Capillary Heat Exchanger Research Articles

Seawater source heat pump system based on capillary heat exchanger for seepage in submarine tunnel: a case study

Abstract The discharge of seepage water from undersea tunnel structures, often treated as wastewater, inherently carries a substantial reservoir of untapped low-grade thermal energy. Unfortunately, comprehensive investigations into harnessing this latent potential remain notably limited. This study introduced an innovative strategy through the design of an undersea tunnel seepage seawater source heat pump system. Distinguished by the integration of a capillary front-end heat exchanger, this system aimed to effectively exploit the frequently disregarded low-grade thermal energy present in the seepage water of undersea tunnel structures. The seawater seepage from the tunnel is transported to the car park at the tunnel entrance, and a seawater energy pool is constructed by storing seawater in its underground space. The use of capillary network placed in the energy pool in the front heat exchanger, water source heat pump units, circulating water pumps and fan coil end device composed of underground undersea tunnel seepage seawater source heat pump system for the building heating and cooling. Furthermore, a comparative assessment was conducted, contrasting this novel system with the traditional air-conditioning setup that utilizes chillers and gas boilers as cooling and heating sources. The aim was to evaluate its capacity for energy conservation and emission reduction. The findings from the study strongly affirmed the viability of the proposed seepage seawater source heat pump system within undersea tunnels. It boasted the potential to achieve annual savings of 53.55 tce, highlighting a noteworthy energy-saving rate of 21.2%. Concurrently, reductions in CO2, SO2, and particulate emissions amounted to 132.28 t/a, 1.07 t/a, and 0.54 t/a, respectively. This study not only stands as a reference for the strategic utilization of seepage seawater from undersea tunnel structures, prioritizing energy conservation and emission reduction, but also pioneers innovative approaches toward resource optimization and environmental sustainability, meeting the inherent needs of carbon peaking and carbon neutrality goals.

Impact of groundwater seepage on thermal performance of capillary heat exchangers in subway tunnel lining

The metro tunnels are currently facing challenges associated with thermal pollution, which significantly affects their operational safety and energy consumption. The subway source heat pump system (SSHPS) with front-end tunnel lining capillary heat exchangers (CHEs) is an effective technology to address this problem. The heat transfer characteristics of SSHPS have been previously investigated; however, there is a lack of research on the thermal performance of tunnel lining CHEs under groundwater seepage. This study established a numerical simulation model of tunnel lining CHEs based on a demonstration project of SSHPS. Subsequently, the influence of groundwater seepage on the heat transfer characteristics of tunnel lining CHEs was comprehensively analyzed. The findings suggest that an increase in groundwater seepage velocity leads to the initial expansion and subsequent contraction of the thermal influence range caused by CHEs. Simultaneously, the performance of CHEs is enhanced with increasing flow velocity. When the seepage velocity increases from 0 to 1 × 10−4 m/s, the heat exchange capacity per meter and efficiency of the CHEs during the cooling season increase by 450 % and 446 %, respectively. During the heating season, they experience a growth of 320 % and 310 %, respectively. In addition, the heat exchange performance of CHEs is also influenced by the relative flow direction between the fluid within CHEs and the groundwater. The counter flow arrangement exhibits a 7.7 % higher efficiency of heat exchange during the cooling season compared to the parallel flow arrangement, and a 3.9 % higher efficiency of heat exchange during the heating season. Therefore, it is highly recommended to evaluate groundwater seepage conditions during the design phase of SSHPS in order to enhance the performance of tunnel lining CHEs. This study serves as a valuable theoretical reference for the design of tunnel lining CHEs.

Effect of laying length on dynamic heat transfer characteristics of capillary heat exchangers in urban subway tunnel lining

The subway tunnel is a typical semi-enclosed underground space, which poses challenges in dissipating the waste heat generated by subway operations and may result in thermal environment deterioration of urban underground spaces. However, by incorporating heat exchangers into the tunnel lining, the subway can be transformed into an “energy tunnel” that effectively addresses this issue. Capillary heat exchanger (CHE) is a common type of tunnel lining heat exchanger. Recent studies have investigated its heat transfer properties and influencing factors under the assumption of homogeneity, neglecting the variability of heat transfer properties along the CHE tube. The present study developed a numerical model of tunnel-lined CHE utilizing water as the circulating medium, based on a demonstration project in Qingdao. Subsequently, an analysis was conducted to investigate the evolution of heat transfer properties for different laying lengths. The simulation results indicate that as the laying length increases from 2 to 10 m, the fluctuation range of outlet temperature decreases significantly from 10.6 °C to a mere 0.4 °C. Moreover, there is a noticeable decrease in average heat transfer per unit length from 1.9 W/m to only 0.4 W/m; however, this is accompanied by an impressive increase in average heat transfer efficiency from 28 % up to an impressive level of around 61 %. Based on the aforementioned results, it is recommended that the optimal length range for a tunnel-lined CHE should fall within 4 to 6 m. The findings of this study can serve as a valuable theoretical reference for the practical engineering applications of CHEs in urban tunnel lining.

