Two-phase Closed Thermosyphon Research Articles

Single-phase thermosyphon heat transport device, a future prospect for heat removal applications: An experimental comparison with heat pipe and two-phase closed thermosyphon

Over the years, the passive mode of heat transportation has gained immense popularity, which includes two-phase devices viz. Heat pipe (HP) and Two-phase closed thermosyphon (TPCT). But recently, the thermosyphon heat transport device (THTD) entered the pool, wherein the heat transfer fluid (HTF) in liquid phase transports heat by natural circulation (thermosyphon). Hence, to gauge their efficacy, in the present experimental investigation, the three devices (with the same geometry and HTF) are compared by imposing isothermal (40°C–90 °C) and isoflux (50–900 W) heating conditions.In isothermal mode, HP transports heat with a minimal temperature gradient compared to TPCT and THTD. The thermal resistance (TR) of TPCT and THTD is 5.4 to 10.1 times and 6.6 to 10.1 times respectively greater than HP. For isothermal temperatures below 80 °C, the TR of THTD is less than TPCT (viscous limit). Later the trend reverses as the TPCT outperforms, while THTD experiences HTF boiling. Further, the heat extracted by HP is 2.1 to 3.9 times and 3 to 3.5 times greater than TPCT and THTD respectively. Hence, its application is justified if cost is not a concern, while THTD (with high-temperature HTF) is suggested over TPCT on an economic basis. Even the isoflux mode showed a similar trend of TR. The system response (based on time constant) depicted both HP and TPCT have similar and nearly constant values over the heat load range, while in THTD, it decreases with an increase in heat load and approaches the other two. Hence, a high-temperature HTF would lead to competitive results. Further, concerning the heat transport distance, as the TR of THTD remained the same compared to the increment in TPCT, the application of the former is justified in long-distance heat transportation. Ultimately, HP is the front-runner, while TPCT and THTD perform on par depending on operating conditions and geometry.

Effect of evaporator curvature on the local non-equilibrium heat regulation in two-phase closed thermosyphon embankment in permafrost regions

The evaporator curvatures can affect the heat transfer performance of two-phase closed thermosyphons (TPCTs) in permafrost regions. To explore the working features and control performance of TPCTs with different evaporator curvatures, two types of TPCTs with horizontal and inclined evaporator were designed. Then, a series of scale embankment models were conducted to test the thermal and deformation performance of the two TPCTs, including a TPCT embankment with inclined evaporator, a TPCT embankment with horizontal evaporator, and a contrast embankment without any TPCTs. The results show that the thermal regulation performance of the TPCT with horizontal evaporator is much better than that with inclined evaporator. The thermal regime difference caused by the vapour-liquid interface interaction occurs in the whole pipe domain above the liquid pool for the TPCT with inclined evaporator while it mainly exists in the condenser domain for the TPCT with horizontal evaporator. Meanwhile, the TPCT with horizontal evaporator can decrease the temperature gradient, and thus weaken the frost heave of the embankment. The TPCT with horizontal evaporator is more appropriate to adjust the local non-equilibrium heat process in embankment. The findings of this study supply a scientific reference for the design of TPCTs embankment in permafrost regions.

Two-phase closed thermosyphons (TPCT) for geothermal energy extraction: A computationally efficient framework

Two-phase closed thermosyphons are efficient passive heat transfer devices that have received substantial attention in ground heat extraction. However, modelling thermal energy recovery using thermosyphons is often associated with a high computational cost due to prolonged operation. This study proposes an efficient computational framework for geothermal utilization using two-phase closed thermosyphons. A 1+1D reduced order transient heat conduction equation is numerically solved in the ground and is coupled with the thermosyphon and a coiled heat exchanger through a thermal network equation. Newton–Raphson method is used to solve the resulting nonlinear thermal resistance equation to determine the total heat transfer rate. The 1+1D algorithm is verified and validated against numerical and experimental studies on geothermal energy extraction and energy storage. The proposed framework facilitates the optimization of design and operational parameters for thermosyphons, providing a runtime range of 4–14 h for 20-year simulations of heat extraction. Parametric studies reveal the pivotal role of ground temperature in influencing the thermal performance of the thermosyphon, wherein doubling the evaporator length results in an increase in thermal power output by up to 45%. Various combinations of cooling flow rates and inlet water temperatures at the helical coil are tested to evaluate the system’s potential for electricity generation. Under different conditions, the thermosyphon exhibits the capability to generate thermal power, with a maximum observed production rate of 109.55 W/m for the base case in the analysis. Raising the inlet water temperature to 80 °C, in conjunction with maintaining a circulating coolant fluid flow rate in the helical coil below 10 lpm, qualifies thermosyphons installed at depths ranging from 150 to 250 metres in the ground for clean power generation. Overall, this study highlights the potential and importance of utilizing natural circulating systems with low environmental impact and high efficiency to mitigate the effects of climate change.

