High Density Polyethylene Pipe Research Articles

The sewer advances: How to select eco-friendly pipe materials for environmental protection

Sewer pipe materials exhibit diverse inner-surface features, which can affect the attachment of biofilm and influence microbial metabolic processes. To investigate the role of the type of pipe material on the composition and metabolic capabilities of the adhering microorganisms, three sets of urban sewers (High-Density Polyethylene Pipe (HDPE), Ductile Iron Pipe (DIP), and Concrete Pipe (CP)) were constructed. Measurements of biofilm thickness and environmental factors revealed that the thickest biofilm in CP pipes reached 2000 μm, with ORP values as low as −325 mV, indicating a more suitable anaerobic microbial habitat. High-throughput sequencing showed similar relative abundances of genera related to carbon and sulfur metabolism in the DIP and CP pipes, whereas HDPE exhibited only half the relative abundance compared to that found in the other pipes. To explore the impact of pipe materials on the mechanisms of microbial response, a metagenomic approach was used to investigate the biological transformation of carbon and sulfur in wastewater. The annotations of the crucial enzyme-encoding genes related to methyl coenzyme M and sulfite reductase in DIP and CP were 50 and 110, respectively, whereas HDPE exhibited lower counts (25 and 70, respectively). This resulted in significantly lower carbon and sulfur metabolism capabilities in the HDPE biofilm than in the other two pipes. The stability of wastewater quality during the transmission process in HDPE pipes reduces the metabolic generation of toxic and harmful gases within the pipes, favoring the preservation of carbon sources for sewer systems. This study reveals the variations in carbon and sulfur metabolism in wastewater pipe systems influenced by pipe materials and provides insights for designing future sewers.

Application of Pattern Search and Genetic Algorithms to Optimize HDPE Pipe Joint Profiles and Strength in the Butt Fusion Welding Process

The rapid spread of the use of high-density polyethylene (HDPE) pipes is due to the wide variety of methods for connecting them. This study keeps pace with the developments of butt fusion welding of HDPE pipes by exploring the relationship between the performance of the weld joints by studying ultimate tensile strength and exploring the joint welding profiles by studying the shape of the joint at the outer surface of the pipe (height and width of the joint cap) and the shape of the joint at the internal surface (height and width of the joint root). Welding pressure, heater temperature, stocking time, and cooling time were the parameters for the welding process. Regression was analyzed using ANOVA, and an ANN was used to analyze the experimental results and predict the outputs. Two optimization techniques (pattern search and genetic algorithm) were applied to obtain the ideal operating conditions and compare their performance. The results showed that pattern search and genetic algorithms can determine the optimal output results and corresponding welding parameters. In comparison between the two methods, pattern search has a limited relative advantage. The optimal values for the obtained outputs revolved around a tensile strength of 35 MPa (3.45 and 4.5 mm for the cap and root heights, and 8 and 6.98 mm for the cap and root widths, respectively). When comparing the effects of welding parameters on the results, welding pressure had the best effect on tensile strength, and plate surface temperature had the most significant effect on the welding profile geometries.

Enhancing PEHD pipes reliability prediction: Integrating ANN and FEM for tensile strength analysis

In the pipe industry, pressure pipes have long made use of High-Density Polyethylene (HDPE), which is used extensively. Currently, HDPE pipes are installed in higher numbers in comparison with other plastic pipes. The purpose of this study is to evaluate and compare the predictive capabilities of two methods, including the finite element method (FEM) and artificial neural network (ANN) techniques, for predicting the tensile strength of HDPE pipes used in water distribution systems. Attempts have been made to improve prediction models to better predict the mechanical behavior of these pipes by improving our understanding of the structure and surface characteristics as well as the interactions between the interface and the operating environment. The results show that experimental trial results are in perfect agreement with machine learning techniques. The findings of this study highlight the benefits of using ANN to predict the behavior of HDPE pipes, which may have significant ramifications for the plastics and water distribution industries.

