Search Results (8)

The development of high-salinity geothermal resources is hindered by corrosion, scaling, and high water treatment costs, posing severe challenges for conventional hydrothermal systems. Deep borehole heat exchanger (DBHE) technology, which extracts heat without water production and avoids contact with high-salinity fluids, offers a promising alternative. In this study, field experiments on a coaxial DBHE were conducted in well SQ-1 (over 3000 m deep) in the Xining Basin, targeting its dual structure: a shallow high-salinity aquifer and a deep high-temperature low-permeability basement. Results show that the deep Proterozoic metamorphic basement has extremely low permeability (10⁻⁸ cm/s), with a bottom hole temperature of 113 °C and an average geothermal gradient of 3.39 °C/100 m, confirming it as a stable solid heat source. Under steady-state operation, the heat extraction rate averaged 150 W/m (144–155 W/m, with an uncertainty of approximately ±3 W/m). A marginal effect of flow rate was observed: increasing flow from 40 to 50 m³/h increased heat extraction by only 1.5%, indicating a threshold flow-rate range beyond which continued flow increases yield diminishing returns. No groundwater extraction, corrosion, or scaling was observed during the 13-day test, confirming the short-term operational reliability of the system under the tested conditions. This study validates shifting heat extraction to deep low-permeability basements to avoid high-salinity issues, providing a scientific basis for clean geothermal utilization in similar regions globally. Full article

Rapid economic growth has directly driven up energy demand, and the gradual depletion of traditional fossil fuels has severely hindered sustainable development. Developing green and efficient geothermal exploitation technologies constitutes a crucial measure for tackling this sustainable development issue. This paper presents a field test associated with a clean energy system conducted in the Guanzhong Basin, China, with the core component of a coaxial casing deep geothermal well. A distributed temperature sensing system (DTS system) with over 3000 m-depth optical fiber installed and adopted to monitor near-wellbore formation temperature changes. Combining information on the inlet/outlet water temperature and flow rate monitored by an integrated temperature–pressure monitoring system, the heat transfer patterns during the operation of the deep geothermal well are deeply investigated. The research results demonstrate that a higher operation parameter of flow rates has a significant increasing effect on the heat transfer capacity of heat exchangers for coaxial casing deep geothermal wells. Although the increase in inlet temperature has minimal effect on the outlet temperature, it leads to a continuous decline in heat transfer capacity. In addition, as heat exchange duration extends, the geothermal gradient of the near-wellbore formation progressively declines. Full article

To investigate the influence of injector recess depth on the combustion characteristics of air heaters, high-speed shadowgraph imaging technology combined with numerical simulation was employed. Targeting a tripropellant coaxial direct-flow single injector, three test cases with recess depths of 0 mm, 5 mm, and 10 mm were designed to systematically study the ignition process, flame propagation characteristics, quasi-steady combustion, and flow field evolution mechanisms. Experimental results indicate that the recessed structure can expand the liquid mist distribution range before ignition: the dimensionless spray width ratios of the 5 mm and 10 mm recess cases are increased by 57.5% and 64.9% respectively compared to the non-recessed case, with an obvious “saturation effect” observed. Injectors with recess exhibit the characteristic of “jet head priority ignition”, which shortens the ignition time and improves ignition efficiency. The 5 mm shallow recess case achieves the optimal combustion stability with the smallest chamber pressure fluctuation (±0.1 MPa). Although the 10 mm deep recess enhances near-field mixing and combustion intensity, it tends to induce flame oscillation and combustion instability. Simulation results verify the experimental observations: the recess depth regulates droplet atomization, component mixing, and combustion heat release processes by altering the recirculation zone range, velocity gradient, and gas–liquid momentum exchange efficiency. This research provides experimental and theoretical support for the structural optimization of injectors in combustion-type air heaters. Full article

