Search Results (7)

Compared to shallow geothermal systems, coaxial medium-deep and deep borehole heat exchangers (MDBHE and DBHE) offer higher temperatures and heat extraction rates while requiring less surface area, making them attractive options for sustainable heat supply in combination with ground-source heat pumps (GSHP). However, existing simulation tools for such systems are often limited in computational efficiency or open-source availability. To address this gap, we propose a rapid modeling approach using the open-source Python package “pygfunction” (v2.3.0). Its workflow was adjusted to accept the fluid inlet temperature as input. The effective undisturbed ground temperature and ground thermophysical properties were weight-averaged considering stratified ground layers. Validation of the approach was conducted by comparing simulation results with 12 references, including established models and experimental data. The proposed method enables fast estimation of fluid temperatures and heat extraction rates for single boreholes and small-scale bore fields in both homogeneous and heterogeneous geological conditions at depths of 700–3000 m, thus supporting rapid assessments of the coefficient of performance (COP) of GSHP. The approach systematically underestimates fluid outlet temperatures by up to 2–3 °C, resulting in a maximum underestimation of COP of 4%. Under significant groundwater flow or extreme geothermal gradients, these errors may increase to 4 °C and 6%, respectively. Based on the available data, these discrepancies may result in errors in GSHP electric power estimation of approximately ±10%. The method offers practical value for GSHP performance evaluation, geothermal potential mapping, and district heating network planning, supporting geologists, engineers, planners, and decision-makers. Full article

Medium-deep borehole heat exchanger (BHE) systems represent an emerging form of ground source heat pump technology. Their heat transfer process is significantly influenced by geothermal gradient and soil stratification, typically simulated using segmented finite line source (SFLS) models. However, this approach involves computationally intensive procedures that hinder practical engineering implementation. Building upon an SFLS model adapted for complex geological conditions, this study proposes a comprehensive simplified algorithm: (1) For soil stratification: A geothermally-weighted thermal conductivity method converts layered heterogeneous media into an equivalent homogeneous medium; (2) For geothermal gradient: A temperature correction method establishes fluid temperatures under geothermal gradient by superimposing correction terms onto uniform-temperature model results (g-function model). Validated through two engineering case studies, this integrated algorithm provides a straightforward technical tool for heat transfer calculations in BHE systems. Full article

The global challenges of rising energy consumption and carbon emissions underscore the urgent need for efficient and sustainable heating solutions in the building sector. The implementation of high-performance buildings that envelope insulation and the increasing adoption of low-temperature radiant heating systems have significantly reduced the water temperature required from heat sources, enabling greater compatibility with renewable energy systems. In this study, we propose a renewable energy heating system incorporating a solar-assisted medium-depth ground heat exchanger (MDGHE). A dynamic simulation model of the solar-assisted MDGHE system was developed in TRNSYS, featuring a novel MDGHE module specifically developed to improve simulation accuracy. A case study of a residential building in China was conducted to evaluate the performance of the proposed system. The simulation results demonstrate that while the standalone MDGHE covers 71.9% of the building’s heating demand, integrating solar collectors with the MDGHE can increase this coverage to 99.9%, enabling full compliance with heating requirements without relying on conventional heat pumps. The results revealed that the system’s COP reached 9.26. Compared with the traditional medium-depth ground source heat pump system with the COP of 4.84, the energy efficiency of this system has been enhanced by 47.7%. A static payback period of 7 years has been obtained compared with the cost of central heating service for residential buildings. These findings highlight the potential of solar-geothermal hybrid systems as a sustainable alternative to traditional heating methods. Full article

Ground-source heat pump (GSHP) systems with medium-depth and deeply buried pipes in cold regions are highly important for addressing global climate change and the energy crisis because of their efficient, clean, and sustainable energy characteristics. However, unique geological conditions in cold climates pose serious challenges to the heat transfer efficiency, long-term stability, and adaptability of systems. This study comprehensively analyses the effects of various factors, including well depth, inner-to-outer tube diameter ratios, cementing material, the thermal conductivity of the inner tube, the flow rate, and the start–stop ratio, on the performance of a medium-depth coaxial borehole heat exchanger. Field tests, numerical simulations, and sensitivity analyses are combined to determine the full-cycle thermal performance and heat-transfer properties of medium-depth geological formations and their relationships with system performance. The results show that the source water temperature increases by approximately 4 °C and that the heat transfer increases by 50 kW for every 500 m increase in well depth. The optimization of the inner and outer pipe diameter ratios effectively improves the heat-exchange efficiency, and a larger pipe diameter ratio design can significantly reduce the flow resistance and improve system stability. When the thermal conductivity of the cementing cement increases from 1 W/(m·K) to 2 W/(m·K), the outlet water temperature at the source side increases by approximately 1 °C, and the heat transfer increases by 13 kW. However, the improvement effect of further increasing the thermal conductivity on the heat-exchange efficiency gradually decreases. When the flow rate is 0.7 m/s, the heat transfer is stable at approximately 250 kW, and the system economy and heat-transfer efficiency reach a balance. These findings provide a robust scientific basis for promoting medium-deep geothermal energy heating systems in cold regions and offer valuable references for the green and low-carbon transition in building heating systems. Full article

