Leland Weiss

Power Output Force Generation by a MEMS Phase Change Actuator

A microelectromechanical-systems-based phase change actuator has been developed and tested for high-speed mechanical power output and force generation. This actuator is well suited for a variety of advanced devices like tactile displays or micro fluidic systems. The device features two thin membranes that bound a cavity filled with working fluid. The working fluid boils at low temperature. Two sizes of actuator are tested, an actuator with membrane sidelengths of 5 mm and an actuator with top membrane sidelength of 10 mm. Two top membrane materials are explored consisting of 2 μm thick silicon and 300 nm thick silicon nitride. Heat addition is through the lower membrane which is fabricated with novel capillary structures designed to increase the efficiency of actuator operation. The actuator is shown to produce up to 2.6 mW of mechanical power output and generate an applied force of 43 mN. Operating speeds up to 100 Hz are demonstrated.

MEMS-Based Boiler Operation from Low Temperature Heat Transfer and Thermal Scavenging

Increasing world-wide energy use and growing population growth presents a critical need for enhanced energy efficiency and sustainability. One method to address this issue is via waste heat scavenging. In this approach, thermal energy that is normally expelled to the environment is transferred to a secondary device to produce useful power output. This paper investigates a novel MEMS-based boiler designed to operate as part of a small-scale energy scavenging system. For the first time, fabrication and operation of the boiler is presented. Boiler operation is based on capillary action that drives working fluid from surrounding reservoirs across a heated surface. Pressure is generated as working fluid transitions from liquid to vapor in an integrated steamdome. In a full system application, the steam can be made available to other MEMS-based devices to drive final power output. Capillary channels are formed from silicon substrates with 100 µm widths. Varying depths are studied that range from 57 to 170 µm. Operation of the boiler shows increasing flow-rates with increasing capillary channel depths. Maximum fluid mass transfer rates are 12.26 mg/s from 170 µm channels, an increase of 28% over 57 µm channel devices. Maximum pressures achieved during operation are 229 Pa.

Fabrication of a MEMS-Based Solar Thermal Collector for Energy Sustainability

A micro solar thermal collector has been fabricated by chemical electrodeposition technique. The coating is made of black nickel-tin selective absorber deposited on a copper substrate. The fabrication technique is presented. The electrodeposition was performed using processes that are less hazardous than many common approaches at a near-neutral pH. The procedure was also performed at room temperature. The environmentally friendly plating procedure makes it a potential replacement for black chromium. The final coating has a dendrite crystal structure of about 100 nm in diameter. The use of this selective coating significantly improved the ability of the fabricated solar collector to convert incident radiation to thermal energy. The performance of the collector plate in a simulated solar environment was examined. An increase in the stagnation temperature of the collector of 10 °C was observed when the selective coating was applied to the base collector plate.Copyright

Phase Change Insulation for Energy Efficiency Based on Wax-Halloysite Composites

Phase change materials (PCMs) have gained extensive attention in thermal energy storage. Wax can be used as a PCM in solar storage but it has low thermal conductivity. Introducing 10% halloysite admixed into wax yields a novel composite (wax-halloysite) which has a thermal conductivity of 0.5 W/mK. To increase the base conductivity, graphite and carbon nanotubes were added into the PCM composite improving its thermal energy storage. Thermal conductivity of wax-halloysite-graphite (45/45/10%) composite showed increased conductivity of 1.4 W/mK (3 times higher than the base wax-halloysite composite). Wax- halloysite-graphite-carbon nanotubes (45/45/5/5%) composite showed conductivity of 0.85 W/mK while maintaining the original shape perfectly until 91 °C (above the original wax melting point). Thermal conductivity can be further increased with higher doping of carbon nanotubes. This new composites are promising heat storage material due to good thermal stability, high thermal/electricity conductivity and ability to preserve its shape during phase transitions.

Fabrication, Testing, and Enhancement of a Thermal Energy Storage Device Utilizing Phase Change Materials