The Use of Renewable Energy and Capillary Heat Exchangers for Energy Savings in the Existing Apartment

Abstract The use of capillary heat exchangers with a large area means the lowest heat carrier temperature – typically less than 30 °C for heating. This determines the very efficient use of the installed heat pump due to an increase in the coefficient of performance (COP) with a decrease in the provided water temperature. The aim of the study was to find out whether renewable energy sources and radiant capillary heat exchangers increase energy efficiency. In order to achieve the maximum energy efficiency during the renovation of the existing apartment, a combination of two green approaches was used. One of them is the replacement of the existing high-temperature radiator heating system with radiant capillary mats. The same system is also used for cooling, which was not possible with the existing system. The necessary energy is provided from renewable aerothermal energy by installing a heat pump together with PV panels to ensure electricity consumption. The measurements were made in the apartment to determine the thermal transmission properties of all boundary structures and an air exchange rate using long-term monitoring. Experimental data were used to create the heat balance, to estimate the heating and cooling powers, as well as the seasonal energy needs. The amount of capillary heat exchangers and the size of the heat pump were chosen accordingly. The very first energy consumption data allow concluding that the planned improvement of the energy efficiency has been successfully achieved by using technology combination described without improvement of the thermal properties.

Energy and exergy efficiency of a daily heat storage unit for buildings heating

This paper deals with a daily solar storage system conceived and built in Laboratoire de Maîtrise des Technologies de l’Energie (LMTE, Borj Cedria). This system consists mainly of (i) a heat storage unit which consists of a wooden case with dimension of 5 m3 (5 m x 1 m x 1 m) filed with fin sand, (ii) a heat collection unit which consists of 5 m2 of south-facing solar collector mounted at a 37° tilt angle and (iii) a network of a polypropylene capillary heat exchanger with an aperture area equal to 5 m² buried inside the heat storage unit and connected to the heat collection unit. The circulation of the heated water between the heat collection unit and the polypropylene capillary heat exchanger allows heating the sand inside the heat storage unit. In order to evaluate the system efficiency during the charging period (during the day) and discharging period (during the night) an energy and exergy investigation were applied. Results showed that during the charging period, the average daily rates of thermal energy and exergy stored in the heat storage unit were 400 and 2.8 kW, respectively. It was found that the net energy and exergy efficiencies in the charging period were 32 % and 22 %, respectively. During the discharging period, the average daily rates of the thermal energy and exergy recovered from the heat storage unit were 2 kW and 2.5 kW, respectively. The recovered heat was compared to the total heat requirements of a tested room (4 m x 3 m x 3 m). The results showed that 30 % of the total heating requirement of the tested room can be achieved from the heat storage system during the whole night until in cold seasons.

Rapid performance prediction model for capillary heat exchangers in subway tunnel linings

Subways are typical underground spaces that help alleviate urban traffic problems; however, they also face challenges such as high energy consumption and deterioration of the thermal environment, which can seriously impact their safe and efficient operation. A subway source heat pump system (SSHPS) with tunnel-lining capillary heat exchangers (CHEs) is effective for addressing these issues. Current research has primarily focused on the heat transfer performance of tunnel lining CHEs, yet there remains a dearth of rapid prediction models for their heat flux that are suitable for efficient engineering design. In this study, numerical simulation systems for tunnel lining CHEs in rectangular and circular subway tunnels were established to analyse their heat transfer performance under different design conditions. The results show that the heat flux of flat and circular CHEs installed in tunnel lining ranges from 47.03 to 117.65 W/m2, and from 36.06 to 115.09 W/m2 in the cooling season, respectively. In contrast, in the heating season, the heat flux of flat and circular CHEs ranges from 27.03 to 89.90 W/m2, and from 19.35 to 87.77 W/m2, respectively. Based on the analysis of these results, rapid prediction models for the CHE heat flux were established using multivariate linear fitting. The established models passed a series of statistical tests, indicating high statistical significance. This study provides a theoretical basis for the engineering design and applications of tunnel lining CHEs.

