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The investigation of a capillary-based heat exchanger is presented for potential integration with thermoelectric devices and thermal energy harvesting. The exchanger is a microfabricated device, designed to promote phase change of low boiling-point working fluids and enhance heat transfer from thermoelectric devices as a result. Several heat exchanger designs are studied and compared in these experiments. All designs rely on capillary channels to pump working fluid from surrounding reservoirs out across a heated exchanger surface. First, a baseline silicon device is fabricated using standard RIE techniques to produce silicon capillary channels. Channel widths of 100 μm are studied with varying heights. In addition to this base silicon device, SU-8 is used to further vary channel height. Heat exchangers are characterized based on operating temperature and heat input using 3M™ HFE 7200 working fluid. Maximum mass transfer rate recorded is 5.50 mg/s for Si channels operating above the boiling point of the working fluid. By contrast, SU-8 channels are shown to be more effective at temperatures below the boiling point. Maximum mass transfer rate is 2.29 mg/s for SU-8 based channels operating at these reduced temperatures.

Solar energy is a renewable and sustainable energy source that has a promising potential for the rapidly growing energy demands across the world. Large scale power generation from the energy of the sun is well established utilizing both direct thermal energy conversion and conversion to electricity via photovoltaic processes. Solar thermal systems have been limited to macro systems, even though they operate at higher efficiency compared to photovoltaic systems. Solar energy harvesting requires the use of collector plates to capture incident radiation. The surface area exposed to incident radiation is critical in solar thermal energy harvesting. In this work, we have integrated micro technology processes and solar thermal energy to design and fabricate a micro solar thermal system for power generation. This work specifically examined surface area enhancement using MEMS-based techniques to maximize solar thermal absorption. Selective absorber coating and enhanced surface areas due to the incorporation of micro structures on the collector substrates were utilized. In this manner, an important component to an autonomous micro power supply is investigated. Advanced microfabrication and electrochemical deposition techniques were employed to generate a selective absorber surface with enhanced surface area on a silicon substrate. Microchannels were used to enhance the surface area on the substrate. The selective absorber coating consists of a bimetallic structure consisting of tin and nickel.