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Experimental Study of the Conversion of Heat to Electricity Using Movement of a Magnet Inside an Oscillating Heat Pipe

Moradi, Sepehr | 2023

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 56403 (08)
  4. University: Sharif University of Technology
  5. Department: Mechanical Engineering
  6. Advisor(s): Shafii, Mohammad Behshad
  7. Abstract:
  8. Heat sources with temperatures less than 100 °C are available in various processes in the form of waste heat and from renewable sources such as solar energy. One of the methods of gaining benefit from these ubiquitous and abundant heat sources is using thermal harvesters that convert low-temperature heat into electrical energy without the need for the power grid. Portable harvesters can be a reliable and low-cost option for providing stable energy for low-power electronic devices such as wireless sensors. Many studies have been conducted at the global level to design and develop efficient low-temperature heat harvesting mechanisms. However, each of them is associated with fundamental weaknesses and shortcomings. Heat pipes have been known as effective heat transfer devices and widely used in this field. Furthermore, from a thermodynamic point of view, a less studied feature of heat pipes is their ability to convert part of the transfer heat flux between two points with a small temperature difference into kinetic energy in the form of a two-phase fluid flow inside the pipe. In this research, we introduce a heat pipe-based energy harvester with a portable size. In this device, the circulating-pulsating flow formed inside a single-turn oscillating heat pipe with an asymmetric structure is used to convert the heat input into mechanical work and then into electrical energy. A glass capillary tube with a 5 mm internal diameter shaped as a closed square loop forms the main body of this harvester. Inside the tube, there is a spherical neodymium magnet with low clearance. The magnet circulates with the fluid current and induces an electric potential difference by passing through the center of the two solenoids wrapped on the tube. After observing the internal two-phase flow pattern for three fluids, pure water, ethanol, and acetone, water was selected as the main working fluid. Afterward, the formation of the steady circulating-oscillating flow was observed and reported. Then, the average magnet circulation frequency, equivalent thermal resistance, and average voltage produced in each solenoid were studied for two different condenser placements, various filling ratios, and several heat fluxes. Based on the quantitative data, the condenser placement and the filling ratio that gave the best performance in terms of heat conduction and power generation were identified. Then, the maximum average electrical power produced and the heat-to-electricity conversion efficiency were measured for the identified optimal working condition. According to the results of the experiments, the fabricated device worked effectively with the temperature difference of the hot and cold parts less than 25 °C and a small transfer flux of 20 W. Placing the condenser at the highest level and filling 50 % of the tube volume with liquid water and the rest 50 % with water vapor brought the best operating results. In these optimal conditions, with an input heat rate of 40 W, the magnet circulated through the tube at an average speed of 27 cm/s, and the peak-to-peak open-circuit voltages of 0.52 V and 0.49 V, were induced in the solenoids on average. Moreover, the equivalent thermal resistance of the harvester decreased to 0.14 K/W, which is of the order of the thermal resistance of oscillating heat pipes. The total average power output of the coils in the optimal working conditions reached 5.0 µW, which means a thermal-to-electrical conversion efficiency of 1.3×10-5. The efficiency of the constructed harvester is more than one order of magnitude higher than the previous similar research. Therefore, the fabricated device has the capability of efficient heat transfer and simultaneous low-power electricity production
  9. Keywords:
  10. Two Phase Flow ; Electromagnetic Induction ; Energy Harvesting ; Pulsating Heat Pipe ; Low-Temperature Heat Recovery ; Heat Engine

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