Air Tunnel Research Articles

Long-term operational characteristics of subway source heat pump system under various tunnel internal heat source intensities

In recent years, subway systems have effectively alleviated urban traffic congestions. However, thermal pollution of underground spaces is a prevailing problem, which poses a serious threat to the safe and efficient operation of subways. The subway source heat pump system (SSHPS), equipped with a front-end tunnel lining heat exchanger, is an effective solution to tackle the aforementioned problem. However, the design methodology and operational strategy of the system is not yet established, which necessitates an analysis of its long-term operational characteristics. In this study, an SSHPS simulation model was developed using the TRNSYS simulation tool based on a demonstration project and the model was subsequently validated against measured data. The validated model was used to evaluate the long-term performance and operational characteristics of the subway system under various internal heat source intensities ranging from 0 W/m to 240 W/m. The simulation results showed that the tunnel air temperature steadily increased as the internal heat source intensity increased. When the internal heat source was 0 W/m, the average annual decrease in tunnel air temperature was 0.11 °C. However, the average annual tunnel air temperature did not fall below the minimum subway design specification temperature of 5 °C at any point in time. When the internal heat source was 120, 180, and 240 W/m, the tunnel air temperature exceeded the maximum temperature of 40 °C stipulated in subway design guidelines on multiple occasions. As the internal heat source intensity increased, the performance of both the heat pump unit and SSHPS progressively decreased during the cooling season and gradually increased during the heating season. The runtime of the heat pump unit gradually decreased from 92.72 % to 77.57 % during the cooling season, whereas it increased from 46.92 % to 89.41 % during the heating season. The heat pump unit exhibited an average energy efficiency ratio (EER) of 5.07, 4.95, 4.87, 4.80 and 4.76 whereas the SSHPS demonstrated an average EER of 3.04, 3.01, 3.00, 2.99 and 2.98. Throughout its operational years, the SSHPS consistently maintained stable and cyclically high performance. However, the system did not fully meet the specified cooling and heating loads under all operational conditions, indicating that there was a source–load mismatch issue within the system. Hence, it is recommended to incorporate auxiliary cold and heat sources, thereby establishing a composite heating and cooling system based on subway source heat pump technology, with the aim of enhancing the reliability of energy supply for the system. This study can serve as a valuable reference for formulating high-efficiency and energy-saving operational strategies for SSHPSs.

Investigation of temperature field variations induced by the air thermodynamic behavior when trains pass through a high-speed railway tunnel in cold regions

High-speed railway tunnels in the cold region, whose numbers are increasing, are widely exposed to the threat of frost damage caused by extremely negative temperatures. The operation of high-speed trains has made this problem even more pronounced. But there are fewer studies focusing on the dynamic changes in temperature during train operation. In this paper, the air thermodynamic behavior of trains passing through a tunnel and its consequences were investigated with the help of the CFD method. In numerical simulations, conditions of different natural airflows, different train speeds, different blockage ratios, as well as two trains intersecting in the tunnel were involved. The following valuable results were obtained. In cold environments, train-induced pressure wave effects will affect the air thermodynamic behavior, but the overall trend in air temperature is mainly influenced by the heat transfer between cold and warm air. The high-speed train will lead to massive cold air being carried into the tunnel and some warm air inside being pushed out. Heat transfer between cold and warm air will cause a significant decrease in tunnel air temperature. This air thermodynamic behavior is particularly prominent in the case of large blockage ratios. When there are same-direction natural airflows, they will be coupled with train-induced airflows, causing a greater decrease in tunnel air temperature. In contrast, opposite-direction natural airflows can reduce the negative effects of the train. Taking the case of opposite-direction natural airflow velocity of 5 m/s as an example, the temperature changes of the three cross-sections are only 16.7 %, 14.8 %, and 42.1 % of those in the case of no natural airflow. Whatever the direction of natural airflows, coupling effects between them and train-induced airflows will be more significant as the natural airflow speed increases. Greater train speed means more cold air entering the tunnel and stronger heat transfer between cold and warm air, causing a more significant temperature decrease. When two trains intersect through the tunnel, airflows caused by them will be coupled. As a result, only a little air enters the tunnel from the entrance and exit, and the heat transfer between air with different temperatures is weak. In this case, the air temperature doesn't decrease but slightly increases as the air is compressed.