Improving the thermal performance of vertical ground heat exchanger by modifying spiral tube geometry: A numerical study

Ground heat exchanger (GHE) is the most crucial element of a ground source heat pump (GSHP) system for building cooling and heating by the utilization of geothermal energy. Therefore, intending to enhance the performance of GHE, the present study conducts a computational investigation of the thermal performance of modified spiral tube vertical GHEs. Several modifications of uniform-pitched spiral GHE are made to increase its thermal performance. Some modifications are introduced as variable-pitched spiral tube GHE where spiral inlet pipes are densified in the lower part of GHEs by reducing pitch distance. Conversely, in some modifications, the position of the outlet straight pipe is changed. Water is considered as the working fluid and the inlet temperature of the water is maintained fixed at 300.15 K. After extensive analysis, it is evident that, when the outlet pipe is placed outside of the spiral coil, there is a 7.67 % enhancement in the thermal performance than a traditional uniform-pitched spiral tube GHE. However, modifications like variable-pitched spiral tube GHEs are not significant to improve the thermal performance due to the quick saturation of the ground soil temperature around the GHE pipes. To have a balance between heat transfer rate and pressure drop, thermal performance capability (TPC) and coefficient of performance improvement (COPimprvt) criterion were evaluated and it is found that the uniform-pitched spiral tube GHE along with the outlet pipe at the outside of the spiral provides maximum thermal performance with a maximum TPC value of 1.062 and provides the positive value of COPimprvt criterion. The positive values of COPimprvt indicate that the spiral tube GHEs are energy efficient based on heat transfer and pressure drop. Moreover, spiral GHE with high-density polyethylene (HDPE), concrete pile, and sandy clay outperform the other materials for pipe, backfill, and soil, respectively. Specifically, HDPE pipe, concrete backfill, and sandy clay as soil offer around 7 %, 5 %, and 7.8 % higher thermal performance compared to polyethylene, sand silica, and clay, respectively.

Mechanical Behavior of Buried HDPE Pipe Subjected to Surface Load: Constitutive Modeling and Finite Element Method Simulations

Abstract In this paper, the Sherwood–Frost constitutive model was first used to simulate the stress response and deformation process of buried high-density polyethylene (HDPE) pipe subjected to surface load, where parameters in this model were obtained by fitting the results of uniaxial tensile tests with different rates and the pipe–soil model was conducted in abaqus. Apparent stress concentration and large deformation are observed in pipe cross section and are closely related to the magnitude and location of surface load. The increments of surface load and offset displacement have opposite effects on the mechanical behavior of pipes. Additionally, the location of the maximum stress appears to shift from the top or bottom to the left and right sides of the pipe cross section with the increment of surface load, and the region of peak hoop stress will show a decreasing trend of counterclockwise rotation. Then, based on stress failure criterion, the relationship between the ultimate bearing capacity of the pipe and the offset displacement was determined, which decided by the angle between the ground and the line connecting load center and cross section center of pipe. Finally, an offset of 0.6 m is a value of interest. When the offset between the load position and the pipe exceeds 0.6 m, the ultimate bearing capacity of the pipe will increase significantly with the increase of the offset. The results of the above research could provide the reference for the safety evaluation and maintenance strategy of gas polyethylene pipe under the surface load.

Crack layer modeling of butt-fusion joints induced slow crack growth in high-density polyethylene pipes

The design and reliability of high-density polyethylene (HDPE) pipes can be dictated by the damage tolerance of their butt-fusion joints. A slow crack growth (SCG) model based on the crack layer theory for HDPE pipes internal and external circumferential and butt-fusion joints cracks is developed to investigate the discontinuous SCG behavior and lifetime tf variations. Using the developed model, the discontinuous SCG patterns, jump lengths and lifetime tf can be accurately simulated. In addition, the effects of pipes standard dimension ratio, internal pressure, and temperature on the SCG behavior and tf are investigated. Compared to pipe cracks, it was found that butt-fusion induced SCG can reduce the pipe tf > 60 %. Unlike longitudinal cracks, tf of external circumferential cracks, were found shorter than the internal ones, mandating an earlier evaluation. The new SCG model shows good accuracy with the experimental results. The same substitute geometry approach used can be followed for othercomplex designs, which aids in establishing a fundamental methodology for more realistic lifetime predictions.

