Sand-Based Thermal Storage System for Human-Powered Energy Generation: A Review

Abstract

Sand-based thermal energy storage systems represent a paradigm shift in sustainable energy solutions, leveraging Earth’s most abundant mineral resource through advanced nanocomposite engineering. This review examines sand-based phase change materials (PCM) systems with emphasis on integration with human-powered energy generation (HPEG). Silicon-based hierarchical pore structures provide multiscale thermal conduction pathways while achieving PCM loading capacities exceeding 90%. Carbon-based nanomaterial doping enhances thermal conductivity by up to 269%, reaching 3.1 W/m·K while maintaining phase change enthalpies above 130 J/g. This demonstrated cycling stability exceeds 1000 thermal cycles with <8% capacity degradation. Thermal energy storage costs reach ~$20 kWh⁻¹—60% lower than lithium-ion systems when normalized by usable heat capacity. Integration with triboelectric nanogenerators achieves 55% peak mechanical-to-electrical conversion efficiency for direct pathways, while thermal-buffered systems provide 8–12% end-to-end efficiency with temporal decoupling between intermittent human power input and stable electrical output. Miniaturized systems target off-grid communities, offering 5–10× cost advantages over conventional batteries for resource-constrained deployments. Levelized storage costs remain competitive despite efficiency penalties versus lithium-ion alternatives. Critical challenges, including thermal cycling degradation, energy-power density trade-offs, and environmental adaptability, are systematically analyzed. Future directions explore biomimetic multi-level pore designs, intelligent responsive systems, and distributed microgrid implementations.

1. Introduction

Renewable energy is essential for global economic development, technological innovation, and societal welfare. The renewable energy sector encompasses three key stages: energy generation, energy storage, and energy utilization. However, the global transition toward renewable energy faces a critical challenge: the temporal mismatch between intermittent energy supply and constant demand [1,2]. This challenge is particularly acute for distributed energy systems, where conventional storage solutions remain economically prohibitive and technically inadequate for buffering highly variable power sources. Human-powered electricity generation (HPEG) exemplifies this challenge at the distributed scale. Unlike solar or wind with predictable patterns, HPEG produces 200–500 W peak power during sporadic 10~30 min sessions, creating extreme temporal variability that conventional electrochemical storage cannot efficiently buffer. This operational profile—characterized by high power intermittency and resource-constrained deployment contexts—demands fundamentally different storage solutions than grid-scale applications.

Energy storage engineering represents a multidisciplinary frontier encompassing energy conversion, reservoir storage, and environmental engineering, playing an increasingly critical role in global energy transition strategies. Beyond traditional electrochemical batteries, diverse storage technologies span multiple scales—from geological formations for large-scale applications to compact thermal systems for distributed generation [3,4,5,6].

For distributed HPEG applications, thermal energy storage offers distinct advantages over electrochemical systems: inherent tolerance to variable power input without complex conditioning circuits, significantly lower costs ($20/kWh vs. $150–200/kWh for lithium-ion), and operational robustness under harsh environmental conditions typical of off-grid deployments. Among various storage technologies, thermal energy storage using phase change materials (PCM) has attracted significant attention due to their high energy storage density and isothermal operation during phase transitions [7,8]. PCMs absorb and release substantial thermal energy at nearly constant temperatures through latent heat storage mechanisms, making them ideal for smoothing intermittent energy flows. However, conventional PCM systems suffer from three critical limitations that restrict their practical deployment: low thermal conductivity (typically 0.2–0.5 W/m·K), resulting in slow charging/discharging rates, liquid leakage during phase transitions, causing containment failures, and high manufacturing costs, limiting scalability [9,10]. These technical and economic barriers have prevented widespread adoption, particularly in resource-constrained settings, where cost-effectiveness is paramount.

To overcome these barriers, sand-based thermal energy storage systems—termed “sand batteries”—have emerged as a transformative solution by leveraging Earth’s most abundant mineral resource combined with advanced nanocomposite engineering [11,12]. Their fundamental innovation lies in utilizing silicon-based porous structures (natural sand) as host matrices for PCMs, where hierarchical pore networks ranging from nanometers to micrometers provide multiscale thermal conduction pathways while physically confining PCMs to prevent leakage [13,14]. This approach achieves PCM loading capacities exceeding 90% while maintaining structural integrity. Recent advances in nanocomposite engineering have dramatically enhanced performance through incorporation of carbon-based nanomaterials—including graphene nanosheets (GNPs), carbon nanotubes (CNTs), and MXene—into silicon dioxide frameworks. These composites achieve thermal conductivity enhancements exceeding 269%, reaching values up to 3.1 W/m·K, while maintaining phase change enthalpies above 130 J/g [15,16]. The performance improvements originate from synergistic phonon transport through carbon networks, capillary confinement within hierarchical pores, and anisotropic heat flow channels [17,18]. Mechanical stability has been demonstrated through over 1000 thermal cycles with less than 8% capacity degradation, addressing long-standing interfacial delamination challenges. Economically, recent commercial demonstrations in Finland have achieved thermal energy storage costs of $20/kWh—60% lower than lithium-ion systems—while the world’s largest 100 MWh installation validates technical feasibility at grid scale [19].

The unique properties of sand-based PCMs make them particularly suitable for buffering intermittent energy sources, especially Human-Powered Energy Generation (HPEG) in off-grid applications. HPEG systems—encompassing triboelectric nanogenerators (TENGs), piezoelectric devices, and mechanical exercise equipment—generate 200–500 W instantaneous power during operation [20,21]. Sand-based thermal storage uniquely addresses three critical HPEG requirements: (1) Temporal buffering—absorbing sporadic 10~30 min mechanical inputs and releasing steady 20–50 W electrical output over 4–8 h via thermoelectric conversion, matching evening demand profiles in off-grid communities. (2) Economic viability—at $20/kWh, sand systems cost 7.5–10× less than batteries, making them feasible for resource-constrained deployments where per capita annual income is $400–800. (3) Environmental resilience—field trials in Kenya demonstrate > 1000 thermal cycles with <8% degradation versus 40–60% capacity fade for batteries under equivalent harsh conditions (35–45 °C, 20–90% humidity). Triboelectric nanogenerators achieve 55% instantaneous efficiency in direct mechanical-to-electrical conversion [22,23]. When integrating sand-based thermal storage for temporal buffering, the complete pathway (mechanical → thermal storage → thermoelectric conversion) achieves 8–12% end-to-end efficiency under steady operation, where thermal storage serves as the energy buffer rather than the primary conversion mechanism. This trade-off between efficiency and temporal decoupling enables continuous power output from intermittent mechanical inputs [24,25]. This integration addresses a distinct market gap: off-grid distributed energy storage for resource-constrained settings. While large-scale sand batteries (100 MWh) target grid-level applications, miniaturized sand-based PCM systems (0.1–5 kWh) specifically serve communities where grid access is limited but mechanical energy is abundant. Approximately 1.2 billion people in sub-Saharan Africa and South Asia lack reliable electricity access [26,27], and community energy hubs powered by HPEG equipment (exercise bicycles, hand cranks, foot pedals) could provide essential services including phone charging, LED lighting, and water pumping. The integration of sand-based thermal storage with HPEG specifically addresses the temporal mismatch problem: converting intermittent 10–30 min human exercise sessions into stable 4–8 h energy output for evening use. At this scale (0.1–5 kWh), sand batteries offer 5–10× cost advantages over lithium-ion systems ($20/kWh vs. $150–200/kWh), making them economically viable for resource-constrained deployments. Despite growing interest in both sand-based thermal storage and HPEG technologies, systematic examination of their integration remains absent. Existing reviews focus either on grid-scale sand batteries (>10 MWh) or on HPEG devices without addressing storage coupling. Critical knowledge gaps persist: design principles for miniaturized thermal storage (0.1–5 kWh) optimized for HPEG intermittency, system-level integration strategies balancing thermal management with thermoelectric conversion, and techno-economic frameworks for community-scale deployment. This review addresses these gaps by examining sand-based thermal storage specifically through the lens of HPEG integration.

Despite these promising developments, critical challenges remain for widespread adoption in dynamic HPEG applications. Thermal cycling degradation mechanisms—including nanofiller agglomeration, pore structure collapse, and PCM decomposition—limit practical lifetimes to 500–1000 cycles, falling short of the 3000–5000 cycles required for daily use applications over 10-year service lives [28,29,30]. The fundamental trade-off between energy density and power density remains unresolved, with existing designs achieving only 65% of theoretical performance limits due to competing requirements for high PCM loading (maximizing energy storage) versus high thermal conductivity pathways (maximizing power output) [31]. Environmental factors, including humidity exposure (causing hygroscopic degradation of porous structures), temperature fluctuations (inducing thermal stress), and mechanical vibration (from HPEG operation) cause significant performance degradation, restricting deployment contexts [32]. Furthermore, the integration of sand-based thermal storage with HPEG and thermoelectric conversion introduces system-level complexities in thermal management, impedance matching, and transient response optimization that remain inadequately addressed in the current literature [33].

This review comprehensively examines sand-based phase change thermal storage materials with emphasis on HPEG integration. Section 2 introduces the fundamental development of phase change thermal storage materials in energy storage, including solid–liquid and solid–solid phase change mechanisms and historical application scenarios. Section 3 presents the basic principles and technical classification of sand batteries, covering core mechanisms of sand as thermal storage medium and various sand-based system configurations. Section 4 discusses current research progress and representative achievements in performance optimization and energy conversion integration. Section 5 identifies key challenges and technical bottlenecks at both material and system levels. Section 6 explores future development directions including novel composite materials and system-level innovation applications. By establishing structure–property–performance relationships across scales, this review guides next-generation sand battery development for resource-constrained and off-grid applications worldwide.

2. Development of Phase Change Thermal Storage Materials in Energy Storage

Having established the context and significance of sand-based thermal storage systems, this section traces the fundamental development of phase change materials that underpin these innovations. Understanding the evolution from basic phase change mechanisms to advanced nanocomposite systems provides essential context for appreciating how sand-based technologies represent a convergence of multiple research streams.

2.1. Fundamental Principles and Technological Evolution of Phase Change Materials

2.1.1. Solid–Liquid and Solid–Solid Phase Change Mechanisms

The thermal storage capacity of phase change materials (PCMs) originates from the latent heat absorbed or released during phase transitions, with solid–liquid and solid–solid phase change mechanisms exhibiting significant differences in thermodynamic behavior and engineering applicability. Solid–liquid PCMs achieve latent heat storage through crystal structure melting, with unit mass heat storage densities reaching 200–400 J/g, though liquid leakage risks limiting cycling stability [34]. In contrast, solid–solid PCMs rely on lattice structure reorganization for energy storage. While their latent heat values are typically lower than solid–liquid systems, volume change rates can be controlled within 5% with no liquid-phase migration issues, making them particularly suitable for long-term cycling scenarios [35]. Figure 1a illustrates the contrasting mechanisms: solid–liquid PCMs undergo complete bond dissociation with 15–20% volume expansion and liquid leakage risk, while solid–solid systems achieve phase change through lattice reorganization with minimal volume change (<5%). The enthalpy–temperature profiles (Figure 1b) reveal that solid–liquid PCMs exhibit sharp transitions with peak values of 150 J/g across narrow temperature windows, whereas solid–solid systems display broader phase change zones (~15–25 °C) with moderate enthalpy values (~100 J/g), reflecting the progressive nature of lattice distortion. The phase transition temperature stability differences also warrant attention: solid–liquid systems often exhibit temperature drift due to phase separation after multiple cycles, while solid–solid systems can maintain temperature fluctuation tolerance within ±1 °C through intermolecular force reconstruction [36].

Figure 1. Phase change mechanisms in PCM systems. (a) Solid–liquid PCMs undergo bond dissociation with 15–20% volume expansion versus solid–solid PCMs with lattice reorganization and minimal volume change (<5%). Interfacial binding energies: −2.3 eV/nm² for solid–solid systems [35]. (b) Enthalpy–temperature profiles from DSC measurements (10 °C/min, N₂): solid–liquid systems show sharp transitions (150 J/g, 5–10 °C range) while solid–solid systems display broader zones (~100 J/g, 15–25 °C) [34,35,36]. Cycling tests (500 cycles): ±1 °C stability for solid–solid versus ±5 °C for solid–liquid systems [36].

Figure 1. Phase change mechanisms in PCM systems. (a) Solid–liquid PCMs undergo bond dissociation with 15–20% volume expansion versus solid–solid PCMs with lattice reorganization and minimal volume change (<5%). Interfacial binding energies: −2.3 eV/nm² for solid–solid systems [35]. (b) Enthalpy–temperature profiles from DSC measurements (10 °C/min, N₂): solid–liquid systems show sharp transitions (150 J/g, 5–10 °C range) while solid–solid systems display broader zones (~100 J/g, 15–25 °C) [34,35,36]. Cycling tests (500 cycles): ±1 °C stability for solid–solid versus ±5 °C for solid–liquid systems [36].

The introduction of nanocomposite technology provides new pathways for morphological control of solid–liquid PCMs. By encapsulating carbonate eutectic molten salts within porous ceramic matrices (such as magnesium oxide), three-dimensional capillary networks can be formed to suppress liquid leakage, while graphite doping increases thermal conductivity to above 4.5 W/m·K, approximately 8 times higher than pure molten salt systems [37]. Thermal interface resistance control becomes crucial for such composite materials—experiments demonstrate that when the salt–matrix contact angle is less than 30°, stability exceeding 500 cycles can be achieved, with the mechanism attributed to the uniform distribution of stress from wettability optimization [37]. For solid–solid phase change systems, molecular dynamics simulations reveal that energy storage efficiency is directly related to lattice distortion energy. By designing organic–inorganic hybrid structures with flexible molecular chains, phase change enthalpy can be increased to 2.3 times that of traditional polymer systems [35].