Quantitative analysis of interface heat transport at the Si3N4/SiO2 van-der Waals point contact

Si3N4 and SiO2 have been widely used in lithium batteries and optical devices. The high energy density brought by device integration makes the interfacial heat transport at the nano scale a hot spot. To optimize interfacial heat transfer, the roughness of the device surface is continuously reduced down to the nanometer. However, due to the limitation of spatial resolution, it is difficult to realize the measurement of thermal boundary resistance in the case of nanoscale contact by the existing measurement means. There is a great discrepancy between the measurement and simulation results, making the optimization of interfacial heat transfer lack of experimental verification. In this paper, the measurement of thermal boundary resistance of Si3N4/SiO2 interface at the nano-scale, was firstly realized by the optimized 3ω-SThM system. The effect of air tunnel on the heat transfer between real surfaces was also analyzed. The progress made so far provides an effective experimental tool to measure interfacial heat transport at nanoscale point contact.

Study of air supplement velocity by thermal stack effect and critical velocity under longitudinal ventilation in the uniclinal V-shaped tunnel

This research focuses on the specific uniclinal V-shaped tunnel during a fire, studying the air supplement triggered by the thermal stack effect and further examining the control mechanism of upstream smoke. It revealed how the air supplement velocity generated by the thermal stack effect, influenced by longitudinal ventilation, varied in the tunnel. It also established a prediction model for the critical velocity based on different geometric tunnel parameters. Results indicate that internal longitudinal velocity enhances air entrainment, lowers the temperature field, and significantly reduces the stack effect. As longitudinal velocity increases, the air supplement velocity exhibits an exponential decay, with a decay coefficient identified as 3.22. Higher heat release rates and slope heights increase the internal temperature and height differences, promoting air supplementation. Combining air supplement velocity with the critical velocity prediction for horizontal tunnels, the study quantifies the critical velocity in uniclinal V-shaped tunnels under varying conditions. It introduces a P parameter to decide on the necessity of external mechanical ventilation for smoke control. P ≥ 1 indicates that mechanical ventilation is unnecessary; P < 1 suggests the opposite. These findings offer significant insights into the dynamics of smoke and air in tunnels, providing valuable guidance for tunnel fire smoke management.

Study of dust pollution control effect based on orthogonal test and CFD numerical simulations.

To control the diffusion of high concentrations of coal dust during tunnel boring and minimize the threat to the life and health of coal miners, theoretical analysis, numerical simulations, and field measurements were combined in this study. First, computational fluid dynamic simulation software was used to simulate the generation of dust particles and their transport pattern in the tunnel. Subsequently, an innovative orthogonal test was performed to study the effect of four ventilation parameters [the pressure airflow rate (Q), distance between the air duct center and heading face (LA), distance between the air duct center and tunnel floor (LB), and distance between the air duct center and nearest coal wall (LC)] on dust diffusion. According to the orthogonal test results, the optimal ventilation parameters for effective dust control are as follows: Q = 1400 m3/min, LA = 7 m, LB = 2.8 m, and LC = 1 m. The optimized set of ventilation parameters was applied to the Wangpo 3206 working face. The results show that dust diffusion in the tunnel was effectively controlled and that the air quality was sufficiently improved.

Numerical analysis of heat and mass transfer in separated ventilation of deeply buried long air intake tunnels

Using earth air tunnel heat exchangers (EATHE) to pre-treat air entering underground space is an effective measure for energy-saving. This paper addresses the scarcity of numerical model for heat and mass transfer in long, separated air intake tunnels by establishing a three-dimensional model that considering condensation, specifically analyzing the heat transfer process in tunnels with separated ventilation earth air tunnel heat exchanger (SV-EATHE). The accuracy of the model was verified by engineering measured data. The heat transfer performance of EATHE and SV-EATHE is compared and analyzed by using the model. The results indicate that the annual heat exchange of SV-EATHE is approximately 80 % of that of EATHE, with the latent heat exchange from March to September accounting for 77.2 % of EATHE's. SV-EATHE significantly reduces condensation in traffic tunnels. Increasing ventilation will correspondingly increase the heat exchange and reduce dehumidification, but the rate of change will gradually decrease. The change in partition wall thickness has an approximately linear relationship with the amount of heat exchange. For every 0.1 m increase in diaphragm wall thickness, the heat exchange in the intake duct in August is reduced by approximately 1500 kWh and the dehumidification is reduced by approximately 0.6 kg/h.

