Waste Heat Use Through Microfabrication and Micro System Development

To effectively harvest waste heat from larger devices, a MEMS-based boiler is fabricated to boil working fluids for use in a low temperature steam system. The boiler is designed and fabricated to collect waste heat and drive working fluid phase change through novel microstructures. This boiled working fluid can then be made available for expansion by other MEMS-based components like piezoelectric membranes or cantilevers. Two different boiler designs are studied and compared in these experiments. Both designs rely on capillary channels to pump working fluid from surrounding reservoirs out across heated boiling surfaces. First, a baseline silicon device is fabricated using standard RIE techniques to produce silicon capillary channels. Channel widths of 300 and 100 μm are studied with maximum aspect ratios of 1:1. Improved aspect ratio capillary channels are investigated through the use of SU-8 polymer structures. The maximum aspect ratios of the SU-8 based channels are 20:1 with channel widths down to 10 μm. SU-8 based boilers deliver improved performance compared to their silicon counterparts. The maximum mass transfer rate was 4.49 mg/s for SU-8 channels with 20:1 aspect ratios. By contrast, the maximum mass transfer rate was 3.18 mg/s for silicon capillary channels with 1:1 aspect ratios. Working fluids like 3M™ HFE 7200 are used in these experiments.