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Improving the Efficiency of District Heating and Cooling Using a Geothermal Technology: Underground Thermal Energy Storage (UTES)
Abstract
For efficient operation of heating and cooling grids, underground thermal energy storage (UTES) can be a key element. This is due to its ability to seasonally store heat or cold addressing the large mismatch between supply and demand. This technology is already available and there are many operational examples, both within and outside a district heating network. Given the range of available UTES technologies, they are feasible to install almost everywhere. Compared to other storage systems, UTES have the advantage of being able to manage large quantities and fluxes of heat without occupying much surface area, although the storage characteristics are always site specific and depend on the geological and geothermal characteristics of the subsoil. UTES can manage fluctuating production from renewable energy sources, both in the short and long term, and fluctuating demand. It can be used as an instrument to exploit heat available from various sources, e.g., solar, waste heat from industry, geothermal, within the same district heating system. The optimization of energy production, the reduction in consumption of primary energy and the reduction in emission of greenhouse gases are guaranteed with UTES, especially when coupled with district heating and cooling networks.
... Storage is currently the only way to use volatile renewables such as solar thermal, making use of waste heat to enhance the overall energy efficiency of the heating and cooling market, which is one of the EU's goals. The utilisation of UTES, as a knot for coupling and integration of decentralised renewable energy heat sources, contributes to the overall efficiency, flexibility, and response time of a DHC system; UTES can also be used to cover the peak demand load as well as to provide a backup supply [10]. The coupling of local renewable energy subsystems and installations to onsite thermal energy storage reduces the heat consumption of the system, without any physical connection to the main DHC network and enabling the share of renewable energies to grow massively [11]. ...
... The UTES technologies are divided in four main typologies, with the common feature of storing the surplus heat and cold (usually in large quantities and over several seasons or years). UTES also represents the best storage solution for temperature levels between less than 10 °C and up to 90 °C [7], using different kinds of storage such as subsurface (PTES), aquifers (ATES), borehole heat exchangers (BTES), abandoned coal mines (MTES) or rock caverns (CTES), as described in detail in [10]. Among these, BTES systems are becoming very popular because of their suitability for seasonal storage of thermal energy, thanks to their slow thermal response and large storage capacities [12]. ...
... To this regard, when evaluating the capital expenditure of these technologies (CAPEX), factors to be considered are drilling, storage piping, and all the other components which the system needs to operate as well as its storage efficiency. As highlighted in [10], in district heating and cooling (DHC) grids peak load systems are typically designed to have a low CAPEX with a higher operational expenditure (OPEX) as they are designed to run for only a limited time. To this regard, UTES technologies can reduce the amount of time peak load systems operate by using low-OPEX energy (e.g., from geothermal), and thus can be used to fill peak demand loads as well as to provide backup supply. ...
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.
Since 2019, the European COST funding agency supports the Action CA18219 Geothermal-DHC on the implementation of geothermal energy into heating and cooling networks across Europe. Geothermal-DHC represents a pan European research network participated by more than 30 countries also including non-European countries. Geothermal-DHC aims at investigating both, technological as well as non-technological concepts for a better integration of geothermal energy into heating and cooling networks by covering the whole spectrum of geothermal technologies from ambient (shallow) geothermal towards deep-and unconventional geothermal (e.g. CCUS-Geothermal). The scientific focus of the network addresses the match between the different geothermal concepts and the requirements of heating (and cooling) grids from generation 2 until generation 5. In this context, Geothermal-DHC not just focuses on new grids but also considers existing heating grids, which require a transformation from fossil fuels to renewable energy sources in the upcoming 10 to 30 years in order to fulfill the EU energy and climate goals. Geothermal-DHC wants to demonstrate that geothermal energy has the capacity to raise the renewable energy share including geothermal energy in heating in cooling grids from currently less than 20% to 30% in 2030 and 50% in 2050.
We report on the design of a modular, high-temperature thermochemical energy storage system based on endothermic-exothermic reversible gas-solid reactions for application in concentrated solar power and industrial thermal processes. It consists of an array of tubular reactors, each containing an annular packed bed subjected to radial flow, and integrated in series with a thermocline-based sensible thermal energy storage. The calcination-carbonation of limestone, CaCO 3 ↔ CaO + CO 2 , is selected as the reversible thermochemical reaction for the experimental demonstration. Synthetized 4.2 mm-mean size agglomerates and 2 mm-mean size granules of CaO with 42 %wt sintering-inhibitor MgO support attained reaction extents of up to 84.0% for agglomerates and 31.9% for granules, and good cycling stability in pressure-swing and temperature-swing thermogravimetric runs. A lab-scale reactor prototype is fabricated and tested with both formulations for 80 consecutive carbonation-calcination cycles at ambient pressure using a temperature-swing mode between 830°C and 930°C. The reactor exhibited stable cyclic operation and low pressure drop, and yielded specific gravimetric and volumetric heat storage capacities of 866 kJ/kg and 322 MJ/m 3 for agglomerates, respectively, and 450 kJ/kg and 134 MJ/m 3 for granules, respectively.
