Underground Thermal Energy Storage Systems Research Articles

Thermal energy storage with tunnels in different subsurface conditions

Thermal energy storage with tunnels in different subsurface conditions

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.

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.

Estimation of In Situ Heat Capacity and Thermal Diffusivity from Undisturbed Ground Temperature Profile Measured in Ground Heat Exchangers

Undisturbed ground temperature (UGT), thermal conductivity (TC) and heat capacity (HC) are essential parameters to design geothermal heat pumps and underground thermal energy storage systems, particularly borehole heat exchangers (BHE). However, field methods to assess the thermal state and properties of the subsurface are costly and time consuming. Moreover, HC is often not evaluated in situ but arbitrarily selected from literature considering the geological materials intercepted by boreholes. This work proposes an original empirical approach to reproduce a UGT profile and estimate in situ thermal diffusivity (TD) and HC in the scope of conventional thermal response tests (TRTs). Empirical equations were developed to reproduce a UGT profile measured along a BHE. Experimental coefficients are found with a non-linear least square solver optimization and used to calculate the damping depth, TD and HC. The suggested heat tracing method was verified and validated against other field methods demonstrating to be fast and reliable. The novelty of this new empirical approach relies on the use of a single temperature profile providing a simple way to better assess subsurface thermal properties.

A thermo-hydraulic numerical model for the initial design of an underground thermal energy storage system in a coastal aquifer

A thermo-hydraulic numerical model for the initial design of an underground thermal energy storage system in a coastal aquifer

Site Selection of Aquifer Thermal Energy Storage Systems in Shallow Groundwater Conditions

: Underground thermal energy storage (UTES) systems are well known applications around the world, due to their relation to heating ventilation and air conditioning (HVAC) applications. There are six kinds of UTES systems, they are tank, pit, aquifer, cavern, tubes, and borehole. Apart from the tank, all other kinds are site condition dependent (hydro-geologically and geologically). The aquifer thermal energy storage (ATES) system is a widespread and desirable system, due to its thermal features and feasibility. In spite of all the advantages which it possesses, it has not been adopted in very shallow groundwater (less than 2 m depth) regions, till now, due to the susceptibility of the storage efficiency of these systems to the in-site parameters. This paper aims to find a reliable method that can be used to find the best location to install ATES systems. The concept of the suggested method is based on integrating three methods. They are, the analytical hierarchy process (AHP), the DRASTIC index method, and ArcMap/GIS software. The results from this method include a criterion that summarizes the best location to install an ATES system. This criterion is depicted by ArcMap/GIS software, producing raster maps that specify the best location for the storage system. The suggested method can be used to find the best location to install the thermal storage, especially in susceptible aquifers.

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

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

Solar district heating with underground thermal energy storage: Pathways to commercial viability in North America

Abstract Underground Thermal Energy Storage (UTES) has emerged in both specific applications and within energy policy literature as a promising technology for meeting thermal loads with locally collected and stored solar energy, as well as several other potential applications, such as time-shifting of grid-based wind and solar power to better align variable generation with loads. In Europe, UTES systems have experienced increased deployment in connection with district heating systems. But despite this academic attention and several demonstration projects, the commercial market viability of UTES systems has yet to be established in North America, and the finance world uses different conceptions of viability than engineering or academic studies. This study explores, through the conventions of finance and risk-mitigation, what capital costs North American UTES systems would need to exhibit to achieve market viability; which is to say, the investment cost at which a UTES system represents an attractive investment when compared with natural gas-based systems for the provision of residential space heating.

Coupled cooling method and application of latent heat thermal energy storage combined with pre-cooling of envelope: Optimization of pre-cooling with intermittent mode

The coupled cooling method combining latent heat thermal energy storage and pre-cooling of the envelope (PE) is a new free-cooling method that is suitable for exposure to high temperatures and other types of harsh environments. PE plays the most critical role in the coupled cooling method. Long-term, continuous PE cannot only reduce energy storage capacity, but it also causes numerous energy waste. Thus, an intermittent operational mode is firstly proposed to improve the heat transfer performance and reduce energy consumption. A simplified numerical model of intermittent thermal storage is established, and the subsequent effects of intermittent ratio (IR) and intermittent period (IP) on cold storage performance have been systematically investigated. Furthermore, the operational period is divided into a cold storage period (CSP) and a cold preservation period (CPP), each with their own respective evaluation indices. Long-term intermittent PE is optimized, and an interchanging continuous/intermittent cold storage strategy is proposed. Under the current operating conditions, as compared with the conventional continuous mode, the duration of CSP is extended by 0–26%, yielding an annual cold storage energy consumption reduction of 68–78%. Thus, the current study demonstrates the significant potential of intermittent operational mode application in underground thermal energy storage systems.

