Borehole Thermal Energy Storage System Research Articles

Sensitivity Analysis of Borehole Thermal Energy Storage (BTES): Examining Key Factors for System Optimization

Borehole thermal-energy storage (BTES) systems have garnered significant attention owing to their efficacy in storing thermal energy for heating and cooling applications. Accurate modeling is paramount for ensuring the precise design and operation of BTES systems. This study conducts a sensitivity analysis of BTES modeling by employing a comparative investigation of five distinct parameters on a wedge-shaped model, with implications extendable to a cylindrical configuration. The parameters examined included two design factors (well spacing and grout thermal conductivity), two operational variables (charging and discharging rates), and one geological attribute (soil thermal conductivity). Finite element simulations were carried out for the sensitivity analysis to evaluate the roundtrip efficiency, both on a per-cycle basis and cumulatively over three years of operation, serving as performance metrics. The results showed varying degrees of sensitivity across different models to changes in these parameters. In particular, the round-trip efficiency exhibited a greater sensitivity to changes in spacing and volumetric flow rate. Furthermore, this study underscores the importance of considering the impact of the soil and grout-material thermal conductivities on the BTES-system performance over time. An optimized scenario was formulated and juxtaposed against the base case, with a comparative assessment based on a 10-year simulation. The analysis revealed that, at the end of the 10-year period, the optimized BTES model achieved a cyclic efficiency of 83.4%. This sensitivity analysis provides valuable insights into the merits and constraints of diverse BTES modeling methodologies, aiding in the selection of appropriate modeling tools for BTES system design and operation.

Energy Performance Analysis of the Renovation Process in an Italian Cultural Heritage Building

Renovating buildings with cultural heritage significance is an important step toward achieving sustainability in our cities. The benefits are not only energy-related but also encompass social aspects that make these renovations a high apriority. The present work investigates the renovation process of a cultural heritage building in the Municipality of Trento in Italy, specifically focusing on achieving energy savings and renewable energy integration by implementing various renovation actions. These renovation actions include improvements to the building envelope, such as roof insulation and window replacements. Additionally, the renovation actions for active systems involve the installation of a ground-source heat pump for heating/cooling coupled with a borehole thermal energy storage system, which is an innovative technology for the renovation of cultural heritage buildings. The electrical systems of the building are upgraded through the addition of standard rooftop photovoltaics, innovative building-integrated photovoltaics (shingles), and the installation of an LED lighting system. The baseline and the renovation scenarios are studied using the dynamic simulation tool INTEMA.building, written in the programming language Modelica. This tool simulates both the building envelope and the energy systems with a high level of detail, using advanced control systems and adjustable time steps. According to the simulation analysis, the primary energy demand is reduced by 30.49%, the final energy demand by 36.74%, and the net electricity demand by 8.72%. Results from this study can be useful to interested stakeholders (e.g., building owners, architects, construction companies, public agents, and urban planners) dealing with the renovation of cultural heritage and protected buildings. Also, the results can be exploited for estimating energy savings by applying advanced renovation strategies for cultural heritage buildings.

Review of borehole thermal energy storage systems in unsaturated soils: Materials, mechanism, and future development

AbstractBorehole thermal energy storage (BTES) is an innovative renewable energy technology for building heating and cooling. The lack of studies about BTES in unsaturated soils acts as a barrier to further implementation. In this study, the research obstacles, progress, hotspots, and differences between countries of BTES systems in unsaturated soils were described in detail based on a literature review, bibliometric analysis, technology roadmap, and data visualizations. The results show that the process of coupled heat and moisture transfer has been a research hotspot in recent years, and the thermo‐hydraulic properties increase the complexity of analysis. There are few studies on experimental data and engineering practices of BTES systems in unsaturated soils. China is leading in publications number, but European and North American countries perform better in international cooperation and policy incentives. Technology development will be directly driven by policy momentum, followed by the gradual establishment of a complete research, design, and evaluation system, which helps accelerate the replacement of traditional fossil energy with renewable energy. In an attempt to improve society's awareness and move forward with scientific foresight of BTES systems in unsaturated soils, this review has revealed the evolutionary process, identified its current status, and depicted the future development from political, economic, social, and other dimensions.

