Performance Prediction and Analysis of Solar-Assisted Ground-Source Heat Pump Systems in Typical Rural Areas, China

Abstract

The increasingly severe energy crisis and associated environmental issues pose new challenges for the efficient and rational utilization of renewable energy. The solar-assisted ground-source heat pump (SAGSHP) system is a novel heating system that effectively combines the advantages of both solar and geothermal energy. In this study, an SAGSHP system was established through TRNSYS simulation software to provide winter heating and year-round domestic hot water for a residential building. By varying the area of solar collectors (A) and the number (n) and the depth (H) of the borehole heat exchangers (BHEs), the system operational performance, including the system energy consumption, ground temperature attenuation, and heat pump efficiency, was investigated. A comparison with a single ground-source heat pump (GSHP) system was also conducted. After 20 years of operation, the parameter optimization resulted in a reduction of approximately 60 MWh and 70 MWh in system energy consumption, equivalent to saving 7.37 t and 8.60 t of standard coal, respectively. At the same time, the total costs over 20 years can be reduced by 48.20% and 33.77%, respectively. The proposed design method and simulation results can serve as the reference for designing and analyzing the performance of the SAGSHP system.

1. Introduction

As globalization and sustainable development concepts continue to evolve, the persistent increase in global energy demand has emphasized the strategic importance of integrating renewable energy sources, such as solar, geothermal, and wind power, to replace the existing energy structure, a major direction for energy development, aiming to reduce the environmental impact of energy, enhance energy security, and promote sustainable development capabilities [1,2]. The increasing demand for indoor environmental quality, driven by population growth, has resulted in a steady escalation of energy usage in buildings. Therefore, optimizing the existing building energy supply mode has become more and more important [3,4]. The energy efficiency of rural buildings is notably inferior to that of their urban counterparts, primarily attributable to the dearth of comprehensive knowledge among rural households concerning the optimization of building envelopes, energy-efficient appliances, and strategic planning [5,6,7]. The thermal performance of buildings in rural northern China is relatively poor, mainly relying on wood and coal as the primary fuels, resulting in low energy utilization efficiency and significant pollution. Therefore, providing a suitable solution for supplying rural heating has become an urgent priority [8,9].

Solar energy and geothermal energy, due to their clean, environmentally friendly, and renewable characteristics, are currently widely applied in multiple fields, including heating, cooling, and domestic hot water (DHW) supply [10,11]. GSHP systems, characterized by high energy utilization efficiency and stable operation, are experiencing rapid development [12,13]. Due to the widespread distribution and large storage capacity, geothermal energy is widely utilized for winter heating [14,15]. However, in most regions, the ground temperature attenuates with prolonged operation, resulting in inadequate heating capacities and the instability of ground-source heat pump systems, thereby compromising the system’s ability to sustain indoor thermal comfort [16,17]. Solar energy is also a commendable renewable energy source with multiple advantages [18,19]. Nevertheless, the output power of solar energy systems is susceptible to weather and climate conditions, which leads to the instability of the operational performance and restricts their standalone application in building heating supply. Furthermore, solar energy systems are also prone to the impact of elevated temperatures on solar panels, resulting in decreased conversion efficiency [20,21,22]. Therefore, achieving the coupling solar and geothermal energy to establish a multi-energy complementary system can not only avoid the decrease in the coefficient of performance (COP) of GSHP systems during long-term operation but also ensure a stable power output of solar energy systems. This hybrid approach is instrumental in advancing the deployment and reliability of renewable energy infrastructures.

Most existing research has concentrated on the engineering, design, performance analysis, and application aspects of these systems [23,24,25]. Weeratunge et al. performed an analytical study on an SAGGSHP system. The results demonstrate that the integrated thermal storage coupled system improves peak-shaving performance, while optimized operation reduces operating costs by 7.8% [26]. By comparing and evaluating the hybrid configurations combining solar collectors, photovoltaics, and PVT with GSHP, with a focus on analyzing their electricity, hot water, and heating demands, the study reveals that the hybrid system with integrated photovoltaics and ground-source heat pumps has higher efficiency than other types [27]. Ballerini et al. analyzed the performance of an SAGSHP system installed in a school in Milan, Italy. The system implements seasonal thermal energy storage technology, enabling the utilization of summer-stored heat for winter heating. The results show that integrating solar collectors increased the heat pump’s seasonal coefficient of performance (SCOP) by approximately 5% [28]. Yan et al. designed and studied a PV-GHE system located in Tikanlik, China. The system operates by transferring heat from the solar panel to the ground using water, provided that the panel’s temperature exceeds 50 °C. When the panel temperature drops below 48 °C, the water pump is turned off. Compared with the conventional PV system, this setup shows an increase in PV efficiency by 4.1–11.1%, alongside a 7.9% improvement in annual power generation [21]. Putrayudha et al. used a fuzzy logic control system to optimize building energy demand. The PVT-GSHP, equipped with this control, achieves an 18.3% reduction in yearly energy consumption [29].

