Urban heat island effects and climate change are worsening outdoor thermal environments, particularly for groups required to wear personal protective equipment (PPE) during outdoor work, thus increasing their heatstress risk. A summer field study was conducted in a hot-humid region to assess individuals' thermal and physiological responses under two clothing conditions (PPE and ordinary clothing) and four cooling scenarios, along with concurrent measurements of thermal environmental parameters. Machine learning models were developed and interpreted with Shapley Additive Explanations (SHAP) to quantify the influence of microclimatic, physiological and personal variables on thermal sensation. Among the tested algorithms, CatBoost yielded the highest prediction accuracy, followed by XGBoost. For PPE wearers, mean skin temperature predominates at neutral thermal sensation levels (SHAP = 0.900), whereas BMI and air temperature become the main drivers at higher thermal sensation levels. Under ordinary clothing, BMI remains the most influential factor across all levels. Across cooling scenarios, BMI shows the largest overall impact, with its SHAP value peaking (0.259) at the highest thermal sensation level under the shading-only scenario, indicating higher heat-stress risk for individuals with higher BMI. Under the misting + shading scenario, gender is a key factor at neutral thermal sensation levels, whereas high humidity becomes the primary driver at the highest thermal sensation levels when forced convection is absent. Therefore, effective outdoor cooling strategies should consider individual characteristics and prioritize composite approaches combining shading, fans, and misting to enhance both heat mitigation effectiveness and equity.

Groundwater flow significantly influences the evolution of the subsurface temperature field and the heat exchange performance of shallow ground-source heat pump systems. Moreover, the coupled hydraulic-thermal interactions between the above-ground pipe network and the underground borehole array substantially affect the load distribution behavior of borehole heat exchanger (BHE) arrays. To systematically investigate thermal interference and load-shifting phenomena within BHE arrays affected by groundwater flow, this study developed a coupled model based on the open-source software OpenGeoSys (OGS) and the thermodynamic toolkit TESPy. The proposed model simultaneously considers underground seepage characteristics, heat transfer within the borehole array, and the hydraulic and thermal balances of the ground pipe network. Compared with the decoupled model, incorporating pipe network coupling into the 25-BHE array simulation revealed that the decoupled model underestimates the heating load of the edge BHE by 26% and overestimates the outlet temperature of the central BHE by 1.9K at the end of the 15th cooling season. Further simulations under different seepage scenarios demonstrate that as the Darcy velocity increased from 0 to 1×10−6 m/s, the load-shifting behavior in the BHE array first decreased and then increased. At a Darcy velocity of 1×10−6 m/s, asymmetric load redistribution intensified, causing the most upstream BHE to bear a 41.4% excess thermal load. Nevertheless, increasing the inter-borehole spacing from 3m to 6m under high Darcy velocity (1×10−6 m/s) reduced the peak shifted load by 63%, stabilized seasonal soil temperature fluctuations within 9K, and kept the long-term cumulative soil temperature rise below 2K at the end of the 15th year, while improving load distribution uniformity. These findings emphasize the need to account for load-shifting behavior in practical BHE projects influenced by groundwater flow and provide guidance for project design.

This dataset presents the heating performance of an innovative short-term thermal energy storage strategy with the Medium-Deep Borehole Heat Exchanger (MDBHE). The existing thermal energy storage strategy is long-term, which charges the thermal energy into the underground during the non-heating season and discharges the thermal energy during the heating season via the MDBHE. The long-term thermal energy storage suffers from a low energy storage efficiency due to the cross-season heat dissipation from the energy-storage zone to the surrounding soil. The proposed short-term thermal energy storage strategy extends the thermal energy storage resource of the MDBHE from the non-heating season to the heating season, which alternatively switches the energy-charging and energy-discharging modes during the heating season. Since the thermal interaction time between the energy-storage zone and the surrounding soil is limited to the switching period (i.e., one day in the demonstration scenarios with different charging temperatures and charging-discharging ratios), the proposed strategy achieves high charging and discharging efficiencies, leading to high overall energy-storage efficiency and large heating capacity. This study presents data on the heating performance of the proposed strategy, including the soil temperature, inlet and outlet temperatures, heating capacity, and energy storage efficiency under different scenarios, as well as the comparative results with the traditional intermittent operation of the MDBHE without thermal energy storage and the existing long-term thermal energy storage of the MDBHE.

