Borehole Heat Exchanger Systems Research Articles

A numerical study on the sustainability and efficiency of deep coaxial borehole heat exchanger systems in the cold region of northeast China

Numerical simulations utilizing OpenGeoSys have been conducted on a deep coaxial borehole heat exchanger situated in the Songliao Basin. The findings reveal that the outlet temperature exhibits a downward trend as circulation flow increases, while it rises with an increase in inlet temperature. Additionally, the heat transfer power escalates with higher flow rates but diminishes as inlet temperature rises. Notably, the long-term operation results in a reduction of outlet temperature and heat transfer power. After 30 years, the decline of them is approximately 0.3 °C and 8 kW under current situation. Furthermore, long-term operation induces a decrease in formation temperature, predominantly around the vicinity of well, where the maximum temperature decline is nearly 30 °C. The reduction in formation temperature is directly proportional to the increase in flow rate and inversely proportional to inlet temperature. Within the depth range of well, the maximum influence radius expands in correlation with both the depth and duration of mining, while remaining relatively stable with variations in flow rate and inlet temperature. Following three decades of extraction, the influence radius is approximately 95m. The results provide vital scientific insights for the retrofitting of abandoned wells in Northeast China into geothermal heat exchange wells for building heating purposes.

Utilizing a CNN-RNN machine learning approach for forecasting time-series outlet fluid temperature monitoring by long-term operation of BHEs system

The Borehole Heat Exchanger (BHE) plays a pivotal role in enhancing heat exchange efficiency within Ground Source Heat Pump (GSHP) systems. The accurate prediction of the BHE's outlet fluid temperature is crucial for optimizing GSHP performance, energy storage, and resource conservation. However, conventional machine learning methods encounter challenges in manual feature extraction, learning complex nonlinear relationships, and adapting to real-world scenarios. To address these limitations, this research proposes a crossbreed model integrating Convolutional Neural Network (CNN) and Recurrent Neural Network (RNN) architectures to forecast long-term outlet fluid temperature in BHE systems. The model framework encompasses data preprocessing, utilizing refined data in the CNN module for temporal feature extraction, subsequently passed to the RNN module to capture sequential and temporal patterns from each dataset. Specifically, the advanced CNN-RNN architecture is designed to establish a comprehensive input-output mapping, leveraging essential input features such as inlet fluid, ambient air, and subsurface temperatures at varying depths (0, 10, and 20 m). Performance evaluation metrics, including R2, RMSE, MAE, and AARE, are employed to compare and assess prediction accuracy across various models, including LSTM, CNN, and SimpleRNN. The obtained results demonstrate the superior performance of the proposed model, achieving an RSME of 0.818, MAE of 0.642, AARE of 0.0305, and an R2 value of 98.75 %. This surpasses the performance of traditional prediction models (LSTM, CNN, and SimpleRNN) by 3.01 %, 5.80 %, and 19.52 %, respectively. Notably, the remarkably low MAE of 0.642 exhibited by a CNN-RNN model underscores its capability to outperform traditional approaches, especially when handling large datasets. These findings emphasize the significance of the developed model in facilitating efficient operation, positioning it as a valuable tool for advancing the long-term sustainability of BHE systems.

Characteristics and Application Analysis of a Novel Full Fresh Air System Using Only Geothermal Energy for Space Cooling and Dehumidification