In this study, a SAGSHP system is established for the winter heating and year-round DHW supply of the rural residential building in Beijing and Changchun. To comprehensively analyze the influence of design parameters on the operational efficiency of the system, various simulation cases are selected by varying the solar collector area (A), number (n), and depth (H) of the borehole heat exchangers. Subsequently, based on the simulated energy consumption, the heat extraction attenuation of the borehole heat exchanger, and the COP of the heat pump, the operational performance of each case is analyzed and evaluated. The proposed research methodology and resulting conclusions aim to provide a theoretical foundation and methodological guidance for the practical design of the SAGSHP systems.

2. System Description

2.1. Target Building Description

The target building selected for this study is a typical residential architecture in Beijing and Changchun (representing the cold and severely cold regions in China, respectively). The case building is a one-floor rural residence with a ceiling height of 3 m, consisting of two bedrooms, a living area, kitchen, bathroom, and a vestibule. The total area subjected to space heating measures approximately 102.36 square meters. Thermal properties of the building envelope are detailed in

Table 1. Thermal parameters of the building envelope.

Building Envelopes	Materials	Heat Transfer Coefficient (W·m−2·°C−1)
External walls	Cement mortar, bricks, insulation layer	0.455
External door	Exterior metal door	3.150
Ceiling	Reinforced concrete, cement mortar, composite roof panel	0.579
Windows	Low-E glazed	2.52

. The hourly outdoor temperature and solar radiation intensity on the horizontal plane throughout the year for both cities are shown in Figure 1 and Figure 2.

The analyzed dwelling is occupied by a three-person household and features an indoor heated space of approximately 102.56 m², with a ceiling height measured at 2.9 m. The heating season in Beijing and Changchun was set from November 15th to March 15th of the following year and from October 20th to April 6th of the following year, respectively. By placing the building under meteorological conditions in rural areas of the two cities, the hourly heating load over a year (8760 h) can be obtained, as illustrated in Figure 3. The highest load of the two buildings during a year both occur in January, reaching 6.573 kW and 10.416 kW, respectively. And the peak DHW loads reach 1.45 kW and 1.66 kW, respectively.

This study examines the winter heating load and annual DHW demand for the rural buildings. Due to the scattered nature of residential buildings, the centralized heating commonly found in cities may not be as suitable for rural areas. Additionally, these regions have vast expanses of land with minimal obstructions, rendering them highly conducive to the adoption of clean energy like geothermal energy and solar energy [30,31]. Thus, it is feasible to couple geothermal energy and solar energy to meet the winter heating and DHW needs of rural dwellings.

2.2. Coupled System Description

A SAGSHP system was designed for residences in rural areas in cold and severely cold regions. The schematic configuration of the integrated system is illustrated in Figure 4. The system primarily consists of a solar collector loop (red lines), a borehole heat exchanger loop (dark blue and rose red lines), a GSHP loop (yellow and green lines), and a DHW loop (light blue lines). In this system, the heat obtained from the U-pipe borehole heat exchangers will be utilized to supply the heat through the GSHP throughout the heating period. The solar energy is utilized to assist the GSHP in providing winter heating, while it is also used to meet the year-round DHW demand of the building and return excess heat back into the ground during non-heating seasons.

3. Simulation System and Operational Strategies

System design and operational strategies are crucial factors for achieving high performance efficiency in the coupled system. This section outlines the various operating modes during different periods and provides relevant data from the main modules, serving as the foundational model for subsequent research.

3.1. Simulation System Introduction

The SAGSHP system was simulated using TRNSYS 18.0 software. Figure 5 illustrates the primary modules of the coupled system, including the U-pipe BHE, the GSHP, the solar collector, two water tanks, and the circulating pumps. There are shared design parameters across the two cities. The TRNSYS types and design parameters for each component are summarized in

Table 2. Design parameters of primary modules in TRNSYS system.