Investigation of Creep Behavior of HDPE Pipe Butt Fusion Welded Joints Using a Stepped Isostress Method.

The utilization of high-density polyethylene (HDPE) pipes is prevalent in water transportation due to their exceptional durability, resistance to corrosion, and ease of installation. Traditionally, butt fusion welding has been employed to connect HDPE pipes. In this study, scanning electron microscopy (SEM) was utilized to examine the microstructure of butt fusion welded joints of HDPE pipes, while the stepped isostress method (SSM) was employed to investigate their creep behavior at 100 °C in ambient air. SEM results revealed a significant presence of craze or lamellae in the base material, whereas minimal occurrences of craze or lamellae were observed in the melt zone. The results obtained from the SSM indicated that the creep life of butt fusion welded joints of HDPE pipes was not adversely affected by the welding bead, and their creep life was no less than that of the base material when ductile creep failure occurred.

High-Density Polyethylene Pipe Butt-Fusion Joint Detection via Total Focusing Method and Spatiotemporal Singular Value Decomposition

High-density polyethylene (HDPE) pipes are widely used for urban natural gas transportation. Pipes are usually welded using the technique of thermal butt fusion, which is prone to manufacturing defects that are detrimental to safe operation. This paper proposes a spatiotemporal singular value decomposition preprocessing improved total focusing method (STSVD-ITFM) imaging algorithm combined with ultrasonic phased array technology for non-destructive testing. That is, the ultrasonic real-value signal data are first processed using STSVD filtering, enhancing the spatiotemporal singular values corresponding to the defective signal components. The TFM algorithm is then improved by establishing a composite modification factor based on the directivity function and the corrected energy attenuation factor by adding angle variable. Finally, the filtered signal data are utilized for imaging. Experiments are conducted by examining specimen blocks of HDPE materials with through-hole defects. The results show the following: the STSVD-ITFM algorithm proposed in this paper can better suppress static clutter in the near-field region, and the average signal-to-noise ratios are all higher than the TFM algorithm. Moreover, the STSVD-ITFM algorithm has the smallest average error among all defect depth quantification results.

Improvement of butt-fusion welding procedure and performance evaluation method for thick-walled bimodal polyethylene pipes

High-density polyethylene (HDPE) pipes have been continuously improved by the progress made by the resin industry and pipe manufacturers. Together with the advanced technology of welding equipment in terms of structure reliability and automations, HDPE pipes have been gradually applied to higher pressure level, bigger diameter, and thicker walls in operations which have become the preferred piping system for nuclear safety application in transporting cooling seawater. There are substantial differences in parameter settings when applying heat and force during different welding procedures, as well as the complicated evaluation methods for mechanical properties, which causes communication barriers between engineers and researchers. Thus, it is necessary to comprehensively study the welding procedures and evaluation methods to standardize the evaluation protocols in the industry. In this paper, based on the assumptions of temperature effect and molecular chain diffusion dynamics at the welding interface, the optimization of the welding procedure is proposed. Some well-established evaluation methods have been used for performance assessment and their feasibilities are discussed. The idea of creating crack along the weld interface is adopted as new method to evaluate the material ability of fracture resistance. Specimens with unilateral notch and weld interface crack are proposed and subjected for quasi-static axial tensile test. The fracture stress and fracture energy at the welding interface are used to differentiate the quality of the joint from different welding procedures. The fracture morphologies of the weld interface are analyzed. A complete picture is drawn for the mechanical fracture resistance of the weld interface. The method used in this paper can be corelated to the pipeline design conditions, therefore could be applied to the real-time engineering applications. Moreover, the evaluation method proposed in this paper is simple, reliable, and easy to implement.