Geothermal energy, as a clean, low-carbon, widely distributed, renewable and environmentally friendly energy source, plays an important role in the transition from traditional energy sources dominated by coal and oil to clean energy. Ground source heat pump technology is a key technological tool for developing geothermal energy for widespread use. Coaxial-cased heat exchangers are the core component of the ground source heat pump system, and their heat transfer performance directly affects the heat transfer efficiency and service life of the ground source heat pump system. According to the actual working conditions of coaxial-cased heat exchangers in fractured aquifers, the coupled pressure–temperature model of the heat transfer outside the borehole was created by considering the influence of the Joule–Thompson effect. For heat transfer inside the wellbore, a multi-layer long concentric cylinder wall model was developed to obtain the fluid temperature distribution within the wellbore. Results show that the heat transfer efficiency increases with the increase of thermal conductivity, water production and effective permeability of fractures. The positive and negative values of the Joule–Thompson coefficient reflect the trend of fluid temperature changing with pressure. The larger the absolute value is, the greater the temperature change is. The increase in the initial temperature of the injected water will lead to a decrease in the theoretical heat transfer. With the increase of the water injection rate and horizontal wellbore length, the heat recovery power will also increase significantly, but the optimal value needs to be considered comprehensively. The findings of the study can not only lay a theoretical foundation for the performance evaluation and optimal design of coaxial-cased heat exchangers but also have great significance in promoting the efficient development of geothermal energy. Full article

The design and performance of a shallow geothermal system is influenced by the geological and hydrogeological context, environmental conditions and thermal demand loads. In order to preserve the natural thermal resource, it is crucial to have a balance between the supply and the demand for the renewable energy. In this context, this article presents a case study where an innovative system is created for the storage of seasonal solar thermal energy underground, exploiting geotechnical micropiles technology. The new geoprobes system (energy micropile; EmP) consists of the installation of coaxial geothermal probes within existing micropiles realized for the seismic requalification of buildings. The underground geothermal system has been realized, starting from the basement of an existing holiday home Condominium, and was installed in dry subsoil, 20 m-deep below the parking floor. The building consists of 140 apartments, with a total area of 5553 m², and is located at an altitude of about 1490 m above sea level. Within the framework of a circular economy, energy saving and the use of renewable sources, the design of the geothermal system was based on geological, hydrogeological and thermophysical analytical studies, in situ measurements (e.g., Lefranc and Lugeon test during drilling; Rock Quality Designation index; thermal response tests; acquisition of temperature data along the borehole), numerical modelling and long-term simulations. Due to the strong energy imbalance of the demand from the building (heating only), and in order to optimize the underground annual balance, both solar thermal storage and geothermal heat extraction/injection to/from a field of 380 EmPs, with a relative distance varying from 1 to 2 m, were adopted. The integrated solution, resulting from this investigation, allowed us to overcome the standard barriers of similar geological settings, such as the lack of groundwater for shallow geothermal energy exploitation, the lack of space for borehole heat exchanger drilling, the waste of solar heat during the warm season, etc., and it can pave the way for similar renewable and low carbon emission hybrid applications as well as contribute to the creation of smart buildings/urban areas. Full article

Raman-based distributed temperature sensing (DTS) is a valuable tool for field testing and validating heat transfer models in borehole heat exchanger (BHE) and ground source heat pump (GSHP) applications. However, temperature uncertainty is rarely reported in the literature. In this paper, a new calibration method was proposed for single-ended DTS configurations, along with a method to remove fictitious temperature drifts due to ambient air variations. The methods were implemented for a distributed thermal response test (DTRT) case study in an 800 m deep coaxial BHE. The results show that the calibration method and temperature drift correction are robust and give adequate results, with a temperature uncertainty increasing non-linearly from about 0.4 K near the surface to about 1.7 K at 800 m. The temperature uncertainty is dominated by the uncertainty in the calibrated parameters for depths larger than 200 m. The paper also offers insights into thermal features observed during the DTRT, including a heat flux inversion along the borehole depth and the slow temperature homogenization under circulation. Full article

