A rigorous and systematic literature search was conducted to capture a comprehensive scope of scholarly and grey literature related to advanced thermal energy systems. The search spanned major scientific databases, including Scopus, Web of Science, ScienceDirect, IEEE Xplore, Google Scholar, and SpringerLink, as well as authoritative grey literature sources such as reports and publications from the International Energy Agency (IEA), International Renewable Energy Agency (IRENA), United Nations Environment Programme (UNEP), World Bank, and Intergovernmental Panel on Climate Change (IPCC). The search strategy employed Boolean logic and advanced keyword combinations encompassing terms such as “thermal energy systems,” “Rankine cycle,” “thermal energy storage (TES),” “phase change materials (PCMs),” “supercritical steam power,” “waste heat recovery,” “organic Rankine cycle (ORC),” “solar-thermal hybrid systems,” “AI thermal optimization,” “biomass CHP,” “nano-enhanced TES,” and “digital twins in thermal energy.” This method yielded a total of 262 relevant records, covering literature published between 2015 and 2025, with a deliberate focus on contemporary advancements that reflect in Innovations in thermal energy systems, bridging traditional and emerging technologies for sustainable energy.

The initial pool of identified records underwent a structured screening process, beginning with a review of titles and abstracts to assess alignment with predefined inclusion criteria. Studies were included if they focused on thermal energy systems applied to power generation, heating, cooling, or hybrid configurations, particularly those incorporating technological innovations such as advanced Rankine cycles, TES, artificial intelligence (AI) applications, or renewable energy integration. Emphasis was placed on literature presenting quantitative data on system performance, energy efficiency, emissions reduction, or techno-economic feasibility. Conversely, studies were excluded if they focused solely on conventional fossil fuel combustion without addressing sustainability enhancements, lacked empirical, simulation-based, or techno-economic evidence, or were limited to grid-tied systems without exploring integration with renewable or smart technologies. After applying these criteria, 193 articles were deemed relevant and selected for full-text evaluation.

Full-text articles were thoroughly assessed to determine their eligibility based on stringent scientific and technical criteria. Priority was given to studies demonstrating scientific rigor, including peer-reviewed publications with substantial citation impact. Technical relevance was evaluated in relation to next-generation TES, advanced Rankine cycle variants, and hybrid configurations integrating renewable sources or digital technologies. Eligible studies also had to clearly articulate outcomes related to environmental performance, energy efficiency, or digitalization, with a particular emphasis on their applicability to global and regional decarbonization strategies, especially within the context of developing economies. Articles that lacked empirical validation, exhibited conceptual ambiguity, or failed to address the integration of hybridized or AI-enhanced TES systems were excluded. Following this comprehensive assessment, 163 high-quality studies were retained for detailed synthesis.

The final inclusion phase resulted in a curated selection of 163 high-quality sources, comprising 129 peer-reviewed journal articles, 12 technical conference proceedings, 8 pilot project and feasibility study reports, and 14 policy and institutional documents. This diverse set of literature provided both empirical evidence and conceptual frameworks essential for the thematic synthesis. The selected sources underpin the analysis of advancements in TES technologies, innovative hybrid system architectures, applications of digital intelligence such as AI and digital twins, and sustainable deployment models aligned with global and regional decarbonization objectives.

Fossil fuel-based thermal systems have long been the dominant source of global energy supply, powering a wide range of applications including electricity generation, industrial heating, and mechanical work (Chen et al., 2023). The primary fossil fuels, coal, oil, and natural gas, have underpinned energy-intensive sectors due to their high energy density, ease of storage and transport, and compatibility with established technologies and infrastructure (Nurdiawati and Urban, 2021; Tuller, 2017). Thermal power plants utilizing these fuels operate by converting chemical energy into heat through combustion, which is then transformed into mechanical and electrical energy via thermodynamic cycles such as the Rankine and Brayton cycles (Tuller, 2017). Coal-fired power plants as shown in Figure 3, typically burn pulverized coal in boilers to produce high-pressure steam, which drives steam turbines (Khalid et al., 2025). Oil-based systems are used in both stationary and mobile applications, including backup generators and marine propulsion, while natural gas, owing to its cleaner combustion profile and flexibility, has become the preferred fuel for modern combined cycle gas turbine (CCGT) plants (Seo et al., 2019). These CCGT plants use gas turbines followed by steam turbines to achieve higher efficiencies, often exceeding 60%, and are well-suited for load-following and peaking operations in modern power grids.

Despite these advantages, the environmental drawbacks of fossil fuel combustion are profound. The process emits large quantities of greenhouse gases (GHGs), most notably carbon dioxide (CO₂), which is the primary driver of anthropogenic climate change (Perera, 2018). Methane (CH₄), a potent GHG with a global warming potential over 25 times that of CO₂ over 100 years, is frequently released during natural gas extraction and transport (Balcombe et al., 2017). In addition to GHGs, fossil fuel combustion emits sulfur oxides (SO_x), nitrogen oxides (NO_x), volatile organic compounds (VOCs), and particulate matter (PM), which contribute to acid rain, smog formation, respiratory diseases, and premature mortality. Beyond atmospheric pollution, the lifecycle of fossil fuel use, from extraction and processing to transportation and combustion, poses significant ecological risks (Howarth et al., 2021; Stern, 2020). Coal mining, particularly through surface and mountaintop removal techniques, leads to deforestation, soil erosion, and water contamination (Burke et al., 2023). Oil drilling, including offshore operations, threatens marine ecosystems through spills and habitat disruption. Hydraulic fracturing (fracking) for shale gas extraction has raised concerns over groundwater contamination, induced seismicity, and land degradation (Deng and Guo, 2024; Xing et al., 2024).

