Borehole Thermal Energy Storage 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.

Optimal control of a heat pump-based energy system for space heating and hot water provision in buildings: Results from a field test

This paper presents and discusses results from a field test that uses model predictive control (MPC) to optimise the operation of a multi-source heat pump-based energy system for space heating and hot water provision in a building. The objective of the optimisation is to minimise the electricity consumption of the heat pump using flexibility from the heat sources, the space heating and domestic hot water tanks while ensuring end-user comfort and system constraints. The field test was performed in the context of the RES4BUILD project. The integrated system consists of photovoltaic-thermal (PVT) collectors, an advanced vapour-compression multi-source heat pump and water buffer tanks for space heating and hot water needs. The heat pump utilises a solar buffer, a borehole thermal energy storage (BTES) and air as heat sources. The solar buffer is supplied with heat from the PVT collectors. The optimal operation of the system is obtained by monitoring and controlling the interactions between the different system components using MPC, taking into account the weather and heat load forecasts. Results have shown that MPC has a potential and added value for the optimal operation of multi-source heat pumps in real-life while considering system constraints and user behaviour to ensure thermal comfort. However, significant effort and expert knowledge are needed to develop the sufficiently accurate system models required by this control approach. The outcomes and conclusions of this work are therefore a basis for further development of such control approaches to improve their replicability and feasibility on a large scale, considering the diverse nature of energy system components in buildings.

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.

Experimental assessment of a solar photovoltaic-thermal system in a livestock farm in Italy

This paper presents an experimental evaluation of the performance of a solar photovoltaic-thermal (PVT) system in a swine farm at Mirandola in Italy. In this project named RES4LIVE, funded by the EU’s Horizon 2020 program, a PVT system is installed to replace fossil fuel consumption in one of the barns on the farm. The electrical energy from the collectors is utilized to operate the heat pump and provide electricity to the barn, whereas the thermal energy from the collector is stored in a borehole thermal energy storage (BTES) for further use by a 35 kW heat pump. The hybrid solar field consists of 24 covered PVT flat plate collectors (7.68 kWel and 25 kWth) with a total aperture area of 39.3 m2, which can increase the temperature of the heat transfer fluid (HTF) to up to 40 °C. The PVT system is connected to a modular solar central (SC) with a standardized design that can also be used for other similar applications. The hybrid solar system complemented by energy storage is expected to save approximately 20,850 kg CO2/year. The data collected from the PVT system, SC, and BTES are rigorously analyzed to evaluate its overall performance. A comprehensive performance assessment reveals the capability of the solar system to reduce carbon emissions and effectively replace fossil fuel consumption in the agricultural sector.

Short-cycle Borehole Thermal Energy Storage: Impact of Thermal Cycle Duration on Overall Performance

Short-cycle Borehole Thermal Energy Storage: Impact of Thermal Cycle Duration on Overall Performance

Thermal analysis of borehole thermal energy storage in unsaturated soil

The thermal performance of soil borehole thermal energy storage (SBTES) systems in unsaturated soils is investigated to address three primary objectives: (1) to explore the impact of subsurface moisture content condition on the SBTES thermal performance, (2) to assess the effect of seasonal surface pressure variation on the SBTES thermal performance, and (3) to examine the influence of soil porosity on the thermal performance of SBTES in homogeneous and layered soils. Results indicate that a higher subsurface moisture content leads to a lower average temperature in the thermal storage domain. Notable hydrodynamic fluctuations are observed in the shallow depth of SBTES domain because of seasonal surface pressure variations although the average temperature and heat storage in SBTES remain mostly unaffected. The results show that a low soil porosity leads to superior SBTES thermal performance, while high porosity soils experienced more pronounced hydrodynamic fluctuations under surface pressure variation conditions. The impact of soil porosity on SBTES thermal performance is minimal in layered soil, except when one soil layer dominates and has a high volume fraction.

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.

Sensitivity analysis of fifth generation district heating and cooling coupled with borehole thermal energy storage with respect to cooling adoption

Fifth generation district heating and cooling (5GDHC) is a promising solution for decarbonising building thermal demands, as it allows for simultaneous heating and cooling through a bi-directional network. This paper aims to investigate the general performance of 5GDHC under uncertainty. Initially, an uncertainty analysis is performed on a 5GDHC model in a case study, revealing significant variation in economic, technical, and environmental performance due to uncertain parameters. Subsequently, variance-based global sensitivity analysis is used to rank the uncertain parameters based on their impact on the economic performance of 5GDHC. The results identify cooling adoption rate, borehole pricing, and discount rate as the most influential parameters. Lastly, factors mapping demonstrates that cooling adoption rate within 0.6-0.9, low borehole costs, low discount rates, and high gas prices will drive the switch to 5GDHC. Overall, this study emphasizes the general applicability of 5GDHC.

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.

