Coupled Study on the Building Load Dynamics and Thermal Response of Ground Sources in Shallow Geothermal Heat Pump Systems Under Severe Cold Climate Conditions

Abstract

To address thermal imbalance and ground temperature degradation in shallow geothermal heat pump (GSHP) systems in severely cold climates, this study analyzes a typical logistics building using an hourly dynamic load model. Multiyear simulations were conducted to investigate the coupling between building load variation and soil thermal response. The results indicate that with a cumulative heating load of 14.681 million kWh and cooling load of 6.3948 million kWh, annual heat extraction significantly exceeds heat rejection, causing ground temperature to decline by about 1 °C per year. Over five and ten years, the cumulative drops reached 2.65 °C and 4.71 °C, respectively, leading to a noticeable reduction in borehole heat exchanger performance and system COP. The study quantitatively evaluates ground temperature and heat exchange degradation, highlighting the key role of load imbalance. To mitigate long-term thermal deterioration, strategies such as load optimization, summer heat reinjection, and operational adjustments are proposed. The findings offer guidance for the design and sustainable operation of GSHP systems in cold regions.

1. Introduction

The global energy crisis and climate change have emerged as critical challenges that urgently demand solutions, pose serious threats to the sustainable development of socioeconomic systems, and impose greater requirements for the transformation of the energy structure [1,2]. Statistical data indicate that thermal energy consumption for building heating and cooling accounts for more than 50% of the global final energy demand [3]. Against the backdrop of international efforts to achieve carbon peaking and carbon neutrality, reducing building heating energy consumption is widely recognized as a key pathway toward a low-carbon transition [4]. As major contributors to building energy consumption, heating systems not only consume vast amounts of energy [5] but also represent one of the principal sources of carbon emissions. Traditional building heating methods still rely heavily on fossil fuels, leading to substantial greenhouse gas emissions and exacerbating energy security issues [6]. Therefore, the development and promotion of clean and renewable energy technologies to replace conventional heating methods are imperative to effectively mitigate carbon emissions, alleviate environmental pressures, and increase the resilience and sustainability of energy systems.

As a rich, stable, and environmentally friendly renewable energy source, geothermal energy has received increasing attention in the building energy sector in recent years because of its broad geographical availability and low environmental impact [6]. Ground-source heat pump (GSHP) technology, as a key method for exploiting geothermal energy, has undergone rapid deployment both in China and globally [7], particularly because of its significant energy savings and environmental benefits in building heating applications in cold regions. However, traditional groundwater-source heat pump systems are limited in areas with water scarcity or extreme climatic conditions, where their heavy dependence on groundwater resources limits their broader application [1]. In contrast, shallow geothermal heat pump systems (typically with burial depths ranging from 20 to 200 m) offer enhanced adaptability and application potential in heating and cooling owing to their stable operation and minimal reliance on large-scale groundwater extraction.

Despite the promising prospects of shallow GSHP systems in cold regions, they face critical challenges under severe cold climate conditions, where the annual heating demand predominantly exceeds the cooling demand. This results in long-term net heat extraction from the ground, leading to a continuous depletion of thermal energy in the surrounding soil and a progressive decline in ground temperature. This process weakens the heat exchange capacity of borehole heat exchangers, reduces the system’s overall energy efficiency, and may ultimately cause system failure. The issues of source-side thermal imbalance and ground temperature degradation have thus become key technical obstacles to the long-term stable operation and large-scale promotion of shallow GSHP systems in cold climates.

To address these challenges, extensive research has been conducted in recent years on the thermal response characteristics of the ground, the influencing factors of heat transfer performance, and strategies for mitigating thermal imbalance. Jiang et al. [8], through ground thermal response tests and numerical simulations, revealed the seasonal degradation characteristics of thermal performance in single-borehole and borehole group systems and proposed integrated control methods that combine heat extraction and heat storage. Wang et al. [9] and Gao et al. [10] developed and validated models for medium-shallow array borehole heat exchangers (MSABHEs) and quasi-three-dimensional heat transfer, respectively, and systematically analyzed the impacts of borehole spacing, flow rate, and geothermal gradient on heat exchange efficiency and explored dynamic load-based operational optimization strategies. He et al. [11] proposed a hybrid borehole heat exchanger array (BHEmix) concept to further exploit the hierarchical utilization potential of geothermal resources. Figueira et al. [12] integrated shallow geothermal technologies into district heating and cooling networks. Li et al. [13] optimized coaxial borehole heat exchanger structures to improve overall thermal performance, whereas Bryś et al. [14] conducted a nine-year experimental study on soil thermal flux dynamics, providing empirical data for assessing the sustainability of ground source systems. Additionally, auxiliary heat reinjection and seasonal load management technologies have been proposed and partially validated to mitigate source-side thermal imbalance [15,16,17,18].

Although significant progress has been made in understanding ground thermal response characteristics and exploring mitigation measures, several key gaps remain: (1) most existing studies assume steady-state loads or adopt simplified boundary conditions, failing to accurately capture the cumulative long-term impacts of realistic hourly dynamic building loads on the ground thermal field; (2) systematic and quantitative analyses of the multiyear dynamic evolution of ground temperature under extreme load imbalance in severely cold regions are still insufficient; and (3) there is a lack of in-depth analysis and quantitative validation, on the basis of dynamic simulations, of the degradation mechanisms of heat exchange performance and system energy efficiency due to thermal imbalance. These limitations significantly hinder the optimized design, operational management, and sustainable development of GSHP systems in cold regions.

To address these research gaps, this study focuses on a typical logistics building located in a severely cold climate zone. By integrating real meteorological data, an hourly dynamic building load model was developed, and multiyear dynamic simulations of the source-side borehole field were conducted. Through systematic analysis of ground temperature degradation and heat exchange performance evolution over annual, 5-year, and 10-year operation periods, this study dynamically quantified the cumulative effects of thermal imbalance and clarified the dominant role of load structure imbalance in the source-side thermodynamic degradation process. Compared to prior studies that often adopt simplified or steady-state load assumptions, this study uniquely integrates hourly dynamic load modeling with long-term ground-side thermal response simulation, enabling a realistic assessment of cumulative degradation. Few previous works have systematically quantified year-by-year thermal decay or examined degradation mechanisms under high heating-to-cooling load ratios, particularly in severe cold regions. On the basis of these findings, comprehensive regulation strategies combining load structure optimization, auxiliary summer heat reinjection, and fluid operational parameter adjustments are proposed, providing theoretical support and practical guidance for the efficient design and sustainable operation of shallow GSHP systems in severely cold regions.