Medium-shallow borehole ground heat exchangers (BGHEs) utilize a burial depth ranging from 200 to 600 m. The heat exchange capacity of a single medium-shallow BGHE is higher than that of a single shallow BGHE. Compared to medium-deep BGHEs, the cost of medium-shallow BGHEs is lower, and both heating and cooling can be achieved, while the former can only be used for heating. However, there is a relative lack of research on the heat transfer characteristics of medium-shallow BGHEs, especially on the influence of the working fluid type on the heat transfer performance of BGHEs. This study aimed to investigate the impact of different working fluids on the performance of medium-shallow BGHEs. First, a heat transfer model for medium-shallow BGHEs was established considering the ground temperature gradient and geothermal heat flow, and its accuracy was validated using experimental test data. Second, the model was used to compare and analyze the effects of various working fluids on the heat transfer performance, pressure loss, and potential environmental benefits of BGHEs. Based on economic analysis, CO₂ was determined to be the most suitable working fluid among the organic fluids considered. Finally, the influence of the number of boreholes and the type of working fluid on the heat transfer performance of borehole clusters consisting of 2 and 4 boreholes was analyzed using the superposition principle. The results indicated that CO₂ could provide the highest heat transfer among the various working fluids selected in this study, as its heat extraction and heat dissipation were approximately 15% and 12% higher than those achieved by water. Isobutane (R600a) achieved the highest net heat and emission reduction, surpassing water by 66.7% and 73.6%, respectively. Regarding the four boreholes, the outlet temperature of the BGHEs gradually decreased at the end of each heating season. After 10 years of operation, the value decreased by approximately 2 °C. The results in this paper provide a theoretical basis and technical guidance for the rational selection of working fluids and improvements in the heat transfer performance of BGHEs, which could promote the development and application of medium-shallow geothermal energy sources. Full article

Deep borehole heat exchangers (DBHEs) extract heat from the medium-depth geothermal energy with the depth of 2–3 km and provide high-temperature heat source for the medium-depth geothermal heat pump systems (MD-GHPs). This paper focuses on the heat transfer performance of DBHEs, where field tests and simulation are conducted to analyze the heat transfer process and the influence factors. Results identify that the heat transfer performance is greatly influenced by geothermal properties of the ground, thermal properties and depth of DBHEs and operation parameters, which could be classified into external factors, internal factors and synergic adjustment. In addition, the long-term operation effects are analyzed with the simulation, results show that with inlet water temperature setting at 20 °C and flow rate setting at 6.0 kg/s, the average outlet water temperature only drops 0.99 °C and the average heat extraction drops 9.5% after 20-years operation. Therefore, it demonstrates that the medium-depth geothermal energy can serve as the high-temperature heat source for heat pump systems stably and reliably. The results from this study can be potentially used to guide the system design and optimization of DBHEs. Full article

Recently, the medium-depth geothermal heat pump systems (MD-GHPs) have been applied for space heating in China. Theoretically, the MD-GHPs use deep borehole heat exchangers (DBHEs) to extract heat from the medium-depth geothermal energy with the depth of 2~3 km, thus, improving the energy performance of whole systems obviously. This paper conducts field tests of nine conventional shallow-depth geothermal heat pump systems (SD-GHPs) and eight MD-GHPs to analyze the energy performance of heat pump systems, as well as heat transfer performance of ground heat exchangers. Then the comparative studies are carried out to analyze the difference between these two ground coupled heat pump systems. Field test results show that the outlet water temperature of DBHEs in MD-GHP can reach more than 30 °C with heat extraction of 195.2 kW~302.8 kW per DBHE with a depth of 2500 m, which are much higher than that of SD-GHPs. However, the heat pumps and water pumps in the ground side should be specially designed to fit the high-temperature heat source instead of following operation mode of SD-GHPs. Then with variable speed compressor which has high energy efficiency under a wide range of load rate and compressor ratio, and with the ground-side water pumps which efficiently operate under high water resistance and low flow rate, the COP of heat pumps and COP_s of whole systems could reach 7.80 and 6.46 separately. Thus, the advantage of high-temperature heat source could be fully utilized to achieve great energy-saving effects. Full article