Geothermal heat pumps to reduce diesel consumption in an off-grid subarctic community: Comparison of solar assisted systems with optional underground energy storage

Overdependence on fossil fuels for heating and electricity poses significant challenges for remote communities in northern Quebec. This study evaluates the efficiency, heating costs and relevance of solar-assisted geothermal systems in a subarctic climate. Detailed subsurface data collected from Whapmagoostui–Kuujjuarapik were used to simulate heat pumps coupled to borehole heat exchangers, taking into account the possibility of thermal energy storage. Results show that a conventional configuration of borehole heat exchanger effectively meet low heating demands, while borehole thermal energy storage enables the annual balancing of ground loads and meets the high unbalanced heating demands. This work offers valuable insights for optimizing ground heat exchanger configurations in a harsh climate.

Techno-economic analysis of fifth-generation district heating and cooling combined with seasonal borehole thermal energy storage

Fifth-generation district heating and cooling (5GDHC) is a promising solution to decarbonise future thermal energy systems. This paper investigates the potential of 5GDHC integrated with borehole thermal energy storage (BTES) in unlocking the synergies between heating and cooling via a bidirectional thermal network. A model for system design and operation simulation of 5GDHC is developed. In a case study, the proposed 5GDHC supplied by BTES facilitates the long-term balancing of heating and cooling demands despite the poor simultaneity between them. This enables 5GDHC to achieve overall advantages over modern heat pump driven systems, including lower levelized cost of energy, less greenhouse gas emission, higher exergy efficiency, and lower electricity peak load. One possible barrier of adopting 5GDHC is the high upfront cost, which mostly attributes to borehole drilling and booster heat pump installation. Demand scenarios with varying heating and cooling demand structures are analysed and show that the ratio of cooling to heating substantially affects the economic performance of 5GDHC coupled with BTES. When cooling to heating ratio is within the optimal range of 0.4–1.0, 5GDHC reaches lower levelized cost compared with modern heat pump driven systems and present-day gas-fired heating systems.

Backfill thermal properties improvement using granular phase change materials and graphite

Phase change materials (PCMs) are ideal for thermal energy storage due to latent heat release or absorption at target temperatures. Hence, adding PCMs into backfill materials of ground heat exchangers for example, can enhance the borehole thermal energy storage capacity and the corresponding shallow geothermal system performance. However, the heat transfer efficiency between the borehole and surrounding ground might be reduced because of the low thermal conductivity of PCMs. Incorporating additives with high thermal conductivity can reduce the lowering effect of PCMs on the overall thermal conductivity of the mixture while maintaining a desirable thermal capacity. This work mixes encapsulated PCMs (EPCMs), graphite and recycled glass fines, aiming to develop green backfill materials with excellent thermal properties. The heat capacity and thermal conductivity of the mixtures with different fractions of the components are measured in a laboratory. The measured data agree with simulated results. Better thermal properties can be achieved with specific fractions of components, and the findings could help geothermal systems design for better energy utilisation.
 Geothermal energy is one of the promising renewable energy sources and its utilisation can be an alternative to fossil fuels. A geothermal system requires a ground source heat exchanger with pipes installed inside a borehole, and backfill materials filling the gap between the pipe heat exchanger and the ground. Thus, the thermal properties of backfill materials are essential to the geothermal energy extraction process and influence the corresponding shallow geothermal system performance [2]. PCMs are used to improve the thermal storage capacity of backfill materials because they can absorb and release heat at a relatively constant temperature [1]. To avoid problems of leakage and volume change of PCMs during a phase change, encapsulated PCMs (EPCMs) are better suited in backfill materials [5]. However, mixtures including PCMs may hinder heat transfer because of their low thermal conductivity. Therefore, other additives with high thermal conductivity can be used to resist the low heat transfer rate [3]. Even though the thermal properties of backfill materials are fundamental to the geothermal energy extraction process, their dependence on the amount of added PCMs and other additives has not been investigated yet. This work mixes EPCMs, graphite and recycled glass fines, aiming to develop green backfill materials with excellent thermal properties. The heat capacity (), thermal conductivity () and thermal diffusivity () of the mixtures with different fractions of the components are measured in a laboratory.
 SEM images of materials used, recycled glass fines, EPCMs and graphite, are shown in Figure 1(a). Cylindrical containers with a diameter of 44 mm and a height of 57 mm are used to prepare samples of mixtures. Each sample has the controlled volume fractions of components indicated by Figure 1(b) while keeping a constant overall porosity of 0.47. A decagon KD2 Pro thermal analyzer with a dual thermal needle measures the thermal properties of samples kept at 4 °C when EPCMs are in the solid phase and 45 °C when EPCMs are in the liquid phase. The geometrical mean model is used for theoretical estimations of values for comparison against experimental results [4].
 