Component/TRNSYS Type	Parameters	Value 1
Borehole heat exchanger/557a	Pattern	Single U-pipe
	Number of boreholes	2
	Borehole spacing	7 m
	Borehole depth	150/200 m
	Borehole radius	0.089 m
	Outer radius of pipe	0.016 m
	Inner radius of pipe	0.013 m
	Pipe thermal conductivity	0.42 W·m−1·K−1
	Fill thermal conductivity	2.2 W·m−1·K−1
	Rock–soil thermal conductivity	1.80/2.12 W·m−1·K−1
	Rock–soil heat capacity	2420/1800 kJ·m−3·K−1
	Thermal gradient	0.0280/0.0394 °C/m
Ground-source heat pump/225	Rated heating power	27.3/44.1 kW
	Rated heating capacity	6.5/10.5 kW
Solar collector/165	Collector slope	50°/54°
	Collector area	15/20 m2
Domestic hot water tank/158	Heat loss coefficient	0.4 W·m−2·K−1
	Tank volume	0.20/0.25 m3
Thermal storage tank/158	Heat loss coefficient	0.4 W·m−2·K−1
	Tank volume	2/3 m3

¹ The notation “value1/value2” denotes the respective parameters for the systems in the Beijing and Changchun systems, respectively. In cases where the results are consistent across both regions, only a single value is reported.

[32,33,34,35].

3.2. Operational Strategies

3.2.1. Solar Collector Loop

Whenever there is sufficient solar radiation intensity, the solar collector will transport heated water to the water tank for thermal storage. The TRNSYS diagram and control logic diagram of the solar collector loop are shown in Figure 6. When the outlet temperature of the collector (t_S) exceeds the temperature of the thermal storage tank (t_T) by more than 8 °C, Pump5 starts and the loop begins to operate. If the loop is already in operation, when the temperature difference between t_S and t_T falls below 2 °C, Pump5 shuts down and the loop ceases operation. Based on the temperature start–stop control described above, it can effectively prevent increased heat loss, decreased heat extraction efficiency, and unnecessary energy consumption resulting from the continuing operation during insufficient solar radiation intensity time.

3.2.2. Domestic Hot Water Loop

The DHW tank extracts thermal energy from the storage tank to provide hot water to users, meeting the DHW demand. As shown in Figure 7, when t_T exceeds the temperature of the DHW tank (t_DHW) by 5 °C, the thermal storage tank initiates heat supply to the DHW tank, activating Pump4. Once the temperature difference between the thermal storage tank and the DHW tank falls below 2 °C, Pump4 shuts down.

Due to the inevitable intermittency and variability of the solar energy, the electric auxiliary heating is introduced for the DHW tank to ensure the water supply temperature. The electric auxiliary rated heating capacities in the Beijing and Changchun systems are 1.5 kW and 1.8 kW, respectively. Once the DHW tank temperature drops below 45 °C, the electric auxiliary heating begins to supply the heat. To avoid frequent starts and stops, a dead band of 2 °C is set in the control module, and the auxiliary heating stops once the DHW tank temperature exceeds 47 °C. In addition, to maintain a consistent temperature for the DHW supply, a bypass (Type 11 h and Type 11 f) is incorporated into the DHW loop. The flow rate in the bypass is regulated through mixed water control to consistently achieve a temperature of 45 °C, as set. The introduction of the bypass and mixed water control will reduce the heating demand of the DHW tank as well, thereby decreasing the energy consumption and improving the system efficiency.

3.2.3. Borehole Heat Exchanger Loop and Ground-Source Heat Pump Loop

In this research, the single U-pipe BHE is utilized to extract heat from shallow rock and soil. Subsequently, the heat transfer fluid, further heated by the GSHP, is delivered to terminal users for winter heating. The borehole heat exchanger operates continuously throughout the season. To avoid the attenuation of the ground temperature resulting from long-term continuous operation, which could impact the thermal extraction efficiency of the BHE and the operational stability of the system, a design utilizing surplus solar energy for thermal recharging in non-heating seasons is introduced. As illustrated in Figure 8, while providing heating to the DHW tank, if the thermal storage tank temperature remains above 65 °C, the heated water will be transported to the BHE to facilitate ground heat recovery. The process will cease when the heat storage tank temperature drops below 60 °C.