To effectively reduce building energy consumption, a novel full fresh air system with a heat source tower (HST) and a borehole heat exchanger (BHE) was proposed for space cooling and dehumidification in this paper. The cooling system only adopts geothermal energy to produce dry and cold fresh air for space cooling and dehumidification through the BHE and HST, which has the advantage of non-condensate water compared to BHE systems integrated with a fan coil or chilled beam. Based on the established mathematical model of the cooling system, this paper analyzed the system characteristics, feasibility, operation strategy, energy performance, and cost-effectiveness of the proposed model in detail. The results show that the mathematical model has less than 10% error in estimating the system performance compared to the practical HST–BHE experimental set up. Under the specific boundary conditions, the cooling and dehumidification capacity of this system increases with the decrease in the air temperature, air moisture content, and inlet water temperature of the HST. The optimal cooling capacity and the system COP can be achieved when the air–water flow ratio is at 4:3. A case study was conducted in a residential building in Shenyang with an area of about 1800 m2. It was found that this system can fully meet the cooling and dehumidification demand in such a residential building. The operation strategy of the cooling system can be optimized by adjusting the air–water flow ratio from 4:3 to 3:2 during the early cooling season (7 June–1 July) and end cooling season (3 August–1 September). As a result, the average COP of the cooling system during the whole cooling season can be improved from 6.1 to 8.7. Compared with the air source heat pump (ASHP) and the ground source heat pump (GSHP) for space cooling, the proposed cooling system can achieve an energy saving rate of 123% and 26%, respectively. Considering that the BHE of the GSHP can be part of the proposed HST–BHE cooling system, the integration of the HST and GHSP for space cooling (and heating) is strongly recommended in actual applications.

Study on thermal radius and capacity of multiple deep borehole heat exchangers: Analytical solution, algorithm and application based on Response Factor Matrix method (RFM)

The deep borehole heat exchanger systems (DBHE) are offering potential low carbon heating for buildings. However, the study of DBHE is hindered by both modeling and system complexity. Although some analytical models of DBHE are presented in literature, the involved expression of complex integral calculation on the contrary slows down the computation speed and many calculations are redundant in algorithm. In this study, a Response Factor Matrix method (RFM) is proposed based on simple analytical expression and well-designed data structure for DBHE. The RFM can be prefabricated and then used in each time step updates of temperature field. A superposition algorithm is designed for fast computation of DBHE array. The proposed model and algorithms are verified by measurements from two independent engineering projects. Based on the new tool, both thermal radius and heating capacity of DBHE array are investigated in details. The step-ascending pattern of thermal radius is recognized. The correlation of borehole spacing and system heating capacity is identified. In the end, the computational efficiency of the new tool is justified and some outlooks are proposed for a further development and applications.

Analysis of the subsurface and fluid temperature monitoring by long-term operation of borehole heat exchangers

Owing to the prevailing escalation in energy costs, escalating energy demands, depletion of conventional resources, and escalating greenhouse gas emissions, there has been an expeditious upsurge in the worldwide proliferation of alternative energy sources. Geothermal systems, in particular, are highly coveted as environmentally friendly and sustainable renewable energy reservoirs, offering efficacious cooling and heating capabilities for various structures (Xu et al., 2020; Ghasemi-Fare & Basu, 2013). The energy exchange efficiency of geothermal systems is significantly contingent upon the ground temperature, enabling the discharge of energy into the ground for cooling purposes in summer and its retrieval for heating requirements during winter. However, long-term operation of geothermal energy systems can engender an imbalance in thermal recovery through the heating and cooling processes, consequently influencing the thermomechanical behavior of the subsurface over time (Baek et al., 2017). Geothermal systems, also known as ground source heat pumps (GSHPs), are designed to operate for around 20 years, while the heat source component can be utilized for an extended duration ranging from 30 to 50 years. Understanding heat transfer characteristics and long-term performance of geothermal energy systems is crucial for optimal utilization and efficiency. However, research often lacks field investigation data, specifically for long-term monitoring and validation of fluid temperatures and subsurface temperature profiles. This research conducted comprehensive long-term measurements to assess fluid and subsurface temperature behavior in a borehole heat exchanger (BHE) system. The study examined the impact of sustained BHE performance on surface and subsurface temperature dynamics. A sophisticated three-dimensional numerical model was developed based on empirical insights from the field study and validated using monitored data. The analysis included inlet/outlet fluid temperature, spatial heterogeneity of ground temperature profiles, thermal recovery processes, and alterations caused by residual thermal energy.
 Furthermore, additional temperature sensors were deployed at various depths to monitor temperature changes at distinct levels within the subground.
 In this study, an advanced three-dimensional (3D) numerical model was developed utilizing the Finite Element Method (FEM) to effectively capture the intricate heat transfer processes occurring around BHEs. The geometry mesh was generated using a specialized triangular mesh generator, enabling spatial variation of element sizes to accurately represent the geometry and temperature distribution patterns. To address the specific requirements of U-tube heat exchanger loops, where the steepest temperature gradient is anticipated, a meticulously chosen element size of 0.7 mm, was employed. As the model extends towards the lateral boundaries, particularly in the radial direction, the element size gradually increases to ensure faithful representation of the system's characteristics. To properly account for diverse vertical temperature gradients, the number of elements and the vertical distance between slices were systematically adjusted within a range of 0.01 to 5 m. This meticulous variation facilitated accurate consideration of the distinct vertical temperature profiles. Notably, the finest discretization was applied near the bottom of the BHE (approximately z = -155 m) and at the top surface, characterized by a significant temperature gradient.
 The depicted Figure 2 showcases the model's performance in analyzing the fluid profile in comparison to the monitored data over a duration of 600 hours. This comparison reveals that the maximum relative difference between the monitored and simulated values for the inlet and outlet fluid temperature does not exceed 2.649 and 2.296 degrees Celsius, respectively. These findings offer compelling evidence of the exceptional accuracy of the proposed numerical modeling approach, thus affirming its suitability for examining the long-term sustainability of BHE systems.