2. Research Methodology

2.1. Study Object and Building Load

The simulation in this study focuses on a modern logistics complex located in Changchun, China. The project covers a total land area of 95,721 m², with an overall building floor area of approximately 300,000 m². The complex comprises a diverse range of functions, including a hotel, office buildings, and a corporate culture center. The diversity of building types results in significant variations in heating and cooling loads, making it an ideal case study for investigating the thermal response of shallow geothermal heat pump (GSHP) systems under severe cold climate conditions. The architectural layout of the complex is shown in Figure 1.

To accurately capture the variations in building energy consumption, an annual hourly dynamic building load simulation model was developed via TRNSYS software (version 18.02 [18]) on the basis of typical meteorological year (TMY) weather data. As shown in Figure 2, the model input parameters include the thermal performance of the building envelope, indoor design temperature and humidity standards, internal occupancy density, and thermal disturbance loads from lighting and equipment. In accordance with the Design Standard for Energy Efficiency of Public Buildings (GB 50189—2015) [19] and the Code for Design of Heating, Ventilation, and Air Conditioning of Civil Buildings (GB 50736—2012) [20], building energy consumption simulations and load analyses were conducted. The model outputs hourly heating and cooling load curves for the entire year, providing essential input data for subsequent ground-source heat pump (GSHP) system modeling.

2.2. Building Envelope and Internal Load Settings

The thermal performance of the building envelope directly affects the overall energy consumption of the building. On the basis of the actual design documentation, both the office and hotel buildings adopted wall, roof, and window systems with good thermal insulation performance. The key parameters are as follows:

Office building: The overall thermal transmittance (U value) of the roof is 0.26 W/(m²·K), the external walls have a U value of 0.34 W/(m²·K), and the external windows use double silver low-E insulated glazing with a U value of 1.4 W/(m²·K).

Hotel building: The thermal insulation performance of the envelope is superior to that of the office building, with the roof and external walls achieving U values of 0.26 W/(m²·K) and 0.34 W/(m²·K), respectively.

Indoor design parameters were set according to the functional zones of the building, including the following: summer indoor temperature settings: 26 °C for office areas and 24 °C for hotel rooms; winter indoor temperature settings: 18 °C for office areas and 22 °C for hotel rooms; and fresh air ventilation rates: 30 m³/h·person for office buildings and 40 m³/h·person for hotel buildings.

With respect to internal loads, the following parameters were adopted: occupancy density: 10 m²/person for office areas and 25 m²/person for hotel rooms; lighting power density: 9 W/m² for office areas and 7 W/m² for hotel areas; and equipment power density: 15 W/m². The sensible and latent heat gains per person were set at 66 W/person and 68 W/person, respectively. Hourly occupancy rates, as well as lighting and equipment usage rates, were adjusted according to weekdays and holidays. The operating periods for the heating and cooling systems were defined as follows: heating season: 20 October to 6 April of the following year, totaling 169 days; cooling season: 20 May to 20 August, totaling 92 days.

Heating operations in office areas were set to continuous 24 h operation, while cooling was set for 12 h per day. In hotel areas, both heating and cooling are operated continuously 24 h per day throughout the year. On the basis of the above settings, hourly building load calculations were conducted. The annual cumulative heating load for the office, hotel, and underground spaces totaled 23.10 million kWh, whereas the cumulative cooling load reached 7.56 million kWh. The basic design loads are summarized in

Table 1. Logistic building design load summary.

Designation	Covered Area (m2)	Total Heat Load (kW)	Base Heating Load (kW)	New Air Heat Load (kW)	Total Cold Load (kW)
Office + exhibition hall	150,536.97	7076.44	2959.9	4116.5	9761.80
Hotel	81,726.43	7354.4	3909.4	3445	6084.60
Substructure	71,464.6	7717.3	937.3	6780	/
Total	303,728	22,148.14	7806.6	14,341.5	15,846.40

.

2.3. Ground-Source Heat Pump System and Borehole Heat Exchanger Design

Key assumptions for the ground thermal model include that (1) the soil is homogeneous and isotropic; (2) the initial ground temperature is uniform with depth below seasonal influence (assumed 11 °C); (3) thermal properties are constant over time; (4) no lateral groundwater flow is considered; and (5) heat transfer is governed by conduction only.

To meet the year-round heating and cooling load demands of buildings in severely cold regions and ensure the efficient and stable operation of ground-source heat pump (GSHP) systems, it is essential to design ground-side borehole heat exchanger systems appropriately. In this study, the design parameters of the borehole system and the thermal characteristics of the circulating fluid were determined on the basis of the results of thermal response tests (TRTs) and relevant codes and standards. The specific details are as follows:

The thermal properties of the subsurface soil and rock layers were determined through TRT measurements, with a comprehensive thermal conductivity of 1.93 W/(m·K) and a volumetric heat capacity of 2250 kJ/(m³·K). The initial ground temperature below the surface was approximately 11 °C. Considering the minimum operating temperature during the winter season under severe cold climate conditions, a double-U vertical borehole heat exchanger design was adopted, with an effective borehole depth of 150 m and a borehole diameter of 180 mm. The boreholes were arranged with 5 m × 5 m grid spacing.

The circulating fluid selected was a 25% mass concentration propylene glycol–water solution to ensure the freezing protection of the circulation system under extreme winter temperatures. The thermal properties of the circulating fluid at −5 °C include a thermal conductivity of 0.446 W/(m·K), a specific heat capacity of 3.71 kJ/(kg·K), and a viscosity of 3.89 mPa·s.