Medium-deep borehole heat exchanger (MBDHE) is a clean building heating solution that offers improved indoor thermal comfort, but it risks heat extraction capacity deterioration due to insufficient geothermal heat flux recovery. Cross-seasonal heat storage can mitigate heat extraction capacity deterioration. However, previous studies focused on optimizing the heat storage parameters without considering the mutual inhibition between geothermal heat flux recovery and the heat storage. This paper proposes a soil heat deficit regulation-based cross-seasonal heat storage method, which expands the soil heat deficit to enhance the geothermal heat flux recovery and heat storage simultaneously for improved heating performance of the MDBHE. The effectiveness of the proposed method is demonstrated by using an MDBHE model validated against actual engineering data. The results show that the flow rate-based soil heat deficit regulation is more effective than the temperature-based soil heat deficit regulation. Expanding the soil heat deficit by lowering the heat extraction temperature effectively increases the heat extraction capacity of MDBHE by up to 44.82% with a relatively stable heat storage efficiency. Expanding the soil heat deficit by increasing the heat extraction flow rate increases the heat extraction capacity by up to 55.93% and improves the heat storage efficiency by up to 18.71%. Furthermore, the robustness of the soil heat deficit regulation is verified under different heat storage conditions. Therefore, the proposed method can effectively improve the heating performance of MDBHE, enhancing indoor thermal comfort and contributing to the advancement of low-carbon building heating.

Against the backdrop of typical geological settings in Shaanxi Province, this study developed 3D heat transfer models for coaxial deep borehole heat exchanger (C-DBHE) and U-type deep borehole heat exchanger (U-DBHE) using the OpenGeoSys (OGS) numerical simulation platform. The models do not account for the influence of groundwater flow on thermal performance and assume a steady-state heat extraction process for the boreholes. A comparative analysis was conducted on the heat recovery capabilities of these two DBHE types under the condition of identical drilling depths. Additionally, integrating drilling cost data, the economic efficiency of heat extraction for C-DBHE and U-DBHE configurations with varying horizontal spacings was assessed. The results show that while the inlet and outlet water temperatures of C-DBHE decrease year by year during long-term operation, the rate of this temperature decline gradually slows over time. After 15 years of service, the heat-influenced radius reaches 60 meters, with the drilling cost per unit heat extraction measured at $6.92 \text{CNY} / \mathrm{W}$. For U-DBHE, heat recovery performance improves as the horizontal section length increases; however, this enhancement is accompanied by a corresponding rise in unit heat extraction costs. Specifically, when the horizontal section length is extended from 200 meters to 1400 meters, the unit cost surges from 7.18 CNY/W to 9.58 CNY/W, representing a 33.48 % increase. The study concludes that C-DBHE should be prioritized for cost reduction in scenarios with moderate heat demands, whereas U-DBHE demonstrates greater superiority in high-load heat supply situations. Consequently, the selection of DBHE type should be tailored to the specific heat requirements of the project.

As an effective approach for deep geothermal energy utilization, medium-deep ground source heat pump (MDGSHP) systems have attracted increasing attention in northern China as a low-carbon solution for district heating. However, traditional MD-GSHP systems exhibit insufficient demand-side flexibility, limiting their capacity for real-time coordination with power grid operations. The incorporation of thermal energy storage (TES) into MD-GSHP systems presents a promising pathway to enhance energy flexibility and support demand response. In this study, an MD-GSHP system integrated with latent heat TES was proposed. Firstly, a dynamic simulation model of the integrated system was established based on TRNSYS platform and validated against field data. Sensitivity analysis was then conducted to analyze the influence of PCM storage tank parameters on system performance. Finally, techno-economic optimization was performed to identify the optimal design parameters and the system's potential for performance improvement. The results show that the optimal PCM charging temperature of the heat pump is strongly dependent on the storage tank volume and PCM melting temperature. As the melting temperature and charging temperature increase, both lifecycle economic performance and energy efficiency exhibit a nonmonotonic trend-rising initially and then declining. Under the joint optimization of economic performance and energy efficiency, the system achieves a levelized cost of energy of $0.481 \text{CNY} / \text{kWh}$ and an SPF of 6.02 after 20-year operation. The research results provide a theoretical basis for guiding the design and operation of deep borehole geothermal systems with integrated TES.