Long-term performance of a subway source heat pump system with two types of front-end heat exchangers

The rapid development of subways and resulting deterioration of the thermal environment in tunnels have led to thermal pollution of underground spaces, which is a problem that cannot be ignored. The subway source heat pump system (SSHPS) with a tunnel-lining capillary heat exchanger (CHE) is an effective technology for solving this problem. Existing researches mainly focused on the short-term performance of the SSHPS. Few studies have analyzed the long-term performance of the system under complex periodic boundary conditions. In this study, a simulation model of an SSHPS, which conforms to an actual demonstration project located in Lingshanwei Subway Station of Qingdao Metro Line 13, was built by coupling a flat CHE for the open-cut section and a circular CHE for the concealed excavation section of a subway tunnel. Field test results of the SSHPS performance were compared with those of the simulation. The maximum relative error is 13.96%, which shows that the simulation model has high accuracy and can satisfy the requirements of practical engineering applications. Based on the verified model, the system performance under complex periodic boundary conditions was simulated for 10 consecutive years. The results indicate that the average heat flux of the flat and circular CHEs has little variation during the continuous operation of the system for 10 years, and the variations during the cooling and heating seasons are less than 1.24 W/m2 and 0.36 W/m2, respectively. The annual air temperature increments in the tunnel with flat and circular CHEs are approximately 0.226 °C and 0.324 °C, respectively. The average coefficients of performance of the heat pump unit in the cooling and heating seasons are 4.43 and 5.11, respectively, indicating that the heat pump unit operates efficiently throughout the year. The main reason for the long-term high performance of SSHPS is the annual thermal balance of the subway tunnel. The thermal balance of the subway tunnel during long-term operation should be considered when designing an SSHPS system. This study can provide a reference for optimizing the design and operation of the SSHPS.

Long-term performance of a front-end capillary heat exchanger for a metro source heat pump system

In recent years, the number of subways has rapidly increased, as they can effectively alleviate urban traffic congestion. However, subways can also result in thermal pollution problems in underground spaces. The long-term operation of a subway causes thermal accumulation in the surrounding rock of a tunnel, severely affecting the safe and efficient operation of the subway. The metro source heat pump technology is an effective approach to address the foregoing problem. However, currently, the research on its systematic design methods remains inadequate. In particular, studies on the evolution law of heat transfer characteristics of its front-end heat exchangers during years of operation are limited.In this study, a numerical model of a front-end capillary heat exchanger (CHE) is formulated. The model is based on a practical subway source heat pump demonstration project located in a cold area in China. The annual heat transfer characteristics of the CHE under periodic boundary conditions are simulated and analysed. The results reveal that the temperature of the CHE and its vicinity fluctuates periodically and increases annually under typical design conditions. The fluctuation range of CHE wall temperature gradually decreases from 25.0 °C at 0 m from the CHE inlet to 9.6 °C at 10 m from the CHE inlet in the 10th year. A distinct ‘inertia’ phenomenon is observed when the temperature wave is transmitted to the rock surrounding the tunnel. With the increase in the surrounding rock depth from 0 to 25 m, the amplitude of temperature wave gradually decreases from 4.277 to 0.013 °C, and the reduction rate gradually declines from 24 % to 14 %. Meanwhile, the phase delay of temperature wave increased from 0° to 282.40°. Moreover, a critical heat transfer point exists along the CHE tube length, and the heat transfer law and efficiency before and after the critical point differ. Statistics show that the ratio of CHE average heat flux after the critical point to that before the critical point ranges from 1.11 % to 46.16 % in the heating season and from 30.19 % to 46.12 % in the cooling season, respectively. The recommended CHE tube length is determined to be 4.9–6.3 m based on the analysis results. This research can provide reference data and a theoretical basis for the design of front-end heat exchangers of subway source heat pump systems.