The heat transfer coefficient inside the borehole pipes was calculated via the Gnielinski empirical correlation (applicable for Reynolds numbers Re = 2300–10⁶ and Prandtl numbers Pr_x = 0.6–10⁵), as expressed in Equation (1) [18].

$$v_{f,Gn}=\left[\left({\displaystyle \frac{\frac{f}{8}·\left(\mathrm{Re}-1000\right)\cdot {\mathrm{Pr}}_f}{1+12.7\times \sqrt{\frac{f}{8}}\times \left({\mathrm{Pr}}^{2/3}-1\right)}}\right)\times \left(1+{\left[{\displaystyle \frac{d}{l}}\right]}^{2/3}\right)\right]\times c_t$$

(1)

where f—Darcy friction factor; Re—Reynolds number; and P_r—Prandtl number. $c_t={\left[{\displaystyle \frac{{\mathrm{Pr}}_f}{{\mathrm{Pr}}_w}}\right]}^{0.01}$ and ${\displaystyle \frac{{\mathrm{Pr}}_f}{{\mathrm{Pr}}_w}}$ = 0.05~20.

The estimation of the convective heat transfer coefficient in the turbulent flow regime was further refined via the Filonenko friction factor correlation [21], as shown in Equation (2).

$$f={\left(1.82\times {\log }^{\left(\mathrm{Re}\right)}-1.64\right)}^{-2}$$

(2)

where the Reynolds number (Re) is defined as $\mathrm{Re}={\displaystyle \frac{{\rho }_f\times u_f\times d}{{\mu }_f}}$, where ρ is the fluid density (kg/m³), v is the average flow velocity (m/s) of the characteristic diameter, typically the inner diameter of the pipe (m), and μ is the dynamic viscosity of the fluid (Pa·s).

The convective heat transfer coefficient between the circulating fluid and the inner wall of the borehole pipe is calculated as:

$$h_{m,Gn}={\displaystyle \frac{{\lambda }_f\times v_{f,Gn}}{d}}$$

(3)

Taking a polyethylene (PE) pipe with an outer diameter of 25 mm and an inner diameter of 19 mm as an example, the calculated Reynolds number of the circulating fluid is 9862, and the Prandtl number is 5.42. The resulting convective heat transfer coefficient was calculated to be 1672 W/(m²·K). On the basis of the annual building base load requirements, the total required drilling length for the ground-source heat pump (GSHP) system is approximately 252,000 m, corresponding to 1682 double-U vertical borehole heat exchangers. The total net land area required for the borehole field is approximately 42,100 m², with an average heat extraction load of approximately 27 W per meter of borehole length. Through the above design parameters and simulation approach, a solid foundation was established for the dynamic coupling simulation of shallow GSHP systems and building load variations under severe cold climate conditions.

Although long-term field measurements were not available, the simulation outputs were benchmarked against typical performance trends reported in existing studies [18] and calibrated based on standard Chinese building energy guidelines. The COP degradation trajectory also aligns with reported empirical observations under similar climatic conditions.

3. Results and Discussion

3.1. Dynamic Thermal Response of a Shallow Ground-Source Heat Pump System in a Single Operation Cycle

To analyze the dynamic heat transfer performance of shallow ground-source heat pump (GSHP) systems during a typical heating season, this study conducted numerical simulations of single-U and double-U borehole heat exchangers under various flow rates and borehole depths over a full heating period (169 days). The focus was placed on examining the evolution of the heat exchange capacity and source-side ground temperature with operation time, aiming to reveal the distinct characteristics of heat transfer behavior during the initial rapid change phase and the subsequent stabilized phase.

3.1.1. Variation Characteristics of the Borehole Heat Exchange Capacity

As shown in Figure 3, the variation process of the heat exchange capacity for a single-U borehole heat exchanger under different flow rates and depths can be divided into three distinct stages. In the initial rapid decline stage (0–7 days), at the beginning of system operation, the surrounding soil has not yet undergone significant heat extraction, resulting in a large initial temperature difference and a rapid rise in heat transfer capacity to its peak value. The subsequent rapid temperature drop in the near-borehole region led to a sharp decrease in the heat exchange capacity. Gradual transition stage (8–21 days): During the second to third weeks of operation, thermal conduction from the far-field to the near-field regions becomes established, resulting in a reduced rate of decline in heat exchange capacity. Quasi-linear slow decline stage (22–169 days): As the thermal field around the borehole approaches dynamic equilibrium, the heat exchange capacity nearly linearly and slowly decreases over time. Increasing the borehole depth significantly enhances the heat exchange capacity. Compared with that at a depth of 120 m, the heat transfer capacity of a single borehole at a depth of 150 m is improved by approximately 10%, which is attributed mainly to the higher undisturbed ground temperatures at greater depths and the reduced influence of seasonal surface temperature fluctuations [18]. The flow rate also plays an important role in heat transfer performance. Increasing the flow rate from 0.2 m/s to 0.4 m/s resulted in a nearly 10% improvement in the heat exchange capacity, indicating a pronounced heat transfer enhancement. However, further increasing the flow rate to 0.8 m/s leads to a progressively smaller gain in heat transfer capacity, resulting in a typical diminishing marginal effect. Considering the increased energy consumption of the circulation pump at higher flow rates, comprehensive energy efficiency analysis suggests that the optimal flow rate for single-U borehole systems should be maintained between 0.4 m/s and 0.8 m/s, balancing heat exchange efficiency and energy consumption for economic operation.

As shown in Figure 4, the initial peak heat exchange capacity of the double-U borehole heat exchanger is greater than that of the single-U configuration, but its rate of decline is more rapid. Specifically, in the initial stage (0–14 days), the heat exchange capacity sharply decreases, indicating that the double-U configuration extracts thermal energy from the near-borehole region at a faster rate within a short period. Mid-to-late stage (15–169 days): The rate of decline gradually stabilizes, exhibiting a quasilinear slow decreasing trend similar to that observed in the single-U heat exchanger. The influence of the flow rate on the heat transfer performance of the double-U configuration follows a similar trend to that of the single-U system but is more pronounced. Under low flow rate conditions (0.2 m/s), although the initial heat exchange capacity is high, the accelerated depletion of thermal energy in the near-borehole region leads to a rapid deterioration in the heat transfer capacity in the later stages. When the flow rate is increased to 0.4–0.8 m/s, the improvement in heat exchange capacity becomes progressively less significant, and the heat transfer performance tends to converge across different flow rate conditions during the later stages. Overall, the double-U borehole configuration is suitable for short-cycle and high-intensity heating demands. However, under long-term continuous operation, it is necessary to reasonably control both the flow rate and the initial load to avoid excessive cooling and subsequent degradation of the heat transfer capacity in the near-borehole region.