Numerical modelling of a high temperature borehole thermal energy storage system: Norway case study

Global warming is threatening life on earth. Utilising renewable energy is considered as the most effective measure to minimise anthropogenic CO2 emissions. High-temperature borehole thermal energy storage (BTES) systems have a world-wide potential to reduce energy consumption, increase energy utilisation of waste heat and provide efficient seasonal heat storage. Hybrid application of BTES and solar energy leads to net zero emissions. In this study HT-BTES is evaluated for seasonal thermal heat storage and recovery. To this end, a CMG STARS model was built and validated using the existing 100-wells BTES system in Norway. Then, the model was used to evaluate BTES thermal performance and thermal recovery efficiency. Sensitivity analysis was also conducted to study the dynamics of storage temperature in the BTES under different operating conditions, such as heat carrier flow rate, injection temperature, and charging period. Results of this case study show that the model-predicted temperatures during charging and discharging are in good agreement with the existing BTES system. In 5 years of operation, 35.5% of the heat injected into the BTES system was recovered, while the significant heat remained in the borehole region and lost to surrounding rock (64.5%). BTES was found very sensitive to flow rate, the charging period and injection temperature. Borehole depth has a minimal effect on BTES storage temperature at constant studied injection temperature.

Simulation of borehole thermal energy storage (BTES) systems using simplified methods

This paper presents two complementary approaches for simulating the thermal performance of borehole thermal energy storage (BTES) systems. The first approach uses the concepts of heat exchange and storage efficiencies as a function of the state-of-charge of the BTES. The second method employs a technique similar to thermal response factors used to model bore fields in ground-source heat pump systems. It utilizes non-dimensional average storage temperatures and bore field thermal resistances of the BTES over time. The two methods rely on numerical experiments to obtain the required performance curves. As shown through examples, the two approaches are relatively simple to use, and it is shown that they are in good agreement with the well-known duct ground storage model. In one of the examples, it is suggested that the design of a BTES could be performed using three successive thermal pulses, similar to the approach used to determine the length of vertical ground heat exchangers in a ground-source heat pump systems.

Coupling a road solar thermal collector and borehole thermal energy storage for building heating: First experimental and numerical results

The adoption of more efficient technologies that integrate renewable resources for heating buildings is a key action for increasing sustainability in the residential sector in the European Union. Borehole thermal energy storage and road thermal collector systems, which have mainly been integrated in colder countries to develop renewable systems, aimed at preventing the freezing of road surfaces in winter, could also be used in warmer countries to develop sustainable heating systems for buildings. In this experimental–numerical study, the possibility of integrating these two systems for the heating of buildings located in the Mediterranean region is investigated for the first time. To this end, a pilot plant was built at the facility test site of the University of Palermo, with the aim of demonstrating the possibility of storing solar energy in summer and recovering it in winter. A new method is proposed to characterize both the thermal conductivity and diffusivity of the different materials in the design phase of the plant. The results of simulations conducted with a validated numerical model show that the proposed system, characterized by an average annual collector efficiency of 10% and a seasonal storage efficiency of 80%, can reduce the length of borehole heat exchangers by about three times compared with a conventional geothermal heat pump plant.