Current research focuses on balancing the contradictory relationship between heat storage density and engineering reliability. Solid–liquid systems leverage high latent heat values for advantages in compact thermal storage devices, but require microencapsulation or fiber reinforcement to compensate for mechanical strength losses, typically resulting in a 15–20% reduction in effective heat storage space [36]. While solid–solid systems excel in leakage prevention, their thermal conductivity is generally below 1 W/m·K, necessitating anisotropic thermal conduction networks to overcome heat transfer bottlenecks. For example, embedding graphene nanosheets in layered perovskite structures can increase thermal conductivity to 2.8 W/m·K while maintaining phase change enthalpy loss rates below 7% [35]. Regarding interface stability, solid–liquid composite materials are prone to nanofiller agglomeration during thermal cycling, leading to cumulative contact thermal resistance increases of approximately 0.05 m²·K/W per hundred cycles, while solid–solid systems, due to higher solid–solid interface binding energies, can reduce interface degradation rates by an order of magnitude [34,35].

Engineering implementations reveal distinct design paradigms for each phase change mechanism. Solid–liquid PCMs prioritize volumetric energy density (200–400 J/g), employing microencapsulation or hierarchical porous confinement to mitigate leakage—strategies that sacrifice 15–20% effective storage volume but enable compact device architectures [36]. Conversely, solid–solid systems address thermal transport limitations through nanocomposite engineering: embedding graphene nanosheets in layered perovskites achieves 2.8 W/m·K thermal conductivity while preserving > 93% phase change enthalpy [35]. Interface degradation mechanisms fundamentally differ—solid–liquid composites experience nanofiller agglomeration-induced thermal resistance growth (~0.05 m²·K/W per 100 cycles), whereas solid–solid systems exploit higher interfacial binding energies (−2.3 eV/nm²) to suppress degradation by an order of magnitude [34,35], positioning them as preferred candidates for extended cycling applications such as sand-based thermal batteries.

2.1.2. Thermal Conductivity Enhancement Strategies for Nanocomposite Phase Change Materials

Traditional PCMs have long been limited in thermal energy storage applications by their inherently low thermal conductivity coefficients (typically <0.5 W/m·K), which not only cause thermal response lag but also severely restrict energy storage and release efficiency [34]. In recent years, constructing multi-level thermal conduction networks through nanocomposite technology has become the core pathway to overcome this bottleneck, with technological evolution primarily along two dimensions: material compositing and structural design.

In the nanomaterial compositing dimension, the synergistic effects of carbon-based materials and metal oxides demonstrate unique advantages. Research shows that composite systems of graphene nanosheets (GNPs) with alumina (Al₂O₃) can significantly reduce interfacial thermal resistance: GNPs achieve longitudinal heat conduction through in-plane phonon vibration, while Al₂O₃ nanoparticles form lateral heat conduction bridges by filling polymer matrix defects. Synergistic GNP/Al₂O₃ doping significantly enhances thermal conductivity in polymer-matrix PCMs [38], with the mechanisms detailed in Section 3.2.2 for sand-based implementations. As illustrated in Figure 2a, GNP/Al₂O₃ systems demonstrate superior thermal conductivity performance (0.587) while maintaining high enthalpy retention (0.511), approaching the commercial viability threshold of 0.8. This interface optimization mechanism manifests at the molecular scale as enhanced adsorption energy—surface-functionalized carbon nanotubes (CNTs) show a 37% increase in interfacial binding energy with polyvinyl alcohol, effectively suppressing phonon scattering [39]. Notably, the localized surface plasmon resonance effect of metal nanoparticles can further enhance the coupling efficiency of photothermal conversion and heat conduction. Metal nanoparticle-induced dual-carbon networks demonstrate exceptional photothermal conversion efficiency through localized surface plasmon resonance effects [40].

Three-dimensional network design of porous matrices provides a new paradigm for thermal conduction path optimization. Polyvinyl alcohol/carbon nanotube porous films prepared by centrifugal electrospinning construct three-dimensional continuous thermal conduction networks through orientation-aligned microfibers, achieving in-plane thermal conductivity of 1.82 W/m·K while maintaining an ultra-thin thickness of 0.12 mm [39]. Molecular dynamics simulations reveal that nanoconfinement effects on pore walls can induce the ordered arrangement of paraffin molecules along pore channels, extending the phonon mean free path by 2.3 times [41]. Biomimetic hierarchical pore structures, inspired by diatom morphology, significantly enhance thermal diffusivity through multi-scale pore synergy [34]—a design principle elaborated in Section 3.2.3 for sand-based implementations.

Figure 2. Thermal conductivity enhancement in nanocomposite PCMs. (a) Three-dimensional heat conduction networks: hierarchical SiO₂ matrix (porosity 65–75%) with GNPs (5–10 μm), CNTs (20–40 nm diameter), and Al₂O₃ nanoparticles (30–50 nm) [38,39,41], measured via transient hot-wire method (ASTM D7896 [42], 25 °C). (b) Performance heatmap of six nanofiller systems: GNPs/Al₂O₃ achieves 269.5% conductivity enhancement with 84% enthalpy retention [38]. Metrics are from hot-disk method (ISO 22007-2 [43]), DSC (5 °C/min), and 1000-cycle testing. (c,d) Thermal boundary conductance (TBC) of h-GST with 5 nm tungsten contact measured by TDTR spectroscopy across temperature range 0–400 K, spanning amorphous (a-GST), cubic (c-GST), and hexagonal (h-GST) phases. (d) TBC comparison with 2 nm tungsten contact thickness, demonstrating contact layer thickness effects on interfacial thermal transport. Both panels show experimental TBC values (solid diamonds) and theoretical lower bounds (dashed red line), revealing 10⁻⁸ to 10⁻⁶ m²·K/W variations across phase transitions; surface modification strategies such as silane treatment can reduce interfacial thermal resistance by up to 38% [38,44].

Figure 2. Thermal conductivity enhancement in nanocomposite PCMs. (a) Three-dimensional heat conduction networks: hierarchical SiO₂ matrix (porosity 65–75%) with GNPs (5–10 μm), CNTs (20–40 nm diameter), and Al₂O₃ nanoparticles (30–50 nm) [38,39,41], measured via transient hot-wire method (ASTM D7896 [42], 25 °C). (b) Performance heatmap of six nanofiller systems: GNPs/Al₂O₃ achieves 269.5% conductivity enhancement with 84% enthalpy retention [38]. Metrics are from hot-disk method (ISO 22007-2 [43]), DSC (5 °C/min), and 1000-cycle testing. (c,d) Thermal boundary conductance (TBC) of h-GST with 5 nm tungsten contact measured by TDTR spectroscopy across temperature range 0–400 K, spanning amorphous (a-GST), cubic (c-GST), and hexagonal (h-GST) phases. (d) TBC comparison with 2 nm tungsten contact thickness, demonstrating contact layer thickness effects on interfacial thermal transport. Both panels show experimental TBC values (solid diamonds) and theoretical lower bounds (dashed red line), revealing 10⁻⁸ to 10⁻⁶ m²·K/W variations across phase transitions; surface modification strategies such as silane treatment can reduce interfacial thermal resistance by up to 38% [38,44].

Multi-scale composite strategies synergistically amplify heat transfer advantages across different dimensions while maintaining high energy density (>130 J/g) and achieving significant thermal performance improvements. Experimental data shows that composite systems with micrometer-scale porous alumina frameworks loaded with 4 wt% carbon nanotubes can achieve effective thermal conductivity of 3.1 W/m·K, with only 7.2% degradation after 300 thermal cycles [41]. This stability originates from multiple interactions between nanofillers and matrices: metal oxides chemically bond carbon-based materials, while micrometer-scale frameworks physically confine nanoparticle agglomeration, forming composite structures with both mechanical strength and thermal stability [38]. Notably, by controlling the spatial distribution gradient of nano/micro components, recent research has achieved intelligent control of thermal conductivity in the thickness direction, providing new possibilities for dynamic thermal management [35].

Figure 2 illustrates thermal conductivity enhancement mechanisms in nanocomposite PCMs through multi-scale integration. Three-dimensional networks combining graphene nanosheets, carbon nanotubes, and Al₂O₃ nanoparticles (Figure 2a) achieve synergistic heat transfer, with GNPs/Al₂O₃ systems demonstrating optimal performance balance (Figure 2b). Temperature-dependent interfacial thermal conductance measurements reveal order-of-magnitude variations across GST crystal phase transitions (amorphous, cubic, hexagonal) from 0 to 400 K. Direct comparison between 5 nm tungsten contacts (Figure 2c) and 2 nm contacts (Figure 2d) demonstrates thickness-dependent thermal boundary resistance modulation: thicker tungsten layers exhibit pronounced TBC enhancement in the crystalline phases (~1000 MW/m²·K at 300 K for 5 nm vs. ~200 MW/m²·K for 2 nm), while maintaining comparable lower-bound TBC values (~10–30 MW/m²·K) across all phase regimes. These measurements validate interface engineering and contact layer optimization as critical strategies for enhancing phonon transport in sand-based nanocomposites.

While nanocomposite enhancement strategies have proven effective for sand-based systems, it is instructive to contextualize these approaches relative to alternative PCM thermal conductivity enhancement methods. Metal foam integration and finned encapsulation represent two prominent strategies widely investigated in the recent literature [45,46]. Metal foam–PCM composites leverage open-cell metallic structures (copper or aluminum, porosity 85–97%) to create continuous three-dimensional heat conduction pathways, achieving thermal conductivities of 5–8 W/m·K—substantially higher than sand-based nanocomposites (2–3.5 W/m·K) [47,48]. However, metal foams impose significant penalties: 15–30% reduction in volumetric energy density due to PCM displacement, 2.5–4× mass increase (copper density: 8960 kg/m³ versus sand: 2650 kg/m³), and material costs exceeding $100/kg compared to <$0.5/kg for sand [49,50]. Similarly, finned encapsulation employs metallic fins (longitudinal, annular, or tree-shaped) to enhance heat transfer area, reducing melting time by 60–78% compared to finless systems [51,52]. Yet finned designs encounter mechanical fatigue under thermal cyclin, orientation-dependent performance variability (25–40% difference between horizontal/vertical configurations), and manufacturing cost escalation for miniaturized geometries [53,54].

Sand-based systems occupy a strategically differentiated niche optimized for resource-constrained HPEG applications. Compared to metal foams and fins, sand matrices prioritize the following: (1) material abundance and cost-effectiveness ($20/kWh versus $45–65/kWh for metal foam, $30–50/kWh for finned systems), (2) superior gravimetric energy density crucial for portable devices, (3) isotropic thermal enhancement obviating orientation sensitivity, and (4) simplified processing without specialized infiltration equipment [55,56]. While absolute thermal conductivity lags in metal foam systems, the 2–3.5 W/m·K range achieved by sand nanocomposites proves sufficient for the HPEG operational window (0–40 °C, 5–50 W intermittent input, 15~60 min cycles), yielding < 10% thermal lag losses while maintaining 85–90% volumetric energy density and enabling scalable deployment in off-grid contexts where capital cost constraints supersede peak power density requirements. The technoeconomic inflection point favors sand-based architectures for distributed thermal storage applications, with metal foams and finned systems better suited to high-power-density scenarios (e.g., electric vehicle thermal management) where performance justifies cost premiums.

Beyond laboratory optimization, the practical value of these thermal conductivity enhancements is best understood through examining how phase change thermal storage has been deployed across diverse real-world applications.

2.2. Historical Application Scenarios of Phase Change Thermal Storage Technology

2.2.1. Solar Thermal Storage and Building Energy Conservation

The applicability of phase change thermal storage technology in solar thermal storage and building energy conservation stems from its unique thermal energy adsorption–release kinetics and adjustable phase change temperature windows. Systems based on solid–liquid phase change materials (PCMs) can absorb excess thermal energy during peak solar radiation periods and release energy during nighttime or low-temperature periods through latent heat storage mechanisms, achieving dynamic control of building thermal environments [44]. This energy time-shifting function has been validated in solar water heating systems—by integrating paraffin-based PCMs with vacuum tube collectors, system thermal efficiency increases by 23%, with nighttime water temperature decay rates reduced to 0.8 °C/h [44].

In building envelope applications, porous clay-based composite PCMs demonstrate unique advantages: the layered structure of montmorillonite clay not only controls organic PCM leakage rates below 0.6 wt%, but its natural pore network also achieves heat storage densities of 189 J/cm³ while imparting additional compressive strength (28 MPa) and fire resistance to building materials [57]. Recent advances show that after embedding graphene-modified Na₂SO₄·10H₂O/bentonite composites in gypsum boards, wall unit thermal response rates increase by 3.2 times, maintaining 94% of initial enthalpy values after 500 thermal cycles, attributable to rapid heat conduction channels established by graphene networks at the microscale [37,57]. Typical cases demonstrate that hierarchical porous magnesium oxide-supported Li₂CO₃-Na₂CO₃ eutectic salt systems, through optimized pore size distribution (50 nm–5 μm gradient structures), successfully extend day–night temperature buffering amplitude to 14.5 °C, reducing building air conditioning energy consumption by 37% [37]. These technological breakthroughs establish the physical foundation for phase change thermal storage to transition from laboratory to engineering applications, while revealing quantitative correlations between material microstructural design and macroscopic thermodynamic performance. The silicon-based hierarchical pore structures developed for building-scale applications (50 nm–5 μm gradient architectures) provide direct material precedents for miniaturized systems. As PCM integration scales from building envelopes to compact electronics, the same capillary confinement and sand-based architectures prove transferable to localized heat dissipation challenges.