Optimization of wind-and-water coordinated dust reduction device for coal mine return airway based on CFD technology

High-concentration respirable dust is generated in fully mechanized mining faces, causing health hazards to operators in return airways. To effectively solve this problem, we developed a wind-and-water coordinated dust reduction device. Firstly, the effectiveness of the numerical simulation was validated through spray experiments. Next, the orthogonal experiment method was employed using software such as Fluent and CFD-Post to conduct numerical simulations. This enabled the analysis of the influence of the fan airflow rate (Q), spray pressure (Pw), and nozzle diameter (D) on the coverage capability of the return air roadway and the distribution pattern of the mist field. The results of the numerical simulations indicated that the optimal combination of parameters was Q = 500 m3/min, D = 1.6 mm, and Pw = 5 MPa. Through field application and actual measurement in the return air tunnel of 117 fully mechanized mining face of Jinjitan Coal Mine, the average dust reduction efficiency reaches 91.05%. After dust reduction, the respiratory dust concentrations in the return airway at 20 m, 50 m and 80 m from the air inlet of the return airway are 0.214 mg/m3, 0.172 mg/m3 and 0.153 mg/m3 respectively, which effectively solves the problem of dust pollution in the return airway and reduces dust harm.

Research on high precision flow control technology of low speed wind tunnel air supply system

Aiming at the shortcomings of traditional analog control valves and digital valves, a high-pressure air supply system with accurate flow control is designed. Firstly, the ER5000 controller and pilot proportional valve drive several diaphragm reducing valves to achieve the preliminary adjustment of air pressure. Secondly, a needle valve based on the PCM digital valve and equal percentage flow characteristics is used to achieve accurate control of gas flow. Then, the Venturi flow meter with a honeycomb and damping network is used to accurately measure the airflow. Finally, a PID flow control algorithm based on nonlinear self-tuning is proposed. The results show that the flow regulation range of the system reaches 0.1∼8 kg/s, the absolute control accuracy is better than ±3 g/s, and the relative control accuracy is better than ±0.04%. Moreover, the system has the characteristics of short regulation time, small overshoot, and good dynamic performance, which can achieve strong support for wind tunnel air supply tests.

Investigation of The Effect of Phase Angle on The Aerodynamic Performance of Three-Bladed Helical Savonius Wind Turbines Using Computational Fluid Dynamics Method

In this study, the effect of the phase angle of the semicircular blades between the stages on the aerodynamic performance of the three-blade, double-stage helical Savonius wind turbines (HSWT), which are vertical axis wind turbines, was examined by the computational fluid dynamics (CFD) method. In the Savonius wind turbine system, the drag force effect provides the most significant contribution to aerodynamic performance. Performance improvements that can affect drag force can provide significant advantages. For this purpose, three-bladed double-stage helical Savonius rotors with eccentricity L/H=1/2 and phase angles of the semicircular blades between the stages Ɵ=0°, 45° and 90° were designed. Solidworks R2018 is used for designs and ANSYS-Fluent 18.1 programs are used for analysis. The turbine with L/H=1/2 and Ɵ=90° was produced on a 3D printer and tested experimentally. Experiments were carried out in the T-490 air tunnel. The results obtained were used as a reference for numerical analysis and the ideal turbine model was tried to be determined. 10 different air velocities ranging between 3.83-20.35 m/s were used in the numerical analysis. As a result, an 11.64% increase in the drag force was observed by changing the phase angle from 0° to 45° in HSWT 1/2s. By changing the phase angle from 0° to 45° in HSWT 1/2s, a 10.77% rise in the drag coefficient was observed. It has been evaluated that the HSWT efficiency improved with the increment in drag force.