Radiant capillary heat exchangers – power calculation for optimal heating and cooling

The issue of switching to renewable energy sources becomes very actual and it is important not only to change the energy source, but also the reduce the final energy needs by improving the energy efficiency of buildings and usage of efficient heating systems. Heat pumps as the most popular renewable energy source are widely used, but their energy efficiency is depending on temperature of the supplied energy carrier. The most efficient are radiant capillary heat exchangers with a large surface area and a low temperature, which typically does not exceed 30°C. Another advantage of radiant capillary heat exchangers is the possibility to operate them in both - heating and cooling modes. Unlike the underfloor heating solution, where the role of thermal convection is very important, the built-in radiant capillary heat exchanger systems provide the energy mainly due to thermal radiation. This study explores two modelling approaches for determination of required power and corresponding area of radiant capillary heat exchangers to be installed in a room to provide heating and cooling: simplified approach, which allows to create the heat balance with a minimum amount of input data and a precise standard-based approach. Calculations were made for three different rooms with variable glazing area and spatial orientation using both approaches. Analysis of the calculation results shows the limits of the simplified method, which overestimates heating need and underestimates cooling need, and the main reason for such differences is simplification of room orientation and subsequent solar heat gains. As the calculated cooling power is less than heating power, therefore the heating estimation is sufficient to estimate the amount of radiant capillary heat exchangers in small/medium rooms for providing both heating and cooling in the climatic conditions of Riga. The use of complex, comprehensive modelling approaches is necessary for rooms with large glazed areas, where the simplified method gives incorrect estimations.

Performance of capillary box heat exchangers buried in seabed for seawater-source heat pumps

An innovative seawater-source heat pump system (SWHPs) was developed by employing capillary box heat exchangers (CBHEs) buried in the seabed as the front-end heat exchanger. Field experiments were firstly presented to test the efficiency of the SWHPs in a practical project. The experimental results show that even under extreme cold/hot climate conditions, the 24-hour COP/EER of the heat pump unit can reach respectively 2.96 and 5.39, which is superior to an alternative air source heat pump (ASHP). Furthermore, an unsteady state three-dimensional seepage and heat transfer model was established to study the thermal performance of CBHEs buried in the seabed using ANSYS FLUENT software. Based on the model, the influences of seawater seepage velocity, thermophysical characteristics of backfilled sand, and flow velocity in the capillary tube on the thermal characteristics of CBHEs were investigated. Moreover, considering the per area heat transfer rate of CBHEs as the evaluation target, the sensitivities of five representative factors including the flow velocity in capillary tubes, seawater seepage velocity, specific heat capacity, density and thermal conductivity of the backfilled sand were discussed using the single factor sensitivity analysis to detect the significant influence parameters.

Resistance-capacitance model of the capillary heat exchanger in subway tunnels

The subway operation generates a large amount of waste heat, which will lead to a disruption of the underground thermal balance. To remove the heat, a ground source heat pump system adopting capillary net as the front-end heat exchanger can be built in the tunnels to recycle the waste heat and use it for building heating. Different simulation method was used to analyze the heat transfer process in the tunnel and the surrounding rock. The existing simulation method usually takes a long time for calculation and can not be used for a real-time regulation of the system. Based on the concept of thermoelectric analogy, a resistance-capacitance (RC) model was proposed for rapid prediction and analysis on the heat transfer of the capillary net in subway tunnel. The annual thermal response in the tunnel was analyzed, and the influence of the tunnel air temperature and the capillary inlet water temperature was examined using this model. The maximum heat transfer flux of the capillary in summer (release heat) and in winter (extract heat) was 55.27 W/m2 and 38.33 W/m2, respectively. The effect of capillary inlet water temperature was more remarkable than that of the tunnel air temperature on the capillary heat exchange process.

Onsite measurement and performance analysis of capillary heat pump system used in subway tunnels

This study proposed a ground source heat pump system using capillary networks as the front-end heat exchanger, to alleviate thermal pollution and utilize waste heat generated in subway tunnels and the surrounding rock. We conducted onsite measurement of the capillary ground source heat pump system based on a real engineering project involving a subway tunnel. Subsequently, the heat transfer performance of the capillary heat exchanger and energy efficiency of the system were analyzed. The heat transfer capacity of the capillary heat exchanger was approximately 45–80 and 100–110 W/m2 in a test with a duration of 7 h in winter and a test with a duration of 14 h in summer, respectively. The average coefficient of performance of the heat pump unit and coefficient of performance of the system were 3.72 and 3.18, respectively, in winters, and 3.19 and 2.64, respectively, in summers. Under a 4-day test with an intermittent operation of 8 h per day in summer, the performance of the system was improved and remained stable for four days during the test. The average coefficient of performance of the heat pump unit and of the system for intermittent operation were 3.67 and 2.96, respectively, which were larger than those for 14 h continuous operation in summer. In practical applications, intermittent operation or additional auxiliary cooling sources are recommended in summer conditions.