2.2.2. Electronic Device Thermal Management and Battery Thermal Protection

As phase change thermal storage technology extends from large-scale energy storage to miniaturized applications, electronic device thermal management becomes a critical breakthrough point. The temperature regulation capability of PCMs originates from dynamic absorption and the release mechanisms of heat flow through phase change latent heat. For example, melamine foam-supported solid–liquid phase change composites achieve 99.1% latent heat retention through vacuum impregnation, suppressing local temperature rise within 5 °C in chip thermal buffering scenarios [58]. This temperature passivation effect is closely related to material microstructure: three-dimensional porous frameworks constructed with graphene networks not only provide 1.67 W/m·K lateral thermal conductivity but also achieve zero-leakage encapsulation of molten PCMs through capillary action in hierarchical channels, maintaining stable thermal buffering performance under dynamic thermal loads from displays [59]. For transient thermal shock in high-power electronic devices, composite systems of nitrogen–oxygen co-doped vertical graphene arrays with carbon nanofibers demonstrate unique advantages, with ultra-thin interfacial thermal resistance (<10⁻⁶ m²·K/W) and adaptive pore contraction characteristics maintaining over 90% shape recovery rate after 1500 thermal cycles [60].

In lithium-ion battery thermal protection, integrated design of PCMs with electrochemical systems requires balancing thermal runaway suppression and energy density. Experiments show that organic phase change systems doped with photoswitchable molecules can establish 200 J/g activation energy barriers, delaying heat release by over 10 h through light-controlled triggering, precisely matching battery module thermal relaxation times [61]. More innovative solutions come from nickel nanoparticle-induced dual-carbon network structures, where localized surface plasmon resonance effects not only enhance 96.9% photothermal conversion efficiency but also reduce electrode interface temperature differences to 2.3 °C through carbon nanotube thermal bridging, significantly delaying thermal runaway chain reactions [40]. Notably, thermally responsive polymers spontaneously form gel barrier layers in phase change temperature zones (60–80 °C). This intrinsic safety mechanism reduces battery peak temperatures by 38% under extreme conditions, with complete recovery of electrochemical performance after cooling [62]. Current technical routes continue to advance in thermal conductivity enhancement and intelligent response dimensions. The fusion of biomimetic porous structures with metal–organic framework materials indicates that next-generation phase change thermal management materials will possess both molecular-level heat flow control and self-healing capabilities [63].

3. Basic Principles and Technical Classification of Sand Batteries

Section 2 reviewed PCM technology’s historical evolution across diverse applications—from building-scale solar thermal storage to chip-level electronic cooling. This section shifts focus to sand-based systems specifically, examining how silicon’s intrinsic material properties (natural porosity, earth abundance, chemical stability) enable distinct design strategies optimized for HPEG’s intermittent power profiles and resource-constrained deployment contexts.

Building upon these fundamental principles and historical applications, sand-based thermal storage systems emerge as a specialized evolution that leverages silicon-based structures’ unique properties to address the limitations of conventional PCMs. This section elucidates the core mechanisms that enable sand to function as an effective thermal storage medium and examines the diverse technical configurations that have emerged.

3.1. Core Mechanisms of Sand as Thermal Storage Medium

3.1.1. Thermal Storage Characteristics of Silicon-Based Porous Structures

Unlike the polymer-matrix and ceramic composites reviewed in Section 2.1, silicon-based porous structures offer inherent advantages for HPEG integration: natural abundance (<$0.5/kg), no synthesis requirements, and intrinsic hierarchical porosity. The thermal storage characteristics of silicon-based porous structures originate from their unique microscopic topology and thermophysical coupling mechanisms. Nanoporous silicon particles prepared through ball milling–etching processes can achieve porosity exceeding 65% [64]. Three-dimensional tomography shows that capillary networks formed by interconnected channels can effectively confine PCMs to pore wall surfaces, increasing unit mass heat storage density to 2.3 times that of traditional composite materials [57]. This enhancement effect not only originates from high specific surface area (>200 m²/g), providing nanoconfinement spaces, but also benefits from hydrogen bonding between surface hydroxyl groups and PCM molecules, which can increase adsorption enthalpy to 1.8 times that of bulk materials [65].

Breakthroughs in thermal conductivity performance depend on synergistic effects from hierarchical channel design. When pore size distribution exhibits bimodal characteristics (5–20 nm micropores coexisting with 100–500 nm mesopores), heat transfer pathways transform from layer-by-layer diffusion to networked conduction, significantly improving thermal conductivity over homogeneous structures [64]. Experiments confirm that micropores in hierarchical channels serve as nucleation sites, reducing phase change supercooling, while mesopores as rapid heat transfer channels can shorten phase change response time to within 13 s [34]. This spatiotemporal synergistic mechanism demonstrates outstanding cycling stability, maintaining 92% of initial heat storage capacity after 600 thermal cycles, attributed to the elastic deformation of channels buffering volume expansion [65,66].

Breakthrough progress in optimization pathways is reflected in interface engineering with nitrogen-doped carbon coatings. Transmission electron microscopy shows that when porous silicon surfaces are coated with 3–5 nm thick amorphous carbon layers, interfacial contact resistance with PCMs decreases to 18% of original values, while thermal expansion coefficient mismatch reduces from 27% to 6% [65]. This atomic-level modification controls the heat storage density degradation rate at 0.15%/cycle in 50 rapid charge–discharge tests, significantly superior to 1.2%/cycle for unmodified systems [64]. Synchrotron radiation X-ray absorption spectroscopy further reveals that pyridine nitrogen sites in carbon layers can stabilize liquid PCMs through coordination, suppressing migration and leakage within porous structures [35].

Figure 3 demonstrates the multi-scale engineering principles discussed above. Biomimetic template synthesis (Figure 3a,b) creates hierarchical pore networks, while high PCM loading stability (Figure 3c) and carbon reinforcement (Figure 3d) enable the thermal–structural synergy essential for sand-based systems.

Figure 3. (a) Synthesis and hierarchical pore structure: Schematic illustration of the diatom-templated synthesis and hierarchical pore replication process, showing the transition from the biological template to the mesoporous silica structure (a, c) and including related structural stages (b1–b3) [67]. (b) Multi-scale SEM images revealing the interconnected pore networks at different magnifications, spanning from 10 μm to 1 μm scales (a–e) [68]. (c) Cross-sectional images of the form-stable composites with high Phase Change Material (PCM) loading (92% to 95%), demonstrating successful leakage-free encapsulation [69]. (d) Macroscopic sample and SEM/elemental mapping showing the carbon distribution within the hierarchical composite structure [70].

Figure 3. (a) Synthesis and hierarchical pore structure: Schematic illustration of the diatom-templated synthesis and hierarchical pore replication process, showing the transition from the biological template to the mesoporous silica structure (a, c) and including related structural stages (b1–b3) [67]. (b) Multi-scale SEM images revealing the interconnected pore networks at different magnifications, spanning from 10 μm to 1 μm scales (a–e) [68]. (c) Cross-sectional images of the form-stable composites with high Phase Change Material (PCM) loading (92% to 95%), demonstrating successful leakage-free encapsulation [69]. (d) Macroscopic sample and SEM/elemental mapping showing the carbon distribution within the hierarchical composite structure [70].

3.1.2. Physicochemical Processes of Thermal Energy Storage and Conversion

The energy storage and conversion efficiency of thermal storage media is fundamentally determined by the synergistic evolution of molecular order and thermodynamic potential during phase change processes. Silicon-based sand undergoes sensible heat storage through lattice vibration mode transitions during heating, while lattice distortion-induced solid–solid phase changes provide high-density latent heat storage capability [35]. Molecular dynamics simulations show that surface hydroxyl groups and internal SiO₂ tetrahedral networks form dynamic hydrogen bonding systems, exhibiting unique stepwise heat absorption characteristics in the 50–200 °C temperature range, directly related to layer-by-layer desorption of surface-adsorbed water and Si-O-Si bond angle relaxation within the lattice [37].

The reversible nature of these physicochemical processes is illustrated through the charge–discharge cycle (Figure 4), where hierarchical pore structures confine PCM phase transitions while carbon-based coatings establish continuous thermal conduction pathways via interfacial hydrogen bonding networks.

Interfacial thermal resistance control in porous silicon-based materials is key to improving energy conversion efficiency. Experiments show that when pore scales decrease to submicron levels, phonon scattering frequency at pore wall interfaces significantly decreases, increasing thermal conductivity by 2.3 times compared to bulk materials [71]. Through three-dimensional thermal conduction networks formed by introducing graphene nanosheets, thermal diffusion rates can be increased to 0.58 m²/s while maintaining porosity > 70%. This phenomenon originates from π-π stacking effects at nanosheet edges enhancing vibrational coupling of cross-pore thermal bridges [37]. This microstructural design enables sand-based materials to maintain 94% of initial heat storage capacity after 20 thermal cycles [41].

Irreversible losses in storage/release processes primarily originate from two entropy change mechanisms: thermodynamic entropy increase caused by local temperature gradients at phase change interfaces and configurational entropy changes caused by gas adsorption–desorption within porous media. Thermodynamic cycle analysis shows that hierarchical pore structures can reduce entropy production rates during heat storage to 0.47 W/(m·K), benefiting from multi-level synergy of macro–meso–micropores, effectively suppressing Marangoni convective disturbances induced by vapor pressure fluctuations [72]. Synchrotron radiation X-ray diffraction further reveals that at the 120 °C critical temperature point, metastable α-quartz phases form within sand bodies, with this phase change process accompanied by negative entropy change of 3.7 kJ/mol, providing an additional driving force for thermal energy storage [73].

The theoretical efficiency of thermal energy storage systems is constrained by thermodynamic fundamentals. For the charge–discharge cycle, the round-trip efficiency (${\eta }_{rt}$) can be expressed as follows:

$${\eta }_{rt}={\eta }_{charge}\times {\eta }_{storage}\times {\eta }_{discharge}=\left({\displaystyle \frac{Q_{stored}}{Q_{input}}}\right)\times \left({\displaystyle \frac{Q_{retained}}{Q_{stored}}}\right)\times \left({\displaystyle \frac{W_{out}}{Q_{retained}}}\right)$$

(1)

where $Q_{stored}$ represents thermal energy absorbed during charging, $Q_{retained}$ accounts for thermal retention after storage duration, and Wout denotes extracted work. For sand-based PCM systems with carbon nanotube networks, experimental measurements yield ${\eta }_{charge}\approx 0.82-0.85$ (accounting for conduction losses through hierarchical pores), ${\eta }_{storage}\approx 0.88-0.92$ over 8 h periods at 25–35 °C ambient conditions [38,39], and thermoelectric conversion efficiency as follows:

$${\eta }_{TE}={\displaystyle \frac{TH-TC}{TH}}\times {\displaystyle \frac{\sqrt{1+ZT}-1}{\sqrt{1+ZT}+\frac{TC}{TH}}}$$

(2)

where TH and TC are hot and cold side temperatures, and ZT is the thermoelectric figure of merit. For typical ΔT = 40–60 K and ZT ≈ 0.8–1.2, ${\eta }_{TE}\approx 0.08-0.12$, yielding overall system efficiency ${\eta }_{rt}\approx 0.06-0.09\left(6-9\%\right)$, consistent with reported 8–12% values when accounting for measurement uncertainties [74,75]. The ΔT = 40–60 K assumption derives from experimental prototypes: TH = 60–80 °C during mechanical charging (resistive heating in CNT networks [38,76]) and TC = 20–35 °C via water/air cooling [74,75]. PCM melting points (55–65 °C for PEG-4000) concentrate heat at thermoelectric interfaces. Field measurements confirm ΔT = 46 ± 5 K under active cooling, 38 ± 6 K passively [77].

Recent research has achieved breakthroughs through biomimetic Janus interface design, constructing temperature-responsive zinc oxide nanocone arrays on sand grain surfaces. When the temperature exceeds set thresholds, nanocones undergo controlled bending, causing surface contact angles to suddenly change from 152° to 38° within a 5 °C temperature rise interval. This wettability transition can increase heat release rates by 217% while reducing interfacial thermal resistance to 0.02 K·m²/W [34]. This breakthrough provides a theoretical basis for developing intelligent sand-based thermal storage systems, potentially enabling intermittent thermal energy storage efficiency approaching practical upper limits (≈30–35% of the Carnot cycle) [73]. These fundamental mechanisms provide the scientific basis for developing diverse sand battery configurations, each optimized for specific application requirements and performance metrics.

3.2. Technical Classification of Sand Batteries

Sand-based thermal storage systems can be categorized by their dominant performance optimization strategy: (1) structural encapsulation for leakage prevention, (2) nanofiller doping for conductivity enhancement, or (3) hierarchical pore engineering for capacity maximization. Table 1 summarizes quantitative performance trade-offs across these classifications.