3.1.2. Ground Temperature Variations Around Borehole Heat Exchangers

The temperature variations of the source-side ground surrounding the borehole heat exchangers directly affect the system’s heat transfer capacity and the operational efficiency of the heat pump units. In this study, numerical simulations of the ground temperature fields were conducted for both single-U and double-U borehole heat exchangers under various flow rates (0.2 m/s, 0.4 m/s, 0.6 m/s, and 0.8 m/s) and borehole depths (120 m and 150 m) throughout the entire heating season. The results are presented in Figure 5 and Figure 6, respectively. Figure 5 illustrates the temperature distribution of the ground surrounding the single-U borehole heat exchanger under different flow rates and depths. The overall trends can be summarized as follows: rapid temperature decreases in the near-borehole region (0–1 m). At the initial stage of operation, the circulating fluid rapidly extracts heat from the surrounding soil, leading to a significant temperature decrease close to the borehole. The lowest temperatures are consistently observed near the central axis of the borehole under all flow rate conditions. Gradual temperature recovery in the middle to far-field region (1–2 m): With increasing radial distance, the magnitude of temperature reduction decreases significantly, and the temperature gradient becomes less steep. The influence of the flow rate is evident: under higher flow rate conditions (0.8 m/s), the decrease in the near-borehole temperature is more pronounced, indicating that increased heat transfer intensifies the local thermal depletion process [22]. The effect of the borehole depth on the overall temperature distribution trend is relatively minor; however, at a depth of 150 m, a slightly greater overall temperature drop is observed, reflecting a greater heat exchange potential at greater depths.

Figure 6 presents the ground temperature distribution characteristics around the double-U borehole heat exchanger. Compared with the single-U configuration, the double-U system results in the following distinct features: a more severe local temperature drop. Due to the greater initial heat transfer capacity of the double-U configuration, the maximum temperature drop near the borehole central axis reaches approximately 5 K. Increased temperature distribution curvature is more concentrated near the borehole wall, forming a more pronounced local thermal disturbance zone. A similar thermal influence radius, beyond 2 m from the borehole, the ground temperature largely recovers to its initial value, and the temperature curves under different flow rates gradually converge. Overall, the double-U configuration can achieve higher heat transfer density in the short term; however, it also leads to a faster local ground temperature decline, necessitating careful consideration of the risk of long-term thermal imbalance.

The ground temperature variation is controlled by two key processes. In the heat extraction process, the circulating fluid extracts heat from the near-borehole soil through convective heat transfer at the borehole wall, creating a localized temperature decrease. Heat replenishment process: Heat from the far-field ground is gradually supplied to the near-borehole region through thermal diffusion mechanisms [22]. In the near-borehole zone (0–1 m), the heat extraction rate significantly exceeds the heat replenishment rate, resulting in a rapid temperature decrease. In the middle zone (1–2 m), thermal conduction becomes dominant, leading to a slower rate of temperature change. Beyond 2 m, a thermodynamic equilibrium state is achieved. Increasing the flow rate intensifies local heat extraction and accelerates the decrease in the near-borehole temperature but has a relatively limited effect on the far-field ground temperature.

3.2. Coupled Annual Response of the Building Load and Source-Side Ground

The performance of ground-source heat pump (GSHP) systems is jointly constrained by the dynamic variations in the building load and the thermal evolution of the surrounding ground. To reveal the dynamic response characteristics of shallow GSHP systems under actual annual operating conditions, this study conducted dynamic simulations of the ground source system for both the heating season (20 October to 6 April of the following year) and the cooling season (20 May to 20 August) on the basis of the building’s hourly load simulation results.

3.2.1. Simulation Results Under Winter Heating Conditions

During the winter heating operation, the shallow GSHP system extracts thermal energy from the subsurface through borehole heat exchangers to meet the indoor heating demand of the building. The circulating fluid absorbs heat from the surrounding soil as it flows through the borehole pipes and then enters the condenser side of the heat pump unit, where the absorbed heat is transferred to the building space, thereby achieving indoor heating. The variation patterns of the inlet and outlet water temperatures and the ground temperature during system operation are shown in Figure 7, demonstrating a clear seasonal dynamic evolution.

In the early stage of the heating season (20 October to mid-November), the outdoor temperatures in Changchun remained relatively high, and the initial ground temperature was approximately 10.63 °C. Consequently, the inlet and outlet water temperatures of the borehole system remained relatively high, with a small temperature difference of approximately 2 °C. The system operated stably, and the heat exchange efficiency was favorable. During this phase, the circulating fluid exhibited strong heat absorption capacity, effectively supporting the heating load requirements.

In the middle stage (mid-November to the end of February), as the outdoor temperature gradually decreased, the borehole system continued extracting heat from the ground, leading to a synchronized decrease in both the inlet and outlet water temperatures. The inlet temperature decreased from approximately 9 °C to approximately 5 °C, whereas the outlet temperature decreased from approximately 6 °C to nearly 0 °C, making heat extraction increasingly challenging. Particularly, from late January to late February, the depletion rate of the ground’s thermal energy accelerated significantly, causing the temperature difference between the inlet and outlet waters to increase to approximately 5 °C. Moreover, the minimum inlet water temperature lagged behind the outdoor temperature minimum by approximately one month, indicating the significant thermal inertia of the ground, which acts as a large-scale heat buffer.

In the late stage of the heating season (March to early April), with the gradual rise in outdoor temperatures, the inlet and outlet water temperatures also rebound. By the end of the heating season, the inlet temperature had increased to approximately 7.5 °C, and the outlet temperature had increased to approximately 6.0 °C, with the difference in the inlet–outlet temperature narrowing to approximately 1.5 °C and the system operation becoming more stable.

