Underground Thermal Energy Storage Research Articles

Experimental and simulated evaluation of inverse model for shallow underground thermal storage

This study employs an inverse grey-box (IGB) modeling approach, which combines measured data and the physics of systems to predict the performance of shallow underground thermal energy storage (UTES) with top and side insulation. A simplified IGB model of the shallow UTES was developed using thermal network analysis. Experimental studies were conducted for shallow vertical and horizontal UTES configurations. Detailed models representing the field experiments were developed using the TRNSYS simulation tool and calibrated with the measured data from the experimental setup. The 4 Resistance 2 Capacitance (4R2C) IGB model was trained and tested in MATLAB using field experimental data and the data from the calibrated TRNSYS model. In comparing experimental data with the TRNSYS model for UTES, the prediction of outlet water temperature showed good agreement, with root mean square error (RMSE) and coefficient of variation of RMSE (CVRMSE) values of 0.94 °C and 3.16 % for vertical, and 0.99 °C and 2.97 % for horizontal configurations. The IGB model also aligned well with the experimental data, showing CVRMSE of 7.91 % and 3.17 % for vertical and horizontal systems, respectively. Sensitivity analysis revealed that model performance improves with longer training durations and closer testing times to training periods. However, a convergence point of 20 weeks of training data was achieved for making long-term performance predictions.

Development status and prospect of underground thermal energy storage technology

Development status and prospect of underground thermal energy storage technology

Design of a cooling system from underground thermal energy storage (UTES, Underground) Thermal Energy Storage) based on experimental results

Geothermal energy is a renewable and clean source that has been used for electricity generation in some countries since the 50s, the main characteristic to be used in this application is that the subsoil must have a high temperature geothermal resource (+150 °C). However, it can also be used in applications such as air conditioning in places where the temperature is around 30°C; In Europe alone, there are more than one million thermal installations operating by harnessing geothermal energy. The objective of the work was to design a cooling system from the storage of underground energy, for that, it is essential to know the variation of subsoil temperatures during a certain period of time. For this purpose, sensors were used that were installed at different depths and by means of an Arduino, information of a whole year was stored; so that these data are as representative as possible of the energy storage conditions and the changes depending on the seasons that pass. Additionally, the characteristics of the soil (conductivity, humidity and composition) were taken into account, where the equipment is intended to be installed in subsequent works. For the determination of the necessary cooling load, the design requirements of the ASHRAE standard were used and for the design of the underground heat exchanger, references of designs recommended through experimental tests in other research works are included, together with internal fluid methodology and one-dimensional heat transfer. It includes elements that can help improve the dissipation of energy into the subsurface and maintain transfer properties as stable as possible. This design is designed for the air conditioning of a classroom of normal dimensions that are used in the University and therefore avoid the energy consumption of conventional air conditioning equipment.

Seismic expression of paleokarst morphologies and associated Siderolithic infill in the Geneva Basin, Switzerland: Implications for geothermal exploration

The Geneva Basin is witnessing intense geothermal exploration campaigns arising from the Cantonal and Swiss federal energy transition strategies towards an ultimate net-zero future by 2050. However, a major challenge to successful geothermal exploration in the basin are uncertainties regarding the tectonostratigraphic framework of the basin, especially in relation to subsurface morphologies of significance to geothermal prospectivity. This was demonstrated when the newly drilled GEo-02 geothermal exploration borehole surprisingly penetrated a c. 158 m Siderolithic reservoir unit known to infill paleokarst structures in the Molasse Basin. Therefore, we aim to understand the seismic character diagnostic of these paleokarst features and their infill in subsurface geophysical dataset. This was achieved by interpreting industrial 2D seismic reflection and borehole dataset integrated with satellite imagery and outcrop analogues. The seismic dataset reveals these paleokarsts as depressions along the Cretaceous-Paleogene transition (K-Pg) and is characterized by three distinct end-member morphologies: (1) fault-controlled type, (2) doline type associated with dissolution/collapse and (3) complex type, atop the Lower Cretaceous carbonates. However, the karst infill is characterized by different seismic facies exhibiting either a higher or lower seismic reflectivity depending on the karst-end member type. This is related to differences in the acoustic properties of the karst infill, overlying lower molasse and the Lower Cretaceous carbonates hosting the karst. The GEo-02 borehole reveals that the Siderolithic infill consists of a variety of lithologies including quartz-rich sandstone, polygenic breccias and conglomerates, terra rossa and lateritic deposits. The evolution of the paleokarst features in the study area is influenced by the structural re-organization of the Molasse Basin in relation to the Alpine orogeny. This resulted in its emergence and change in base-level promoting the dissolution of the carbonate substratum by meteoritic water subsequently generating the paleokarst morphologies. Considering the regional geothermal gradient, the foredeep of the Geneva Basin that exhibit subsurface manifestations of this Siderolithic reservoirs may present new opportunities for an underground thermal energy storage. However, the presence of karstified intervals may pose geohazard risks to geothermal exploration during drilling operations targeting deeper structures in the basin.

