Research progress and challenges of energy diaphragm wall

Abstract

Energy diaphragm walls (EDWs) harness shallow geothermal energy through the internal circulation of fluid in heat exchange pipes, thereby providing buildings with energy-efficient, low-carbon, and sustainable energy solutions. However, the influencing factors of EDWs are complex and are subject to the coupling effects of multiple physical fields. To deeply understand the operational mechanism of EDWs and promote the development and engineering application of this technology, this paper comprehensively reviews the current state of research on engineering cases, experimental studies, and numerical calculations concerning heat exchange efficiency, thermodynamic behavior, analysis/design methods, and multi-field coupling of the walls. A review of previous research indicates: 1) the optimal spacing between HEPs in EDWs can be designed based on the anticipated geothermal energy extraction cost; 2) the stress caused by temperature changes in the wall is greater than that caused by excavation, and the thermal stress within the wall is unevenly distributed, leading to the creation of bending moments; 3) reducing the spacing between pipes can improve heat exchange efficiency in the short-term, but has minimal long-term impact and increases costs. This work can provide technical references and directions for development for researchers and related practitioners.

Graphical abstract

Keywords

EDW / geothermal energy / heat transfer efficiency / thermo-mechanical response / multi-field coupling

1 Introduction

The current world is facing challenges in energy transformation, climate change, and sustainable development, among other areas. In addition to the development of solar, wind, hydrogen, and biomass energy, geothermal energy has become one of the important new sources of energy to address these issues due to its advantages of abundant reserves, stability and high efficiency, energy saving, emission reduction, and renewability. Geothermal energy mainly comes from the decay of magma and radioactive materials inside the Earth, with a small portion of energy coming from the sun. It is estimated that geothermal energy accounts for about 66% of the total global renewable energy, which is approximately 170 million times the global coal reserves [1–3]. Developing geothermal energy according to local conditions can enhance the local energy supply capacity and ensure regional energy security. Geothermal energy is a clean energy source with zero pollution, zero emissions, and low energy consumption, which can make a significant contribution to the goal of carbon neutrality. The use scenarios of geothermal energy have already involved geothermal power generation, heating and cooling, industrial production, agricultural utilization, and hot spring bathing, among other fields, and its range of applications is expected to expand further and achieve cascading utilization in the future [4–6]. As various countries implement and improve policies related to the development of geothermal energy, the engineering technology for researching, exploiting, and using geothermal energy becomes particularly important.

To address the continuously increasing global energy demands and the growing urgency of sustainable development goals, the construction sector has begun to focus on the integration of high-efficiency energy utilization and green technologies. Utilizing economical, efficient, and clean shallow geothermal energy is an effective approach to solving building energy issues, primarily facilitated by the deployment of ground source heat pump (GSHP) systems within energy geostructures (EGS) [7,8]. EGS as an innovative renewable energy technology, are playing an increasingly important role in the energy supply of buildings [9–11]. This technology, by combining structural support with geothermal energy exchange capabilities, realizes the energy activation of civil engineering structures, not only enhancing the functionality of the infrastructure but also improving the efficiency of energy use.

In 1855, the Austrian mining engineer Peter Ritter von Rittinger first systematically set forth the principle of the heat pump, and then developed a GSHP system suitable for the underground environment [12]. In the 1980s, a new technology that involved embedding heat exchange pipes (HEPs) within concrete components to form underground heat exchangers (HEs) allowed deep foundation structures to be used for the first time in the extraction of geothermal energy [13,14]. To overcome the challenges of traditional GSHP structures, which occupy a large area of underground space and require high initial investment costs, the concept of dual-purpose EGS with HEPs embedded in underground foundation components was proposed. Such as energy piles, energy base slabs, energy diaphragrm walls (EDWs), energy tunnels, etc. [15–21]. An illustrative diagram of the heat exchange systems for various EGSs is shown in Fig.1. These EGSs have advantages such as resource conservation, good economic benefits, and strong heat transfer performance, and thus have the potential for widespread application in densely populated urban areas. Additionally, concrete has good thermal conductivity and heat storage capacity, making it an ideal medium for HEs [22]. EGS consists of HEPs bound to reinforcing cages to form HEs, which exchange heat with the shallow ground through the circulation of fluid within the pipes. When heating is required, the circulating fluid absorbs heat from the ground to warm the indoor space; when cooling is needed, it releases the heat from the indoor space into the ground. These energy underground structures, which exploit and utilize geothermal energy, have been integrated into district heating networks and are capable of meeting the wider energy needs of users within the region [23].

The majority of engineering applications for energy underground structures utilize energy piles, and pile foundation is the most commonly used building foundation component [24–27]. The research carried out in the field of energy underground structures on energy piles is also the most systematic and comprehensive. Currently, there are over 20 proposed and constructed energy tunnel projects globally, with most of these projects located in Europe and East Asia. In recent years, many researchers have entered the field of energy tunnels, involving extensive research work including analysis, experimentation, and numerical studies [28–31]. The EDW, which embeds HEPs within the wall to form a HE, is the novel energy underground structure that this article focuses on. To date, some experts have accumulated technical knowledge through experiments, numerical simulations, and theoretical methods on aspects such as heat exchange efficiency, thermodynamic behavior, analysis/design methods, and multi-field coupling of EDWs [32–34]. Moreover, several countries have successfully applied this technology in practical engineering. EDWs offer significant low-carbon and economic benefits for heating and cooling building spaces and hold broad application prospects in areas such as energy transformation, energy conservation and emission reduction, and foundation enclosure construction. Therefore, in-depth research into this structure is of great significance.

