Power-to-Heat and Seasonal Thermal Energy Storage: Pathways Toward a Low-Carbon Future for District Heating

Abstract

Power-to-Heat and Seasonal Thermal Energy Storage are emerging technologies that facilitate the integration of variable renewable energy sources into building and district energy systems. This review synthesizes recent advancements in technologies, integration strategies, and case studies, with a particular focus on nearly zero-energy buildings and nearly zero-energy districts. A structured literature survey, prioritizing sources from 2020 to 2025, was conducted to map available options. The analysis includes Power-to-Heat systems, primarily electric boilers and heat pumps, as well as various seasonal thermal energy storage configurations, including Aquifer Thermal Energy Storage, Borehole Thermal Energy Storage, Pit Thermal Energy Storage, Tank Thermal Energy Storage, and Packed Bed Thermal Energy Storage. The findings indicate that coupling renewable energy with Power-to-Heat and seasonal thermal energy storage can significantly enhance the flexibility of buildings and district systems, reducing the curtailment of renewable sources by utilizing surplus electricity from renewable generation, particularly during periods of low demand, and lowering the environmental impact of buildings and district heating networks.

1. Introduction

The urgent need to address climate change, the exponential growth in global energy demand, and the depletion of fossil fuel resources highlight the importance of developing and implementing sustainable energy solutions. Meeting the future energy needs of a growing global population requires a comprehensive shift toward environmentally friendly energy systems. Although significant progress has been made in deploying renewable electricity and heat generation technologies, further developing energy storage technologies is essential for transitioning to low-carbon systems. In this context, Power-to-Heat (P2H) and Seasonal Thermal Energy Storage (STES) can integrate variable renewable energy (VRE) sources, such as wind and solar, into traditional heating systems (both individual and district) that rely heavily on fossil fuels. The integration of P2H, STES, and VRE is particularly important in the household sector, as it accounts for a significant share of final energy consumption and greenhouse gas (GHG) emissions. In 2023, the residential sector accounted for 26.2% of the EU’s final energy consumption. Households primarily used energy for space heating and hot water preparation, accounting for 62.5% and 15.1% of the residential sector’s final energy consumption, respectively (see Figure 1a). Most of the energy consumption was covered by natural gas (29.5%) and electricity (25.9%). Renewable energy sources accounted for 23.5% of the sector’s energy consumption, followed by petroleum products and derived heat (10.3% and 8.5%, respectively). Finally, a small proportion was covered by solid fossil fuels at 2.2% (see Figure 1b) [1]. The scope and stringency of minimum performance standards and building energy codes are increasing across countries, and the use of efficient solutions and renewable building technologies is accelerating. However, the sector requires more rapid changes to align with the Net Zero Emissions by 2050 (NZE) scenario, which is a normative framework that outlines a pathway for the global energy sector to achieve net-zero CO₂ emissions by 2050 [2].

Figure 1. Final energy consumption in the residential sector by the usage (a) and fuel (b).

Nearly zero-energy buildings (NZEBs) and nearly zero-carbon buildings (NZCBs) provide promising options to reduce the environmental impact of the building sector. Building optimization is a promising technique for evaluating the design choices of NZEBs and NZCBs. It has been adopted for selecting the optimal solution for achieving zero energy performance by optimizing an objective function related to energy (e.g., thermal loads, renewable energy generation, and energy savings), environment (e.g., CO₂ emissions), and/or economy (e.g., life-cycle cost, net present value, and investment cost) [3]. As a result, both NZEBs and NZCBs use energy-efficient technologies (HVAC, lighting, and appliances) and utilize renewable energy sources (RES). Furthermore, NZCBs are achieved when the embodied carbon emissions from producing and transporting construction materials are offset [4]. Figure 2 illustrates the primary steps necessary to achieve the NZEB standard.

Figure 2. The most general steps required to achieve the NZEB standard.

NZEBs can be considered high-quality if they employ passive strategies and energy-efficient technologies to the maximum extent and utilize available renewable energy resources within the building footprint. To further decarbonize urban environments, the concept of NZEB is evolving into nearly zero-energy districts (NZEDs). This shift acknowledges that maximizing energy efficiency and reducing carbon emissions at the building level is insufficient to achieve broader climate and sustainability objectives. NZEDs aim to leverage synergies between buildings, infrastructure, and energy systems to achieve greater overall efficiency and resilience by scaling up from individual buildings to entire neighborhoods or districts. This shift involves optimizing building design and performance, as well as integrating decentralized renewable energy sources alongside district heating and cooling networks, energy storage, and smart grid technologies. Furthermore, NZEDs support the development of flexible, interactive energy systems that balance supply and demand in real time, reduce peak loads, and integrate electric vehicles and other emerging technologies. The move towards NZEDs represents a holistic approach to urban planning and energy management that contributes significantly to climate neutrality, energy security, and an improved quality of life in cities.

Energy generation in NZEDs is typically based on solar photovoltaics, solar thermal systems, shallow and deep geothermal energy, biomass, and small-scale wind turbines. These technologies are often embedded in low-temperature district heating and cooling networks, which facilitate efficient energy distribution and integration of multiple heat sources [5,6]. Energy demand reduction is achieved through passive building design, highly insulated building envelopes, efficient HVAC systems, and demand-side management strategies supported by digital energy platforms [7,8]. To overcome the temporal mismatch between renewable energy supply and heating/cooling demand, NZEDs employ both short- and long-term energy storage. Battery systems often provide short-term balancing, while STES plays a key role in ensuring the flexibility and resilience of heat supply [9]. Furthermore, sector coupling strategies such as Power-to-Gas (P2G), Power-to-Heat (P2H), and Power-to-Hydrogen (P2H2) are increasingly integrated, enabling the use of surplus renewable electricity for thermal or chemical storage and thereby supporting grid stability and decarbonization of multiple sectors. Finally, smart grids and demand-response mechanisms facilitate energy sharing and enhance district-level self-sufficiency, while also supporting grid integration at the urban scale [10].

Considering the potential for coupling various generation and storage technologies, the high potential for connecting VRE (including photovoltaic panels and wind turbines) with P2H is evident. P2H technologies are solutions that convert electrical energy into thermal energy for use in buildings and industries. Due to their ability to integrate VRE sources into the power system, these technologies are considered key enablers of the low-carbon energy transition. P2H systems utilize surplus electricity from renewable generation, particularly during periods of low demand, to reduce fossil fuel consumption and enhance overall energy system flexibility. Technological maturity, cost-effectiveness, and scalability make them especially suitable for integration with low-temperature district heating and cooling networks, which are increasingly applied in NZED concepts. Moreover, P2H contributes to grid stability by absorbing excess renewable electricity during periods of high generation, while simultaneously reducing reliance on fossil-based heat production [11,12]. The general idea of integrating VRE, STES, and P2H solutions into the district network is shown in Figure 3.

Figure 3. The concept of integrating VRE, STES, and P2H solutions into the district network (adopted from Ref. [11]).

Summarizing this part, as global decarbonization efforts intensify, VRE sources are becoming increasingly central to electricity generation. However, the inherent variability of their output, driven by weather conditions and seasonal variations, poses significant challenges to grid stability, energy security, and the efficient use of surplus electricity. This variability creates an urgent need for flexible, demand-side solutions and long-duration energy storage technologies that can absorb, store, and redistribute energy over different time scales. In this context, integrating P2H and STES technologies is crucial for efficiently and cleanly utilizing energy generated by RES for residential and district heating.

Despite the growing body of literature on individual components such as P2H systems and TES, comprehensive analyses addressing their combined application—particularly in connection with VRE sources—remain limited. Most existing studies focus either on technology-specific advancements or on isolated case studies, without systematically examining how integrating P2H with STES can enhance flexibility, decarbonization, and system-level resilience across different spatial scales.

To address this gap, the present review synthesizes recent progress in P2H and STES technologies, with an emphasis on integration strategies, operational synergies, and practical applications in NZEBs and NZEDs. The review consolidates findings from both numerical modeling and experimental research to identify emerging trends, unresolved challenges, and pathways toward large-scale implementation. It establishes a comprehensive framework that links renewable electricity generation, thermal storage, and sector coupling as key enablers of a low-carbon, flexible, and resilient heating sector. The review specifically focuses on

• Examining the technical principles and current state of the art of existing P2H and TES technologies, including innovative approaches to integrating VRE systems (photovoltaics and wind energy) with STES;
• Analyzing the strategic role of these technologies in decarbonizing residential and district heating systems through enhanced flexibility and renewable integration;
• Identifying and discussing numerical and experimental studies that propose and validate novel configurations tailored to NZEBs and NZEDs.

2. Methodology

To ensure the credibility and clarity of the present paper, the review followed a systematic and structured approach comprising three main stages: (i) literature search and retrieval, (ii) content analysis, and (iii) synthesis and discussion of domain knowledge. The collected data were then systematically compared to identify key trends, research gaps, and synergies among the reviewed technologies.

2.1. Criteria Applied to Ensure the Credibility and Clarity of the Review

The literature was retrieved from leading academic databases—Scopus, Web of Science, ScienceDirect, IEEE Xplore, and EBSCO—and supplemented with information from carefully selected, topic-relevant websites. The following criteria were applied to refine the results:

• preference was given to peer-reviewed journal articles and conference proceedings—152 (98.7% of all references);
• articles published in the last 10 years were primarily considered for a total of 147, including 110 articles published in the period of 2020–2025 and 37 articles published in the period of 2015–2019;
• articles from leading editorial sources were mainly cited, including Elsevier, MDPI, Springer Nature, Taylor&Francis, Wiley, IEEE, Frontiers, ASCEE, and IOPscience.

2.2. The Main Stages of the Literature Review Process

The review spans a broad time frame, with emphasis on recent publications to capture the state of the art. Carefully selected earlier studies are included for essential context. The literature review process involved several stages:

• titles, keywords, and abstracts were first reviewed to evaluate their relevance to the topic under consideration;
• full-text articles were assessed to identify relevant scientific and technical contributions, methodologies, and findings;
• the key findings were thematically organized and allocated to the appropriate sections.

This structured approach ensured a comprehensive and unbiased overview of the current state of the art and provided a robust basis for the comparative synthesis presented in the subsequent sections.

