Thermoelectric Energy Harvester – Device Structure
In order to use a thermoelectric generator(TEG) as an energy source, it is necessary to improve the output power. In general, the way to increase the power density(power per area) of a TEG is to improve the performance(figure of merits, ZT) of the material. However, the power density of the TEG is affected not only by the material characteristics but also by the temperature difference across the TEG. If the total temperature difference of the system is not sufficiently applied to the TEG, great amount of heat is lost and the power output decreases. Therefore, the structure of the device should be optimized in consideration of the entire heat system. Various structural factors of thermoelectric elements such as height, FF(Fill Factor), contact resistance, solder, and electrode characteristic are complexly combined and each factor has optimum value for the highest power. To find the optimum design, the electrical and the thermal model of the TEG system are designed, and the power density can be calculated by the equation extracted by the model. The calculated results are well fitted with experiments. The improvement of the power density will widen the application range of TEG.
Thermoelectric Energy Harvester – Material
Recent studies on thermoelectric (TE) devices have sought to develop them as efficient electric power supplies for sensors, wearable appliances, and other applications based on body area networks (BANs) or wireless sensor networks (WSNs), as well as for recovering waste heat. Flexible TE generators (f-TEGs) in particular are attracting significant attention because they can be applied to curved or uneven surfaces. F-TEGs are mostly designed for room temperature applications. So, Bi-Te based materials are usually used for f-TEGs because they exhibit the highest TE properties at room temperature.
Thermoelectric Energy Harvester – Mass Production
To increase the conversion efficiency researchers primarily focus on improving the figure of merit (ZTMAT = S2T/ρκ) of the coupled TE materials, where S is the Seebeck coefficient, ρ is the electrical resistivity, κ is the thermal conductivity, and T is the absolute temperature. Studies on improving ZTMAT have mainly focused on the choice of bulk single-crystal materials, and their growth methods, such as the Bridgman and zone-melting, or powder-metallurgy methods, such as hot-pressing, or hot-extrusion. However, the conventional methods of fabricating these bulk TE materials are unfavorable for cost-effectiveness and large-scale mass production. Using screen printing technique for the fabrication of f-TEGs can improve the scalability of the devices to enhance their suitability for various applications. Moreover, this technique is also suitable for mass production. The TE material can be rapidly formed using a simple printing method, and there is no size limitation for the f-TEG or the slicing and dicing process of the bulk TE material. We have improved the mechanical & TE properties of screen-printed thermoelectric materials (sp-TEs) based on the low process cost and high productivity of screen-printing technique. The TE properties of the current sp-TEs have improved by 90% compared to commercial bulk TE materials applied to real TE devices.
Thermoelectric Energy Harvester – Application
When a thermoelectric device (TED) experiences temperature difference, it can function as an energy harvester by using Seebeck effect. Flexible thermoelectric generator (f-TEG) has advantage on energy harvesting efficiency comparing with other typical TEGs because most of the heat sources does not have flat surface but has curved one. One of the convenient application of f-TEG as energy harvester is Wireless Sensor Network system (WSN) in factories, which gains energy from heat pipes of the factories and uses the harvested energy for detecting gas leakage and any other possible danger in the factory.
When electrical current is applied to a TED, it can function as a temperature controller by using Peltier effect. Depending on the direction of current, TED can operate as a heater or a cooler; depending on the amount of current flow, the level of the temperature change can be controlled. TED can be applied in the virtual reality application, medical field, military, etc. Despite on its usefulness as temperature controller, TED is limited on detail temperature expression since it can only control the temperature of a dull plane. In order to solve this problem, a research of arranging multiple TEDs in two dimensional array is in progress.