A typical fossil fuel-based thermal processing system is illustrated in Figure 4. Fossil fuel dependence exposes nations to geopolitical and economic vulnerabilities (Gupta and Chu, 2018). Energy supply disruptions due to political instability, trade restrictions, or resource monopolization can lead to energy insecurity and price volatility (Bordoff and O'Sullivan, 2023). Many fossil fuel-rich countries face the “resource curse,” where overreliance on hydrocarbon revenues hampers economic diversification and social development. In recognition of these environmental and socio-economic impacts, international frameworks such as the Paris Agreement (2015) have catalyzed global efforts to decarbonize energy systems (Ives et al., 2021; Cochran and Pauthier, 2019). Governments and regulatory bodies are increasingly implementing carbon pricing mechanisms, emissions trading schemes, and stringent air quality standards to curb fossil fuel use. Financial institutions and investors are divesting from fossil fuel assets in favor of cleaner energy portfolios, reflecting a broader shift toward sustainable development (Samuel, 2025; Narassimhan et al., 2018). Nonetheless, the transition away from fossil fuel-based thermal systems remains a complex and regionally variable challenge. Many developing countries continue to rely on fossil fuels for economic growth and energy access, given the affordability and availability of these resources (Garcia et al., 2024). Bridging this transition requires integrated approaches that combine technological innovation, policy reform, international cooperation, and investment in low-carbon alternatives such as renewables, carbon capture and storage (CCS), and energy efficiency improvements (Eze et al., 2024a; Eze et al., 2024b). Table 1 compares the three fossil fuel-based thermal systems of performance and impact.

The Rankine cycle is a cornerstone of classical thermodynamic engineering and remains one of the most widely utilized systems for thermal-to-mechanical energy conversion, particularly in large-scale power generation (Arabkoohsar, 2020). At its core, the Rankine cycle involves the heating of a working fluid, typically water, into high-pressure, high-temperature steam, which expands through a steam turbine to perform mechanical work before being condensed back into liquid form for recirculation (Konur et al., 2022). This closed-loop process is the operational foundation of most steam-based power plants, including fossil-fueled and nuclear facilities. In its basic configuration, the Rankine cycle consists of four principal processes: isentropic compression in a feedwater pump, constant-pressure heat addition in a boiler, isentropic expansion in a turbine, and constant-pressure heat rejection in a condenser (Ohji and Haraguchi, 2022; Figure 5). The efficiency of the cycle is governed by the temperature and pressure differentials between the heat source and sink, constrained by the second law of thermodynamics. Over the decades, considerable research and development have led to the evolution of the Rankine cycle into more advanced variants, namely, supercritical (SC) and ultra-supercritical (USC) cycles (Mohamed et al., 2020). In these systems, the operating pressures and temperatures exceed the critical point of water (22.1 MPa and 374 °C), eliminating the liquid-vapor boundary and enabling higher thermal efficiencies, typically in the range of 40%–45% compared to 33%–38% for subcritical units. These advancements reduce fuel consumption per unit of electricity generated, thereby lowering emissions intensity and operational costs.

Further improvements include reheating and regenerative feedwater heating, both of which enhance cycle efficiency. Reheat Rankine cycles involve partially expanding the steam in the turbine, returning it to the boiler for reheating, and then expanding it further, thus reducing moisture content and mechanical stress in low-pressure turbine stages (Tiktas et al., 2022). Regenerative cycles preheat feedwater using steam extracted from intermediate turbine stages, reducing the heat input required in the boiler and improving overall efficiency. Despite these technological enhancements, the Rankine cycle’s traditional deployment remains largely dependent on non-renewable heat sources such as coal, oil, and nuclear fission (Bagherian and Mehranzamir, 2020). This dependence not only contributes to greenhouse gas emissions and resource depletion but also limits the adaptability of the cycle to decarbonized energy systems. Although renewable alternatives such as CSP and geothermal systems have begun to implement modified Rankine cycles, their global penetration remains limited due to high capital costs and geographical constraints (Aridi, 2023).

Another critical limitation of Rankine-based systems is their significant water consumption, both for steam generation and for cooling in the condenser. Power plants using once-through or wet recirculating cooling systems withdraw large volumes of freshwater, making them vulnerable in arid or drought-prone regions (Zhu et al., 2025). This raises sustainability and operational risks, particularly in the context of increasing climate variability and water scarcity. Dry cooling systems, though available, entail efficiency penalties and increased capital costs. The Rankine cycle’s compatibility with Carbon Capture and Storage (CCS) technologies in coal-fired plants introduces a potential pathway for cleaner operation. However, the energy penalty associated with CO₂ capture and compression (typically reducing net plant efficiency by 7–10 percentage points) has hindered widespread deployment. In conclusion, while the Rankine cycle remains a mature and reliable workhorse of global thermal power generation, its future viability in a low-carbon energy landscape depends on the integration of cleaner heat sources, efficiency-boosting innovations, hybridization with renewables, and water-conscious cooling technologies. A transition toward flexible, low-emission adaptations of the Rankine cycle is imperative to align with contemporary energy and environmental sustainability targets (Lu et al., 2022). Table 2 compares the features of Rankine and advanced steam cycle technologies in thermal systems.