Energy conversion through deep borehole heat exchanger systems: Heat storage analysis and assessment of threshold inlet temperature

The heating capacity of deep borehole heat exchanger (DBHE) systems gradually attenuates due to the cold accumulation of rock and soil during long-term operation. Therefore, heat storage in rock and soil is crucial to alleviate thermal attenuation and ensure stable operation. However, the mechanism and design methods for heat storage are unclear. In this paper, heat extraction and storage models for DBHE systems are established, the effects of heat storage on system operation are revealed, and a design method for the threshold inlet water temperature of heat storage is proposed. The results show that the fluid in the DBHE should inflow from the inner pipe to release more heat into the rock and soil when adopting heat storage. The heat storage power under fluid inflow from the inner pipe is 10.81 kW higher than that from the annular space under the same operating conditions. Increasing the inlet water temperature is more beneficial than increasing the flow rate to enhance the heat storage power. After 2880 h of operation, the heating power of the DBHE is increased by 3.44% for the next heating season when the storage flow rate is increased from 8 m3/h to 38 m3/h. Thus, a high heat storage inlet temperature with a low flow rate should be adopted during the heat storage period. Furthermore, the threshold inlet water temperature of heat storage is highly dependent on the depth of the DBHE and the geothermal gradient, with percentage contributions of 57.14% and 39.26%, respectively. The multiple linear regression model (R2 = 0.972) has shown high predictive accuracy in the assessment of threshold inlet water temperatures, with a maximum relative error of −5.60%. The findings of this paper provide technical support for designing heat storage using DBHE systems.

Study on long-term performance sustainability of medium deep borehole heat exchanger based on simplified one-dimensional well model

Heating buildings and residences is a low-cost and high-efficiency renewable energy utilization method through the coupling of medium deep borehole heat exchanger and ground source heat pump systems. Since the performance of the medium deep borehole heat exchanger system will gradually decrease during long-term operation, its sustainability becomes a key issue in current research. In this paper, a comprehensive numerical model with a simplified one-dimensional well is established using COMSOL Multiphysics by considering the influence of factors including layered strata, groundwater flow, thermophysical properties, and geothermal gradient. The rationality of the model is verified by the thermal response test data of the medium deep borehole heat exchanger system located in Daxing District, Beijing, China. The outlet temperature of the medium deep borehole heat exchanger system declined by 0.48 °C (3.1%) next 20 years of continuous operation. The area greatly affected by heat transfer process is basically within 20 m around the borehole. In the fifth, tenth, fifteenth, and twentieth years, the heat transfer impact scope is about 43 m, 56 m, 74 m, and 84 m, respectively. This study can provide a reference for the performance sustainability of long-term operation of medium deep borehole heat exchanger systems.

