Latent thermal energy storage for solar process heat applications at medium-high temperatures – A review

Overview

Latent thermal energy storage (LTES) represents a critical mechanism for enhancing the efficiency and dispatchability of solar process heat applications, particularly within the medium-to-high temperature ranges essential for industrial operations. The scholarly review "Latent thermal energy storage for solar process heat applications at medium-high temperatures – A review" provides a comprehensive analysis of this technology, focusing on the integration of phase change materials (PCMs) with solar thermal systems to bridge the gap between intermittent solar irradiance and consistent industrial heat demand. This body of work examines the thermodynamic principles governing latent heat storage, where energy is absorbed or released during the phase transition of a material, primarily from solid to liquid and vice versa, without a significant change in temperature. This characteristic allows for high energy density and near-isothermal operation, which are advantageous for maintaining stable process temperatures in sectors such as food processing, textiles, and chemical manufacturing.

The scope of the review encompasses the identification and evaluation of suitable PCMs that exhibit optimal thermal properties for medium (100–300 °C) and high (>300 °C) temperature applications. Key parameters analyzed include latent heat of fusion, melting point, thermal conductivity, and supercooling behavior. The literature highlights that while organic PCMs, such as paraffins and fatty acids, offer good chemical stability and low corrosion, they often suffer from low thermal conductivity. Conversely, inorganic PCMs, including salt hydrates and binary/ternary eutectic mixtures, provide higher latent heat capacity and thermal conductivity but may face challenges related to phase separation and supercooling. The review also addresses the thermal energy storage density, which can be approximated by the formula Q=m⋅ΔHf, where Q is the stored energy, m is the mass of the PCM, and ΔHf is the latent heat of fusion. This relationship underscores the importance of selecting materials with high ΔHf values to maximize storage capacity within compact system footprints.

Furthermore, the article investigates the design and performance of heat exchangers used in LTES systems, which are crucial for efficient charging and discharging of the stored thermal energy. Common configurations include shell-and-tube, plate, and finned-tube heat exchangers, each offering distinct advantages in terms of surface area-to-volume ratio and fluid dynamics. The review also discusses the integration of LTES with various solar collector technologies, such as parabolic troughs and linear Fresnel reflectors, which are well-suited for achieving the required temperature levels. By synthesizing recent advancements and identifying persistent technical challenges, the review serves as a foundational resource for engineers and researchers aiming to optimize solar thermal systems for industrial process heat, thereby contributing to the decarbonization of the industrial sector through the increased utilization of solar energy.

What is latent thermal energy storage?

Latent thermal energy storage (LTES) is a thermodynamic method for capturing and releasing heat energy through the phase transition of specific materials, known as phase change materials (PCMs). Unlike sensible heat storage, which relies on temperature changes within a single phase, LTES utilizes the latent heat absorbed or released during a change of state—most commonly from solid to liquid and vice versa. This mechanism allows for the storage of significant amounts of energy at a nearly constant temperature, offering high energy density and isothermal operation, which are critical advantages for solar thermal systems and building climate control.

The fundamental principle governing LTES is the enthalpy change associated with phase transitions. When a PCM absorbs heat from a source, such as solar collectors, it melts, storing energy as latent heat. Conversely, when the stored energy is needed, the PCM solidifies, releasing the heat. The total heat storage capacity (Q) can be described by the summation of sensible heat before and after the phase change, and the latent heat during the transition. For a melting process, the total energy stored is often approximated by integrating the specific heat capacity (cp) over the temperature range and adding the latent heat of fusion (L):

Q = m * [∫(c_s * dT) + L + ∫(c_l * dT)]

Where m is the mass of the PCM, cs and cl are the specific heat capacities of the solid and liquid phases, respectively, and L is the latent heat of fusion. This formulation highlights that the energy density of LTES is primarily driven by the magnitude of L, allowing for more compact storage systems compared to sensible heat storage technologies.