Biomass has been a fundamental source of thermal energy since prehistoric times, representing one of humanity’s earliest fuels for cooking, heating, and even rudimentary industrial applications (Zhang et al., 2020). It includes a diverse range of organic materials such as wood, agricultural residues (e.g., crop stalks, husks), animal manure, and other biodegradable waste products (Ericsson and Werner, 2016). Biomass combustion releases stored solar energy captured through photosynthesis, making it a potentially sustainable and renewable energy source when managed within a closed carbon cycle (Jaiswal et al., 2023). Traditionally, biomass has been predominantly used in rural and developing regions for domestic cooking and space heating due to its local availability and affordability (Ahmad et al., 2022; Mehetre et al., 2017). However, traditional combustion technologies, such as open fires, simple mud stoves, and basic kilns, are characterized by low thermal efficiency (often below 10%–15%) and incomplete combustion. These inefficiencies generate substantial amounts of smoke, particulate matter, carbon monoxide, and other harmful pollutants, contributing to indoor air pollution that severely impacts respiratory health, especially among women and children (Zhang et al., 2021). The World Health Organization estimates that household air pollution from inefficient biomass combustion causes 3.8 million premature deaths annually worldwide (Balmes, 2019).

Table 3 extensively compares the characteristics of biomass and traditional heating systems. In modern contexts, biomass combustion has been integrated into CHP systems and industrial boilers, where controlled combustion produces both electricity and useful thermal energy with higher efficiencies as depicted in Figure 6 (Rezaei et al., 2021). Biomass CHP plants typically achieve electrical efficiencies around 20%–30% and total efficiencies (electric plus thermal) exceeding 70%, offering an attractive option for decentralized energy generation in rural areas and agro-industrial zones (Uzoagba et al., 2024). Despite these benefits, biomass use is not without significant environmental and sustainability challenges. Unsustainable harvesting practices, including over-reliance on woodfuel, can accelerate deforestation, soil erosion, and habitat loss, undermining biodiversity and ecosystem services (Sreelekshmi and Nandan, 2025). This is particularly concerning in regions with high population pressures and limited forest management policies. The carbon neutrality of biomass combustion hinges on sustainable feedstock management; otherwise, net carbon emissions may exceed those of fossil fuels when land-use changes and biomass regrowth delays are factored in (Makepa and Chihobo, 2024).

To address the limitations of traditional biomass use, there has been a growing emphasis on the development and deployment of advanced biomass conversion technologies (Adams et al., 2018). Gasification converts solid biomass into a combustible synthesis gas (syngas) through partial oxidation, enabling cleaner and more efficient combustion in engines or turbines (Ram and Mondal, 2022). Gasifiers can support small to medium-scale distributed power generation with reduced emissions. Similarly, pellet stoves and boilers utilize densified biomass pellets with standardized moisture and energy content, improving combustion efficiency, fuel handling, and emissions control (Pradhan et al., 2018). Sustainable biomass supply chains are critical to ensuring long-term viability and environmental integrity. This includes responsible sourcing from managed forests and agricultural residues, minimizing competition with food production, and integrating life cycle assessments to optimize carbon and energy balances. Innovations such as anaerobic digestion for biogas production, agroforestry systems, and biochar application further enhance the sustainability of biomass energy (Subbarao et al., 2023). In conclusion, while biomass remains a vital renewable energy resource, particularly in developing regions, its broader adoption and environmental benefits depend on transitioning from inefficient traditional practices to advanced combustion and conversion technologies, supported by sustainable resource management and policy frameworks. These improvements can enhance energy access, reduce health impacts, and contribute meaningfully to climate mitigation goals (Silva et al., 2025). Table 4 presents a comparative analysis of fossil fuel systems, Rankine cycle technologies, and biomass-based systems, as synthesized from sources (Makepa and Chihobo, 2024; Chen et al., 2023; Nurdiawati and Urban, 2021; Tuller, 2017; Khalid et al., 2025; Seo et al., 2019; Perera, 2018; Balcombe et al., 2017; Howarth et al., 2021; Stern, 2020; Burke et al., 2023; Deng and Guo, 2024; Xing et al., 2024; Gupta and Chu, 2018; Bordoff and O'Sullivan, 2023; Ives et al., 2021; Cochran and Pauthier, 2019; Samuel, 2025; Narassimhan et al., 2018; Garcia et al., 2024; Eze et al., 2024a; Eze et al., 2024b; Arabkoohsar, 2020; Konur et al., 2022; Ohji and Haraguchi, 2022; Mohamed et al., 2020; Tiktas et al., 2022; Zhu et al., 2025; Lu et al., 2022; Zhang et al., 2020; Ericsson and Werner, 2016; Jaiswal et al., 2023; Ahmad et al., 2022; Mehetre et al., 2017; Zhang et al., 2021; Balmes, 2019; Rezaei et al., 2021; Uzoagba et al., 2024; Sreelekshmi and Nandan, 2025; Mohanty et al., 2025; Adams et al., 2018; Ram and Mondal, 2022; Pradhan et al., 2018; Subbarao et al., 2023; Silva et al., 2025).