DEVELOPMENT OF ENERGY-EFFICIENT AIR DEDUSTING

Among the sources of internal pollution one can point to dust, breath products of passengers, various emissions from finishing materials of trains and metro stations, etc. This problem is usually solved in two ways: wet cleaning of tunnels and stations and using powerful tunnel vacuum cleaners. Both of these methods have common disadvantages. Firstly, dust removal can only be carried out at night, when there are no trains on the line. Secondly, the dust removal installations themselves are expensive and are effective only at low speeds of dust movement through the tunnel. All metros in the world have a problem with dust removal. Research methods include analytical and computational studies of air distribution in the metro ventilation network using mathematical modeling methods using graph theory and flow algorithms, as well as experimental studies of air flow parameters in natural conditions of the Almaty metro.This article proposes an innovative method for dust removal of tunnel air based on the use of piston action of trains and labyrinth filters installed in station ventilation joints. The parameters of the calculation model of the metro line are substantiated on the basis of a decomposition approach to the mathematical modeling of aerodynamic processes by moving from a linear model of the metro line to a periodic open-loop model. Computational aerodynamics methods were used to determine the patterns of changes in air speed through the ventilation vent and the available pressure drop depending on the speed and mode of trains passing through the station.

Experimental study on the effects of branch tunnel ventilation on the smoke movement and temperature characteristics in bifurcated tunnel fires

A series of fire tests were conducted in a bifurcated tunnel model with branch tunnel ventilation. Three fire locations were considered. The characteristics of smoke movement and temperature profile under various fire source location scenarios were analyzed. The results indicate that when the fire source is not at the intersection, the ventilation airflow in the branch tunnel is diverted at the intersection before it is imposed on the fire source, which results in the actual velocity of ventilation air imposed on the fire source being less than the velocity of ventilation air in the branch tunnel. A new parameter, named the diversion coefficient, was introduced for convenient analysis. By substituting the diversion coefficient values into a classic maximum temperature rise prediction model for the single-line tunnel fires, good prediction results were observed. Moreover, a new dimensionless smoke temperature decay model was developed. The applicability of the model was validated using full-scale fire test data from previous research. The dimensionless temperature decay law of smoke was analyzed based on the newly established model. Results show that when the fire is positioned at the intersection, the decay coefficient of the smoke downstream of the fire decreases linearly as the ventilation rate increases, while decay coefficient of the smoke upstream of the fire decreases slowly at first and then increases with the increase of ventilation rate. When the fire source is not at the intersection, the branch tunnel ventilation conditions could be classified into three categories: sub-critical, critical, and super-critical velocity. Changes in the ventilation velocity within the branch tunnel have little influence on temperature decay process of smoke in the main tunnel under critical and sub-critical velocity conditions. However, under super-critical velocity conditions, the smoke temperature decreases rapidly after reaching the intersection. Furthermore, predictive models for the critical velocity of the branch tunnel under various fire locations were established based on theoretical analysis and experimental data. Within a specific range of heat release rate, the critical velocity is proportional to the cube root of the heat release rate. Compared to when the fire is located at the intersection, the critical velocity is reduced significantly when the fire is not in the intersection.

Analytical and Numerical Models of a New Hybrid System of Earth Air Tunnel Integrated Solar Air Heater With Sensible Storage

Abstract A novel combination of an earth air tunnel (EAT) and a sensible thermal storage-aided solar air heater has been proposed when the sensible storage medium is taken to be rocks filled within the air flow passage. Both time-dependent and steady-state models are reported. The former describes the exact thermal performance of the system for a two-month wintertime for the city of Baghdad, Iraq. The latter describes average performance of the system. The implicit finite difference method and method of separation of variables have been used to solve the pertinent equations in the respective models. Acceptable level of accuracy was obtained between the steady-state numerical and analytical solutions as well as between the numerical and the published data. It is revealed that preheating by a sufficiently long EAT module improves the effective power as well as the output temperature from the rock bed solar air heater by about 35% and 9% respectively for the present set of parameters considered. It is also observed that for the present set of parameters considered, the temperature gradients in the direction normal to air flow in the solar air heater are insignificant and may be ignored. A parametric study is also carried out that assesses the impact of system parameters on the quality as well as quantity of the energy extracted from the system. This work is hence the first to couple an EAT with a sensible thermal storage equipped solar air heater and may pave the way for future studies.