Ground source heat pump (GSHP) system are widely recognized as an energy-efficient renewable energy utilization system, particularly for public buildings. In the GSHP systems where multiple units operate jointly, the control strategy for these units plays a key role in enhancing energy efficiency. This study proposes a novel control strategy for the joint operation of multiple heat pump units under varying load conditions, using a hotel building in Xi'an and an office building in Harbin as illustrative examples. The proposed control strategy was analyzed and compared with traditional average load method. The simulation results demonstrate that the proposed control strategy significantly improved the average coefficient of performance (COPs) of GSHP units and reduced the total energy consumption of the systems applied in both of the studied cases, by raising average part-load ratios (PLRs). The proposed control strategy effectively meets the buildings' load demand while simultaneously decreasing system energy consumption, thereby offering significant practical engineering value in terms of energy savings for the air conditioning systems.

Medium-deep U-shaped butted well (MUBW) is one of the most potential geothermal-heating way. However, the thermal interference may happen during long-term geothermal extraction period, which results in a heating performance attenuation. In this work, the heat transfer performance and surrounding rock-soil temperature distribution characteristics of MUBW in whole life cycle were investigated. The three-dimension full-size heat transfer model for long-term thermal extraction was proposed and the different structure sizes were considered. The results showed that the thermal interference is more likely to happen in a high heating load condition for the conventional structure size. By increasing horizontal length from 200 m to 600 m, the average specific heating load could increase by 26.1 %. When vertical depth increases to 3000 m, the heating load increases by 47.0 % at least. It is noted that the annual average inlet and outlet temperatures decrease by 5.1 °C in the whole life cycle, which should be considered in design on system operation. Additionally, the horizontal length of 600 m is large enough to avoid thermal interference in whole life cycle for MUBW with depth of 2000∼3000 m. This study has a practical meaning for ensuring a sustainable thermal extraction of MUBW.

U-type medium-deep borehole heat exchanger (U-MDBHE) is a sustainable building heating technology. Current studies assess the long-term thermal performance of U-MDBHE using typical meteorological year weather data. The conclusions indicate a discernible deterioration in the thermal performance of U-MDBHE attributed to heat extraction attenuation. The thermal performance deterioration leads to the oversize of U-MDBHE and hinders the widespread application of U-MDBHE. This study introduces a novel idea that the long-term thermal performance of U-MDBHE should be evaluated considering climate change (CC) and verifies that the favorable effects of CC on the thermal performance of U-MDBHE can effectively mitigate the adverse effect of heat extraction attenuation. The favorable effects of CC include reducing the heating demand (due to the reduced building heating load (BHL) caused by CC) and improving the heating supply capacity (due to the enhanced outlet temperature caused by CC). In addition, the reduced BHL under CC enhances the inlet temperature of U-MDBHE, thereby improving its operation safety. CC mitigates the heat extraction attenuation of U-MDBHE, with the strongest effect in the ascending well, followed by the descending well, and then the butted well. Case studies using experimentally validated simulations on the 30-year operation of U-MDBHE demonstrate that by mitigating the adverse effect of the heat extraction attenuation, CC reduces the accumulated energy consumption by 14.31%-26.59% and improves the operation safety by up to 100% in Harbin (severe cold region) and Beijing (cold region). This study significantly contributes to improving the long-term thermal performance of U-MDBHE.

The medium -deep borehole heat exchanger (MDBHE) is a clean solution for building heating with renewable geothermal energy. The long-term performance evaluation with the design time scale same as the life span of the MDBHE is generally adopted by the MDBHE design for high heating performances. However, the long-term performance evaluation largely increases the computational load of the MDBHE design, resulting in practical inconvenience. This study proposes the idea that properly shortening the design time scale could maintain the information of the long-term performance evaluation for the MDBHE design with high heating performances while reducing the computational load. The idea is verified by the MDBHE design for building heating in the severe cold zone of China. The multiple design parameters of the MDBHE are simultaneously optimized considering climate change. The results show that under different geological conditions, economic conditions, and system safety requirements, compared with the life span of the MDBHE of 30-50 years, the design time scale properly shortened to 10 years achieves similar high heating performances regarding the energy consumption, cost, and system safety, and reduces the computational load by 66.1 %-79.3 %. This study contributes to high computational -efficient and high-performance design of the MDBHE for building heating.