2.3. Keyword Selection

To conduct a comprehensive literature search across the selected databases, a set of keywords and their combinations was formulated using Boolean operators (“AND”, “OR”) to ensure precision and coverage. The selected terms included: Power-to-Heat, PtH, P2H, Thermal Energy Storage (TES), Seasonal Thermal Energy Storage (STES), Aquifer Thermal Energy Storage (ATES), Borehole Thermal Energy Storage (BTES), Tank Thermal Energy Storage (TTES), Pit Thermal Energy Storage (PTES), Packed Bed Thermal Energy Storage (PBTES), Latent Heat Thermal Energy Storage (LHTES), Thermochemical Seasonal Storage (TCSS), Variable Renewable Energy (VRE), renewable electricity utilization, district heating, sector coupling, sector integration, nearly zero-energy districts (NZEDs), and decarbonization, among others. These terms were combined and refined iteratively to capture a broad yet relevant range of studies addressing the integration of P2H and STES technologies within VREs.

2.4. Limitations of the Review

The following limitations were identified during literature collection and review preparation:

• the review focuses on journal articles and conference proceedings written in English and may overlook contributions written in other languages;
• the review includes scientific papers and may exclude those described in technical reports and other non-scientific works;
• the reliance on keyword searches may inadvertently exclude some relevant studies due to the variations in terminology.

3. P2H Technologies and Their Potential Integration with VRE and TES

P2H technologies are applications that use electrical energy to generate heat for buildings (including individual houses and district heating systems), industrial processes, and other sectors. P2H systems convert surplus electricity into thermal energy through established principles, enabling rapid integration with existing district heating infrastructure. Heat generated by P2H can be coupled with TES and utilized in a controlled manner to enhance energy efficiency or integrate renewable resources [13]. This section introduces P2H technologies and their potential integration with various renewable energy sources, as well as various thermal energy storage options. The data presented here summarizes existing knowledge and provides an initial basis for examining the novel concept of integrating VRE-driven P2H systems with STES installations.

3.1. Overview of P2H Technologies

Nowadays, a range of P2H technologies is employed across various solutions, including electric boilers, heat pumps, radio-frequency heaters, microwave heaters, mechanical vapor recompression, infrared heaters, resistance furnaces, induction furnaces, and electric arc furnaces [14]. These technologies enable the direct conversion of electrical energy into thermal energy. Electric boilers and heat pumps are commonly used for building heating and in district heating systems. The primary advantage of electric boilers lies in their ability to convert electricity into heat with 95–99% efficiency, as radiation losses from exposed surfaces are minimal [15]. They have electrical capacities from a few kW up to >70 MW [16]. On the other hand, heat pumps represent an alternative electrification technology that offers even higher efficiency [17]. Large-scale heat pumps can be integrated centrally into district heating networks, using low-grade environmental sources such as air, groundwater, or waste heat. If fossil-based post-heaters are not employed, the system must be capable of supplying at least 90 °C. Alternatively, decentralized heat pumps can be installed at block or building level within low-temperature distribution networks. In low-energy buildings, domestic hot water (DHW) typically requires a temperature of around 60 °C, while space heating can be provided at even lower levels, such as 35–45 °C [18]. Commercially available heat pumps typically have a coefficient of performance (COP) ranging from 2.4 to 5.8 and heating capacities from 2 kW to 20 MW [19]. Both electric boilers and heat pumps can be considered clean and efficient alternatives to fossil-fuel systems, such as gas- and coal-fired boilers. Furthermore, depending on the size, these devices can be installed at different levels: building, network, and plant (see Figure 4) [20]. It increases the flexibility in the district heating sector by coupling the power and heat sectors.

Figure 4. The example of building and network levels of heat pump installations (1—central heat pump, 2—individual heat pumps, 3—underground thermal energy storage).

Radio frequency and microwave heaters, in turn, are commonly used in processes that require uniform and rapid heating, such as food processing and drying materials. They have been identified as novel physical heating methods, providing fast and volumetric heating [21]. Radio frequency units are characterized by key attributes such as high reliability, predictable and controllable performance, and the ability to power multiple nodes simultaneously, making them a preferred solution for applications with lower anticipated power consumption and numerous nodes [22,23]. On the other hand, microwave heaters are widely applied as a rapid, efficient, and environmentally friendly heating technique in fields such as food sterilization, material synthesis, and chemical engineering. Nonetheless, it still faces challenges, including limited heating efficiency, non-uniform temperature distribution, and low compactness, which in turn hinder ease of fabrication and system integration [24,25].

The next P2H technology, infrared heaters, has been widely adopted in various engineering applications due to its low cost, high energy efficiency, and rapid response time. It has been successfully implemented, e.g., in food processing, food drying, and metal forming [26,27,28]. Unlike conventional heating methods, IR heating transfers energy directly through radiation, enabling faster heating rates, reduced energy losses, and more precise temperature control. In the food industry, IR heating is particularly advantageous because it not only provides efficient thermal processing but also exerts a sterilizing effect by inactivating microorganisms, thereby improving both safety and quality [29]. The example of infrared fruit drying using radiant heaters is shown in Figure 5.

Figure 5. Illustration of infrared fruit drying using radiant heaters [30].

Resistance and induction furnaces are critical in metallurgy and materials processing due to their high-temperature capabilities and precise control. Resistance furnaces generate heat through electrical resistance in heating elements, providing a uniform temperature distribution and a straightforward design that suits continuous operations such as annealing, sintering, and heat treatment. However, they tend to exhibit lower energy efficiency and slower heating rates. In contrast, induction furnaces utilize electromagnetic induction to heat materials directly, enabling faster heating, higher energy efficiency, and cleaner operation without combustion byproducts. Induction heating offers superior energy transfer efficiency, localized heating, rapid temperature rise, and consistent thermal profiles—qualities that are especially beneficial in melting, alloying, and high-throughput industrial processes [31,32,33].

The last example, electric arc furnaces, operate primarily on electrical energy, supplemented by moderate amounts of chemical energy, to generate the heat needed to melt recyclable scrap. Generated temperature reaches a level of 1500–1550 °C [34]. Thermal energy is mainly produced by the electric arc formed between the graphite electrodes and the scrap or the molten bath. Considering the structure of electric arc heaters, they are composed of a steel shell with water-cooled panels and a lower vessel, a hearth lined with refractory material, and a roof that houses the electrodes. Nowadays, electric arc furnaces play a central role, e.g., in steel recycling. They enable flexible steel production with significantly lower CO₂ emissions compared to conventional methods, while simultaneously facilitating the recycling and efficient utilization of scrap resources [35,36]. The electric arc furnace is illustrated in Figure 6.

Figure 6. Illustration of an electric arc heater [37].

In general, P2H technologies represent a highly promising option for decarbonizing the energy and industrial sectors while supporting the achievement of global climate targets. They also enhance the flexibility of energy systems by integrating variable renewable energy sources. A brief characterization of power-to-heat technologies and their typical applications is summarized in Table 1.

Table 1. A brief characterization of power-to-heat technologies and their typical applications.

Technology	Brief Characterization	Typical Applications	Sources
Electric boilers	Electric boilers use electric resistance to heat water or generate steam. They are commonly used for district heating and industrial applications.	District heating, hot water supply, and industrial steam systems	[38]
Heat pumps	Heat pumps are devices that use electricity to extract heat from a source, such as the air, ground, or water. Then, they amplify and transfer that heat to a sink. This method is highly efficient for space and water heating.	Residential/commercial heating, district heating	[39]
High-frequency heaters	High-frequency heaters, also known as radio-frequency heaters, use electromagnetic fields at radio frequencies to generate heat within materials. They are often used for drying and curing.	Textile, paper, food drying, and composite curing	[40]
Microwave heaters	Microwave heaters utilize microwave radiation to heat dielectric materials directly. They are typically used in food and material processing.	Food processing, chemical synthesis, drying	[41]
Infrared heaters	Infrared heaters emit infrared radiation that heats surfaces directly. They are suitable for surface drying and heating applications.	Commercial heating, plastic forming, and drying processes	[42]
Resistance furnaces	Resistance furnaces use resistive elements to reach high temperatures for industrial heating and material processing.	Glass, ceramics, and metal heating	[43]
Induction furnaces	Induction furnaces use electromagnetic induction to heat conductive materials. They are widely used in metalworking.	Steel hardening, forging, and metal melting	[44]
Electric arc furnaces	Electric arc furnaces utilize an electric arc between electrodes to melt metals, particularly to produce steel.	Steelmaking, scrap metal recycling	[45]

Among other technologies, heat pumps, electric boilers, and electric resistance heaters are widely recognized as the three most promising options for typical P2H solutions. These systems can also be combined into hybrid configurations that integrate two or more heating methods, enhancing flexibility, efficiency, and system responsiveness [46].

Furthermore, P2H technologies offer significant advantages for advancing the energy transition. One key benefit is its ability to convert surplus VRE into heat, thereby supporting grid regulation and reducing reliance on fossil fuels. As VRE penetration grows across the European power grid, issues related to voltage stability, transient response, small-signal behavior, and frequency regulation are becoming more prevalent. P2H mitigates these issues by providing a flexible and controllable demand-side solution [47].

3.2. Integration of P2H Technologies with VRE

Widespread adoption and integration of P2H technologies offer significant potential to reduce CO₂ emissions and accelerate industrial electrification. These solutions contribute to a more sustainable future by harnessing renewable energy and enhancing energy efficiency, thereby reducing environmental impact and increasing economic resilience.

Renewable electricity can be generated from diverse sources, including solar photovoltaics, concentrating solar power, and onshore or offshore wind. A key advantage of renewable energy is its high flexibility in deployment across different spatial and functional scales. In grid-connected (on-grid) systems, renewable electricity can serve as both a central supply for district energy networks and a direct source for individual end-users, with surplus generation often fed back into the grid. In contrast, off-grid installations operate independently of centralized infrastructure and are typically deployed in remote or isolated areas to meet local electricity demands [48].

