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Deep geothermal energy has become widely exploited in recent years through the use of closed loop systems for building heating. Intended to meet high heating demand in densely populated neighbourhoods, an enhanced U-tube borehole heat exchanger (EUBHE) system, in which a deviated deep borehole is connected with another vertical one to form a closed loop, is introduced in this work. For capacity and efficiency analysis of applying EUBHE systems to extract deep geothermal energy, a 3D numerical model is implemented and established based on the OpenGeoSys software. Through evaluation by thermal performance tests and thermal response tests on the EUBHE system, the maximum sustainable heat extraction rate is found to be 1.2 MW in a single heating season and 1.1 MW in 10 years, which can provide heating to more than 35,000 m2 of residential buildings located in northern China. Moreover, the 10-year system thermal performance and efficiency are evaluated when coupled with a ground source heat pump (GSHP), and compared with the two deep borehole heat exchanger (2-DBHE) array system that has the same total borehole length as the EUBHE system. Results show that GSHP-coupled EUBHE system is more efficient than the 2-DBHE array system, as it consumes 27% less electricity.

Medium-deep borehole heat exchanger (MDBHE) has attracted considerable attention because of its ability to acquire large quantities of geothermal energy. Geological conditions in the medium-deep reservoir significantly affect thermal extraction. This study proposes a transient heat transfer model for MDBHE; the accuracy of the model is validated using experimental data. Using this model, the thermal performance of MDBHE under different geological parameters is investigated based on the operation characteristics of MDBHE. Sensitive analysis highlights the importance of the effects of geothermal gradient, thermal conductivity, and heat capacity of the rock soil on thermal extraction load (TEL) and thermal effect radius (TER) of the MDBHE. Results indicate that geothermal gradient affects the TEL mostly and heat capacity has the greatest impact on TER. In regions with low rock-soil heat capacity, borehole spacing should be sufficiently wide to prevent thermal interference between adjacent boreholes. For continuous operations of MDBHE, the annual average TEL declines with operating year and the declining proportion is the largest in the second year for 4%–6%. Besides, the maximum TER increases, with maximal increasing proportion in the second year up to ∼80%. Under different geological parameters, the declining proportion of TEL and increasing proportion of TER within the same year are almost identical. In addition, the thermal performance of MDBHE under layered subsurface is presented. Layered RTCs and geothermal gradients have a significant effect on the TEL of MDBHE; and layered subsurface with segmented RTCs and rock-soil heat capacities considerably affects the TER. This study will enable the wide promotion of MDBHE application in different regions.

Geothermal energy stored in the 1~2 km depth, which has advantages in terms of its continuity and sustainability, has attracted more attention in recent decades in the building heating industry. To meet the carbon-neutral prospect in the building sector, a new-type deep borehole heat exchanger (DBHE) is proposed to extract geothermal energy and has been utilized in Europe and northern China. The DBHE is typically drilled to more than 2000 m depth, and it is usually coupled with a heat pump to supply heat to the buildings. In this work, a dual-continuum finite element method was implemented in the open-source software OpenGeoSys to mimic the heat transfer process between the DBHE and the surrounding subsurface. After validating against in-situ experimental data of the pilot DBHE heating project in Xi’an, a heat pump model was also included in the OpenGeoSys model so that the entire heating system can be simulated over the long-term operation. Results show that the circulation temperatures of the DBHE have a decreasing trend during the long-term operation. Through the energy analysis, the amount of heat extracted by the DBHE was found to be mainly supplied by the energy stored within the surrounding soil. With different drilling depths of the DBHE, long-term simulation results illustrate that the heat extraction rate increases with deeper depth. After considering the electricity consumption of the heat pump and circulation pump, the Levelized cost of heating (LCOH) of the DBHE heating system was evaluated over its life-span cycle, and the optimal drilling depth of the DBHE was found to be 2600 m based on the specific geological properties of Weihe Basin, Xi’an. The proposing evaluation method provides a reference for decision-makers when designing the DBHE heating system.