Investigating the effect of MXene/ethanol nanofluid in thermosyphon cooling of hybrid photovoltaic/thermal (PV/T) panels

Current research focuses on employing MXene/ethanol nanofluid (NF) as a working fluid in an array of two-phase closed thermosyphons (TPCT) for cooling the PV panel of hybrid solar photovoltaic/thermal (PV/T) system from the rear. Unique features such as high electrical conductivity, capacitive capacity, and hydrophilicity were the advantages of MXene nanoparticles. Moreover, comparisons between PV systems are provided in terms of performance and efficiency parameters. The results revealed that the optimum inclination angle for the panels was identified as 30°, and the optimum filling ratio was determined as 50 %. Applying NFs up to an optimal concentration of 1 % instead of the base fluid increased the temperature drop behind the panel, improved the panel output power, and enhanced overall efficiency. A temperature drop of 19.21 °C and an increase in electrical output of 1.62 W were observed behind the module at a 1 wt% MXene concentration. Therefore, the presented study discloses very high performance in PV/T systems.

Numerical investigation of surface wettability effect on two-phase closed thermosyphon

A two-phase closed thermosyphon (TPCT) is promising for heat removal due to its simple structure and high performance in heat transfer. However, investigating the phase change process in TPCT through experiments with current technology is still challenging. To study the various surface wettability effects on TPCT, this is the first systematical volume of fluid (VOF) method-based numerical study. The wettability in terms of contact angles (5°, 40°, 120°, and 175°) is endowed on the surfaces of TPCT. The numerical results of heat transfer coefficients of the TPCT show good agreement compared with the correlations. The simulation results reveal that the hydrophilic evaporator and the hydrophobic condenser benefit heat transfer. Within the input power of 376.14 W, the hydrophilic evaporator can reduce the thermal resistance of the evaporator to 35.3 % and delays the vapor film formation. The hydrophobic condenser decreases the thermal resistance of the condenser at 3.64 % and prevents filmwise condensation. The surface wettability is critical to the temperature distribution along the evaporator, but the main temperature drop is located between the adiabatic section and the condenser. It is also noted that the thermal resistance of the condenser is dominant to the overall thermal resistance. This study provides a detailed physical insight into the surface wettability effect on TPCT and benefits the future design in thermal engineering applications.

Seasonal ground cold energy storage potential for data center cooling using thermosyphon: A comparative study of five cities in Canada for carbon footprint reduction

Artificial Ground Freezing (AGF), using two-phase-closed-thermosyphon (TPCT) device, is an emerging technique for storing the cold energy of the winter season in the ground. The stored energy can later be used in the summer season for data center cooling, building comfort, etc. However, this energy storage potential has not been quantified so far. This study is conducted using an experimentally validated numerical model that simulates the ground freezing phenomenon in five Canadian cities over a 2-years period for 1%, 5%, 10% and 20% soil porosity. Each city offers a different energy storage potential based on ambient conditions and soil porosity — ranging from 3.31 – 23.19 MWh per TPCT, annually. In addition, up to 4,851 kg of carbon footprint can be reduced per TPCT in each city. This study examines and ranks the cities based on the potential for cold energy storage and associated carbon footprint for new and commissioned (already operating) cooling-intensive facilities. Yellowknife is ranked number one for commissioned cooling-intensive facilities and Montreal is ranked number one for new cooling-intensive facilities. It also provides a pre-feasibility guideline for selecting the geographic location to implement the novel idea in cities with similar weather conditions. A new parameter, carbon emission efficiency, is also introduced and used for ranking.