Comparative study on the performance of butt fusion-welding processes for nuclear safety class large-diameter thick-walled PE pipes

New technologies in polymer synthesis and pipe extrusion equipment have led to the commercialization of high-performance, large-diameter, thick-wall high density polyethylene (HDPE) pipes. They have been used in the field of seawater transport and cooling to replace metal pipes, due to their advantages of high corrosion resistance and extensibility. Connection of HDPE pipe is important as it determines the safety of the entire piping system. Butt fusion welding is commonly used for HDPE pipe connection but may cause the formation of weak points in the welded joints, interfering the reliability of the pipeline system in the application of nuclear power plants. At present, there is a lack of research on evaluating the performance of welded joint for large-diameter thick-wall HDPE pipes made by butt fusion-welding. The purpose of this study is to investigate the influence of three different butt fusion-welding processes, i.e., single low pressure (SLP), single high pressure (SHP) and dual low pressure (DLP), by evaluating the performance of their welded joints, including characterizing tensile strength, extensibility, crystallinity and hardness. In specific, a thick-wall HDPE pipe with OD of 812.8 mm and wall thickness of 74 mm which is certified for nuclear safety class was used for study. Representative specimen from the outer, middle and inner part across the wall of the main pipe body and welded joints were taken for testing. Different test methods and specimens were designed to assess the feasibility of evaluating the welding performance from different welding process. The results showed that the mechanical properties of different locations of the welded joints were different, and the tensile strength and fracture energy of the middle part of the joint were lower than that of the inner and outer parts, which could be caused by the difference in the crystallinity and thickness of the melting zone influenced by welding processes, as can be seen from the analysis of DSC test and morphology observation. Hardness testing was conducted on the section of the welded joints, and it revealed that the micromechanical properties of the welded joints in the region of the heat-affected zone were enhanced significantly, which may be due to the annealing effect caused by welding process. In summary, The DLP process resulted in the best extensibility of the welded joints among three processes, suggesting that the joining pressure from welding process plays an important role in affecting the extensibility of the welded joints.

Effect of carbon black incorporation methods on the initial creep/slow crack growth resistance and properties of high-density polyethylene (HDPE) pipes

The increasing utilization of polyethylene (PE) pipes in diverse industries has driven a focused exploration into the properties and behaviors of PE materials. In response to this heightened interest, this study aims to investigate the impact of different carbon black (CB) incorporation methods on the properties of PE100 polyethylene, particularly its resistance to slow crack growth (SCG). Two distinct methods, direct incorporation and utilization of a commercial masterbatch, were employed to introduce CB into the PE100 matrix. Results revealed that direct incorporation led to superior dispersion of CB, thereby enhancing the mechanical properties of the resultant compound. Furthermore, SCG resistance exhibited improvement when CB was directly incorporated, contrasting with a degradation in SCG resistance observed when a CB masterbatch was used compared to neat PE. Comprehensive rheological and thermal analyses were conducted to elucidate these findings. The study concludes that the presence of low molecular weight substances and inadequate CB dispersion within the CB masterbatch significantly contributed to the deteriorated SCG resistance observed in PE100 material.

DEVELOPMENT OF FRICTION STIR WELDING TOOL FOR HIGH-DENSITY POLY-ETHYLENE (HDPE)–CASE STUDY: FIBERGLASS COMPOSITE MATERIAL

This study aims to develop an effective Friction Stir Welding (FSW) method for composite material of High-Density Poly-Ethylene (HDPE) Pipes. The development of welding tool, there was the addition of an external heating source on the shoulder and probe/pin to overcome the problem of lack of heat resulting from friction between the tool and the material to be welded. The case study was conducted to join the short fiberglass-HDPE composite with a type of ratio of 30% by weight of short fibre and 70% by weight of HDPE, which optimizing parameters such as rotating speed, welding speed, and preheating temperature. The FSW joining process for short fiberglass-HDPE composite sheets was carried out using a Fanuc Series 21i-M CNC milling machine as the driving tool with rotational speed (ω) varied in 3 conditions, namely 600 rpm, 800 rpm and 1000 rpm, and welding speed (v) or feeding at 5 mm/minutes and 10 mm/minutes. The temperature was controlled according to the liquid point of High-Density Poly-Ethylene, which was 130oC, and raised to 150oC and 170oC. The 12 pieces of thermocouple were used along the track on the material and jig plates at the top, middle and bottom. Then, the results of joining the sheets were made in the form of specimens with sizes according to ASTM 3039. The tensile tests of the specimens were carried out at a rate of 0.01 mm/s. The results showed the highest tensile strength was an average value of 24.52 MPa at a rotational speed of 800 rpm, the feeding of 5 mm/min and the temperature of 130oC. The lowest tensile strength was an average value of 17.54 MPa at a temperature of 170°C with a speed of 600 rpm.