PCMs represent a critical advancement in TES by exploiting the latent heat absorbed or released during phase transitions, typically solid-liquid, to store and release thermal energy at near-constant temperatures, enabling significantly higher energy densities than conventional sensible heat storage (Togun et al., 2024; Patil et al., 2025). This is especially advantageous for applications requiring compact form factors or precise thermal management, such as building energy systems, solar thermal power plants, and electronics cooling. PCMs are categorized into organic (e.g., paraffins, fatty acids), inorganic (e.g., salt hydrates, metallic alloys), and eutectic mixtures, each with unique thermal, chemical, and physical properties (Mehling, 2024). Organic PCMs offer chemical stability and minimal supercooling but have low thermal conductivity and flammability risks; inorganic PCMs generally exhibit higher latent heat and thermal conductivity but suffer from phase segregation, corrosion, and subcooling, complicating long-term durability; eutectics provide customizable melting points but can combine the limitations of both classes (Jebasingh and Arasu, 2020). Key performance criteria include phase transition temperature aligned with operating conditions, high latent heat capacity to maximize energy storage per volume, thermal conductivity to ensure efficient charging/discharging rates, chemical inertness, and cycle stability over thousands of phase changes (Vitorino et al., 2016). Table 5 outlines representative examples and typical compositions of various PCMs used in thermal energy storage applications.

In practical deployments, PCMs have been successfully integrated into building envelope components, such as wallboards, ceiling panels, and window films, where their latent heat absorption reduces peak cooling loads by buffering diurnal temperature swings, leading to up to 30% reductions in HVAC energy use (Draou and Brakez, 2024). In CSP plants, molten salt-based PCMs enable thermal energy to be stored during peak insolation and dispatched during cloudy periods or at night, enhancing plant capacity factors and grid reliability; notable commercial installations like the Gemasolar plant in Spain demonstrate multi-hour storage with molten salt PCMs enabling 24-h power output (Elkelawy et al., 2024). Additionally, PCMs are utilized for thermal regulation in lithium-ion batteries to prevent overheating, extending battery life and safety, and in cold chain logistics for maintaining precise temperature control of pharmaceuticals and perishable goods (Weng et al., 2022). The working principles of PCM are illustrated in Figure 7; Table 6 shows the performance, costs, scalability, and deployment.

Despite these advantages, PCM technologies face significant barriers. The inherently low thermal conductivity of many PCMs limits rapid heat transfer, requiring the integration of conductive fillers such as expanded graphite, carbon nanotubes, or metal foams, as well as engineered heat exchanger geometries to enhance heat exchange rates (Wu et al., 2020). Phase separation and subcooling in salt hydrates degrade storage capacity and reliability over cycling, necessitating encapsulation techniques and chemical stabilizers to maintain integrity. Moreover, chemical incompatibility between PCMs and containment materials poses corrosion or leakage risks, necessitating rigorous material compatibility assessments and protective coatings. The relatively high cost of advanced PCMs and challenges in scalable manufacturing further restrict widespread adoption, particularly in large-scale industrial TES applications (Huang et al., 2021). Current research trends emphasize hybrid TES configurations that combine PCMs with sensible heat storage media, like molten salts or concrete, to synergistically balance energy density and cost while optimizing operational flexibility. Cutting-edge developments in nano-enhanced PCMs, which incorporate nanomaterials to improve thermal conductivity and stability, and “smart” PCMs with tunable phase change properties responsive to external stimuli, are poised to address current limitations and expand PCM applicability across emerging sectors such as wearable thermal management and electric vehicle thermal regulation (Abdullah et al., 2025; Mohtasim and Das, 2024).

TEGs exploit the Seebeck effect, a fundamental thermoelectric phenomenon where a temperature difference across certain conductive or semiconductive materials induces a voltage difference, thus enabling direct conversion of thermal energy into electrical energy without mechanical moving parts (Jaziri et al., 2020). This solid-state energy conversion mechanism offers several intrinsic advantages, including high reliability, minimal maintenance, compactness, and scalability, making TEGs well-suited for applications requiring robust and long-lasting power sources. The operational principle of a TEG involves creating a temperature gradient across a thermoelectric module composed of pairs of n-type and p-type semiconductor elements connected electrically in series and thermally in parallel (Siddique et al., 2017). Heat absorbed at the hot junction generates charge carrier diffusion, driving an electrical current through an external load at the cold junction.

Nanofluids are engineered colloidal suspensions consisting of nanometer-sized particles, typically metals (e.g., copper, silver), metal oxides (e.g., Al₂O₃, TiO₂), carbides (e.g., SiC), or carbon-based materials such as graphene and carbon nanotubes, dispersed in conventional heat transfer fluids like water, ethylene glycol, or mineral oils (Hussain, 2016). These nanoparticles significantly modify the thermophysical properties of the base fluids, most notably enhancing thermal conductivity, specific heat capacity, and convective heat transfer coefficients (Solangi et al., 2015). The underlying mechanisms for these enhancements include increased surface area for heat exchange, intensified Brownian motion, liquid layering at the solid–liquid interface, and altered micro-convection patterns within the fluid medium. Consequently, nanofluids have emerged as promising candidates for boosting heat transfer performance in a wide array of applications, including microchannel heat sinks for electronics cooling, compact automotive radiators, solar thermal collectors, industrial boilers, and high-efficiency heat exchangers (Mehta et al., 2022).