To promote the development and application of EDW technology, the following text provides a comprehensive review of EDWs in terms of engineering, experiments, and numerical research, building upon a systematic introduction of the research background and operating principles of EDWs. The overall flow chart of writing this article is shown in Fig.2. First, it summarizes the engineering cases that have been constructed worldwide, and introduces the project background, structural parameters, and reflections on experience for each project. Subsequently, it reviews the progress of experimental research on EDWs from three aspects: heat exchange efficiency, thermodynamic behavior, and analysis/design methods. In addition, the related test devices and methods are summarized. Finally, it provides a comprehensive overview of the numerical research achievements in EDWs, covering heat exchange efficiency, thermodynamic behavior, analysis/design methods, and multi-field coupling analysis. Currently, EDW technology has made certain research advances in engineering applications, experimental research, and numerical analysis. However, practical engineering cases of underground energy walls are still rare, challenges include but are not limited to improving heat exchange efficiency and providing reasonable analysis/design methods. This article provides a comprehensive review of existing research on EDWs, offering technical references and theoretical foundations for relevant professionals. Additionally, it promotes the technological development and engineering applications of EDWs, providing an energy-efficient, effective, and sustainable solution to building energy supply problems.

2 Basic principle

Diaphragm walls are a common type of retaining structure used in deep foundation pit projects, notable for their excellent waterproofing properties and structural continuity. These structures serve not only as temporary supports during construction but can also act as permanent side walls for basements [35]. Due to their wide range of applications, deep placement, and large contact area with the soil, diaphragm walls with embedded HEPs for harvesting shallow geothermal energy have become an important direction in the development of energy geotechnical structures. Currently, engineering cases where diaphragm walls of structures such as subway stations, office buildings, museums, art centers, hotels, and residential buildings are equipped with HEPs to exploit shallow geothermal energy are evident [36]. The depths of these EDWs vary from 15 to 55 m, with many around 30 m deep. The heat exchange processes carried out by these EDWs are primarily used for heating and cooling indoor spaces.

The heat exchange system in an EDW includes the HEPs embedded within the underground continuous wall (primary circuit) and the piping used for heating/cooling the indoor space (secondary circuit) [37]. These two pipeline circuit systems are connected by a heat pump which controls the operation mode and output power of the heat exchange system. The HEPs are bound to the reinforcement cage and buried within the underground continuous wall, with circulating fluid running inside each pipeline. The primary circuit is divided into two parts based on the boundary conditions of the wall side of the underground continuous wall: one part of the wall is fully embedded in soil, while the other part of the wall only contacts soil on one side. The thermal performance of the fully embedded wall part is mainly influenced by the soil’s coefficient of thermal conductivity, the condition of the groundwater, and the wall/soil contact surface conditions. The other part of the heat exchange circuit is affected not only by the soil side but also by the thermal conditions of the indoor space on the excavation side, which impacts the circuit's heat exchange efficiency. The operation of the secondary circuit varies according to the indoor heating and cooling demand; in winter, the heat pump absorbs geothermal energy obtained from the ground through the primary circuit to heat the indoor space, while in summer, it absorbs the heat from the indoor space and stores it underground through the heat pump and primary circuit [38–40]. The working principle of the EDW comprehensive system is illustrated in Fig.3.

Geothermal energy reserves are abundant, and the heat underground can be conducted and convected through mediums such as soil and groundwater, resulting in a relatively stable temperature condition around the EDW [41,42]. Moreover, the seasonal heating and cooling demands of building interior spaces can maintain a relative balance of energy underground. The primary circuit, secondary circuit, and heat pump in the heat exchange system act as heat transfer intermediaries, providing a connective pathway for thermal transfer between the building’s interior spaces and the subterranean environment. Consequently, the heat exchange system of the EDW is based on the principle of heat transfer from high-temperature regions to low-temperature regions, successfully utilizing shallow geothermal energy for heating and cooling the building’s interior spaces.

EDW technology exhibits significant advantages in terms of energy, economy, and efficiency. Energy-wise, geothermal energy is a renewable resource, boasting abundant reserves, high stability, and energy security [43]. The heat exchange system uses geothermal energy for heating and cooling, helping to reduce carbon dioxide emissions and promoting sustainable energy development. Economically, the EDW operates by consuming only a minimal amount of electrical energy to circulate the fluid within the pipes, saving a considerable amount of electricity compared to air source heat pumps and significantly reducing the cost of building heating and cooling [44–47]. Although the initial capital investment for EDW heat exchange systems is substantial, potential government subsidies and tax incentives can reduce the fund input, and the long-term operational costs are much lower than those of traditional heating and cooling systems. In terms of efficiency, the EDW serves as a building foundation and basement sidewall, not occupying additional underground space. The heat exchange system transfers underground thermal energy directly for heating and cooling, minimizing energy consumption in the intermediate process and resulting in a high energy utilization rate. Presently, numerous scholars have researched the heat exchange efficiency, thermodynamic behavior, and design methods of EDW [34]. Some countries have already successfully constructed actual EDW projects, and several planned projects are in the works. Overall, EDW technology provides buildings with an energy-saving, efficient, and sustainable energy solution, and it demonstrates tremendous potential for economic benefits, energy efficiency, and environmental impact. With continuous technological development and improvement, as well as the support of sustainable development needs and carbon reduction policies from various nations, it is expected that this technology will gain broader application.

3 Projects

The first case of an EDW with comprehensive material documentation was constructed in Switzerland in the 1990s [12]. Subsequently, countries such as Austria, the UK, China, and Italy also saw the application of EDW projects [48,49]. Among these, Austria and the UK have a relatively higher number of construction projects and research. At present, the installation of this type of GSHP primarily targets the diaphragm walls around special building basements and subway stations. As the technology is still immature and requires significant upfront investment, it has not been widely applied in engineering projects. Given the limited number of actual EDW projects, it is essential to review and summarize the completed projects to provide valuable experience and technical support for subsequent design and construction. The following will compile a summary of the EDW projects, as shown in Tab.1, and systematically introduce and evaluate these project cases.

