Phase-change material
A phase-change material (PCM) is a substance that absorbs or releases a significant amount of latent heat during a phase transition, typically between solid and liquid states, enabling efficient thermal energy storage and temperature regulation with minimal volume change.[1] These materials are characterized by high energy storage density—often 5 to 14 times greater than sensible heat storage methods—due to the isothermal nature of the phase change process, which occurs at a nearly constant temperature.[1] PCMs are broadly classified into three categories: organic (e.g., paraffins and fatty acids, with latent heats of 150–250 kJ/kg and melting points from 20°C to 60°C), inorganic (e.g., salt hydrates, offering 170–330 kJ/kg and higher thermal conductivity of 0.5–1.0 W/m·K but prone to supercooling and corrosion), and eutectic mixtures (combinations of the above for tailored phase transition temperatures and higher volumetric latent heat storage densities).[2] Desirable properties include congruent melting to prevent phase separation, chemical stability, non-toxicity, and low flammability, though challenges like low thermal conductivity in organics (0.1–0.3 W/m·K) often necessitate enhancements such as encapsulation or composite formulations with expanded graphite or nanoparticles.[1][2] The primary applications of PCMs leverage their ability to moderate temperature fluctuations and store renewable energy, including passive building cooling (e.g., integration into walls or ceilings to reduce peak loads by up to 35%), solar thermal systems (e.g., in Trombe walls for off-peak heat release), electronics and battery thermal management (e.g., preventing overheating in lithium-ion cells), and niche uses like spacecraft temperature control or protective textiles for firefighters.[1][2] Recent advances focus on shape-stabilized composites and microencapsulation to address leakage and improve heat transfer, expanding their role in sustainable energy solutions amid growing demands for energy efficiency.[2]Fundamentals
Definition and Principles
Phase-change materials (PCMs) are substances designed to store and release large amounts of thermal energy through reversible phase transitions, typically solid-liquid or solid-solid, occurring at a nearly constant temperature. These materials exploit the high latent heat of fusion inherent in the phase change process, allowing them to absorb heat when melting (endothermic) and release it when solidifying (exothermic) without significant temperature variation. This isothermal behavior makes PCMs particularly effective for thermal management where stable temperatures are required.[3][4] The thermodynamic foundation of PCMs lies in latent heat storage, distinct from sensible heat storage in materials like water or concrete, where energy is absorbed or released primarily through temperature changes (Q = m * c * ΔT, with c as specific heat capacity). In contrast, latent heat for PCMs is quantified by the equation ΔH = m * L, where ΔH is the enthalpy change, m is the mass, and L is the specific latent heat of fusion, often ranging from 100 to 300 kJ/kg for common PCMs. This mechanism provides a volumetric energy storage density up to 10 times higher than sensible heat methods at equivalent temperature ranges, enabling compact and efficient thermal energy systems.[5][6] A representative example of a solid-liquid PCM is paraffin wax, which transitions from solid to liquid at temperatures around 50–60°C, storing approximately 200 kJ/kg of latent heat while maintaining phase stability over multiple cycles.[4] The concept of PCMs gained prominence in the 1970s during the global energy crisis, when research focused on their use for solar thermal energy storage to address intermittent supply challenges; the term "phase-change material" emerged in this context.[5]Phase Transition Mechanisms
Phase-change materials (PCMs) primarily utilize solid-liquid transitions for thermal energy storage, as these involve substantial latent heat absorption or release during melting and freezing at a relatively constant temperature. This process is governed by thermodynamic principles where the material absorbs heat to overcome intermolecular forces, transitioning from an ordered solid lattice to a more disordered liquid state without significant temperature change until the phase transition is complete. Solid-solid transitions, involving structural rearrangements within the solid phase (e.g., from one crystalline form to another), are less common due to their typically lower latent heat values and slower kinetics, though they avoid issues like leakage associated with liquidity. Liquid-gas transitions, while offering higher latent heats, are rarely employed in practical PCM applications because they entail large volume expansions (up to 1000 times) and require high pressures to contain the vapor phase, rendering them inefficient for compact storage systems. Solid-gas transitions, such as sublimation, are even more impractical owing to extreme volume changes and low energy densities under ambient conditions.[7] The mechanisms underlying these phase transitions begin with nucleation, the initial formation of stable phase embryos, followed by growth where the new phase propagates through the material. In solid-liquid PCMs, heterogeneous nucleation often dominates during freezing, initiated at impurities, container walls, or additives that lower the energy barrier for crystal formation; homogeneous nucleation, occurring spontaneously in the bulk, requires significant undercooling and is less common. During melting, the reverse process involves dissolution of the solid lattice, with growth rates influenced by heat transfer rates and material purity. Supercooling, a key kinetic effect, occurs when the liquid phase persists below the equilibrium freezing temperature due to insufficient nucleation sites, potentially delaying energy release and reducing system efficiency; this can exceed 10-20% of the melting point in some materials. Hysteresis refers to the temperature offset between melting and freezing points, arising from supercooling, kinetic barriers, or structural changes, which can widen the effective transition range and impact thermal management precision.