Deep borehole heat exchangers (DBHEs) coupled with heat pump systems present a promising solution for building space heating. However, conventional heating systems have limited demand flexibility and ignored the potential of direct heating using DBHEs. Thus, this study proposed a hybrid DBHE heating system by integrating latent heat thermal energy storage (LHTES) and borehole direct heating (BDH), and evaluated its performance in terms of energy, exergy, economy, and flexibility. This work aimed to achieve a matching operation between the LHTES and the heat pump and quantify the performance improvement potential of the hybrid system. Firstly, a thermodynamic analysis was carried out based on a pilot project, showing that the seasonal performance factor (SPF) of the system can reach 4.3 under the high-temperature heat storage mode, with a 45.5% improvement in exergy efficiency. Based on the field-measured data, a transient model of the hybrid system was developed using TRNSYS and MATLAB. With the established model, the single and interactive impacts of multiple critical parameters on the system performance were explored. Subsequently, a multi-objective optimization was performed using artificial neural networks and a genetic algorithm by considering several scenarios with different geographical locations and electricity tariffs. The optimization results revealed a trade-off between the levelized cost of energy (LCOE) and the flexibility factor, both of which were highly sensitive to the tank volume of the LHTES. The case studies showed that the LCOE of the optimized hybrid system was decreased by up to 8.5%, while the SPF, exergy efficiency, and flexibility factor were improved by up to 20.3%, 3.0%, and 998.7% respectively, in comparison with the conventional system. Compared with the hybrid system without using LHTES, integrating LHTES led to a decrease in LCOE by up to 5.0% and an increase in SPF by up to 3.8%. This study demonstrated the potential of using such hybrid systems for building heating decarbonization.

With the development of the economy and society, energy problems have become a great concern. The heat pump-coupled thermal energy storage (TES) system is a potential form of building heating, which can improve the stability of the grid and promote the consumption of renewable energy. Phase change materials (PCMs) are widely used in the field of building heating, but there are still some problems such as unsatisfactory melting points, low thermal conductivity, phase separation, and supercooling, which limit the application of PCMs in heat pump heating systems. Therefore, it is very important to improve PCMs by a performance improvement method. This work first summarizes the classification, advantages and disadvantages of PCMs, and introduces the connection between PCMs and heat pumps. Then, a detailed summary of PCMs applied in heat pump heating systems is presented, and a comprehensive review of the performance improvement methods for PCMs, which include additives, encapsulation, and eutectic compounds, is discussed. Finally, the existing problems, solutions, and future research directions are proposed. The emphasis of the research is to clarify the influence of PCMs on heat pump performance and the effect of different performance improvement methods on PCMs, and to illustrate the future development direction for PCMs in heat pump heating technologies, including the matching of heat pumps and PCMs, multi-standard decision methods and advanced control strategies.

The deep borehole heat exchanger (DBHE) coupled ground source heat pump (GSHP) system is widely developed in recent years to extract geothermal energy for building heating. However, its operating mechanism and energy flows within different components have not been comprehensively investigated. Meanwhile, the high outlet temperature of DBHE has the potential to meet the direct heating requirement but may exceed the GSHP's temperature threshold. Therefore, this study first presented a hybrid DBHE heating system transient model with direct heating and coupled GSHP approaches using TRNSYS and MATLAB software. The novelty of this system is that DBHE was also coupled with a heat exchanger (HEX) to realize direct heating. Taking a residential building in Xi'an and DBHE with the depth of 2000 m as a scenario, the operation behavior and energy performance of the system were analyzed. The results showed that HEX will operate for several days before operating GSHP at the beginning of heating season, and annual HEX operating time gradually decreases from 27.1 days to 8.8 days over 10 years. Annual energy fraction of heat extracted from the soil by the HEX and GSHP in the total heating demand gradually decreases from 0.14 to 0.05 and increases from 0.72 to 0.78 over 10 years, respectively. Further, the importance of HEX in system performance was evaluated. Compared with the hybrid system, the GSHP's COP will be overestimated by 11.3 % and the heat extraction amount will be significantly underestimated when HEX is not designed in the DBHE system. Moreover, the effects of different building heating load characteristics (including actual load and constant load) on system performance were discussed. It was found that the minimum inlet temperature of DBHE during the heating season under constant load will be overestimated, with a maximum deviation of 8.4 degrees C. Dynamic building load also results in a parabolic soil temperature isotherm at the end of heating season.