Due to the relatively low temperatures in shallow soil, typically ranging between 10 °C and 30 °C, the outlet water temperature of the borehole heat exchanger cannot meet the required heating water temperature. The GSHP is used to transfer heat from the low-temperature heat source at the outlet of the BHE to the high-temperature fluid to be used for terminal heating. Meanwhile, the heat pump is the component with the highest power consumption in this coupled heating system. Its operating duration will significantly affect the cumulative energy expenditure and efficiency of the system. In the heating season, solar energy will supplement the geothermal heat pump’s operation to enhance heating efficiency. If the thermal storage tank has heated the DHW tank and its temperature remains above 65 °C, the hot water from the thermal storage tank will be used to provide heating to the users. Moreover, the bypass control is employed to maintain the heating water temperature at approximately 45 °C.

4. Results and Discussion

Following the aforementioned parameter design and operational strategies in TRNSYS, the performance of the coupled systems in Beijing and Changchun was simulated for continuous operation over a period of 20 years. The literature review indicates that the solar collector area (A), number of boreholes (n), and borehole depth (H) are the most frequently studied system parameters in solar–geothermal heating systems [36,37,38]. The solar collector area is the primary determinant of harvestable solar thermal energy, while the number and depth of boreholes significantly affect the geothermal extraction efficiency and surrounding ground temperature variations. Moreover, the synergistic interaction among these parameters poses challenges for coupled system design optimization. Therefore, this study systematically investigates the individual and synergistic effects of these three key parameters on overall system performance.

4.1. Energy Consumption of Ground-Source Heat Pump

Energy consumption is one of the important factors reflecting the efficiency of the system operation, with the total energy consumption of the coupled system in this study being calculable as follows:

$$P=P_{GSHP}+{\displaystyle \sum P_{pump,i}}+P_{AUX}$$

(1)

where P_GSHP, P_pump,i, and P_AUX, respectively, represent the energy consumption of the ground-source heat pump, each circulating pump, and the electric auxiliary heating for the DHW tank.

As mentioned before, the GSHP is the component with the highest energy consumption in the system, and its impact on total energy consumption is significant. Therefore, it is essential to conduct an analysis and comparison of heat pump energy consumption across various operational conditions, as illustrated in Figure 9a,b. Different curves represent variations in the heat extraction of borehole heat exchangers with different numbers of boreholes and collector areas.

In both Beijing and Changchun systems, the variations in the energy consumption of the heat pump can be categorized into three groups of curves due to different collector areas. This is because higher solar energy collection and storage will significantly reduce the operating time of the GSHP, directly affecting its energy demand. Simultaneously, the decreased operating time of the heat pump mitigates the decline in ground temperature throughout a single heating season, resulting in a lesser decrease in the outlet water temperature from the borehole heat exchanger. Consequently, as the thermal demand on the heat pump decreases, the system experiences a notable decline in energy consumption.

It is observed that, regardless of the cases, the energy consumption of the heat pump consistently decreases with the increasing of H. However, from Figure 9b, it can be found that when H, n, and A are all relatively low, the reduction in the energy consumption of the heat pump with the increase in these three design parameters is not significant. For residential buildings located in severe cold regions (Changchun) with high loads, in this study, most of the solar energy is used to meet the DHW load. Additionally, shallowly buried pipes result in a low outlet water temperature from the BHE. Thus, the GSHP needs to operate at a high load over extended periods to meet the heating demand of users. This leads to generally higher energy consumption results.

As the borehole depth increases further, the variation in energy consumption gradually decreases. More significantly, when there are more buried pipes or larger collector areas, the trend of energy consumption variation becomes more moderate, as shown in Figure 9a,b. Taking the case of n = 1 and A = 15 m² in the Beijing system as an example, when H increases from 100 m to 150 m, the energy consumption of the heat pump decreases from 32,044.48 kWh to 31,293.92 kWh, a decrease of 2.34%, while when H increases from 200 m to 250 m, the energy consumption decreases from 30,594.57 kWh to 30,051.93 kWh, a decrease of 1.77%. The overall decrease is 6.22%. For the case of n = 3 and A = 25 m² in the Beijing system, the overall decrease is only 2.33% during the increase in H. Additionally, when H and A are constant, the energy-saving effect of increasing n from 1 to 2 is significantly higher than further increasing the number of the boreholes.