A CFD modelling for optimizing geometry parameters for improved performance using clean energy geothermal ground-to-air tunnel heat exchangers

In this numerical study, different geometric parameters have been optimized for improved performance using a clean energy geothermal Ground to Air Tunnel Heat Exchanger (GATHE). A commercial computational fluid dynamic code ANSYS Fluent was used, and the code has been validated with the available experimental and analytical results. Three soil samples were used to analyze the thermal properties, each with a thermal conductivity of 0.65, 1.25 and 3.50 Wm−1K−1. The critical soil thickness was found to be 10 times the pipe diameter and the lowest outlet temperature of 300.1 K was achieved for a 60 m pipe with a soil thermal conductivity of 3.50 Wm−1K−1. To further enhance the performance of the system, multiple fin arrays were introduced into the ductwork at 0.5 m, 1.0 m and 2.0 m pitches. For a 10 m long pipe with fins installed every 0.5 m, an outlet temperature of 306.7 K was observed. With 4 fins for temperature variation, performance increased by 19.4 % compared to the case without fins. When the fin spacing is 1 m, the temperature drop increases by 5.6 %, and when the fin spacing is 2 m, the temperature drop increases to 7 %, with an outlet temperature of 306.1 K. Finally, a novel set of multiple blocks in diverging and converging patterns was introduced to restrict airflow and further improve heat dissipation. Compared to a single-fin block, the outlet temperature of the 4-fin block was reduced by 14.52 % due to the increased contact area, thus improving the thermal performance of the heat exchanger system.

Experimental and model-based comparison of wind tunnel and inverse dispersion model measurement of ammonia emission from field-applied animal slurry

Volatilization of ammonia from field-applied animal slurry is a significant problem. Accurate emission measurements are needed for inventories and research, but are not provided by all measurement methods. Wind tunnels may give emission values substantially above or below micrometeorological results, which have been shown to be accurate. This limitation reduces the utility of wind tunnel results, which make up a large fraction of available measurements. The present work focused on understanding wind tunnel measurement error by comparing micrometeorological and wind tunnel measurements with the aid of a semi-empirical model. Ammonia loss from digestate after field application was measured in high time resolution in three field trials using wind tunnels and the backward Lagrangian stochastic (bLS) dispersion technique simultaneously. Differences in measured emission were interpreted using the ALFAM2 model, and measurements were used to evaluate the model. Results showed that wind tunnel and bLS methods provided different cumulative emission estimates, although there were similarities in measured emission dynamics. The ALFAM2 model was generally able to reproduce emission dynamics for both measurement methods, but only when differences in mass transfer between the two methods were incorporated in an experimental parameter set. This important result suggests that: 1) the simple structure of the ALFAM2 model captures the essential physical and chemical processes controlling emission and 2) the two measurement methods differ only (or mainly) through mass transfer above the slurry/soil surface and rain. Therefore, with careful selection of wind tunnel air flow it should be possible to approximately match emission that occurs under open-air conditions. But without temporal variation in air flow, actual emission dynamics cannot be captured. This work provides a template for integrating and comparing measurements from different methods, and suggests it is possible to use wind tunnel measurements for model evaluation and even parameter estimation.

Self-Tuning Controller Using Shifting Method

This paper presents a newly implemented self-tuning PID controller that uses a relay feedback identification using a recently designed relay shifting method to determine the mathematical model of the process and subsequently adjust the controller parameters. The controller is applicable to proportional and integrating systems and is even applicable to systems with transport delays if steady-state oscillation can be achieved in the relay control of the system. After briefly introducing the relay shifting method, the current paper describes the hardware (HW) and software (SW) of the proposed controller in detail. The relay feedback identification and control of a laboratory setup by an automatically tuned controller is demonstrated on a real laboratory device called “Hot air tunnel”. The evaluation of the experiment and the characteristics of the controller are presented at the end of the paper. The advantage of the relay method is that it is not as computationally intensive as other identification methods. It can thus be implemented on more energy-efficient microcontrollers, which is very important nowadays.

Fuzzy prediction of the mine's ventilation structure's tunnel air volume

For the problem that it is difficult to accurately measure the air volume in the tunnel where the mine ventilation structure is located. Firstly, the tunnel air volume value is expressed by the interval number, and the air volume value of the tunnel where the ventilation structure is located is obtained by the law of nodal air balance. Then the interval number is transformed into a triangular fuzzy number to represent the tunnel air volume, the generalized induced ordered weighted average operator is introduced, the minimum-maximum closeness is used as the optimal quasi-measure, and the preference coefficient is introduced to transform the multi-objective nonlinear optimization problem into a single-objective optimization problem. And consider the temporal characteristics of the error series and the trend of the error value change for the error correction of air volume prediction error, and construct the interval combination type air volume fuzzy prediction model. Using the air volume data of a mine tunnel in Inner Mongolia, the example calculation of the model shows that the interval combination type prediction method can effectively improve the accuracy of air volume prediction in the tunnel of ventilation structures. Keywods: Error correction, Interval combination type, Interval number, Preference coefficient, Triangular fuzzy number

CFD-based simulation study of dust transport law and air age in tunnel under different ventilation methods.