3.2.1. Sand-Based Composite Materials Based on Solid–Solid Phase Changes

Sand-based composite materials designed for solid–solid phase changes use silicon-based porous frameworks as structural cores, fixing phase change substances through dual actions of chemical bonding and physical adsorption, achieving morphological stability of storage units during cycling processes. Research shows that selecting three-dimensional silicon dioxide networks with high specific surface areas as carriers can effectively enhance phase change substance loading rates and suppress volume expansion [37]. For example, nanoporous silicon-based frameworks prepared through sol–gel methods, when pore size distribution is controlled within the 2–50 nm range, can form capillary force-locking effects at the microscale, maintaining geometric integrity of PCMs during melting–solidification processes [41]. Interface binding mechanism analysis shows that hydrogen bonding between silicon-based surface hydroxyl groups and organic phase change substances significantly enhances the thermodynamic stability of composite materials, with interfacial binding energies verified through molecular dynamics simulation reaching −2.3 eV/nm² [35]. Form-stable composites achieve high PCM loading without leakage, as demonstrated in Figure 3c, where loading ratios from 92 to 95% maintain structural integrity.

Enhancement of thermal cycling stability relies on multi-scale structural engineering, particularly the synergistic effects of hierarchical channel design and nano-reinforcement phases. Experiments demonstrate that when materials contain both micrometer-scale heat transfer channels and nanometer-scale heat storage units, thermal stress during cycling can be effectively dispersed by three-dimensional networks, controlling heat storage density degradation rates within 5% after 500 thermal cycles [37]. Taking graphene quantum dot-modified sand-based composites as an example, two-dimensional nanofillers not only increase thermal conductivity to 1.8 W/(m·K) (320% improvement over pure PCMs), but also alleviate interfacial thermal mismatch problems through lattice matching mechanisms [41]. Synchrotron radiation X-ray tomography shows that these nano-reinforcement phases form continuous heat conduction paths in matrices, enabling heat to penetrate 5 mm thick samples within 2.7 s [35]. The core–shell architecture (Figure 5a) demonstrates how carbon coatings (2 nm) on SiO2 shells (5–8 nm) encapsulate PCM cores (50 nm), enabling this thermal conductivity enhancement.

In typical preparation cases, core–shell structured sand-based composites constructed using spray drying technology demonstrate excellent engineering application potential. By precisely controlling the ratio of silica sol to sodium nitrate in precursor solutions (3:7 mol%), materials form interpenetrating network structures after sintering at 900 °C, achieving volumetric heat storage density of 306 kWh/m³, a 41% improvement over traditional solid–liquid phase change systems [37]. Notably, the synergistic effect of hierarchical pore systems (62% micropores, 28% mesopores, 10% macropores) enables these materials to maintain a stable thermal conductivity of 0.89 W/(m·K) across a wide temperature range of 50–900 °C while achieving 95.3% phase change enthalpy retention [35].

3.2.2. Nano-Doped Sand-Based Phase Change Systems

The development of nano-doped sand-based phase change systems fundamentally aims to achieve synergistic optimization of heat storage density and thermodynamics through precise control of microscopic interfaces. Carbon-based nanomaterials represented by graphene and carbon nanotubes become preferred dopants due to their unique phonon transport properties and high specific surface areas. Their surface dangling bonds can form three-dimensional thermal conduction networks with silicon dioxide matrices through hydrogen bonding [76]. Low-dosage CNT doping (0.5–1 wt%) substantially enhances sand-based composite thermal conductivity while preserving high phase change enthalpy [78]. These nanocomposite mechanisms are visualized in Figure 3d, showing carbon network distribution in hierarchical matrices. More innovative is the core–shell structure design strategy: heterogeneous nanoparticles (SZZ) with carboxylated carbon nanotubes as cores and metal–organic framework ZIF-8 as shells demonstrate dual functions in sand-based systems. The outer porous shell structure anchors molten PCMs through capillary action, while the inner carbon core establishes rapid heat conduction paths, enabling the composite system to maintain 92% of initial enthalpy values after 1000 thermal cycles [73]. This synergistic mechanism is illustrated through the three-dimensional thermal network shown in Figure 5b, where graphene nanosheets and carbon nanotubes form interpenetrating conduction pathways.

Gradient doping technology provides new approaches to solving nanoparticle agglomeration challenges. Concentration gradient doping systems constructed through layer-by-layer deposition form dense-to-sparse nanoparticle distributions within sand-based materials: high-concentration regions (5 wt% ZnO nanowires) primarily handle heat flow transfer functions, while low-concentration regions (1 wt%) mainly maintain structural stability. This design achieves adaptive thermal conductivity adjustment from 0.45 W/m·K to 0.68 W/m·K in the 20–80 °C working range [79]. Molecular dynamics simulations reveal that carbon atom lattice vibration coordination effects induced by nano-doping can reduce interfacial thermal resistance by 47%. When doping particle sizes are below 50 nm, increased contact area between nanoparticles and sand-based media raises interfacial binding energy to 3.2 eV/nm² [76]. Quantitative relationships between nanofiller content and thermal conductivity (Figure 5h) reveal that MXene achieves 2.34 W/(m·K) at 5 wt%, outperforming GNP and CNT at equivalent loadings.

Figure 5. Technical classification and performance characteristics of sand-based thermal storage systems. (a) Core–shell solid–solid PCM structure with carbon coating and SiO₂ shell. (b) Three-dimensional nano-doped thermal conduction network combining graphene and CNT. (c) Biomimetic hierarchical pore architecture with macro–meso–micropore gradient. (d–f) SEM images showing multi-scale porous structures at different magnifications [68,80]. (g) Cycling stability showing enthalpy retention and thermal conductivity evolution over 1000 cycles. (h) Thermal conductivity enhancement as a function of nanofiller content (GNP, CNT, MXene). (i) Performance map correlating energy density, power density, and PCM loading capacity.

Figure 5. Technical classification and performance characteristics of sand-based thermal storage systems. (a) Core–shell solid–solid PCM structure with carbon coating and SiO₂ shell. (b) Three-dimensional nano-doped thermal conduction network combining graphene and CNT. (c) Biomimetic hierarchical pore architecture with macro–meso–micropore gradient. (d–f) SEM images showing multi-scale porous structures at different magnifications [68,80]. (g) Cycling stability showing enthalpy retention and thermal conductivity evolution over 1000 cycles. (h) Thermal conductivity enhancement as a function of nanofiller content (GNP, CNT, MXene). (i) Performance map correlating energy density, power density, and PCM loading capacity.

Breakthroughs in interface engineering significantly enhance the practical value of nano-doped systems. Covalent cross-linked networks constructed using UV-induced “thiol-ene” click chemistry successfully combine nano-reinforcement phases with sand-based carriers, achieving a tensile strength of 2.3 MPa while reducing interfacial thermal resistance to 1/5 of traditional physical mixing methods [78]. More forward-looking is biomimetic hierarchical pore structure design: fractal pore networks inspired by plant vascular bundles precisely position nano-dopants at heat flow path nodes, shortening macroscopic thermal response time to 30% of pure sand-based materials while maintaining a stable thermal conductivity of 1.9 W/m·K under dynamic mechanical loads [35]. These innovations establish material foundations for developing miniaturized thermal storage modules for HPEG, particularly demonstrating unique advantages in their rapid storage/release thermal response in intermittent mechanical energy input scenarios.

3.2.3. Multi-Level Pore Structure Sand-Based Thermal Storage Devices

Multi-level pore structure sand-based thermal storage devices achieve simultaneous optimization of heat storage density and dynamic performance through the synergistic effects of hierarchical pores. Research shows that gradient-distributed micrometer–submicrometer pore networks can effectively regulate PCM infiltration behavior and mass transfer paths. For example, capillary penetration rates of lithium/sodium carbonate eutectic molten salts in silicon-based porous carriers follow power law relationships with pore diameters [37], while nanoscale pores significantly suppress molten salt leakage through surface tension effects. This combination of multi-scale pore structures not only enhances the thermodynamic stability of thermal storage systems but also increases overall thermal conductivity by 230% through reduced heat transfer interface resistance [41]. The hierarchical pore architecture (Figure 5c–f) demonstrates macro → meso → micropore transitions, with SEM imaging revealing interconnected channels from 8 μm to sub-micrometer scales.

Regarding preparation processes, template methods and freeze casting have become mainstream technologies for constructing controllable multi-level pore structures. Sand-based carriers prepared using diatomite as biological templates can precisely control natural hierarchical channels to pore size ranges of 50 nm to 5 μm, as shown in the biomimetic template structures of Figure 3a,b [67,68,81]. Notably, vertically oriented channels formed through ice crystal growth induction in freeze casting can increase axial thermal conductivity to 3.6 times radial values, with this anisotropic characteristic particularly suitable for axial heat transfer requirements in HPEG [35].

Synergistic effects of surface modification and nano-doping demonstrate unique advantages in multi-level pore systems. Experiments show that graphene quantum dot-modified pore walls can increase the heterogeneous nucleation site density of molten salts by two orders of magnitude while reducing activation energy barriers for phase change processes through π-π interactions [34]. When combined with silicon carbide nanowire pore channel filling, composite systems maintain 92% heat storage capacity after 1000 thermal cycles, with performance degradation rates 67% lower than unmodified systems [41]. Typical device cases show that sandwich-structured multi-level pore thermal storage modules can achieve continuous thermal energy release for up to 8 h at 50 W/kg power density, with interfacial contact thermal resistance controlled below 0.05 K·m²/W [37], validating the engineering feasibility of hierarchical structure design in dynamic thermal management scenarios. With the theoretical foundations and technical classifications established, recent experimental advances have begun translating these concepts into practical performance improvements and functional prototypes. The comprehensive performance map (Figure 5i) demonstrates that optimized sand-based systems achieve energy densities exceeding 140 kJ/kg at power densities of 2.5–3.0 kW/m³, representing the state-of-the-art balance between storage capacity and discharge rate. Table 1 summarizes the structural characteristics and performance metrics of the three sand-based thermal storage classifications, providing a quantitative comparison to guide system selection for specific applications.

Table 1. Comparative analysis of sand-based thermal storage system classifications.

Classification	Structure	Thermal Conductivity (W/m·K)	PCM Loading (%)	Cycle Life (Cycles)	Main Advantage	Optimal Use	Refs.
Solid–Solid	Core–shell SiO2; 2–50 nm pores	0.89–1.8	92–95	500–3000	Zero leakage; <5% volume change	Stationary storage; building integration	[35,37,41,69]
Nano-Doped	GNPs/CNTs/MXene; 1–5 wt%	1.9–2.34	75–92	1000–2000	269% conductivity boost; 2.7 s response	Rapid charge–discharge; HPEG	[35,73,76,78,79]
Hierarchical Pore	Macro–meso–micro; 50 nm–5 μm	0.82–3.6	90–98	1000–3000	Highest loading; anisotropic (3.6:1)	High energy density; solar thermal	[34,35,41,82,83]

Table 1. Comparative analysis of sand-based thermal storage system classifications.

Classification	Structure	Thermal Conductivity (W/m·K)	PCM Loading (%)	Cycle Life (Cycles)	Main Advantage	Optimal Use	Refs.
Solid–Solid	Core–shell SiO2; 2–50 nm pores	0.89–1.8	92–95	500–3000	Zero leakage; <5% volume change	Stationary storage; building integration	[35,37,41,69]
Nano-Doped	GNPs/CNTs/MXene; 1–5 wt%	1.9–2.34	75–92	1000–2000	269% conductivity boost; 2.7 s response	Rapid charge–discharge; HPEG	[35,73,76,78,79]
Hierarchical Pore	Macro–meso–micro; 50 nm–5 μm	0.82–3.6	90–98	1000–3000	Highest loading; anisotropic (3.6:1)	High energy density; solar thermal	[34,35,41,82,83]

While these technical classifications demonstrate diverse structural configurations, the practical implementation of sand-based thermal storage systems critically depends on systematic PCM selection considering operational requirements and material compatibility constraints.

3.3. PCM Selection Criteria for Sand-Based Thermal Storage Systems

3.3.1. Operating Temperature Range and PCM Classification

The selection of appropriate PCMs for sand-based thermal storage systems in HPEG applications is fundamentally governed by the operating temperature window, which directly determines phase transition behavior, energy storage density, and system efficiency. For HPEG applications, the practical operational window is constrained to 0–40 °C by typical ambient conditions for wearable devices and community energy hubs [30], where thermal energy harvested from human activities—such as walking, cycling, or manual labor—generates intermittent heat loads with temperature peaks rarely exceeding 45 °C. This moderate temperature regime necessitates careful PCM selection to ensure phase transitions occur within the working range while maintaining adequate thermal buffering capacity.

PCMs suitable for this temperature window can be classified into three categories based on their phase transition temperatures: low-temperature organic compounds (15–30 °C), mid-range organic–inorganic hybrids (30–45 °C), and extended-range salt eutectics (40–60 °C). Paraffin waxes, such as n-octadecane (Tm = 28 °C, ΔH = 244 J/g) and n-eicosane (Tm = 36.8 °C, ΔH = 247 J/g), dominate the low-temperature category due to their high latent heat capacity and chemical stability [84]. Fatty acid eutectics, including capric–lauric acid mixtures (Tm = 21–32 °C, ΔH = 140–150 J/g), offer cost advantages but exhibit 12–15% lower energy density compared to paraffins [85]. Polyethylene glycol (PEG) with molecular weights of 4000–6000 (Tm = 55–65 °C) and inorganic salt hydrates like CaCl₂·6H₂O (Tm = 29 °C, ΔH = 191 J/g) provide alternatives for slightly elevated temperature applications, though their integration with sand matrices requires additional encapsulation strategies to prevent leakage and dehydration [86]. The critical design parameter is matching the PCM’s phase transition temperature to 5–10 °C above the average operating temperature to maximize latent heat utilization while maintaining sufficient sensible heat storage capacity during temperature fluctuations.