4.2. Energy Consumption of Electric Auxiliary Heating

The electricity consumption of the electric supplementary heating system for the DHW tank represents a key factor affecting the system’s total energy use. Figure 10 illustrates the electric auxiliary heating energy consumption for the system operating over 20 years under different cases. It can be found that the increase in the collector area strongly influences the electricity consumption. Taking the system in Changchun with n = 1 and H = 100 m as an example, when the collector area increases from 15 m² to 25 m², the electricity consumption decreases by 45.68%.

As H and n increase, the affected range of soil and rock surrounding the borehole heat exchanger will also expand. Therefore, in non-heating periods, the proportion of solar energy used for ground reinjection will increase, leading to a rise in the electricity demand for the auxiliary heating of the DHW tank to maintain the appropriate water supply temperature. Meanwhile, due to the high load of the Changchun system, the variation in the proportion of solar energy allocated to meet the DHW load across different cases is less pronounced. Compared to the Beijing system, although the electric auxiliary energy consumption is higher under the same case in the Changchun system, the increase with the rise in H and n is comparatively gradual.

4.3. Total Energy Consumption

According to the analyses in Section 4.1 and Section 4.2, with the increase in H and n, there is an inverse trend in the variation in heat pump energy consumption and electric auxiliary heating energy consumption. Given the various factors influencing the total energy consumption of the system, it is essential to select the optimal system design parameters. Figure 11 demonstrates that the variation in the solar collector area has the greatest effect on the overall energy usage of the system. This occurs because increases in solar heat collection and storage can concurrently reduce energy consumption for both the heat pump and auxiliary electric heating elements—the two dominant contributors to total energy consumption. In the simulated cases, as the collector area increases, the reduction in the total energy consumption can reach up to 20%. However, this decreasing trend is also modulated by H and n. When all three design parameters increase simultaneously, the rising energy consumption of the electric auxiliary heating progressively outweighs the reduction in the heat pump energy consumption, consequently decelerating total energy consumption decline. Taking the condition of n = 3 and H = 250 m as an example, increasing the collector area from 15 m² to 25 m² results in a mere 16.41% reduction in total energy consumption. For the higher-load Changchun system, this reduction merely reaches 14.47%.

When H and n increase, different cases show varying trends in total energy consumption. By comparing Figure 11a,b, it can be found that for situations with smaller heating loads (Beijing system), the total energy consumption shows a slight increase, ranging from 1% to 8%. For the Changchun system with higher heating load dominance, increasing H and n demonstrates more pronounced efficacy in reducing heat pump energy consumption for heating, thereby driving down the overall system energy consumption. After increasing the number of the boreholes, the total energy consumption decreases steadily with increasing depth; however, the rate of reduction decreases as the collector area and borehole depth increase.

To further investigate the varying trends in total energy consumption, Figure 12 compares the overall energy demand during the heating and non-heating seasons for several cases while keeping the collector area constant (20 m²). The total energy demand during heating and non-heating seasons exhibits opposite trends as the design parameters change. The calculation based on the data in Figure 12a,b shows that with the increase in H and n, the proportion of energy consumption during the heating season to the total energy demand over twenty years gradually decreases. For the Changchun system, this decreased from 92.51% to 86.94%, while for the Beijing system, it decreased from 86.45% to only 75.56%. This reflects a declining proportion of the GSHP energy consumption for terminal heating. Concurrently, non-heating season electric auxiliary heating consumption shows a 97.18% surge—a trend that may drive overall energy consumption upward. Evidently, when the proportion of heating season energy use or heat pump consumption to total energy demand decreases significantly, increasing H and n provides diminishing returns for total energy reduction. Moreover, given the substantial initial investment required for system upscaling, parameter design must be rigorously tailored to actual load profiles.

The sensible design of system parameters will significantly affect the system’s energy consumption and further impact the operational electricity costs. However, it is important to note that the system’s operational performance is numerous and complex. Consequently, pursuing energy reduction as a sole objective represents an oversimplified approach to system optimization.

