Latest Advances in Thermal Energy Storage for Solar Plants

Abstract

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. The objective of this review paper is to explore significant research contributions that focus on practical applications and scientific aspects of thermal energy storage materials and procedures. For each type of storage, different materials have been examined, taking into consideration the most recent studies, both for medium and long-term storage and, when possible, comparing methodologies for the same purpose. It has been observed that TCHS systems have the potential to reduce the volume of chemical storage tanks by 34 times using chemical reactions. Among the SHS materials, water, molten salts, and graphite exhibit the highest energy density, with graphite also possessing remarkable thermal conductivity. Nanoparticles can enhance the thermophysical properties of TES materials by increasing their thermal conductivity and wettability and improving intermolecular characteristics. The use of biobased PCMs for applications that do not require very high temperatures allows for maximizing the efficiency of such storage systems.

1. Introduction

The importance of clean energy has never been greater than it is today. The current methods of energy production and consumption are unsustainable. Considering the ongoing global growth in both the economy and population, there is an even greater urgency to address this issue [1,2]. Climate change, caused by human-generated greenhouse gas emissions, poses one of the most critical challenges humanity faces. Approximately two-thirds of global greenhouse gas emissions and carbon dioxide emissions come from the combustion of fossil fuels [3,4]. As a result, the energy sector must take a leading role in combating climate change. The increasing use of fossil fuels in emerging countries is leading to deteriorating air quality, which has severe consequences for public health.

Currently, the world’s energy demand is continuously expanding, primarily due to population growth. It is expected to grow by nearly 30% in the next three decades. However, petroleum-based fossil fuels, which currently account for approximately 80% of the world’s energy supply, are projected to decline rapidly over the next 50 years. This has raised awareness of energy crises and significant environmental concerns, such as global warming and urban air pollution [2]. Extensive research has been conducted on new technologies for harnessing renewable energy sources, which are gaining increasing interest worldwide due to their abundant availability and low environmental impact. Additionally, the broader adoption of energy-saving systems is crucial for reducing greenhouse gas emissions and meeting the growing global energy demand. Therefore, it is essential to envision a comprehensive transformation of the economy, where decarbonization, renewables, the circular economy, efficiency, and the rational and equitable utilization of natural resources and enabling technologies serve as objectives and tools for a sustainable energy and economic model.

The ongoing debate on the evolution of energy systems to favor the transition towards more environmentally friendly forms initially emphasized the promotion of energy generated exclusively from renewable sources, while discouraging the use of gas, particularly of hydrocarbon origins, due to its evident polluting nature. Renewables have a substantially zero production cost since sunlight and wind are freely available, as opposed to gas, which incurs defined extraction and production costs. Based solely on this economic consideration, renewables should simply replace gas [4,5].

However, it is widely known that power generation from renewable sources is subject to strong fluctuations, making it unable to ensure stability and continuity of supply to meet the overall energy demand without appropriate energy storage solutions. In recent times, the international energy system has been impacted by various extraordinary phenomena, collectively referred to as a ‘perfect storm’, which can be attributed to multiple causes. The first factor was the post-COVID-19 recovery, which led to a sudden global surge in energy and raw material demand, causing shocks in supply and demand mechanisms. This was further compounded by geopolitical factors, mainly related to the Russia–Ukraine war, which strained international relations with Russia, the largest gas supplier to Europe. Simultaneously, the delayed commissioning of the Nord Stream 2 gas pipeline, directly connecting Russia to Germany, created a significant imbalance between gas supply and demand, particularly during the winter months [6]. The outbreak of the war in Ukraine intensified the already high tensions in the energy markets, fueling strong speculative activities. However, these events represent only the visible aspects of the rise in gas prices. Structural issues within the industry have also contributed to the soaring cost of gas, more than tripling its previous levels.

Addressing these industrial concerns is crucial as the escalating cost of natural gas also exacerbates the global warming problem. Therefore, it is imperative to continue the efficient development of energy from renewable sources. Currently, carbon dioxide is the primary cause of the human-induced greenhouse effect, accounting for over 50% of its impact and making it the main driver of climate change. The combustion of fossil fuels is the primary source of carbon dioxide emissions. Due to the time lag between emissions and their effects, the full extent of the consequences of climate change is yet to unfold in the coming decades, posing an escalating threat to global economic stability and our way of life.