Long-term borehole energy storage by the inlet position control for low temperature heat source application

Borehole thermal energy storage (BTES) systems are gaining attention due to their potential to efficiently store and release thermal energy. However, the conventional BTES systems often suffer from reduced efficiency under low heat source temperature conditions. This study aims to address this issue by introducing and evaluating the inlet position control method, a novel approach to enhance the BTES performance for low-temperature heat source applications. A single-cycle 3D simulation model, substantiated by experimental data from a BTES system installed in the YeonCheon region, Korea, is developed using the ANSYS Fluent program. The model investigates the heat transfer from the water heat source to the soil and compares the performance of the conventional method with the inlet position control method in terms of heat gradient contours and average temperature difference in the soil. The inlet position control method is found to improve the BTES storage efficiency by 11.90% during the early winter season of the thermal release period in the third year, compared to the conventional method. The standard deviation for simulation data is ± 1.787 with a peak error of 2.4%, indicating the effectiveness of the simulation model. Furthermore, the inlet control method enhances the BTES storage efficiency under low heat source temperature conditions and promotes faster thermal equalization, resulting in a 4.2% improvement in thermal energy release efficiency. We demonstrated the novelty of the inlet position control method in improving BTES performance and its applicability to low-temperature heat source applications. The findings have significant implications for the design and operation of future BTES systems, with the potential to expand their application in various industries and climates.

Collective behavior of boreholes and its optimization to maximize BTES performance

Borehole layout strongly affects the behavior of borehole heat exchangers (BHEs) and changes the performance of a borehole thermal energy storage (BTES). This study investigates the existence and importance of the optimum collective behavior of BHEs to maximize the performance of BTES. Charge benefit ratio, storage efficiency and configurational benefit factor are proposed as performance indicators for better and finer performance evaluations of BTES systems. A small-scale BTES consisting of ten boreholes arranged on a concentric double-ring layout is considered as an application. Performance variations with the inner and the outer radii of the borehole field are analyzed for the first five years of operation. The temperature fields of different configurations show the transition from collective to individual behavior of boreholes, which leads to an optimal radial configuration maximizing the performance indicators. It is seen that the indicators strongly depend on both inner and outer radii and they reach their maximums for the same distinct radial configuration. The optimum arrangement can almost double the thermal performance indicators. It is thus of great importance to optimize collective behavior to maximize the usage of stored thermal energy. The results are qualitatively general and represent the common behavioral patterns of BTES systems.

Development of a Pilot Borehole Storage System of Solar Thermal Energy: Modeling, Design, and Installation

Borehole thermal energy storage systems represent a potential solution to increase the energy efficiency of renewable energy plants, but they generally have to comply with strict regulatory frameworks, mainly due to the deliberate modification of the subsoil’s natural state. This paper presents the design, testing, and monitoring phases carried out to set up a borehole thermal energy storage (BTES) system able to exploit the excess solar heat from photovoltaic thermal (PVT) collectors. The case study is the refurbishment of a pig nursery barn, hosting up to 2500 weaners, in Northern Italy. This study aims to define a BTES suitable to develop a heating system based on renewable energy, ensuring environmental protection and long-term sustainability. The retrofitting intervention includes the installation of a dual-source heat pump (DSHP), in order to recover the solar heat stored in summer during winter. Specific constraints by the Environmental Authority were as follows: maximum storage temperature of 35 °C, authorization to intercept the shallowest aquifer at a maximum depth of 30 m, obligation of BHE grouting, and the definition of a strategy for continuous measuring and monitoring of the groundwater’s thermophysical properties. The results were used as inputs to optimize the design and installation of the integrated system with PVT, BTES, and DSHP.

Reduced order 1 + 3D numerical model for evaluating the performance of solar borehole thermal energy storage systems