Austria constructed several EDW projects over nearly ten years from around 1997 to 2007 [68]. The applications of these EDWs included enclosure structures for deep foundations of large buildings and subway station perimeter walls. The Arts Centre in Bregenz, Austria, was the second engineering case in history to employ an EDW [12]. The building covers an area of 33500 m², with the depth of foundation pit excavation reaching 11 m. The retaining structure primarily employs a 28-m-deep diaphragm wall. A total of 24000 m of polyethylene piping was embedded within the walls to serve as the primary loop for a GSHP, providing heating and cooling for the building’s interior spaces. The art center’s collection has specific requirements for temperature and humidity control in its storage environment. The use of EDWs to harness shallow geothermal energy for heating/cooling offers environmental and economic benefits, presenting a new solution for the design of buildings with high energy demands. The Uniqa Tower, located in the center of Vienna, is an office building with a European Union green building label [13]. The building has a five-story basement, enclosed by 35 m deep diaphragm walls. Embedded within 7800 m² of the diaphragm wall are HEPs, which can meet one-third of the building’s energy needs. In addition to embedding HEPs within the deep foundations of the aforementioned buildings for heating and cooling using geothermal energy. The extension of Vienna’s U2 subway line utilized GSHP technology at its four stations, representing the first comprehensive application of this technology in global subway projects. Among them, the U2 Taborstraße station included a geothermal cooling system using an EDW of 1865 m² [12]. This system is capable of utilizing the substantial waste heat produced by subway operations or transferring it into the soil via HEPs. In 2006, Lot LT44 of the Lainzer tunnel, located near Vienna, began the construction of an energy plant with the support of the Vienna municipal authorities [13]. During the design phase, technicians determined the optimal spacing for HEPs in the EDW based on the cost of extracting geothermal energy at €25/MWh. This optimal spacing was only suitable for the geometric, economic, and thermal boundary conditions of Lot LT44. However, this method could provide a viable route for the optimization of other EGS.

After about a decade of exploration in Austria, the UK successfully applied the EDW technology to the construction of the Bulgari Hotel in 2012 [55]. Subsequently, the technology was applied to several important projects. The Bulgari Hotel project in Knightsbridge, London, was the first in the UK to use GSHP technology in conjunction with diaphragm walls [54,56]. Construction commenced in 2009, and by 2010 the installation of HEPs within the first continuous wall was completed, leading to the hotel’s smooth opening in 2012. The EDWs of the Bulgari hotel are embedded to a depth of 36 m and has a width of 0.8 m. As almost two-thirds of the diaphragm wall is exposed to the basement, the heat transfer before and after the excavation of the basement is reduced. The EDW at Dean Street Station was the UK’s second project of this kind [58,59]. The EDW’s depth reached 41 m, and its width was 1 m, with HEPs bound to the reinforcing cage. When operational, the heat exchange system could provide a heating capacity ranging from 0.2 to 1 MW for station space heating and water heating. As part of the UK’s largest civil engineering project, the Crossrail Project employed EDW technology for heating and cooling at Tottenham Court Road Station in London [56]. Additionally, the Moorgate shaft project, which commenced service in London in 2022, also incorporated this green low-carbon EDW technology [62].

In addition to the successful application of EDW technology in actual projects in Austria and the UK, there are also related engineering cases in countries such as China and Italy. The Shanghai Museum of Natural History, located at Jing’an Garden in Shanghai, China, has a construction area of 12029 m² and an excavation depth for its foundation pit of 18.5 m [63,64]. To meet energy and low-carbon requirements, the building incorporates HEPs within the diaphragm wall around the foundation pit, utilizing shallow geothermal energy through the circulating fluid in the pipes for heating and cooling. Subsequently, on-site tests indicated that when the HEPs are arranged optimally, the heat transfer performance of the EDWs can be improved by 40%. In the center of Tradate in the province of Varese in Northern Italy, a zero-energy residential building was constructed with three underground levels, down to a depth of 10.8 m [65–67]. The foundation pit was surrounded by EDWs that served a dual purpose, with a wall thickness of 0.5 m and an embedded depth of 15.2 m. The HEPs were tied to the steel reinforcement cages on the soil side, at a distance of 5 to 8 cm from the soil surface. The part of the diaphragm wall that is fully embedded underground accounts for only one-third of the wall height. Operational results indicate that when a large portion of the EDW is exposed to the basement space on one side, it has a significant impact on the heat exchange performance of the energy system. Therefore, the impact caused by such factors requires further in-depth research.

4 Experiments

The diaphragm walls equipped with HEPs not only serve as foundations for the structures above and enclosures for basements but also provide heating/cooling for adjacent buildings through heat exchange [18]. These factors result in an exceptionally complex working environment for the EDWs, operating under the coupled action of thermo-hydro-mechanical. Experimental studies can reflect the actual service environment of the structures more accurately and can effectively reveal the underlying mechanisms of interaction. To date, several scholars have conducted experimental research on the thermal efficiency, thermodynamic behavior, as well as analytical and design methods of EDWs.

4.1 Heat transfer efficiency and its influencing factors

For EGS, heat exchange efficiency is one of the focal points of interest [69]. A deeper understanding of the factors influencing the heat exchange efficiency of EDWs and their patterns can provide a basis for the rational design of these structures, thereby improving the economic benefits of heating/cooling. At present, many scholars have studied the heat exchange efficiency of EDWs and their influencing factors through experimental methods. The following is a systematic introduction and assessment of these works, with the related research content compiled in Tab.2.

