Coaxial Borehole Heat Exchanger Research Articles

GROUND TEMPERATURE MONITORING FOR A COAXIAL GEOTHERMAL HEAT EXCHANGERS FIELD: PRACTICAL ASPECTS AND MAIN ISSUES FROM THE FIRST YEAR OF MEASUREMENTS

Ground temperature at shallow depth (< 50 m) is not stable, nor in space, neither in time, and its behaviour is the result of superimposition of effects of heat pulses of different origin: solar, geothermal and anthropic. The correct assessment of ground temperature is a crucial point when designing a shallow geothermal energy system. In geothermal closed loop projects, more the borehole heat exchangers are short, more the contribution of the ground temperature variability on the heat exchange is prominent. Monitoring ground temperature can be very useful to correctly understand the behaviour of a shallow geothermal reservoir subjected to heat extraction and/or injection by a ground source heat pump system. The present work illustrates the practical aspects and main issues occurred in the installation, testing and working phases of a monitoring system realised to record ground temperature in a geothermal application. The case study is a field of eight coaxial borehole heat exchangers, 30 m long, connected to a novel prototype of dual source (air and ground) heat pump.

Thermal performance of a co-axial borehole heat exchanger

Thermal performance of a co-axial borehole heat exchanger

Transient numerical simulation of the coaxial borehole heat exchanger with the different diameters ratio

In this paper, the thermal and hydrodynamic performance of the coaxial borehole heat exchanger (CBHE) with different diameters ratio, during heat injection into the ground are investigated. For this purpose, the energy and Reynolds Averaged Navier-Stokes (RANS) equations with SST k-ω turbulence model are numerically solved to simulate transient fluid flow and heat transfer. Numerical simulation results are validated with results of available experimental data. The reduction of heat transfer between the annular and central region and the increase of heat transfer between the annular region and ground is the main objective of this study to improve the thermal performance of the CBHE. The results show that the temperature difference between the inlet and outlet of the CBHE rises with the decrease of diameter ratio. In order to reduce costs, instead of using a CBHE with a high diameter ratio and more borehole depth, it is better to use a CBHE with a low diameter ratio and borehole depth. Also, the optimal diameter ratio is calculated for the lowest pumping power. Of course, due to the insignificance of the pumping power against the heat transfer at the different diameters ratio, the CBHE with a lower diameter ratio is recommended because it has better thermal performance and cooling.

Analysis and optimization of underground thermal energy storage using depleted oil wells

Underground thermal energy storage (UTES) is an important technology to utilize the industrial waste heat and the fluctuating renewable energy. This paper proposed a new deep UTES system by using single depleted oil well (DOW), and the coaxial borehole heat exchanger with insulation is introduced to retrofit the DOW for seasonal TES. At first, a comprehensive model combing wellbore and formation heat transfer was built, evaluating the performance of heat storage in summer and space heating in winter. Monte Carlo method was used for parameters sensitivity analysis, to obtain the correlation between storage efficiency and five influence parameters. The results indicated that inlet temperature showed the strongest influence on the storage efficiency. By the optimal design, it was found that for an existing heat source, there was an optimal well depth to obtain maximum storage efficiency. During the annual operation, total heat storage for the DOW with depth of 2000 m was about 4.7 × 106 MJ and total heat extraction was about 2.9 × 106 MJ more than 8 × 105 MJ from that without previous heat storage progress. Consequently, the DOW-UTES system could be used to implement seasonal heat storage or waste heat recovery in summer to provide space heating in winter.

Temperature sensor module for groundwater flow detection around borehole heat exchangers

We present a temperature sensor module (TSM) for detecting the groundwater flow velocity and direction in the vicinity of a borehole heat exchanger (BHE). We demonstrate the method for double U-tube BHEs, which can also be applied to coaxial BHEs. TSMs can be installed together with a BHE to monitor the temperature distribution in the horizontal plane of a BHE. They consist of 16 digital temperature sensors equally distributed on two concentric rings and a compass, which provides the TSM orientation. While the eight sensors located on the inner ring mainly map the temperatures of the inlet and outlet tubes, the temperatures measured on the outer ring are influenced by advective heat transport due to groundwater flow. We study the sensitivity of the TSM to groundwater flow by numerical simulations and suggest a method for deriving both the groundwater flow velocity and direction. Moreover, we designed a TSM prototype and tested it in a sandbox experiment. This test confirmed the operational principle of the TSM and the method for determining the groundwater flow velocity and direction. The digital temperature sensors used in the TSM prototype resolve temperature changes down to 0.02 K. Both the numerical simulations and the experimental measurements show that such a temperature resolution enables the detection of groundwater flow velocities above 10−7 m s−1 (> 30 m a−1).