Beyond their superior thermal conductivity, nanofluids also offer operational benefits such as reduced pumping power requirements for equivalent thermal outputs, owing to improved convective heat transfer performance and lower viscosity increases compared to conventional additives (Younes et al., 2022). These advantages make nanofluids particularly appealing for compact and energy-efficient systems, especially where enhanced heat dissipation is essential under space-constrained or high-heat-flux conditions. However, the practical implementation of nanofluids is currently hindered by several challenges. These include nanoparticle agglomeration and sedimentation over time, which compromise fluid stability and thermal performance; increased risk of component erosion and fouling due to particle abrasion; and uncertainties related to the environmental toxicity and health implications of nanoparticle exposure (Gul et al., 2025). To mitigate these limitations, ongoing research focuses on advanced nanoparticle synthesis methods (e.g., sol-gel, hydrothermal, and laser ablation techniques), surface functionalization using surfactants or chemical coatings to improve colloidal stability, and the development of eco-friendly, biodegradable nanomaterials (Mohite et al., 2024). Furthermore, comprehensive lifecycle analyses and techno-economic evaluations are being conducted to assess the scalability, safety, and sustainability of nanofluid technologies in real-world systems. As these scientific and engineering barriers are systematically addressed, nanofluids are poised to play a transformative role in next-generation thermal management systems, contributing significantly to the efficiency and sustainability of both conventional and renewable energy platforms (Gul et al., 2025; Mohite et al., 2024). Table 8 illustrates the performance, cost, scalability, and deployment status of Nanofluids.

The integration of AI, ML, and advanced optimisation methodologies is fundamentally transforming the design, control, and operational efficiency of thermal energy systems (Shaban et al., 2024; Eze et al., 2025a). These intelligent technologies enable the development of predictive and adaptive control frameworks capable of managing the dynamic behavior of complex thermal infrastructures, including district heating networks, HVAC (Heating, Ventilation, and Air Conditioning) systems, CHP units, and industrial process heat exchangers (Mira et al., 2023). AI-driven models, leveraging historical and real-time data from sensors, supervisory control systems, and external environmental inputs (e.g., weather forecasts, occupancy levels, production schedules), are used to forecast thermal loads, optimize dispatch strategies, identify operational anomalies, and proactively recommend control actions (Thapa, 2022). Advanced optimisation algorithms, including Genetic Algorithms (GA), Particle Swarm Optimization (PSO), Ant Colony Optimization (ACO), and Reinforcement Learning (RL), have been successfully applied to improve thermal energy distribution efficiency, reduce energy consumption, minimize operational costs, and enhance the longevity of thermal equipment (Li et al., 2023). Reinforcement learning, in particular, supports autonomous decision-making in highly dynamic environments by learning optimal policies through interaction with system states and feedback mechanisms. Moreover, hybrid models combining physics-based simulations with data-driven AI approaches have shown superior performance in capturing the nonlinear and time-variant characteristics of thermal processes (Sarmah, 2019). Comparative Analysis of AI and Optimization Techniques in Thermal Energy Systems is shown in Table 9.

AI also plays a critical role in facilitating the seamless integration of intermittent renewable energy sources, such as solar thermal collectors, geothermal units, and biomass boilers, into thermal networks, along with energy storage systems (e.g., phase change materials, hot water tanks) (Ukoba et al., 2024). Through real-time optimization and load balancing, AI enables enhanced flexibility and resilience of thermal grids under varying supply and demand conditions. The advent of digital twin technologies, virtual replicas of physical systems, augmented with AI and IoT connectivity, further strengthens operational intelligence. These platforms offer high-fidelity simulations, real-time diagnostics, and predictive maintenance capabilities, significantly reducing unplanned downtime, extending asset lifespans, and improving system-level reliability and economic performance. As AI and optimisation techniques continue to evolve, their role in achieving sustainable, efficient, and intelligent thermal energy systems is becoming increasingly indispensable (Raghuvanshi et al., 2025). Table 10 provides a comparative analysis of emerging technologies in thermal energy systems, highlighting their operational principles, efficiency metrics, advantages, and limitations.

Figure 9 presents a comparative analysis of traditional and emerging thermal energy technologies in terms of efficiency and capital cost. Traditional systems, fossil-fuel boilers (≈35%), steam turbines (≈42%), and CSP (≈32%), show moderate efficiencies but relatively higher cost ranges. Emerging technologies display a wider performance spectrum: PCMs achieve notably higher efficiency (≈80%) at the lowest capital cost, underscoring their potential for transformative thermal storage. In contrast, thermoelectric generators (≈12%) and nanofluids (≈18%) remain at lower efficiency levels, reflecting their developmental stage. AI-based optimization systems (≈28%) provide moderate gains with only marginal cost increases (+5–20%). Overall, the comparison highlights PCMs as the most promising solution while emphasizing the need for further R&D to enhance other next-generation approaches.