Xia et al. [63] conducted four sets of experiments leveraging the Shanghai Natural Museum project to study the effects of HE types (W-type and single U-type) for EDWs, water flow velocities (eight different speeds ranging from 0.25 to 1.5 m/s), inlet water temperatures (32.0, 35.0, and 38.0°C), and operational modes (intermittent or continuous) on the heat transfer performance of the structure. Kürten et al. [70] carried out a series of indoor experiments to investigate the impact of pipeline layout type, fluid velocity within the pipes, groundwater flow, ground temperature, and soil’s coefficient of thermal conductivity on the heat exchange efficiency of the EDWs. In addition, they compared the experimental results with calculations from a new model proposed in this paper to validate the model’s effectiveness. Sterpi et al. [67] studied the heat transfer process, thermal boundary conditions, and thermal response patterns of this energy system through full-scale structural field tests, finding that the thermal conditions of basements negatively affect the energy performance of the structure. Sterpi et al. [65] conducted full-scale field monitoring tests on a six-story residential building in Italy, measuring the inlet and outlet fluid temperatures and the corresponding circulation flow rates of the diaphragm wall to study the effects of pipeline layout types, the ratio of exposed to fully immersed parts of the wall, and the thermal boundary conditions on the excavation side on the heat exchange efficiency of the EDW. To understand the energy efficiency and heat transfer processes of the geothermal system, Angelotti and Sterpi [66] used a comprehensive data acquisition system to monitor inlet and outlet fluid temperatures and flow rates, heat pump power consumption, and long-term temperature changes in the wall and stratum, studying the impact of thermal boundary conditions and seasonal temperature variations on the EDW. The coefficient of thermal conductivity of concrete structures is one of the main factors affecting the heat exchange efficiency of EDWs. Elkezza et al. [71] designed and manufactured a set of experimental apparatus for large-scale model tests targeting EDWs made with graphite-enhanced thermally conductive concrete. They studied the heat exchange efficiency, stiffness, thermal expansion coefficient, and wall-soil interaction of this type of EDW. The research indicates that the use of thermally enhanced concrete pouring EDWs significantly improves heat exchange efficiency compared to typical conventional concrete, without adversely affecting stiffness and compressive strength.

4.2 Thermodynamic response and its influencing factors

The diaphragm wall with the HE not only has energy supply requirements but also needs to meet the structural mechanical performance to ensure the safe use of the overall structure. Therefore, the thermodynamic response of the EDW is another key issue under study. Currently, only a few scholars have conducted experimental research on the thermodynamic response of the EDW and its influencing factors through experimental methods.

Dong et al. [72] conducted indoor experiments on the thermo-mechanical issues of EDWs, using stress, strain, and temperature sensors to detect behavior under thermal loads, and studied the mechanical behavior of the structure and surrounding soil under cyclic temperature effects. The study found that temperature increases lead to an increase in axial strain of the wall and soil pressure at the wall-soil interface; additionally, uneven thermal expansion of the wall causes bending moment. You et al. [73] used laboratory centrifuge test methods to monitor the changes in temperature, deformation, and soil pressure of the wall, studying the thermodynamic response of EDWs buried in sandy foundations under thermo-mechanical coupling conditions, and their interaction with the soil under heating conditions. The study found that the stress generated by temperature changes in the wall is greater than that caused by excavation, and thermal stress is unevenly distributed within the wall.

4.3 Analysis and design methods

The engineering application of EDWs is still in its early stages, and the design and construction of the actual project have an exploratory nature [74]. To accurately predict and optimize the thermal performance of EDWs, it is necessary to improve the analysis and design methods of this structure and provide technical support for the promotion and application of this technology. Currently, some scholars have proposed analysis and design methods suitable for EDWs based on experimental verification. The following will systematically introduce and evaluate these methods, and summarize the related research work in Tab.3.

Sun et al. [64] established a two-dimensional heat transfer model for the semi-buried and fully buried sections of the EDW. The model was then analyzed and solved to propose the method for calculating hourly heat exchange in the heat exchange system and the design procedure of the heat exchange system. Relying on the EDW heat exchange system of the Shanghai Natural Museum, on-site measured data was compared with the analytical solution to validate the reliability of the model’s calculation method. However, the model has assumptions such as constant soil temperature at any position and zero thermal contact resistance at each interface. Therefore, this design method has limitations.

The thermal resistance model is commonly used for the simplified calculation of borehole HEs [78,79], but it cannot be directly applied to EDWs due to the lack of rotational symmetry. Kürten et al. [70,75] considered the resistance of the structure itself and the pipelines, developed a thermal resistance model for designing EDWs, and implemented it into the finite difference program SHEMAT-Suite [80]. In the thermal resistance model, T₁, T₂, T₃, and T_F represent the temperatures of the ground, adjacent rooms, external surface of the pipeline, and fluid inside the pipe, respectively, and R_a, R_b, R_c, and R_p represent the thermal resistances between the corresponding regions. The thermal resistance model can be represented using the triangular connections, or it can be transformed into star-shaped connections suitable for numerical simulations. By comparing with indoor experiments and COMSOL Multiphysics numerical simulations, the effectiveness of this new semi-analytical design method has been validated. This method offers significant time efficiency compared to fully discrete numerical simulations and allows for consideration of all parameters of the heat exchange system and structural components in the design process. Using this method to analyze the parameters, it has been found that temperature difference, fluid flow rate inside the pipeline, groundwater flow rate, pipe cover thickness, and pipeline layout type significantly affect the heat exchange efficiency of the structure. In further research, the influence of groundwater flow rate and seasonal temperature fluctuations near the surface should be considered in this model.

Shafagh et al. [76] considered the influence of pipe wall geometry, boundary conditions, and heterogeneous materials, and developed a dynamic thermal network model with three boundary interfaces. The basic parameters of this network model include temperature (T_i(t)), flux (Q_i(t)), coefficient of thermal conductivity on the admittive path (K_i), and constant coefficient of thermal conductivity on the transmittive path (K_ij), defining three boundary interfaces for the EDW system, namely the pipe system, ground, and associated building basement. The dynamic thermal network is modeled using relatively simple coding and can be implemented through independent applications or the TRNSYS simulation environment. By conducting on-site experiments with a self-developed heat response test system on two full-size EDWs in Barcelona, Spain, the effectiveness of the model was verified. The research indicates that the model can predict the thermodynamic response of the EDWs under a range of conditions with high accuracy. Furthermore, compared to traditional finite volume or finite element modeling methods, the dynamic thermal network model significantly improves computational efficiency.

Lee et al. [77] proposed a new structural form of embedding geothermal exchangers in diaphragm walls composed of steel pipe cast-in-place piles. The applicability of this structural form was verified through in situ tests, and a complementary CFD model was developed using COMSOL Multiphysics. By establishing the CFD model, a design method for diaphragm walls composed of energy cast-in-place piles with steel pipes was provided and compared with additional CFD models. However, further field tests involving at least three energy cast-in-place pile units are necessary to validate the design equation of this new EDW.