Fatigue crack growth and slow crack growth of HDPE pipes under internal pressure and flat plate compression

High-Density Polyethylene (HDPE) pipes are widely used in city gas and water systems as buried pipelines for their excellent performance. However, in addition to the internal pressure, it also suffers loads from traffic vehicles, foundation settlements, and illegal buildings. In this paper, fatigue crack propagation of HDPE pipes with a longitudinal external crack was experimentally studied under different load ratios in two cases. Pulsatile internal pressure was implemented in the first case. In the other one, pulsatile flat plate compression and constant internal pressure were applied. The crack mouth opening displacements were recorded by the digital image correction technique. The crack depth was obtained with compliance calibration. It shows that the fatigue crack growth rate decreases as the load ratio increases. In both cases, the slow crack growth kinetics were gained by extrapolating the load ratio to 1.0, representing constant internal pressure or flat plate compression.

Exciting and Detecting Higher-Order Guided Lamb Wave Modes in High-Density Polyethylene Structures Using Ultrasonic Methods.

High-density polyethylene (HDPE) pipes are becoming increasingly popular, being used in various fields, such as construction, marine, petroleum, water transfer, process water, methane gas collection, oil and gas gathering, gas distribution systems, mining, acid and wet gas lines, offshore oil and gas and in nuclear power plants. Higher-order guided Lamb wave (UGW) modes can be used to detect various defects in complex structures. We will apply this methodology to one of the types of plastic-the structure of high-density polyethylene (HDPE). However, the excitation of UGW modes faces numerous challenges, especially when there is a need to identify which mode is excited. It is essential to note that, in the higher frequency range, multiple different higher-order modes can usually be excited. This can make it difficult to determine which modes have actually been excited. The objective of this research was to successfully excite and receive various higher-order UGW modes in high-density polyethylene structures using both ultrasonic single-element transducers and a phased array. Theoretical calculations were performed using a variety of methods: semi-analytical finite element (SAFE) method, 2D spatial-temporal spectrum analysis and finite element modeling (FEM). The results obtained from both measurements and simulations clearly demonstrate the possibility of efficiently exciting and receiving different Lamb wave modes possessing different phase velocities.

A study on pipe failure analysis in water distribution systems using logistic regression

Abstract This study presents a pipe failure analysis that aims to identify key factors affecting pipe failure in a pilot study area (PSA). For this purpose, pipe failures that occurred in the PSA were recorded for 2 years. Recorded pipe failures were integrated into the existing geographical information systems of the PSA and then visualized. Then, the logistic regression model that calculates failure probability with unbalanced and small- and medium-sized data was performed to analyze pipe failures. Pipe material, age, diameter, burst pressure, and average water pressure are the factors used in the study. The developed model achieved a success rate of 70.1%, suggesting that it can predict a considerable portion of pipe failures with a relatively satisfactory performance. Findings show that the failure probability of pipes with an age greater than 20 years is 1.675 times higher than that of pipes with an age less than 20 years. Moreover, polyvinyl chloride pipes are 5.265 times and high-density polyethylene pipes are 9.027 times more likely to fail than ductile pipes. Further research should delve into the impacts of the number of service connections, pipe lengths, and traffic load on pipe failures.