Solar energy is presently the most accessible and abundant among other forms of renewable energy sources. Solar thermal systems (STS) are considered one of the best alternatives for energy production from renewable sources and serve to mitigate the problem of climate change. Solar energy is widely regarded as a symbol of clean and renewable energy, whereas other non-renewable forms of energy contribute to air pollution. STS aims to efficiently convert solar energy into heat, representing a sustainable energy production method. Studies confirm that the sun will continue to provide thermal energy for another 4 billion years. Under cloudless sky conditions, the average incident radiation on a solar panel is around 1000 W/m², which is sufficient to produce domestic hot water. However, for higher temperature thermal energy demands, solar concentration panels are necessary [7,8]. There are several promising methods for providing heating from renewable energy resources. Greenhouse gases, especially carbon dioxide, are recognized as one of the primary environmental challenges of our time. The use of fossil fuels contributes to greenhouse gas emissions, leading to increased environmental impacts, costs, and health issues. Moreover, the continuous utilization of fossil fuels depletes finite resources over time. Thus, it is essential to integrate fossil fuel resources with renewable energy sources and energy storage options to reduce society’s dependence on fossil fuels.

Thermal storage is a fundamental component of a solar system as it bridges the gap between resource availability and energy demand, enhancing the economic viability of the system. An important aspect in evaluating the efficiency of a storage tank is stratification, which refers to the existence of a temperature gradient that facilitates the separation of fluid at different temperatures based on their varying densities. A stratified tank minimizes losses to the environment and promotes efficient collector operation. The use of thermal energy storage reduces energy costs, enhances energy consumption efficiency, increases the flexibility of energy production processes, reduces plant operating costs and size for the same power output, improves air quality by reducing pollutant emissions, mitigates the greenhouse effect, and preserves fossil fuel reserves. The graph presented in Figure 1 shows the metric tons of CO₂ emitted annually worldwide from 1900 to 2022 [9]. Energy production remains the primary contributor to greenhouse gas emissions, with the energy industry and other industrial sectors showing the highest increases. It is in these sectors that action must be taken to ensure improvement in climate conditions (Figure 2). Industrial processes can be categorized into three groups according to the process temperature range: low temperature (below 150 °C), medium temperature (150–400 °C), and high temperature (above 400 °C) (

Table 1. Industrial processes suitable for solar heat applications and their temperature ranges [10].

Industrial Sector	Processes	Temperature Range [°C]
Chemical industry	Soaps	200–260
	Syntethic rubber	150–200
	Processing heat	120–180
	Distilling	110–300
	Industrial Furnace	500–1000
Plastic industry	Preparation	120–140
	Distillation	140–150
	Separation	200–220
	Extension	140–160
	Drying	180–200
	Blending	120–140
	Moulding	100–500
Food and beverage	Drying	30–90
	Washing	60–90
	Pasteurising	60–80
	Boiling	95–105
	Sterilising	60–120
	Heat treatment	40–60
All industrial sectors	Pre-heating of boiler feed water	30–100
	Industrial solar cooling	55–180
	Heating of factory buildings	30–80

). Therefore, it will be necessary to find the right compromise between the required storage duration and the suitable temperature for the process.

Therefore, the purpose of this paper is to provide an overview of the current state of thermal energy storage (TES), exploring its applications, plant technologies, materials, and the state of research. TES plays a pivotal role in the deployment of renewable energies due to their intermittent nature and availability in remote geographical areas.

The review is structured as follows: Section 2 discusses various types of energy storage classified by storage duration, heat exchange type, and storage time interval. Section 3, Section 4 and Section 5 provide a detailed analysis of the different technologies available for sensible, thermochemical, and latent energy storage, respectively. Section 6 provides a comparison of different storage technologies. Section 7 summarizes global trends. Finally, Section 8 presents the most significant conclusions of the article.

2. Energy Storage

2.1. Solar Plant

In the simplest configuration (Figure 3), 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.

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 [10,11]. 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 4). 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 [7]. In contrast, non-concentrated collectors have a similar interception and absorption area for solar radiation.

2.1.1. 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).

2.1.2. 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 5), linear Fresnel reflector (LFR), parabolic dish reflector (PDR), and central receiver (e.g., Figure 6) 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.

From an economic perspective [7] (Figure 7), concentration collectors offer advantages in terms of the solar collection surface area because the reflective surface requires less material [12]. 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 5, 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.

2.2. 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 8). 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 [13].