This study presents a computationally efficient numerical model for solving the fluid flow and heat transfer phenomena in co-axial boreholes heat exchangers arranged on a rectangular grid (N × N) drill hole pattern. Such systems of boreholes are an integral component of a Solar-Borehole Thermal Energy Storage System for seasonal thermal storage application. The numerical model couples a one-dimensional fluid flow and convective heat transfer model in the heat exchanger with a three-dimensional model of conductive heat transfer in the strata. In addition to solving mass and heat transfer in borehole systems, it integrates a solar thermal collector system and building dynamic thermal load to design a Solar- Borehole Thermal Energy Storage system for a 182-unit residential apartment building. Simulation also considers and evaluates the conductive, convective, and radiative thermal losses of the system. The model is validated using experimental data from a coaxial borehole heat exchanger and compared to the results of a commercial finite volume solver for multiple co-axial boreholes. The computational cost of developed numerical model is reduced by a factor of 194 compared to a commercial finite volume solver. Solar- Borehole Thermal Energy Storage system simulation is performed for a period of five years considering hourly fluctuations in solar irradiance and building dynamic thermal energy demand. Parametric study is performed on depth and spacing of boreholes, and mass flowrate in the solar thermal collectors. It is found that a system of 100 boreholes, each 100 m deep, linked to 674 solar thermal collectors can provide the heating for the building with a total thermal demand of 5600 GJ per year.

Techno-Economic Analysis of a Seasonal Thermal Energy Storage System with 3-Dimensional Horizontally Directed Boreholes

Geothermal energy storage provides opportunities to store renewable energy underground during summer for utilization in winter. Vertically oriented systems have been the standard when employing boreholes as the means to charge and discharge the underground. Horizontally oriented borehole storage systems provide an application range with specific advantages over vertically oriented systems. They are not limited to the surface requirements needed for installation with vertical systems and have the potential to limit storage losses. Horizontal systems can be incorporated into the built environment and utilize underground storage sites below existing infrastructure. An experimental study examines configurations using a mix of renewable energy (photovoltaic panels) and grid energy to charge a storage system during summer for use during winter. A comparison of five different borehole configurations at three different loading temperatures was composed using an experimentally validated numerical model. The horizontal systems studied and analyzed in this work showed improved performance with scale and charging temperature. This paper supports further exploration into specific use cases for horizontal borehole thermal energy storage systems and suggests applications which would take advantage of better performance at scale.

Repurposing a deep geothermal exploration well for borehole thermal energy storage: Implications from statistical modelling and sensitivity analysis

Borehole thermal energy storage (BTES) is an important technology to minimise greenhouse gas emissions by storing surplus heat from industrial processes, space cooling or even excess summertime renewable wind or solar energy. This paper investigates the efficiency of BTES via a single deep ex-geothermal exploration well in Newcastle, retrofitted using a coaxial deep borehole heat exchanger (DBHE) completion. Previously, few studies have investigated 1) the use of a single DBHE for thermal energy storage or 2) the retrofitting of an ex-geothermal exploration well; therefore, this study investigates deep BTES through numerical modelling on MATLAB by testing the impacts of 10 design parameters on operational performance of a DBHE using both global and local sensitivity analyses.Under base-case conditions, a DBHE operating at a depth of 920 m could achieve a heat extraction rate in excess of c.54 kW recorded at the end of a 6 month (winter) heat production phase. When applying a 6 month (summer) thermal charge phase prior to extraction (recorded as 250 kW at the end of the charge period), the thermal yield recorded at the end of extraction was increased to a minimum of c.69 kW. In total, over an annual cycle, 1.23 GWh of heat was injected into the formation, and 0.46 GWh was extracted. Across all local sensitivity simulations, the average heat extraction rate was increased by 9.5–55.6 kW following a 6 month period of charge. The global sensitivity analysis demonstrated that thermal recovery was most influenced by parameters such as the undisturbed geothermal gradient, flow rate, inlet temperature during charge and inlet temperature during extraction. Most of these are operational parameters, indicating deep BTES systems can be optimised through careful engineering. The study concludes that single DBHEs have some capacity to store surplus heat. However, the additional heat yield during extraction is only a modest proportion of the heat reinjected to the formation during the charging phase (calculated as <20 % using the new storage efficiency metric proposed in this study). This approach is only likely to be viable where there is a large source of surplus heat with little alternative value, and where there is an existing deep borehole suitable for retrofitting. If these conditions do not exists, more conventional, shallower, multi-borehole arrays are likely to be more suitable for BTES.