 The thermal conductivity () of the mixtures increases with increasing graphite volume while decreasing with increasing EPCMs volume, as shown in Figure 1(c). The difference between under solid EPCMs and liquid EPCMs is limited. The heat capacity () rises with increasing EPCMs volume but declines with the increase of graphite volume, and it is higher when EPCMs are liquid than solid. It can be observed that a trade-off between the improvement of and the reduction of exists. Hence, the thermal diffusivity (), which depends on the ratio of to , is also investigated as presented in Figure 1(d). It decreases with the increase of EPCMs volume but grows with increasing graphite volume. Besides, it is higher under solid EPCMs than that under liquid EPCMs. This indicates the backfilling materials have a faster thermal response to the change of surrounding temperature under solid EPCMs.

Thermal and structural response of a pavement solar collector prototype

Thermal and structural response of a pavement solar collector prototype

Assessing the potential of low-transmissivity aquifers for aquifer thermal energy storage systems: a case study in Flanders (Belgium)

The Member States of the European Union pledged to reduce greenhouse gas emissions by 80–95% by 2050. Shallow geothermal systems might substantially contribute by providing heating and cooling in a sustainable way through seasonally storing heat and cold in the shallow ground (<200 m). When the minimum yield associated with the installation of a cost-effective aquifer thermal energy storage (ATES) system cannot be met, borehole thermal energy storage, relying mostly on the thermal conductivity of the ground, is proposed. However, for large-scale applications, this requires the installation of hundreds of boreholes, which entails a large cost and high disturbance of the underground. In such cases, ATES systems can nevertheless become interesting. This paper presents a case study performed on a Ghent University campus (Belgium), where the feasibility of ATES in an area with a low transmissivity was determined. The maximum yield of the aquifer was estimated at 5 m3/h through pumping tests. Although this low yield was attributed to the fine grain size of the aquifer, membrane filtering index tests and long-term injection tests revealed that the clogging risk was limited. A groundwater model was used to optimize the well placement. It was shown that a well arrangement in a checkerboard pattern was most effective to optimize the hydraulic efficiency while maintaining the thermal recovery efficiency of the ATES system. Hence, for large-scale projects, efficient thermal energy storage can also be achieved using a (more cost-effective) ATES system even in low-permeability sediments.

Performance evaluation and control scenarios for targeted heat injection and extraction in an existing geothermal borehole field in Norway

This work presents the calibration, validation, and analysis of a borehole thermal energy storage (BTES) in building performance simulation using operational data from an existing borehole field in Kalnes, Norway. The data used in this study comprises the results of a thermal response test as well as operational data from the borehole field, which is used to cover the heating and cooling load of a hospital complex. The first part of this study focuses on the calibration and validation of the borehole field model using the IDA ICE software. Then, the validated model is used to explore the impact of different operational strategies in which the charging/discharging of the three sections of the borehole field are prioritized in different orders and compared to the most recent operational strategy. This analysis is carried out on a single year and a 20-year perspective to evaluate long-term temperature trends. This investigation aims to evaluate the current and future impacts of the existing operational strategy and whether it could be improved given that the real-life system struggles with below zero brine temperature at the end of the heating season. The simulation results show that the outgoing brine temperature in each borehole section is strongly dependent on the mass flow rate used in the BTES but that the temperature in an individual section had little impact on the neighboring one. When a section was prioritized by the control logic for heat extraction or injection (both in terms of order and mass flow), there was a notable increase or decrease in the outgoing brine temperature, indicating that it was possible to have targeted heat injection/extraction. In the 20-year operational horizon, the simulation results predicted a gradual warming trend of the outgoing brine temperature of approximately 1 °C due to the additional heat injected in all three sections and regardless of the scenario. The study shows that the most recently implemented operational strategy, in which all sections are charged and discharged simultaneously, is the most viable operation scenario for the borehole thermal energy storage at Kalnes, thus confirming previous findings in literature. Since none of the sections had a superior storage or heat regeneration capacity, prioritizing sections would only lead to more significant temperature swings in the ground. On the other hand, the current operation strategy leads to an overall higher temperature in the ground and reduces the risk of low outgoing brine temperatures.

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.

Modeling of an Integrated Renewable-Energy-Based System for Heating, Cooling, and Electricity for Buildings

An integrated numerical model that describes the operation of a renewable-energy-based system for a building’s heating, cooling, and domestic hot water needs is described in this study. The examined energy system includes a vapor compression multi-source heat pump, PVT collectors, borehole thermal energy storage, and water tanks. Energy balance equations for the collectors and the tanks are coupled with correlations for the heat pump and the piping losses within a thermal network approach. The non-linear system of equations that arises is solved by employing in-house software developed in Python v. 3.7.3. The performance of the numerical tool is validated against measurements collected during the pilot operation of such a system installed in Athens (Greece) for two 5-day periods (summer and winter). It is shown that the proposed model can predict, both qualitatively and quantitatively, the building’s energy system performance, whereas limited deviations from the experimental findings are mostly observed when highly transient phenomena occur. The numerical tool is designed with flexibility in mind and can be easily adapted to accommodate additional energy-system configurations and operational modes. Thus, it can be utilized as a supporting decision tool for new energy systems’ designs and the optimization of existing ones.

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.