Photovoltaic technology is one of the most widely used renewable energy technologies due to its scalability and compatibility with urban environments. Its principle is based on converting solar radiation directly into electricity via the photovoltaic effect. This conversion occurs within PV panels, which are composed of interconnected cells, most commonly fabricated from silicon. PV panels can be integrated into rooftops, façades, shading devices, and even building materials as building-integrated photovoltaics (BIPV). PV electricity can typically serve local building loads, supply district infrastructure, be stored in batteries for short-term balancing, or be utilized for electric vehicle charging [49,50]. Wind energy complements PV by providing electricity during periods when PV output is lower, such as at night or in winter. Wind energy systems convert the kinetic energy of airflow into electricity, providing a renewable and sustainable power source. Their key components, wind turbines (WTs), utilize rotor blades to capture wind energy and drive a generator through mechanical rotation, producing electricity via electromagnetic induction. At the district scale, wind power can typically be harnessed through mid-sized onshore turbines located near urban peripheries or offshore farms connected to local grids. Small-scale urban wind turbines are also being developed, although their adoption remains limited [51]. In P2H systems, wind is typically used alongside PV, ensuring a more stable renewable supply profile. However, areas located in coastal or windy regions can benefit substantially from wind integration, though urban siting remains constrained. The integration of solar and wind resources into hybrid renewable energy systems with or without energy storage can be achieved through various configurations, including

• co-located systems in which PV panels and WTs are installed at the same site and share a grid connection, providing a steadier energy supply;
• microgrids in which hybrid solar–wind systems operate as stand-alone systems;
• integrated control in which advanced controllers balance solar and wind output by directing surplus energy to storage or immediate use;
• optimization algorithms that determine the optimal solar–wind mix based on weather, demand, and storage conditions;
• demand response based on matching generation with consumption patterns through real-time demand-side management [52,53,54].

PV and WT systems offer several advantages, but they also face challenges related to upfront costs, storage requirements, and intermittency. The most important strengths and weaknesses of these systems are summarized in Table 2.

Table 2. The most important strengths and weaknesses of PV and WT systems [55,56,57].

Parameter	Photovoltaics	Wind Turbines
Strengths	Predictable seasonal and daily patterns; Scalability for various applications; Low maintenance and operating costs; Possibility of centralized or decentralized generation; Lack of greenhouse gas emissions and air pollution during operation; Reduction of dependence on fossil fuels.	Predictable output over the long term; Scalability by adding additional turbines; Low operating costs; Possibility of centralized or decentralized generation; Lack of greenhouse gas emissions and air pollution during operation; Reduction of dependence on fossil fuels.
Weaknesses	Energy generation limited to daylight hours; Location dependence of energy generation and seasonal variations in solar energy output; Energy generation affected by cloudy, rainy, or heavily shaded conditions; High initial costs; Challenges in the proper disposal and recycling of PV panels.	Energy generation variable due to fluctuations in wind speed and direction Low energy generation in calm conditions andshutdowns in high wind; Noise, aesthetic, land, and health considerations; High initial costs and maintenance challenges; Challenges in the proper disposal and recycling of wind turbines.

An alternative to classic PV panels is Concentrated Solar Power (CSP) technologies. These technologies are currently under development and are not yet as reliable as conventional PV panels. CSP technologies use optical elements, such as mirrors and lenses, to concentrate direct solar radiation onto a receiver to produce high-temperature heat. Heat-transfer fluids (HTFs), such as molten salts, synthetic oils, or water/steam, are heated to temperatures often exceeding 400–600 °C and then used for direct process heat, electricity generation, or charging TES systems. Common CSP configurations include linear Fresnel reflectors, parabolic troughs, solar towers, and parabolic dishes with Stirling systems [58]. Multiple factors, including geographic conditions, solar resource characteristics, project scale, and specific energy demand profiles, influence the decision between CSP and PV. CSP technologies are particularly advantageous for large-scale installations that require dispatchable power and thermal storage. In contrast, PV is generally more cost-effective and better suited for decentralized or distributed generation applications [59].

Considering the connection of various renewable energy technologies, both from the standpoint of energy generation and utilization, the system connecting PV/WTs with HPs can be proposed (see Figure 7). In such hybrid systems, electricity from PV and WTs powers large-scale or decentralized HPs, which then provide heating and cooling through district heating networks or building-level systems. This configuration directly links intermittent renewable electricity production with thermal energy demand, offering several advantages: it enables the effective use of variable renewable generation, reduces reliance on fossil-based backup systems, and enhances overall system efficiency through sector coupling. Furthermore, when combined with TES, the mismatch between renewable supply and heating/cooling demand can be further mitigated, allowing for stable operation and improved energy self-sufficiency.

Figure 7. The idea of the system connecting heat pumps (HP) with a PV installation and wind turbines, where 1—wind turbines; 2—PV panels; 3—power plant; 4—AC/DC converter; 5—inverters; 6—central heat pump; 7—individual heat pumps.

3.3. Integration of P2H Technologies with TES Systems

Due to the day-night cycle and weather fluctuations (including overcast conditions, sunlight, and wind speed), renewable energy sources are intermittent and variable. These variations make achieving a stable, reliable energy output challenging, complicating their integration into the power grid and their use by end-users. There is also often a mismatch between the timing and location of energy demand and supply, leading to inefficient energy use and suboptimal performance. Consequently, grid operators sometimes disconnect inverter-based VRE plants, curtailing significant amounts of renewable generation. The solution to these challenges is energy storage [60]. As illustrated in Figure 8, energy storage can be broadly categorized into four main types: thermal, chemical, mechanical, and electromagnetic/electrostatic.

Figure 8. Classification of energy storage technologies according to their storage mechanism [61].

Nowadays, with the growing popularity of PV and WTs, significant efforts are being directed toward direct electricity storage, using technologies such as conventional batteries [62], supercapacitors [63], compressed air energy storage (CAES) [64], and pumped hydro storage [65]. On the other hand, considering the coupling of P2H technologies with energy storage, the most interesting option is TES. There are three general types of TES systems: sensible heat storage (SHS), latent heat storage (LHS), and thermochemical storage (THS). Among other options, SHS can be considered the most straightforward and commonly used form of TES. It relies on materials that have high thermal conductivity and good temperature stability. These materials should also exhibit negligible volume change during heat storage, be cost-effective, and be durable over the long term. The amount of heat stored is proportional to the temperature change and the medium’s heat capacity. Typically used materials for SHS include water, concrete, brick, rock, ceramics, molten salts, cast steel, cast iron, synthetic oils, and mineral oils [66]. LHS systems use latent heat, most often associated with the phase transition of a material. The classification of a given material as a phase change material (PCM) is typically based on its phase transition temperature. It must be within the operating range of the heat storage system. The most commonly used compounds for this purpose are organic compounds, such as paraffin (melting point ~50 °C), or inorganic materials, such as salt hydrates (melting points ranging from ~30 °C to ~120 °C). A phase transition enables an increase in the energy density of heat storage, provided the thermal storage’s operating temperature is close to its phase transition temperature. In this case, energy is stored in the heat of phase transition, which occurs within a narrow temperature range. The ability to store latent heat results in a high energy density for PCMs, making LHS more compact than SHS. The advantage of these materials is also that the outlet temperature of the heat transfer fluid can remain nearly constant during the discharging cycle, providing more stable and predictable performance [67]. Finally, THS systems use reversible chemical reactions to store thermal energy. In this case, water paired with silica gel, lithium bromide, lithium chloride, magnesium sulfate, and sodium chloride is considered the most prominent material [68]. In THS systems, the most commonly used reaction is the hydration of salts or hydroxides. The dehydration process is endothermic. By supplying energy to the material, salt or hydroxide is converted into a lower-grade hydrate or a dehydrated molecule. The hydration process is exothermic. When water is introduced into the dehydrated compound, it forms hydrates and releases some energy. Water must be supplied to the storage in gaseous form to fill the bed’s pores evenly. Due to the low temperatures during hydration and dehydration, there must be low pressure inside the storage. THS systems offer the highest thermal energy storage density, but they are more complex and often require further technological development and refinement through scientific research before they can be widely commercialized [69]. A brief comparison of Sensible, Latent, and Thermochemical Storage is presented in Table 3.

Table 3. A brief comparison of TES technologies [66,68,69,70].

Parameter	SHS	LHS	THS
Storage mechanism	Energy stored as a temperature difference in solid or liquid media	Energy stored using phase change materials	Energy stored in chemical bonds
Materials	Water, molten salts, oils, steel, iron, rocks, and concrete	Paraffins, salt hydrates, etc.	Metal oxides, ammonia-based, hydrothermal
Energy density	Moderate	High	Very high
Storage temperature	Moderate to high (90 °C for water, 500 °C for molten salts, and 1000 °C for rocks)	Low (20–150 °C)	Low to high (60–200 °C for adsorption or hydration, and >600 °C for redox reactions
Typical duration of energy storage	Short to medium-term (hours to days)	Medium to long-term (days to months)	Long-term (months to years)
Cycle efficiency	High	Moderate to high	High
Technical complexity	Simple	Medium	Complex
Maturity	Commercially available	Pilot scale	Laboratory stage
Typical applications	Residential heating, district heating, and industrial processes	Residential heating, solar thermal, industrial processes	Residential heating, industrial high-temperature processes, seasonal storage
Expected costs	Low to moderate (depending on materials)	Moderate to high (especially for organic PCMs)	High (materials and complexity)

Recent advances in materials and system design are expanding the capabilities of TES technology, making its integration into residential, district, and industrial heating applications increasingly feasible. In this context, the most promising option is seasonal thermal energy storage. STES is the process of capturing thermal energy during periods of surplus energy supply (e.g., summer), intending to use it during periods of high demand (e.g., winter). STES can operate properly whenever large amounts of energy can be stored over a long period. THS will have an advantage in this respect. Storing energy through a chemical reaction results in almost zero storage losses. However, the containment and conditioning of the reactants (e.g., keeping them sealed, dry, and at the required pressure) can introduce indirect losses, including leakage and the auxiliary electricity needed for pumps or compressors. SHS and LHS systems are less resistant to energy losses to the surroundings. Despite their different classifications, their operations are similar. To extend the storage period, it is necessary to ensure adequate thermal insulation for both cases. The advantage of LHS over SHS is its higher energy density, which results in a smaller heat-exchange surface with the surroundings. However, classic SHS systems may prove to be more effective than LHS for long-term heat storage, despite their lower storage capacity. This refers to storage using packed bed materials.