Numerical study of Two-Phase flow behavior during dryout in a Two-Phase closed thermosyphon under thermal boundary conditions

This paper presents a two-dimensional axisymmetric two-phase closed thermosyphon (TPCT) model filled with R-134a refrigerant using the Volume of Fluid (VOF) method, Lee model, and Continuum Surface Force (CSF) method within Computational Fluid Dynamics (CFD) and simulates the flow characteristics inside the TPCT during the occurrence of dry out. Specifically, it discusses the two-phase flow in the condensation and evaporation sections when dry out occurs. The results indicate that the condensation section exhibits varying forms of vapor–liquid interface waves at different times. Phenomena such as liquid carryover and entrainment become increasingly prominent over time, contributing significantly to refrigerant blockages and dryout in the evaporation section. The mechanism behind dryout in the evaporation section is revealed to be the result of the initially continuous falling film being disrupted by fluctuations in the liquid–vapor interface, leading to the premature drying of thin liquid films in the troughs of these waves.

Experimental research on the cooling effect of a novel two-phase closed thermosyphon with semiconductor refrigeration in permafrost regions

Hybrid two-phase closed thermosyphon (TPCT) artificial ground freezing technique is an effective cooling strategy adaptable to different geological and meteorological conditions in permafrost regions. Previous studies have focused on the feasibility of hybrid TPCT with little attention given to the cooling characteristics and long-term effects of this technique under actual environmental conditions. In this study, a novel structured semiconductor refrigeration device, with simple structure, small size, and wide operating temperature range, was designed and manufactured to act as the active condenser of a hybrid TPCT, and the cooling effect of this device was investigated during a two-year field test under stable power supply. This study showed that: 1) The minimum (maximum) average temperature of the semiconductor refrigeration two-phase closed thermosyphon (SRTPCT) evaporator was 0.5 (2.1) °C lower than the TPCT evaporator. The average ground temperature decreased by 0.3 °C, and the permafrost table rose to 0.2 m at 1 m from the evaporators around SRTPCT. The semiconductor refrigeration device added more than 9.75 MJ of cold storage to the soil around the TPCT evaporator during an operating cycle. 2) The optimal working period for the semiconductor refrigeration device was from June to October, and the cooling effect was better when the ambient temperature was low in the warm season. 3) As active condenser of hybrid TPCT in permafrost regions, semiconductor refrigeration device has a larger heat transfer power enhancement for TPCTs than vapor compression refrigeration device in the warm season, and vice versa in the cold season, as well as a higher heat transfer power enhancement for TPCTs than adsorption refrigeration device. The results of this study provide relevant parameters and references for the design and engineering application of future SRTPCT.

Climate warming is likely to weaken the performance of two-phase closed thermosyphon on the Qinghai–Tibet Plateau

Over the years, numerous geotechnical approaches have been implemented to mitigate the adverse effects of climate warming on various infrastructures in the permafrost region of the Qinghai–Tibet Plateau (QTP), such as the Qinghai–Tibet Highway and Railway, and achieved the expected engineering outcomes. However, little attention has been given to whether the performance of these geotechnical approaches has changed during the ongoing process of climate warming. To investigate the performance variation of one of these geotechnical approaches, which is two-phase closed thermosyphon (TPCT), during sustained climate warming, we conducted a statistical analysis of soil temperature monitoring data in 2003–2020 from eight regular embankments and six TPCT embankments in our permafrost monitoring network. The results indicate that TPCT undeniably has a cooling effect on the permafrost beneath embankments, even rapidly eliminated previously formed taliks beneath embankment. However, further analysis reveals that the performance of TPCT has been weakening during sustained climate warming, which has confirmed by the re-forming of the taliks beneath embankment where they had been previously eliminated. Based on the current understanding, we attributed the weakening of thermosyphon performance to a significant reduction in the air temperature freezing index caused by ongoing climate warming. Through this study, we aimed to draw attention to the evolving performance of geotechnical approaches in permafrost regions amid climate warming, prompting necessary engineering innovations to address this situation and ensure the sustainable development of the permafrost region on the QTP.