4.4 Test device and method

Conducting indoor or on-site experiments is an important means of studying EDW systems, which can comprehensively simulate the actual operation condition and complex interactions of structures, obtaining parameters and performance more in line with real conditions. Currently, although some scholars have conducted experimental research, the experimental devices and methods are still very unified and cannot yet meet the needs of multi-scale, multi-physics fields, and complex working conditions. Below is a summary of relevant indoor experimental platforms and on-site testing devices, as shown in Tab.4. Subsequently, an introduction and evaluation of these experimental devices and methods will be provided to serve as a reference for future experimental research and equipment development.

To verify the new thermal resistance model, Kürten et al. [70] constructed an indoor experimental platform and carried out a series of experiments. The overall dimensions of the experimental setup were 3 m × 3 m × 2 m, consisting of independent internal and external circulation systems. This setup allowed control over parameters such as the inlet temperature and flow rate of the HEPs, ground temperature, and groundwater flow rate. Two different pipeline configurations (U-shape, W-shape) were alternately arranged to reduce thermal interference between systems. To prevent the generation of dark currents that could weaken the impact of groundwater, the direction of groundwater flow was kept parallel to the wall surface. Subsequently, measurements were taken for outlet temperatures, as well as the coefficient of thermal conductivity of the soil and concrete. Additionally, Kürten et al. [75] conducted thermal response tests on EDWs using the same experimental setup. The setup was surrounded by insulating materials to prevent heat loss. Different HEP structures and groundwater flow conditions were used in the experiments, maintaining constant inlet temperatures and flow rates while measuring parameters such as outlet temperatures, output energy, and soil temperatures. However, issues such as temperature sensors not being sensitive enough, low liquid flow rates within the pipes, and thermal interactions between the pipelines exist in the experimental setup, indicating the need for further improvement in subsequent research on the experimental setup and methods. To reveal the thermodynamic response of EDWs and the surrounding soil, Dong et al. [72] conducted indoor model experiments using self-developed equipment. HEPs were installed in a concrete continuous wall (2 m × 1.8 m × 0.2 m) placed in a cubic steel box filled with uniform dry sand, with the temperature around the box controlled at (10 ± 2) °C. The wall was heated for 75 h with 50 °C circulating water at a flow rate of 0.03 m³/h. Temperatures at various locations on the wall and soil were measured using sensors, as well as soil pressure and wall strain at the soil/wall interface. You et al. [73] placed a small-scale model of an EDW in a centrifuge to simulate the lateral pressure it would experience under actual working conditions. The cantilevered diaphragm wall was made of copper, with the centrifuge acceleration controlled within 50 g. The experimental setup used strain gauges to monitor the vertical deformation of the wall, soil pressure sensors on both sides of the wall to monitor soil pressure, and thermocouple temperature sensors to measure the temperature of the model and the environment. Additionally, a camera was used to monitor soil movement. To evaluate the thermal performance of thermally enhanced concrete EDWs and the thermal interaction between the wall and soil, Elkezza et al. [71] conducted indoor experiments using a self-made fully instrumented test rig (1 m × 1 m × 1 m). The test rig consisted of a testing tank, a geo-energy structure, and a data acquisition device. The side walls of the setup were insulated to reduce the influence of ambient temperatures. A drainage system was installed at the bottom of the setup to evenly apply groundwater, and a peristaltic pump was used to control the liquid flow rate within the pipes at 67 L/h. T-type thermocouples were used in the experiments to monitor the inlet and outlet temperatures of the HEPs, concrete temperatures, soil temperatures, and ambient temperatures. Additionally, the lateral earth pressure on the wall was monitored using pre-placed earth pressure cells.

Located in the north-west of Italy, a residential building’s diaphragm walls and bottom plates are all equipped with HEPs. Since the geothermal system was activated, Sterpi et al. [65,67] have been continuously monitoring it through a comprehensive sensing and data collection system. A large number of PT100 platinum resistance thermometers are placed on the steel cages of the diaphragm walls at different depths on both sides and on the anchor rods to monitor temperature changes. A flow meter integrated into the circulation pump monitors the fluid flow rate, while temperature sensors installed at the inlet and outlet of the HEPs monitor the fluid temperature. The data acquisition and management system are integrated into specialized software developed by a company in Italy [81]. However, due to only having monitoring data for the first two years of the EDW system, it cannot reflect its long-term operational status. Angelotti and Sterpi [66] conducted further testing and research on the zero-energy residential building using a comprehensive data collection system. The experiment monitored the temperature and flow rate of the pipeline inlets and outlets, as well as the heat pump electricity consumption, and used temperature sensors to measure the temperatures on both sides of the walls and at different positions of the anchor rods. Long-term temperature monitoring of the EDWs and the surrounding soil revealed any deviations caused by the unbalanced heating and cooling demands. However, due to various influencing factors affecting the monitoring data, the thermal properties of the soil cannot be accurately determined, resulting in uncertainties in the research findings. To validate the newly developed dynamic thermal networks model, Shafagh et al. [76] used a self-developed thermal response test system to conduct a series of on-site experiments on two EDWs in Barcelona, Spain. The experimental setup consisted of a water tank with electric heaters, circulating pumps, instrumentation, and data recording devices. The experiments monitored the temperature and flow rates at the inlet and outlet of the HE. To test the thermal performance of the new type of EDW, Lee et al. [77] set up a test bed on the engineering site. The testing apparatus consists of data logger equipment, data monitoring devices, and a constant-temperature water bath. The fluid inlet temperature inside the HEP is maintained at 5 °C using the constant-temperature water bath, with a flow rate controlled at 11.2 L/min. T-type thermocouples are used to monitor the fluid temperatures at the inlet and outlet of the HEP every 2 min.

5 Numerical simulation

In addition to conducting experimental research on EDWs, numerical simulation can carry out a series of targeted analyses in a relatively short period. Due to the lack of actual engineering cases and the complex structural characteristics of EDWs, numerical simulation is currently the main approach for studying them. Many scholars have conducted relevant analytical research on EDWs.