STES enables the large-scale temporal balancing of heating and cooling needs, enhancing the integration of intermittent renewable energy into energy systems. Several types of STES have been developed so far, including Aquifer Thermal Energy Storage (ATES), Borehole Thermal Energy Storage (BTES), Tank Thermal Energy Storage (TTES), Pit Thermal Energy Storage (PTES), Packed Bed Thermal Energy Storage (PBBTES), Latent Heat Thermal Energy Storage (LHTES), and Thermochemical Seasonal Storage (TCSS). Each technology utilizes a distinct storage medium and configuration, offering unique advantages and challenges in terms of efficiency, cost, scalability, and geological or spatial requirements [71]. A brief discussion of SHS solutions in the context of STES is provided below.

3.3.1. Aquifer Thermal Energy Storage

ATES is a technology that uses aquifers as natural underground reservoirs to store and recover thermal energy seasonally, enabling year-round heating and cooling. ATES systems operate based on the porosity and permeability of aquifer formations, which facilitate the circulation and retention of groundwater within the pore network of the geological medium. In the summer, excess heat is transferred to the groundwater and injected into a warm well where it is stored underground. Conversely, during winter operation, water, naturally or mechanically cooled, is injected into the cold well. Thermal demand is met by pumping from the proper well—warm water in winter for heating, cold water in summer for cooling. Wells are located far apart from each other, so their thermal plumes do not overlap. Figure 9 provides a schematic representation of ATES operation.

Figure 9. The general idea of ATES technology (adopted from Ref. [72]).

The successful implementation of an ATES system depends on several key preconditions, including geological, hydrogeological, and hydrogeochemical characteristics. Suitable geological settings are typically found in sedimentary basins, where unconsolidated aquifers are the most favorable storage medium. However, both fractured and unfractured aquifers can also be utilized. Ideally, aquifers consist of medium- to fine-grained sands, gravels, or other porous sediments and rocks with sufficient groundwater to sustain the system [73]. ATES systems rely on groundwater to function properly. Areas with limited groundwater storage are less suitable because a single well typically needs to yield approximately 70 m³/h. ATES can be applied in both fresh and saline groundwater systems, though saline environments require corrosion-resistant equipment. Cold-water injections generally have negligible effects on water chemistry. Still, the injection of warm water can alter the concentrations of As, P, K, Si, and Mo, as well as pH and dissolved organic carbon. Therefore, continuous monitoring of these parameters is essential to ensure water quality in ATES applications [74]. In summary, ATES allows storing large amounts of thermal energy in groundwater, leveraging its vast mass and high heat capacity, enabling storage of heat or cold for many months with minimal losses. However, the large storage volume results in a low charging rate, making the system suitable mainly for seasonal energy storage.

3.3.2. Borehole Thermal Energy Storage

BTES technology uses boreholes with closed-loop fluid circuits (borehole heat exchangers) to transfer heat or cold to the subsurface, where thermal energy can be stored seasonally. The depth of borehole heat exchangers typically ranges from a few meters to over 200 m, depending on factors such as energy demand, available surface area, and subsurface conditions [75]. Due to their large storage capacity and slow thermal response, BTES systems are being increasingly adopted for seasonal thermal energy storage applications. Borehole heat exchangers are particularly suitable for applications requiring significant underground heat exchange capacity, as subsurface environments at sufficient depths provide relatively stable temperatures and predictable thermal gradients [76]. The idea of BTES technology is shown in Figure 10.

Figure 10. The general idea of BTES technology (adopted from Ref. [72]).

The operation of BTES relies on the capacity of the geological matrix to store and transfer heat. Efficient storage with minimal losses is determined by the thermal properties of the subsurface, which are influenced by factors such as mineral composition, storage depth, temperature, heat carrier temperature, fluid saturation, and groundwater flow in the vicinity of the boreholes [77]. Moreover, the influence differs between systems designed solely for energy extraction and those intended for energy storage. In extraction-only applications, higher groundwater flow enhances thermal recharge of the subsurface, thereby improving the efficiency of borehole heat exchangers. In contrast, for storage applications, excessive groundwater flow can result in substantial heat losses through advection, thereby reducing storage efficiency and overall system performance [78,79]. Similarly to ATES, the large storage capacity of the storage medium and its low thermal conductivity allow energy to be stored for long periods. Unfortunately, this also results in a very long charging time for the storage system.

3.3.3. Pit Thermal Energy Storage

PTES is an artificial underground storage system typically constructed at depths of 5–15 m. The storage volume is created by an excavated pit, where the bottom and inclined sidewalls form the reservoir, eliminating the need for additional structural support. The ground-level surface is sealed with either a fixed or a floating cover to minimize heat loss [80]. In PTES systems, both liquid and liquid–solid storage media can be employed. The latter, typically consisting of a water–gravel mixture with a gravel content of 60–70%, provides greater structural stability. However, because gravel has a significantly lower heat capacity than water, the overall storage capacity of the mixture is significantly reduced. Consequently, a PTES using a water–gravel medium must be approximately twice the size to store an equivalent amount of energy [81]. The general idea of PTES technology is shown in Figure 11.

Figure 11. The general idea of PTES technology (adopted from Ref. [82]).

The performance of a PTES system is strongly influenced by factors such as operating temperature levels and the degree of thermal stratification within the storage fluid. Effective stratification can be achieved through the design of appropriate diffuser devices (inlet/outlet) that minimize turbulence in the water, as well as by applying high-quality thermal insulation at the surface to prevent heat loss from the uppermost water layer. Due to the large scale of PTES systems and the associated demand for construction materials, these factors can significantly affect project profitability, making the planning and design process both complex and challenging [83,84].

3.3.4. Tank Thermal Energy Storage

TTES technology typically employs reinforced concrete or stainless-steel tanks as storage vessels, with water used as the primary heat storage medium. The geometric design of hot water tanks should be optimized to minimize heat losses and thereby maximize storage efficiency. For buried thermal storage systems, vertical cylindrical tanks represent the most efficient configuration; however, spherical geometries can also be a suitable alternative. Thermal losses can be minimized by applying insulation layers up to one meter thick, typically composed of materials such as glass wool, polyurethane, extruded polystyrene (XPS), or expanded polystyrene (EPS). Furthermore, heat dissipation from water tanks can be reduced by embedding them in sand or other low-thermal-conductivity backfill materials. Water in tanks can be used to store large amounts of heat at temperatures typically ranging from ambient levels to 90 °C. The general idea of TTES technology is shown in Figure 12.

Figure 12. The general idea of TTES technology (adopted from Ref. [85]).

From a technical perspective, thermal buoyancy in the tank creates a temperature gradient, resulting in thermal stratification. Such stratification is advantageous for thermal energy storage systems, as it minimizes mixing between low- and high-temperature fluid layers, thereby enhancing storage efficiency. Nowadays, hot water tanks are the widely used thermal storage option, as they provide a practical solution that can be implemented in any location [86,87,88].

3.3.5. Packed Bed Thermal Energy Storage

PBTES is a system consisting of an insulated tank filled with a solid filler material, serving as the storage medium. A wide range of packing materials has been investigated as candidates for such systems, including silica sand, crushed rocks, alumina beads, steel slags, ceramics, and perforated concrete blocks. Heat is transferred via a heat transfer fluid (HTF) that flows directly through the packed bed, enabling thermal stratification within the system. When a liquid HTF is used, it also allows dual storage of both the fluid and the solid medium. A key characteristic of packed-bed TES is the ability to use air as the HTF, which flows through the porous medium during charging and discharging cycles. During charging, hot air enters the packed-bed unit through the designated inlet and exits through the outlet, transferring heat to the filler material until the bed is fully charged. During discharging, the flow direction is reversed: cold air is introduced into the heated bed to recover the stored energy. In this way, hot and cold fluids alternately pass through the same channel, exchanging heat with the storage medium [70,89]. It is also possible to separate the HTF circuit by using a heat exchanger placed inside the packed bed. This reduces heat-exchange efficiency but allows more advanced control of the extracted heat output, which is no longer directly dependent on the bed’s porosity and permeability. The general operating principle of the PBTES is illustrated in Figure 13.

Figure 13. The general idea of the operating principle of the PBTES (adopted from Ref. [90]).

Packed-bed systems have several advantages, the most notable being the ability to operate over a wide temperature range. The absence of water in the storage tank enables significantly higher heat storage temperatures. Depending on the material, it can reach temperatures of up to 1000 °C, which significantly increases the energy density of this storage system [91]. The chemical stability of the heat transfer fluid and the solid storage material primarily limit this range. PBTES are recommended for various applications, including solar installations, industrial waste heat recovery, and geothermal energy utilization. Depending on the filler material and heat transfer fluid chosen, PBTES often rely on materials that are abundant, locally available, environmentally benign, and non-reactive, making them a sustainable alternative to other storage options.

3.3.6. Summary of the Discussed SHS Technologies

Summarizing the SHS solutions discussed above, the most important parameters were analyzed and compared in Table 4. As may be concluded, each of the solutions discussed can be applied to district heating. At the same time, some of them can utilize waste heat, industrial heat, surplus solar energy, and other sources. On the other hand, due to differences in storage medium type, the maximum temperature is typically limited to 95 °C. High-temperature thermal storage can be considered only in the case of PBTES technology.

Table 4. A brief characterization of STES technologies and their typical applications [14].

Technology	Storage Medium	Temperature Range	Typical Applications	Sources
Aquifer Thermal Energy Storage (ATES)	Groundwater in an aquifer	5–90 °C	District heating, industrial recovery	[92]
Borehole Thermal Energy Storage (BTES)	Soil/rock via vertical boreholes	5–90 °C	Residential heating, solar storage	[93]
Pit Thermal Energy Storage (PTES)	Water in insulated pits	5–95 °C	District heating, campuses	[5,94]
Tank Thermal Energy Storage (TTES)	Water in insulated tanks	5–95 °C	Urban heating networks	[95]
Packed Bed Thermal Energy Storage (PBTES)	Silica sand, crushed rocks, alumina beads, steel slags, ceramics, and concrete blocks	100–1000 °C	Industrial waste heat, solar thermal power, and long-duration storage	[96]

3.4. Potential for Further Development of P2H and STES Technologies

Many P2H and TES technologies are well-established and play a significant role in the European energy transition. However, connecting P2H technologies with STES remains an interesting option that warrants further investigation for both domestic and district heating systems. Integrating these technologies has the potential to increase the flexibility and efficiency of heating systems, allowing them to more effectively match energy demand with renewable energy supply over extended periods. By shifting electricity into the heating sector during periods of high renewable generation and low demand, P2H reduces the need for curtailment and mitigates fluctuations in instantaneous VRE penetration. Additionally, P2H offers flexibility to electricity markets by operating economically during hours of low or even negative prices. Leveraging seasonal storage enables excess heat generated during peak renewable production, such as summer, to be stored and used during the high-demand winter months. This significantly reduces reliance on fossil fuels and enhances grid stability. Furthermore, this approach could contribute to decarbonizing heating networks, enhance energy security, and support broader net-zero emission objectives in the heating sector.