Phase Change Materials (PCMs)

The performance of a latent heat storage system is intrinsically linked to the properties of the selected PCM. PCMs are categorized into three main classes: organic, inorganic, and eutectic mixtures. Organic PCMs, such as paraffins and fatty acids, are favored for their chemical stability, corrosion resistance, and supercooling behavior. Inorganic PCMs, including salt hydrates and metallic alloys, typically offer higher thermal conductivity and latent heat capacity but may suffer from phase separation and corrosion issues. Eutectic mixtures combine two or more components to achieve a specific melting point and enhanced thermal properties.

Key selection criteria for PCMs include a melting point appropriate for the application, high latent heat of fusion, high thermal conductivity to facilitate rapid charging and discharging, and long-term thermal reliability. The integration of PCMs into solar energy systems enables the decoupling of energy collection and utilization, thereby smoothing out the intermittent nature of solar irradiance and enhancing the overall efficiency of thermal energy management.

Solar process heat applications

Latent thermal energy storage (LTS) is particularly effective for solar process heat applications requiring medium to high temperatures, typically ranging from 100 °C to 300 °C. In these applications, solar thermal collectors concentrate or focus radiation to heat a phase change material (PCM), which absorbs energy during the melting phase and releases it during solidification. This mechanism allows for a more compact storage system compared to sensible heat storage, as the temperature remains nearly constant during the phase transition, ensuring precise thermal regulation for industrial processes.

The energy density of LTS is defined by the latent heat of fusion, expressed as Q=m⋅L, where Q is the stored energy, m is the mass of the PCM, and L is the specific latent heat. For solar applications, the selection of PCM is critical. Paraffin waxes are commonly used for lower temperature ranges (60–100 °C), while salt hydrates and organic esters are preferred for medium temperatures (100–200 °C). At higher temperatures (200–300 °C), inorganic salts such as nitrate mixtures become viable, offering high latent heat capacities and stability under repeated thermal cycling.

Integration with Solar Collectors

In solar process heat systems, LTS is often integrated with parabolic trough or linear Fresnel reflectors. These collectors focus sunlight onto a receiver tube containing the PCM or a heat transfer fluid (HTF) that circulates through a PCM storage tank. The HTF absorbs solar energy and transfers it to the PCM, which melts and stores the thermal energy. During periods of low solar irradiance or at night, the stored heat is extracted by circulating the HTF through the solidifying PCM, delivering consistent temperature output to the industrial process.

Challenges in this integration include the relatively low thermal conductivity of many PCMs, which can limit the charging and discharging rates. To address this, enhanced heat transfer techniques such as finned tubes, metal foams, and encapsulation in spherical or cylindrical shells are employed. These methods increase the surface area for heat exchange, improving the efficiency of energy storage and retrieval. Additionally, thermal management systems are designed to minimize heat losses and ensure uniform temperature distribution within the storage unit.

The application of LTS in solar process heat offers significant advantages for industries such as food processing, textiles, and chemicals, where consistent temperature control is essential. By leveraging the high energy density and isothermal characteristics of latent heat storage, solar thermal systems can achieve greater flexibility and reliability, reducing dependence on auxiliary heating sources and enhancing the overall economic viability of solar energy utilization.

Medium-high temperature ranges

Medium-high temperature latent thermal energy storage systems operate in the range of approximately 100 °C to 350 °C, bridging the gap between low-temperature building applications and high-temperature industrial processes. This temperature band is particularly significant for concentrating solar power (CSP) plants, where parabolic troughs and linear Fresnel reflectors typically deliver heat transfer fluids within this spectrum. The selection of phase change materials (PCMs) for this range is critical, as the material must maintain thermal stability over thousands of melt-freeze cycles while offering a high volumetric energy density.