The evolution of the source-side ground temperature exhibited a three-stage pattern: a slow decline in the early stage, an accelerated decline in the middle stage, and a stabilization trend in the late stage. By the end of the heating season, the ground temperature had decreased from the initial 10.63 °C to approximately 7.8 °C, resulting in a cumulative decrease of nearly 3 °C. These dynamic changes are attributed to the imbalance between the heat extraction rate and the ground’s thermal diffusion capacity. During the high-load winter period, rapid extraction of thermal energy from the near-borehole zone outpaced heat replenishment from the far field, causing a rapid local temperature decline. Additionally, owing to the inherent thermal capacity and conductivity of the ground, a significant time lag effect exists in the system’s response to external temperature changes. As the ground temperature decreases, the temperature difference between the circulating fluid and the ground narrows, increasing the heat transfer resistance and reducing the system’s operational efficiency.

On the basis of the above analysis, the following engineering optimization recommendations are proposed: In the early heating season, the system should be activated early to fully utilize the advantages of higher ground temperatures and delay subsequent ground temperature decline. In the mid-to-late heating season, building load control strategies should be implemented to moderately reduce peak loads, thereby avoiding excessive cooling of the source-side ground. During the system design phase, it is crucial to account for the thermal lag characteristics of the ground and appropriately size the borehole field to ensure sufficient heat exchange capacity at the end of the heating season, thus maintaining indoor thermal comfort and system operational efficiency.

3.2.2. Simulation Results Under Summer Cooling Conditions

During the summer cooling operation, the shallow ground-source heat pump (GSHP) system discharges excess heat into the underground soil via borehole heat exchangers, thereby achieving indoor space cooling. As the circulating fluid flows through the boreholes, it releases heat to the surrounding ground, cools down, and then returns to the evaporator side of the heat pump unit to absorb additional indoor thermal loads for cooling purposes. Figure 8 presents the variations in the inlet and outlet water temperatures and the source-side ground temperature during the summer cooling season.

In the early cooling season (late May to mid-June), during the transition between spring and summer, the outdoor air temperature remained relatively low. At this stage, the inlet water temperature of the borehole system was approximately 17 °C, whereas the outlet water temperature was approximately 12 °C, maintaining a temperature difference of approximately 5 °C, indicating favorable heat exchange conditions and high system operational efficiency. The initial ground temperature at the source side was relatively low (approximately 11 °C), providing an excellent heat sink environment for heat pump cooling.

As summer progressed and outdoor temperatures gradually increased, the heat rejection load of the borehole system rose, leading to a simultaneous increase in both the inlet and outlet water temperatures. During the peak temperature period in July, the inlet water temperature reached a maximum of approximately 32 °C, and the outlet water temperature rose to approximately 27 °C; however, this high-temperature period was relatively short.

As the outdoor temperature gradually decreased, the source-side water temperature also tended to decrease, with the inlet temperature decreasing to approximately 27 °C and the outlet temperature to approximately 22 °C by early September, when the cooling season ended.

Throughout the cooling season, the temperature difference between the inlet and outlet waters remained relatively stable at approximately 5 °C, indicating that the heat exchange system consistently maintained good heat rejection capability. The source-side ground temperature continuously increased during the cooling season, displaying an approximately linear growth pattern. The temperature initially increased from 11 °C to approximately 12.8 °C by the end of the cooling season, resulting in a cumulative temperature increase of approximately 1.8 °C. This phenomenon demonstrates that during the summer cooling period, the borehole heat exchanger system significantly replenished the thermal energy in the near-borehole region through continuous heat injection, effectively contributing to the recovery of the source-side ground temperature.

In addition to heat injection during the cooling season, during the spring and autumn transitional periods, the higher temperatures of the far-field ground also facilitated heat replenishment near the boreholes through thermal diffusion effects. This dual heat storage mechanism plays a positive role in mitigating long-term ground heat depletion during the heating season and helps delay ground temperature degradation.

3.3. Thermal Imbalance Characteristics of the System Under Multiyear Operation

During continuous multiyear operation, shallow ground-source heat pump (GSHP) systems experience gradual thermal imbalances in the source-side ground due to the long-term accumulation of heating and cooling load disparities. This evolving trend in ground temperature variation directly impacts system performance and energy efficiency levels. To reveal the long-term dynamic behavior of the ground source system, this study conducted numerical simulations on the typical building load conditions of the logistics complex, analyzing the borehole heat exchanger system over 5-year and 10-year operational cycles.

The variation curves of the inlet and outlet water temperatures of the borehole system, as well as the average ground temperature over time, are presented in Figure 9, Figure 10, Figure 11 and Figure 12.

As shown in Figure 9, during the 5-year continuous operation cycle, the inlet and outlet water temperature curves of the borehole system exhibit highly consistent seasonal periodicity across each year. Overall, under summer cooling conditions, the maximum inlet water temperature gradually decreases annually, from 38 °C in the first year to 36 °C in the fifth year, accompanied by a synchronous decline in outlet water temperatures. Similarly, under winter heating conditions, both the inlet and outlet water temperatures decrease annually, reflecting the cumulative impact of a continuous decrease in ground temperature on the system’s heat exchange capacity. Although the amplitude of temperature fluctuations during each cooling and heating season remains relatively stable, the absolute temperature levels clearly decrease each year.

The evolution trend of the source-side ground temperature over time is illustrated in Figure 10. The overall curve displays a characteristic wavy pattern, reflecting the dynamic response of the ground to seasonal load variations. Each summer, heat injection leads to a localized rise in ground temperature, whereas each winter, heat extraction results in a temperature decrease, resulting in a pronounced annual cyclic oscillation. However, with the extension of the number of operational years, the baseline of the temperature oscillations gradually shifts downward, indicating that the system is in a state of net thermal loss. By the end of the fifth year, the average ground temperature had decreased to approximately 7.98 °C, representing a reduction of approximately 2.65 °C compared with the initial ground temperature of 10.63 °C, corresponding to an average annual decline rate of approximately 0.53 °C per year.

As shown in Figure 11, during the 10-year continuous operation period, the variation patterns of the inlet and outlet water temperatures of the source-side borehole heat exchanger system remain generally consistent but exhibit a clear year-to-year downward trend. For both summer cooling and winter heating operations, the inlet and outlet water temperatures gradually decrease with an increasing number of operational years, indicating continuous thermal depletion of the surrounding ground and a progressive weakening of the heat exchange potential. Particularly, during the winter heating season, the year-by-year decline in inlet water temperature leads to a reduced heat transfer temperature difference in the heat pump unit, thereby causing a decrease in the system’s coefficient of performance (COP).