The technologies and storage concepts reviewed in this section collectively provide the technical foundation for implementing integrated P2H and STES systems. Their performance characteristics, including temperature operating ranges, conversion efficiencies, and scalability, govern their applicability across diverse energy contexts, from single-building configurations to community-scale and district-level heating networks. At present, electric boilers and large-scale heat pumps dominate commercial deployment due to their technological maturity and near-complete energy conversion efficiency. In contrast, PBTES, TCSS, and hybrid configurations represent emerging solutions with significant potential for long-duration, high-temperature applications.

Beyond technical metrics, the integration potential of these systems is shaped by system-level design choices, regulatory frameworks, and market structures that appropriately value the flexibility and ancillary services provided by TES. P2H units can serve as dispatchable electrical loads, while TES components enable temporal decoupling between renewable electricity generation and heat demand. Their coordinated operation yields synergistic benefits, including demand-side management, reduced operational costs, and lower carbon intensity across thermal networks.

Despite technological advancements, several challenges remain. These include the development of cost-effective, high-capacity storage materials; the implementation of advanced control strategies capable of dynamic operation under variable renewable supply; and the optimization of system configurations tailored to specific climatic and economic conditions. The subsequent section builds upon this technological foundation by analyzing numerical simulations, experimental investigations, and real-world demonstration projects. This synthesis not only evaluates technical performance and efficiency but also provides insights into cost-effectiveness, reliability, and the broader role of P2H–STES systems in enabling low-carbon, renewable-based district heating infrastructures.

4. Pathways Toward Low-Carbon District Heating: Overview of the Numerical and Experimental Works

Worldwide literature emphasizes the need for the energy transformation of the economy, including the building sector. The available works focus on both single buildings (including new and existing structures) as well as entire districts, cities, and countries. NZEDs can improve regional energy efficiency, provide a sustainable framework for urban energy modeling, and—when organized as energy-flexible NZEB clusters—enable the development of smart city regions [97,98]. The NZED concept can be considered as a universal approach for urban energy modeling, connecting such aspects as technological innovations, environmental impact, economy, energy market, and safety reasons [99].

4.1. Selected Studies of the District Scale Systems Dedicated to the NZEBs and NZEDs

Wyrwa et al. [100] conducted a techno-economic analysis of converting selected hard coal–based DH systems into sustainable and efficient alternatives. A model of six DH systems in Poland, which produce approximately 8.5 PJ of heat annually, was developed using the IEA-TIMES generator. Two scenarios were analyzed. In the first scenario, the economic and technical parameters were held constant at their 2025 values from 2025 to 2050. In the second scenario, gas prices and CO₂ emission allowance prices were allowed to vary over time. The authors demonstrated that achieving a 20% share of heat production from solar thermal would require installing additional seasonal heat storage facilities with a total capacity of 197 TJ. Moreover, rising natural gas and CO₂ allowance prices were found to accelerate the transition toward greater reliance on solar heating plants. Although the analysis did not account for district heating network operational constraints or field-area limitations, the findings clearly suggested that, in the future, heat production from solar thermal plants integrated with STS could become increasingly price-competitive. Komninos [101] presented a model for transitioning housing districts to NZEDs, demonstrating that under specific population densities, fossil-fuel-based neighborhoods can become self-sufficient and carbon-neutral by utilizing solar energy. While feasible in southern Europe due to a favorable climate and solar yield, NZEDs face challenges in central and northern regions; however, future improvements in PV technology may broaden their applicability. As the author noted, the transition requires addressing seasonal energy imbalances, storage needs, and financial investments, and is expected to take over a decade. The approach combines smart city systems, renewable energy integration, and nature-based solutions, emphasizing the roles of human behavior, community planning, and machine intelligence. Tulus et al. [102] evaluated the possibility of integrating central solar heating plants coupled with seasonal storage in the residential sector in selected European climate zones, including Madrid, Athens, Berlin, and Helsinki, which serve as proxies for the Mediterranean continental, Mediterranean, central European, and Nordic climates, respectively. A multi-objective optimization framework comprising life-cycle cost analysis for economic evaluation and LCA for environmental impact estimation was applied to a residential neighborhood community of 1120 apartments. The technical performance was also evaluated by satisfying both the space heating demand and the domestic hot water service requirements. The results showed that central solar heating plants coupled with seasonal storage can achieve a renewable energy fraction above 90%, with environmental impact reductions of 82.1–86.5% compared to a natural gas heating system. The performance of solar district heating systems with seasonal thermal storage was also studied by Renaldi and Friedrich [103]. The authors analyzed the system deployed in the UK using a case study of the Drake Landing Solar Community in Okotoks, Canada. It was found that reduced solar penetration can be balanced by increasing long-term storage capacity. Furthermore, it was stated that the analyzed system should be implemented to improve the energy performance of newly built houses rather than the current building stock of older homes. The key role of thermal storage units in the efficiency of solar systems was confirmed by Janus et al. [104]. The authors reviewed selected solar energy systems for heat generation and storage and discussed examples of CFD applications in their analysis. The conclusions highlighted the significant role of numerical simulations in improving the performance of such systems. Penttinen et al. [105] explored the feasibility of a large cavern thermal energy storage in a DH system with waste incineration. The authors conducted 62 one-year optimizations for seasonal storage with varying sizes and power. Furthermore, the annual system emissions were estimated. As observed, even small-capacity seasonal storage can significantly reduce system emissions. The return on investment for the most profitable option (i.e., 90 GWh of storage with a power of 200 MW) ranged from 3.6% to 9.4%. Considering economic factors, it was concluded that seasonal energy storage remains less profitable than traditional energy investments; however, this may change as the cost of carbon emissions rises. Rasku and Kiviluoma [106] presented case studies on the potential of P2H flexibility and energy-efficiency measures for a hypothetical 2030 Finnish detached-housing stock, analyzed in the context of the Nordic power system and an isolated Finnish system. The stock was approximated by two archetype dwellings modeled using a lumped-capacitance approach and coupled to a stochastic linear-programming unit-commitment model. The results showed that, under high VRE shares, residential P2H combined with thermal storage could deliver greater system-level cost savings than energy efficiency measures alone. Nevertheless, from the perspective of homeowners, efficiency improvements remained more attractive, as extensive reliance on residential P2H to support the power system could increase heating costs. Hirvonen and Kosonen [107] evaluated the potential of a seasonal BTES system for enhancing the use of waste heat in a residential community. Heat produced from waste incineration during summer was stored underground and used to preheat district heating flows during the colder months. Among 36 analyzed cases, the median storage efficiency was 61%, even when storage temperatures often exceeded 60 °C. The system demonstrated significant potential for emission reduction, achieving up to an 86% reduction in the heat supply of a local heating network in Finland. In the three most successful cases, annual CO₂ emissions from district heating were reduced by 536, 819, and 1109 tons, respectively. Maruf et al. [108] reviewed the most promising P2H and TES technologies, assessing them as both innovative and economically viable for improving energy efficiency and reducing CO₂ emissions. They highlighted electric boilers, heat pumps, resistance heaters, and hybrid systems as the most mature P2H solutions for Europe. In contrast, sensible and latent heat storage was identified as the dominant TES options. High-temperature heat pumps and electrode boilers showed strong potential for industrial applications. In contrast, combined heat and power (CHP) systems can play a key role in integrating power and heat sectors, especially in DH. As was discussed, TES is expected to cover 60% of total heat storage by 2050, supporting flexible energy systems. The study also presented modeling approaches for these technologies, accounting for operational constraints and climate impacts. Jin [109] estimated the role of P2H in South Korea’s 2050 net-zero energy scenario using the EnergyPLAN model. Simulations based on government projections have shown that future infrastructure may be overbuilt, with electricity and heating capacities exceeding demand. VRE is expected to generate a significant surplus—approximately 89 TWh annually—yet only 4.7 TWh could be absorbed by P2H in district heating, covering just 14.5% of heating needs. Sensitivity analyses revealed the limited cost-effectiveness of flexible resources, such as thermal and electricity storage. To improve system efficiency and enhance net-zero feasibility, the author recommended increasing wind power capacity over solar PV, given its better performance in South Korea’s conditions. Adesanya et al. [110] analyzed a large-scale greenhouse in Yeoju-si, South Korea, equipped with a complex hybrid renewable energy and thermal energy storage system to address the intermittency of renewable energy sources. The setup included solar thermal and PVT collectors, hybrid and ground-source heat pumps, and multiple TES units. Performance evaluations demonstrated high efficiency across all components, with heat pumps supplying the majority of the heating energy. The system significantly reduced greenhouse gas emissions and operational costs, demonstrating the potential of integrated RES and TES solutions for sustainable agriculture. Todorov et al. [111] evaluated the integration of ATES and groundwater source heat pumps (GWHP) into district heating and cooling networks in Southern Finland. A novel method was developed to integrate groundwater modeling with publicly available data sources. The case study was conducted for Pukkila, where heat was primarily generated by a 1.5 MW wood chip boiler, supplemented by oil boilers. Groundwater flow and thermal models were calibrated using national datasets and tools like MODFLOW and QGIS. The integration of ATES with seasonally reversible operation and balanced pumping resulted in a minimal environmental impact and demonstrated strong economic feasibility. GWHPs were used to partially meet heating demand, improving efficiency by preheating return flows before boiler input. Simulation data from a typical office building was scaled to match Pukkila’s heating profile. The system demonstrated high performance, with the potential to achieve significant CO₂ reductions and cost savings. Rosato et al. [112] developed a TRNSYS-based model of a hybrid district heating system in Naples, combining solar and geothermal energy with seasonal thermal storage to serve six residential buildings and three schools. The simulation results demonstrate substantial reductions in primary energy use, CO₂ emissions, and operational costs compared to conventional, standalone heating systems. Kim et al. [113] conducted a techno-economic analysis on a hybrid renewable energy system (HRES) designed for an NZED in Jincheon, South Korea. The system integrated solar thermal collectors, STES, heat pumps, and a DH network. Its performance was evaluated through dynamic simulations and validated using experimental data collected over one year in 2017. The study assessed the system’s ability to meet the energy demands of 300 energy-plus residential buildings and six non-residential objects in a humid subtropical climate. Two conventional alternatives—gas-fired boilers (Case 1) and a centralized heat pump system (Case 2)—were used for comparison. Sensitivity analysis across 10 scenarios explored variations in solar collector area and STES volume. Results showed that increasing the solar fraction could reduce CO₂ emissions by up to 61% compared to Case 2 and increase primary energy savings by up to 73% compared to Case 1. The proposed system also achieved a lower levelized cost of heat, a six-year payback period, and a benefit–cost ratio of 1.7. Pastore et al. [114] assessed the potential contribution of expanding DH systems and adopting energy-saving measures to decarbonize Italy’s building stock by 2050. The paper examined the combined effects, interactions, and techno-economic implications across the entire national energy system, using a set of scenarios simulated with the H2RES software (https://h2res.org/ (accessed on 21 September 2025)). The results indicated that heat pumps are the most suitable technology for both centralized and decentralized applications. Expanding district heating was identified as a priority for decarbonization, as it enables cost reductions, facilitates the integration of thermal storage systems, and enhances overall system flexibility. In the most favorable scenario, fourth-generation DH could supply up to 40% of Italy’s heat demand. While energy-saving measures lower both heat demand and primary energy consumption, they come with higher annual costs and substantially greater investment requirements. Finally, the study confirmed the importance of energy-saving measures, emphasizing the need to determine their optimal level of deployment to prevent excessive increases in energy system costs. According to the authors, the presented work may serve as a guide for decarbonizing the building heating sector, supporting both the development of national energy strategies and the design of investment schemes. Nematchoua [115] presented a framework for retrofitting urban districts to achieve NZED status, with a focus on the European Union. A 100-year life-cycle assessment of urban, sustainable, and rural neighborhoods revealed that urban areas have significantly higher energy demand. By combining deep building renovations, electric mobility, solar energy, and climate adaptation, energy demand could be reduced by up to 95% by 2050. The research highlighted the environmental benefits and feasibility of NZEDs, while stressing the need for expert training and improved data collection to support climate-resilient urban development. Sihvonen et al. [116] examined the role of P2H and TES technologies in decarbonizing DH systems through case analyses of the energy grid in Mariehamn. Two decarbonization scenarios for the city’s DH system were developed and compared with the existing system to assess both technical and economic feasibility. The analysis also considered integrating P2H technologies into reserve markets. Using the PLEXOS software (https://www.energyexemplar.com/plexos (accessed on 21 September 2025)), the developed scenarios were simulated for the period of 2018–2022, accounting for variations in demand and electricity market prices. The results revealed that electricity and reserve market prices significantly influence both operating costs and system profitability. Overall, electrification offers strong potential for cost-effective emission reductions in district heating; however, it also increases vulnerability to fluctuations in electricity prices. Finally, the authors concluded that TES is one of the essential ways to mitigate cost risks and reduce investment uncertainty. Hennessy et al. [117] explored the potential flexibility of DH and the use of short-term TES within distribution networks, including approaches to modeling it. In contrast, Guelpa and Verda [118] investigated both short- and long-term TES opportunities in district heating and cooling systems. Furthermore, Leone et al. [119] investigated design differences across six projects (12 case studies) of positive energy districts (PEDs) developed within the European Smart Cities and Communities program. The analysis aimed to uncover the rationale behind energy-related choices in generation, storage, and distribution across various geographical contexts. The study enabled the preparation of a catalogue of energy system solutions implemented in the examined PEDs, accompanied by a critical evaluation of the underlying motivations, thereby identifying general trends in regions with comparable characteristics. Finally, Koohi-Fayegh and Rosen [120] highlighted the current status and future needs for optimizing community-scale STES. While most research has focused on building-level technologies, it underlined a growing need to tailor thermal storage solutions to the unique characteristics of communities. District energy systems—powered by sources like solar thermal, geothermal, industrial waste heat, and cogeneration—can benefit significantly from well-integrated seasonal storage. As the authors concluded, key challenges include determining the optimal type (sensible, latent, or thermochemical), scale, quantity, and duration of storage systems. Often, a mix of short-, medium-, and long-term storage is necessary to maximize performance and flexibility across diverse energy demands.