3.3.2. Chemical Compatibility and Interfacial Stability Considerations

Beyond thermal matching, long-term performance of sand-based PCM composites depends critically on chemical compatibility between the phase change medium and silicon-based porous carriers. The primary compatibility challenge stems from disparate thermal expansion behaviors: silicon dioxide matrices and organic PCMs exhibit significant CTE mismatch, generating interfacial stresses that limit practical lifetimes. This mechanical incompatibility manifests as progressive pore wall cracking and PCM leakage, limiting practical lifetimes to 500–1000 cycles in unmodified systems [87].

Organic PCMs—including paraffins, fatty acids, and PEG derivatives—generally exhibit excellent chemical inertness with SiO₂ frameworks under normal operating conditions, with negligible interfacial reactions observed over 1000+ thermal cycles at temperatures below 80 °C [88]. However, surface hydroxyl groups on sand particles can catalyze oxidative degradation of organic molecules at elevated temperatures or in the presence of trace moisture, necessitating surface salinization treatments or antioxidant additives (0.2–0.5 wt% butylated hydroxytoluene) [89]. Conversely, inorganic salt PCMs pose more severe compatibility concerns due to their inherent reactivity: molten nitrate salts (NaNO₃-KNO₃ eutectics) accelerate the corrosion of silica frameworks through selective Na⁺ ion migration and hydroxide formation at 300–400 °C, while chloride-based salt hydrates (MgCl₂-CaCl₂) induce localized pH reductions that weaken Si-O-Si bonding networks even at moderate temperatures. Recent molecular dynamics simulations have revealed that interface evolution in salt/sand systems follows concentration-dependent kinetics, with corrosion rates proportional to the square root of Na⁺ mobility [90].

Practical PCM selection must therefore balance thermal performance requirements with interfacial stability constraints. For the 0–40 °C operational window characteristic of HPEG applications, organic PCMs (particularly paraffin-based systems with surface-functionalized sand supports) offer optimal combinations of energy density (200–250 J/g), chemical compatibility (>3000 cycles at 95% capacity retention), and cost-effectiveness, while inorganic alternatives remain viable only for specialized high-temperature applications (>100 °C) with appropriately selected carrier materials or protective coatings [91]. The integration of graphene or carbon nanotube interfacial layers (0.5–2 wt%) has emerged as a promising strategy to simultaneously enhance thermal conductivity and provide mechanical buffering against CTE mismatch, achieving up to 40% reduction in interfacial stress accumulation while improving cycling stability.

4. Current Research Progress and Representative Achievements

With the theoretical foundations and technical classifications established, recent experimental advances have begun translating these concepts into practical performance improvements and functional prototypes. This section surveys cutting-edge research that has substantially enhanced material properties and demonstrates successful integration with HPEG systems.

4.1. Performance Optimization of Sand-Based Composite Phase Change Materials

4.1.1. Thermal Conductivity Enhancement Technologies

In optimizing thermal conductivity of sand-based composite PCMs, multi-scale control of low-dimensional nanofillers and construction of three-dimensional thermal conduction networks are considered core technical pathways. Through synergistic combinations of carbon-based nanomaterials (such as graphene nanosheets, carbon nanotubes) with ceramic particles (such as alumina, boron nitride), continuous thermal conduction paths can be formed within silicon-based porous frameworks, significantly enhancing thermal energy transfer efficiency [38,39]. For example, GNPs construct two-dimensional thermal conduction planes in sand-based matrices, while Al₂O₃ particles fill pores and connect interlayer gaps, forming multi-level conduction structures with substantial thermal conductivity enhancement (Figure 2) [38]. This synergistic mechanism originates from spatial distribution optimization of nanofillers and interfacial phonon coupling effects, with the key being controlling filler orientation alignment and surface functionalization to reduce interfacial thermal resistance.

Engineering design of three-dimensional thermal conduction networks requires precise control combined with material preparation processes. Centrifugal electrospinning technology can directionally embed carbon nanotubes (CNTs) into polyvinyl alcohol/polyethylene oxide composite fibers, forming through-type thermal conduction frameworks. When CNT doping reaches 4 wt%, composite film thermal conductivity increases by 77.1% while maintaining the mechanical integrity of the flexible matrices [39]. In sand-based systems, similar strategies can be achieved through template-assisted sintering, utilizing the porous characteristics of silica sand as loading carriers for nanofillers, inducing directional self-assembly of nanofillers through controlled pore gradient distribution, thereby establishing three-dimensional thermal conduction networks with low tortuosity.

Interfacial thermal resistance control strategies directly affect thermal energy storage and release efficiency. Experiments show that after surface modification of Al₂O₃ particles with silane coupling agents, interfacial contact thermal resistance with GNPs decreases by 38%, originating from chemical bonding between surface functional groups and polymer matrices strengthening interfacial phonon transport [38]. Molecular dynamics simulations further reveal that when nanofiller spacing is less than 10 nm, synergistic phonon vibration modes can break through traditional diffusion heat transfer limits, increasing composite material thermal diffusivity to 3.2 times that of pure sand-based materials. By optimizing doping ratios (such as 5 wt% GNPs with 40 wt% Al₂O₃), optimal balance between energy storage density (66.71 J/g) and thermal conductivity (1.82 W/m·K) can be achieved, providing material-level solutions for intermittent thermal energy storage in HPEG [38,39]. The multi-dimensional performance map (Figure 6a) confirms that MXene-based composites excel in cycling stability (1.0) and leakage resistance (0.98), making them ideal candidates for dynamic mechanical loading scenarios.

Predictive modeling of thermal conductivity in nanocomposite PCMs requires accounting for interfacial thermal resistance, which dominates phonon scattering at nanofiller–matrix boundaries. The classical Maxwell–Eucken theory, originally developed for dilute suspensions assuming perfect thermal contact, overestimates $k_{eff}$ by 30–50% in nanocomposite systems [92]. Recent modifications incorporate Kapitza resistance ($R_K$) at phase boundaries through an effective medium approach. Starting from the two-phase effective conductivity relation for spherical inclusions, we obtain the following:

$$k_{eff}={\displaystyle \frac{k_m\left[\left(k_f+2k_m\right)+2\phi \left(k_f-k_m\right)\right]}{\left[\left(k_f+2k_m\right)-\phi \left(k_f-k_m\right)\right]}}$$

(3)

where $k_m$ and $k_f$ are matrix and filler conductivities, and $\phi$ is volume fraction. Introducing interfacial resistance modifies the filler conductivity to an apparent value $k_f^{\ast }$:

$$k_f^{\ast }={\displaystyle \frac{k_f}{\left[1+\frac{2R_k{k}_f}{d}\right]}}$$

(4)

where d is the characteristic filler dimension. For anisotropic fillers (graphene nanosheets, CNTs), the shape factor β replaces the spherical geometry assumption, yielding the generalized form used in this study [92,93]. This approach has been validated across carbon-ceramic nanocomposites with prediction errors < 12% [94].

Applying this interfacial resistance framework (Equations (3) and (4)), the effective thermal conductivity ($k_{eff}$) of hierarchical sand-PCM nanocomposites is expressed as follows:

$$K_{eff}=k_{matrix}\times {\displaystyle \frac{\left[k_{filler}+2k_{matrix}+2\phi \left(k_{filler}-k_{matrix}\right)\left(1+\beta R{Kk}_{matrix}\right)\right]}{\left[k_{filler}+2k_{matrix}-\phi \left(k_{filler}-k_{matrix}\right)\left(1+\beta RK{k}_{matrix}\right)\right]}}$$

(5)

where φ is the nanofiller volume fraction, $R_k$ is the interface thermal resistance (typically 10⁻⁸ to 10⁻⁶ m²·K/W for carbon-silica interfaces), and β is the particle shape factor (β = 3 for spheres, $\beta \to \infty$ for aligned fibers). For GNP/Al₂O₃ systems with ${\phi }_{GNP}=0.03, {\phi }_{Al2O3}=0.15$, and R_k ≈ 2 × 10⁻⁸ m²·K/W (achieved through silane surface treatment), Equation (5) predicts k_eff ≈ 1.75 W/m·K, agreeing with experimental values of 1.82 ± 0.15 W/m·K within measurement uncertainty [38]. This validates the interfacial engineering approach for thermal conductivity enhancement.

4.1.2. Cycling Stability and Leakage Prevention Solutions

Building upon thermal conductivity enhancement, the thermal cycling stability and leakage prevention performance of PCMs directly determine the service life and safety of thermal storage systems. Research shows that microcrack propagation caused by periodic volume deformation during solid–liquid phase changes in silicon-based porous frameworks is the primary cause of capacity degradation [82], particularly in sand-based composites where weak van der Waals forces between silicon dioxide particles cannot withstand repeated phase change stress, leading to pore structure collapse and phase change component exudation [83]. Figure 6b tracks performance retention across 1000 thermal cycles, revealing a critical degradation threshold at 500 cycles where CNT composites experience accelerated decay (from 96% to 78% retention). Addressing this issue, breakthrough progress has been achieved through dual strategies of nanocomposite and structural encapsulation in recent years.

In material modification dimensions, the interfacial strengthening effects of carbon-based nanomaterials significantly enhance the mechanical stability of phase change systems. Taking carbon nanotube (CNT) doping as an example, their three-dimensional network structures not only enhance thermal conductivity but also suppress PCM–matrix interface delamination through pinning effects. CNT doping enables composite fiber films to maintain excellent latent heat retention across hundreds of thermal cycles with minimal leakage [39]. MXene aerogel systems demonstrate superior long-term stability through self-supporting frameworks that suppress interfacial delamination (Figure 6b). Graphene sheets enhance the elastic modulus of sand-based porous structures through π-π interactions, with their wrinkled morphology design capable of absorbing approximately 12.7% of phase change volume deformation [95]. Figure 6a quantitatively compares these enhancement strategies across five key metrics, revealing that composite systems achieve an optimal balance between thermal conductivity (normalized score 1.0) and cost-effectiveness (0.25). This mechanism has also been verified in MXene ordered assembly systems, where potassium ion-induced self-supporting aerogels maintain intact framework structures after 2000 liquid–solid phase changes [96].

In structural encapsulation technology, microencapsulation and polymer crosslinking form multi-level leakage prevention barriers. Biomimetic encapsulation strategies based on wood cell walls confine PCMs within 1–5 μm directional micropores, combined with polydopamine surface coatings reducing leakage by 89% compared to traditional encapsulation [82]. More innovatively, self-healing crosslinked networks achieve in situ damage repair through dynamic hydrogen bonding, with ion-crosslinked salt hydrogels recovering 92% mechanical strength within 30 min after fracture, improving cycling stability to over 500 cycles [83]. Figure 6d quantifies the trade-off between leakage prevention and PCM loading: while microencapsulation reduces leakage to 0.94 wt%, it sacrifices 13% loading capacity (82% vs. 95% for unencapsulated systems), highlighting the need for hierarchical pore designs that achieve both objectives simultaneously. This intelligent repair mechanism’s synergistic action with sand-based porous structures demonstrates only 0.08% mass loss rate per cycle in polyvinyl alcohol/polyacrylic acid interpenetrating networks [39].

Recent breakthroughs reflect multi-scale structural synergistic optimization, such as gradient pore designs with nanofiber reinforcement. Core–shell structured fiber films prepared through centrifugal electrospinning—with outer CNT-doped polyethylene oxide forming rigid protective layers (thickness ≈ 15 μm) and inner porous polyvinyl alcohol encapsulating PCMs—enable thermal cycling exceeding 1500 cycles with leakage rates controlled below 0.05 g·m⁻²·cycle⁻¹ [39]. Biomimetic wood composite systems maintain 200 cycles without leakage at 98.58% solar photothermal conversion efficiency through the synergistic effects of lignin-derived carbon frameworks and MXene [82], establishing foundations for sand-based thermal storage material applications in dynamic mechanical load scenarios. The comprehensive benchmarking in Figure 6 demonstrates that optimized sand-based composites achieve 94% PCM loading with only 0.6 wt% leakage (Figure 6d) while maintaining 80% performance retention after 1000 cycles (Figure 6b), meeting the commercial deployment requirements for off-grid energy storage systems. With material-level performance substantially improved, the focus shifts to demonstrating functional integration within HPEG, wherein intermittent mechanical input poses unique conversion challenges.

Figure 6. Performance benchmarking of sand-based PCM systems. (a). Normalized multi-dimensional performance (n = 5 materials). Dashed line: commercial target. (b). Cycling stability with 95% confidence intervals. (c). Energy-power density trade-off by material category. (d). Leakage rate vs. PCM loading across encapsulation strategies. Data available: [38,39,40,41,82,83,96].

Figure 6. Performance benchmarking of sand-based PCM systems. (a). Normalized multi-dimensional performance (n = 5 materials). Dashed line: commercial target. (b). Cycling stability with 95% confidence intervals. (c). Energy-power density trade-off by material category. (d). Leakage rate vs. PCM loading across encapsulation strategies. Data available: [38,39,40,41,82,83,96].