2.3. Classification

2.3.1. 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 2. Temperature range classification summary [14,15,16,17].

  	Temperature	Applications	Sources
  HTTES	T > 200 °C	Generation of power, heating building	Solar power stations, cogeneration plants, waste energy (waste heat from industrial processes)
  LTTES	10 °C < T < 200 °C	Heating/cooling buildings	Solar collectors, cogeneration plants

  ).
• 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).

2.3.2. 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 [17,18].
• 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. [18] 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 9). 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.

2.3.3. Classification by Type of Heat Exchange

There are primarily three types of TES systems [13,19]: sensible storage systems, latent storage systems, and thermochemical storage systems (Figure 10).

• 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 [19,20]: 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 11), 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 [20]: 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.

3. Sensible Heat Thermal Storage

In this type of storage, energy is stored by changing the temperature of a liquid medium (such as water or oil) or a solid medium (such as rock, brick, sand, or soil) without undergoing any phase change within the designated temperature range. The storage medium’s internal energy varies as a result. For temperatures above 100 °C under normal pressure, substances such as oils, molten salts, and liquid metals are commonly used. For higher temperatures, solid materials such as rocks, minerals, ceramics, metals, and concrete can be utilized, with some materials capable of withstanding temperatures up to 1000 °C. The amount of energy stored is directly proportional to the difference between the final and initial temperatures as well as the storage heat capacity, as depicted by Equations (1) and (2).

$$E=m{\int }_{Tin}^{Tout}{C}_{p}dT$$

(1)

$$\dot{Q_s}=\rho C_{p}V\Delta T_s$$

(2)

3.1. Liquid Substances

3.1.1. Water

The primary element in the category of liquid substances is water, which can serve as a heat transfer fluid in active systems or as a sensible storage medium due to its high specific heat capacity. Water also has the advantage of being able to exist in different phases, allowing its use in various applications. For liquid storage systems, water is utilized to store thermal energy below 100 °C, while pressurized steam in stainless steel tanks is employed for concentration systems. However, corrosion issues may arise when using water as a storage medium.

An example of such a thermal storage mechanism is the use of pressurized saturated water to generate steam [7,10,21] (Figure 12). During the charging phase, the saturated liquid occupies nearly the entire volume of the tank (around 90%). Water has several advantages, including its non-toxic and non-flammable nature, high availability, and the potential to eliminate the need for heat exchangers when used as a fluid in manifolds. However, there are disadvantages to consider, such as the risk of freezing or boiling, its corrosive properties, and difficulties in achieving stratification. Depending on the intended application, storage tanks made of stainless steel, plastic, or similar materials can be utilized. These tanks act as heat exchangers with varying characteristics, aiming to store thermal energy with minimal losses to the surrounding environment.

3.1.2. Thermal Oil

For temperatures above 100 °C, the use of pressurized containers for steam becomes costly [22]. Therefore, alternative substances such as organic oils, molten salts, and liquid metals are preferred in these cases. Certain oils, such as Dowtherm and Therminol, are suitable for temperatures ranging from 100 to 300 °C. However, these oils may degrade over time and pose flammability concerns. Concentrated solar plants often utilize thermal oils, including both edible and non-edible vegetable oils. These oils are colorless, transparent, and can withstand temperatures of up to 400 °C, offering a much broader temperature range compared to water [23]. Synthetic oils are generally preferred over traditional mineral oils because they have better thermophysical properties (lower viscosity and higher thermal conductivity) and are less flammable [24,25,26].

The most common synthetic oil is a eutectic mixture of biphenyl and diphenyl oxide commonly known by their commercial names such as Therminol-VP1, Dowtherm A, and Diphyl. It has the highest thermal stability among organic HTFs and is characterized by low viscosity, making it particularly suitable as an HTF. However, being an organic fluid, it is flammable and has caused some fire incidents in CSP plants. It solidifies around 12–13 °C and can be used up to 400 °C without significant thermal decomposition. This upper temperature is the main limitation of current thermal oil technology [27].

Researchers have tried various coatings and/or additives to hinder decomposition reactions that occur around 400 °C [22,23]. Nanoparticles have also been added to increase thermal conductivity. It is believed that hydrogen formation increases due to the aging of biphenyl and diphenyl oxide in parabolic trough installations over prolonged periods (over 10 years). This is a relevant issue today, especially considering how many commercial plants are operating with thermal oil. The formation of hydrogen degrades the vacuum inside the trough tube enclosure, which is necessary for proper heat transfer. The gas acts as an insulator, reducing thermal conductivity and thus reducing the efficiency of the solar collector. The hydrogen production reaction seems to be catalyzed by both impurities in the oil and the presence of oxide layers on the surface of the steel receiver walls.