[8][9][10] Efficiency in PCM phase transitions is often evaluated using figures of merit that balance latent heat storage against sensible heat losses over the transition temperature range. A key metric is defined as St = \frac{\Delta H}{\Delta T \cdot C_p}, where \Delta H is the latent heat of fusion, \Delta T is the temperature range of the transition (including hysteresis and supercooling effects), and C_p is the specific heat capacity; higher values indicate superior performance by maximizing latent heat relative to sensible contributions. Narrow transition ranges (\Delta T \approx 0-5^\circC) are ideal, as they minimize sensible heat dilution and enable isothermal operation, enhancing overall storage density. This metric underscores the preference for materials with sharp, reversible transitions to optimize energy retention in applications like building thermal regulation. A critical distinction in phase transition behavior is between congruent and incongruent melting, particularly relevant for inorganic PCMs like salt hydrates. Congruent melting occurs when the solid phase transforms directly into a liquid of identical composition, ensuring reversibility and stability over cycles without phase separation. In contrast, incongruent melting involves decomposition into a liquid and a solid phase of different compositions (e.g., anhydrous salt precipitating from a hydrated melt), leading to stratification, reduced latent heat in subsequent cycles, and potential container corrosion. Salt hydrates such as sodium sulfate decahydrate exemplify this issue, where incongruent behavior causes the solid anhydrous phase to settle, necessitating additives or encapsulation to maintain uniformity. This mechanism directly affects long-term reliability, with congruent materials offering higher cycle stability.[11][12]Classification
Organic Phase-Change Materials
Organic phase-change materials (PCMs) constitute a primary class of carbon-based, non-polar compounds that primarily exhibit solid-liquid phase transitions for thermal energy storage. These materials are derived from natural or synthetic sources and include subtypes such as paraffins, fatty acids, polyalcohols, and polymers. Paraffins, consisting of linear alkanes like n-eicosane (C20H42), serve as representative examples with a melting point of approximately 36.9°C and a latent heat of fusion around 247 kJ/kg, making them suitable for applications near room temperature. Fatty acids, such as stearic acid (C18H36O2), offer higher melting points of about 69°C and latent heats of roughly 200 kJ/kg, providing versatility for moderate-temperature storage. Polyalcohols, including fatty alcohols like octadecanol, and polymers such as polyethylene glycol (PEG), further expand the category; PEG, for instance, allows tunable phase transition temperatures through molecular weight variations while maintaining high enthalpies of fusion exceeding 150 kJ/kg.[13][14][15][16][17] These organic PCMs demonstrate several key advantages that enhance their practicality in thermal management systems. They exhibit excellent chemical stability over repeated cycles, with no phase separation or significant supercooling, ensuring consistent performance. Unlike inorganic counterparts, organic PCMs are non-corrosive to common construction materials, possess low toxicity, and undergo congruent melting, which preserves their composition during transitions. Additionally, they cover a broad temperature range from -5°C to 120°C, accommodating diverse applications without the need for complex handling.[15][18][4] Despite these benefits, organic PCMs face notable limitations that can impact efficiency. Their thermal conductivity is inherently low, typically ranging from 0.1 to 0.2 W/m·K for paraffins and fatty acids, which slows heat transfer rates and requires enhancements for practical use. Flammability poses a safety concern, particularly for paraffins, necessitating fire-retardant measures in enclosed systems. Furthermore, phase transitions involve volume changes of 10-20%, leading to potential mechanical stress or leakage if not properly contained.[19][3][15][20] To address sustainability challenges, bio-based organic PCMs derived from vegetable oils and waste fats have emerged prominently since the 2010s, offering renewable alternatives to petroleum-derived options. These materials, such as those processed from palm or soy oils into fatty acid esters, maintain comparable thermophysical properties while reducing environmental impact through biodegradable sourcing. Research highlights their potential for eco-friendly thermal storage, with examples achieving melting points around 30-40°C and latent heats over 180 kJ/kg.[21][22][23]Inorganic Phase-Change Materials