4.2. Energy Conversion Integration in HPEG

4.2.1. Mechanical–Thermal–Electrical Energy Synergistic Conversion Mechanisms

The mechanical–thermal–electrical conversion pathway comprises three sequential stages, each with distinct efficiency characteristics. Primary mechanical-to-electrical conversion via TENGs achieves 55% instantaneous efficiency through contact electrification [77,97], representing the theoretical upper limit for direct conversion. However, practical HPEG applications require temporal buffering due to intermittent human activity patterns. The thermal-mediated pathway involves: (1) mechanical energy dissipation as friction heat (>90% conversion efficiency), (2) thermal storage in sand-based PCMs (82–85% charge efficiency accounting for thermal losses), and (3) thermoelectric reconversion to electricity (8–12% conversion efficiency limited by Carnot constraints). The overall system efficiency of 8–12% reflects this multi-stage conversion, where thermal storage provides essential temporal decoupling—converting 10–30 min exercise sessions into 4–8 h continuous electrical output—rather than optimizing instantaneous efficiency [77,98].

Multi-level energy regulation during thermal energy storage phases relies on the thermal hysteresis characteristics of PCMs synergizing with Seebeck effects in thermoelectric devices. Experiments show that sand-based porous structures loaded with zinc oxide core–shell nanoparticles maintain 68% initial latent heat density after 50 thermal cycles, with their hierarchical pore structures stabilizing cold-end temperature gradients of thermoelectric generation devices above 15 K/mm [72,73]. Through embedded microfluidic thermal switch design, systems can dynamically adjust heat flow paths according to external load demands. When environmental temperature falls below phase change points, radiative cooling effects from thermal storage modules can increase thermoelectric conversion efficiency by 24.89% [74,98].

Energy dissipation control during tri-energy domain coupling requires balancing competitive mechanisms between interfacial polarization and thermal transport at mesoscopic scales. Theoretical models based on modified Maxwell equations reveal that displacement current (∂D/∂t) generated by triboelectric charges and thermal expansion effects (∂$P_s$/∂t) driven by phase change latent heat jointly determine system maximum power points [97]. Through biomimetic serpentine electrode topology optimization, 82% mechanical energy capture rate and 64.74 J/g stable heat storage density are achieved in 2 cm³ miniaturized modules, with temperature fluctuations under transient thermal shock controlled within ±1.5 °C [34,39]. This cross-scale energy matching strategy provides critical technical support for all-weather operation of HPEG.

The reported 8–12% end-to-end efficiency reflects fundamental thermodynamic constraints. For sand-based PCM systems operating at ΔT = 40–60 K (TH ≈ 333–353 K, TC ≈ 293 K), the Carnot efficiency ceiling is 12–17%. With commercial Bi₂Te₃ thermoelectric modules (ZT ≈ 0.8–1.2), the practical conversion efficiency yields 8–12%—representing 50–70% of the Carnot limit, consistent with established low-temperature thermoelectric performance. This efficiency primarily reflects thermoelectric conversion rather than thermal storage losses (82–85% charge efficiency). Approaching theoretical limits (30–35%) would require breakthrough materials with ZT > 3, which remain elusive. The trade-off is explicit: direct TENG conversion achieves 55% efficiency without storage, while thermal-mediated pathways sacrifice efficiency to enable the 4–8 h energy time-shifting essential for off-grid applications.

4.2.2. Miniaturized Thermal Storage Module Design Cases

In miniaturized thermal storage module design, material selection must balance high heat storage density with mechanical stability. Silicon-based porous carriers, with their natural multi-level pore structures and chemical inertness, become ideal phase change material (PCM) support matrices. Research shows that in situ impregnation of paraffin into carbonized polyimide/Kevlar nanofiber composite aerogel nanopores can achieve up to 98.3% PCM loading rates while utilizing the synergistic effects of graphene sheets and silver nanoparticles to increase the axial thermal conductivity to 1.20 W/(m·K), significantly superior to traditional physical mixing processes [75,99].

In typical structural designs, biomimetic bean-pod core–shell microcapsule systems demonstrate unique advantages. Core–shell fiber films prepared using centrifugal electrospinning, with carbon nanotube-reinforced polyethylene oxide as shell layers encapsulating polyethylene glycol phase change cores, achieve 66.71 J/g latent heat values within 200 μm diameter fibers. This structure forms three-dimensional confinement effects through crosslinked polymer networks, controlling leakage rates below 0.2 wt% while maintaining complete encapsulation after 500 bending cycles [39,100]. For thermal conductivity optimization, miniaturized scenarios require nanofillers to establish efficient thermal conduction paths in limited spaces. Experiments show that when graphene sheets are vertically aligned along heat flow directions, radial thermal conductivity can reach 45% of axial values. Combined with magnetron-sputtered silver coatings, interfacial contact thermal resistance can be reduced by 62% [59,99].

When integrated into HPEG, such miniaturized thermal storage modules can absorb 85% of braking kinetic energy converted to latent heat storage under 30 min cycling conditions, achieving 12.3% thermoelectric conversion efficiency with thermoelectric plates. When module thickness is controlled within 3 mm, stable working ranges of 39–42 °C can be maintained despite 20 °C environmental temperature fluctuations, which can be successfully applied to active temperature control systems in smart cycling apparel [75,101]. Despite these promising achievements in laboratory and prototype demonstrations, several critical challenges remain that must be addressed before widespread commercial deployment can be realized.

5. Key Challenges and Technical Bottlenecks

Despite the performance advances demonstrated in Section 4 (thermal conductivity > 3 W/m·K, cycling stability > 1000 cycles), translating sand-based PCM technology from laboratory prototypes to field-deployable HPEG systems requires addressing fundamental material degradation mechanisms and system-level integration complexities, examined below.

5.1. Material-Level Limiting Factors

5.1.1. Thermal Cycling Degradation and Interface Stability Issues

In the continuous optimization pathway of phase change thermal storage material performance, thermal cycling degradation and interface instability consistently remain key bottlenecks restricting long-term stable operation of sand-based batteries. Microscale research shows that repeated crystal nucleation and growth during solid–liquid phase changes lead to cumulative grain boundary sliding and dislocation proliferation within sand-based materials [102], particularly at interfaces between silicon-based porous frameworks and phase change components where periodic volume expansion generates shear stresses reaching 1.5–2.3 GPa magnitudes, causing nanoscale crack initiation and propagation. In situ transmission electron microscopy observations reveal that after 500 thermal cycles, interfacial bonding strength in SiO₂/paraffin composite systems decreases by approximately 37%, directly related to lattice distortion caused by thermal expansion coefficient differences between phases [41].

The essence of interface stability degradation originates from the nonlinear responses of multiphase materials under dynamic thermal loads. Molecular dynamics simulations reveal that CTE mismatch between sand-based carriers and organic PCMs generates substantial interfacial strain under thermal cycling [103], manifesting as vacancy aggregation and atomic diffusion channel blockage that cause macroscopic delamination. Notably, graphene nanosheet-doped composites can increase interfacial thermal stress dispersion efficiency to 2.7 times that of traditional materials through three-dimensional interlocking structures [71], though this method has limited applicability for porous sand-based carriers with porosity exceeding 85%.

Current solutions primarily focus on interface engineering: ① Atomic layer deposition technology can construct 2–5 nm thick Al₂O₃ transition layers on sand grain surfaces, extending SiO₂/fatty acid system cycling life to over 3000 cycles [41]. ② Biomimetic spider web structure carbon fiber networks can confine PCMs within micrometer-scale cells, effectively suppressing liquid leakage and reducing interfacial stress concentration [104]. ③ Core–shell ZnO@SiO₂ nanoparticles as reinforcement phases demonstrate 42% improved stress buffering capacity compared to single components, originating from the shell layer amorphous structure’s adaptive regulation of plastic deformation [73]. However, existing research mostly focuses on static performance testing at laboratory scales, with systematic understanding of failure mechanisms under coupled mechanical vibration and temperature shock in actual operating conditions still lacking.

Notably, recent research has successfully predicted interface evolution patterns in multicomponent salt/sand composite systems under cyclic loading through machine learning-assisted deep potential molecular dynamics methods [103]. This method reveals that selective migration of Na⁺ ions in MgCl₂-CaCl₂ molten salts accelerates sand-based carrier surface corrosion, providing new dimensions for interface stability design. However, achieving precise control of interface dynamics while maintaining high energy density remains an urgent scientific challenge in this field.

Comparative analysis with lithium-ion batteries reveals complementary performance profiles: commercial LIBs achieve 80% capacity retention after 800–2500 cycles at 25 °C, degrading 52–94% faster at 45 °C [105,106], whereas sand-based PCMs maintain > 92% capacity after 1000 cycles at up to 300 °C [15,16]. However, achieving 3000–5000 cycle targets for daily use HPEG [28,29,30] requires advancing interface engineering beyond laboratory-scale demonstrations.

5.1.2. Balance Between Energy Density and Power Density

In the engineering process of phase change thermal storage systems, synergistic optimization of energy density (volumetric heat storage capacity) and power density (energy release rate per unit time) represents a core contradiction determining system practicality. For sand-based thermal storage media, while high specific surface areas of silicon-based porous frameworks can effectively encapsulate PCMs (such as paraffin or eutectic salts) through capillary action, their inherently low thermal conductivity (0.1–0.5 W/m·K) significantly limits power density [37,57]. Experiments show that when nano-graphite doping increases to 15 wt%, composite system thermal conductivity can reach 1.8 W/m·K, but PCM loading rate decreases from 92% to 78%, resulting in a 16% reduction in unit mass latent heat value [82]. This nonlinear trade-off relationship originates from competitive mechanisms between multi-scale heat transfer paths and heat storage unit distribution: while thermal conduction networks formed by nanoparticles accelerate heat diffusion, they also occupy micropore filling spaces for PCMs [35].

Recent breakthroughs in interface engineering provide new approaches to resolving this contradiction. In wood-derived aerogel-supported sand-based composite PCMs, MXene nanosheets form three-dimensional interpenetrating networks with cellulose fibers through hydrogen bonding, maintaining 78% PCM loading rates while increasing axial thermal conductivity to 0.82 W/m·K [82]. This directionally aligned multi-level pore structure (50–200 μm macropores synergizing with 2–5 nm mesopores) enables molten PCMs to form continuous liquid phase channels driven by capillary forces, reducing solid–liquid interfacial thermal resistance. Molecular dynamics simulations show 0.12 eV adsorption energy between surface carboxyl-functionalized carbon nanotubes and paraffin molecules, with this strong interaction suppressing phase separation and enhancing interfacial phonon transfer efficiency [78].

Nanoconfinement effects demonstrate unique advantages in balancing energy storage and release rates. When PCMs are encapsulated within clay interlayer domains below 2 nm, crystallinity increases by 23%, phase change enthalpy reaches 124.2 J/g, and thermal response time shortens by 40% [57,78]. This size effect originates from ordered molecular chain arrangement in confined spaces, both increasing storage density and accelerating phase change kinetics through reduced melting entropy. However, excessive spatial constraint can cause phase change temperature shifts (up to 15 °C maximum), potentially affecting matching efficiency with thermoelectric devices [34].

Current optimal sand-based composite PCMs have achieved energy densities exceeding 150 kJ/kg (corresponding to 1.2 kW/m³ power density), approaching 65% of theoretical limits [35,82]. Breaking through this bottleneck requires developing biomimetic hierarchical heat transfer structures, such as radial gradient pore designs inspired by plant vascular bundles, achieving anisotropic thermal conductivity (axial 1.5 W/m·K, radial 0.3 W/m·K) while maintaining high PCM loading rates (>85%) [34]. Additionally, intelligent responsive phase change systems (such as photothermal-triggered crystallization) can achieve on-demand adjustment of energy release rates through external field control, providing new paradigms for dynamic power management [35,78]. Beyond material-intrinsic limitations, practical deployment faces equally significant challenges at the system integration level, where multiple components must operate reliably under real-world conditions.

Quantitative benchmarking clarifies performance trade-offs: lithium-ion systems achieve 200–300 Wh/kg energy density with 85–95% round-trip efficiency [107], while sand–PCM–thermoelectric pathways reach 36–50 Wh/kg at 8–12% efficiency. Despite this 5–6× energy density gap, sand-based systems offer 60–85% cost advantages ($20/kWh vs. $150–304/kWh for LIBs) [16], positioning them competitively for long-duration (>8 h) off-grid applications where temporal buffering supersedes instantaneous power delivery.

5.2. System Integration Challenges

5.2.1. Dynamic Thermal Management Control Strategies

In integrating sand-based thermal storage systems with HPEG, dynamic thermal management control strategies must address the contradictions between intermittent mechanical energy input and periodic thermal demands. The core lies in real-time adjustment of heat flow paths and phase change rates to balance temperature gradients and energy conversion efficiency in storage units [36,75]. Research indicates that when the interfacial contact thermal resistance of sand-based composite PCMs exceeds 0.05 K·m²/W, system response delays lead to over 15% energy loss [76], requiring control strategies to establish quantitative correlation models between thermal resistance and phase change kinetics. Based on solid–liquid interface movement patterns within porous media, asymmetric pulsed heat flow distribution algorithms can reduce transient thermal fluctuation amplitudes to within ±2.1 °C while maintaining over 92% thermoelectric conversion efficiency [108].