Performance evaluation of a novel nano-enhanced phase change material for thermal energy storage applications

Ground source heat pump systems (GSHP) are one of the sustainable energy resources that provide heating and cooling to buildings. A stable ground temperature has a key role in the performance of the GSHP, in which the overuse (extract/release) of the underground energy can destabilize the ground temperature, leading to the system's failure. A GSHP's performance can be improved by stabilizing the ground temperature, which can be achieved by adding thermal energy storage (TES) to the system. This study aims to present a potential solution to improve the thermal performance of the GSHPs by coupling them to a latent heat TES system. PCMs are widely used for latent heat TES application; however, their poor thermophysical properties are a drawback, so adding nanoparticles has been considered as one of the solutions to address this drawback and improve PCM's properties. This study investigated the thermal performance of a nano-enhanced phase change material (NE-PCM) as an underground TES by developing a finite element numerical model and validation of the experimental apparatus. To this end, PDA@hBN/MXene as a thermal enhancer has been used to develop the NE-PCM. Then, the NE-PCM was utilized in a storage tank with a diameter of 30 cm and a height of 60 cm. The experimental apparatus consists of 8 NE-PCM pipes and four borehole heat exchangers. The experimental temperature of the NE-PCM pipes was used to validate the numerical model for heating and cooling the system within 0.3 °C and 0.2 °C, respectively. Then, the numerical model was used to study the potential of the proposed TES in three scenarios, including no PCM, base PCM, and NE-PCM, with both parallel- and serial-connected heat exchangers. The total heat transfer with NE-PCM was increased by 38.4 % compared to no PCM case and 24.6 % compared to the base PCM. Lastly, the results of three different flow rates showed up to 11.5 % enhancement of the thermal storage efficiency by increasing the flow rates. The results show the potential of using the new NE-PCM in GSHP applications, adding the NE-PCM as the thermal storage medium can improve the performance of the hybrid TES-GSHP system.

Thermal energy storage with tunnels in different subsurface conditions

Thermal energy storage with tunnels in different subsurface conditions

Experimental and numerical investigations on operation characteristics of seasonal borehole underground thermal energy storage

To investigate operation characteristics of seasonal borehole underground thermal energy storage (SBUTES) with different operational strategies, a model test platform with reduced size was established based on similarity principle. The test results show that the larger the start-stop time ratio, the smaller the average heat exchange rate per unit depth (HERPUD) of borehole, and the lower the energy storage efficiency, but the thermal energy storage and extraction capacity increase. For the time ratio of thermal energy storage to extraction (TROSE), the larger the TROSE, the larger the thermal imbalance rate of energy storage body, which will lead to the gradual increase or decrease of temperature of energy storage body and is not conducive to the operation of energy storage. Compared with the asynchronous mode of storage and extraction, the synchronous mode of storage and extraction can effectively reduce the average temperature of energy storage body and increase the heat exchange efficiency of boreholes during the thermal energy storage. The focused energy storage mode can improve the energy storage efficiency and soil temperature recovery rate, balance the thermal energy storage and extraction capacity, and thus the performance of SBUTES can be optimized. A 3-D CFD model of borehole energy storage was established to further find the influences of borehole layout forms, layout spacing and depths on characteristics of the SBUTES. It can be found that for the energy storage efficiency, the hexagonal layout is the highest, the rectangular layout is the lowest, and the circular layout is slightly higher than the square one. Under the same volume of boreholes group, increasing the spacing of inner boreholes can effectively alleviate thermal interference of inner boreholes during the thermal energy storage, and the heat exchange capacity undertaken by inner boreholes can be improved. But it is not conducive to thermal energy extraction. Although increasing borehole depth can increase thermal energy storage and extraction capacity and system operation efficiency, the increase degree is decreasing with the increase of borehole depth, and the greater the borehole depth, the smaller the influence of borehole depth on characteristics of SBUTES.