4.2. Selected Studies of the Island-Scale Systems Dedicated to the NZEBs and NZEDs

Considering island-scale systems, Żołądek et al. [121] presented an energy-economic analysis of an island system that was designed to meet the electrical energy demand of a tourist facility in two selected European locations. The proposed system was equipped with a PV field, a wind turbine, a wood chip gasifier, a battery, an electrolyzer, and a fuel cell. The operation of this system was simulated in TRNSYS 18 software. Three cases were analyzed: one with a high surplus of renewable energy, one with renewable energy similar to load, and one with a lack of renewable energy. It was demonstrated that in Gdansk (Poland), it is possible to meet 99% of user demand with renewable energy sources, with an excess energy of 31%. At the same time, in Agkistro (Greece), a similar result can be achieved with 43% excess energy. Furthermore, the proposed system was shown to be unprofitable for Agkistro, as the simple payback time (SPBT) exceeds 22 years. On the other hand, SPBT at 12.5 years was calculated for the Gdansk case. Notably, the system relies on fossil fuels for less than 1% of its demand, suggesting that future studies may further lower initial costs by excluding gas infrastructure. The findings indicate that when renewable energy sources account for a large share of user demand, system profitability is strongly influenced by local energy vector costs. The results also demonstrate the system’s robustness in supplying energy to users, suggesting that future advancements could enable operation entirely based on renewables. Another example of an energy system for the building—a school located in southern Italy—was investigated by Panno et al. [122]. The authors analyzed the integration of a heat pump for hot water production with a solar system comprising flat plate collectors and a BTES. The study employed TRNSYS simulations based on several months of monitored building data. Over a 20-year operating period, the proposed configuration met more than 76% of the school’s heating demand with solar energy, with the remaining demand covered by electricity (about 18%) and gas (around 6%). Furthermore, the solar collector had an average efficiency of 0.64, the heat pump achieved an annual COP of 5.11, and the storage system operated with an annual efficiency of 0.59. The energy and economic assessments highlighted the benefits of BTES in the Italian context. According to the authors, in regions with abundant solar irradiation and relatively low heating demand, solar plants can serve as the primary energy source for buildings. Achieving this, however, requires STES to address the mismatch between heat demand and solar energy availability. D’Amico et al. [123] proposed an energy district in Palermo centered around three 1950s school buildings, aiming to optimize energy use. To reduce reliance on the national grid, PV systems were integrated, with projected self-consumption and grid-sale strategies. The PV systems were expected to cover 18–33% of the schools’ electricity needs and achieve a payback period of under nine years, owing to favorable solar conditions in southern Italy. Another option, the CHP unit, sized for a thermal load of 718,000 kWt, has shown consistent thermal performance over its 20-year lifespan, though its electrical output is initially insufficient before becoming surplus. Economic analysis estimated savings of €2800 from the CHP system and over €426,000 from the PV systems. Together, they prevent approximately 5750 tons of CO₂ emissions. Despite environmental and energy benefits, the CHP system’s long payback period reflected limited operating hours tied to school schedules. Roumpakias et al. [124] examined a refurbished nearly zero-emission school building in Central Greece that faced grid export limitations due to saturation of renewable energy sources. Despite generating surplus photovoltaic electricity, the lack of battery storage prevented grid integration, leaving the system underutilized. To address this, the authors proposed shifting surplus electricity exports to the evening hours using a daily-discharge battery system. Simulations identified an optimal battery size of 12 kWh/kWp and evaluated TES to enhance heating efficiency. With declining battery costs, the investment demonstrated a positive net present value, providing a viable strategy to enhance renewable integration in overloaded Greek power networks. Nhut et al. [125] proposed a solar-assisted house-heating system with a seasonal underground thermal energy storage tank tailored to local weather conditions on Jeju Island, South Korea. A mathematical model utilizing the thermal response factor method was developed to evaluate the effects of insulation thickness, collector area, and tank volume on system performance. Results showed solar fractions of 45.8% on clear days, 17.26% on intermittent-cloud days, and 0% on overcast days. The approach demonstrated potential for energy, space, and cost savings, supporting the practical application of STES in the analyzed configurations. Ju et al. [126] discussed the potential of short-term TES to reduce peak power demand in district heating systems. While centralized TES is common, substations in high-temperature networks often lack such storage. A 5 m³ stratified thermal storage tank was installed in a Finnish office building substation and simulated using IDA-ICE. By charging the tank during off-peak hours and discharging during peak periods, and by reducing the water supply mass flow, the system achieved significant peak shaving—up to 31.5% on colder days—without compromising indoor comfort. Kasperski and Oladipo [127] evaluated various filling and insulation materials for a seasonal SHS system designed to support a plus-energy house in Poland. Seven material combinations were tested and categorized into advanced, medium, and simple technologies. Using a mathematical model, the study analyzed HVAC performance throughout the year and identified optimal operating temperatures that minimize storage volume and cost. Two recommended solutions emerged: clinker brick with fireplace wool (23 m³, €12,500) and concrete block with glass wool (27 m³, €1700), offering effective and locally available options for thermal energy storage. Gagliano et al. [128] compared various energy system configurations combining heat pumps, solar installations (PV or PV/T), and thermal and electrical storage. Simulations using TRNSYS 17.2 revealed that integrating both storage types with a PV system significantly boosts performance—raising self-consumption from 34.1% to 69.4% and self-sufficiency from 27.9% to 59.9%. Systems with PV/T collectors and dual storage achieved even higher results: 96.2% self-consumption and 86.9% self-sufficiency. These configurations supported the development of NZEBs by minimizing energy imbalance and reducing reliance on the electrical grid. Pazold et al. [129] presented the development and simulation of three TES models—thermally activated building systems (TABS), large water-based storage (WBTS), and high-temperature stone storage (HTSS)—designed to utilize excess electricity in grids with high renewable energy penetration. These models were integrated into WUFI® Plus building simulation software) and validated with experimental data, particularly for TABS and HTSS. The authors highlighted the potential of these systems to cover up to 90% of residential heating demand using surplus electricity, with performance depending on storage size and building geometry. It was finally concluded that despite promising results, real-world deployment faces challenges related to construction specifics, system performance, and user behavior. Dec et al. [130] proposed a hybrid energy system combining PV panels, solar thermal collectors, and a ground-source heat pump, with an emphasis on integrating a BTES. It was stated that by seasonally reversing the flow in the ground heat exchanger, excess solar heat can be stored underground, thereby raising soil temperature and improving the heat pump’s efficiency. In the case of a study of a school building, this approach reduced annual electricity demand and PV system size by 23%, and heat pump energy use by 38%. Sornek et al. [131] introduced a novel configuration of solar chimney with sensible heat storage designed to reduce energy consumption in residential buildings. The chimney uses an accumulation material that stores solar heat during the day and preheats ventilation air. Laboratory tests and TRNSYS-based simulations confirmed its effectiveness, showing that airflow direction, volume, and the number of heated walls significantly influence thermal performance. The model closely matched experimental data, with minimal temperature deviation. The system proved a promising alternative to traditional brick chimneys or a thermal storage wall. Lindholm et al. [132] introduced a novel optimization method for seasonal energy storage that uses interval halving to solve complex mixed-integer linear programming problems. Applied to a system with a reversible solid oxide cell (RSOC), the method enabled efficient conversion of PV electricity into hydrogen and back into electricity and heat. Compared to a reference system without storage, the RSOC and hydrogen setup significantly improved the use of onsite renewable energy. Hailu et al. [133] presented more than one year of monitoring data from a solar-powered TES system in Palmer, Alaska—a region with long, harsh winters and high energy costs. The analyzed system combined PV and solar thermal collectors with an SHS unit installed beneath an unused garage. Temperature sensors and TRNSYS simulations were used to evaluate performance, revealing a peak ambient garage temperature of ~28 °C (compared to ~22 °C in simulations). Findings confirmed that seasonal solar thermal storage is a cost-effective solution for cold climates, such as Alaska, where energy costs are 3–5 times higher than the national average.