Experimental study on solar heating system based on heat pipe technology for mitigating subgrade frost heave

Subgrade frost heave is the main reason that affects the safe operation and comfort of trains during the cold season. Solar heating system based on heat pipe technology is a promising method to mitigate the frost heave by raising the subgrade temperature. In this paper, a field experiment was conducted to evaluate the heating performance of the solar heating system based on heat pipe technology in the cold season. In addition, the heat collection performance and stability of the system are compared with other solar heating systems. Finally, its applicability is discussed. Experimental results revealed that the solar heating system based on heat pipe technology can effectively raising the soil temperature and mitigate the frost heave of subgrade. During the experiment period, the soil temperature at the depth of −0.5 m in the experiment site was 5 °C higher than that of the comparison site, and the maximum freezing depth was 0.4 m, which was 56 % lower than that of the comparison site. The mean daily temperature of heat collection units (HCUs)fluctuated around 30 °C, and the mean daily temperature of the two-phase closed thermosyphons (TPCTs) fluctuated between 15 °C and 30 °C. The heat transfer efficiency of System 1 and System 2 is greater than 0.5, and the total effective heat transfer during the experiment period is 512 MJ and 283 MJ, respectively, and the heat transfer difference is about 2 times. Compared with other solar heating systems, the system proposed in this paper has good heat collection performance and more superior system stability. The mean daily heat transfer of System 1 and System 2 is 6.4 MJ and 3.71 MJ, respectively, which is 62.9 % higher and 5.6 % lower than the mean daily heat loss of subgrade of 3.93 MJ, respectively. The system has the application prospect of low maintenance cost and good long-term anti-frost heave effect.

Thermo-mechanical stability analysis of thermosyphons for pipeline soil and their optimal layout in permafrost regions

In permafrost regions, warm-oil buried pipelines release vast amounts of heat energy to surrounding soils and lead to rapid permafrost degradation. Settlement disasters induced by permafrost degradation significantly increase the probability of pipeline failure. The two-phase closed thermosyphon (TPCT) can cool soil, inhibiting permafrost degradation, but its deployment is still at the experimental stages along the China–Russia Crude Oil Pipeline (CRCOP). Therefore, it is important to determine an optimal layout for TPCTs that can achieve both effective cooling and cost control. In this study, a strategy for a nonlinear thermo-mechanical coupled model is proposed through Python–Abaqus secondary development. Based on it, the thermal and mechanical effects of TPCTs under different spacings, as well as their optimal layout, are numerically investigated. Closer TPCT spacing leads to a lower degradation rate of frozen soil and a lower peak TPCT heat flux. In addition, the peak pipeline von Mises stress in the transition zone increases for greater TPCT axial distances over time. When the TPCT axial distance is ≤8 m, the minimum pipeline principal strains remain below the critical compression strain over 50 years. Finally, a backpropagation neural network is constructed to predict the maximum pipeline compressive strain, and most prediction results fall within the 95% confidence interval.

Influence of cellulose nanofiber fluid on flow instability and heat transfer of two-phase closed thermosyphon

In this research, the geyser phenomenon occurring in a small-diameter two-phase closed thermosyphon (TPCT) was observed and the instability of the device was discussed. Geyser phenomena interfere with the natural circulation of internal working fluids, increasing the thermal resistance of the system and contributing to the instability of the device. This study attempts to improve the thermal performance and stability of the system using cellulose nanofiber (CNF) fluid as the working fluid. The use of CNF fluid was observed to reduce the magnitude of temperature change inside the evaporator of the TPCT significantly. Moreover, it improved the local boiling heat transfer coefficient by 3.1 %, 87.3 %, and 181.2 % on average when the filling ratios are 0.25, 0.5, and 0.75, respectively. Studying the local heat transfer performance and instability will be helpful in designing a more stable TPCT efficiently. Additionally, the findings of this study can be applied to solar thermal power generation or heat pipe research for cooling strategies in computing servers, depending on the input heat load.

A modified dynamic Lee model for two-phase closed thermosyphon (TPCT) simulation

A modified model consisting of a dynamic Lee model, volume of fluid model, and continuum surface force model is developed. The modified model investigates heat transfer characteristics of the vapor–liquid phase change process and details of the two-phase flow during operation of a two-phase closed thermosyphon. The mass transfer time relaxation parameters for the Lee phase change model are the most critical coefficients which determine the rate of the vapor–liquid phase change. A dynamic adjustment of the mass transfer time relaxation parameters for the Lee phase change model is realized based on the amount of mass transfer between the vapor and liquid phases and the values of the mass transfer time relaxation parameters become stabilized. The relative error between the modified model and experimental data for the temperature distribution is 5%, representing an acceptable agreement. Compared with the original model, the maximum thermal resistance errors in evaporation section and condensation section are reduced by 19.3% and 107.1%, respectively. These results indicate that the modified model can provide good corrections with high accuracy.