Improving Efficiency of Underground Heat Storage Systems

Underground thermal energy storage (UTES) systems are very popular in Europe and are gaining popularity in North America. The first large-scale solar borehole thermal energy storage (UTES) system in Canada has been implemented in the town of Okotoks (McClenahan D, 2006). The second UTES project in Alberta is located in the community of Southwoods in Edmonton (Kantrowitz T. et al., 2013), which proposes to use natural gas cogenerators to generate electricity and heat during the winter to improve electricity generation efficiency and hence reduce overall energy costs. During the generation of electricity, unused heat can be collected and used to heat space and water for domestic use. During the summer, the electrical output from cogeneration exceeds demand, resulting in a seasonal and annual thermal imbalance which UTES can control through the transfer and storage of the excess heat to a UTES system. Most soils in the Edmonton region consist of sandy clays with high moisture content and high thermo-conductivity. Thus, storing heat in such soils would be inefficient since most of the heat would escape before it could be used for heating purposes. However, if such soils could be modified to become good insulators via innovations in jet grouting and soilcrete technologies, UTES systems could become more prevalent as a renewable energy source in areas with highly thermo-conductive soils. This paper proposes to develop the jet grouting and soilcrete theory and conceptual models required to create an insulated enclosure around UTES systems in highly thermo-conductive soils.

Laboratory scale geophysical measurements aimed at monitoring the thermal affected zone in Underground Thermal Energy Storage (UTES) applications

Underground Thermal Energy Storage systems have showed to be a useful and increasing technical solution for covering heating and cooling and domestic hot water buildings' demand. Thermal influence of these plants is however not still debated as it deserves and correct monitoring strategies appear to be of major importance both to better understand processes and to highlight their environmental effects into high populated areas. Litho-, hydro- and bio-sphere can indeed be adversely affected by temperature variations induced in the underground by heat storage applications. For this purpose, a geophysical monitoring approach using time-lapse electrical resistivity measurements contemporary to analogical simulations is here tested at laboratory scale. Results of the experiments are reported comparing measured apparent resistivity with direct temperature measurements and numerical simulations of heat propagation. Data presented confirmed that electrical resistivity has powerful relation with temperature variation in monitored media. In addition, they showed that also without performing data inversion valid temperature estimation can be carried out. Post-processing calibration of apparent resistivity data showed to be in acceptable agreement with both temperature measurements and numerical simulations. Simple apparent electrical resistivity variations appear therefore to be a promising, economic, quick and non-invasive tool for mapping thermal modifications induced in the underground by shallow geothermal applications.

Biodegradation of cis-1,2-Dichloroethene in Simulated Underground Thermal Energy Storage Systems.

Underground thermal energy storage (UTES) use has showed a sharp rise in numbers in the last decades, with aquifer thermal energy storage (ATES) and borehole thermal energy storage (BTES) most widely used. In many urban areas with contaminated aquifers, there exists a desire for sustainable heating and cooling with UTES and a need for remediation. We investigated the potential synergy between UTES and bioremediation with batch experiments to simulate the effects of changing temperature and liquid exchange that occur in ATES systems, and of only temperature change occurring in BTES systems on cis-DCE reductive dechlorination. Compared to the natural situation (NS) at a constant temperature of 10 °C, both UTES systems with 25/5 °C for warm and cold well performed significantly better in cis-DCE (cis-1,2-dichloroethene) removal. The overall removal efficiency under mimicked ATES and BTES conditions were respectively 13 and 8.6 times higher than in NS. Inoculation with Dehalococcoides revealed that their initial presence is a determining factor for the dechlorination process. Temperature was the dominating factor when Dehalococcoides abundance was sufficient. Stimulated biodegradation was shown to be most effective in the mimicked ATES warm well because of the combined effect of suitable temperature, sustaining biomass growth, and regular cis-DCE supply.

Geothermal waste heat utilization from in situ thermal bitumen recovery operations.