Heat transfer performance analysis of front-end capillary heat exchanger of a subway source heat pump system

During the operation of subway systems, a significant amount of heat is stored annually in the rock surrounding the tunnel, which results in underground thermal pollution. A subway source heat pump system with a front-end capillary heat exchanger (CHE) is considered an effective technology to address this problem. However, it has not yet been employed in systematic design and operation methods. This study numerically simulates and analyses the heat transfer performance of a tunnel lining CHE for an entire year under the typical conditions of an actual project. The results are expected to provide theoretical guidance for CHE design and operation. Moreover, the proposed model was verified by comparing the simulation results with those of field experiments.The simulation results indicated that both the temperature and heat flux of the CHE and tunnel surrounding rock experienced periodic fluctuations during the heating and cooling seasons and exhibited a strong heat recovery ability in the transitional seasons. The temperature of the surrounding rock (Tmid) gradually decreased with increasing depth of the surrounding rock, and the change in Tmid at greater depths (≥15 m) was less than 0.1 and 0.13 °C during the heating and cooling seasons, respectively. However, phase delays and amplitude attenuation phenomena occurred during the harmonic temperature transfer process. The phase delay of the tunnel wall temperature (Tw) valley and peak during the heating and cooling seasons were 13 and 2.2 h from the CHE wall temperature (TC) valley and peak, respectively. It was suggested that the efficient operation of the CHE should harmonise the two temperature waves of the tunnel wall and CHE. Furthermore, the average heat flux values at the lining-CHE outer wall contact surface during the CHE operation were −16 and 22 W during the heating and cooling seasons, respectively. In addition, the CHE exhibited abnormal heat fluxes during the heating and cooling seasons along the CHE pipe (10 m), indicating that the design and operation of the CHEs must consider the influence of the pipe length.

Heat transfer model of the front-end capillary heat exchanger of a subway source heat pump system

The use of metro source heat pumps can help alleviate the deterioration of the subway thermal environment and convert the “waste heat” into a valuable resource for heating aboveground buildings. At present, this technique represents a highly effective energy-saving method to control the thermal environment in subway tunnels. In this study, the front-end capillary heat exchanger (CHE) of the subway source heat pump system of a demonstration project in Qingdao was considered as the research object. Considering a CHEthat was installed in a multilayer circularcomposite medium, a fluid-thermal coupled heat transfer model was established by combining theoretical analyses and numerical simulation techniques. A module pertaining to the CHE was implemented on the TRNSYS platform based on the corresponding fluid-thermal coupled heat transfer model. The field test results for the performance of the subway tunnel CHE system were compared with the simulation results of the module. The maximum and average relative errors were 9.3% and 3.5%, respectively. The verification results indicated that the developed simulation module is highly accurate and can satisfy the requirements of practical engineering applications. Based on the validated module, the influence of three typical design factors (inlet water temperature, flow velocity and type of surrounding rock) on the heat transfer performance of CHE was analyzed. This study can provide a theoretical basis for the design and operation optimization of front-end CHEs of metro source heat pump systems.

Performance analysis of a hybrid subway source heat pump system using capillary heat exchanger

A large amount of free heat generated by the running of subway is released to the tunnel. Due to arc structure of subway tunnels, conventional ground source heat pumps and large-diameter heat exchangers are difficult to efficiently extract the heat and maintain surrounding-rock thermal balance. To sustainably take advantage of these free heat, in this paper, a hybrid subway source heat pump system is proposed, which is integrated by subway source heat pump system, 4.3 mm diameter capillary heat exchanger and auxiliary cooling system. A dynamic simulation model of the hybrid heat pump system, which includes the heat transfer process between capillary heat exchanger and surrounding-rock was developed on TRNSYS. Field tests were conducted at a subway station of Qingdao (China) to validate this model. Under different strategies, the coefficient of performance of the hybrid heat pump system and characteristics of surrounding-rock temperature field are compared. The results indicated that the hybrid heat pump system can ensure appropriate surrounding-rock temperature without overheating (from 33.9 ℃ to 31.3 ℃). Additionally, the coefficient of performance of the subway source heat pump units was 4.432, increased by 2.52%, the energy consumption of the subway source heat pump system can be decreased by 5.55%.