Evaluation of closed-loop U-Tube deep borehole heat exchanger in the Basal Cambrian Sandstone formation, Alberta, Canada

Closed-loop deep borehole heat exchanger (DBHE) systems for producing heat from geothermal sources have the advantage that the heat transfer fluid is contained within the loop. In this study, for the first time, a U-configuration closed-loop DBHE was examined to evaluate the energy produced per unit energy invested from a 2330-m-deep geothermal reservoir in central Alberta, Canada. A detailed earth model where the system is modeled from the surface to the geothermal source is used in a numerical simulation model to understand the efficiency of the process. The results reveal that the fluid flow reaches its highest temperature in the ascending section of the U-loop rather than the bottom section which implies that the insulation on the working fluid should start in the ascending section of the U-loop. The results demonstrate that the closed-loop system can achieve ratios of the energy produced and energy invested of 7 GJ/GJ. Although this efficiency is promising, the absolute amount of heat energy harvested is limited by the loop’s heat transfer area in the geothermal reservoir.

Evaluation of the Shallow Geothermal Potential for Heating and Cooling and Its Integration in the Socioeconomic Environment: A Case Study in the Region of Murcia, Spain

In order to boost the use of shallow geothermal energy, reliable and sound information concerning its potential must be provided to the public and energy decision-makers, among others. To this end, we developed a GIS-based methodology that allowed us to estimate the resource, energy, economic and environmental potential of shallow geothermal energy at a regional scale. Our method focuses on closed-loop borehole heat exchanger systems, which are by far the systems that are most utilized for heating and cooling purposes, and whose energy demands are similar throughout the year in the study area applied. The resource was assessed based on the thermal properties from the surface to a depth of 100 m, considering the water saturation grade of the materials. Additionally, climate and building characteristics data were also used as the main input. The G.POT method was used for assessing the annual shallow geothermal resource and for the specific heat extraction (sHe) rate estimation for both heating and, for the first time, for cooling. The method was applied to the Region of Murcia (Spain) and thematic maps were created with the outputting results. They offer insight toward the thermal energy that can be extracted for both heating and cooling in (MWh/year) and (W/m); the technical potential, making a distinction over the climate zones in the region; the cost of the possible ground source heat pump (GSHP) installation, associated payback period and the cost of producing the shallow geothermal energy; and, finally, the GHG emissions savings derived from its usage. The model also output the specific heat extraction rates, which are compared to those from the VDI 4640, which prove to be slightly higher than the previous one.

Analysis of economy, thermal efficiency and environmental impact of geothermal heating system based on life cycle assessments

The deep buried coaxial and horizontally-butted borehole heat exchangers are the two typical wells for geothermal utilization in space heating. The economicindexes, heat extraction performance and environmental impact of the two wells are crucial for actual projects. In this study, the Monte Carlo simulation and sensitivity analysis based on life cycle saving are employed to evaluate the economy of the two borehole heat exchanger systems. The heat extraction considering both performance and economic factors is examined by the levelized cost, while the environmental impact is assessed by the carbon intensity analysis. The results showed that the payback periods of a coaxial borehole heat exchanger system and a horizontally-butted borehole heat exchanger system are 6.7–9.2 years and 9.1–11.7 years respectively, and the life cycle saving of a butted borehole heat exchanger may reach a high value after 25 years of operation. The sensitivity of life cycle saving for both coaxial and horizontally-butted borehole heat exchangers is mainly governed by heating income, discount rate, and non-depreciable initial investment. The heat production capacity of the butted borehole heat exchanger system is 1128–1342 kW, which is twice as much as that of coaxial borehole heat exchanger. From the life cycle perspective, the levelized cost of energy and carbon intensity of the coaxial borehole heat exchanger is $ 8.67–9.35/GJ and 70.43–80.86 g(CO2)/kWh, slightly higher than those of the butted borehole heat exchanger. It can be proved that the two borehole heat exchangers are more in line with the low-carbon policy than the traditional heating methods, and the well construction and operation phase contributes a major carbon emission from the perspective of the life cycle.

