several electrical and thermal extrinsic factors that have not been thoroughly investigated. Here, we study the relevant extrinsic effects and a correction scheme for them. A finite element model simulates the electrical potential and temperature fields of a sample, and enables the detailed analysis of electrical and thermal transport. The model predicts that the measurement strongly depends on the materials, sample geometries, and contact resistance of the electrodes. To verify the model, we measure the thermoelectric properties of Bi2-Te3 based alloys with systematically varied sample geometries and either with a point or a surface current source. By comparing the model and experimental data, we understand how the measurement conditions determine the extrinsic effects, and, furthermore, able to extract the intrinsic thermoelectric properties. A correction scheme is proposed to eliminate the associated extrinsic effects for an accurate evaluation. This work will help the Harman method be more consistent and accurate and contribute to the development of thermoelectric materials.

Accuracy of the Harman measurement largely depends on the heat transfer between the sample and its surroundings, so-called parasitic thermal effects (PTEs). Similar to the material evaluations, measuring thermoelectric modules (TEMs) is also affected by the PTEs especially when measuring under atmospheric condition. Here, we study the correction methods for the Harman measurements with systematically varied samples (both bulk materials and TEMs) at various conditions. Among several PTEs, the heat transfer via electric wires is critical. Thus, we estimate the thermal conductance of the electric wires, and correct the measured properties for a certain sample shape and measuring temperature. The PTEs are responsible for the underestimation of the TEM properties especially under atmospheric conditions (10–35%). This study will be useful to accurately characterize the thermoelectric properties of materials and modules.