4.3. Selected Studies of the Testing Prototypes of Systems Dedicated to NZEBs and NZEDs

Considering the requirement to meet a zero-carbon standard by buildings, Homa et al. [134] proposed a solution based on long-term, high-temperature heat storage using a sand bed. This work demonstrated the feasibility of the storage system for multiple charging strategies and air–heat-exchanger discharge. An insulated, 300 kg sand-filled tank was constructed as a laboratory prototype to validate an ANSYS Workbench 2024 R2 model, which was subsequently used to guide efficiency improvements. Continuous, off-peak (G12), and PV-driven charging showed adaptability to changing energy prices and renewable output. The results indicate a cost-effective, low-impact solution applicable to building heating and industrial uses, and promising for scalable, long-term storage. Trevisan et al. [135] investigated an innovative radial-flow high-temperature PBTES system designed for large-scale, fossil-free energy applications. The laboratory-scale prototype had an energy capacity of 49.7 kWh. The system operated with dry airflow at 25–700 °C; tested charge temperatures ranged from 500 to 700 °C, and discharge temperatures ranged from 25 to 100 °C. Experimental tests assessed the impact of varying heat transfer fluid flow rates and inlet temperatures on system performance. The tested system achieved a maximum thermal efficiency of 71.8%, with uniform temperature distribution in the inner core but notable porosity effects in the outer regions. Thermocline degradation was identified as a key challenge, affecting thermal uniformity and efficiency. A 10% reduction in the heat transfer fluid flow rate resulted in a 12.5% decrease in efficiency, underscoring the importance of precise flow control. Abrha et al. [136] explored the thermal performance of a PBTES system designed for small-scale power generation. This system used the air-rock bed. Using ANSYS-Fluent for CFD simulations and a pilot-scale experimental setup, the system was analyzed under both natural and forced convection conditions. The storage unit, filled with granite pebbles, was heated from the bottom to 550 °C. Results showed that natural convection was inefficient, with minimal temperature rise beyond 10 h of charging. In contrast, forced convection—enhanced by a small fan—significantly improved heat transfer, reducing charging time from 60 to 5 h and increasing stored energy from 50 to 70 MJ. Forced convection raised average temperatures more effectively and achieved higher energy storage at various porosities. The study concludes that forced convection is a practical solution for PBTES systems, especially when paired with waste heat or solar collectors. Cascetta et al. [137] presented experimental and numerical research on a PBTES system developed at the University of Cagliari, Italy. The discussed system consisted of an insulated steel tank with a net volume of approximately 0.5 m³, filled with small alumina spheres having an average diameter of 8 mm. Hot air was used as the heat transfer fluid. An electric heater heated the air to 300 °C. Charging and three 3-way valves handled discharge modes. The proposed system was tested under varying operating conditions, including mass flow rate, temperature thresholds, and tank aspect ratio. Two simulation models were developed: a 1D model in Matlab-Simulink and a 2D axisymmetric CFD model in ANSYS Fluent. In both cases, a transient two-equation approach was implemented to resolve fluid and solid temperatures. Moreover, the CFD model incorporated additional factors, including radial porosity, thermal losses, and bead conductivity. Experimental results showed good agreement with simulations, though the 1D model was limited in predicting stored energy due to wall effects. On the other hand, the 2D model provided a more accurate representation of the system’s thermal behavior. Tests revealed that higher aspect ratios improved charging efficiency, whereas imposing outlet temperatures limited energy recovery. In addition to technology assessment, available studies are focusing on the analysis of material properties. For example, Jafari et al. [138] discussed the potential of recycled construction materials—glass, asphalt, ceramic, and concrete—as substitutes for natural sand in low-temperature TES systems. After processing the materials to uniform grain sizes, their chemical and thermal properties were assessed during a six-hour charging cycle at 60 °C. Although sand exhibited the highest thermal conductivity and density, concrete showed the greatest specific heat capacity and thermal storage performance among the recycled materials. Ceramics offered rapid energy release, and sand excelled in volumetric energy density. Overall, the findings suggest that recycled concrete is a promising material for TES applications, particularly for affordable water heating solutions in underserved communities. Anagnostopoulos et al. [139] demonstrated that waste foundry sand can be effectively combined with sodium nitrate (NaNO₃) to create composite phase change materials suitable for thermal energy storage. The resulting material showed thermal stability up to 400 °C, a heat capacity of 628 ± 27 kJ/kg, and a thermal conductivity of 1.38 W/(m·K), indicating suitability for medium- to high-temperature storage and supporting the sustainable reuse of industrial waste. On the other hand, Tetteh et al. [140] examined various models for effective thermal conductivity in sand and selected the most accurate one for numerical simulation of a sand bed. It also explored enhancing sand’s thermal performance by adding discarded metallic chips. Experiments with three types of metals and two integration methods revealed that mixing sand with 20% aluminum chips (by volume) significantly improved heat transfer, achieving a heat rate 1.7 times higher than that of pure sand. Schlipf et al. [141] investigated high-temperature PBTES systems using fine-grained materials, such as silica sand, quartz gravel, and basalt. It was found that particles with grain sizes of 2 mm or less exhibit a sharply defined temperature profile during charging, indicating efficient, localized heat transfer within the storage medium. Diago et al. [142] analyzed seven sand samples from various locations in the United Arab Emirates, focusing on their physical and chemical properties. The authors found that high calcium content and elevated temperatures (650–1000 °C) can hinder heat flow in thermal storage systems. Additionally, the specific heat of sand increases with temperature. Compared to molten salt systems, sand-based heat storage offers a significant cost advantage—up to 90% savings on material—and supports much higher operating temperatures. Niksiar et al. [143] evaluated the thermal performance of silica sand as an SHS medium in a shell-and-tube system using water as the heat transfer fluid. Fine sand, with smaller particles, showed better heat transfer than coarse sand, requiring less time to charge (11.86 h vs. 13.36 h) but slightly more time to discharge (17.58 h vs. 16.55 h). To enhance performance, various copper fin configurations were tested with fine sand. Systems with radial fins showed the greatest improvement—eight radial fins reduced charging and discharging times by 63.74% and 78.5%, respectively. Annular fins also improved efficiency, with twenty fins cutting charging time by 56.24% and discharging by 68.26%. These enhancements demonstrated the potential of finned silica sand systems for efficient thermal energy storage. Ananth and Selvakumar [144] evaluated the thermal properties of Manufactured Sand (M–Sand) and Plaster Sand (P–Sand) as alternatives to River Sand for thermal energy storage applications. Using a custom experimental setup, the authors analyzed particle characteristics, thermal conductivity, heat flow, and mass loss. Results showed that M–Sand and P–Sand exhibited lower mass loss and faster heat stabilization compared to River Sand, with M–Sand offering the highest energy storage capacity. These findings support the use of these sustainable materials to reduce reliance on River Sand, especially under India’s regulatory constraints.