TES technologies play a pivotal role in overcoming the temporal disparities between renewable energy generation and end-use demand, thus ensuring energy reliability and operational flexibility (Elkhatat and Al-Muhtaseb, 2023; Eze et al., 2024c). In solar energy systems, particularly CSP plants, TES is indispensable for enabling dispatchable electricity production. Among the most mature and commercially implemented solutions is molten salt storage, wherein a mixture of sodium and potassium nitrates stores thermal energy at high temperatures exceeding 500 °C (Codd et al., 2020). This heat can later be used to generate steam and drive turbines, even after solar irradiance declines (Yang et al., 2025). Iconic installations such as the Crescent Dunes Solar Energy Plant in the United States and Gemasolar in Spain demonstrate the feasibility of this technology, achieving over 15 h of thermal autonomy and supporting baseload electricity generation (Camacho et al., 2024). In the context of wind energy, which inherently produces fluctuating electrical outputs, TES can serve as an effective indirect storage mechanism. During periods of low demand and high generation, excess electricity can be converted into thermal energy using resistive heating elements, heat pumps, or electro-thermal energy storage units. The resulting thermal energy can then be stored in media such as hot water tanks, PCMs with high latent heat capacities, or thermochemical storage systems that leverage reversible chemical reactions for high-density storage (Togun et al., 2024; Mehling, 2024; Camacho et al., 2024).

Underground Thermal Energy Storage (UTES) systems, including Aquifer Thermal Energy Storage (ATES) and Borehole Thermal Energy Storage (BTES), offer scalable, seasonal solutions by exploiting the natural thermal inertia of subsurface geological formations (Brown et al., 2024). These systems are especially effective for large-scale district heating and cooling networks in urban environments, providing high volumetric energy density, minimal thermal losses, and long operational lifespans. Additionally, ice thermal storage, widely used in commercial buildings and HVAC systems, stores cooling energy by producing ice during off-peak periods, which is then used for air conditioning during peak hours, significantly reducing grid stress and operational costs (Brown et al., 2024; Eze et al., 2025b). Overall, the integration of TES with solar and wind energy enhances the stability, dispatchability, and cost-effectiveness of renewable power systems, facilitating a transition toward resilient and low-carbon energy infrastructures.

Hybrid solar-thermal systems, particularly Photovoltaic-Thermal (PVT) configurations, offer a synergistic approach to maximizing the utility of solar radiation by concurrently generating electricity and harvesting thermal energy within a single integrated system (Verma et al., 2022). In PVT systems, conventional photovoltaic (PV) modules are coupled with thermal collectors, allowing a working fluid, typically air, water, or a refrigerant, to extract waste heat from the PV surface. This dual functionality not only produces usable heat for direct consumption or storage but also enhances electrical efficiency by lowering the operating temperature of the PV cells, which is known to degrade performance under thermal stress (Pandey et al., 2025). These systems are highly effective in cogeneration applications where simultaneous demand for electricity and heat exists, such as in residential water heating, space heating and cooling, greenhouse temperature regulation, and small-scale industrial processes (Sarvar-Ardeh et al., 2024). Their modular design makes them particularly well-suited for deployment in remote, off-grid, or energy-poor regions, where infrastructure for separate thermal and electrical systems is either lacking or economically infeasible. When paired with TES systems, such as water tanks, PCMs, or thermochemical storage, PVT installations can deliver round-the-clock thermal services and buffer the inherent intermittency of solar irradiance (see Figure 10) (Verma et al., 2022; Sarvar-Ardeh et al., 2024).

Recent innovations in Concentrated Photovoltaic-Thermal (CPVT) systems further extend the utility of hybrid technologies by employing optical concentrators (e.g., Fresnel lenses or parabolic mirrors) to focus solar energy onto high-efficiency PV and thermal receivers (Wang et al., 2025). These systems achieve significantly higher energy densities, making them suitable for medium-to high-temperature industrial processes. Additionally, solar-assisted absorption refrigeration technologies, which utilize captured thermal energy to drive cooling cycles via lithium bromide or ammonia-water absorption systems, enable solar-based cooling solutions for both residential and industrial applications. Altogether, hybrid solar-thermal systems represent a promising paradigm for multi-functional, decentralized, and sustainable energy generation, advancing energy efficiency, reducing land use per unit of output, and lowering dependence on fossil-based thermal energy sources (Cameron et al., 2022).

CSP plants represent a cutting-edge integration of renewable energy generation and TES, offering dispatchable and stable electricity in regions with high solar irradiance. A flagship example is the Noor Ouarzazate Solar Complex in Morocco, which stands among the world’s largest CSP facilities. The complex features a combination of parabolic trough collectors and a central solar tower system, both equipped with molten salt TES to store thermal energy at temperatures exceeding 500 °C (Laaroussi et al., 2023). This enables the facility to supply electricity continuously for up to 7–8 h after sunset, providing baseload capacity and significantly reducing reliance on fossil fuel-based peaking plants (Laaroussi et al., 2023; Salime, 2021).

The Noor complex exemplifies how TES empowers time-shifting of solar energy, enabling generation during periods of low or no irradiance and enhancing grid flexibility. Additionally, the plant contributes to national energy security and climate commitments by displacing conventional generation and lowering carbon emissions. A similar achievement is demonstrated by Gemasolar in Spain, which utilizes a central tower receiver and a molten salt storage system with over 15 h of thermal autonomy, achieving near-constant solar power generation. Unlike intermittent photovoltaic systems, CSP-TES configurations provide both energy storage and controllability, making them ideal for large-scale grid integration and hybridization with other renewable or backup sources (Benbba et al., 2024). These high-impact examples underscore the technical viability, economic attractiveness, and environmental benefits of CSP-TES systems. Their success validates their potential as scalable solutions for decarbonizing power sectors in solar-rich countries and as strategic infrastructure for meeting net-zero energy goals, particularly in desert and semi-arid regions where solar resources are abundant and consistent (Laaroussi et al., 2023; Benbba et al., 2024).