Promoting solar energy for heating is of great significance for achieving the national goals of "double carbon" goals. Increasing the heat gain of window space could improve the solar energy utilization in congested urban environments. The paper proposed a new heating type for active solar house by solar louver. Thermal effects validation on solar louver were studied by field-test and simulation analysis. A solar louver model was established on the platform of MATLAB and TRNSYS. The simulated results showing that heat gain of solar louver reached the largest when blades' angle was about 10 degrees. Solar louver with high reflectivity and low thermal emissivity could capture more heat gain. The in-situ experiments of active solar house coupling solar louver were carried out in Ordos. From 24th to 29th Jan. 2022, all-day comparative experiments of thermal effects in actual winter-weather were conducted. The experimental results indicating that solar louver in maximum light transmission mode was preferred in active solar house. Compared with the condition of solar louver in overlapped light transmittance mode, maximum light transmission mode could obtain 17.7% higher heat gain. Indoor temperature was more higher and temperature field was more uniform. Moreover, by comparison with the noncoupled solar louver, solar louver in maximum light transmittance mode decreased active heating supply capacity by 33.8%, which could reduce energy consumption of building. The main work here provided a theoretical basis for performance optimization of active solar house.

The medium-deep borehole heat exchanger (MDBHE) coupled heat pump (HP) is a promising building heating solution for carbon reduction. Existing studies claim that the MDBHE-HP suffers a bottleneck challenge of longterm heating performance degradation because of the heating extraction attenuation of the MDBHE, which largely hinders the practical applications of the MDBHE-HP. This study innovatively proposes and justifies that climate change positively compromises the heating extraction attenuation of the MDBHE and significantly improves the long-term building heating performance of the MDBHE-HP. Climate change reduces the building heating load, which mitigates the heating extraction attenuation of the MDBHE, thereby reducing the unmet level of the building heating load. The mitigated heating extraction attenuation enhances the coefficient of performance of the MDBHE-HP, thereby reducing the heating energy consumption. Case studies of 30-year operation in the severe cold and cold zones of China with different geological conditions show that while the heating extraction attenuation of the MDBHE increases the heating energy consumption by 5.0% - 8.7%, climate change reduces the heating energy consumption by 9.2% - 17.0%. Moreover, climate change reduces the unmet level of the building heating load by 70.5% - 100%. Therefore, the positive effects of climate change on the longterm building heating performance of MDBHE-HP should be considered, which contributes to promoting the practical applications of the MDBHE-HP for low-carbon building heating.

Borehole heat exchanger (BHE) array is a key element in the ground source heat pump system (GSHPS). The BHE array is traditionally simplified by assuming the same heat exchange rate for each BHE, which normally is not correspondent to the real applications. In this investigation, a TRT was firstly conducted for a site located in Shanxi Province (China) to get the local ground thermal properties. A numerical simulation framework was next verified by the TRT results. Subsequently, the ground thermal properties obtained by the TRT were used in the numerical simulation model to investigate the influence of two operation strategies on the ground temperature development of the surrounding soil and the BHE performance. The results showed that both ground temperature and BHE array performance decreased during the 8-year operation period of the BHE array in the heating mode. The ground temperature around the center BHE was lower in the same heat load (SHL) scenario (same heat load for the BHEs in the array) than in the same inlet temperature (SIT) scenario (same carrying fluid inlet temperature for the BHEs in the array), and this difference can be 2 °C in the 7th year. The annual average outlet temperature in the SHL scenario was also lower than that in the SIT scenario, especially in the long term. The investigation shows that SIT working strategy superiors the SHL strategy, providing a reference for the upcoming investigation on the design aspect of the GSHPS.