Figure 12 further illustrates the evolution of the average ground temperature. Overall, the ground temperature exhibited a characteristic annual cyclic oscillation: rising during the summer heat rejection period and falling during the winter heat extraction period. However, as the operational years accumulate, the baseline of the ground temperature oscillations continuously shifts downward, reflecting an overall year-by-year decline in ground temperature and indicating that the system is in a state of sustained thermal deficit. By the end of the tenth year, the average ground temperature had decreased to 5.96 °C, representing a reduction of 4.71 °C from the initial ground temperature of 10.63 °C, with an average annual temperature decline rate of approximately 0.47 °C per year. The detailed changes in ground temperature at the end of each operational year are summarized in

Table 2. Statistics of temperature changes in rock and soil masses annually.

Time (h)	Time	Average Temperature of Soil and Rock Mass (°C)
8760	End of the first year	10.01
17,520	End of the second year	9.45
26,280	End of the third year	8.93
35,040	End of the fourth year	8.45
43,800	End of the fifth year	7.98
52,560	End of the sixth year	7.54
61,320	End of the seventh year	7.12
70,080	End of the eighth year	6.72
78,840	End of the ninth year	6.33
87,600	End of the tenth year	5.96

, further verifying the cumulative effect of thermal imbalance.

The long-term source-side thermal imbalance observed after multiyear operation is attributed primarily to the inherent imbalance between the annual heating and cooling loads of the building. The extracted heat during the heating season substantially exceeds the rejected heat during the cooling season, causing the ground source system to remain in a net heat output state over the long term. Given the limited heat diffusion replenishment rate of shallow ground layers and the insufficient compensation provided by summer heat rejection within a single year, the ground-side thermal energy cannot achieve annual equilibrium, leading to a cumulative year-by-year decrease in the ground temperature. Moreover, as the ground temperature decreases, the heat transfer temperature difference between the circulating fluid and the surrounding soil diminishes, further reducing the heat exchange efficiency and exacerbating the degradation of the system’s operational performance.

3.4. Full-Cycle Dynamic Analysis of the Source-Side Ground Heat Transfer Characteristics of Borehole Fields

During the summer cooling operation, the ground-source heat pump (GSHP) system rejects excess heat into the underground soil. In this case, a lower ground temperature leads to a higher operational efficiency of the heat pump unit, as reflected by an increased coefficient of performance (COP), thereby improving the system’s economic performance. Conversely, during the winter heating operation, the system extracts thermal energy from the ground. In this case, a higher ground temperature results in a higher COP and better economic performance. Thus, the system’s operational economic performance imposes conflicting requirements on the underground ground temperature: lower temperatures are preferable during cooling, whereas higher temperatures are preferable during heating.

To ensure the long-term reliability and economic viability of GSHP systems operating year round for both cooling and heating, maintaining a relatively stable underground ground temperature over time is crucial. In other words, the amount of heat rejected into the ground during the summer cooling season must be approximately equal to the amount of heat extracted during the winter heating season, achieving a thermal balance of the underground ground after each annual cooling–heating cycle.

Importantly, the heat extraction and heat rejection amounts through the borehole heat exchangers are not directly equal to the building’s heating and cooling loads. They also depend on the energy efficiency ratio (EER) or coefficient of performance (COP) of the employed heat pump units. The relationships are as follows:

$$CO{P}_{w\mathrm{int}er}={\displaystyle \frac{Ca{p}_{heating}}{P_{heating}}}$$

(4)

$$CO{P}_{summer}={\displaystyle \frac{Ca{p}_{cooling}}{P_{cooling}}}$$

(5)

If the comprehensive COP (energy efficiency ratio) of the ground-source heat pump system with buried pipes is assumed to be 3.6 in winter and 4.8 in summer, then:

$$Q_{absorbed}=Ca{p}_{heating}-P_{heating}=Ca{p}_{heating}\times \left(1-1/COP\right)=0.72\times Ca{p}_{heating}$$

(6)

$$Q_{reject}=Ca{p}_{cooling}-P_{heating}=Ca{p}_{cooling}\times \left(1-1/COP\right)=1.21\times Ca{p}_{cooling}$$

(7)

where Cap is heat production; P is power; Qabsorbed is the heat taken by the heat pump from the buried pipe side (i.e., underground rock soil); and Qreject is the heat dissipated by the heat pump from the buried pipe side (i.e., underground rock soil). The above relationships indicate that the ratio of the heat rejection load to the heat extraction load through the borehole heat exchanger is even greater than the ratio of the building’s cooling load to the heating load. The magnitude of this difference is strongly influenced by the system’s coefficient of performance (COP). In this project, the ground-source heat pump (GSHP) system is designed to handle a base heating design load of 7806.6 kW and a total cooling design load of 15,846.4 kW. The cumulative base heating load reaches 14.6819 million kWh, whereas the cumulative cooling load totals 6.3948 million kWh.

On the basis of the hourly building load calculations undertaken by the borehole field, the system configuration of the borehole heat exchangers, and the operational parameters of the heat pump units, a dynamic simulation model of the GSHP system was established to evaluate its annual operational performance. When the GSHP system is responsible for meeting the entire cumulative base heating load and cumulative cooling load of the project (i.e., 14.6819 million kWh for heating and 6.3948 million kWh for cooling), the annual variation in the inlet and outlet water temperatures is shown in Figure 13. The temperature trends exhibit typical seasonal characteristics: during the winter heating season (January–April), both the inlet and outlet water temperatures are lower than the surrounding ground temperature, reaching the lowest point in January. At this time, the inlet water temperature decreases to approximately 1 °C, and the outlet temperature increases to approximately 6 °C, resulting in a maximum temperature difference of approximately 5 °C. This indicates that the heat pump system operates under high heat exchange load conditions. As the heating season progresses, the temperature difference gradually narrows, decreasing to approximately 1 °C by the end of the heating season in April. Static period (April to mid-May): After the heating season ends, the system enters a dormant phase, during which the ground temperature surrounding the boreholes gradually recovers due to ambient thermal diffusion. Summer cooling season (May to September): During this period, the system continuously rejects heat into the ground through the borehole heat exchangers, causing a simultaneous rise in both the inlet and outlet water temperatures, reaching a peak in July and August. After the cooling season concludes, the system enters the second static phase. However, due to the elevated ground temperature caused by summer heat rejection, the ground temperature slowly decreases during this dormant period.