There is a growing need to develop technology to harness previously untapped sources of energy and waste heat is one source of energy that provides significant potential to increase the efficiency of overall energy use in multiple applications. Waste heat as a source of energy supply has relatively low availability and a storage system is required to efficiently utilize the energy. One method for harvesting this waste heat is through the use of phase change material (PCM) as a thermal energy storage (TES) medium.This work examines the operation of a thermal energy storage device that collects and stores heat in a PCM. The TES relies on the phase change of icosane wax, which possesses a large latent heat of fusion and high thermal storage capacity. However, the icosane wax typically has low thermal conductivity. This work focuses specifically on enhancing the thermal conductivity of the sink by incorporating an additive possessing high thermal conductivity without significantly reducing the storage capacity (volume) of the TES. The performances of both the TES as well as TES with enhanced conductivity are monitored to validate operation.Three different major experiment sets were performed; one contained only icosane wax as the PCM, a second incorporated a copper foam mesh along with the Icosane to increase the thermal conductivity of the working fluid. Finally, the third comprised of a novel copper matrix with increased surface area and thus, better conductivity for the PCM. The power absorbed by the PCM and the thermal storage potential for each of these tests was also studied in these experiments.The novel copper matrix showed good promise in greatly increasing the thermal conductivity of the system from 0.49 W/mK in the icosane-only test to 3.90 W/mK. A 200% increase in the power absorbed by the device was also achieved with the copper matrix conductivity enhancer. Results indicate an improvement of average thermal conductivity by a factor of 8 due to this work. A steady state power absorbed of 0.9 kW/m2 was achieved for the preliminary test of unmodified icosane. A similar steady state value of 0.96 kW/m2 was achieved with the copper mesh enhancer. The maximum power absorbed was achieved with the novel copper matrix at 2.76 kW/m2.Copyright

Investigation of Microboiler for Discarded Thermal Scavenging

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.© 2013 ASME

Small-Scale Flat Plate Collectors for Solar Thermal Scavenging in Low Conductivity Environments

There is great opportunity to develop power supplies for autonomous application on the small scale. For example, remote environmental sensors may be powered through the harvesting of ambient thermal energy and heating of a thermoelectric generator. This work investigates a small-scale (centimeters) solar thermal collector designed for this application. The absorber is coated with a unique selective coating and then studied in a low pressure environment to increase performance. A numerical model that is used to predict the performance of the collector plate is developed. This is validated based on benchtop testing of a fabricated collector plate in a low-pressure enclosure. Model results indicate that simulated solar input of about 800 W/m2 results in a collector plate temperature of 298 K in ambient conditions and up to 388 K in vacuum. The model also predicts the various losses in W/m2 K from the plate to the surroundings. Plate temperature is validated through the experimental work showing that the model is useful to the future design of these small-scale solar thermal energy collectors.

Broadband magnetic levitation-based nonlinear energy harvester

In this work, development of a broadband nonlinear electromagnetic energy harvester is described. The energy harvester consists of a casing housing stationary magnets, a levitated magnet, oblique mechanical springs, and a coil. Magnetic and oblique springs introduce nonlinear behavior into the energy harvester. A mathematical model of the proposed device is developed and validated. The results show good agreement between model and experiment. The significance of adding oblique mechanical springs to the energy harvester design is investigated using the model simulation. The results from the model suggest that adding oblique springs to the energy harvester will improve the performance and increase the frequency bandwidth and amplitude response of the energy harvester.

A COPPER MICROCHANNEL HEAT EXCHANGER FOR MEMS-BASED WASTE HEAT THERMAL SCAVENGING

The growing necessity for increased efficiency and sustainability in energy systems such as MEMS devices has driven research in waste heat scavenging. This approach uses thermal energy, which is typically rejected to the surrounding environment, transferred to a secondary device to produce useful power output. This paper investigates a MEMS-based micro-channel heat exchanger (MHE) designed to operate as part of a micro-scale thermal energy scavenging system. Fabrication and operation of the MHE is presented. MHE operation relies on capillary action which drives working fluid from surrounding reservoirs via micro-channels above a heated surface. Energy absorption by the MHE is increased through the use of a working fluid which undergoes phase change as a result of thermal input.In a real-world implementation, the efficiency at which the MHE operates contributes to the thermal efficiency of connected small-scale devices, such as those powered by thermoelectrics which require continual heat transfer. This full system can then more efficiently power MEMS-based sensors or other devices in diverse applications. In this work, the MHE and micro-channels are fabricated entirely of copper with 300μm width channels. Copper electro-deposition onto a copper substrate provides enhanced thermal conductivity when compared to other materials such as silicon or aluminum. The deposition process also increases the surface area of the channels due to porosity. Fabrication with copper produces a robust device, which is not limited to environments where fragility is a concern.The MHE operation has been designed for widespread use in varied environments. The exchanger working fluid is also non-specific, allowing for fluid flexibility for a range of temperatures, depending on the thermal source potential. In these tests, the exchanger shows approximately 8.7 kW/m2 of thermal absorption and 7.6 kW/m2 of thermal transfer for a dry MHE while the wetted MHE had an energy throughput of 8.3 kW/m2. The temperature gradient maintained across the MHE bottom plate and lid is approximately 30 °C for both the dry and wetted MHE tests though overall temperatures were lower for the wetted MHE.Copyright

Copper Plated Microchannel Heat Exchanger for MEMS Application

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.Copyright