Numerical analysis of heat extraction performance of a deep coaxial borehole heat exchanger geothermal system

The coaxial borehole heat exchanger (CBHE) can effectively develop deep geothermal resources with larger heat exchange area than traditional U-tube BHE. However, few studies focus on its parameters optimization and temperature field dynamic response. Hence, an unsteady-state heat transfer model is established for a deep CBHE. The finite difference method is adopted to solve the model, and the model is validated with experiment data. The effects of key factors on the CBHE performance are comprehensively analyzed. Simulation results depict that the outlet temperature decreases greatly at the initial stage, and then remains relatively stable. There is a critical value for flow rate to obtain higher thermal power with appropriate pressure drop. The insulated pipe can significantly reduce heat dissipation of working fluid in central pipe. There is an optimal insulation length in terms of heat insulation effect and costs. The thermal conductivity of cement sheath has remarkable influence on the thermal process and partial cementing with high thermal conductivity cement is favorable. The temperature field analysis shows that the impact scope is limited, and the temperature in the reservoir before the next heating period would almost recover its original condition, verifying the sustainability of CBHE system.

Potential application of vacuum insulated tubing for deep borehole heat exchangers

During recent Paris climate agreement, 195 courtiers pledged to reduce the greenhouse gas emissions and pursue effects to limit the global temperature increase to 2 °C. Geothermal resources, as renewable sources of energy, can contribute to electricity, as well as heat production without having any negative impacts on environment or the security of energy supply. One of the ways to extract geothermal heat from the Earth’s crust are Deep Borehole Heat Exchangers, which can be either drilled especially for such purposes or, what is more cost effective, reconstructed from already existing, negative or abandoned oil or natural gas wells. In many depleted boreholes, exploiting underground geothermal waters is impossible or technically complex, therefore potential reconstruction for heat exchanger might be the only possible solution that allows utilizing geothermal heat.Polish energy sector is heavily dependent on coal resulting in large CO2 emission per capita. The country has considerable potential for geothermal energy development, as there are large numbers of deep and abandoned wells that might be utilized for geothermal heat production. Following paper investigates potential reconstruction of abandoned or negative wells into deep coaxial borehole heat exchangers. Main focus was put on selection of coaxial inner column which enables for heat carrier circulation. To maximize the heat uptake from geothermal systems based on borehole heat exchangers, it is advised to use inner column made from material with lowest possible thermal conductivity coefficient. Vacuum Insulated Tubing, as one of the options of insulated inner column, can significantly improve heat production and increase efficient energy use. This technology found plethora of applications in offshore and onshore petroleum industry and is yet to gain more popularity within geothermal sector.

A physical and semi-analytical comparison between coaxial BHE designs considering various piping materials

Abstract This paper presents an improved semi-analytical model for the heat transfer related to a coaxial borehole heat exchanger (BHE). This model, referred to as the composite coaxial (CCx) model, is verified using results from three different thermal response tests performed on full-scale coaxial BHEs having various pipe size and material properties. The model is further used to analyze the performance and length requirements of various coaxial configurations comparing the use of a steel outer pipe to an insulated inner pipe. This comparison is made considering standard HDPE pipe sizes as the status quo, where the parameters are kept within the verified tested limit of the model. The results show, in comparison to using an insulated inner pipe, that a steel outer pipe has a greater impact on reducing the overall length requirement while maintaining system performance.