Overall, since the annual heat balance is dominated by heat extraction (heating loads exceeding cooling loads), the cumulative effect of the annual operation leads to an approximate 1 °C decrease in ground temperature by the end of the year compared with its beginning. The multiyear continuous operation simulations (Figure 9, Figure 10, Figure 11 and Figure 12) further reveal the long-term evolution of the heat transfer characteristics on the source side. The curves of inlet and outlet water temperatures, as well as ground temperatures, display superimposed annual cyclic oscillations with an overall downward trend. By the end of 5 years of operation, the average ground temperature decreased from the initial 10.63 °C to 7.98 °C, corresponding to a total reduction of 2.65 °C, with an average annual temperature decline rate of approximately 0.53 °C/a. By the end of 10 years, the average ground temperature had further decreased to 5.96 °C, resulting in a total reduction of 4.71 °C and an average annual decline rate of approximately 0.47 °C/a. The detailed ground temperature changes at the end of each year are summarized in Table 2, verifying the cumulative nature of the thermal depletion effect. Notably, the rate of ground temperature decline tends to be faster in the early years and gradually slows down in the later years, reflecting the buffering effect caused by enhanced thermal diffusion and heat replenishment from the far-field ground as system operation progresses.

The variation curve of the underground ground temperature is shown in Figure 14. From the perspective of the annual evolution trend, the ground temperature exhibited a typical “double peak and single trough” pattern, with a clear seasonal dynamic response. At the beginning of January, the initial ground temperature is approximately 10.63 °C. During continuous heat extraction under winter heating conditions, the ground temperature subsequently decreases rapidly. By the end of March, the source-side ground temperature reaches its annual minimum, dropping below 9.0 °C, reflecting the thermal depletion effect in the near-borehole region caused by high-intensity heat extraction during the winter season. Between April and May, as the heating season ends, the GSHP system enters a dormant phase where the borehole heat exchangers cease operation. During this period, the ground temperature in the near-borehole region gradually recovers due to ambient thermal diffusion, rising slowly to approximately 9.0 °C by the end of May.

Starting in June, with the progressive activation of the building’s air conditioning system, the GSHP system transitions into cooling operation mode, and heat is continuously rejected into the underground ground. This leads to a rapid rise in ground temperature, reaching an annual maximum of approximately 11.25 °C by the end of August, forming the first peak of the year. From September to mid-October, as the cooling load decreases and the system operation frequency decreases, the GSHP system gradually enters a second dormant period. During this phase, the ground temperature tends to stabilize and shows a slight downward trend. On 20th October, the system re-enters the new heating season. The borehole field resumes heat extraction from the ground, causing the source-side ground temperature to drop rapidly, falling below 10 °C by the end of December and initiating a new annual temperature variation cycle. The full-year simulation results indicate that the shallow ground temperature dynamically varies under the combined effects of building heating and cooling load fluctuations, system operational status, and environmental thermal diffusion conditions. Winter heat extraction causes the ground temperature to decrease, whereas summer heat rejection promotes ground temperature recovery. However, because the annual heating load is significantly greater than the cooling load is, the overall annual heat balance remains negative, leading to a slight downward shift in the baseline ground temperature annually.

The dynamic evolution of the ground heat transfer process within the borehole field has a significant effect on the operational performance of the system. As the ground temperature gradually decreases annually, the heat transfer temperature difference between the circulating fluid and the surrounding ground during the winter heating season decreases, leading to increased thermal resistance. Consequently, the coefficient of performance (COP) of the heat pump units decreases, and the heating capacity becomes constrained. During long-term operation, this can result in reduced heating stability of the system and, under extremely cold conditions, may eventually lead to the system being unable to meet the building’s heating demand. To mitigate ground temperature degradation and ensure the long-term stable operation of the system, multiple comprehensive optimization measures should be implemented during both the system design and operational stages, including reasonably balancing the proportions of heating and cooling loads to increase the summer heat rejection capacity; introducing auxiliary heat reinjection technologies to strengthen summer thermal replenishment; optimizing borehole arrangement and flow rate control to improve source-side heat exchange efficiency; establishing dynamic ground temperature monitoring systems; and implementing intelligent operational scheduling mechanisms to adjust operation strategies in real time and maintain thermal balance within the system.

4. Discussion

The long-term operational performance of shallow ground-source heat pump (GSHP) systems under severe cold climate conditions is significantly influenced by the dynamic evolution of the source-side ground temperature. The preceding analyses systematically revealed the coupled relationship between dynamic building loads and underground thermal responses, the seasonal variations and long-term degradation trends of the ground temperature, and the progressive decline in the source-side heat exchange capacity over time. On the basis of these findings, further discussions were conducted to deepen the understanding of system behavior and to propose targeted optimization strategies.

The simulation results indicated that the year-round imbalance between heating and cooling loads is the fundamental cause of ground temperature degradation. Under the conditions of this study, the annual ratio of heating to cooling loads is approximately 2.3:1, resulting in a long-term net heat extraction state for the GSHP system. This leads to cumulative thermal depletion of the source-side ground and a sustained temperature decline. Compared with mild climate regions, where heating and cooling demands are more balanced, the dominance of heating demand in severe cold regions exacerbates the thermal imbalance, posing significant challenges to the long-term stable operation of GSHP systems.

Strong dynamic coupling exists between the variation in ground temperature and heat exchange performance. As the ground temperature decreases annually, the inlet water temperature of the borehole heat exchangers during the winter heating periods simultaneously decreases, weakening the heat transfer driving force, increasing the thermal resistance, and causing a gradual decline in the coefficient of performance (COP) of the heat pump units. System operation progressively shifts from a high-efficiency zone to a low-efficiency zone, and during winter peak load periods, the degradation of heat exchange capacity becomes particularly pronounced, potentially compromising the stability of the indoor thermal environment.