Research on Fresh and Hardened Sealing Slurries with the Addition of Magnesium Regarding Thermal Conductivity for Energy Piles and Borehole Heat Exchangers

Currently, renewable energy is increasingly important in the energy sector. One of the so-called renewable energy sources is geothermal energy. The most popular solution implemented by both small and large customers is the consumption of low-temperature geothermal energy using borehole heat exchanger (BHE) systems assisted by geothermal heat pumps. Such an installation can operate regardless of geological conditions, which makes it extremely universal. Borehole heat exchangers are the most important elements of this system, as their design determines the efficiency of the entire heating or heating-and-cooling system. Filling/sealing slurry is amongst the crucial structural elements. In borehole exchangers, reaching the highest possible thermal conductivity of the cement slurry endeavors to improve heat transfer between the rock mass and the heat carrier. The article presents a proposed design for such a sealing slurry. Powdered magnesium was used as an additive to the cement. The approximate cost of powdered magnesium is PLN 70–90 per kg (EUR 15–20/kg). Six different slurry formulations were tested. Magnesium flakes were used in designs A, B, C, and magnesium shavings in D, E and F. The samples differed in the powdered magnesium content BWOC (by weight of cement). The parameters of fresh and hardened sealing slurries were tested, focusing mainly on the thermal conductivity parameter. The highest thermal conductivity values were obtained in design C with the 45% addition of magnesium flakes BWOC.

Investigation of the horizontally-butted borehole heat exchanger based on a semi-analytical method considering groundwater seepage and geothermal gradient

The horizontally-butted borehole heat exchanger (BHE) can be used to extract heat efficiently from medium-deep strata, where the groundwater seepage and undisturbed geothermal gradient are critical influence factors. In the application, it is an important and fundamental work to predict the BHE performance for high efficiency geothermal energy utilization. In order to solve its outlet temperature and heat exchange rate, this study develops a semi-analytical solution based on an adjustable multi-layer model associated with the moving finite line source (MFLS) method. The predicted results of a 2505 m horizontally-butted BHE show that the insulation layer and groundwater seepage are beneficial to a better performance. When the insulation depth increases from 0 to 900 m, the outlet temperature can be elevated from 40.5 °C to 48.7 °C, with the fluid volumetric flow rate given at 15 m3·h-1. With a 240 m thick aquifer layer, the groundwater seepage leads to 0.34 °C and 5.73 kW improvements in the outlet temperature and heat exchange rate, respectively. The proposed method successfully solves the problem of predicting the large depth horizontally-butted BHE drilled through various geological formations and aquifers. It can serve as a reference for the rational design of BHE systems for utilizing effectively the geothermal energy.

Defining the Shallow Geothermal Heat-Exchange Potential for a Lower Fluvial Plain of the Central Apennines: The Metauro Valley (Marche Region, Italy)

