At medium-deep and deep parts of the Earth's crust lay vast and so far mostly untapped reserves of geothermal energy. This energy source has the potential to meet humanity's needs for energy for thousands of years. However, the large-scale deployment has been hindered by a lack of effective and dependable exploitation technologies. Gravity heat pipes have recently garnered significant attention due to their exceptional heat transfer capability and their ability to harness geothermal heat without water-mining. Nevertheless, it is a persistent challenge to extend gravity heat pipes to depths that are sufficient to exploit deep geothermal energy. In this study, we introduce an innovative ladder-structure inner pipe design to overcome this shortcoming. This design incorporates a series of stepped ladders and small condensate guiding pipes, allowing for the development of super-long gravity heat pipes (SLGHPs). Combined with an optimal working fluid, this novel structure enables kilometers-long gravity heat pipes. As a direct outcome of this innovation, a 4149-meter-long SLGHP was constructed and installed in a geothermal well using ammonia as the working fluid. The SLGHP geothermal system demonstrated the ability of continuous heat output exceeding 1 MW, with a heat flux across the radial section of the SLGHP reaching 4 × 10⁷ W m⁻². Furthermore, an ammonia vapor-driven power generator was developed and integrated with the SLGHP system. During a 72-hour test, this generator successfully produced electricity at a steady rate of approximately 7 kW. The breakthrough design of the SLGHPs holds the potential to unlock Earth's deep geothermal energy reserves, providing a sustainable and reliable energy source for generations to come.

It is important for the safety and good performance of a Li-ion battery module/pack to have an efficient thermal management system. In this paper, a battery thermal management system with a two-phase refrigerant circulated by a pump was developed. A battery module consisting of 240 18650-type Li-ion batteries was fabricated based on a finned-tube heat-exchanger structure. This structural design offers the potential to reduce the weight of the battery thermal management system. The cooling performance of the battery module was experimentally studied under different charge/discharge C-rates and with different refrigerant circulation pump operation frequencies. The results demonstrated the effectiveness of the cooling system. It was found that the refrigerant-based battery thermal management system could maintain the battery module maximum temperature under 38 °C and the temperature non-uniformity within 2.5 °C for the various operation conditions considered. The experimental results with 0.5 C charging and a US06 drive cycle showed that the thermal management system could reduce the maximum temperature difference in the battery module from an initial value of 4.5 °C to 2.6 °C, and from the initial 1.3 °C to 1.1 °C, respectively. In addition, the variable pump frequency mode was found to be effective at controlling the battery module, functioning at a desirable constant temperature and at the same time minimizing the pump work consumption.

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.

The ultra-long gravity heat pipe has a long heat transfer distance and narrow working fluid flow channel within its tube. Due to these unique design features, the vapor–liquid counter-flow and heat transfer characteristics of these heat pipes are more complex than those found in conventional-size heat pipes. This paper innovatively proposes the design of a segmented visualization window structure of an ultra-long gravity heat pipe, which successfully overcomes the challenge of visualizing the internal flow during operations. A visualization experimental platform, measuring 40 m in height with an inner diameter of 7 mm and the aspect ratio up to 5714, was built to investigate the evolving characteristics of two-phase flows with an increasing heat input and the impact of the phase change flow characteristics on the thermal performance of ultra-long gravity heat pipes. The results obtained can provide guidance for the development of the internal structure of ultra-long gravity heat pipes that are being applied in exploiting geothermal energy. The results show that, at low heat input (200 W, 250 W), there are separate flow paths between the condensate return and the steam, but the high hydrostatic pressure due to the height of the liquid injection results in the presence of an unsaturated working fluid with a higher temperature in the liquid pool area, which has a lower evaporation rate, limiting the heat transfer through the heat pipe. It is found that if increasing the heat input up to 300 W, the evaporative phase change in the heating section becomes intense and stable. At the same time, despite the intermittent formation of liquid columns in the adiabatic section due to the vapor–liquid rolls, which increases the resistance to the vapor–liquid counter-flow, the liquid columns are blocked for a short period of time, and the path of steam rises and the condensate return is smooth, which does not seriously affect the steam condensation and liquid return evaporation. At this point, the overall temperature of the heat pipe is evenly distributed along the tube and the heat transfer performance is optimal. When the heat input further increases (350 W, 400 W), a large amount of condensate is trapped in the upper part of the adiabatic section and the condensing section by long liquid plugs for a long time. At this point, the condensate flow back to the heating section is significantly reduced, and the steam is seriously prevented from entering the condensation section, resulting in a significant increase in the temperature gradient between the lower part of the evaporating section and the upper part of the adiabatic section and deterioration of the heat transfer performance.

Lithium-ion batteries (LIBs) are widely used as power sources for electric vehicles due to their various advantages, including high energy density and low self-discharge rate. However, the safety challenges associated with LIB thermal runaway (TR) still need to be addressed. In the present study, the effects of the battery SOC value and coolant flow rate on the TR behavior in a LIB pack are comprehensively investigated. The battery pack consists of 10 18650-type LIBs applied with the serpentine channel liquid-cooling thermal management system (TMS). The TR tests for various SOC values (50%, 75% and 100%) and coolant flow rates (0 L/h, 32 L/h, 64 L/h and 96 L/h) are analyzed. The retarding effect of the TMS on TR propagation is found to be correlated with both the coolant flow rate and the battery SOC value, and a larger coolant flow rate and lower SOC generally result in fewer TR batteries. Furthermore, the TR propagation rate, evaluated by the time interval of TR occurrence between the adjacent batteries, increases with the battery SOC. The battery pack with 100% SOC shows more rapid TR propagation, which can be completed in just a few seconds, in contrast to several minutes for 50% and 75% SOC cases. In addition, the impact of the battery SOC and coolant flow rate on the maximum temperature of the TR battery is also examined, and no determined association is observed between them. However, it is found that the upstream batteries (closer to the external heater) show a slightly higher maximum temperature than the downstream ones, indicating a weak association between the TR battery maximum temperature and the external heating duration or the battery temperature at which the TR starts to take place.