Although multiple optimization strategies were proposed in this study, including extending the cooling season, introducing auxiliary heat reinjection, and optimizing building load management, each approach faces practical limitations. Increasing cooling loads may lead to higher overall energy consumption; auxiliary heat reinjection technologies require additional investment and increase operational complexity; and building load management must be carefully implemented to maintain indoor thermal comfort. Therefore, future optimization measures should comprehensively consider system energy efficiency, economic feasibility, and operational practicability to develop a dynamic, adaptive integrated control framework.

To address the long-term operational challenges of shallow GSHP systems in severely cold climates, future research should focus on several directions: first, conducting system design parameter sensitivity analyses to systematically evaluate the effects of variations in the flow rate, borehole depth, and borehole spacing on ground temperature evolution and system performance; second, combining simulation and experimental studies to validate the effectiveness and feasibility of different thermal balance optimization strategies (e.g., auxiliary heat reinjection); third, establishing predictive models for COP evolution on the basis of dynamic ground temperature changes to enable system lifecycle energy efficiency assessment; fourth, developing intelligent control strategies on the basis of real-time building load and ground temperature feedback to optimize dynamic flow rates and heat exchange modes; and finally, exploring multi-energy complementary integrations between shallow GSHP systems and solar energy, thermal storage, and waste heat recovery technologies to further enhance overall system efficiency and ground thermal sustainability.

Among the proposed strategies, auxiliary summer heat reinjection using rooftop solar collectors or air-source hybridization and adjusting circulation flow rates (within 0.4–0.8 m/s) are considered the most technically and economically feasible for retrofitting. These measures enhance thermal replenishment without requiring additional drilling, making them suitable for existing system upgrades.

In conclusion, the source-side ground heat transfer characteristics and their long-term degradation trends are core issues affecting the stability and sustainability of shallow GSHP systems in severely cold regions. By systematically analyzing the load–ground temperature coupling characteristics, long-term operational behavior, and optimization strategies, this study provides a solid theoretical foundation and practical engineering guidance for the efficient operation and sustainable development of future GSHP systems.

This study is based entirely on simulation data, without support from long-term field monitoring. While the models are calibrated using design standards and literature benchmarks, the absence of empirical validation introduces uncertainty. Future studies should incorporate real-time monitoring data to validate degradation rates and refine model inputs.

The assumption of spatially uniform soil thermal properties may overlook localized anomalies or groundwater effects, potentially underestimating lateral thermal recharge. However, in the absence of detailed subsurface variability data, such assumptions offer a practical and accepted modeling baseline.

5. Conclusions

This study focused on a typical logistics building under severe cold climate conditions, combining hourly dynamic building load modeling with multicycle simulations of the source-side borehole system. The long-term coupling characteristics between dynamic building loads and ground thermal responses were systematically investigated, the evolution laws of ground temperature degradation and heat exchange performance were quantitatively analyzed, and corresponding comprehensive optimization strategies were proposed. The main conclusions are as follows:

(1)

The cumulative base heating load of the building reached 14.6819 million kWh, whereas the cumulative cooling load totaled 6.3948 million kWh, resulting in a heating-to-cooling load ratio of approximately 2.3:1. This imbalance caused the system to operate in a net heat extraction state throughout the year. The source-side ground temperature decreased by approximately 1.0 °C over a complete annual operation cycle, indicating a significant trend of thermal imbalance.

(2)

Multicycle dynamic simulation results show that after five consecutive years of operation, the source-side ground temperature decreased from the initial 10.63 °C to 7.98 °C, with a cumulative reduction of 2.65 °C. After ten years of operation, the temperature further decreased to 5.96 °C, resulting in a total reduction of 4.71 °C, with an average annual decline rate of approximately 0.47–0.53 °C per year. Ground temperature degradation led to a 4–6 °C decrease in borehole outlet water temperatures during the winter heating season, a reduction of more than 30% in the circulating water inlet–outlet temperature difference, and an average decline of approximately 12% in the heat pump COP.

(3)

The fundamental causes of the decrease in the cumulative ground temperature and heat exchange performance degradation were identified as the persistent dominance of heating loads over cooling loads during the annual cycle combined with the insufficient heat replenishment capacity of shallow ground layers. The most affected region was concentrated within a radius of approximately 2 m around the boreholes, where thermal depletion was the most significant.

(4)

By optimizing the allocation of building heating and cooling loads, extending the summer heat rejection period, increasing auxiliary heat reinjection (recommended to account for 15 to 20% of the annual heat extraction), and reasonably adjusting borehole flow rates (maintained between 0.4 and 0.8 m/s) and borehole layout density, the ground temperature degradation rate can be effectively reduced by approximately 30–40%, extending the system’s high-efficiency operational lifespan by more than five years.

(5)

The simulation results indicate that the current borehole field configuration, with 5 m × 5 m spacing, may be suboptimal for the given thermal load profile. Insufficient ground source sizing and close borehole spacing lead to intensified thermal interference and cumulative temperature decline. Increasing the number of boreholes with a larger spacing (7–10 m) and improving the ground’s moisture content are effective approaches to enhance thermal sustainability.

This study innovatively performs a quantitative analysis of annual and multicycle ground temperature degradation based on hourly dynamic loads under severe cold climate conditions. This study systematically revealed the thermodynamic degradation mechanisms and proposed comprehensive regulation strategies, including load optimization, auxiliary summer heat reinjection, and operational parameter adjustments, providing valuable engineering guidance for practical applications. However, this study was based on dynamic simulations and analyses of a single building case with typical meteorological year data. This method does not consider the effects of multibuilding load superposition, actual meteorological year variability, or source-side borehole interference in dense fields. Additionally, the effectiveness of optimization strategies has been derived primarily from theoretical analysis, lacking systematic experimental validation or large-scale engineering demonstrations. Future research should further incorporate multisource load scenarios, different climatic conditions, and complex hydrothermal coupling effects to better understand the source-side thermal field evolution mechanisms. Moreover, efforts should focus on establishing long-term energy efficiency prediction models on the basis of measured data and exploring intelligent control strategies and multi-energy complementary system integrations to promote the large-scale, intelligent, and efficient sustainable application of shallow GSHP systems in cold regions.