Thermal management instability caused by PCM cycling degradation is particularly prominent. Experiments show that after 500 thermal cycles, local thermal conductivity reduction in sand-based composites due to microcracks can reach 38% [61], disrupting preset heat flow distribution networks. To address this, introducing distributed fiber optic sensor-based temperature feedback systems with machine learning dynamic correction of heat flow redistribution coefficients can reduce efficiency degradation rates from interface failure from 0.12%/cycle to 0.03%/cycle [36]. Notably, environmental humidity poses special challenges to control algorithm adaptability: when relative humidity > 70%, surface-adsorbed water films on sand-based materials reduce effective thermal diffusivity by 19–26% [109], requiring humidity compensation functions coupled in control models and two-phase flow simulation in porous media to optimize cooling channel layouts [75].

Recent advances show that doping magnetic responsive nanoparticles into sand-based phase change systems enables dynamic deflection of heat flow vectors using external magnetic fields. Under 4.0 kW/m² equivalent heat flux density, this active control strategy shortens thermal response time to 1/3 of traditional passive systems while improving the heat flow distribution uniformity index to 0.91 [75]. However, this technology still faces balancing challenges between high-frequency magnetic field energy consumption and nanoparticle agglomeration, requiring development of micro magnetic control arrays self-powered by phase change latent heat, pointing toward innovative directions for next-generation intelligent thermal management systems [36,108].

5.2.2. Environmental Adaptability

Environmental adaptability represents an unavoidable core challenge in sand-based thermal storage system integration, with performance exhibiting highly nonlinear coupling relationships with external environmental parameters. Extreme temperature fluctuations cause thermodynamic imbalance in phase change processes. For example, in desert regions with day–night temperature differences exceeding 50 °C, non-uniform expansion of silicon-based porous structures triggers pore collapse, causing heat storage capacity reductions of 22.3% [73]. More severely, when environmental temperatures exceed PCM solid–liquid phase change windows (such as −30 °C in winter or 80 °C in summer), thermal storage systems degrade to simple sensible heat storage, with energy density reductions reaching 68% [72]. Existing interface stability research shows that introducing graphene oxide nanosheets (5 wt%) can form topological interlocking structures on porous silicon substrate surfaces, controlling PCM leakage rates below 0.9%, though local delamination still occurs under extreme conditions with temperature change rates exceeding 30 °C/min [110].

Humidity environment corrosion mechanisms on silicon-based porous structures exhibit multi-scale characteristics: at mesoscopic scales, water molecule penetration weakens the Si-O-Si framework’s bonding strength, causing specific surface area degradation at 0.15 m²/g·RH% [111]; at microscopic levels, hydroxylation reactions induced by capillary condensation water generate amorphous SiO₂·nH₂O, causing thermal conduction path blockage and 30–40% thermal conductivity loss [78]. Experiments confirm that atomic layer deposition (ALD) of 3 nm thick Al₂O₃ barrier layers on pore walls extends material lifetime in 95% RH environments to over 5000 h, though this process reduces heat storage density by 12.7% [38].

Structural failure under mechanical vibration scenarios originates from multiphysics field coupling effects: periodic loads at 10–50 Hz frequencies induce pore resonance, causing composite material elastic modulus to decrease at 0.8 MPa/√Hz rates [112]. Finite element simulations show that when vibration acceleration exceeds 3 g, microcrack networks form within sand-based composites, with propagation rates negatively correlated with storage modulus logarithms (R² = 0.93). Gradient encapsulation technology—using silicon carbide fiber cloth as substrate with layer-by-layer deposition of polyimide/silicon dioxide hybrid coatings—can increase interfacial binding energy to 2.1 J/m², maintaining 92% initial heat storage capacity after 3000 cycles under 5 g vibration conditions [113]. However, existing protection schemes generally face excessive mass-to-power ratio bottlenecks, with typical values reaching 0.37 kg/W, severely restricting mobile vehicle applications [35]. This exceptional cycling stability, maintaining >90% enthalpy retention over 1000 cycles (Figure 5g), validates the structural integrity of multi-level pore designs under repeated phase change stresses.

Breaking through environmental adaptability bottlenecks requires establishing material–system synergistic optimization frameworks: at material levels, developing self-healing ZIF-8@MXene composite coatings can achieve in situ microcrack repair through dynamic coordination bond reorganization, extending lifetime in humid-heat environments to 8000 h [34]; at system levels, constructing multi-parameter coupled control models based on LSTM neural networks, through real-time adjustment of heat flow distribution ratios in storage units, can control output power stability within ±2.1% despite ±40 °C environmental fluctuations [114,115]. Material-level thermal stability tests demonstrate sand-based composites maintain structural integrity across −30 °C to 300 °C (depending on PCM melting point and SiO₂ matrix sintering temperature), though operational efficiency varies significantly.

For HPEG applications, the practical operational window is 0–40 °C (typical ambient conditions for wearable devices and community hubs). Within this range, experimental validation shows material-control synergistic strategies achieve 81.3% round-trip efficiency (defined by Equation (1)) with ±2.1 °C temperature uniformity across modular arrays, representing a 23.6 percentage point improvement over traditional schemes lacking active thermal management [40,114]. Beyond this window, systems transition to sensible heat storage with 60–70% efficiency degradation, limiting extreme-temperature deployments.

Economic analysis reinforces this competitiveness: thermal energy storage averages $232/kWh capex for 8 h duration systems, 24% lower than lithium-ion’s $304/kWh for 4 h systems, translating to 5–8× lower levelized costs ($0.12–0.18/kWh vs. $0.35–0.55/kWh) for off-grid HPEG deployments [116]. Economic viability reconciles efficiency–cost trade-offs: For 2 kWh off-grid systems, sand-based storage costs $60 total (10-year lifecycle: $40 capital + $20 maintenance over 1000 cycles) versus $450 for lithium-ion ($300 capital + $150 replacements at 500-cycle lifetime under 40 °C/85% RH conditions [105,106]). Despite 6–9% system efficiency versus 85–95% for batteries, 7.5× lower capital cost and 2.8× superior cycle life yield $0.12–0.18/kWh LCOS versus $0.35–0.55/kWh [116], positioning thermal storage where upfront capital—not energy efficiency—constitutes the binding constraint in resource-limited deployments [31].

6. Future Development Directions and Innovation Pathways

Overcoming the identified challenges requires not only incremental improvements but also paradigm-shifting innovations in both material design and system architecture. This section explores emerging research directions that promise to transcend current limitations, focusing on biomimetic design strategies, intelligent responsive systems, and novel integration approaches for next-generation applications.

6.1. Development of Novel Sand-Based Composite Materials

6.1.1. Biomimetic Multi-Level Pore Structure Design

Biomimetic multi-level pore structure design draws inspiration from nature’s exquisite hierarchical pore systems in biological materials, such as wood’s vascular bundle systems achieving fluid transport through millimeter-scale conduits, bone’s micrometer-scale trabecular networks providing mechanical support, and nanoscale hydroxyapatite pores regulating ion exchange [34]. These cross-scale pore synergistic mechanisms provide theoretical paradigms for three-dimensional structural optimization of sand-based thermal storage materials: constructing macroscopic heat storage chambers through macropores (>50 μm) to enhance PCM loading rates, strengthening PCM–matrix interfacial binding energy through mesopore channels (2–50 μm), and forming nanoconfinement effects through micropore structures (<2 nm) to suppress volume expansion during phase changes [64]. Based on this, template-assisted self-assembly technology demonstrates significant advantages, such as constructing inverse opal structure macropore frameworks using polystyrene microsphere templates, introducing mesoporous SiO₂ coatings through sol–gel methods, and finally, growing carbon nanotube micropore networks on pore walls via chemical vapor deposition, forming three-level continuously interconnected thermal conduction paths [117].

Experimental research shows that when macropore porosity is controlled at 65–75%, mesopore diameter distribution concentrates at 20–30 μm, and micropore specific surface area reaches 800 m²/g, sand-based composite material phase change enthalpy values can exceed 320 J/g, an approximately 40% improvement over traditional homogeneous structures [83]. This performance leap results from multi-level pore synergistic effects: macropore frameworks ensure efficient encapsulation of high-enthalpy PCMs like PEG, mesopore channels suppress molten PCM leakage through capillary forces, while micropore networks increase thermal conductivity to 1.8 W/m·K through interfacial phonon coupling effects, 239% higher than non-hierarchical systems [117]. Particularly noteworthy is that biomimetic porous structures can also impart dynamic response characteristics to materials, such as achieving precise phase change temperature adjustment within 25–80 °C ranges through pore curvature radius control, providing critical material foundations for matching the intermittent heat generation characteristics of HPEG [35].

Recent breakthrough research reveals the deep impacts of pore topology on thermal storage performance. Three-dimensional pore network reconstructions using synchrotron radiation X-ray tomography show that dendritic pore structures with fractal characteristics can increase thermal diffusion rates to 2.3 times those of isotropic pore structures [64]. This biomimetic optimization design successfully achieves 98% phase change enthalpy retention after 3000 thermal cycles in sand-based composites, providing new approaches to solving thermal degradation challenges in traditional PCMs [83]. With advances in cryo-electron tomography and other advanced characterization techniques, future prospects include atomic-scale resolution of lattice vibration modes at multi-level pore interfaces, establishing precise structure–property relationships between pore topology and thermodynamic performance [34].

6.1.2. Intelligent Responsive Phase Change Systems

Building upon biomimetic multi-level pore structure design, intelligent responsive phase change systems achieve a dynamic balance between heat storage density and energy release rates by introducing external field (light, magnetic, temperature) programmable control of active components. The core lies in constructing composite phase change materials (PCMs) with multi-modal response capabilities, such as through the photoisomerization mechanisms of azopyridine polymers, enabling molecular configuration transitions under specific wavelength illumination to precisely control the storage and release timing of phase change enthalpy [118]. Such material systems typically employ nanocomposite strategies to enhance response sensitivity: graphene or carbon nanotube networks not only increase thermal conductivity above 3.5 W/(m·K) [40], but their π-π conjugated structures also serve as photothermal conversion media, achieving 98% light absorption efficiency in near-infrared bands, providing rapid thermal excitation channels for light-controlled phase changes.

Magnetic nanoparticle doping opens new dimensions for magnetic–thermal synergistic control. Fe₃O₄@SiO₂ core–shell structures can generate directional heat flow in alternating magnetic fields through localized surface plasmon resonance effects, enabling 0.5 K/s precise temperature control in spatially confined scenarios [34]. This dynamic control capability, combined with hierarchical pores in sand-based porous carriers, successfully improves PCM cycling stability to over 300 cycles while maintaining high phase change enthalpy values of 130 J/g [119]. Notably, the molecular-level reversible deformation characteristics of photoisomerization materials (such as azobenzene derivatives) enable microscopic thermodynamic state switching without triggering macroscopic phase changes, providing innovative solutions to leakage problems in traditional solid–liquid PCMs [61].

Interface engineering is particularly critical in miniaturized thermal storage–generation integrated devices. Constructing 2 nm thick Al₂O₃ interface layers on carbon nanotube surfaces through atomic layer deposition can reduce contact thermal resistance between solid–solid PCMs and thermoelectric conversion modules to 10⁻⁶ m²·K/W levels [36]. However, interface stability under multi-field coupling conditions remains challenging: lattice distortions caused by photothermal cycling lead to nanoscale cracks at graphene–PCM interfaces, with thermal conductivity degradation reaching 22% after 500 cycles [35]. Future development of in situ self-healing interface technologies, combined with dynamic covalent bond design, is needed to achieve long-term stable operation of storage units under complex operating conditions. Complementing these material innovations, system-level architectural advances are necessary to realize the full potential of sand-based thermal storage in practical energy applications.

Actionable thermoelectric targets include: (1) materials achieving ZT > 1.5 at 300–500 K—n-type Mg₃Sb₂ alloys (ZT ≈ 1.3) and p-type BiCuSeO systems (ZT ≈ 1.5)—through dual-doping [120,121]; (2) module-level interfacial resistance <10⁻⁶ m²·K/W via diffusion-barrier contacts [122]; (3) production costs <$5/W (vs. current $15–20/W for Bi₂Te₃). Achieving ZT = 1.5 would elevate system efficiency to 12–15%, reducing required storage capacity by 20–25% for equivalent electrical output.

6.2. System-Level Innovation Applications

6.2.1. Off-Grid Community Energy Hubs: Human-Powered Storage Integration

Within system-level innovation frameworks, wearable integrated thermal storage-generation devices break traditional physical boundaries between energy storage and collection modules, achieving all-weather capture and on-demand distribution of human mechanical energy through synergistic topology of multi-modal energy flows. The core design concept integrates phase change thermal storage units with energy harvesting elements in biomimetic hierarchical structures on flexible substrates, forming composite systems with both energy buffering and instant conversion capabilities [73,110]. Taking fiber-based PCM multilayer fabric structures as carriers, internal three-dimensional networks absorb binary eutectic phase change gels (such as lauric acid/myristic acid systems) through capillary action, maintaining 124.6 J/g phase change latent heat while achieving 96.5% photothermal conversion efficiency. Outer functionalized P(VDF-HFP) triboelectric dielectric layers convert mechanical motion to electrical energy output through contact–separation mechanisms [73,110]. This conformal integration of porous sand-based thermal storage modules with TENG enables devices to maintain stable thermal–electrical coupling output characteristics under periodic strain from human body motion.

Material design breakthroughs reflect synergistic optimization of nanoconfinement effects and dynamic interface control. Inspired by wood’s unidirectional transport characteristics, anisotropic porous sand-based frameworks encapsulate PCMs, achieving a balance between heat storage density and mechanical flexibility through biomimetic channel design. Research shows that nickel-induced dual-carbon network (CH@Ni-CNTs) composite phase change systems not only increase photothermal conversion efficiency to 96.9%, but their unique carbon nanotube shuttle structures also impart superelastic deformation capabilities, maintaining stable phase change enthalpy of 131.0 J/g after 300 thermal cycles [40]. This structural advantage combined with TENG flexible electrodes (such as MXene-modified silver nanowire networks) controls energy conversion efficiency degradation within 5% under 90° bending conditions, establishing foundations for practical wearable device applications.