The integration of TES into smart thermal grids, primarily district heating and cooling systems, constitutes a transformative advancement in urban energy infrastructure. These grids are fundamental to achieving decarbonization, operational flexibility, and cost efficiency in increasingly electrified and renewable-based energy systems. By embedding TES solutions within these networks, cities can effectively decouple heat generation from consumption, enabling better alignment of thermal supply with variable demand profiles while harnessing intermittent renewable sources.

In Denmark, a global leader in district heating innovation, cities such as Aalborg exemplify this integration through the deployment of large-scale hot water storage tanks and UTES facilities (Johan et al., 2022). UTES leverages geological media such as aquifers or boreholes to store thermal energy seasonally, enabling heat captured during warmer months, often from excess renewable generation or waste heat, to be retained and dispatched during colder periods. The coupling of TES with AI-driven demand forecasting and optimization algorithms allows the system to dynamically adjust heat production from sources like solar thermal arrays, biomass boilers, and wind-powered heat pumps. These advanced control strategies optimize operational scheduling, reduce reliance on fossil fuel peaking units, and improve cost-effectiveness by shifting energy use to off-peak periods (Johan et al., 2022).

Sweden’s district energy networks showcase further innovation by integrating seasonal TES with large-scale heat pump technology and extensive industrial waste heat recovery infrastructure (Brange et al., 2016). These networks benefit from sophisticated ML and AI frameworks that continuously analyze vast datasets, from meteorological forecasts to consumer behavior, to optimize thermal supply chains holistically. By managing generation assets, TES dispatch, and user consumption patterns, these intelligent systems enhance load matching, minimize peak demand spikes, and maximize renewable heat utilization (Brange et al., 2016). The digital convergence of TES, AI, and renewable heat sources significantly improves system resilience, allowing the grid to adapt in real-time to fluctuating external conditions such as weather variability, grid constraints, and dynamic occupancy levels. Moreover, this integration supports demand response initiatives and predictive maintenance, which collectively lower operational costs and extend infrastructure lifespans (Fernqvist et al., 2023).

Importantly, smart thermal grids act as critical enablers for urban sustainability and climate action plans by enabling deep decarbonization of heating and cooling, a sector traditionally reliant on fossil fuels and representing a major share of urban carbon emissions (Khaleel et al., 2024). Through the flexible integration of TES and digital controls, these grids reduce energy waste, promote efficient utilization of renewable resources, and foster the transition towards net-zero emissions in cities. Overall, the Danish and Swedish cases provide replicable models demonstrating how the fusion of digitalization, TES technologies, and renewable energy integration forms the backbone of next-generation thermal grids, essential for sustainable, resilient, and low-carbon urban energy systems globally (Fernqvist et al., 2023; Khaleel et al., 2024).

Industrial sectors in Germany and Japan have been at the forefront of adopting advanced waste heat recovery technologies, driving significant improvements in energy efficiency, cost reduction, and carbon emissions mitigation.

In Germany, particularly within the cement and steel manufacturing industries, the integration of ORC systems has become widespread. These systems efficiently convert medium-temperature waste heat, often in the range of 150 °C–300 °C, into electricity by utilizing organic working fluids with low boiling points (Pili et al., 2020). The implementation of ORC technology in these plants has led to enhanced energy self-sufficiency, reducing grid electricity dependence and increasing overall process efficiency by up to 20%. When combined with TES units, the recovered thermal energy can be temporally shifted, stored, and dispatched during peak load periods or used to supply adjacent industrial or district heating facilities, maximizing utilization and system flexibility (Shahid et al., 2025; Larrinaga et al., 2021).

In Japan, the Energy Conservation Center has catalyzed the diffusion of waste heat recovery solutions across diverse sectors, including chemical manufacturing, automotive production, and general manufacturing (Dou et al., 2018). Leading corporations such as Toyota have integrated CHP systems alongside TES to capture and recycle thermal energy from processes such as engine testing and paint curing. This holistic approach not only curtails energy expenses but also delivers substantial reductions in CO₂ emissions, bolstering the environmental and economic sustainability of manufacturing operations (Thekdi and Nimbalkar, 2015). These exemplary projects underscore the strategic value of coupling TES with waste heat recovery technologies to foster industrial symbiosis, where excess thermal energy from one process is efficiently redirected to others, thereby embodying circular economy principles. Moreover, these initiatives contribute to national decarbonization agendas and energy security objectives by decreasing fossil fuel consumption, enhancing energy system resilience, and promoting sustainable industrial competitiveness. Collectively, they illustrate a scalable pathway for industrial sectors worldwide to transition toward low-carbon, energy-efficient production paradigms.

Governments and international organizations worldwide are increasingly adopting comprehensive regulatory frameworks and incentive mechanisms designed to accelerate the integration of renewable energy and improve overall energy efficiency, with thermal energy technologies playing a critical role in this transition. Key policy instruments such as feed-in tariffs (FITs), renewable portfolio standards (RPS), carbon pricing mechanisms, and investment tax credits directly stimulate capital flows into TES, CSP, and industrial waste heat recovery systems by improving project economics and reducing financial risk (Filho et al., 2023).