5.1 Heat transfer efficiency and its influencing factors

Currently, many scholars have conducted numerical simulation studies on the heat transfer efficiency and influencing factors of EDWs. The following is a systematic introduction and evaluation of these works, with relevant research content summarized in Tab.5.

di Donna et al. [62] combined numerical computations with statistical analysis to study the influence of various structural parameters of EDWs on heat transfer efficiency. The research revealed that reducing the spacing between pipes can temporarily enhance heat transfer efficiency, but the long-term impact is minimal. Moreover, the temperature surplus between the wall and excavation, as well as the coefficient of thermal conductivity of concrete, significantly affect heat transfer efficiency. Due to the insufficiently systematic consideration of parameters, this study is still unable to accurately and comprehensively evaluate the heat transfer efficiency of this structure. Rammal et al. [82] utilized the finite difference software FLAC3D to establish a two-dimensional numerical model, analyzing the impact of parameters such as groundwater flow rate, effective length of continuous walls, and types of thermal loads. The study indicated that groundwater flow and increasing the effective length of walls have a positive impact on heat transfer efficiency. Additionally, the type of thermal load directly affects the thermal performance of EDWs. Makasis and Narsilio [83] used experimentally validated three-dimensional numerical models to conduct multiple parameter analyses on EDWs [88], studying the effects of pipe layout, pipe spacing, ground thermal conductivity, and wall depth on thermal performance. The study found that smaller pipe spacing had little effect on thermal performance improvement but increased costs; the choice between horizontal or vertical pipe layouts depended on the wall dimensions. Makasis et al. [84] employed numerical simulation methods to investigate the impact of boundary conditions between soldier pile energy retaining walls and the air inside underground spaces on system heat transfer efficiency. The study indicated that the selection of these boundary conditions significantly influenced the system’s heat transfer efficiency. This research is also applicable to EDWs. Sterpi et al. [65] used the ABAQUS finite element software to calibrate the wall model with on-site test data, conducted a series of numerical analyses, evaluated the influence of excavation boundary conditions and thermal-physical properties of system materials, and proposed two pipeline layout types that help improve heat transfer performance. Angelotti and Sterpi [66] performed three-dimensional EDW numerical simulations using the ABAQUS finite element software, studying the effects of thermal boundary conditions around the wall and seasonal temperature variations on EGS. The research indicates that the temperature conditions in the basement have an adverse effect on heat transfer in geothermal systems. Furthermore, setting the wall boundary conditions reasonably is crucial for correctly simulating the heat transfer process. Zeng et al. [85] established a three-dimensional finite element model using COMSOL Multiphysics software, considering the influences of wall type, pipelines, groundwater, and indoor temperature, and studied the thermal performance of EDWs adjacent to air-conditioned spaces.

5.2 Thermodynamic response and its influencing factors

The thermodynamic response of EDWs is one of the key issues under study. Currently, many scholars have researched the thermodynamic response of EDWs and their influencing factors through numerical simulation methods. The following provides a systematic introduction and evaluation of these works, summarizing the relevant research content in Tab.6.

Bourne-Webb et al. [89] used ABAQUS finite element software to conduct numerical simulation analysis, studying the influence of the thermal characteristics of wall-cavity surfaces on the thermodynamic response of EDWs. The study found that the mechanical behavior of EDWs is mainly influenced by seasonal temperature variations, with a relatively small impact from heat transfer. However, in this model, the temperature distribution of the entire wall system is relatively uniform, therefore no additional internal forces are generated, which differs somewhat from the actual situation. To reveal the energy performance and thermal effects of EDWs on the structural behavior, Sterpi et al. [90] carried out a numerical analysis considering the long-term operation of a geothermal system and studied the impact of circulating temperature change on the mechanical behavior of EDW and the wall-soil interaction. Dong et al. [72] performed finite element analysis using ANSYS software to investigate the mechanical behavior of EDWs and surrounding soil under cyclic temperature effects. The study found that temperature increases lead to thermal expansion of the wall, thereby increasing the lateral soil pressure at the wall-soil interface. To explore the influence law of thermodynamic parameters of EDWs, Rui and Yin [54] carried out a series of thermo-hydro-mechanical coupled finite element analyses. They studied the variations of soil permeability coefficient, soil heat transfer coefficient, and concrete thermal expansion coefficient under thermal and mechanical loads, as well as the impact of these coefficient variations on the wall-soil interaction. The study found that the mechanical response of the wall is mainly affected by the thermal expansion and contraction of concrete, with minor effects from changes in soil and water properties. Zhou et al. [57] used the COMSOL Multiphysics finite element software to establish a three-dimensional numerical model for studying the thermally-induced mechanical behavior in deep EDWs. They conducted parametric analyses over long periods on thermal load patterns, buried pipe locations, ambient underground space temperatures, and excavation levels. The research findings indicate that the elastoplastic effects of soil have a significant impact on the thermally induced mechanical properties of EDWs.

5.3 Analysis and design methods

The energy supply of the HE is a key parameter for designing the EDW, which can be used to evaluate the thermal performance of the structure, thereby providing a geothermal design that meets the project requirements. Currently, some scholars have proposed analysis and design methods suitable for this dual-purpose foundation by combining numerical analysis methods. The following will systematically introduce and evaluate these methods, and summarize the relevant research work in Tab.7.

To reduce the simulation time and cost required for thermal analysis of EDWs, Makasis et al. [92] proposed a model that predicts the temperature changes of fluid inside pipelines based on the heat provided by the system. The use of this model in conjunction with numerical simulations can significantly reduce the time needed for designing EDWs and quickly determine the energy supply for HEs.

The heat transfer between the EDW and the surrounding media can be estimated using Eq. (1) [94,95].

(1)Q_{\mathrm{t}\mathrm{o}\mathrm{t}\mathrm{a}\mathrm{l}}=Q_{\mathrm{i}\mathrm{n}}-Q_{\mathrm{o}\mathrm{u}\mathrm{t}}=m_{\mathrm{f}}\cdot C_{\mathrm{f}}\cdot \left(T_{\mathrm{i}\mathrm{n}}-T_{\mathrm{o}\mathrm{u}\mathrm{t}}\right),

where Q_in and Q_out are the heat exchange capacity per unit time at the inlet and outlet respectively, m_f and C_f are the mass flow rate and the specific heat capacity of the fluid in the pipe respectively, and T_in and T_out are the inlet and outlet pipe temperatures, respectively.