Experimental evaluation of IDA ICE and COMSOL models for an asymmetric borehole thermal energy storage field in Nordic climate

• A large-scale asymmetric borehole field was modelled in IDA ICE and COMSOL. • Simplification of asymmetric borehole field layout was investigated in 3-D model. • Borehole field models were validated by DTS-measured brine temperatures. • Validation of borehole field models showed good agreement. • Simplified-geometry borehole field model can reduce computational work. Appropriate modeling of borehole thermal energy storage (BTES) system is crucial for accurate prediction of BTES-coupled ground source heat pump performance with accessible computational load. This study aims to validate 3-D numerical models developed in IDA ICE 4.8 and COMSOL for a large-scale asymmetric borehole field consisting of 74 groundwater-filled boreholes with average depth of 310 m in Otaniemi, Finland. Comparisons were conducted between the detailed-geometry IDA ICE and COMSOL borehole field models and between the detailed-geometry and simplified-geometry IDA ICE borehole field models. All these models were validated by 1.5-years brine temperatures measured by temperature sensors and a distributed temperature sensing monitoring system. The results show the detailed-geometry IDA ICE model can estimate the BTES inlet and outlet brine temperatures as well as the COMSOL model. The average inlet and outlet brine temperature differences against the measurement data in the detailed-geometry IDA ICE and COMSOL models were both within 1 ℃. The simplified-geometry IDA ICE models can predict the borehole field inlet and outlet brine temperatures with a similar high accuracy (within 1 ℃ against the measurement) and can reduce the computational time by 72 % less than the detailed-geometry IDA ICE model.

Assessment of seasonal Borehole Thermal Energy Storage in the seawater intrusion region of a carbonate aquifer

In coastal areas the seawater intrusion region underlying freshwater aquifers represents a low quality but wide and deep geo resource. Seasonal thermal energy storage and recovery is an important component of district heating and cooling system to manage renewable energy fluctuations, such as solar irradiance or waste heat from industrial processes, and the corresponding mismatch of thermal energy demand and supply. A numerical tool to evaluate the performance of seasonal Borehole Thermal Energy Storage (BTES) system to store and recover solar energy in the seawater intrusion region, underlying the shallow freshwater aquifer and with thermally and hydraulically insulated upper borehole section is developed and applied to the coastal carbonate aquifer of the metropolitan area of Bari (Italy). The hourly thermal demand of the University Sport Centre of Bari is used as benchmark. The design and performance of the BTES system is strongly dependent on the geological and hydrogeological context as well as on the environmental and operational conditions. The aquifer characterization suggests to locate BTES zone at a depth higher than 100 m from the freshwater – saltwater interface where carbonate unit appears less fractured and karstified showing a value of bulk permeability less than 10−12 m2 and the groundwater flow is slow (∼10−3 md−1). Rayleigh number criterion is used as constraint to determine the maximum heat storage temperature (∼70°C) in order to preserve the lateral thermal stratification and the thermal impact on the shallow freshwater resource. A novel mathematical and computational model is developed to help the design of BTES system and to evaluate its efficiency. The results indicate that the thermal losses within thermally insulated zone influence the effective thermal recovery factor which, according to the baseline scenario, is equal to 47% after five years of operation. The heat energy production of the solar heating system, covers the heat demand with percentage range of 79–117%. The location of BTES in deep seawater region attenuates the decreases of efficiency due to the groundwater flow which became significant at specific discharge around 10−2 md−1. Changing the operation schedule with a shorter heating storage period increases the thermal recovery factor of the BTES of 11.19% after five years of operation, but at the same time the trend of the heat energy production shows a surplus during the midseason and deficit during the winter and summer season respect to the thermal demand. Great care must be taken on the maximum heat storage temperature. A low heat storage temperature ensures a wider safety margin with regards to the thermal stratification and the thermal impact on the shallow aquifer. Anyway, a decrease of 10°C on the maximum heat storage temperature produces a deficit of the heat production respect to the thermal demand in the range of 15%–21%.