Inorganic phase-change materials (PCMs) encompass a diverse class of substances that undergo solid-liquid phase transitions, offering high energy storage capacities suitable for thermal management applications. These materials are primarily categorized into salt hydrates, pure salts, and metals or alloys, each exhibiting distinct thermophysical properties that make them advantageous for specific temperature ranges. Unlike organic PCMs, inorganics generally provide superior volumetric energy density due to their higher densities and latent heats, though they present unique stability challenges.[12][24] Salt hydrates, such as sodium sulfate decahydrate (Na₂SO₄·10H₂O), represent a prominent subtype with phase transition temperatures around room conditions, for instance, melting at 32.4°C and delivering a latent heat of 239–254 kJ/kg. Pure salts like sodium nitrate (NaNO₃) target higher temperatures, with a melting point of 306.4°C and latent heat of 178.6 kJ/kg, making them viable for industrial heat recovery. Metals and alloys, exemplified by gallium, operate at low temperatures (melting point 29.8°C, latent heat 80.1 kJ/kg) but compensate with high density (approximately 5.9 g/cm³), yielding a volumetric heat storage of about 488 kJ/L. These subtypes collectively enable applications from building cooling to electronics thermal regulation, leveraging their inherent high latent heats up to 300 kJ/kg and thermal conductivities in the range of 0.5–1 W/m·K, which facilitate faster heat transfer compared to organics. Additionally, their low cost (often 1–20 $/kWh thermal storage capacity) and non-flammable nature enhance safety and economic viability.[12][25][26][24] Despite these benefits, inorganic PCMs face significant limitations that can impair long-term performance. Salt hydrates are particularly prone to phase segregation during incongruent melting, where the anhydrous salt separates from the water, resulting in a progressive loss of latent heat over repeated cycles—up to 20–30% degradation after 100 cycles in untreated materials. Supercooling, another common issue, can delay solidification by 10–20°C below the melting point, reducing the effective operating temperature range and energy recovery efficiency. Furthermore, many inorganics, especially chlorides and nitrates, exhibit corrosiveness toward common container metals like steel, with corrosion rates reaching 70 mg/cm² after extended cycling, necessitating protective coatings or compatible materials. Eutectic salt mixtures, such as NaCl-Na₂CO₃, address some stability concerns for high-temperature applications above 200°C, like concentrated solar thermal power, maintaining a latent heat of 311.6 kJ/kg at 635°C over 1000 cycles with minimal property degradation.[27][12][24][28]Solid-Solid and Eutectic Phase-Change Materials
Solid-solid phase-change materials (PCMs) represent a class of materials that undergo a phase transition between two solid states, typically from crystalline to amorphous, without passing through a liquid phase. This transition allows them to store and release latent heat while maintaining structural integrity, making them particularly suitable for applications where leakage is a concern. Polymeric materials, such as cross-linked polyethylene glycol (PEG), are commonly used examples, exhibiting phase transitions in the range of 40-60°C with latent heats of 100-200 kJ/kg.[29][30] The primary advantages of solid-solid PCMs include the absence of leakage during phase change, as no liquid intermediate forms, and minimal volume change, typically less than 1%, which enhances compatibility with surrounding structures. These properties enable their use without encapsulation in certain scenarios, simplifying integration into composites or coatings. However, limitations such as relatively lower latent heat compared to solid-liquid PCMs, more complex synthesis processes involving crosslinking, and higher production costs can restrict widespread adoption.[31][32][33] Eutectic phase-change materials, on the other hand, are mixtures of two or more components—often organic and inorganic substances—that exhibit a sharp melting point at a specific composition, behaving as a single phase during transition. These mixtures form at the eutectic point in phase diagrams, where the liquidus lines of the constituent components intersect, resulting in a lowest possible melting temperature for the system. A representative example is the eutectic mixture of capric acid and lauric acid, which transitions at approximately 20°C with a latent heat of 180 kJ/kg, making it suitable for low-temperature thermal management.[34][35]Properties and Selection
Thermophysical Properties
Phase-change materials (PCMs) exhibit several key thermophysical properties that determine their efficacy in thermal energy storage. The melting or freezing temperature (Tm) typically ranges from below 0°C to over 100°C, depending on the material type, enabling selection for diverse temperature regimes. The latent heat of fusion (ΔHf) is a primary attribute, generally falling between 100 and 250 kJ/kg for most organic and inorganic PCMs, such as 128–244 kJ/kg for paraffins and 105–231 kJ/kg for salt hydrates. Specific heat capacity (Cp) varies from 1 to 3 kJ/kg·K in both solid and liquid phases, influencing sensible heat storage. Thermal conductivity (k) is often low, ranging from 0.1 to 0.5 W/m·K for pure organic PCMs and up to 1 W/m·K for inorganics, limiting heat transfer rates. Density (ρ) spans 700–1600 kg/m³, with organics around 800–900 kg/m³ and inorganics higher at 1400–1600 kg/m³. Volume expansion during phase change is notable, typically 5–20%, with minimal changes (around 5–10%) for organics like paraffins and larger (10–20%) for inorganics, necessitating container design considerations.[36][37][38]| Property | Typical Range | Example Values (Organic PCMs) |
|---|---|---|
| Melting Temperature (Tm) | -5°C to 200°C | 45–55°C (paraffins) |
| Latent Heat (ΔHf) | 100–300 kJ/kg | 160–170 kJ/kg (RT series) |
| Specific Heat (Cp) | 1–3 kJ/kg·K | 2 kJ/kg·K (solid/liquid) |
| Thermal Conductivity (k) | 0.1–1 W/m·K | 0.2 W/m·K (paraffins) |
| Density (ρ) | 700–1600 kg/m³ | 770–880 kg/m³ (liquid/solid) |
| Volume Expansion | 5–20% | 10% (paraffins) |