4.4. Heat Extraction of Borehole Heat Exchanger

As the operational years increase, there will be a gradual decline in the ground temperature surrounding the BHE. The progressive reduction in the temperature difference between the ground and the circulating fluid in the BHE leads to a continuous attenuation of heat extraction. This phenomenon subsequently affects multiple subsurface thermal parameters and compromises the long-term operational stability of the geothermal system. In this study, in non-heating seasons, the surplus solar energy is utilized for thermal reinjection through borehole heat exchangers to facilitate ground temperature recovery. Figure 13a,b compare the ground temperature trends with and without thermal reinjection under the operational condition of H = 100 m, n = 1, and A = 25 m². For the Beijing system, the ground temperature declined from an initial 13.68 °C to 11.93 °C following 20 years of operation. Thermal reinjection effectively mitigated this depletion, maintaining the temperature at 13.61 °C by year 20. For the Changchun system, a significant ground temperature decline occurred regardless of thermal reinjection implementation, with temperatures decreasing to 8.81 °C and 7.89 °C by year 20, respectively.

To further indicate the systemic impacts of long-term ground temperature depletion, Figure 14a,b illustrate the variations in heat extraction for 10 selected cases (including situations without thermal reinjection) in both Beijing and Changchun systems for 20 years.

From Figure 14a, it can be observed that greater H and n result in higher levels of heat extraction. However, their respective increments do not increase monotonically with the increase in H and n. For example, the greatest increment occurs in the case where H = 100 m, n = 3, and A = 25 m². After 20 years of operation, the heat extraction increases from 3577.43 kWh to 3596.38 kWh. In addition, in the Beijing system, apart from the cases without thermal reinjection, the heat extraction for all other conditions demonstrates a gradual increasing trend, which slows down with the increase in operational years. In contrast, the two sets of cases without supplementary heating show the noticeable decrease in heat extraction after 20 years of operation. This indicates that for the design parameters concerning the studied buildings and heating systems, relying solely on natural ground temperature recovery is insufficient to maintain a constant ground temperature.

For the Changchun system, due to the higher load, thermal reinjection remains insufficient according to Figure 14b. When H and n are large, it implies that the rock and soil surrounding the BHE have a higher heat capacity. Consequently, while the heat extraction is greater, its rate of decay is lower. Compared with the Beijing system, the two cases without thermal reinjection exhibit a more significant trend of heat extraction attenuation. When H = 100 m, n = 1, and A = 25 m² (no thermal reinjection), after 20 years of operation, the heat extraction decreased by nearly 165 kWh. The rate of decline in heat extraction increased from 1.44% to 2.00%. Thus, to avoid a series of potential issues caused by ground temperature attenuation, introducing thermal reinjection is proven necessary. And the properly matched design parameters (H, n, and A) can achieve significant enhancement in thermal reinjection performance, thereby promoting ground temperature recovery and ensuring long-term operational stability.

4.5. COP of Ground-Source Heat Pump

The COP, used to reflect the efficiency of the heat pump, is one of the key performance indicators for system operation. It can be calculable as follows:

$${\mathrm{COP}}_{GSHP,20}={\displaystyle \frac{Q_{GSHP}}{P_{GSHP}}}$$

(2)

where Q_GSHP and P_GSHP respectively represent the heat supply and energy consumption of the GSHP during 20 years of operation.

The COP chart of the GSHP operating under various conditions for 20 years is depicted in Figure 15. As H, n and A increase, the COP of the heat pump shows a rising trend. Where the increase in H has the most significant impact on the COP of the heat pump. When n = 1 and A = 15 m² in Beijing system, as H increases from 100 m to 250 m, the COP increases from 4.07 to 4.33, with a rise of 6.39%.

Moreover, as the values of the three system parameters increase, the rate of COP improvement progressively declines. In some results points of Figure 15, there is a phenomenon of close to each other. This phenomenon is more pronounced in the Beijing system with lower loads. Taking n = 2, A = 25 m² and n = 3, A = 15 m² as examples, when H for both groups of cases is set to 100 m, the COP of the heat pump is 4.272 and 4.270, respectively. However, for the n = 2, A = 25 m² condition, the rate of COP increase with increasing H is higher. Therefore, when H = 250 m², a notable difference in the COP of the heat pump is observed between the two aforementioned cases. Other curves also display similar patterns, which implies that if there is a need to increase the borehole depth, keeping the remaining two parameters within a smaller range of values will result in a more significant increase in the heat pump’s efficiency. In addition, reducing n and A would effectively decrease the initial investment and the payback period of the system, especially the number of boreholes. Therefore, such patterns of variation are desirable.

