To address the growing problem of pollution and global warming, it is necessary to steer the development of innovative technologies towards systems with minimal carbon dioxide production. Thermal storage plays a crucial role in solar systems as it bridges the gap between resource availability and energy demand, thereby enhancing the economic viability of the system and ensuring energy continuity during periods of usage. Thermal energy storage methods consist of sensible heat storage, which involves storing energy using temperature differences; latent heat storage, which utilizes the latent heat of phase change materials; and thermochemical heat storage, which utilizes reversible chemical reactions through thermochemical materials.
1. Solar Plant
In the simplest configuration (Figure 1), there are three main components: solar collectors, a circulation system for heat transfer fluid, and a storage tank. The solar collectors capture solar energy and convert it into heat. The circulation system transfers the heat to the working fluid, which can be either air or water. The storage tank’s role is to store the collected energy and make it available for use. Additionally, depending on specific implementations, other elements such as circulation pumps, valves, control systems, and exchangers may be included in the system.
Figure 1. General scheme of solar plant.
In solar thermal applications, the solar collector plays a crucial role in absorbing solar radiation and converting it into heat, which is then transferred to the working fluid 
. The thermal energy collected can be utilized directly for supplying hot water or for heating and cooling systems in buildings. Alternatively, it can be stored in a thermal energy storage unit for later use during periods without sunlight or on cloudy days. Therefore, energy storage is necessary to ensure the availability of energy at different times and locations. The advancements in technology have led to the development of two types of solar thermal collectors based on concentration ratios: concentrated solar thermal collectors and non-concentrated solar thermal collectors (Figure 2
). Concentration-based systems utilize concave reflective surfaces to capture and focus solar radiation onto a smaller collection area, resulting in reduced losses and potential integration with thermal energy storage 
. In contrast, non-concentrated collectors have a similar interception and absorption area for solar radiation.
Figure 2. Types of solar thermal collectors.
1.1. Non-Concentrating Collectors
Non-concentrating collectors are positioned to maximize the collection of solar radiation. The positioning of these collectors is determined by specific angles of inclination and orientation, which depend on the geographic latitude. Generally, non-concentrating solar thermal collectors are divided into three types: flat plate collectors (FPC), stationary compound parabolic collectors (CPC), and evacuated tube collectors (ETC).
1.2. Concentrating Collectors
The addition of an optical system that connects the incident solar radiation and the absorbing exterior can concentrate the incoming radiation onto a slightly smaller collection area. This arrangement reduces heat losses and allows for higher temperatures compared to flat plate collectors (FPC). Concentrating collectors consist of concentrators and receivers. There are many commercially available designs for concentrators and receivers. Concentrators can be refractive or reflective, continuous or non-continuous, and cylindrical or parabolic. Additionally, the receiver can be flat, convex, concave, or cylindrical, and it can be with or without glass. In concentration collectors, the positioning of an optical system is crucial due to the sun’s movement throughout the day. In general, concentration collectors are divided into four categories: parabolic trough collector (PTC) (e.g., Figure 3), linear Fresnel reflector (LFR), parabolic dish reflector (PDR), and central receiver (e.g., Figure 4) or heliostat field reflector (HFR). A CSP plant can achieve higher thermal efficiency because the working fluid can reach higher temperatures due to a reduced heat dissipation area compared to a non-concentration collector system with the same surface area. CSP systems capture a smaller amount of diffuse radiation, which depends on the concentration ratio.
Figure 3. Planta Solar 10 (PS10) parabolic trough collectors in Tabernas, Almeria, Spain.
Figure 4. Planta Solar 10 (PS10) central receiver in Tabernas, Almeria, Spain.
From an economic perspective 
), concentration collectors offer advantages in terms of the solar collection surface area because the reflective surface requires less material 
. Therefore, the cost per unit of the solar collection surface area is lower in a CSP system compared to a non-concentration collector system. The solar surface reflectance degrades over time, necessitating periodic cleaning. The tower in Figure 3
, installed in Tabernas, Almeria, Spain, standing at a height of 115 m, houses a “solar furnace” that generates electricity through a generator (essentially a large “dynamo”) powered by a steam turbine. The steam is produced through the thermal exchange between water and molten salt, which is heated by the sun to a temperature of 650 °C in specially insulated storage systems, ensuring thermal exchange with water for steam production even during periods without sunlight for several days. Specifically, the system generates saturated steam at 275 °C, capable of driving the steam turbine. Additionally, the system also produces hydrogen through electrolysis, utilizing a portion of the electricity generated in a completely green manner.
Relation between installation cost and process temperature range 
2. Thermal Storage System
Thermal energy storage (TES) systems have the potential to enhance the efficient utilization of thermal energy equipment and facilitate a large-scale transition. They are commonly employed to address the imbalance between energy supply and demand. The methods for storing thermal energy can be categorized as active or passive. Active methods can further be classified as direct or indirect.
In direct active methods, a liquid with similar characteristics is used both in the storage material and the solar collector. Indirect active methods overcome the limitations of direct methods by employing different liquids for storage and solar collection. On the other hand, passive methods utilize solid materials that absorb heat from the liquid through a charge and discharge process. Common materials used for passive storage include phase change materials (PCMs), concrete, and rocks.