Deep borehole heat exchangers (DBHEs) are a state-of-the-art and feasible apparatus for building heating and renewable energy utilization. Conventional BHEs with long-term operation may experience notable performance degradation, DBHE may provide an alternative way to overcome this issue. In this paper, a numerical model was developed by considering the ground temperature gradient in the axial direction and multilayer thermal properties of rock and soil and was used to simulate the temperature distribution and the performance characteristics of coaxial DBHEs. The effectiveness of the model was validated against the experimental data collected from a demonstration project. It was shown that the simulation results agreed well with the experimental data. The DBHEs could offer a higher heat exchange capacity in comparison to conventional BHEs. In the intermittent operation for 10 years, the decreasing proportions of the outlet temperature under four different operation modes with the run-stop ratio (i.e. the ratio of the running time to the stopping time in a day) of 8:16, 12:12, 16:8 and 24:0, were no more than 3.57%. The rock temperature profiles in the heating mode of both commercial and residential buildings were presented and the annual decreasing proportions were less than 4.0%. The findings obtained from this study could be used as a reference for sustainability research of DBHEs under different working conditions and operation modes.

Due to its sustainability, continuity and low carbon emissions, the utilization of geothermal energy is gaining more attention all around the world. Shallow geothermal energy is usually extracted through borehole heat exchangers (BHE) with a maximum length up to 150 m. Such systems typically require large space areas, thus limiting its application in built-up urban areas. This study presents a case where deep borehole heat exchanger (DBHE) with a depth down to 2500 m was constructed to extract geothermal energy for building heating purposes. A double-continuum finite element based numerical model was set up to simulate the heat transport process within and around the DBHE. The model has been validated by the experimental data in a demonstration project located in Fengxi, Xi’an China. The heat extracting performance of DBHE under different types of boundary conditions (including the Dirichlet condition and Neumann condition) are evaluated. The amount of thermal recharges from top, sides and bottom of the domain were differentiated and quantified. It is found that different types of boundary conditions will lead to deviations in the simulated heat fluxes and corresponding thermal recharge. The numerical simulations also suggest that the sustainable heat extract capacity of DBHE is mainly determined by the stored heat from the surrounding subsurface, and thermal recharge takes only a limited contribution. According to the calibrated modelling results, the proper heat extraction rate of DBHE in the long-period operation modes is analyzed.

Recently, an Enhanced U-type Borehole Heat Exchanger (EUBHE) system has been installed in Xi’An, China. The EUBHE system is composed on one vertical and a second deviated borehole. The bottom ends of them are connected at a depth of 2.5 km and form a closed “U-type” loop system. During the heating season, water is circulated through the two boreholes to extract geothermal energy for building heating purpose. In this work, a numerical model was developed based on the OpenGeoSys (OGS) software, in which the boreholes of the EUBHE system are represented by the line elements and the soil/rock surrounding them is discretized with 3D prism elements. With this dual-continuum model, the operation of the EUBHE system can be efficiently simulated, particularly for long-term scenarios. The developed numerical model was verified against analytical solution in a benchmark proposed by Ramey. To simulate the long-term operation of EUBHE, a Direchlet-type boundary was imposed at the inlet of the system. Temperature difference between inlet and outlet were calculated based on the building thermal demand. The impact of parameters of the vertical and deviated boreholes and flow rate of the circulating water are further investigated. Preliminary modelling results showed that the sustainable specific heat extraction rate of the EUBHE system can reach up to 200 W/m. With higher grout and pipe conductivities, the system performance will be improved. The heat extraction efficiency of EUBHE system is higher than the traditional Deep Borehole Heat Exchangers (DBHE). However, electricity consumption from the circulating pump is elevated. The developed numerical model presented in this work can also be utilized for the design and optimization of the EUBHE system.

