This paper presents detailed fabrication and operational characterization of a MEMS based microboiler designed to scavenge waste thermal energy from sources like transportation or industry. Microboiler operation is based on capillary action that drives the working fluid from surrounding reservoirs to the surface that is heated by the waste thermal energy. As a result of phase change from liquid to vapor, pressure is created inside the enclosed central steamdome. This pressurized vapor can be made available to another MEMS device (PZT membranes, thermoelectric, etc.) to produce useful power output. In contrast to previous work, the new miniature microboiler design has undergone several modifications that improve operating efficiency. Capillary channels that were designed in linear fashion have been upgraded to a radial layout. This modification facilitated boiler enhancements in capillary flow, operating pressure, and rate of mass transfer. Capillary channels are formed from the silicon substrate with 50 μm widths and 100 μm depths. Power inputs of 2W, 3W and 4W are utilized to characterize performance. Maximum energy absorption via phase change of working fluid was 1.6 mW given a source temperature of 128 °C. The maximum steady state operating pressure achieved during testing was 3.65 kPa.

Thermal energy is a leading topic of discussion in energy conservation and environmental fields. Specifically for large-scale applications solar energy and concentrated solar power (CSP) systems use techniques that include thermal energy storage systems and phase change materials to harvest energy. However, on the smaller centimeter scale, there have been historically fewer investigations of these same techniques. The main goal of this paper is to investigate thermal energy storage (TES) as applied to a small scale system for thermal energy capture. Typical large-scale TES consists of a phase change material that usually employs a wax or oil medium held within a conductive container. The system stores the energy when the wax medium undergoes a phase change. In typical applications like buildings, the system absorbs and stores incoming thermal energy during the day, and releases it back to the surrounding environment as temperatures cool at night. This paper presents a new TES unit designed to integrate with a thermoelectric for energy harvesting application in small, cm-scale applications. In this manner, the TES serves as a thermal battery and source for the thermoelectric, even when originating power supply is interrupted. A unique feature of this TES is the inclusion of internal heat pipes. These heat pipes are fabricated from copper tubing and filled with working fluid, mounted vertically, and immersed in the wax medium of the TES. This transfers heat to the wax by means of thermal conductivity enhancement as an element of the heat pipe operation. This represents a first of its kind in this small-scale, thermal harvesting application. As tested, the TES rests atop a low temperature (60 °C) heat source with a heat sink as the final setup component. The heat sink serves to simulate thermal energy rejection to a future thermoelectric device. To measure the temperature change of the device, thermocouples are placed on either side of the TES, and a third placed on the heat source to ensure that the energy input is appropriate and constant. Heat flux sensors (HFS) are placed between the heat source and the TES and between the TES and heat sink to monitor heat transferred to and from the device. The TES is tested in a variety constructions as part of this effort. Basic design of the storage volume as well as fluid fill levels within the heat pipes are considered. Varying thermal energy inputs are also studied. Temperature and heat flux data are compared to show the thermal absorption capability and operating average thermal conductivities of the TES units. The baseline average thermal conductivity of the TES is approximately 0.5 W/mK. This represents the TES with wax alone filling the internal volume. Results indicate a fully functional, heat pipe TES capable of 8.23 W/mK.

Demand for increased density circuit architecture, micro- and nano-scale devices, and the overall down-scaling of system components has driven research into understanding transport phenomena at reduced scales. One method to enhance transport processes is the utilization of mini-, micro-, or nano-channels which drive uniform temperature and velocity profiles throughout the system. This work specifically examines a unique heat exchanger. The exchanger is developed as a closed system, with 300μm width channels, fabricated entirely with copper. The heat exchanger has been designed for widespread use in varied environments. Further, the exchanger is working fluid non-specific, allowing for different fluids to be specified for various temperature ranges. The system design can be used equally well as a standalone heat exchanger or coupled with another device to provide a thermal energy storage system. Fabricating the heat exchanger with copper for the substrate as well as the channels themselves allows the exchanger to maintain a high thermal conductivity which aides in the fluid energy transference. The exchanger was fabricated to be a closed system removing any excess equipment such as pumps. In testing, the exchanger showed thermal absorption of 2.2kW/m2 given input of 2.63kW/m2 and working fluid amounts of 37μL. The general design and use of copper in the exchanger allowed maximum absorption of 84% of the input with operation below the boiling point of the working fluid.

Hydrogen sulfide (H2S) is rapidly emerging as a biologically significant signaling molecule. In recent studies, sulfide level in blood or plasma has been reported to be in the concentration between 10 and 300 μM suggesting it acts in various diseases. This work reports progress on a new Lab-on-a-Chip (LOC) device for these applications. The uniquely designed, hand-held device uses advanced liberation chemistry that releases H2S from liquid sample and an electrochemical approach to detect sulfide concentration from the aqueous solution. The device itself consists of three distinct layers of Polydimethylsiloxane (PDMS) structures and a three electrode system for direct and rapid H2S concentration measurement. In this work specifically, the oxidation of sulfide at the gold (Au) and platinum (Pt.) electrodes has been examined. This is the first known application of electrochemical H2S sensing in an LOC application. The analytical utility and performance of the device has been assessed through direct detection using chronoamperometry (CA) scan and cyclic voltammetry (CV). An electrocatalytic sulfide oxidation signal has been recorded for sulfide concentration range vs, Ag/AgCl at different pH buffers at the trapping chamber. The calibration curve in the range 1 μM to 1 M was obtained using this electrode setup. The detection limit was found to be 0.1 μM. This device shows promise for providing fast and inexpensive determination of H2S concentration in aqueous samples.

Hydrogen sulfide (H2S) detection is an important capability for applications that range from environmental to biomedical use. In medical application, hydrogen sulfide may be an effective marker for various cardiovascular diseases. This work reports progress on H2S detection using a unique lab-on-a-chip device designed specifically for both environmental and biomedical applications. The chip consisted of three distinct layers of PEO/PDMS structures which have been bonded using various techniques including Reactive Ion Etching (RIE). First layer consisted of capillary channels to organize the flow of the sample. Also, liberation of the sulfide took place at this layer. The second layer was a H2S selective membrane. The third layer consisted of trapping chamber where trapped H2S samples were withdrawn for the quantification of H2S concentration. Fabrication of the first layer was accomplished using photolithography technique. Specifically, the chip incorporated unique design features and operation with advanced liberating chemistry that effectively released H2S from aqueous solutions introduced to the device. Mixture of poly-dimethylsiloxane-ethylene oxide polymeric (PDMS-b-PEO) and polydimethylsiloxane (PDMS) was cast on Su8 mold which produced super hydrophilic channels that allowed liquid flow via capillary action. The chip has been both fabricated and characterized as reported in this work. For each sample, 150 μL of the reaction volume was loaded in an HPLC vial and analyzed by a Shimadzu Prominence HPLC equipped with fluorescence detection and an eclipse XDB-C18 column. Sulfide transfer increased steadily at a rate of approximately 2% per minute until peaking at approximately 60% at 30 minutes. Percent transfer data show that sulfide diffused into the trapping chamber in a reproducible manner and that it was stable once it reached its peak at 30 minutes. Characterization and testing of the fully assembled device indicates significant promise and utility. Additional improvements may be made by optimizing parameters such as the decreasing ratio of the chamber volumes to the membrane area and the membrane thickness. The performance of this microfludic device was attributed to hydrophilic surface of PEO/PDMS, strong bonding of the chip using 3M transfer tape and well suited PDMS membrane that allow selective diffusion of hydrogen sulfide.