Community-scale HPEG systems integrated with sand-based thermal batteries address critical energy access challenges for the 1.2 billion people lacking reliable electricity [26,27], where conventional grid extension costs ($5000–15,000/km) exceed local capital capacity. The fundamental innovation lies in matching sand batteries’ exceptional cost advantage ($20/kWh versus $150–200/kWh for lithium-ion systems, validated in Finland’s 100 MWh commercial demonstration [19]) with the intermittent mechanical energy characteristics inherent to human-powered generation. Distributed HPEG equipment—exercise bicycles and hand-crank generators producing 200–500 W instantaneous power [20]—undergoes sequential energy conversion: primary mechanical-to-electrical conversion at 55% efficiency [77], electrical-to-thermal storage in hierarchical porous composites achieving > 90% charge efficiency through resistive heating in carbon nanotube networks [39,76], and thermal-to-electrical reconversion via thermoelectric modules at 8–12% efficiency under ΔT = 40–60 K gradients [74,75]. A representative 30 min exercise session demonstrates the complete energy budget. Given average mechanical power Pm = 150 W over duration t = 1800 s, the input mechanical energy is as follows:

$$Em=Pm\times t=150\mathrm{W}\times 1800\mathrm{s}=270\mathrm{k}\mathrm{J}$$

(6)

Assuming frictional heat generation efficiency η friction ≈ 0.90 (10% mechanical losses), thermal energy available for storage $Q_{available}=0.90\times 270 \mathrm{k}\mathrm{J}=243 \mathrm{k}\mathrm{J}$. With sand–PCM charge efficiency ${\eta }_{charge}=0.82$ measured for CNT-doped hierarchical porous composites [39,74], stored thermal energy becomes

$$Q_{stored}={\eta }_{charge}\times Q_{available}=0.82\times 243\mathrm{k}\mathrm{J}\approx 199\mathrm{k}\mathrm{J}$$

(7)

Over an 8 h retention period at 25–35 °C, thermal retention efficiency ${\eta }_{storage}\approx 0.85$ yields $Q_{retained}\approx 169\mathrm{k}\mathrm{J}$. With thermoelectric conversion efficiency ${\eta }_{TE}=0.10\left(\mathrm{E}\mathrm{q}\mathrm{u}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}\left(2\right),\mathsf{\Delta }T\approx 50\mathrm{K}\right),$

$$Eelectric={\eta }_{TE}\times Q_{retained}=0.10\times 169\mathrm{k}\mathrm{J}\approx 17\mathrm{k}\mathrm{J}$$

(8)

The overall efficiency is ${\eta }_{overall}={\displaystyle \frac{Eelectric}{Em}}={\displaystyle \frac{17}{270}}\approx 0.063\left(6.3\%\right)$, falling within the reported 8–12% range when accounting for optimized thermal management and higher-performance thermoelectric modules (ZT > 1.0). This calculation demonstrates that thermal-buffered pathways trade efficiency for essential temporal decoupling capability.

Miniaturized sand battery modules (0.1–5 kWh capacity) achieve technical performance through material–system synergistic design optimized for daily thermal cycling. Hierarchical pore structures enable 92–95% PCM loading while carbon nanomaterial doping (4–5 wt% CNT/MXene) reduces thermal response time to 180–300 s, matching 10–30 min HPEG session durations [68]. Experimental validation demonstrates that 2 m³ sand-based thermal storage modules maintain > 85% energy retention over 8 h periods at 25–35 °C ambient temperatures, with thermal conductivity of 1.8–3.1 W/(m·K) enabling efficient heat extraction [38,41]. System integration employs distributed sensor networks with model predictive control algorithms that reduce temperature non-uniformity to ±2.1 °C across modular arrays [36]. Laboratory demonstrations show rolling TENG-based bicycle systems generate 425 V open-circuit voltage over 21 s of mechanical driving, with sand-based PCM modules converting friction heat to latent storage and enabling subsequent 417 s stable power output [77].

The required thermal storage capacity ($Q_{req}$) for temporal decoupling can be estimated from power mismatch and desired output duration:

$$Q_{req}={\displaystyle \frac{\left(P_{out}\times t_{out}\right)}{{\eta }_{TE}}}=P_{out}\times t_{out}\times \left({\displaystyle \frac{TH}{TH-TC}}\right)\times \left[{\displaystyle \frac{\sqrt{1+ZT}+\frac{TC}{TH}}{\sqrt{1+ZT}-1}}\right]$$

(9)

For continuous 5 W output over t_out = 8 h with ΔT = 50 K and ZT = 1.0, $Q_{req}\approx 1.44\mathrm{M}\mathrm{J}$ (400 Wh thermal). With a sand–PCM volumetric energy density of 150 kJ/L (based on 90% PCM loading and 130 J/g latent heat), the required storage volume is approximately 9.6 L. Accounting for the heat exchanger and insulation, practical module dimensions reach 15–20 L (0.015–0.020 m³), consistent with reported community-hub designs utilizing 2 m³ arrays supporting 50-person usage [116,123]. This scaling relationship guides modular system design for specific HPEG applications.

Technoeconomic analysis reveals capital costs of $400–600/kWh for miniaturized systems, representing a 5–8× reduction compared to lithium-ion microgrids due to abundant feedstock materials (<$0.5/kg processing costs), simplified thermal management, and extended cycle life (>1000 cycles with <8% degradation) [15]. Levelized cost of storage calculations yield $0.12–0.18/kWh versus $0.35–0.55/kWh for conventional electrochemical storage in off-grid contexts [116]. However, broader implementation requires addressing socioeconomic factors, including community participation models and operational sustainability, necessitating rigorous field validation and comprehensive impact assessments across diverse deployment contexts before scaled adoption [31].

6.2.2. Distributed Microgrid Energy Storage Nodes

As energy hubs, distributed microgrids impose dual requirements on energy storage systems: satisfying high energy density for long-duration power supply while possessing rapid power response capabilities to smooth fluctuations from intermittent renewable energy [116,124]. Sand batteries, with their solid–solid phase change thermal storage characteristics, demonstrate unique advantages in the 15–25 kWh/m³ energy density range, with thermal inertia-driven slow-release characteristics complementing the rapid discharge characteristics of supercapacitors [123,125]. Through the topological arrangement of sand-based thermal storage modules, pulsed mechanical energy from HPEG (peak power approximately 200–500 W) can be converted to stable heat flow, outputting 2–5 W/m² of continuous electrical energy through thermoelectric conversion units, achieving over 72 h energy spatiotemporal transfer [74,126].

System architecture innovation reflects three-level synergy: the bottom layers employ biomimetic porous sand-based thermal storage bodies to enhance thermal diffusion efficiency (thermal conductivity reaching 3.2 W/m·K), the middle layers configure dual-mode thermal switches (mechanical phase change valves combined with electrostrictive membrane structures), and the top layers achieve multi-node dynamic scheduling through improved branching in dual Q-network algorithms [127,128]. Experiments demonstrate that microgrid node clusters comprising 50 power-generating bicycles, supported by 3.6 m³ of sand-based thermal storage bodies, can maintain 5 kW-level loads continuously for 12 h with system cycling efficiency of 61.7% [116,123]. In actual deployment in an off-grid community in Tibet, this system successfully transferred 18.6 kWh of daytime exercise-generated energy to nighttime lighting and heating, validating robustness against extreme temperature differences (−15 °C to 25 °C) and environmental humidity fluctuations (30–85% RH) [36,124].

The quantum design of the thermal–electrical interfaces breaks through traditional efficiency bottlenecks: introducing gradient-doped Bi₂Te₃/Sb₂Te₃ superlattice thermoelectric materials achieves 13.2% thermoelectric conversion efficiency under ΔT = 80 K conditions, a 47% improvement over traditional modules [125,128]. Combined with intelligent heat flow distribution algorithms based on model predictive control, systems can dynamically adjust the thermal coupling strength of 24 independent storage units, completing 0.5–5 kW power step responses within 10 s [124,126]. This spatiotemporal decoupled storage characteristic enables system capacity utilization rates 2.3 times higher than lithium battery solutions when addressing solar–bioenergy hybrid power supply scenarios [116,123]. These future-oriented innovations, combined with demonstrated performance achievements, position sand-based thermal storage as a transformative technology for distributed energy applications; this multidimensional innovation strategy is visualized in Figure 7. Deployment specifications should target the following: Phase 1 (0–18 months)—0.5–2 kWh units with >80% charge efficiency over 4–8 h, validated through 500-cycle testing; Phase 2 (18–36 months)—manufacturing costs <$0.8/kg for composites and <$8/W for thermoelectric modules, achieving $250–400/kWh installed cost; Phase 3 (36–60 months)—field systems demonstrating > 1000 cycles with <10% annual degradation and <50 kg CO₂-eq/kWh lifecycle emissions.

7. Conclusions

From material performance validation to system integration, sand-based phase change materials have demonstrated significant technical feasibility. In thermal conductivity enhancement, through biomimetic multi-level pore structure design and nano-doping strategies (such as MXene composite systems and carbon nanotube network construction), material thermal conductivity has exceeded 0.82 W/m·K, 4.6 times higher than traditional organic PCMs, achieving anisotropic heat transfer optimization. Cycling stability through interface crosslinking reinforcement and microcapsule encapsulation technology maintains 91.5% phase change enthalpy retention after 200 thermal cycles, with supercooled stability technology achieving thousand-cycle operation without degradation. In human-powered generation scenarios, the integration of sand-based thermal storage units with triboelectric nanogenerators demonstrates a mechanical-to-electrical energy conversion efficiency of 8–12% (with a thermal storage efficiency of 82.3%), where the primary conversion occurs via direct triboelectric mechanisms rather than thermal-to-electrical pathways. The thermal storage component primarily serves as an energy buffer with >80% charge–discharge efficiency, while thermoelectric conversion contributes an additional 2–5% under optimal temperature gradients (ΔT = 40–60 K), combined with tri-modal energy storage mechanisms achieving an energy density of ≈ 140–180 J g⁻¹ and volumetric energy storage of ≈ 40–60 kWh m⁻³, sufficient for wearable and micro-device applications.

However, scalability in developing regions faces material sourcing challenges. High-purity silicon processing and carbon-based nano-dopants (graphene, CNTs, MXene) require supply chains concentrated in industrialized areas, potentially offsetting cost advantages. Localized production using biomass-derived carbons or agricultural waste-derived porous materials could reduce import dependence by 60–75%, though this necessitates parallel development of regional manufacturing capabilities.

A commercialization potential assessment shows that sand-based thermal storage systems’ levelized energy storage costs have decreased to the $20/kWh critical point, 60% lower than lithium-ion batteries. Its raw material cost advantages are 10 times higher compared to graphite-based thermal batteries, with scalable production costs of $0.5/kg achievable through mechanical alloying processes. In mobile application scenarios, their modular design enables a thermal storage unit power density of 2.05 g/cm³, successfully achieving the development of a wearable thermal–electrical co-generation system combined with intelligent fabric integration technology. Commercialization potential depends on overcoming critical barriers: (1) reducing field-validated cycling life from current the 500–1000 to target 3000+ cycles through improved encapsulation, (2) establishing supply chains for nano-doped sand–PCM production in resource-constrained regions, and (3) developing technical training programs for installation and maintenance. Technoeconomic analyses indicate levelized costs of $0.12–0.18/kWh for miniaturized systems versus $0.35–0.55/kWh for lithium-ion microgrids [116], providing an economic incentive for off-grid deployment. However, comprehensive field trials in diverse climatic and socioeconomic contexts remain essential before scaled adoption.

To achieve scaled application of sand batteries in HPEG, breaking traditional disciplinary boundaries is essential, establishing multi-dimensional collaborative innovation paradigms across materials science, mechanical engineering, and energy systems. In materials, surface, and interface engineering—based on carbon atom vibration coordination mechanisms revealed by molecular dynamics simulations combined with thermodynamic analysis to construct directional thermal conduction networks—these batteries can achieve atomic-level matching between thermal storage media and functional coatings through biomimetic multi-level pore structure design.

Mechanical transmission system integration innovation can reference injection molding processes for flexible phase change materials, developing miniaturized thermal storage modules with self-adaptive characteristics while utilizing triboelectric nanogenerator theoretical frameworks to optimize mechanical–thermal synergistic conversion paths. Energy conversion efficiency breakthroughs require establishing thermal–electrical–mechanical multi-physics field coupling models, referencing uncertainty cognition methods from photovoltaic thermal systems and developing energy flow optimization algorithms under dynamic conditions. Intelligent control should integrate real-time optimization strategies from modular battery systems, achieving precise thermal storage power regulation through reconfigurable power supply topology. Environmental sustainability requires integrating environmentally friendly processes for silicon nanostructure preparation, establishing full lifecycle assessment systems from nano-doping to system integration, ensuring the environmental compatibility of sand-based thermal storage materials through their manufacturing, use, and recycling stages. This deep multidisciplinary intersection not only requires breaking the cognitive boundaries of traditional thermal management technologies but also demands the establishment of cross-scale collaborative design methodologies, ultimately achieving full-chain innovation from molecular vibration control to system energy management.