The above equation considers the effect of the fluid inside the pipe but does not assess the impact of surrounding soil and groundwater on heat exchange. To more comprehensively evaluate the thermal performance of EDWs, Rammal et al. [82] proposed a new method for estimating the heat exchange power between EDWs and surrounding soil, with the calculation expressions shown in Eqs. (2) and (3).

(2){\overrightarrow{P}}_{\mathrm{c}}={\overrightarrow{j}}_{\mathrm{c}\mathrm{o}\mathrm{n}\mathrm{d}}=-{\lambda }_{\mathrm{S}}\cdot \overrightarrow{grand}T,

(3){\overrightarrow{P}}_{\mathrm{v}}={\overrightarrow{j}}_{\mathrm{a}\mathrm{d}\mathrm{v}}={\rho }_{\mathrm{w}}\cdot C_{\mathrm{w}}\cdot {\overrightarrow{q}}_{\mathrm{w}}\cdot T,

where {\overrightarrow{q}}_{\mathrm{w}} is the vector representing the fluid-specific discharge that stands here for the velocity of groundwater flow (m/s), {\overrightarrow{j}}_{\mathrm{c}\mathrm{o}\mathrm{n}\mathrm{d}} and {\overrightarrow{j}}_{\mathrm{a}\mathrm{d}\mathrm{v}} are the vectors representing the conductive and advective terms of heat exchange.

Using the above calculation method to establish a two-dimensional numerical model in FLAC3D software, analyze the influence low of parameters such as groundwater flow, effective length of diaphragm walls, and types of thermal loads. Studies have shown that groundwater flow and the activation of longer walls have a positive impact on heat exchange. In addition, the type of thermal load directly affects the thermal performance of EDWs.

di Donna et al. [93] considered the effects of ground temperature, coefficient of thermal conductivity, and groundwater flow rate. They used the FEFLOW© finite element software [62] to establish a thermo-hydro coupling analysis model and developed energy capacity charts for the design phase of EDWs. This method can estimate the heat that can be extracted and injected by EDWs without the need for comprehensive numerical simulations. The study using this method found that groundwater and temperature differences in the structure are important parameters to consider in the design of EGS. In addition, the excavation boundary conditions have a significant impact on the energy performance of the structure.

5.4 Multi-field coupling influence analysis

The heat exchange system of EDWs involves the interaction and effects of multiple physical fields during operation, including pressure, deformation, and heat transfer between the diaphragm walls, soil, pore water, and excavated side air. The principle of multi-field coupled analysis of EDWs is shown in Fig.4.

Conducting multi-physics field coupling analysis can comprehensively reveal the interaction mechanisms and mechanical responses of various parts of the system, providing a theoretical basis for the structural design and thermal performance analysis of EDWs. Currently, many research works have used numerical simulation methods to carry out thermo-mechanical coupling, thermo-hydro coupling, and thermo-hydro-mechanical coupling analyses of EDWs. The following will systematically introduce and evaluate these studies, and the relevant research content is summarized in Tab.8.

Some scholars use the method of thermo-mechanical coupling numerical analysis to study the mechanical response of EDWs. Coletto and Sterpi [96] by establishing a thermo-mechanical coupling numerical analysis model, studied the influence of heat transfer on soil temperature, internal actions of the wall, and soil-structure interaction of EGS. The study shows that additional thermal loads have little impact on the overall stability and safety of the structure, but the development of internal special actions caused by thermal loads should be considered in the optimization design. Sterpi et al. [90] studied the influence of the heat transfer process on the mechanical behavior and soil-structure interaction of EDW through three-dimensional thermo-mechanical coupling finite element analysis. It was found that factors affecting the heat transfer process will affect the temperature changes of the structure and soil, thereby influencing the mechanical behavior of EDWs. Barla et al. [91] used the finite difference software FLAC to study the effect of cyclic temperature changes on the bending moments and displacements at different locations of an EDW using a two-dimensional plane strain thermo-mechanical coupled analysis method. The study found that the bending moments and horizontal displacements at the top of the diaphragm wall increased by 16% due to thermal activation, while the horizontal displacements at the bottom and baseplate, generated by constraints, could be neglected.

To investigate the impact of the groundwater effects caused by the cyclic temperature on the EDWs, some scholars have used the method of thermal-water coupled numerical analysis to study the energy performance of EDWs. Barla et al. [91] used the method of thermo-hydro coupled numerical analysis to study the influence of the surrounding soil response caused by cyclic temperature on the heat exchange efficiency of EDWs. The study found that favorable groundwater flow conditions can significantly improve heat exchange efficiency. In addition, the temperature variation of groundwater flow has an impact on the surrounding ground within 7 °C. Based on the consideration of hydraulic, coefficient of thermal conductivity, groundwater temperature, and flow rate, di Donna et al. [93] established a thermo-hydro coupled finite element analysis model and proposed an energy capacity chart for the design phase of EDWs. The study indicates that the temperature difference between groundwater and structure is an important parameter to consider in the design of EGS. Additionally, the excavation boundary conditions have a significant impact on the energy performance of the structure.