Geothermal thermal energy with a medium-deep borehole heat exchanger (MDBHE) is a clean solution for building heating. There lacks a proper design of the MDBHE based on long-term performance evaluation, which is essential to promoting the efficient application of geothermal thermal energy for building heating. This study proposes a design optimization of the MDBHE for building heating under climate change. First, the long-term heating energy performance of the MDBHE under climate change is evaluated. Second, the operation cost is evaluated based on the long-term heating energy performance, considering the discount rate and inflation rate. Third, an optimization algorithm is employed to determine the optimal design of the diameter ratio of the inner pipe to the outer pipe, pipe depth, and inlet velocity of the MDBHE. The optimization method minimizes the cost of initial and operational expenditure while satisfying the system safety requirement. Results show that climate change should be considered, which helps to robustly reduce energy consumption and cost by 9.3%–44.4% and 3.9%–21.8% respectively, under different design conditions compared with the typical meteorology. This study contributes to the proper design of the MDBHE as a clean building heating solution.

Medium-deep borehole heat exchangers (MBHEs) have received increasing attention with respect to building heating. To avoid the thermal interference of adjacent MBHEs, the temperature distribution characteristics of medium-deep rock soil were investigated in this work. The evolution of the maximum rock-soil thermal affected radius (MTAR) over a full lifecycle was analyzed. The results showed that the rock-soil thermal affected area (RTAA) continuously expanded in both the radial and vertical directions when the MBHE continuously extracted geothermal energy during a heating season. The factors of the thermal extraction load, fluid velocity, geothermal gradient, and pipe length, impacted the RTAA in the vertical direction, while rock-soil thermal conductivity affected the RTAA in both the radial and vertical directions. Furthermore, the thermal affected radius (TAR) in deeper formations was larger, reaching even 96 m, such that thermal interference between adjacent MBHEs was more likely to occur. The MTAR in shallow formations was limited to 20 m. Consequently, a new layout form, achieved by inclining the borehole, was proposed to increase the distance between adjacent MBHEs in deep formations. The recommended incline angle was equal to or larger than four times the TAR angle. This work provides a scientific reference for promoting the application of multiple MBHE arrays.

To reduce carbon emission and achieve carbon neutrality, deep geothermal energy has been widely extracted for building heating purpose. In recent years, deep borehole heat exchanger (DBHE) heating system has gained more attention, especially in densely populated urban areas in Weihe Basin, northern China. The long-term performance and the economic feasibility are essential for the system application. In this work, the DBHE model implemented in OpenGeoSys software is verified against an analytical solution and a comprehensive economic analysis approach is further proposed. Then the short-term thermal performance tests are conducted to obtain the tentative heat extraction capacity for long-term simulation. The long-term simulations are further performed with the heat pump unit under the adjusted tentative heat extraction rate imposed on the DBHE. Finally, a comprehensive economic analysis is applied to the DBHE heating system over 15 heating seasons. Results show that the minimum coefficient of performance value of the heat pump is 4.74 over the operation of 15 heating seasons. With the increase of depth for the DBHE, the total electricity consumption of heat pumps and circulation pumps has a prominent promotion. With the comprehensive approach of economic analysis, the depth of 2,600 m has the lowest levelized cost of total heating amount, which is the best system design for the application in Weihe Basin. The present results are specific to the conditions in Weihe Basin, but the proposed economic analysis approach is generic.

To meet the prospect of carbon neutrality, Deep borehole heat exchanger (DBHE) shows good potentiality in extracting deep geothermal energy for building heating, especially in densely populated urban areas of northern China. To investigate the influence on different soil thermal properties and system layouts of the DBHE array, a comprehensive numerical model has been established by OpenGeoSys software coupled TESPy toolkit and a series of scenarios are simulated. Results show that thermal conductivity lay a more important influence on heat extraction performance for DBHE array, rather than volumetric heat capacity. The thermal plume of DBHE array will grow larger along with higher thermal diffusivity. For typical geological parameters in Xi'an, the inter-borehole spacing should not be set below 15 m or it will bring a risk of freeze in circulation. The heat extraction performance and long-term sustainability of single-line layout are obviously better than other layout patterns, also with a smaller ground area needed to deploy the boreholes. This study implies that soil thermal conductivity is the core factor in determining the heat extraction performance of DBHE array and also gives suggestions for the system design of DBHE array in the aspect of borehole spacing and system arrangement. (c) 2021 Published by Elsevier Ltd.