Application of a concrete thermal pile in cooling the warming permafrost under climate change

Permafrost degradation caused by climate warming is posing a serious threat to the stability of cast-in-place pile foundations in warm permafrost regions. Ambient cold energy can be effectively utilized by two-phase closed thermosyphons (TPCTs) to cool the permafrost. Therefore, we installed TPCTs in a cast-in-place pile foundation to create a unique structure called a thermal pile, which effectively utilizes the TPCTs to regulate ground temperature. And we conducted a case study and numerical simulation to exhibit the cooling performance, and optimize the structure of the thermal pile. The purpose of this study is to promote the application of thermal piles in unstable permafrost regions. Based on the findings, the thermal pile operated for approximately 53% of the entire year and effectively reduced the deep ground temperature at a rate of at least −0.1 °C per year. Additionally, it successfully raised the permafrost table that is 0.35 m shallower than the natural ground level. These characteristics prove highly beneficial in mitigating the adverse effects of permafrost degradation and enhancing infrastructure safety. Expanding the length of the condenser section and the diameter of the TPCT in a suitable manner can effectively enhance the cooling capability of the thermal pile and ensure the long-term mechanical stability of the pile foundation even under climate warming.

Effect of inner diameter on the confinement of two-phase closed thermosyphon

A two-phase closed thermosyphon (TPCT) is a heat transfer device that effectively transfers heat through the phase change of a working fluid. While the working fluid circulates smoothly in a nominal TPCT, compact TCPT has a greater shear force effect between the upward vapor flow and the falling liquid. The inner diameter and type of working fluid significantly affect the confinement effect, but the effects of the inner diameter on the confinement remain unclear. In the case of Co > 0.5, the TPCT was fully confined, so the heat was dominantly transferred by axial conduction through the confined bubbles and liquid slugs. For Co < 0.5, the working fluid circulated smoothly, and the heat was mainlytransferred by latent heat. In the unconfined case, the heat transfer coefficient at the evaporator slightly increased at high Co owing to the mixing effect within the liquid pool. In the condenser, the cold surface was covered with liquid at high Co values owing to the re-rising of falling liquid by the high shear rate. As a result, the overall thermal resistance of the TPCT increased at high Co values due to the significant degradation in the thermal performance of the condenser.

Experimental and computational investigation of thermal performance and fluid flow in two-phase closed thermosyphon

Over the last two decades, energy recovery has become increasingly important in the transition from fossil fuels to green energy. Waste heat recovery, such as the two-phase closed thermosyphon (TPCT) type of heat exchanger, is a highly efficient energy recovery technique. However, accurately predicting the thermal performance and flow patterns in TPCTs is challenging due to the complex physics of phase-change phenomena. A new predictive model is suggested that can provide improved accuracy when predicting thermal performance and flow patterns in TPCTs. An experimental study was conducted to acquire the data, and the results were compared with computational results obtained from the new proposed model. The model uses the volume of fluid (VOF) model with the Lee model for the phase change process in OpenFOAM v2106. To increase accuracy, the saturation temperature and vapor density were defined as a function based on the mass balance in the TPCT. In addition, a simulation was conducted to predict the thermal performance and flow patterns in a TPCT, during which the effects of the empirical value of the mass transfer intensity factor were investigated. The study is expected to provide insights into phase-change modeling with high accuracy and an understanding of the thermal characteristics of TPCTs.

Numerical Investigation of the Two-Phase Closed Thermosyphon Operating with Non-Uniform Heat Flux Profiles

The two-phase closed thermosyphon (TPCT) or wickless heat pipe has been widely considered as an extremely effective and low-cost heat removal device for various applications. A computational fluid dynamics (CFD) investigation of the TPCT can provide detailed information regarding its design and development. In this study, the effect of non-uniform heat-input profiles on a vertical TPCT has been investigated. A CFD model has been built to simulate the evaporation and condensation processes within the TPCT investigated, using a solver based on OpenFOAM which has been modified and validated against experimental data reported in the open literature. Four non-uniform heating profiles of the TPCT have been investigated, and the effects these have on the internal flow field within the pipe are discussed. Simulation results show that the non-uniform heat flux profiles can impact thermal performance depending on the overall heat loading and the heat flux profile.

Energy and environmental investigation on photovoltaic system performance by application of square cross-sectional two-phase closed thermosyphon.

