Investigation of a MEMS-based capillary heat exchanger for thermal harvesting

This article describes theoretical and experimental studies of MEMS-based capillary heat exchangers, designed for use as the heat sink in a waste thermal energy harvesting system currently under development. Specific goals include the understanding of the impact of thermal conductivity and capillary dimensions. Experimental studies were done using silicon and SU-8 based heat exchangers each with rectangular capillaries having width of 100 μm. The depth of silicon capillaries was maintained at either 64 or 100 μm while that of SU-8 capillaries was varied from 48 to 134 μm to 160 to 375 μm. The overall footprint of each microdevice was 38 mm by 13 mm. The fluid used was 3M™ HFE 7200. Heat exchanger performance was characterized based on operating temperature, mass transfer rate, heat flux, and thermal resistance. Studies were done for two different operating temperatures, one below the boiling point and another at the boiling point of the working fluid. These two operating temperatures were referred to in this article as low-temperature and high-temperature operation mode. Based on the experimental study the fluid was found to wet both silicon and SU-8 with contact angle measured to be 6°. From the experimental studies conducted it was found that the pumping effect of the capillaries increased with increasing wall height. Based on the thermal tests it was observed that the highest mass transfer rate and heat flux under the low-temperature and high-temperature operation mode were obtained for an SU-8 heat exchanger with capillary height of 134 μm and silicon heat exchanger with capillary height of 100 μm, respectively. Based on this study it was found that under low-temperature operation mode the capillary dimension was more influential than thermal conductivity; however, under high-temperature operation mode the opposite was proved to be the case. The thermal resistances were found to vary from 0.011 to 0.09 m2CW and from 0.0073 to 0.0096 m2CW for low-temperature and high-temperature operation mode respectively. Also it was found based on comparison that the conceptualized MEMS-based capillary heat exchanger was found to have lower thermal resistance than existing heat sinks. Using the experimentally validated theoretical model developed in this work it was found that the performance of the MEMS-based capillary heat exchangers improved with increasing thermal conductivity of the substrate as well as the capillary dimensions. The model has helped conclude that developing SU-8 capillaries on aluminum or copper substrates would optimize the influence of thermal conductivity and capillary dimensions on the performance of the MEMS-based heat exchanger.