Hydraulic and thermogeological design differences between two-loop vertical and inclined coaxial borehole heat exchangers

Abstract One of the most common geoexchange system in practice today is a vertical two-loop heat exchanger. There is also a growing trend of inclined drilling of multiple 40–50 m boreholes with coaxial heat exchangers, which are efficient when installing in areas where there is a favourable groundwater flow. Special care should be taken in hydraulic design of coaxial system, since laminar flow often occur in annular channel, due to glycol mixture implementation. In this research, vertical and inclined coaxial borehole exchangers were compared in identical geological environment, to express differences in heat extraction and thermal resistances, as well as in flow regimes and pressure drop. Extended Steady-State Thermal Response Step Test was implemented, which incorporate series of power steps to determine borehole extraction rate at steady-state heat transfer conditions. Results show that 2U-heat exchanger has advantages over coaxial heat exchangers, in terms of greater steady state heat extraction rate and more favourable hydraulic. Defined extraction rate at 0/-3 °C borehole temperature, in dominantly clay ground, was 43,5 W/m for coaxial system, compared to 54,5 W/m for vertical 2U-loop. Borehole resistance analysis showed almost double value for coaxial system compared to 2U-loop, due to frequent problems in cementation of inclined boreholes.

Experimental and analytical investigation on pipe sizes for a coaxial borehole heat exchanger

This paper investigates the use of a vertical coaxial borehole heat exchanger (BHE), focusing on those consisting of standard geothermal piping material, as a component in a ground-source heat pump system. The results of a lab-scale experiment are used to verify the trends exhibited by a recent semi-analytical model, referred to as the composite coaxial (CCx) model, considering short-term behavior when laminar flow is experienced in the annular space of a coaxial heat exchanger. The discussion on pipe sizes is then expanded upon using the suggested model along with a modified design procedure to compare the performances realized by an example heat pump. A comparison is made here between configurations having various nominal inner pipe diameters while maintaining the same outer pipe. The results of the analysis show that increasing the inner pipe diameter, within the verified limit of the composite coaxial model, will reduce the required length of heat exchanger and increase the overall coefficient of performance realized by the heat pump.

Investigation of dynamic heat transfer process through coaxial heat exchangers in the ground

Recently, researchers are focussing on using ground coupled heat pump systems as a heat source or sink rather than air source heat pumps for HVAC needs due to the stable temperature and the high thermal inertia of the soil. The investment cost of these systems is too expensive therefore the precise thermal analysis, design and parameter optimization are essential. For an accurate design, the maximum of physical phenomena such as: axial effects, seasonal effects, underground water flow and BHE dynamic behaviour must be accounted for in order to reflect exactly the real physical situation. In the present paper thermal interferences are investigated under seasonal effects and a dynamic heat flux for a vertical coaxial borehole heat exchangers field. This enables to avoid thermal interferences by predicting efficient period of operation corresponding to the beginning of the studied phenomena (interferences) for a given separation distance between two boreholes. To reach this purpose, as a first step, a transient 2D Finite volume method (FVM) for a single borehole heat exchanger was built using MATLAB, which accounts for accurate axial and seasonal effects and a dynamic heat flux that is function of depth and time. This model has been validated against the Finite Line Source (FLS) analytical solution and good agreement between analytical and numerical methods has been obtained. Then the model has been extended to a quasi-3D model in order to investigate thermal interferences between two neighbouring boreholes. After 500 h and at the mid-point of the separating distance (1.5 m) where interferences are the strongest, the temperature is 50% (6.64 °C) lower than the case where there are no interferences.

Efficient simulation of multiple borehole heat exchanger storage sites

In this paper, an adapted model is developed for borehole heat exchangers (BHEs) to simulate geothermal applications such as heat storage on a large scale efficiently and with high accuracy. The adapted numerical model represents all BHE components, allowing for a detailed representation of the governing processes. The approach is calibrated and validated for a single U-tube BHE using a high-resolution experimental data set from a laboratory thermal response test. It is found that the computational effort can be reduced by factors of ~50, ~50 and ~25 for single U-tube, double U-tube and coaxial BHEs, respectively, if an absolute deviation of less than 1 % compared to a conventional fully discretised model is allowed. Computation times can be reduced further by accepting higher deviations. The adapted modelling approach allows for a detailed and correct representation of the temporal and spatial temperature distribution under highly transient conditions by applying it to a high-temperature heat storage scenario using multiple BHEs. The model is especially suited to represent coupled flow and heat transport processes, to account for groundwater flow in the BHE region as well as geological heterogeneities and especially interaction between a large number of BHEs.