In situ thermal methods for bitumen extraction introduce a tremendous amount of energy into the reservoirs raising ambient temperatures of 13 °C to as high as 200 °C at the steam chamber edge and 50 °C along the reservoir edge. In essence these operations have unintentionally acted as underground thermal energy storage systems which can be recovered after completion of bitumen extraction activities. Groundwater flow and heat transport models of the Cold Lake, Alberta, reservoir, coupled with a borehole heat exchanger (BHE) model, allowed for investigating the use of closed-loop geothermal systems for energy recovery. Three types of BHEs (single U-tube, double U-tube, coaxial) were tested and analyzed by comparing outlet temperatures and corresponding heat extraction rates. Initial one year continuous operation simulations show that the double U-tube configuration had the best performance producing an average temperature difference of 5.7 °C, and an average heat extraction of 41 W/m. Given the top of the reservoir is at a depth of 400 m, polyethylene piping provided for larger extraction gains over more thermally conductive steel piping. Thirty year operation simulations illustrate that allowing 6 month cyclic recovery periods only increases the loop temperature gain by a factor of 1.2 over continuous operation. Due to the wide spacing of existing boreholes and reservoir depth, only a small fraction of the energy is efficiently recovered. Drilling additional boreholes between existing wells would increase energy extraction. In areas with shallower bitumen deposits such as the Athabasca region, i.e. 65 to 115 m deep, BHE efficiencies should be larger.

Experiment and simulation of temperature characteristics of intermittently-controlled ground heat exchanges

Because of poor heat transfer coefficients of soil/rock, ground source heat pumps (GSHP) or underground thermal energy storage (UTES) systems always occupy a large area and need many ground heat exchangers. This initial energy investment is so heavy that it cannot be used on a large-scale. Intermittent operation can reduce the extreme temperatures around the ground heat exchangers (GHEs) and keep the temperature in reasonable range. The aim of this study is to implement an experiment and develop a dynamic model of hydronic heating systems of GSHP in order to get a more fair comparison of energy efficiency between continuously controlled and intermittently controlled systems. Factors such as thermal inertia, temperature levels and lag time are also considered to see how they affect the efficiency. It is shown that temperature variation is related to the intermittent period and that intermittence prolongs the heat transfer without reaching at an utmost temperature (operation limitation). An effectively controlled intermittent process can optimize the capacity of heat exchange units so as to achieve better application of the ground energy. Additionally, the intermittent control can decrease the number of GHEs of GSHP and UTES systems and keep better working conditions.

Combination of ground source heat pumps with chemical dehumidification of air

In the present paper the possible synergies provided by the combination of an underground thermal energy storage (UTES) system with a desiccant based air handling unit (AHU) are analysed. Differently from the conventional solutions, the summer humidity control is obtained here by chemical dehumidification of the ventilation airstream performed by liquid desiccants in a packed column. Being the water temperature of the boreholes heat exchangers generally suitable to meet the sensible load without any integration with the chillers, the plant can operate in a complete free-cooling mode. In winter, the main benefits are due to the higher temperature level at which the UTES works and to the AHU configuration allowing sensible and latent heat recovery. For the same reasons, the required UTES size is sensibly smaller, reducing in this way not only the operation but above all the investment costs. The UTES system competitiveness is then increased. The described solution is investigated by a computer simulation referring to a modern office building in the climate of northern Italy and its performance has been compared to a traditional HVAC plant and to a traditional ground source heat pump (GSHP) system. Finally, some economic evaluations are reported, showing the competitiveness of the proposed configuration.

地中地盤蓄熱の動向 大地は身近な資源であり，エンジニアリングの対象としての有効な蓄熱素材となる

Large amount of thermal energy storage is essential for proceeding with further energy saving and contributing to environmental issues. The long term of weekly, monthly and seasonal thermal energy containment is useful to link between urban thermal energy demands and thermal energy sources-heat exhaust from industries, incinerations, power generations, air-conditioned buildings and other social infrastructures, and various natural heat sources such as seasonal warm and cold temperatures and solar energy. While we have to consider that we have both global limited resources-fossil fuel resources and environmental resources which is indispensable for human life, we have to continue our engineering efforts to establish the social structures of energy circulation by putting an energy source to joint use of industries and urban life through cascade use of thermal energy by each temperature level of thermal demands. Thus total energy consumption will be decreased among industries, social capitals and urban life. Now that huge thermal energy storage systems are not feasible to build on the ground, Underground thermal energy storage system (UTES) is one of the best effective solutions to the above issues, requiring us further engineering development about analysis technology on thermal analysis of underground soil behaviors and heat transfers, underground soil structure survey, underground thermal energy storage systems, low cost bore hole construction technology, underground heat exchanger technology, underground water movement control, more progressed heat pump technology, system integration of effective thermal energy consumption and storage types and peak load shavings, system operation and control technology and environmental evaluations. UTES is able to be combined with application of geo-thermal energy. Research and development efforts are continued to provide the solutions to above engineering subjects.