4.6. Summary of Results and Comparison

As evidenced by Section 4.1, Section 4.2, Section 4.3, Section 4.4 and Section 4.5, the selection of design parameters significantly impacts system performance. Unreasonable equipment sizing compromises long-term operational stability—particularly for shallow borehole depths (≤100 m), where over-extraction may occur. Figure 16 illustrates the heat extraction decline characteristics across varying borehole depths ranging from 50 m to 250 m. It can be found that, conditions with shallow burial depths (50 m and 100 m) exhibit significantly more pronounced heat extraction decline rates compared to systems with greater depths. This phenomenon occurs because the thermal storage capacity of shallow soil surrounding BHEs is insufficient to meet the specified heating load demands in the study. Under this condition, the design of thermal reinjection becomes even more essential.

To ensure timely ground temperature recovery while effectively reducing total system energy consumption, three sets of results were selected as optimal cases for both the Beijing and Changchun systems. And economic estimate was conducted for the selected cases. The total cost during 20 years of operation can be calculated through:

$$C_{\mathrm{total}}=C_I+C_o$$

(3)

$$C_I=C_d+C_p+C_c$$

(4)

$$C_O={\displaystyle \sum _{i=1}^T{P}_i{a}_i}$$

(5)

where C_I and C_O represent the initial investment and the operating electricity cost, respectively. C_d, C_p and C_c are the cost of drilling, pipeline lying and solar collectors. Because the main focus here is on comparing total costs under different cases, components such as the thermal storage tank, system fittings and so on, which have consistent prices among different cases, are not included in Formula (4). P_i and a_i represent hourly total energy consumption and electricity price, respectively. The hourly electricity prices of Beijing and Changchun are as illustrated in

Table 3. Hourly electricity prices of Beijing and Changchun.

Beijing	Time periods	Electricity price, CNY/kWh
Peak pricing	14:00~17:00, 19:00~22:00	0.9857
Shoulder pricing	8:00~14:00, 17:00~19:00, 22:00~24:00	0.6021
Valley pricing	0:00~8:00	0.3
Changchun	Time periods	Electricity price, CNY/kWh
Peak pricing	8:00~21:00	0.562
Valley pricing	21:00~8:00	0.329

.

In addition, to illustrate the benefits of introducing solar energy, a standalone GSHP heating system are employed as the control group. The schematic diagram of the single GSHP system is illustrated in Figure 17 (red lines for user side and purple lines fir BHE side), with the design parameters illustrated in

Table 4. Design parameters of the single GSHP system.

Component/TRNSYS Type	Parameters	Value 1
Borehole heat exchanger/557a	Pattern	Single U-pipe
	Number of boreholes	2
	Borehole spacing	7 m
	Borehole depth	150/200 m
	Borehole radius	0.089 m
	Outer radius of pipe	0.016 m
	Inner radius of pipe	0.013 m
	Pipe thermal conductivity	0.42 W·m−1·K−1
	Fill thermal conductivity	2.2 W·m−1·K−1
	Rock-soil thermal conductivity	1.80/2.12 W·m−1·K−1
	Rock-soil heat capacity	2420/1800 kJ·m−3·K−1
	Thermal gradient	0.0280/0.0394 °C/m
Ground-source heat pump/225	Rated Heating Power	27.3/44.1 kW
	Rated Heating Capacity	6.5/10.5 kW

¹ The notation “value1/value2” denotes the respective parameters for the systems the Beijing and Changchun systems, respectively. In cases where the results are consistent across both regions, only a single value is reported.

. The comparison results are illustrated in

Table 5. Results comparison (Beijing system).

Beijing	H = 100 m, n = 2, A = 20 m2	H = 150 m, n = 1, A = 25 m2	H = 250 m, n = 2, A = 25 m2	Control Group
Energy consumption, kWh	49,035.78	43,350.22	46,822.96	101,578.70
Heat extraction decay rate 1	−0.39%	−0.45%	−0.52%	0.51%
COP of GSHP	4.24225	4.23859	4.42155	4.28314
Initial investment, CNY	38,000	31,000	88,750	49,500
Operating electricity cost, CNY	27,396.29	24,375.83	26,431.56	57,412.28
Total cost, CNY	65,396.29	55,375.83	115,181.56	106,912.28

¹ Negative value means an increase in heat extraction.

and

Table 6. Results comparison (Changchun system).