Some scholars have adopted the method of thermo-hydro-mechanical coupling numerical analysis to comprehensively analyze the performance and influencing factors of EDWs. Rui and Yin [58] based on the EDW project set up at a London subway station, conducted a series of thermo-hydro-mechanical coupling numerical analyses using internally developed finite element code, studying the short-term and long-term effects of the operation of GSHP systems on the mechanical properties of walls and soil, as well as the interaction between walls and soil. The study found that the thermal expansion of the wall concrete significantly affects its mechanical properties, and the thermodynamic effects of the soil have little influence on the structural performance. Yin and Rui [97] adopted a thermo-hydro-mechanical coupling model to conduct extensive finite element analysis on the EDW, to study the impact of a GSHP on the mechanical properties of a diaphragm wall of a certain subway station in London. The research found that due to the imbalance of soil heat extraction in winter and heat storage in summer, surface subsidence caused by soil temperature decrease and shrinkage still occurred after 20 years of system operation. Additionally, the change in bending moments within the wall was primarily caused by the temperature difference on either side of the wall. To assess the long-term mechanical performance of EDWs in stiff clays (including the structural behavior of the walls and the response of the surrounding ground), Dai and Li [59] conducted a coupled thermo-hydro-mechanical finite element analysis based on a real engineering case, taking into account changes in air temperature, soil temperature, and seasonal geothermal effects. The results of the study indicate that the geothermal system has an impact on the long-term mechanical performance of the EDWs, but does not lead to serious safety issues. Sailer et al. [98] carried out a series of coupled thermo-hydro-mechanical finite element simulations on EDWs to analyze and determine the complex and highly nonlinear interaction mechanisms between the wall and the soil. The research found that under cyclic temperature conditions, the mechanical behavior of the wall is highly transient. Additionally, changes in thermally induced pore water pressure significantly affect the stress state of the wall. To investigate the influence law of thermodynamic parameters of EDWs, Rui and Yin [54] conducted a series of thermo-hydro-mechanical coupled finite element analyses. They studied the variations in soil permeability coefficient, soil heat transfer coefficient, and concrete thermal expansion coefficient under thermal and mechanical loads, as well as the impact of these coefficient changes on the interaction between the wall and soil. To reveal the complex thermo-hydro-mechanical coupling phenomena in soil and the interaction mechanism with EDWs, Sailer and others [100] used the Imperial College finite element program to perform fully coupled simulations of thermo-hydro-mechanical behavior. The research found that thermal expansion of the soil, volumetric deformation caused by pore water, and mechanical interactions are the main factors affecting the structural response of the wall and that this structural response is highly transient. Therefore, when designing EDWs, it is essential to thoroughly assess the hydraulic and thermal parameters of the soil.

6 Conclusions and prospect

6.1 Conclusions

This paper systematically summarizes the research progress and challenges in exploiting shallow geothermal energy using the technology of EDWs and provides directions for further research. It mainly focuses on the engineering, experimental, and numerical research work related to EDWs, offering a comprehensive review of the research progress of heat exchange efficiency, thermodynamic behavior, analysis/design methods, and multi-field coupling aspects. EDWs have significant advantages in energy transformation, energy conservation and emission reduction, and foundational enclosure fields. Studying this technology can provide buildings with an energy-saving, efficient, and sustainable energy solution. The review work in this paper contributes positively to the development and engineering application and promotion of EDW technology.

Upon reviewing the achievements of previous studies, some main conclusions have been drawn.

1) In the field of engineering, technical personnel can design the optimal spacing between HEPs for EDWs based on the anticipated extraction price of geothermal energy. At this point, the heat exchange efficiency of the EDWs can be enhanced by 40%.

2) In terms of experiments, the stress caused by temperature changes in the wall is greater than the stress caused by excavation, and thermal stress is distributed unevenly within the wall, thereby generating bending moments. The thermal conditions of the basement have a negative impact on the energy performance of EDWs. Moreover, enhancing the heat transfer capability of concrete can improve its heat exchange efficiency.

3) In terms of numerical simulation, reducing the spacing between pipes can temporarily enhance heat exchange efficiency, but the long-term impact is minimal and it also increases costs. The mechanical behavior of EDWs is primarily influenced by seasonal temperature variations that cause thermal expansion and contraction of the concrete. Beneficial groundwater flow conditions can significantly improve heat exchange efficiency; therefore, when designing EDWs, it is essential to thoroughly assess the soil’s hydraulic and thermal parameters.

6.2 Prospect

It is noteworthy that the scope of this review is constrained by the timeliness and accessibility of existing published literature. The field of EDWs is rapidly evolving, with new technologies and projects constantly emerging. There is a significant gap between research results, published data, and practical applications. The factors influencing EDWs are complex and subject to the coupling effects of multiple physical fields. Although the long-term operational costs of EDWs are significantly lower than those of conventional heating and cooling systems, the initial construction investment is substantial, which to some extent hinders the engineering promotion of this technology. Therefore, it is necessary to further enhance the heat exchange efficiency of EDWs and provide reasonable analysis and design methods.

In summary, the EDW is an energy-saving, economical, efficient, and sustainable engineering technology, with broad application prospects and development potential. However, current research on EDWs is still insufficient, facing numerous challenges in theory, experimentation, numerical studies, engineering practice, and materials. Based on extensive literature review work, directions for further research in the field of EDWs are predicted, with the main directions listed as follows.

1) Establish a more systematic and precise theoretical analysis model, considerations may include thermal conduction, boundary conditions, groundwater, near-surface seasonal temperatures, and adjacent spatial conditions. This provides a theoretical foundation for the structural design and analysis of EDWs.

2) Conduct multi-scale, multi-physical field indoor model tests. Investigate the meso- and macro-scale thermodynamic response of EDWs under the coupled effects of thermo, hydro, and mechanical fields.

3) Conduct a systematic parametric evaluation and analysis of the performance of EDWs, taking into account parameters such as coefficient of thermal conductivity, heat capacity, thermal resistance, temperature control, and flow control to enhance energy utilization efficiency.

4) Relying on the EDW project, carry out long-term behavioral monitoring. This can help better understand and evaluate the long-term performance of EDWs, thereby laying a foundation for their widespread engineering applications and promotion.

5) Research materials with a high coefficient of thermal conductivity and strong durability to enhance the heat exchange efficiency and structural reliability of the EDW system.

6) Investigating the deterioration mechanisms and durability of EDW under multiscale and multi-physical field coupling. This is one of the key factors currently restricting the promotion of EDWs.

7) Develop more optimized software design platforms, and establish integration methods for the EDW heat pump system with the building energy management system. Achieve dynamic regulation of heat exchange power, and optimize the operational state based on real-time data.

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