To solve the problem of high-concentration dust pollution in a bored tunnel, we conducted a simulation study on the dust transport law and air age of the wind flow in a bored tunnel under different ventilation methods. Air age was innovatively introduced as an index for evaluating tunnel air quality. The results show that dust pollution is serious under conditions of press-in ventilation, which is unfavorable to personnel operations. Following the installation of an on-board dust-removal fan, an effective dust-control air curtain forms in the tunnel, and the high-concentration dust is essentially controlled within the range of Z = 13m from the working face. The dust concentration in the working area on the left side of the tunnel is CD < 200mg/m3, and the dust-control effect is obvious. At the same time, the air age on both sides of the tunnel is reduced by 35.5% following the use of the on-board dust-removal fan. Taking into account dust control by ventilation and dust removal by fan, spraying dust reduction measures are added, and we developed automated wind-mist synergistic wet high-frequency oscillation dust-capturing technology for tunnel boring. This could effectively improve the problem of high levels of coal dust pollution in tunnels.

Cooling underground substations worldwide using heat pumps

The use of underground space in urban environments has increased at an accelerated rate over the past few years, particularly to satisfy transport infrastructure needs. Transport tunnels are being constructed at increasingly greater depths in the already congested subsurface. While transport tunnels have demonstrated significant potential as sustainable heat sources for heating under - and above-ground spaces, due to the large area in contact with the ground [4,5], cooling the substations that serve the tunnels, containing plants and machinery, is a task that poses a significant economic and environmental challenge. To tackle this, a new method that takes advantage of airflow in the tunnels and the thermal mass of the ground surrounding the tunnels has been recently introduced [2]. Similar to energy tunnels [1], these systems use water-filled high-density polyethylene (HDPE) pipes integrated into the tunnel space to exchange heat, however using the pipes to reject heat from the substations to the tunnels. This work adopts four cities, namely Sydney in Australia, Guangzhou in China, London in the UK, and Stuttgart in Germany, and investigates the suitability of this approach to cool substations for different meteorological and geological conditions around the world. Moreover, varying heat exchanger lengths and tunnel air temperatures are incorporated in the analysis, to assess the impact of these key parameters on system efficiency and performance.&#x0D; A detailed 3D finite element heat and mass transport model is utilised in this work, simulating the tunnel lining, surrounding soil, the airflow within the tunnel, the pipes attached to the tunnel linings (tunnel air side), and the circulating fluid within the pipes [2]. The location farfield temperature and material properties are shown in Table 1, for the soil conditions in the four locations and for the common materials (shaded in grey). A parametric analysis is undertaken to investigate the performance of these systems under varying conditions. For each of the locations, five different pipe leg length values are used: 50 m, 100 m, 150 m, 200 m, and 300 m and the air velocity is assumed as 0.5 m/s, typical of these tunnels. In addition, two different distributions for the temperature of the air entering the tunnel are used: the estimated tunnel air temperature () distributions based on available literature and measurements [3, 6, 7], as well as the surface air temperatures (), both shown in Figure 1-left. The latter is used to understand the impact of natural ventilation of tunnels on the system, which could inform the placement of the pipes with respect to the positions of ventilating shafts along the tunnel length.&#x0D; The results are shown in Figure 1-right, in terms of the cooling thermal power that a single pipe loop can provide (total pipe loop length being ), when operating constantly over 20 years – to replicate the real conditions in cooling substations. These values are obtained such that the temperature of the fluid does not reduce below 5 °C, such that the heat pump efficiencies remain high. The locations are shown based on the line colour and the air temperature distribution used for each location is represented by the line type (solid:, dashed:). The results show that the cooling provided can vary significantly based on the investigated parameters and suggest that the temperature of the air flowing in the tunnel is very influential to the system performance. The most favourable outcome is achieved for Stuttgart (green), which has the lowest tunnel air and farfield temperature, and the least favourable for Guangzhou (red), which has the higher air and farfield temperature. Comparing the two air temperature distributions used (solid vs dashed), using instead of , which is lower in all cases, increases the performance of the system. This is most obvious for the case of London, with an increase between 2.5 kW and 5 kW, likely due to the low value of the farfield temperature as opposed to the significantly high tunnel air temperatures (). The length of the pipes used in all cases increases the amount of energy, however that increase is logarithmic in nature and therefore after about 150 m to 200 m the investment might not be justified. Overall, this work shows that there is significant potential to utilise heat pump technologies to cool tunnel substations, especially in cooler climates such as central-northern Europe.

