Low-grade heat to electricity conversion using the self-circulation of a magnet inside a single-turn pulsating heat pipe

Abstract

Low-temperature (≤100°C) heat sources are abundantly and ubiquitously available in nature, such as solar energy, and in the form of untapped waste heat. This work aims to harness low-grade heat sources by introducing a portable harvesting system capable of effectively converting small spatial temperature gradients to mechanical and then electrical energy while maintaining a relatively high heat transfer capability. The designed and studied pair of harvesters are based on single-turn pulsating heat pipes with different condenser locations. Asymmetrical heating and cooling of the looped capillary glass tube gives rise to the bulk circulation of the internal fluid with occasional oscillation. The thermally-driven two-phase flow carries a spherical neodymium magnet fitted inside the tube. Two external solenoids generate a small amount of electricity every time the magnet passes through them. The proper functioning of energy-harvesting pulsating heat pipes (EH-PHPs) was investigated for pure water, acetone, and ethanol as the working fluids. Only water demonstrated satisfactory results and was thus selected as the main working fluid. Subsequently, high-frame-rate imaging of the internal flow along with time-domain temperature and open-circuit voltage data were employed and discussed to study the working principles of the harvester. In the next step, the mechanical, thermal, and electrical performance of the two EH-PHPs were quantitatively investigated for five different filling ratios (FRs) between 20% to 80% and five levels of heat input from 20.0W to 40.0W for each FR. Among the studied conditions, the EH-PHP with horizontal condenser (HC) configuration and 50% FR demonstrated the best performance in all three aspects. Accordingly, when 40.0W of heat was supplied to this ideal case, the magnet mean circulation frequency around the loop, effective thermal conductivity, and average induced open-circuit voltage (peak-to-peak) reached their maximum values of 0.46Hz, 11.5kW/(mK), and 0.53V, respectively. Although the integration of electromagnetic generators had reduced the heat transfer performance, the thermal conductivity of the EH-PHPs was comparable to that of conventional heat pipes and at least 25 times better than copper. Finally, the EH-PHP with HC configuration and 50% FR, the superior case, produced up to 5μW of electrical power while its solenoids were connected to load resistances with an optimum impedance. Therefore, even in its early stage of development, the portable EH-PHP can be used to supply low-power electronics such as a quartz watch. The generated electricity corresponds to about 0.01% relative Carnot efficiency. The remarkable thermal conductivity and capability of electricity production with less than 5°C temperature difference across the tube distinguish the EH-PHPs from mature energy harvesting technologies with higher energy conversion efficiencies.