Energy storage not only helps reduce the gap between energy supply and demand but also enhances the efficiency and reliability of energy systems. It plays a crucial role in energy conservation by enabling fuel savings and improving the competitiveness of production systems through waste energy recovery. Various storage systems are available to store energy in different forms, including chemical, mechanical (potential or kinetic), magnetic, and thermal energy. In thermal energy storage systems, heat is transferred to the storage medium during the charging phase and released during the discharge phase.
The complete process typically involves three stages: charge, storage, and discharge (Figure 6
). Some phases may occur separately or simultaneously, such as the charge and storage phases, and they can be repeated within the same storage cycle. The choice of materials for energy storage depends on the specific storage system, temperature range, and intended application 
Figure 6. Stages of the accumulation process.
3.1. Classification by Operating Temperature Range
- High temperature thermal energy storage: This includes systems operating at temperatures typically above 200 °C and plays a vital role in renewable energy technologies and the recovery of waste heat from other processes (Table 1).
- Low temperature thermal energy storage: This category operates between 10 °C and 200 °C. Its most frequent applications include the heating and cooling of rooms and buildings, solar cooking, solar boilers, air treatment systems, and greenhouses. It is often combined with solar collectors or cogeneration plants (Table 2).
Temperature range classification summary 
3.2. Classification by Accumulation Time Interval
There are two main categories of thermal energy storage based on their storage duration:
- Short-term thermal storage: This category includes systems with a daily cycle and those with a storage capacity ranging from a few hours to a maximum of one week. The thermal energy in these systems is typically maintained at temperatures high enough to allow direct exchange with the user at the required temperature. These systems are suitable for meeting immediate and short-term energy demands .
- Long-term or seasonal heat accumulation: This category addresses the mismatch between high solar radiation during the summer and higher heat demand in winter. These systems are designed to store thermal energy over longer periods, usually from summer to winter, to balance out the seasonal variations in energy supply and demand. These systems often utilize large-volume water storage, which makes them economically viable despite the higher installation costs.
As regards this classification, Vecchi et al. 
studied the latest solution for thermo-mechanical energy storage: this study has extensively characterized the latest solution for thermomechanical energy storage (TMES) solutions for future applications such as long-duration energy storage (LDES). The results demonstrate that traditional TMES systems (mainly ACAES and LAES) are more suitable for short-term storage durations of around 8 h, while ACAES also meets the cost objectives for LDES.
However, caution is advised especially for traditional TMES systems that are influenced by standby losses (Figure 7). New TMES technologies that offer compact storage, limited losses, and cost-effective storage materials represent a promising proposition for long-term seasonal heat accumulation, with lower efficiencies offset by a favorable investment cost structure and reduced capacity contributions. In particular, the use of hydration/dehydration reactions of CaO and oxidation/reduction of metals appear to be promising paths for the development of TCES.
ACAES and LAES with modelling approach 
3.3. Classification by Type of Heat Exchange
There are primarily three types of TES systems 
: sensible storage systems, latent storage systems, and thermochemical storage systems (Figure 8
Figure 8. Thermal energy storage for CSP plants.
- Sensible heat storage: defined as storage that exploits the physical properties of a material to store thermal energy at the expense of a temperature rise of the material itself, due to the temperature variation fluid used.
- Latent heat storage : the second form of storage that exploits the physical properties of a material to store energy due to phase change fluid used (the heat regards melting, solidification, vaporization, and condensation). This kind of storage, as opposed to sensible accumulation, however, is not focused on increasing temperature, but rather aims to cause a complete phase transition (solid–liquid, typically) of the material used.
- The main attractiveness of this system lies in the amount of energy this process requires: comparing the same mass quantity of material for sensitive and latent accumulation, the latter requires a higher sensitive and latent energy content, and the latter can accumulate up to 2/3 more than the sensitive counterpart, generating large savings in storage volume. No less important is the fact that the phase change takes place at an approximately constant temperature (Figure 9), an aspect which is of great importance in all applications where a heat source with little time variation is required. Latent storage is possible due to the presence of multiple substances with a melting temperature in the range of interest for civil and industrial applications.
- Thermochemical energy (breaking and formation of molecular bonds) due to the absorption/release of chemical binding energy by shifting the reaction equilibrium of the reactants constituting the storage medium : when talking about chemical storage, it is understood that what we want to store is always thermal energy, but the way to obtain this exploits chemical reactions between two materials. In general, the reactions of interest are absorption reactions in which substance A (called absorbent) and substance B (called sorbate) interact with each other through weak physical bonds, such as Van der Waals forces or hydrogen bridges. Depending on whether the heat flow is into or out of the system, the reaction is called endothermic or exothermic. The physical state in which the two substances occur differs depending on the application, but the most standard solution involves the absorbent in solid form and the sorbate (typically water) changing from liquid to vapor state and vice versa. This type of reaction is possible because the material chosen for absorption is microporous, with internal cavities that allow sorbate molecules to settle on their surface.
Figure 9. Solid–liquid phase transition.