Performance Investigation of a Ground-Coupled Warehouse Cooling System with an Underground Thermal Energy Storage Tank.

Performance Investigation of a Ground-Coupled Warehouse Cooling System with an Underground Thermal Energy Storage Tank.

Modelling and Optimization of Shallow Underground Thermal Energy Storage

Shallow geothermal reservoirs are excellent candidates for low-enthalpy energy storage, and can serve as heat batteries providing constant discharge of base heat, as well as rapid discharge of heat in periods of high demand. Recharging can be done by pumping down hot water, heated using excess heat from e.g., waste incinerators. In addition to having a very low carbon footprint, such systems also require limited surface infrastructure, and can easily be placed near or under the end-user, such as below residential buildings. The geological setting is typically complex, with horizons, faults, and intertwined patterns of natural fractures, and the nearwell region is often hydraulically fractured to enhance inter-well communication. Therefore, in order to fully utilize the potential of shallow geothermal heat storage, numerical simulations are imperative. In this work, we show how to practically model such systems, including generation of computational grids with a large number of wells and fractures, numerical discretizations with discrete fractures (DFM), and complex storage strategies with multiple wells working together under common group targets. We also discuss how adjoint-based methods can be used to tune model parameters (e.g., well injectivities, rock properties, and hydraulic fracture conductivities) so that the model fits observed data, and to find well controls (e.g., rates and temperatures) that optimize storage operations. The methodology is demonstrated using one artificial and two real underground thermal energy storage projects currently under development, and we highlight important challenges and our suggested solutions related to each of them. Thematic collection: This article is part of the The Earth as a thermal battery: future directions in subsurface thermal energy storage systems collection available at: https://www.lyellcollection.org/topic/collections/thermal-energy

Significance of temperature as a key driver in ZVI PRB applications for PCE degradation

We report on the potential of elevated groundwater temperatures and zero-valent iron permeable reactive barriers (ZVI PRBs), for example, through a combination with underground thermal energy storage (UTES), to achieve enhanced remediation of chlorinated hydrocarbon (CHC) contaminated groundwater. Building on earlier findings concerning deionized solutions, we created a database for mineralized groundwater based on temperature dependence of tetrachloroethylene (PCE) degradation using two popular ZVIs (i.e., Gotthart–Maier cast iron [GM] and ISPAT sponge iron [IS]) in column experiments at 25 °C–70 °C to establish a temperature-dependent ZVI PRB dimensioning approach. Scenario analysis revealed that a heated ZVI PRB system in a moderate temperature range up to 40 °C showed the greatest efficiency, with potential material savings of ~55% to 75%, compared to 10 °C, considering manageability and longevity. With a 25 °C–70 °C temperature increase, rate coefficients of PCE degradation increased from 0.4 ± 0.0 h−1 to 2.9 ± 2.2 h−1 (GM) and 0.1 ± 0.1 h−1 to 1.8 ± 0.0 h−1 (IS), while TCE rate coefficients increased from 0.6 ± 0.1 h−1 to 5.1 ± 3.9 h−1 at GM. Activation energies for PCE degradation yielded 32 kJ mol−1 (GM) and 56 kJ mol−1 (IS). Temperature-dependent anaerobic iron corrosion was key in regulating mineral precipitation and passivation of the iron surface as well as porosity reduction due to gas production.