In this work we assessed the shallow geothermal heat-exchange potential of a fluvial plain of the Central Apennines, the lower Metauro Valley, where about 90,000 people live. Publicly available geognostic drilling data from the Italian Seismic Microzonation studies have been exploited together with hydrogeological and thermophysical properties of the main geological formations of the area. These data have been averaged over the firsts 100 m of subsoil to define the thermal conductivity, the specific heat extraction rates of the ground and to establish the geothermal potential of the area (expressed in MWh y−1). The investigation revealed that the heat-exchange potential is mainly controlled by the bedrock lithotypes and the saturated conditions of the sedimentary infill. A general increase in thermal conductivity, specific heat extraction and geothermal potential have been mapped moving from the coast, where higher sedimentary infill thicknesses have been found, towards the inland where the carbonate bedrock approaches the surface. The geothermal potential of the investigated lower Metauro Valley is mostly between ~9.0 and ~10 MWh y−1 and the average depth to be drilled to supply a standard domestic power demand of 4.0 kW is ~96 m (ranging from 82 to 125 m all over the valley). This investigation emphasizes that the Seismic Microzonation studies represent a huge database to be exploited for the best assessment of the shallow geothermal potential throughout the Italian regions, which can be addressed by the implementation of heating and cooling through vertical closed-loop borehole heat exchanger systems coupled with geothermal heat pumps.

Field test and numerical investigation on deep coaxial borehole heat exchanger based on distributed optical fiber temperature sensor

The thermal performance of DCBHE is superior to conventional borehole heat exchanger (BHE) systems. In the current study, the field test using a DCBHE (2044 m) is conducted, based on a distributed optical fiber temperature sensor (DOFTS). The temperature of the circulating fluid in terms of depth and operation time is monitored in real-time. The thermal extraction performance of the DCBHE is evaluated over an operation period of 60 d. Moreover, the temperature distribution characteristics, thermal process and impact scope of the formation are analyzed. The results demonstrate that the temperature of the bottom borehole is measured as 107.3 °C via DOFTS, and the average geothermal gradient is determined as 0.0507 °C/m. The thermal power and the coefficient of system performance are able to reach 238.0 W/m and 7.40 following 60 d of operation, respectively. The inner pipe made of polyethylene has exhibits a high level of thermal insulation for the outlet fluid. The thermal process between the soil and fluid is enhanced close to the borehole. And the soil temperature profiles exhibit fluctuations around the borehole within 1 m due to power failure. Furthermore, the formation impact scope is mainly within radius 5 m for operation 60 d.

Long-term high frequency monitoring of a large borehole heat exchanger array

Borehole heat exchangers are a key technological element of geothermal energy systems and modelling their behaviour has received much attention. The aim in the work reported here has been to produce a reference data set that can be used in analysis of large borehole heat exchanger systems and validation of models of such. A monitoring exercise to collect high frequency data from a large ground heat exchanger array consisting of 56 boreholes over 38 months since the start of operations is reported. The system is associated with a mixed-use university building that has both heating and cooling loads. Ground heat exchange was found to be dominated by rejection of heat over the monitoring period and modest seasonal increases in temperatures. The ground heat exchanger installation has been additionally characterised by analysis of thermal response test data to estimate the effective ground and grout thermal properties. The utility of the measurements as a reference data set by presenting a model validation study is furthermore demonstrated. This has highlighted some features of the data that are more significant in systems of larger scale. These reference data are being made openly available for further work on performance analysis and model validation.

In Situ Experiment and Numerical Model Validation of a Borehole Heat Exchanger in Shallow Hard Crystalline Rock

Accurate and fast numerical modelling of the borehole heat exchanger (BHE) is required for simulation of long-term thermal energy storage in rocks using boreholes. The goal of this study was to conduct an in situ experiment to validate the proposed numerical modelling approach. In the experiment, hot water was circulated for 21 days through a single U-tube BHE installed in an underground research tunnel located at a shallow depth in crystalline rock. The results of the simulations using the proposed model were validated against the measurements. The numerical model simulated the BHE’s behaviour accurately and compared well with two other modelling approaches from the literature. The model is capable of replicating the complex geometrical arrangement of the BHE and is considered to be more appropriate for simulations of BHE systems with complex geometries. The results of the sensitivity analysis of the proposed model have shown that low thermal conductivity, high density, and high heat capacity of rock are essential for maximising the storage efficiency of a borehole thermal energy storage system. Other characteristics of BHEs, such as a high thermal conductivity of the grout, a large radius of the pipe, and a large distance between the pipes, are also preferred for maximising efficiency.