4.4. Case Studies of the Existing Seasonal Thermal Energy Storage Installations

Regarding seasonal thermal energy storage, numerous studies on existing installations have been reported in the literature. Paksoy et al. [145] reported a feasibility study for an ATES system in a hospital in Adana, Turkey. This system was designed to combine solar energy and provide heating and cooling to the hospital by storing solar heat underground in the summer and cold in winter. As shown, approximately 7000 MWh of energy can be stored annually in the cold section of the aquifer. This energy can be injected during winter over a period of approximately 2000 h. Then, in the summer, approximately 6500 MWh of the stored energy can be utilized to cool the hospital for around 3000 h (17 h per day). This significantly reduces reliance on conventional chillers, resulting in electricity savings of around 3000 MWh. Additionally, the analyzed system can contribute to preheating ventilation air by extracting 7000 MWh of thermal energy from the warm part of the aquifer. This process allows for substantial fuel savings—it was estimated to be around 1000 m³ of oil annually. Another example of an ATES system used in a hospital was discussed by Vanhoudt et al. [146]. In this case, an aquifer thermal energy storage system was combined with a heat pump to provide heating and cooling for the ventilation air in a hospital in Belgium. Groundwater flow and temperature, along with the energy performance of heat pumps and the building’s overall energy demand, were monitored. The resulting energy balance showed that the heat pump system’s primary energy consumption was 71% lower than that of a conventional setup using gas-fired boilers and water chillers. This reduction translated into a total CO₂ savings of 1280 tons over the monitoring period. Consequently, seasonal performance factors (SPFs) of 5.9 for heating and 26.1 for cooling were estimated. Guo et al. [147] discussed a 50,000 m³ BTES system developed in China to harness both industrial waste heat and solar energy. Supply temperatures varied by source, ranging from 60 °C to 90 °C. The core ground temperature reached 40 °C, with further increases expected over time. During the first heat injection phase, 33,458.6 GJ of thermal energy was stored, increasing the average soil temperature from 10.0 °C to 35.6 °C. The core reached 40.2 °C. The study highlighted the technical viability of large-scale BTES systems for enhancing district heating flexibility, resilience, and energy efficiency. Other examples of BTES systems dedicated to meet heating, cooling, and domestic hot water needs in buildings were discussed by Giordano et al. [148]. The authors highlighted examples from Canada and Northern Europe that demonstrate the reliability and convenience of such systems, resulting in both energy and economic savings. Moreover, a field-scale BTES lab was built in unsaturated alluvial deposits near Torino, Italy, to study thermal injection and storage processes. In its first year (from April to the middle of October), 9.1 GJ of thermal energy was stored, raising the ground temperature by 2 °C. During the analyzed period, the ground collected approximately 17% of the total thermal energy. The lab showed that the Po Plain’s soil is suitable for BTES and can support future efficiency improvements. The direct coupling of solar panels and boreholes proved ineffective; however, short-term storage could potentially improve performance. Simulations revealed that a dual-cycle operation (charge and discharge phases) could reduce thermal impact, and that insulation around boreholes could double efficiency. Zhu et al. [149] developed and experimentally tested a solar STES system connected with a ground-source heat pump in a group of new buildings of the new campus in Tianjin, China. The system comprised 1500 m² of solar thermal collectors and 580 boreholes, each 120 m deep. A mathematical method was introduced to calculate the coefficient of performance (COP) for both the ground-source heat pump and the combined system. After one year of operation, soil temperature increased by 0.21 °C, reversing the previous trend of annual temperature decline. The COP of the overall system and the heat pump was improved by 3.4% and 2.4%, respectively, compared to the operation data without the solar seasonal storage process. Although the performance boost was modest, it helped prevent further deterioration of COP. The study suggests that long-term use of solar seasonal storage can improve the efficiency of ground-source heat pumps. Compared to traditional urban heating and gas boiler systems, the combined approach offered better environmental and economic benefits. The results obtained by the authors may be useful for further development and implementation of such systems for sustainable building energy solutions. In contrast to ATES and BTES installations, Hesaraki et al. [150] studied the performance of a stratified seasonal hot water storage tank with a heat pump in a single-family house in Stockholm, Sweden. The system was tested with three types of space-heating emissions—medium, low, and very low temperature—to evaluate its efficiency and design parameters. A MATLAB-based model was developed to simulate hourly heat demand, solar heat production, and the operation of a backup heat pump. A total of 108 scenarios were analyzed, varying collector area (30–50 m²), storage volume-to-collector area ratio (2–5 m³/m²), and tank height-to-diameter ratio (1.0–2.0). Results showed that the most efficient configuration included a 50 m² collector area, a 5 m³/m² storage ratio, and a 1.0 height-to-diameter ratio. It was stated that very low-temperature heating systems significantly reduced heat pump workload due to higher solar fraction and COP. The buried tank design minimized thermal losses by maintaining a stable ground temperature. In general, the study discussed highlighted the importance of optimizing system parameters based on the type of heat emission to maximize seasonal storage efficiency. Bauer et al. [151] discussed the example of the first solar DH system with STES for existing buildings in Eggenstein-Leopoldshafen, Germany. This system was developed to supply solar heat for space heating and hot water to a retrofitted school, sports center, fire station, and swimming pool. The system was equipped with 1600 m² of solar thermal collectors and a 4500 m³ PTES made of gravel/water, chosen for its cost-effectiveness and safety. The PTES was integrated into the schoolyard, featuring an open-air classroom and playground. A 15 kW heat pump enhanced storage discharge by lowering the usable temperature to 10 °C, thereby increasing thermal capacity. Moreover, the conventional backup system included two 600 kW gas boilers and a 30 m³ buffer tank. The system achieved a planned solar fraction of 35–40% and demonstrated how STES can be effectively integrated into existing infrastructure. Another SHS technology, packed–bed thermal energy storage, was analyzed by Sambo et al. [152]. The authors investigated the feasibility of preheating lumpy manganese ores to 600 °C using air heated to 750 °C in a packed-bed system. Experimental tests were conducted in a pilot-scale shaft column, and a one-dimensional three-phase numerical model was developed to simulate the heat transfer process. The model, validated using temperature data, showed good agreement with experimental results, with maximum errors below 25 °C. Convection was identified as the dominant heat transfer mechanism, while radiation and particle-to-particle conduction had minimal impact. Although the model lacked radial temperature distribution data, it effectively predicted solid-phase temperatures. The study highlighted the potential of using air as a heat-transfer fluid and provided valuable data for the design and control of manganese ore preheating systems. Szewerda et al. [153] described the world’s first commercial “sand battery” launched in Kankaanpää, Finland, to provide sustainable heating. The system stores thermal energy in a steel container filled with 100 tons of sand, capable of maintaining temperatures of 500–600 °C for months. Powered by surplus or renewable electricity, especially from photovoltaics in summer, the stored heat is used in winter to warm residential houses, offices, and a public pool, and can also support industrial processes. With a heating capacity of 100 kW and an energy capacity of 8 MWh, the sand battery provides a durable, cost-effective solution for seasonal energy storage. Another example of a high-temperature thermal storage system is Magaldi Green Thermal Energy Storage (MGTES), which uses fluidized sand beds to store thermal energy. The system uses silica sand to store heat for periods ranging from 8 h to several weeks with minimal losses. It operates in three phases: charging with renewable or grid electricity while sand is fluidized; storing by deactivating fluidization to reduce heat loss; and discharging by releasing the stored energy as superheated steam for industrial use. The setup enables efficient, high-temperature thermal storage above 600 °C, supporting P2H applications [154].

4.5. Long-Term Outlook

The above-discussed literature converges on a central insight: integrating P2H with appropriately scaled STES is a high-impact strategy for decarbonizing buildings and district heating systems. This potential is contingent upon three critical conditions: (i) market mechanisms that recognize and reward flexibility and curtailment mitigation; (ii) control systems capable of dynamic, responsive operation; and (iii) technology choices aligned with local climate conditions, load profiles, and siting constraints. When these prerequisites are met, integrated systems consistently achieve high renewable energy shares, operational resilience, and cost competitiveness. In their absence, performance outcomes diverge, leading to inconsistencies or contradictions.

Further studies on integrating P2H, STES, and VRE technologies are needed to fully explore the potential of integrating various sources, storage, and consumers into a comprehensive system. The above-described solutions are reliable, mature, and relatively inexpensive, making them the most attractive options compared to others. From this standpoint, they may be treated as rapid solutions for the energy transformation process. On the other hand, integrating growing volumes of VRE into the grid through P2H and STES could be significantly enhanced by leveraging widespread residential demand-side management (DSM). It could shift heat production from periods of low electricity prices and high VRE output to times of higher demand, all while maintaining thermal comfort. When managed by predictive control systems—such as model-predictive or data-driven algorithms that incorporate, for example, weather, pricing, and occupancy forecasts—these setups can reduce energy curtailment, flatten peak loads, alleviate distribution grid congestion, and lower both operational costs and emissions. However, widespread adoption faces several barriers, including high initial storage investment costs, limited device compatibility, unclear valuation of flexibility under current market structures, and concerns about user comfort and cybersecurity. Advancing these systems will depend on the development of standardized communication protocols, dynamic pricing models, incentives for flexibility, neighborhood-scale virtual power plant aggregation, and the technical expertise required for installation and maintenance.

The long-term outlook for integrating VRE, STES, and P2H technologies is highly promising. Considering currently available technologies and ongoing research, one of the most attractive options is the integration of renewable energy sources (such as solar and wind) with power-to-heat solutions (electric heaters or heat pumps), combined with seasonal high-temperature heat storage (such as “sand batteries”). This configuration, supplemented by predictive control algorithms, could provide an effective pathway toward nearly zero-energy and positive-energy districts.

5. Conclusions

The integration of Power-to-Heat and seasonal thermal energy storage technologies with variable renewable energy sources presents a promising pathway toward low-carbon energy systems. Despite notable advancements in renewable electricity and heat generation, energy storage remains a critical component for overcoming the intermittency and variability of VRE. P2H technologies, by converting surplus renewable electricity into thermal energy, offer a flexible and scalable solution for reducing fossil fuel dependency, particularly in residential and district heating applications. Their compatibility with low-temperature networks and NZED concepts further enhances their relevance.

STES systems, capable of storing thermal energy over extended periods, complement P2H by addressing seasonal mismatches between energy supply and demand. Among the various TES options, sensible heat storage stands out for its simplicity and widespread use. In contrast, other forms, such as latent and thermochemical storage, offer additional benefits in terms of efficiency and scalability. The diversity of STES configurations—such as ATES, BTES, PTES, and TTES—allows for tailored applications depending on geological and spatial conditions.

Although existing studies demonstrate the feasibility of integrating these technologies, real-world implementations remain limited. Expanding their deployment, especially in NZEB and NZED contexts, requires further research and development. Overall, the synergistic integration of VRE, P2H, and STES holds substantial potential for decarbonizing the heating sector, improving energy efficiency, and fostering a resilient and sustainable energy future.

Looking ahead, future research should focus on large-scale demonstration projects that combine P2H and STES with VRE sources. Particular attention should be given to (i) the development of high-temperature and hybrid TES materials, (ii) advanced control algorithms and digital twins for real-time system optimization, and (iii) comprehensive techno-economic assessments reflecting regional climatic and regulatory conditions. Furthermore, policy frameworks and market incentives should recognize the flexibility that TES systems provide. A coordinated approach integrating technological, economic, and policy perspectives will accelerate the deployment of P2H–STES systems and contribute decisively to the realization of fully decarbonized, resilient district heating networks.