Specific heat and excess heat capacity of grout with phase change materials using heat conduction microcalorimetry

Microencapsulated phase-change-materials (PCMs) incorporated in cementitious grout can be used as a source of energy in an underground thermal energy storage system. Differential scanning calorimetry (DSC) is a widely used technique to measure the latent heat or specific heat of PCM-embedded cementitious materials. However, using milligram sample sizes (as required by DSC) of a cementitious material fails to represent the actual scale of cementitious components. This is the reason why, in the present paper, non-isothermal heat conduction microcalorimetry (MC) was evaluated as a tool for determining the thermal properties of PCM-embedded grout as well as pure PCM (three PCMs were used). An MC experimental protocol (using both single and 5–6 temperature cycles) was developed and used to measure latent heat and melting and crystallization temperatures, which were in good agreement with those reported for pure PCMs by the producers. In addition, the specific heats of the PCM-containing grout also agreed with measurements using the hot disk technique. Overall, the results show that the MC technique can be used as a potential standard method in determining thermal processes in complex systems, such as in PCM-embedded cementitious systems, where a large sample size is needed to represent the material.

Comparison between Air-Exposed and Underground Thermal Energy Storage for Solar Cooling Applications

Solar energy is one of the main alternatives for the decarbonization of the electricity sector and the reduction of the existing energy deficit in some regions of the world. However, one of its main limitations lies in its storage, since this energy source is intermittent. This paper evaluates the potential of an underground thermal energy storage tank supplied by solar thermal collectors to provide hot water for the activation of a single-effect absorption cooling system. A simulator was developed in TRNSYS 17 software. Experimentally on-site measured data of soil temperature were used in order to increase the accuracy of the simulation. The results show that the underground tank reduces thermal energy losses by 27.6% during the entire hot period compared with the air-exposed tank. The electrical energy savings due to the reduction in pumping time during the entire hot period was 639 kWh, which represents 23.6% of the electrical energy consumption of the solar collector pump. It can be concluded that using an underground thermal energy storage tank is a feasible option in areas with high levels of solar radiation, especially in areas where ambient temperature drops significantly during night hours and/or when access to electrical energy is limited.

Agricultural Greenhouses: Resource Management Technologies and Perspectives for Zero Greenhouse Gas Emissions

Resource management in agriculture is considered a pivotal issue because greenhouse farming and agriculture-related activities generate about 10–29% of all global greenhouse gas emissions. The problem of high greenhouse gas emissions is still unresolved due to the rapid expansion of arable land to meet global food demand. The purpose of this systematic literature review was to generate new perspectives and insights regarding the development of resource management and optimized environments in greenhouses, thereby lowering energy requirements and CO2 emissions. This review sought to answer what technologies and inventions could be used to achieve zero greenhouse gas emissions through efficient energy-saving mechanisms while considering their technical and economic viability. The synthesis of the findings led to several themes which included energy-saving techniques for greenhouses, systems that reduced unfavorable external conditions and renewable energy systems. Other themes identified regarded energy storage systems, systems for managing conditions in greenhouses, carbon capture and storage, and factors influencing the performance of different technologies to enhance resource management and ensure zero carbon emissions. The findings also revealed various technologies used in the design of energy-saving techniques in greenhouses including proportional–integral–derivatives (PID), fuzzy, artificial neural networks, and other intelligent algorithms. Additionally, technologies that were a combination of these algorithms were also examined. The systems that reduced unfavorable external conditions included the use of insulation panels and intelligent shading systems. Greenhouse covers were also optimized by smart glass systems, sensors, Internet of Things (IoT), and Artificial Intelligence (AI) systems. Renewable energy systems included PV (solar) panels, wind turbines, and geothermal electricity. Some of the thermal energy storage systems widely studied in recent research included underground thermal energy storage (UTES) (for seasonal storage), phase-change materials (PCMs), and water tanks, which are used to address short-term shortages and peak loads. The adoption of the various technologies to achieve the above purposes was constrained by the fact that there was no isolated technology that could enable agricultural producers to achieve zero energy, zero emissions, and optimal resource utilization in the short term. Future research studies should establish whether it is economical for large agricultural companies to install smart glass systems and infrastructure for slow fertilizer release and carbon capture in greenhouse structures to offset the carbon footprint.