Techno-economic analysis of a hybrid district heating with borehole thermal storage for various solar collectors and climate zones in Pakistan

Fossil fuels are being used to accommodate domestic heating needs all over the world, and the alarming rise in carbon footprint is demanding the world to shift towards renewable energy technologies. A key strategy to lessen household fossil fuel consumption is a solar hybrid district heating network integrated with seasonal thermal energy storage (TES). The objective of this study was to analyze and compare the thermo-economic performance of solar hybrid district heating systems integrated with borehole TES systems in ' 'Pakistan's five climate zones and identify the best suitable solar thermal collector technology. Based on the solar energy incident on different types of thermal collectors, a validated TRNSYS model was used to investigate the dynamic performance indices i.e., solar fraction, system efficiency, and thermal storage efficiency. The heat demand model and weather-related data were changed as input to the system. Based on mentioned performance indices, out of 5 different cities having different climates, climate Zone-C(Quetta), having a continental semi-arid climate, is selected as the most suitable, and Zone-E(Karachi) has tropical desert climate as the least favorable among five different zones with a seasonal storage efficiency of 61% and 19% respectively. The solar fraction is about 91%, also up to the mark as the proposed system utilizes both solar energy and auxiliary systems. Further, the implementation of different thermal collector technologies Parabolic Trough Collector, Evacuated Tube Collector & Flat Plate Collector, revealed that the thermal efficiency of ETC is best among all, with the highest value of 65%, followed by PTC at 44%, and FPC shows the least efficiency of 36%. The economic analysis shows that the model embedded with ETC has a minimum payback period of 6.90 years, followed by an FPC of 7.22 years and a maximum for PTC of 7.38 years. The analysis presented in the current study provides guidelines for researchers and policymakers to identify acceptable location-based best solar thermal collector technologies coupled with seasonal thermal storage for district space heating.

Design optimization of a district heating and cooling system with a borehole seasonal thermal energy storage

The optimal design of borehole thermal energy storage systems can ensure their techno-economical goals are met. Current design optimization methods either employ detailed modelling unsuitable for numerical optimization or use simplified models that do not consider operational conditions. This paper proposes an optimization-oriented model and a non-convex optimization formulation that, differently from other studies in the literature, can consider the influence of the seasonal storage size and temperature on its capacity, losses, heat transfer rate, and efficiency of connected heat pumps or chillers. This methodology was applied to a case study, considering two scenarios: storing only the rejected heat from cooling and integrating solar thermal generation. Results show that, with varying boundary conditions such as the electricity CO2 intensity profile, cooling demand, and price of carbon emissions, not only the optimal seasonal storage size changes but also its optimal operating conditions. The potential reduction of CO2 emissions was found, under standard boundary conditions, to be limited (up to 6.7%), but an increase in cooling demand and an enhancement of the CO2 intensity seasonal variation led to a reduction of 27.1%. Integration of solar generation further improved it to 43.7%, with a comparably small increase in annual cost, up to 6.1%.

Finite element simulation of permeable fault influence on a medium deep borehole thermal energy storage system

Solutions for seasonal energy storage systems are essential for the reliable use of fluctuating renewable energy sources. As part of the research project SKEWS, a medium deep borehole thermal energy storage system with a depth of 750 m is under construction at Campus Lichtwiese in Darmstadt, Germany, to demonstrate this innovative technology. Prior to the design of SKEWS, the geological context in the surroundings of the project location was investigated using archive drilling data and groundwater measurements. The geologic survey suggests the assumption that the uppermost part of the intended storage domain is crosscut by a normal fault, which displaces the Permian rocks east of Darmstadt against granodioritic rocks of the Odenwald crystalline complex. A 3D finite-element numerical model was implemented to estimate the effect of the potentially higher hydraulic conductivity of the fault zone on the planned storage system. For this purpose, a storage operation over a time span of 30 years was simulated for different parametrizations of the fault zone. The simulations reveal a limited but visible heat removal from the storage region with increasing groundwater flow in the fault zone. However, the section of the borehole thermal energy storage system affected by the fault is minor compared to the total depth of the system. This only constitutes a minor impairment of the storage efficiency of approximately 3%. In total, the amount of heat extracted varies between 320.2 GWh and 326.2 GWh for the different models. These findings can be helpful for the planning and assessment of future medium deep borehole thermal energy storage systems in fractured and faulted crystalline settings by providing data about the potential impact of faults or large fractures crosscutting the storage system.

