Abstract
The installation and operation of enhanced geothermal systems (EGS) involves many challenges. These challenges include the high cost and high risk associated with the investment capital, potential large working-fluid leakage, corrosion of equipment, and subsiding land. A super-long heat pipe can be used for geothermal exploitation to avoid these problems. In this paper, a high aspect-ratio heat pipe (30 m long, 17 mm in inner diameter) is installed vertically. Experiments are then carried out to study its heat-transfer performance and characteristics using several filling ratios of deionized water, different heating powers, and various cooling-water flowrates. The results show that the optimal filling-ratio is about 40% of the volume of the vaporizing section of the heat pipe. Compared with a conventional short heat pipe, the extra-long heat pipe experiences significant thermal vibration. The oscillation frequency depends on the heating power and working-fluid filling ratio. With increasing cooling-water flow rate, the heat-transfer rate of the heat pipe increases before it reaches a plateau. In addition, we investigate the heat-transfer performance of the heat pipe for an extreme working-fluid filling ratio; the results indicate that the lower part of the heat pipe is filled with vapor, which reduces the heat-transfer to the top part. Based on the experimental data, guidelines for designing a heat pipe that can be really used for the exploitation of earth-deep geothermal energy are analyzed.
Highlights
A heat pipe is a highly-efficient heat-transfer device that uses liquid-to-vapor phase change and the associated fluid flow to transfer heat from a hot to a cold region
Four experimental procedures were carried out to study the heat-transfer performance and characteristics of the super-long heat pipe: (i) At a fixed heating-power and cooling-water flowrate, the heat pipe was tested with different working-fluid filling volumes to find the optimal fill-ratio. (ii) At a fixed heating-power and a certain workingfluid filling volume, the heat pipe was tested with different cooling-water flowrates. (iii) At extreme working-fluid filling volume and a certain cooling-water flow rate, the heat pipe was tested with different heating-powers. (iv) At a certain working-fluid filling volume and cooling-water flowrate, the heat pipe was tested with different heating powers to investigate the temperature oscillation characteristics
For a certain heating power, the optimal filling volume for the highest heat-transfer rate and lowest thermal resistance was determined to be about 40% of the volume of heated section
Summary
A heat pipe is a highly-efficient heat-transfer device that uses liquid-to-vapor phase change and the associated fluid flow to transfer heat from a hot to a cold region. In contrast with highly-conductive materials such as copper, heat pipes can be designed to move larger quantities of heat over longer distances, through narrower spaces, and using lower temperature differences [2]. The effective thermal conductivity can approach 100 kW/(m·K) for long heat pipes, compared to approximately 0.4 kW/(m·K). The heat pipe consists of a closed structure, which is filled with a working-fluid in two phases. Since the liquid and its vapor coexist in equilibrium, the pressure inside the container is equal to the vapor pressure that corresponds to the Sustainability 2021, 13, 12481.
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