In the drive to achieve net-zero carbon emissions, decarbonisation of heating is essential. This can be facilitated by geothermal energy, but drilling geothermal wells is associated with high risks and costs. The use of preexisting wells (e.g., exhausted hydrocarbon wells or failed geothermal exploration boreholes) offsets this cost while potentially turning liabilities into assets. The Newcastle Science Central Deep Geothermal Borehole (NSCDGB) is a geothermal exploration well that was drilled to target the Carboniferous Fell Sandstone Formation at 1418.5 to 1795 m depth. However, low hydraulic conductivities prevented the development as a conventional “wet” geothermal abstraction well; therefore, new alternative methods of development are being explored. This work investigates the repurposing of the NSCDGB as a deep borehole heat exchanger (DBHE), focusing on the sustainable operation of the system in the long term by employing a constant heat load designed to contribute to local buildings or a heat network. Numerical modelling was undertaken by using OpenGeoSys software to analyse the thermal and hydraulic performance of the system. Both homogeneous and heterogeneous models were developed to compare the influence of lithological layering in contrast to a homogeneous (nonstratified) subsurface geological model. Results from homogeneous simulations modelling the DBHE to a depth of 922 m show that a 50-kW heat load can be supported for a lifetime of 25 years. This corresponds to a 65-kW building load when coupled to a heat pump with a coefficient of performance of 4.33. Thus, the DBHE could meet up to 72% of the heat demand of the adjacent urban sciences building. Rather than being a purely hypothetical case study, this work considers a real existing borehole, adjacent to a building cluster which could make use of the geothermal heat. Heterogeneity, which has been considered for the first time at the NSCDGB site, exhibits a minor impact in comparison to homogeneous simulation results. Flow direction and mass flow rate also exhibited small effects on the system performance, whereas if the exploration well could be repurposed to increased depths, the heat load could be increased. This is the first study of a coaxial DBHE at the NSCDGB site considering long-term effects of mass flow rate, heterogeneity, and flow direction. The study evaluates the feasibility of repurposing an exploratory geothermal well in the UK as a DBHE that can be used as a low-carbon heat source for space heating, thus converting liabilities into potential “green energy” assets. Full article

This work concerns borehole heat exchangers and their testing using apparatus for thermal response tests. In the theoretical part of the article, an equation was derived from the known equation of heat flow, on which the interpretation of the thermal response test was based. The practical part presents the results of several measurements taken in the AGH Laboratory of Geoenergetics. They were aimed at examining the potential heat exchange capacity between the heat carrier and rock mass. Measurement results in the form of graphs are shown in relation to the examined, briefly described wells. Result analysis made it possible to draw conclusions regarding the interpretation of the thermal response test. The method of averaging the measurement results was subjected to further study. The measuring apparatus recorded data at a frequency of one second, however such accuracy was too large to be analyzed efficiently. Therefore, an average of every 1 min, every 10 min, and every 60 min was proposed. The conclusions stemming from the differences in the values of effective thermal conductivity in the borehole heat exchanger, resulting from different data averaging, were described. In the case of three borehole heat exchangers, ground properties were identical. The effective thermal conductivity λ_eff was shown to depend on various borehole heat exchanger (BHE) designs, heat carrier flow geometry, and grout parameters. It is important to consider the position of the pipes relative to each other. As shown in the charts, the best (the highest) effective thermal conductivity λ_eff occurred in BHE-1 with a coaxial construction. At the same time, this value was closest to the theoretical value of thermal conductivity of rocks λ, determined on the basis of literature. The standard deviation and the coefficient of variation confirmed that the effective thermal conductivity λ_eff, calculated for different time intervals, showed little variation in value. The values of effective thermal conductivity λ_eff for each time interval for the same borehole exchanger were similar in value. The lowest values of effective thermal conductivity λ_eff most often appeared for analysis with averaging every 60 min, and the highest—for analysis with averaging every 1 min. For safety reasons, when designing (number of BHEs), safer values should be taken for analysis, i.e., lower, averaging every 60 min. Full article