Selection of Suitable Pipe Material for New Water Distribution Network Using Analytic Hierarchy Process – A Case Study of Athivilai Village of India

This study was made to design a new water distribution network for the Athivilai village of Tamilnadu, India. The associated problems of the village are improper water supply for growing water requirements during the summer season. The cost, service life, and material type play an important role when designing a new network. In this study, the suitable pipe material is chosen by using Analytical Hierarchy Process. The criteria selected for Analytic Hierarchy Process include Cost, Service life, Ease of maintenance, Corrosion, Leakage, and Internal pressure. The results show that the Ductile Iron pipe ranks top position followed by High Density Polyethylene, Polyvinyl Chloride, and Galvanized iron. After selecting the pipe material, the design of water distribution network is done in EPANET 2.0 software. The results show that the diameter of pipes ranges from 40 mm to 250 mm. Since the corrosion of smaller diameter Ductile Iron pipes are faster, the High Density Polyethylene pipe which acquires the second rank can be used for diameters smaller than 80 mm. For pipes with a diameter greater than 80 mm, Ductile Iron pipes are used. Furthermore, since a continuous system of supply is designed, the pressure rises above 60 m during non peak hours which indicates that the pipes are constantly subjected to internal pressure. Both the selected pipes can withstand high internal pressure which confirms the results obtained by the Analytic hierarchy process are suitable.

Comparison of the thermal and hydraulic performance of single U-tube, double U-tube and coaxial medium-to-deep borehole heat exchangers

Decarbonisation of heat is essential in curbing carbon dioxide emissions and can be achieved through the use of geothermal systems. Recently, single-well, closed-loop, deep borehole heat exchangers, using a coaxial design, have become the focus of attention, partly due to the potential to repurpose existing infrastructure (such as oil and gas wells); however, few have investigated the potential for other types of heat exchanger for middle-deep geothermal systems. Therefore, in this study, a comprehensive numerical analysis was undertaken using OpenGeoSys software to investigate the thermal and hydraulic performance of coaxial, single U-tube and double U-tube middle-deep borehole heat exchangers (MDBHEs). The purpose of this paper is to test the maximum operational depth for each type of pipe configuration as few wells have been completed to depths exceeding 500 m using single/double U-tube configurations. The best performing MDBHEs should minimise parasitic and hydraulic losses, whilst maximising thermal output. Furthermore, ground sourced heat pumps require electricity; therefore, at times where electricity prices are high (and drilling costs can be minimised) it may be more beneficial to utilise MDBHEs to encounter greater temperatures.Results indicate that coaxial MDBHEs provide the best performance in terms of specific heat extraction and lowest pressure/parasitic losses. Double U-tube MDBHEs can provide a similar thermal performance to the coaxial design, but have significantly greater hydraulic pressure losses across all simulations, which translates to greater parasitic pumping power costs. Single U-tube MDBHEs demonstrate the poorest performance in terms of heat extraction and pressure losses. At the end of the 25-year base case scenario for a 800 m MDBHE, coaxial, U-tube and double U-tube configurations, all with a fluid circulation rate of 5 L/s, provided specific heat extraction rates of 39.1 W/m, 32.8 W/m, and 36.0 W/m, respectively, with the fluid inlet temperature set as a constant of 5 °C. For these simulations, pressure losses were estimated as 85 kPa (coaxial), 1.46 MPa (single U-tube) and 423 kPa (double U-tube)—the single U-tube value being close to the nominal 16 bar (1.6 Mpa) pressure rating of SDR11 high density polyethylene pipe. Further parametric analysis was also undertaken, investigating depth, flow rate, rock thermal conductivity, pipe diameter and shank spacing.