Aquifer thermal energy storage (ATES) systems with groundwater heat pumps (GWHP) provide a promising and effective technology to match the renewable energy supply and demand between seasons. This paper analyses the integration of an ATES and GWHP system in both district heating (DH) and district cooling (DC) networks in terms of system's efficiency, techno-economic feasibility and impact on the surrounding groundwater areas. To that end, a novel method of holistic integration of groundwater modeling is proposed and demonstrated for retrieving and analyzing data from a variety of open Finnish public data sources. A case study is presented, where the ATES integration is examined within an existing district heating network in Southern Finland. It is concluded that combining heating and cooling, with seasonally reversible ATES operation and balanced pumping volumes during summer and winter periods, had low impact on the aquifer area and is economically feasible. Finally, the study concludes that even with limited data, obtained from open public data sources, it is possible to assess the ATES integration with an acceptable accuracy.
To generate performance predictions of borehole thermal energy storage (BTES) systems for both seasonal and short-term storage of industrial excess heat, e.g., from high to low production hours, models are needed that can handle the short-term effects. In this study, the first and largest industrial BTES in Sweden, applying intermittent heat injection and extraction down to half-day intervals, was modelled in the IDA ICE 4.8 environment and compared to three years of measured storage performance. The model was then used in a parametric study to investigate the change in performance of the storage from e.g., borehole spacing and storage supply flow characteristics at heat injection. For the three-year comparison, predicted and measured values for total injected and extracted energy differed by less than 1% and 3%, respectively and the mean relative difference for the storage temperatures was 4%, showing that the performance of large-scale BTES with intermittent heat injection and extraction can be predicted with high accuracy. At the actual temperature of the supply flow during heat injection, 40 °C, heat extraction would not exceed approximately 100 MWh/year for any investigated borehole spacing, 1–8 m. However, when the temperature of the supply flow was increased to 60–80 °C, 1400–3100 MWh/year, also dependent on the flow rate, could be extracted at the spacing yielding the highest heat extraction, which in all cases was 3–4 m.
Large-scale seasonal thermal energy storage (TES) can help maximize renewable energy integration into district heating and cooling (DHC) systems. However, expertise and concrete projects in the field is limited and there is currently a lack of reliable and adequate analysis tools and cost data to assess the technical-economic potential of aquifer thermal energy storage (ATES) or pit thermal energy storage (PTES). This paper provides a review summary for current ATES and PTES systems, including design concepts, application criteria, specific cost comparisons of various storage systems, as well as a summary of applicable modelling and design tools currently available.
Buildings consume approximately 3/4 of the total electricity generated in the United States, contributing significantly to fossil fuel emissions. Sustainable and renewable energy production can reduce fossil fuel use, but necessitates storage for energy reliability in order to compensate for the intermittency of renewable energy generation. Energy storage is critical for success in developing a sustainable energy grid because it facilitates higher renewable energy penetration by mitigating the gap between energy generation and demand. This review analyzes recent case studies - numerical and field experiments - seen by borehole thermal energy storage (BTES) in space heating and domestic hot water capacities, coupled with solar thermal energy. System design, model development, and working principle(s) are the primary focus of this analysis. A synopsis of the current efforts to effectively model BTES is presented as well. The literature review reveals that: (1) energy storage is most effective when diurnal and seasonal storage are used in conjunction; (2) no established link exists between BTES computational fluid dynamics (CFD) models integrated with whole building energy analysis tools, rather than parameter-fit component models; (3) BTES has less geographical limitations than Aquifer Thermal Energy Storage (ATES) and lower installation cost scale than hot water tanks and (4) BTES is more often used for heating than for cooling applications.