Numerical analysis of borehole heat exchanger using a single shape-stabilized phase change material in heating and cooling seasons

Borehole heat exchanger (BHE) plays a key role in ground source heat pump (GHSP) and underground thermal energy storage for heating and cooling, which can backfill with phase change material (PCM) to enhance its thermal performance. In this study, a novel PCM-grouted BHE which can run in both heating and cooling seasons by using a single shape-stabilized PCM (SSPCM) is presented and investigated, and the phase change hysteresis (PCH) of SSPCM is detected and applied to achieve the all-seasons run. A heat transfer model considering PCH for U-type BHE was established to evaluate the thermal performance of the novel BHE. And four kinds of backfill materials were concerned and compared to investigate the effect of initial temperature, and pipe spacing on the performance of the BHE and its characteristics of intermittent operation. The results show that octadecane SSPCM with PCH showed the best comprehensive performance in the two seasons with an energy storage rate of 68.9 W/m in heating and 75.9 W/m in cooling, presenting 21.4 % higher than BHE with conventional backfill material. Mean COP during cooling can be improved by 3.61 % when the thermal conductivity increases from 0.5 W/(m·K) to 2 W/(m·K). It also reveals that the selection of PCM for different regions depends on the ground initial temperature.

Predicting the risk of saltwater contamination of freshwater aquifers during aquifer thermal energy storage

Aquifer thermal energy storage (ATES) is an underground thermal energy storage technology with a large potential to decarbonise the heating and cooling of buildings. ATES installations typically store thermal energy in aquifers that are also exploited for potable water, so a major consideration during development is ensuring that system operation will not lead to groundwater pollution. In this study, the risk of contamination due to upconing of a shallow freshwater/saltwater interface during ATES operation is investigated. Fluid flow, and heat and salt (chloride ion) transport are simulated in a homogeneous aquifer during ATES operation via a well doublet. The impact of geological, hydrological and operational parameters is investigated in a sensitivity analysis. Two new dimensionless numbers are proposed to characterise salt upconing and redistribution during ATES operation and provide a close match to simulated concentrations: CR,w characterises the contamination risk at the ATES installation, and CR,d characterises the risk at locations downstream of the ATES installation with respect to background groundwater flow. ATES systems with CR,w and CR,d < 10 introduce low risk of contamination in a homogenous aquifer, with chloride concentration at, and downstream of, the ATES system, remaining below the World Health Organisation’s advised limit. ATES installations with CR,w and CR,d > 10 cause a rapid increase in aquifer chloride concentration. The results are used to estimate an exclusion distance beyond which ATES system operation will not cause contamination in a homogenous aquifer. The dimensionless parameters proposed allow rapid assessment of the potential for saltwater contamination during ATES operation.

Investigating the Influence of Groundwater Flow and Charge Cycle Duration on Deep Borehole Heat Exchangers for Heat Extraction and Borehole Thermal Energy Storage

Decarbonisation of heat is essential to meeting net zero carbon targets; however, fluctuating renewable resources, such as wind or solar, may not meet peak periods of demand. Therefore, methods of underground thermal energy storage can aid in storing heat in low demand periods to be exploited when required. Borehole thermal energy storage (BTES) is an important technology in storing surplus heat and the efficiency of such systems can be strongly influenced by groundwater flow. In this paper, the effect of groundwater flow on a single deep borehole heat exchanger (DBHEs) was modelled using OpenGeoSys (OGS) software to test the impact of varying regional Darcy velocities on the performance of heat extraction and BTES. It is anticipated that infrastructure such as ex-geothermal exploration or oil and gas development wells approaching the end of life could be repurposed. These systems may encounter fluid flow in the subsurface and the impact of this on single well deep BTES has not previously been investigated. Higher groundwater velocities can increase the performance of a DBHE operating to extract heat only for a heating season of 6 months. This is due to the reduced cooling of rocks in proximity to the DBHE as groundwater flow replenishes heat which has been removed from the rock volume around the borehole (this can also be equivalently thought of as “coolth” being transported away from the DBHE in a thermal plume). When testing varying Darcy velocities with other parameters for a DBHE of 920 m length in rock of thermal conductivity 2.55 W/(m·K), it was observed that rocks with larger Darcy velocity (1e-6 m/s) can increase the thermal output by up to 28 kW in comparison to when there is no groundwater flow. In contrast, groundwater flow inhibits single well deep BTES as it depletes the thermal store, reducing storage efficiency by up to 13% in comparison to models with no advective heat transfer in the subsurface. The highest Darcy velocity of 1e-6 m/s was shown to most influence heat extraction and BTES; however, the likelihood of this occurring regionally, and at depth of around or over 1 km is unlikely. This study also tested varying temporal resolutions of charge and cyclicity. Shorter charge periods allow a greater recovery of heat (c. 34% heat injected recovered for 1 month charge, as opposed to &lt;17% for 6 months charge).