Thermal evaluation of coaxial deep borehole heat exchangers

This paper presents a performance study of deep borehole heat exchangers. The coaxial borehole heat exchanger (BHE) has been selected because for the present conditions it has a better performance than the conventional U-tube BHE. A numerical model has been developed to study the coaxial BHE. The model predictions are compared to detailed distributed temperature measurements obtained during a thermal response test. The model is found to accurately predict the behavior of a coaxial BHE. The influence of the flow direction of the mass flow is studied for BHEs in the range 200 m–500 m. A parametric performance study is then carried out for the coaxial case with different borehole depths, flow rates and collector properties. The results clearly show a significant increase in the system performance with depth. In addition, it is shown that with increasing borehole depth, the heat load that can be sustained by the BHE is significantly increased. An overall performance chart for coaxial BHEs for the depths of 300–1000 m is presented. The chart can be used as a guide when sizing deep BHE installations.

Transient heat transfer in a coaxial borehole heat exchanger

Ground-source heat pumps often use vertical boreholes to exchange heat with the ground. A transient heat transfer model has been developed for a thermal response test on a pipe-in-pipe coaxial borehole heat exchanger. The analytical model calculates the vertical temperature profiles in the fluid flowing through the pipes, which are coupled to the surrounding grout and ground. The model is verified against measured vertical temperature profiles in the circulating fluid during a distributed thermal response test. The comparison with measured data indicates that the proposed model gives a more accurate estimate of the borehole thermal resistance than the conventional analytical model that uses a mean temperature approximation. The model demonstrates how strongly the shapes of the temperature profiles are dependent on the thermal resistance of the internal pipe wall and the flow direction.

Comparative Heat Transfer Efficiency Study of Coaxial and U-loop Boreholes

Abstract—This paper presents the results of comparative heat transfer efficiency study based on in situ full scale tests conducted on conventional U-loop borehole heat exchangers (BHE) and a coaxial BHE. The results show that the coaxial BHE has considerably higher heat transfer efficiency, while consuming less pumping power than the U-loop’s. The implication of a high efficiency BHE on reducing borehole loop length and initial investment cost is also discussed. It is concluded that the higher efficiency coaxial BHE has the potential to reduce borehole length 30% to 50%.

Study of the effect of medium flow parameters on heat transfer in the laboratory coaxial model of a borehole heat exchanger

Borehole heat exchangers (BHE) are more and more commonly applied for recovering energy from rock mass renewable sources and for rational heat managing in the heating/ cooling installations. BHE exploitation simulations can be either provided by mathematic methods, included in examples [8, 9] or by carrying out tests at laboratory stands. BHEs located in the Laboratory of Geoenergetics in Faculty of Drilling, Oil and Gas AGH University of Science and Technology are being used to conduct research in area of presented interests [1]. A research on the efficiency of heat transfer in a coaxial borehole heat exchanger is described in this paper. In recent times theme of adapting old or not used anymore oil and gas wells is highly increased. The most important fact is that every deep well that is being transferred into BHE has to be of centric construction. This is for minimizing of pressure [7]. The objective of the study is to adjust the materials and the configuration of a vertical borehole heat exchanger in such a way that the heat transfer processes can be analyzed. A series of experimental measurements were performed for various capacities. Three various pipe materials were used in the experiments: polyethylene (PE), polyvinylchloride (PVC), polypropylene (PP) or a combination there of. The vertical borehole heat exchanger operated at the rock mass temperature. In laboratory conditions the rock mass heat source was substituted with 4 heating blankets, 80 W each. The polyethylene and polyvinylchloride pipes constituted flow channels in two zones. Their diameter differed depending on the applied combinations. The inner diameter of the heat exchanger equaled to 25.3 mm. The

Measurements and Design Calculations for a Deep Coaxial Borehole Heat Exchanger in Aachen, Germany

