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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.

In this paper, the shear strength of small-area welded contacts was optimized. Using design of experiments (DoE) optimization techniques, copper power contacts with 4 mm2 feet were welded to both copper and nickel/gold-plated copper substrates where pressure, amplitude and deformation were the control factors. For the DoE, the setting combinations were chosen based on the concept of classical Screening Design. This design was best suited because it allowed an elementary exploration of interaction relationships for a wide range of factors. The pressure was varied from 1.4 to 1.8 bar, the amplitude from 90 to 100%, and the deformation from 0.06 to 0.1 mm. These limits were chosen based on prior welding experience with the equipment and similar parts. Each bond was optically inspected to confirm high-quality bonds. Using common shear techniques, over 200 bonds were sheared; strengths were recorded as the DoE dependent variable. The initial step found that pressure was the most significant factor. Pressure and deformation were also proven to affect strength inversely to each other. Interestingly, the planarity of the contacts had no measurable effect on the welding strength. Consistent bond strengths were achieved regardless of the bonds' place within the established bond order. For both copper-to-copper and copper-to-nickel/gold-plated bonds, the optimal settings were: pressure at 1.8 bar, amplitude at 90%, and deformation of 0.1 mm, while achieving shear strengths of 40.5 kgf and 39.1 kgf, respectively. This doubled the previous benchmark weld strengths.

This article investigates the use of advanced, high porosity thermally conductive foams and a thermal energy storage (TES) device for small scale thermal energy harvesting. In final application, it may be employed in various real world situations that include existing systems like thermoelectric generators (TEGs) and thermal scavenging systems that provide power output from freely available thermal sources. Experimental tests were conducted using various porosity metallic copper foams ranging from 85 % to 89 % porosity. Copper foams were selected to serve as the heat exchanger innards and examined for several key attributes. These included the ability of the foams to yield capillary action with working fluids like water or 3M™ HFE7200. Thermal energy absorption by the exchanger to the working fluid was also monitored. These results were compared to other exchangers based on capillary channel fabrication techniques as previously reported by the research team. Full characterization was based on operating temperature, measured thermal input, mass transfer rate, and heat transfer capability. Preliminary investigation of a matching, small-scale TES unit designed to integrate with the heat exchanger and a future thermoelectric for energy harvesting application was also conducted. Thermal storage was accomplished via solid-liquid phase change of a paraffin wax within the TES device forming a so-called “thermal battery.” In a final design, the TES includes what is defined by thermodynamics as heat pipes. The integrations of several heat pipes, made of copper tubing and filled with working fluid, mounted vertically and immersed in the wax medium will transfer heat to the wax by means of thermal conductivity and phase transition. This represents a first of its kind in this small-scale, thermal harvesting application. The specific tests performed in this initial work included one TES unit filled with a paraffin wax medium and a second that contained several copper vertically placed tubes surrounded by the paraffin wax. The overall thermal conductivity of the phase change medium (wax) was investigated for both constructions as was the ability of each to absorb thermal energy directly. Results indicated capillary action of the working fluid was possible via incorporation of copper foams within the heat exchanger. Maximum heat flux observed in exchanger tests was 0.27 kW/m2 given an operating temperature of 76.6 °C and 2.5W thermal input. Thermal storage tests indicated a maximum thermal capture rate of 0.91 W and phase change material thermal conductivity of 1.00 W/mK for the TES device constructed with copper tubing innards. This compared favorably to the baseline wax conductivity of approximately 0.32 W/mK. Future efforts will fully incorporate both the heat exchanger and matching TES device for a complete harvesting and thermal capture system. The ability of the exchanger to provide thermal energy for storage to the “thermal battery” will be monitored.