• Energy Research

AbstractGroundwater convection is commonly observed in real-world projects, particularly in coastal and groundwater-abundant regions. To accurately evaluate the heat extraction capacity of the deep borehole heat exchanger (DBHE) considering groundwater flow, a conduction–convection coupled numerical model of the DBHE is established by OpenGeoSys (OGS) software. Then, the variation of the DBHE circulation temperature and the heat extraction capacity affected by different groundwater conditions, including Darcy velocity, location of the aquifer, and porosity of the aquifer, are quantitatively analyzed. The results show that the porosity and location of the aquifer have a limited effect on the heat extraction capacity of the DBHE. With the given scenario in this study, when the Darcy velocity reaches more than $$1\times 10^{-7}\,{{\textrm{m}}/{\textrm{s}}}$$ 1 × 10 - 7 m / s , it has a distinguishable effect on the heat extraction capacity of DBHE under the influence of groundwater. In addition, long-term simulations of multiple DBHEs considering the characteristics of the ground pipe network are performed in different directions of groundwater flow. The results indicate that groundwater flow can alleviate cold accumulation around the boreholes, and the thermal plume is pushed much towards the downstream direction. The cross-flow groundwater results in a higher circulation temperature than the parallel flow for the DBHE array. The maximum temperature difference between the two configurations is $${1.98}\,^{\circ }{\textrm{C}}$$ 1.98 ∘ C , which occurs at the end of the 15th operating year based on the given parameters. The results of this study can be used as a guide for project engineers and decision-makers to accurately assess the heat extraction capacity of DBHE and strategize the layout of the DBHE array, taking into account the influence of groundwater flow.

To contribute to the goal of carbon neutralization, the closed-loop borehole heat exchanger system is widely applied to use geothermal energy for building cooling and heating. In this work, a new type of medium-shallow borehole heat exchanger (MSBHE) is proposed, which is coaxial type and has a depth range between 200 m to 500 m. To investigate the long-term performance of MSBHE in the area with unbalanced cooling and heating load of buildings and the sustainable load imbalance ratio under different design parameters, a comprehensive numerical model is established. The results show that the drilling depth significantly influences the sustainable load imbalance ratio of MSBHE. As the drilling depth is increased from 200 m to 500 m, the load imbalance ratio of the MSBHE increases from 20.76% to 60.29%. In contrast, the load imbalance ratio is always kept at the same level with different inlet velocities and operation modes. Furthermore, in a 9-MSBHE array system, the heat exchanger located in the middle of the array has the lowest load imbalance ratio of 48.97%, which is 15.98% lower than the borehole in the edge location. This is caused by the significant influence of the shifted-load phenomenon among MSBHEs in an array system. The findings of the work imply that this newly proposed MSBHE can sustain a notable load imbalance ratio, which is particularly applicable to the areas with a strong imbalance of annual building load.

The provided scripts implement a workflow for the parametric optimization of a two-phase geothermal source ORC power plant by applying different search algorithms to a TESPy model of an ORC cycle. Besides parameter influence analysis, both single and multivariate, optimization procedures are available. For more information please refer to the README and the TESPy online documentation. Feel free to contact us for further questions.

Abstract Among various groundwater heat pump (GWHP) system configurations, the single-well circulation (SWC) groundwater heat pump system is totally different with other conventional GWHP systems. In recent years, the SWC system has gained increasing interest due to the fact that it is a cost-effective technology providing large quantities of heating and cooling to buildings. The performance of a SWC system is closely related to the hydrogeological and thermogeological properties. Also, the proper system design and installation are crucial to maintain long-term sustainability and efficiency. In this work, a series of scenarios were simulated to evaluate the influence of different parameters on the outlet temperature evolution and efficiency of a SWC system. It was found that the W-S mode with active cooling will be beneficial to the efficiency of the heat pump and the recovery of the subsurface. It is also demonstrated that the sealed section length plays the most influential role in the design of a SWC system. In addition, the present of groundwater flow will have positive effect on the system performance. In contrast to other factors, the thermal conductivity, volumetric heat capacity and porosity are considered to have a minor influence on the sustainability and efficiency of the system.

For two-phase geothermal resource, Organic Rankine Cycle (ORC) based binary plant is often applied for power production. In this work, a network topology was built with the Thermal Engineering Systems in Python (TESPy) software to simulate the stationary operation of the ORC plant. With this topology, the performance of nine different working fluids are compared. From the thermodynamic perspective, the gross and net power output is optimized respectively. Results show that R600 has the highest gross power output of 17.55MW, while R245fa has the highest net power output of 12.93MW. However, the turbine inlet temperatures for these two working fluids need to be designed at the upper limit of 131℃. It is also found that R245ca and R601a (Isopentane) require the heat exchange rates of IHE to be larger than 1.51MW and 0.99MW to satisfy the re-injection temperature limit, which are smaller than the R600 (6.7MW) and R245fa (6.0MW) cases. Besides, in order to establish a stable ORC plant, the lower geo-steam fraction, the working fluid with lower critical state is preferred. The workflow for the ORC design and optimization in this work is generic, and can be further applied to thermo-economic investigation.

Pressure-relieved coal mining method has been successfully carried out in many mines in China, especially in high gassy mines. The motivation for the paper is to help engineers fill the knowledge gap on how to evaluate how the permeability changes and effect on improving low permeability prior to its construction. A new method is proposed in a combined theoretical and experimental way in laboratory. On the tests, the volumetric strain-dependent permeability is obtained based on triaxial compression tests. Then, we have designed a physical modeling experiment to know the mining field deformation especially in the subsidence data, which is easily measured and reliable ignoring the continuum concept hypothesis. Another, based on semi-infinite integral model, the theoretical formulas of subsidence are solved consistent with the testing result from the physical modeling. The volumetric strain derived from the semi-infinite integral model is proposed to establish the relationship between permeability and volumetric deformation of mining field. Finally, the pressure-relieved effect on changing permeability is studied and discussed. The results show that the combined method is a good way to determine the distribution on permeability changing with advancing distances, also a good tool for improving the traditional predraining borehole layout and opportunity to extract gas methane.

Abstract 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.