Experimental investigation into the effects of voids on the response of buried flexible pipes subjected to incrementally increasing cyclic loading

In this study, large-scale fully instrumented laboratory tests were conducted to investigate the behaviour of buried flexible high-density polyethylene (HDPE) pipes, in sand beds with and without voids subjected to incrementally increasing cyclic loading. Voids with a predetermined size were created at one side of the spring-lines of the pipes, which were buried at variable depths, H, of 1.5, 2.0 and 2.5 times the diameter of the pipe, D. Results showed that increasing the pipe burial depth, H/D, contributed to decreasing the settlement of the footing, deformation of the pipe crown and invert, lateral displacement of the spring-line, and the stress and strain generated along the pipe crown and invert. Void presence led to a significant increase in the footing settlement, which ranged from 3 % up to 18 %, according to H/D. Furthermore, void presence led to a sharp increase in the crown, invert, and spring-line settlements, which ranged from 34 % to 52 %, 10 %–12.5 %, and 13 %–38 %, respectively. Increasing pipe burial depth was found to be highly effective in protecting buried pipes, minimising inevitable consequences of the presence of voids. However, this was combined with an increase in the pressure at the pipe spring-line that led to a positive horizontal support at the pipe's spring-lines resulting in reducing pipe deformation.

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.
 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.
 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.

Full-scale investigation of the short-term continuous and intermit-tent operation of an energy piled wall section

The growing demand for highly efficient renewable energy technologies has positioned shallow geothermal energy as an attractive alternative. The initial high capital cost of traditional ground heat exchangers (GHEs), mainly associated with drilling, has extensively hindered their implementation. Conventional geostructures overcome this drawback, and their use as GHEs is getting more attention. Piles [4, 8, 10], pavements [7], and walls [1, 5] are some of the structures that are being used. Energy piles have been the most popular among them, and their implementation has increased steadily since the 1980's [5]. After more than two decades of intensive research, a large dataset of full-scale energy pile foundation observations is available [6], and a thermo-mechanical conceptual framework [2, 3] has been formulated.
 Piles are not limited to foundations; they are used in retaining walls. In contrast to foundations, retaining walls mainly work under lateral stress conditions instead of axial loads. In principle, energy piled retaining wall's initial state, circuits configuration and boundary conditions control their response. Nevertheless, in the context of (energy) piles, observations appear limited to a few full-scale thermo-mechanical studies to date [1, 5]. More attention to this energy geostructure from the scientific and engineering practice communities is required, and the documentation of additional case studies is needed to promote the energy piled walls implementation more broadly.
 In this work, a series of full-scale thermo-mechanical field testing has been performed in an energy piled wall section. The retaining wall system section consists of three piles equipped with pipes to work as heat exchangers. Each houses 4 U-loops in series made of high-density polyethylene (HDPE) pipe with an outer diameter of 25 mm and HDR of 11. The pipe circuits are connected in parallel through a common header manifold operated on the surface. Two of these piles have been instrumented with 10 pairs of vibrating wire rebar strainmeters with temperature sensors located on diametrically opposing sides of the pile. One is close to the excavation, whereas the other is to the ground side - see more details in [10].
 The thermal activation of the wall section occurred at 34.7 m depth and was achieved through an in-situ Thermal Response Test (TRT) unit. The unit heats a carrier fluid through constantly powered electrical heating components and pumps the fluid in the pipe loops within the GHEs. To emulate a conventional thermal load (e.g., energy piles 30-70 W/m, [9]), the entire wall section is supplied with a heating power of 5.5 kW over 5 days active period, and the thermal load is provided under either continuous or intermittent conditions (i.e., 5 thermal cycles, 12 hours on/off). Five recovery days allowed the ground to return partially to its initial thermal equilibrium. The testing period lasted 26 days of constant monitoring.
 The obtained measurements in one pile are presented in Figure 1. It compares the measured strains at different construction stages (i.e., excavation to 32.5, 36.4, and 34.7 m depths) against those observed at the end of both operation modes (i.e., 5 days, 5th cycle). It is noticed that the temperature-induced strains recorded by the available sensors (i.e., 11/20 in this case) are, in most cases, within the axial strain's envelope derived from previous excavation and construction stages. Only one sensor at 33 m indicates tensile axial strains larger than those observed at different excavation depths; nevertheless, it represents a minor deviation compared with the increment recorded at the same depth during the last excavation and anchors removal. Based on these observations and assuming a thermoelastic response, thermally induced axial stresses remain within the design ranges delimited by design envelopes.