This study aims at evaluating the feasibility of an installation for space heating and cooling the building of the university in the center of the city Aachen, Germany, with a 2500 m deep coaxial borehole heat exchanger (BHE). Direct heating the building in winter requires temperatures of 40°C. In summer, cooling the university building uses a climatic control adsorption unit, which requires a temperature of minimum 55°C. The drilled rocks of the 2500 m deep borehole have extremely low permeabilities and porosities less than 1%. Their thermal conductivity varies between 2.2 W/(m·K) and 8.9 W/(m·K). The high values are related to the quartzite sandstones. The maximum temperature in the borehole is 85°C at 2500 m depth, which corresponds to a mean specific heat flow of 85 mW/m2–90 mW/m2. Results indicate that for a short period, the borehole may deliver the required temperature. But after a 20-year period of operation, temperatures are too low to drive the adsorption unit for cooling. In winter, however, the borehole heat exchanger may still supply the building with sufficient heat, with temperatures varying between 25 and 55°C and a circulation flow rate of 10 m3/h at maximum.

Borehole resistance and vertical temperature profiles in coaxial borehole heat exchangers

Ground source heat pump systems are often coupled to the ground by circulating a fluid through vertical Borehole Heat Exchangers (BHEs). The design of a system requires estimates of the ground thermal conductivity and the borehole thermal resistance, which are usually determined by an in situ thermal response test on a completed borehole. The usual test interpretation methods average the inlet and outlet fluid temperatures and use this mean temperature as the average temperature along the borehole length. This assumption is convenient but does not strictly apply. For a coaxial heat exchanger this paper develops an analytical model for the vertical temperature profiles, which can be used instead of the mean temperature approximation to estimate borehole resistance. The model is verified with measured temperatures on a BHE, where an optical technique allows continuous measurements along a coaxial borehole during a distributed thermal response test. A sensitivity study shows that the proposed method corrects errors in the mean temperature approximation, which overestimates the borehole resistance in a coaxial borehole.

Short time-step performances of coaxial and double U-tube borehole heat exchangers: Modeling and measurements

Several authors have investigated the long time-step performance of borehole heat exchangers. In that time scale, the borehole thermal capacitance is generally neglected, since the time span of interest is on the order of months or years. The borehole thermal capacitance consists both of grouting material and heat carrier fluid, and it mostly affects the short time-step behavior, when hourly or shorter time intervals are considered. Some models are available in the literature for short time-step simulations of borehole heat exchangers. In this article, short time-step analyses of coaxial and double U-tube heat exchangers are performed by means of numerical capacity resistance model. The capacity resistance model is compared with field measurements, analytical solutions available in the literature, and commercial software based on the finite-element method. Further comparisons between coaxial and common double U-tube heat exchangers are carried out by means of measurements performed in a suitable plant system. Furthermore, the series and parallel arrangements for double U-tube heat exchangers are investigated considering the borehole thermal capacitance.

Distributed thermal response tests on a multi-pipe coaxial borehole heat exchanger

In a distributed thermal response test, distributed temperature measurements are taken along a borehole heat exchanger during thermal response tests, allowing the determination of local ground thermal conductivities and borehole thermal resistances. In this article, the first results from six heat injection distributed thermal response tests carried out on a new, thermally insulated leg type, multi-pipe coaxial borehole heat exchanger are presented. The borehole heat exchanger consists of 1 insulated central and 12 peripheral pipes. Temperature measurements are carried out using fiber-optic cables placed inside the borehole heat exchanger pipes. Unique temperature and thermal power profiles along the borehole depth as a function of the flow rate and the total thermal power injected into the borehole are presented. A line source model is used for simulating the borehole heat exchanger thermal response and determining local variations of the ground thermal conductivity and borehole thermal resistance. The flow regime in the peripheral pipes is laminar during all distributed thermal response tests and average thermal resistances remain relatively constant, independently of the volumetric flow rate, being lower than those corresponding to U-pipe borehole heat exchangers. The thermal insulation of the central pipe significantly reduces the thermal shunt to the peripheral pipes even at low volumetric flow rates.[Supplemental Materials are available for this article. Go to the publisher's online edition of HVAC&R Research to view the supplemental file.]