Correct design of vertical borehole heat exchanger systems through the improvement of the ASHRAE method

In this article, the ASHRAE method for borehole heat exchanger field design is deeply discussed in order to trace its theory and physical meaning. The aim of the article is corroborate previous studies in order to demonstrate that the current version of the method is not suitable for a reliable sizing of the borehole field since the criteria for calculating the temperature penalty term fail to large extent from a correct estimation. Based on the above findings (demonstrated by a comprehensive comparison with reference results pertaining a vast number of borefield configurations), a new version of the temperature penalty calculation procedure is presented that is able to maintain the ASHRAE formalism and simplicity while enhancing the accuracy of the design results.

Experimental verification and programming development of a new MDF borehole heat exchanger numerical model

Numerical models of borehole heat exchanger (BHE) systems are of high practical importance in borehole thermal energy storage technologies. Therefore, this article shows the experimental verification and results comparison of Multi-Degree-of-Freedom (MDF), Diersch, quasi-three-dimensional (q3D) and ANSYS CFX numerical models of heat transport in a single U-tube vertical BHE. Emphasis has been put on verification of a new MDF model and its programming development in ANSYS software using user programmable features (UPFs). Accordingly, a significant reduction of CPU time and good agreement with experiment data are gained, thus rendering the MDF and Diersch model suitable for utilisation in engineering practice.

Thermal performance analysis of borehole size effect on geothermal heat exchanger

Thermal performance was the most important factor in the development of borehole heat exchanger utilizing geothermal energy. The thermal performance was affected by many different design parameters, such as configuration type and borehole size of geothermal heat exchanger. These eventually determined the operation and cost efficiency of the geothermal heat exchanger system. The main purpose of this work was to assess the thermal performance of geothermal heat exchanger with variation of borehole sizes and numbers of U-tubes inside a borehole. For this, a thermal response test rig was established with line-source theory. The thermal response test was performed with in-line variable input heat source. Effective thermal conductivity and thermal resistance were obtained from the measured data. From the measurement, the effective thermal conductivity is found to have similar values for two-pair type (4 U-tubes) and three-pair type (6 U-tubes) borehole heat exchanger systems indicating similar heat transfer ability. Meanwhile, the thermal resistance shows lower value for the three-pair type compared to the two-pair type. Measured data based resistance have lower value compared to computed result from design programs. Overall comparison finds better thermal performance for the three-pair type, however, fluctuating temperature variation indicates complex flow behavior inside the borehole and requires further study on flow characteristics.

Long-term performance of an irregular shaped borehole heat exchanger system: Analysis of real pattern and regular grid approximation

A reliable evaluation of long-term performance of a heat pump coupled with a borehole heat exchanger (BHE) field is necessary to verify the stability of its heat exchange capability over the time. The BHE field pattern is often assumed to be regular (e.g., rectangular, L-shaped, T-shaped, etc.), or is assumed to be adequately approximated by one of these shapes. Moreover, a planned geothermal system is often designed regardless of the presence of other existing or planned BHE systems. In order to evaluate the validity and the possible limitations of these assumptions commonly made by the designer, a number of 25-year time span simulations have been carried out by means of 2D finite element modeling. In particular, the case of a real 28 BHE field, irregularly shaped and related to a building located in Northern Italy, has been studied together with its 7-by-4 regular grid approximation and a series of 28 BHE fields having different shapes. Besides the real annual thermal load profile characterized by quasi-balanced winter heating and summer cooling, two other profiles characterized by increasingly unbalanced operation have been taken into account. The numerical study shows that (i) the regularly shaped approximation, a common choice in BHE design, seems to be reasonable under the condition that groundwater flow is absent for all the thermal load profiles; (ii) if a strong heating/cooling imbalance occurs, the thermal footprint of a BHE field can be very extensive, preventing the installation of future nearby BHE systems.