Numerical investigation on the thermal performance of energy tunnels under groundwater flow and tunnel airflow

The integration of underground tunnels with ground heat exchangers (GHEs), also known as energy tunnels, is a promising technology that has gained attention in energy geotechnics research. This study investigates the coupled effects of groundwater and tunnel-air flows on the energy tunnel system via 3D thermo-hydraulic numerical modelling. It is found that the combination of groundwater flow parallel to the tunnel and limited airflow velocity results in reduced operational efficiency of ground source heat pump (GSHP) due to the strong thermal interference along the tunnel. The study also highlights the importance of real-scale modelling to evaluate the thermal yield of long energy tunnels, particularly when dealing with parallel groundwater flow in geothermal tunnel design.&#x0D; The use of shallow geothermal energy is a renewable solution towards net-zero carbon emissions. In recent years, a variety of conventional geotechnical structures, such as piles, retaining walls and tunnels, have been transformed into energy geostructures to also serve as GHEs to harness renewable energy for space heating and cooling purposes [7]. The thermal performance of energy tunnels is significantly affected by the presence of groundwater flow [1, 6] and the airflow in the tunnel [2, 4]. The groundwater flow and tunnel airflow can both be present in the field, however, their coupled effects on the energy tunnel system are hardly explored in the literature. This study aims to bridge this gap via real-scale numerical investigations.&#x0D; This study uses 3D thermo-hydraulic numerical modelling developed with the finite element package COMSOL Multiphysics [3]. The numerical model was successfully validated against an energy tunnel model-scale laboratory experiment [5]. Figure 1 shows the overall geometry and boundary conditions of the numerical model. A 48 m long tunnel section is thermally activated with the embedment of HDPE pipes (outer diameter of 25 mm and 2.3 mm thickness) placed in the lining segments at a spacing of 200 mm. A single tunnel ring consists of six segments with absorber pipes joined between them. Every four adjacent rings are connected in series to form a complete loop (with a single inlet and outlet), which is then connected in parallel to a flow and return header pipe(s). The entire thermally activated section comprises six loops, with identical fluid inlet temperature and flow rates, as they are fed by the same flow header pipe. The outlet fluid from each loop is mixed along the return header pipe to the GSHP. A constant 36 kW cooling thermal load is applied, and the simulation is run for ten years to allow the system to reach a steady state condition considering the minimum groundwater flow and airflow. The thermal properties of the ground are prescribed as 2 W/(m·K) and 1100 J/(kg∙K) for thermal conductivity and specific heat capacity, respectively.&#x0D; Figure 2 shows the entering water temperature (EWT, header outlet) and the coefficient of performance (COP) of the GSHP after 10 years of operation for different groundwater flow velocities and directions for tunnel air velocities = 0.05 m/s and 2 m/s.&#x0D; Results show that an increase in and leads to a decrease in EWT and an increase in COP. However, the effect of groundwater direction and velocity is more significant when airflow is limited ( = 0.05 m/s). The parallel groundwater flow leads to a higher EWT and lower COP than when the flow is inclined and perpendicular to the longitudinal tunnel axis. Consequently, the thermal performance of the system under parallel flow with a higher can be even worse than the perpendicular and inclined flow with a lower . When = 0.05 m/s, the EWT of parallel flow increases by 15% to 20% compared to the perpendicular flow, resulting in a maximum decrease of 15% in the COP. The effect of groundwater flow is much less significant when airflow is fast ( = 2 m/s): for increases from 0.05 to 2 m/d, EWT and COP change by no more than 13% and 5%, respectively, and the impact of groundwater flow direction is minor.

Thermal energy storage with tunnels in different subsurface conditions

Thermal energy storage with tunnels in different subsurface conditions