The role of molten salts, sands, and phase change materials in energy conservation

by Marco Arezio

Thermal storage represents one of the most significant challenges in the energy transition . With increasingly distributed and intermittent generation— think photovoltaic and wind power, for example —there's a growing need for systems capable of storing energy in the form of heat, then releasing it when demand dictates.

In this scenario, scientific research in recent years has led to the experimentation and development of innovative materials and solutions: from molten salts to high-temperature sands, to phase change materials (PCMs). These approaches not only expand storage possibilities, but also offer efficiency, durability, and environmental sustainability.

Molten salts: high temperature accumulation

The use of molten salts is currently among the most established solutions for thermal storage. These mixtures of nitrates and carbonates, when heated to temperatures between 250°C and 600°C, can store large amounts of energy in the form of sensible heat. Concentrated solar power (CSP) systems have already demonstrated the effectiveness of this technology, with tanks that can release thermal energy for several hours even in the absence of solar radiation.

From an academic perspective, numerous studies published in the last five years have explored the chemical stability of salts, corrosion issues related to metal containers, and the efficiency of loading and unloading cycles. Research advances have led to new, less corrosive and more cost-effective mixtures, improving the competitiveness of these systems.

Sand as a low-cost storage medium

A particularly interesting line of research is the use of sand for thermal storage. Sand is an abundant, inexpensive material that is stable at very high temperatures (up to 1000°C). Recent studies have shown that sand-based storage systems can be a valid alternative to molten salts, especially in industrial settings requiring high-temperature process heat.

The principle is simple: heat is introduced into the sand bed through electrical resistors or heat transfer fluids and can be subsequently recovered with exchangers. Scientific research is evaluating the thermal properties of sand (conductivity, storage capacity) and optimal containment methods, also considering the internal fluid dynamics of the granules. Looking ahead, this technology could offer more sustainable, less expensive, and long-lasting systems.

Phase Change Materials (PCMs): Latent Storage

Phase change materials (PCMs) occupy a central position among the most promising thermal storage technologies. Unlike systems based on sensible heat—which store energy by simply increasing the material's temperature—PCMs exploit the latent heat associated with phase transitions, typically between solid and liquid states.

This means that, during melting, a PCM can absorb a large amount of energy while maintaining a nearly constant temperature; similarly, during solidification, it releases the same amount of heat without significant temperature variations.

Characteristics and types of PCM

An effective PCM must possess several key properties: a transition temperature compatible with the application, a high enthalpy of fusion to store large amounts of energy, good cyclic stability (i.e., the ability to maintain performance even after thousands of melting/solidification cycles), and safe, non-corrosive chemical behavior.

The main PCM families are :

Organic: paraffins and fatty acids, characterized by good stability and non-corrosiveness, but with relatively low thermal conductivity and, sometimes, flammability.

Inorganic: Inorganic hydrated salts and eutectics, which offer high enthalpy and low costs, but may suffer from supercooling or phase separation phenomena.

Composites and hybrids: materials that combine polymer matrices, fibers, or conductive nanoparticles with PCMs to improve mechanical stability, increase thermal conductivity, and reduce the risk of leakage into the liquid phase.

Integration into storage systems

PCMs are extremely versatile in their application. In construction, they are integrated into panels, plasters, or cladding to improve the thermal inertia of spaces, helping to reduce internal temperature peaks and thus energy consumption for air conditioning.

In the industrial sector, PCMs are used to recover process heat and redistribute it at times of peak demand.

In energy systems, they can be used as buffers in solar thermal and photovoltaic systems, allowing the supply of heat or electricity to be extended even in the absence of sun.

Other fields of application include the cold chain for food and pharmaceutical transport, thermal management of electronic devices, and even lithium batteries, where PCMs prevent sudden overheating, improving safety and efficiency.

Challenges and future prospects

Despite progress, the widespread deployment of these technologies still requires further development. Molten salts face corrosion and cost challenges, sands require more efficient containment and exchange systems, while PCMs must ensure cyclic stability and environmental compatibility. However, the direction indicated by research is clear: innovative thermal storage will become a cornerstone of the energy transition, alongside electrochemical batteries and other forms of storage.

Future scenarios foresee a growing integration of these solutions into energy grids and industrial plants, promoting the decarbonization of processes and the intelligent use of resources. Looking ahead, the combination of multiple technologies—for example, molten salts and PCM—could generate highly efficient hybrid systems capable of adapting to a wide range of energy needs.

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