Research on the design and application of capillary heat exchangers for heat pumps in coastal areas

This study investigated the optimal design of a capillary heat exchanger device for the heat pump system and its innovative engineering application in a building. The overall aim was to use a capillary heat exchanger to obtain energy in coastal areas for promoting renewable energy in low-carbon building design. Initially, the main factors affecting the efficiency of the capillary heat exchanger were identified, a mathematical model was then established to analyse the heat transfer process. The analysis showed the flow rate and the capillary length are the key factors affecting the efficiency of the capillary heat exchanger. Secondly, to optimize the structural design of the capillary heat exchanger, the heat energy transfer is calculated with different lengths of the capillary under various flow rates in summer and winter conditions, respectively. Thirdly, a typical building is selected to analyse the application of the capillary heat exchanger for extracting energy in the coastal area. The results show the performance of the selected capillary heat exchanger heat pump system, in winter, the heat energy transfer rate is 60 W/m2 when the seawater temperature is 3.7 °C; in summer, the heat energy transfer rate is 150 W/m2 when the seawater temperature is 24.6 °C. Finally, the above field test results were examined using a numerical simulation model, the test and simulation results agree with each other quite well. This paper is conducive in promoting the development of the capillary heat exchanger heat pump as an innovative sustainable technology for net-zero energy and low carbon buildings using renewable energy in coastal areas. Practical application: A recently proposed capillary heat exchanger is used as an energy extraction and utilisation device to obtain energy in coastal areas for promoting renewable energy in low-carbon building design. This paper explores the application of a capillary heat exchanger as both cold and heat sources for application in typical low-rise buildings. The analysis of the heat energy transfer rate of a typical low-rise building located in a coastal area in summer and winter provides guidance for the application of capillary heat exchangers.

Non-typical designs of Polypropylene Capillary Heat Exchangers

Non-typical designs of Polypropylene Capillary Heat Exchangers

Study on key design parameters of subway source heat pump system with capillary exchanger

As an emerging subway waste heat utilization system, capillary network subway source heat pump system has high practical value. At present, there are many researches on capillary network heat exchanger, but little research has been done on the influencing factors and design optimization of the entire system. The orthogonal test method is used to determine the key design parameters affecting the subway source heat pump system. The multi-factor analysis method is used to optimize the system design. The results show that the key design parameters that affect the subway source heat pump system are the cooling and heating load ratio and the flow velocity in the capillary tube. Meanwhile in the future system design, the range of the cold-heat load ratio of the subway source heat pump system is between 1: 1.5 and 1: 2.5 and the flow velocity in the capillary tube is between 0.05 m/s and 0.09 m/s, which can be used as a reference for system design.

Thermal energy storage using phase change materials: Techno-economic evaluation of a cold storage installation in an office building

Utilizing the latent heat of solidification and melting of so-called phase change materials (PCMs) allows higher storage densities and increased process flexibility within energy systems. However, there is an existing gap in the current literature studying simultaneously the technical and economic performance of these thermal energy storages within an actual application. Thus, in this work a comprehensive techno-economic analysis of a full-scale storage with 7000 L salt-hydrate surrounding a polypropylene capillary tube heat exchanger is presented. The storage is located in a multi-story office building in Gothenburg, Sweden and is used for daily peak shaving of the building’s cooling energy demands. The daily utilizable storage capacity for the installation was determined to be 99kWh, which is 36% of the installed capacity given by the storage manufacturer. The major limiting factor were found to be 60–75% smaller charging rates than what was designed by the manufacturer. Using a mixed integer linear programming model (MILP) to yield optimum scheduling, the storage investment cost limit for a 5 year payback time can be estimated as 9804 SEK (≈ 921 EUR). These developed key performance indicators can be readily compared against alternative storage technologies and designs in order to select the optimal storage design for equivalent applications. Future work is needed to investigate reasons behind the lower than expected storage capacity.

Performance analysis of capillary front-end heat exchanger for subway tunnel

The ground source heat pump system can remove the waste heat from the subway tunnel and supply heat to the above-ground building in winter at the same time, which not only alleviates the thermal accumulation of the tunnel surrounding rock, but also effectively utilizes renewable energy. In this study, the heat transfer performance of the capillary front-end heat exchanger for the subway tunnel ground source heat pump system was studied by simulation. The capillary heat exchanger was simplified as a surface heat source, and the one-dimensional unsteady heat transfer model was established by TRNSYS. The heat transfer process between the capillary, the surrounding rock and the tunnel air was simulated. The effect of inlet water temperature, water flow velocity, initial temperature of tunnel surrounding rock and tunnel air temperature on the performance of capillary heat exchanger was analyzed. Based on the actual conditions, the range of the capillary heat flux was given. Besides, the heat transfer performance of the capillary under intermittent operation was studied. It was found that, when the operating time ratio was 2/3, 1/2, 1/3, and 1/6, the heat flux increased by 10.6%, 21.8%, 28.3%, and 36.9% respectively compared with continuous operation.