In order to respond to climatic change, many efforts have been made to reduce harmful gas emissions. According to energy policies, an important goal is the implementation of renewable energy sources, as well as electrical and oil combustion savings through energy conservation. This paper focuses on an extensive review of the technologies developed, so far, for central solar heating systems employing seasonal sensible water storage in artificial large scale basins. Among technologies developed since the late 1970s, the use of underground spaces as an energy storage medium – Underground Thermal Energy Storage (UTES) – has been investigated and closely observed in experimental plants in many countries, most of them, as part of government programmes. These projects attempt to optimise technical and economic aspects within an international knowledge exchange; as a result, UTES is becoming a reliable option to save energy through energy conservation. Other alternatives to UTES include large water tanks and gravel–water pits, also called man-made or artificial aquifers. This implies developing this technology by construction and leaving natural aquifers untouched. The present article reviews most studies and results obtained in this particular area to show the technical and economical feasibility for each system and specifics problems occurred during construction and operation. Advantages and disadvantages are pointed out to compare both alternatives. The projects discussed have been carried out mainly in European states with some references to other countries.
A method for seasonal storage of heat or cold in the bedrock (the HYDROCK concept) is presented and its thermal performance discussed. It involves the use of a fractured bedrock at shallow depths (ca. 50–250 m), where existing fractures are stimulated or new fractures artificially created and used as flow-paths for a heat/cold carrier (usually water). The fracture surfaces are used as heat exchangers and the bedrock is loaded and unloaded to suit the energy needs. Propants are injected into the fractures to keep them open and reduce the energy needed for pumping water through the system. Field tests confirm that stacked parallel fractures can be produced by hydraulic fracturing. The thermal performance of the store is modelled and compared with a ducted ground heat store. It is shown that the HYDROCK store may yield 10–20% more energy during extraction than a ducted ground heat store for similar amounts of injected energy. This indicates that the HYDROCK concept is competitive as a seasonal energy store and may be of particular importance as an alternative energy source where existing methods cannot be economically justified.
In the current energy transition, there is a growing global market for innovative ways to generate clean energy. Storage technologies are potential and flexible solutions to deal with the intermittent nature of renewable resources. Closed mines can be used for the implementation of plants of energy generation with low environmental impact. This paper explores the use of abandoned mines for Underground Pumped Hydroelectric Energy Storage (UPHES), Compressed Air Energy Storage (CAES) plants and geothermal applications. A case study is presented in which the three uses are combined in just one mine. This preliminary study allows estimating an electrical energy generation of 153 and 197 GWh year-1 at the UPHES and CAES systems, respectively, and a thermal energy generation of 0.41 GWh year-1 at the geothermal system, with a total cost of 358 M€. An underground closed mine can be used to store energy for re-use and also for geothermal energy generation, providing competitive renewable energy with a low CO2 footprint. These initiatives aid to ensure sustainable economic development of communities after mine closure.
To meet the global climate change mitigation targets, more attention has to be paid to the decarbonization of the heating and cooling sector. Aquifer Thermal Energy Storage (ATES) is considered to bridge the gap between periods of highest energy demand and highest energy supply. The objective of this study therefore is to review the global application status of ATES underpinned by operational statistics from existing projects. ATES is particularly suited to provide heating and cooling for large-scale applications such as public and commercial buildings, district heating, or industrial purposes. Compared to conventional technologies, ATES systems achieve energy savings between 40% and 70% and CO2 savings of up to several thousand tons per year. Capital costs decline with increasing installed capacity, averaging 0.2 Mio. € for small systems and 2 Mio. € for large applications. The typical payback time is 2–10 years. Worldwide, there are currently more than 2800 ATES systems in operation, abstracting more than 2.5 TWh of heating and cooling per year. 99% are low-temperature systems (LT-ATES) with storage temperatures of < 25 °C. 85% of all systems are located in the Netherlands, and a further 10% are found in Sweden, Denmark, and Belgium. However, there is an increasing interest in ATES technology in several countries such as Great Britain, Germany, Japan, Turkey, and China. The great discrepancy in global ATES development is attributed to several market barriers that impede market penetration. Such barriers are of socio-economic and legislative nature.
The use of thermal energy storage (TES) in buildings in combination with space heating and/or space
cooling has recently received much attention. A variety of TES techniques have developed over the past
decades. TES systems can provide short-term storage for peak-load shaving as well as long-term (seasonal)
storage for the introduction of natural and renewable energy sources. TES systems for heating or cooling
are utilized in applications where there is a time mismatch between the demand and the most economically
favorable supply of energy. The selection of a TES system mainly depends on the storage period required,
economic viability, and operating conditions. One of the main issues impeding the utilization of the
full potential of natural and renewable energy sources, e.g., solar and geothermal, for space heating
and space cooling applications is the development of economically competitive and reliable means for
seasonal storage of thermal energy. This is particularly true at locations where seasonal variations of
solar radiation are significant and/or in climates where seasonally varying space heating and cooling
loads dominate energy consumption. This article conducts a literature review of different seasonal thermal
energy storage concepts in the ground. The aim is to provide the basis for development of new intelligent
TES possibilities in buildings.
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