By increasing solar radiation, the temperature of photovoltaic cells rises, and as a result, the electrical power and lifespan of the panel are reduced. By cooling the panel with two-phase closed thermosyphons (TPCTs), this effect can be minimized. In contrast to typical TPCT, which has a circular cross-section, the thermosyphon used in this study has a square cross-section. In the proposed system, the solar cells place on an aluminum plate to reduce the thermal resistance and improve the heat transfer rate. Investigations have been made on the effects of three different filling ratios, including 25, 45, and 65%. The trial results show that equipped PV panel with square TPCT with a filling ratio of 45% produces the best cooling performance. In this instance, 68.31kJ of heat energy was transmitted to the tank water. Electrically, the equipped panel has been found to produce up to 3.85% greater output power than a conventional one. A new mathematical model to estimate the performance of equipped PV panel with square TPCT is introduced too. Additionally, the research has looked into how varied tank volumes, solar radiation, and wind velocity affect the temperature difference in tank water. Based on environmental investigations, the proposed solar system is used instead of natural gas and fuel oil, it will prevent the annual emission of 106.3 and 159.4kg of CO2 per unit area of the panel to atmosphere.

Numerical investigation on thermal performance enhancement mechanism of tunnel lining GHEs using two-phase closed thermosyphons for building cooling

To date, the tunnel lining GHEs have encountered issues with the relatively low heat exchange rate and the rapid decline of heat exchange rate with time. This is because the huge heat accumulation around the tunnel lining GHEs obstructs the heat transfer between the absorber pipe and the surrounding rock during the GHEs operation. Hence, to improve the heat accumulation and enhance heat exchange rate, the two-phase closed thermosyphon (TPCT), an efficient device for long-distance heat transfer, was employed in the tunnel lining GHEs to build a thermally induced channel between absorber pipes and the surrounding rock, accelerating heat transfer between the absorber pipe and surrounding rock. The heat transfer model of the tunnel lining GHEs using TPCTs was built to analyze the thermal performance enhancement mechanism of tunnel lining GHEs using TPCTs for building cooling with different thermal conductivities of primary lining concrete and surrounding rock, and convective heat transfer coefficients (CHTCs) on tunnel internal walls. The results showed that throughout a 90-day operation, TPCTs dramatically boosted the heat injection rate of tunnel lining GHEs from 16.5 to 27.0 W/m2 with an increase of 63.6%. Thermal conductivities of the primary tunnel lining concrete have significant effects on the heat injection rates of the GHEs with TPCTs. The heat injection rates increased from 27.0 to 36.4 W/m2 as thermal conductivities of the concrete increased from 1.85 to 10.7 W/m K, and the enhancement rate ranged from 63.6% to 94.7% when compared to tunnel lining GHEs without TPCTs. The enhancement rate of tunnel lining GHEs with TPCTs decreased with increasing thermal conductivity of surrounding rock and CHTC, reaching up to 122.1% under the thermal conductivity of 1.4 W/m K and CHTC of 0 W/m2 K. Overall, TPCT greatly enhances the heat injection rate of tunnel lining GHEs, implying that it is a promising technology.

Thermal performance and heat transfer process of an expressway embankment with horizontal-thermosyphons in permafrost regions

Two-phase closed thermosyphon (TPCT) has been considered to be an effective measure to reduce soil temperatures and maintain engineering stabilities in permafrost, but deformations and cracks of a testing expressway with inclined insertion thermosyphon embankment (ITE) were found to be serious and aggravated year by year. Monitoring results from 2011 to 2018 indicate that uneven temperatures and permafrost thawing are main causes for the disease in ITE. To address this issue, a horizontal insertion thermosyphon embankment (HTE) was proposed in this paper. Numerical results show that thermal performance of HTE is much better than that of ITE. In October of the 20th service year, base temperatures of HTE at center, sunny and shady foot can be reduced to −2.3 °C, −1.2 °C and − 1.6 °C, respectively. Low-temperature (≤ − 1 °C) contours in HTE showed a layered and nearly symmetric distribution, covering almost the entire base width range. The width of −1.5 °C isotherm can be increased by 61%, and the estimated annual heat release of deep soils can be increased by 72% compared with ITE. The thermal enhancement of HTE is because the horizontal-thermosyphon doubles the contact surface with soils and heat dissipation, and effectively reduces the heat flux difference. The horizontal-thermosyphon could be considered for application in expressways built on permafrost stratum.