Geothermal heat pump system (GHP) is efficient to utilize renewable energy in cooling and heating of building. However, the major problem is that the coefficient of performance (COP) of GHPs will decay after a long-time operation because of the imbalanced energy injection into and extraction from the ground. It is an effective way to apply solar collectors in the GHPs to collect thermal energy in the non-heating season and deliver the heat into ground, which may relief the imbalance. In our research, a demo project of solar-assisted geothermal heat pump system (SAGHP) in a rural building in Xi’an was built up, which delivers hot water from solar collector into soil from September 1 to November 14, 2018. For analyzing the effectiveness of elevating the ground temperature, a TRNSYS model was established, verified by the measured data in the project, and served for simulating three systems, which including the SAGHPs without accumulating solar energy in soil, the SAGHPs with accumulating thermal energy in soil in summer, and the SAGHPs with accumulating thermal energy in soil in autumn. The comparison of systems showed the significance of thermal storage in Xi’an and the feasibility of thermal injection in autumn for solar collection for improving the heat imbalance of GHPs.

In the context of reducing carbon emission, Deep Borehole Heat Exchanger (DBHE) array has a large potential in extracting geothermal energy to provide building heating in densely populated urban areas. To investigate the thermal interaction among the DBHE, a comprehensive numerical model has been built with the OpenGeoSys software, and it is validated by monitoring data from a pilot project in Xi’an, China. The long-term simulations manifest that the outlet temperature of the DBHE array has a noticeable draw-down of 4.70 ° C over 20 years in comparison to the single borehole setup. The maximum difference of outlet temperature among individual DBHE can reach up to 0.88 ° C over 20 years, which will lead to a shifted thermal load of 23.35 kW (12.25 % of the designed average value). Based on the predicted subsurface temperature distribution, a non-linear correlation can be established between the drawdown in working fluid temperature and the accumulated amount of extracted heat. The finding of work implies that the thermal interaction among individual DBHE is of significance for the sustainability of the system, and comprehensive numerical modeling should be considered in the designing procedure.

When a Borehole Heat Exchanger (BHE) array is coupled with heat pump to provide cooling and heating to the buildings, thermal interaction between BHEs may occur in the subsurface. In the long term, imbalanced seasonal thermal load may lead to low or high temperature zones accumulating in the centre of the array. In this study, numerical models are configured according to a real BHE array project in Leicester, UK, and verified against monitoring data. Based on this reference model, a series of numerical experiments are conducted to investigate the response of circulation fluid temperature to different settings of imbalanced thermal load. It is found that over long-term operation, the sub array with a larger number of installed BHEs is shifting its thermal load towards the other branch with less BHEs installed. Within each sub array, the heat injection rate on the central BHEs is gradually shifted towards those located at the edge. A linear correlation is also found between the working fluid temperature increment and the amount of the accumulated heat injected into the subsurface.

Application of the medium-deep borehole heat exchanger (MDBHE) coupled ground source heat pump (GSHP) system on building heating has been gradually accepted by public and its sustainability becomes a key concern. In this work, a long-term transient heat transfer model for the MDBHE coupled GSHP system is proposed to investigate the effects of various influencing factors on the thermal extraction and energy efficiency of this system. Sensitive analysis highlights the significant effects of the specific heat transfer rate, rock-soil thermal conductivity, geothermal gradient, pipe depth and fluid velocity on the annual decline of the system performance during its long-term operation. Furthermore, the operation optimization for this system should be conducted based on its performance in quasi-steady state. An optimal fluid velocity exists to acquire the maximum specific heat transfer rate of MDBHE under a constant Seasonal Performance Factor of Heat Source (SPF2). In order to guarantee an efficient thermal extraction, the comprehensive effects of both specific heat transfer rate and fluid velocity on the energy efficiency should be taken into account. For the benchmark MDBHE, the operation condition is optimized with the specific heat transfer rate of 142 W/m, and the fluid velocity of 0.7 m/s; the annual SPF2 is always greater than or equal to 4.0 during the long-term operation, which ensures a sustainable and efficient operation.