Complementing these market-based incentives are targeted research, development, and demonstration (RD&D) funding programs that prioritize next-generation materials, innovative TES system architectures, and advanced digital controls, including artificial intelligence and machine learning for real-time optimization. Regions with progressive and coordinated energy policies exemplify effective frameworks: the European Union’s Green Deal mandates ambitious decarbonization targets supported by substantial funding for clean energy infrastructure (Marelli et al., 2025); the U.S. Inflation Reduction Act provides extensive tax credits and grants that catalyze renewable and storage deployment (Guha, 2025); and China’s Renewable Energy Law enforces mandatory renewable capacity additions alongside supportive financial mechanisms (Liu, 2019) (see Figure 11). Beyond fiscal incentives, the development and harmonization of technical standards for TES performance, safety, environmental compliance, and interoperability are fundamental to fostering investor and stakeholder confidence. Standardization accelerates market acceptance by reducing uncertainty and enabling economies of scale in manufacturing and deployment. Collectively, these regulatory and incentive frameworks constitute a multidimensional policy ecosystem that is essential for scaling thermal energy technologies, enhancing grid flexibility, and achieving climate goals at the national and global levels. Table 11 presents the examples of policy measures that have accelerated TES adoption in different countries.

Despite notable technological progress in TES and related systems, the widespread adoption of these solutions continues to face a series of interrelated technical, economic, and social barriers (Simó-Solsona et al., 2021). One of the foremost challenges is the high upfront capital expenditure associated with TES infrastructure, including costs linked to advanced materials, system components, and installation. Moreover, the complexity of integrating TES within existing energy networks, which often requires sophisticated control architectures and compatibility with diverse energy sources and loads, adds to deployment hurdles (Saxena et al., 2024). Material durability also remains a concern, as thermal cycling, corrosion, and long-term stability of phase change materials and heat transfer fluids can affect system reliability and lifespan (Adesusi et al., 2023). Beyond technical and financial constraints, public acceptance issues frequently impede project development. These challenges stem from limited awareness of TES benefits, perceived operational or safety risks, and concerns about potential disruptions to local environments or communities. Such social factors can delay permitting processes and hinder stakeholder engagement, underscoring the need for proactive community involvement and transparent communication.

Addressing these obstacles demands comprehensive policy interventions that include risk mitigation strategies such as government-backed guarantees, targeted subsidies, and streamlined regulatory and permitting frameworks to accelerate deployment. On the innovation front, advances in materials science, notably the development of high-temperature stable phase change materials, thermochemical storage media, and corrosion-resistant fluids, offer pathways to enhance system efficiency and durability. Simultaneously, the design of modular and scalable TES units can reduce installation complexity and lower capital costs. The integration of AI-driven optimization algorithms and predictive analytics facilitates operational improvements by enhancing energy management, forecasting demand, and optimizing charge-discharge cycles, which collectively improve economic viability. To navigate the multifaceted nature of TES adoption, interdisciplinary collaboration among engineers, policymakers, economists, and social scientists is essential. Such cross-sectoral efforts enable the development of holistic solutions that balance technical feasibility, market competitiveness, and social acceptability. Taken together, these challenges and opportunities frame a roadmap for scaling TES and thermal integration technologies within decarbonized energy systems, contributing to resilient, efficient, and sustainable energy infrastructures worldwide.

The advancement of thermal energy systems hinges on sustained innovation across several interdisciplinary domains. Central to this progress is the development of advanced materials, including novel PCMs with enhanced thermal conductivity and stability, thermochemical storage media capable of reversible energy storage through chemical reactions, and corrosion-resistant high-temperature molten salts that extend operational lifetimes and broaden application temperature ranges. These material innovations are critical for increasing TES system efficiency, durability, and economic viability. Simultaneously, the integration of AI and ML into thermal energy systems is poised to transform their management and control. AI-driven techniques enable real-time system optimization, predictive maintenance, and adaptive control of increasingly complex thermal grids, improving performance, reducing downtime, and maximizing energy utilization. These capabilities facilitate dynamic balancing of supply and demand, integration with variable renewable energy sources, and enhanced grid resilience.

Emerging hybrid energy systems that synergize TES with complementary technologies, such as electrochemical battery storage, renewable power generation, and green hydrogen production, represent a frontier for creating flexible and resilient multi-vector energy infrastructures. These integrated systems can leverage the unique strengths of each technology to deliver reliable, low-carbon energy across sectors and timescales. Further research into digital twins and IoT-enabled monitoring promises substantial gains in system modeling fidelity, operational transparency, and fault detection. By creating virtual replicas of physical thermal systems, digital twins allow for scenario testing, performance optimization, and proactive maintenance, supporting more efficient and cost-effective system operation. Beyond technical innovations, interdisciplinary investigations encompassing policy frameworks, economic modeling, and behavioral science are essential to overcoming non-technical barriers and facilitating the scale-up of TES technologies from pilot demonstrations to commercial adoption. Understanding regulatory impacts, market dynamics, and end-user acceptance will guide the design of enabling environments and adoption strategies. Collectively, these research trajectories will drive the emergence of the next-generation of sustainable, efficient, and smart thermal energy infrastructures, which are indispensable for achieving global energy transition goals and fostering resilient, low-carbon energy systems worldwide.