Laboratory and numerical study on innovative grouting materials applicable to borehole heat exchangers (BHE) and borehole thermal energy storage (BTES) systems

In this study, a laboratory-scale prototype of a borehole field has been designed and built to assess various innovative grouting products in a fully controlled environment. Three novel grout formulations are developed and evaluated: enhanced grout, a mixture of enhanced grout and microencapsulated phase change material, and a mixture of enhanced grout and shape stabilized phase change material. The objective is to evaluate the enhancement in their thermal properties (i.e., thermal conductivity and thermal energy storage capacity) compared to those using a commercial reference grout. Besides, three-dimensional numerical modeling is performed to provide a better understanding of the heat transfer and phase transition inside and outside the grout columns and to study the capability of the developed grouts to be used in a borehole heat exchanger or as borehole thermal energy storage system. To the best of the authors' knowledge, there have been just a few numerical studies on using phase change materials inside borehole heat exchangers to assess thermal energy storage applications. The experimental and numerical results showed much higher efficiency of the grout developed with a high thermal conductivity than the reference grout in terms of heat transfer in both the grout column and the surrounding sand. Furthermore, the results indicated the noticeable influence of the microencapsulated phase change material's presence in the grout formulation in terms of heat absorption/storage during the phase transition (from solid to liquid). However, it is concluded that reengineering shape stabilized phase change material should be conducted to make it more appropriate for thermal energy storage applications.

시추공식 계간축열 시스템을 이용한 4세대 지역난방 공급 및 효율 개선 연구

The purpose of this study was to compare the conventional Geothermal Source Heat Pump (GSHP) System, with Borehole Thermal Energy Storage (BTES) utilized heat pump system in cost-wise criteria. The consecutive operation of conventional GSHP performance decreased annually, and eventually discharged the purpose of the system. Deploying the BTES system with heat pump, thereby accumulating the low-cost heat energy from the summer season and using it to ease the heating load peak, can manage the presented problem, as well as supply the sufficient and stable amount of heat energy. Additionally the estimated heat pump capacity of conventional GSHP was 7,864 kW and BTES utilized system showed 1,781 kW. The land area occupancy also decreased by 25%. The BTES system can successfully decrease the initial facility investment cost and make efficient use of ground space.

An inversion method to estimate the thermal properties of heterogeneous soil for a large-scale borehole thermal energy storage system

Borehole thermal energy storage (BTES) is a technology in which the thermal energy generated over summer may be collected and stored in soil for extraction in winter. The difficulty of evaluating the thermal conductivity and volumetric heat capacity of heterogeneous soil introduces considerable uncertainty in BTES system design and operation. This study proposes a method to estimate the thermal properties of heterogeneous soil in a BTES system based on the modified duct ground heat storage model (DST) model. The inversion estimation method was verified using long-term monitoring data of soil temperature distribution and fluid temperatures from a large-scale BTES project in Chifeng, China. Results show that the layered thermal properties of heterogeneous soil led to the irregular distribution of soil temperature at different depths. The cumulative heat storage prediction error decreased from 27.8% to 3.7% when utilizing the proposed method relative to a thermal response test (TRT) - based parameter estimation. The influence of inaccurate estimates of soil thermal properties on the scaled expansion of BTES systems was also examined, and with results emphasizing the importance of correctly estimating soil thermal properties for BTES system design.