Organic and Inorganic PCM Candidates

Inorganic salts, such as binary and ternary eutectic mixtures, are predominant in the medium-high temperature category due to their high latent heat of fusion and thermal conductivity compared to organics. Common examples include mixtures of sodium nitrate and potassium nitrate, which melt around 220 °C. The energy storage capacity per unit volume, Evol, can be expressed as:

Evol=ρ⋅L+ρ⋅Cp⋅ΔT

where ρ is the density, L is the latent heat of fusion, Cp is the specific heat capacity, and ΔT is the sensible temperature range utilized during charging and discharging. Inorganic salts often suffer from supercooling and phase segregation, requiring nucleating agents and thickening agents to ensure reliable cyclic performance.

Thermal Conductivity Challenges

A primary limitation of PCMs in the medium-high temperature range is their relatively low thermal conductivity, which can bottleneck the charging and discharging rates of the storage unit. For salt-based PCMs, thermal conductivity typically ranges from 0.5 to 1.5 W/m·K. To mitigate this, enhanced heat transfer structures such as finned tubes, metal foams, and graphite composites are integrated into the storage tanks. These enhancements increase the effective thermal conductivity, keff, allowing for faster heat flux, q, governed by Fourier’s law:

q=−keff⋅∇T

Graphite-expanded PCMs can increase thermal conductivity by up to an order of magnitude, significantly reducing the temperature gradient within the storage medium and improving the exergy efficiency of the system.

How does this review compare technologies?

The review establishes a rigorous comparative framework for evaluating latent thermal energy storage (LTES) systems, prioritizing thermophysical properties and economic viability over simple capacity metrics. The analysis contrasts phase change materials (PCMs) against sensible heat storage (SHS) and thermochemical storage (TCS), highlighting the superior volumetric energy density of LTES. This density is defined by the latent heat of fusion, L, and the melting temperature range, Tm, allowing for more compact system designs compared to SHS, which relies on specific heat capacity, cp, and temperature differential, ΔT. The review explicitly addresses the trade-off between energy density and thermal conductivity, a critical bottleneck for PCMs. Materials with high latent heat, such as paraffin waxes and salt hydrates, often exhibit low thermal conductivity, necessitating integration with heat exchangers or conductive fillers like graphite.

Material Performance and Stability

A central component of the comparison involves the thermal cycling stability of different PCM classes. The review examines organic, inorganic, and eutectic mixtures, noting that organic PCMs generally offer better chemical stability and supercooling resistance, whereas inorganic PCMs provide higher latent heat per unit volume. The analysis includes an evaluation of phase change consistency over multiple melt-freeze cycles, a key factor for long-term operational reliability in solar thermal applications. The review also compares the cost-per-kilowatt-hour ($/kWh) across these material categories, identifying that while inorganic salts may have lower raw material costs, the encapsulation and corrosion protection requirements can increase the overall system expenditure.

System Integration and Efficiency

The comparative assessment extends to system-level integration, evaluating how different LTES technologies perform within solar thermal collectors. The review contrasts direct contact storage, where the PCM is in direct thermal contact with the heat transfer fluid (HTF), against indirect contact systems using encapsulated PCM units. This comparison highlights the impact of thermal resistance on charging and discharging rates. The analysis further examines the efficiency of heat recovery, defined by the ratio of useful heat output to the latent heat stored. The review concludes that while LTES offers significant advantages in temperature stability during discharge, the selection of the optimal technology depends heavily on the specific operating temperature range of the solar collector and the desired duration of heat retention.

Key findings and conclusions

The scholarly analysis of latent thermal energy storage (LTES) confirms that phase change materials (PCMs) offer a superior volumetric energy density compared to sensible heat storage, making them particularly effective for solar thermal applications. The primary conclusion is that the integration of PCMs into solar collectors significantly enhances system efficiency by reducing thermal losses during intermittent irradiation. This is achieved through the isothermal nature of the phase transition, which maintains a stable temperature profile during charging and discharging cycles.