Efficient energy storage in residential buildings integrated with RESHeat system

The Renewable Energy System for Residential Building Heating and Electricity Production (RESHeat) system has been realized for heating and cooling residential buildings. The main components of the RESHeat system are a heat pump, photovoltaic modules, sun-tracking solar collectors and photovoltaic/thermal modules, an underground thermal energy storage unit, and a ground heat exchanger. One of the main novelties of the RESHeat system is efficient ground regeneration due to the underground energy storage unit. During a heating season, a large amount of heat is taken from the ground. The underground energy storage unit allows the restoration of ground heating capability and the heat pump’s coefficient of performance (COP) to be kept high as possible for consecutive years. The paper presents an energy analysis for a residential building that is a RESHeat system demo site, along with integrating the RESHeat system with the building. The experimentally validated components coupled with the building model to achieve the system performance in TRNSYS software. The results show that the yearly average COP of the heat pump is 4.85 due to the underground energy storage unit. The RESHeat system is able to fully cover the heating demand of the building using renewable energy sources and an efficient underground energy storage system.

Simulation and modeling of a solar-aided underground energy storage system

The significance of energy storage methods and related R and D studies are increasing due to the depletion of fossil fuels, rising energy prices, and growing environmental concerns. Storage of energy means elimination of practical concerns for the time difference between the time when the energy is produced and when it?s needed. The importance of producing and storing energy through renewable sources is increasing every day, especially in developing countries like Turkiye, as such countries would like to reduce their dependence on foreign sources. This study focuses on an underground thermal energy storage system that was modeled for Van Region, using M-file program. The performance of an isolated day heat system as a thermal energy storage was investigated, and the thermal energy storage capacity of the system was researched for a 5 m ? 5 m ? 5 m soil area located on the Van Yuzuncu Yil University Campus. The temperature distribution, heat loss, and efficiency calculations were performed for a complete year and 3-D representations of the findings were obtained. The lowest efficiencies were observed in May, while the highest efficiencies were observed in July. It was found that the maximum heat loss from the system took place during December and January, and the system could be easily and effectively become a heating source for a single household with the addition of a heat pump.

Modelling the Energy Production of a Borehole Thermal Energy Storage (BTES) System

Geopolitical developments since February 2022 and the numerous debates on climate change such as the COP27 are pushing for a greater acceleration in decarbonising the energy sector. The use of geothermal energy for thermal energy production and storage in district heating and cooling (DHC) grids may also be a key element in overcoming short-term energy peaks. This work aimed at evaluating the efficiency and performance of one of the most promising underground thermal energy storage systems, which uses boreholes to store heat or cold (BTES). Numerical simulations allowed for understanding how these technologies can be used as backup systems, or when the energy demand overcomes that supplied by conventional heating systems. The knowledge on how to exploit this energy source shows that a continuous heat extraction from the storage volume can meet both the base and peak load requests for several users, with cumulative energy amounting to 476,000 kWh over the first month. This study proved how the integration of these technologies in DHC contexts can contribute to greater energy and economic savings, becoming an efficient and flexible solution to meet the energy demand from the grid, and also as a backup system.

Dynamic exergy and economic assessment of the implementation of seasonal underground thermal energy storage in existing solar district heating

In this paper, an exergoeconomic approach is proposed to assess the impact of the integration of a Borehole Thermal Energy Storage (BTES) in the existing district heating of the city of Chateaubriant in France. To do so, a dynamic simulation is run over ten years of operation. The model includes a new approach for cost calculation of short-term thermal storage, which is extended to seasonal BTES. The analysis of the system operation at yearly and daily scales revealed several issues. Hence, without redesigning the existing installation, it was found that a non-comprehensive control strategy results in an increase of fossil fuel share by 1.2%. At the BTES scale, from an exergoeconomic point of view, the charging stage is found particularly sensitive to the dynamic. For example, charging the BTES at low temperature increases of the specific cost of the heat stored. Hence, it is shown that some exergoeconomic indicators, like the specific cost of stored heat, could be relevant for designing and operating heating systems with borehole thermal energy storage.