Changchun	H = 150 m, n = 2, A = 20 m2	H = 150 m, n = 2, A = 25 m2	H = 250 m, n = 1, A = 25 m2	Control Group
Energy consumption, kWh	128,747.32	116,659.97	113,558.05	184,221.23
Heat extraction decay rate	1.17%	0.91%	0.66%	1.40%
COP of GSHP	4.19286	4.21323	4.26632	4.26556
Initial investment, CNY	54,500	55,750	47,500	66,000
Operating electricity cost, CNY	56,326.95	51,213.73	50,249.44	81,591.58
Total cost, CNY	110,826.95	106,963.73	97,749.44	147,591.58

.

For the Beijing system, the heat pump efficiency at H = 150 m, n = 1, and A = 25 m² is slightly lower than the other cases, but it has significantly lower energy consumption and total costs. For the Changchun system, the case with H = 250 m, n = 1, and A = 25 m² not only features lower energy consumption and total costs but also ensures a lower rate of heat extraction decay, which is desirable. A comprehensive analysis of simulation results across all operational conditions reveals the following for low-load demand systems (Beijing system): While increasing the borehole quantity (n) and depth (H) reduces heat pump energy consumption, excessive depth leads to elevated total energy consumption and high initial investment. Therefore, the recommended burial depth range is between 100 and 200 m, with no more than two wells. For the Changchun system, since the load is relatively high, it is recommended to increase the burial depth, with design parameters of 200 m or more, which can effectively reduce the heat extraction decay rate over long-term operation.

Moreover, compared to the results of the control group, various aspects of the system performance have shown significant improvements, particularly in reducing energy consumption. After parameter optimization, the energy demand of the two systems decreased by approximately 60 MWh and 70 MWh in comparison with the single GSHP heating system, which is corresponding to savings of saving about 7.37 t and 8.60 t of coal equivalent, respectively.

5. Conclusions

As a reaction to the increasingly severe energy crisis and environmental challenges, it is imperative to conduct the design and improvement of multi-energy coupled systems. This study presents a SAGSHP system established for typical cities in cold and severely cold regions of China, namely Beijing and Changchun. This system is designed to meet the heating load during winter and the year-round domestic hot water load. Through analyzing operational results under different cases, the subsequent conclusions can be inferred:

(1)

The introduction of solar energy has reduced the energy consumption of various components within the heating system. As the area of solar collectors increases (from 15 m² to 25 m²), the overall energy consumption of the system decreases significantly, with a maximum reduction of up to 20%. However, the increase in the number and depth of borehole heat exchangers results in an opposite trend in the energy consumption of the GSHP and the electric auxiliary heating within the DHW tank.

(2)

Utilizing surplus solar energy for ground thermal reinjection during the non-heating seasons can solve the problem of ground thermal attenuation and ensure the relatively stable heat extraction rate of the BHE. For the Beijing system, after 20 years of operation, there will be an increase in heat extraction, with the highest growth rate reaching 0.60%. Due to the higher load in the Changchun system, the selected cases still experienced a decline in heat extraction after long-term operation. However, compared to the cases without thermal reinjection, the rate of decline in heat extraction decreased from 2.00% to 1.44%.

(3)

Increasing the solar collector area, the number and the depth of boreholes all contribute to the improvement of the heat pump’s COP. However, the increment of the efficiency decreases as these factors increase. Therefore, selecting system design parameters reasonably can achieve a relatively higher efficiency of the heat pump while keeping other operational parameters within optimal ranges and controlling reasonable investment costs.

(4)

Based on the comprehensive analysis of results from each case, three optimal conditions are selected for both Beijing and Changchun systems. A comparison with a single GSHP system indicates that for 20 years of operation for the Beijing system, the optimal condition (H = 150 m, n = 1, and A = 25 m²) of the coupled system reduces energy consumption by 57.32% (approximately 60 MWh), equivalent to saving 7.37 t of standard coal. For the Changchun system (H = 250 m, n = 1, and A = 25 m²), the energy consumption savings can reach 70 MWh, equivalent to 8.60 t of standard coal. Additionally, the total cost over 20 years decreases by 48.20% and 33.77% in Beijing and Changchun systems, respectively.

The future work involves designing a year-round energy supply system capable of summer cooling and winter heating, followed by multi-objective collaborative optimization using Response Surface Methodology (RSM). This will provide a data foundation and design experience for the engineering applications of SAGSHP systems.