Thermodynamic Efficiency and Material Selection

Research indicates that the selection of PCM is critical for maximizing exergy efficiency. Paraffin waxes and salt hydrates are identified as leading candidates due to their high latent heat of fusion and chemical stability. The governing principle for heat transfer in these systems is often described by the enthalpy-porosity method, where the effective heat capacity Ceff is modified to account for the latent heat L over the melting temperature range ΔTmelt:

C_eff = C_solid + L * \delta(T - T_melt)

This formulation allows for accurate modeling of the solid-liquid interface. Studies conclude that nano-enhanced PCMs (NEPCMs) can improve thermal conductivity by up to 30-50%, addressing the inherent low thermal conductivity of many organic PCMs. However, the trade-off involves potential sedimentation and increased cost, requiring careful economic analysis.

System Integration and Solar Synergy

The integration of LTES with solar thermal collectors demonstrates a significant reduction in the coefficient of performance (COP) variability for solar-driven heat pumps. By storing excess solar heat during peak irradiation and releasing it during periods of low solar input, LTES systems smooth out the thermal load. This reduces the reliance on auxiliary electric heaters, thereby increasing the overall solar fraction of the system. Conclusions emphasize that proper sizing of the PCM volume relative to the collector area is essential to avoid underutilization or thermal stratification issues.

Economic and Environmental Impact

Economic analyses presented in the literature suggest that while the initial capital cost of LTES systems is higher than sensible storage, the levelized cost of stored energy (LCSE) becomes competitive over a 10-15 year lifespan. The environmental benefit is quantified by the reduction in CO2 emissions due to the increased utilization of solar thermal energy, which displaces fossil-fuel-based heating. The payback period is heavily dependent on local solar irradiance and the cost of the primary fuel source being displaced.

Future research directions

Future research in latent thermal energy storage for solar applications is increasingly focused on overcoming material limitations and system integration challenges. A primary area of investigation involves the development of advanced phase change materials (PCMs) with tailored thermophysical properties. Researchers are exploring composite PCMs, such as microencapsulated salts or paraffins embedded in porous matrices, to enhance thermal conductivity, which is often the bottleneck in latent heat systems. The effective thermal conductivity (keff) of these composites is critical for reducing the charging and discharging times of solar thermal collectors. Studies suggest that integrating high-conductivity fillers, such as graphene or metallic foams, can significantly improve heat transfer rates without substantially compromising the latent heat capacity (ΔHfus).

System Integration and Dynamic Modeling

Beyond material science, future work emphasizes the dynamic modeling of PCM-based solar systems to better predict performance under fluctuating irradiance. Current models often assume quasi-steady states, but advanced numerical simulations incorporating the enthalpy-porosity method are needed to capture the complex phase-front movement. This includes refining the Stefan number (St=ΔHfusCpΔT) to more accurately describe the ratio of sensible to latent heat during transient solar cycles. Improved models will enable better sizing of storage units relative to collector areas, optimizing the cost-per-kWh stored.

Long-Term Stability and Degradation

Long-term thermal cycling stability remains a critical research gap. Many organic and inorganic PCMs suffer from supercooling, phase segregation, or thermal degradation after hundreds of cycles. Future studies aim to quantify the lifespan of encapsulation materials under high-temperature solar fluxes. Investigating the chemical compatibility between the PCM and its container—often stainless steel or polymer shells—is essential to prevent leakage and corrosion. Additionally, the economic viability of large-scale deployment depends on reducing the cost of encapsulation, prompting research into low-cost, durable shell materials that maintain integrity over decades of operation.

Hybrid Storage Configurations

Emerging research also points toward hybrid storage configurations that combine latent and sensible heat storage to leverage the high energy density of PCMs and the rapid response of sensible media. These hybrid systems aim to smooth out temperature fluctuations in solar water heating and concentrated solar power (CSP) plants. Future work will focus on optimizing the stratification within these hybrid tanks to minimize thermal losses and maximize exergy efficiency. The integration of these systems with photovoltaic/thermal (PV/T) collectors is another promising direction, where PCMs regulate the operating temperature of PV cells, thereby enhancing electrical efficiency while storing thermal energy for later use.