3.1.3. Molten Salt

To exceed the temperature limit of thermal oils, molten salts are used: these have a high thermal capacity and great thermal stability. The technical limit reached today is 565 °C when used in a Rankine steam cycle. They have a melting point above 200 °C, so below this temperature, it causes freezing and possible rupture of the pipes; therefore, it is necessary to size them properly. It is therefore strictly necessary to use such fluids with solar concentration systems to keep them always at high temperatures. For temperatures around 300 °C, the following can be considered: Hitec, a eutectic mixture of 40% NaNO₂, 7% NaNO₃, and 53% KNO₂. This has a melting temperature of 145 °C and is usable up to 425 °C, above which decomposition and oxidation occur. Sodium hydroxide, with a melting temperature of 320 °C, is usable up to 800 °C, highly corrosive, and difficult to contain at high temperatures; it is the second most used molten salt [20].

Commonly used salts and their eutectic mixtures include the ternary mixture of salts Hitec (53% KNO₃, 7% NaNO₃, and 40% NaNO₂) and the binary mixture of salts commercially known as “Solar Salt” (60% NaNO₃ and 40% KNO₃). Typically, a molten salt storage system is implemented with two tanks: one serving as the cold tank and the other as the hot tank. The molten salt is pumped between the two tanks for charging and discharging, while the heat is stored in the liquid salt mixture. In indirect systems, a heat exchanger with thermal oil as the heat transfer fluid is used, while in direct systems, the salt is used both as a storage medium and as the HTF.

The main advantages of molten salts are their high energy density, ranging from 70 $\mathrm{k}\mathrm{W}\mathrm{h}/{\mathrm{m}}^3$ to 200 $\mathrm{k}\mathrm{W}\mathrm{h}/{\mathrm{m}}^3$; an operating temperature of up to 565 °C; a high cyclability with over 10,000 cycles; and a lifespan of over 20 years. A drawback is that the system is limited to working in the steam Rankine cycle, which means that efficiencies are lower compared to the Brayton cycle. However, the molten salts currently used decompose beyond 560 °C, causing critical stability issues after thermal cycles. Furthermore, high temperatures require alloys with high corrosion resistance, which affects the final cost of energy. Therefore, corrosion is a problem that needs to be addressed to balance the benefits of high-temperature operating conditions with the use of more expensive construction materials and higher maintenance costs.

3.1.4. Liquid Metal

Liquid metals are used for concentration solar plants because they have a melting point often below the environmental temperature and a high boiling point (

Table 3. Liquid substances.

Substance	Type of Fluid	Temperature [°C]	Cp [J/kgK]	Comments
Water	--	0–100	4190	above 100 °C pressurized tank
Glycol-Water	--	0–374	3470	--
HT43	Oil	10–315	2300	non-oxidizing at high temperatures
Therminol 55	Oil	18–315	2400	--
Therminol 66	Oil	9–343	2100	--
Dowtherm A	Oil	12–260	2200	eutectic mixture
Hitec	Molten salt	150–590	1550	--
Draw salt (50% NaNO3–50% KNO3)	Molten salt	250–590	1550	--
Sodium	Liquid metal	125–760	1300	reacts violently with water, oxygen

). Therefore, they have an elevated temperature range for thermal accumulation: among them are sodium (Na) with a conductivity coefficient of 64.9 $\mathrm{W}/\mathrm{m}\mathrm{K}$ and eutectic bismuth lead with a coefficient of 10,600 $\mathrm{W}/{\mathrm{m}}^2\mathrm{K}$, but the latest study [23] reported values significantly higher than 47,000 (Na) and 24,000 (eutectic bismuth lead) $\mathrm{W}/{\mathrm{m}}^2\mathrm{K}$ using alloys of these skewers. There are several negatives in the use of liquid metals, such as corrosion, flammability, toxicity, and their high cost. Unlike molten salts, liquids do not have the same limitations in terms of upper and lower operating temperature limits. They can solidify at temperatures below 0 °C and reach boiling temperatures above 1600 °C, eliminating the problem of molten salt freezing inside the pipes. Additionally, liquids can operate at low pressures while still achieving the required temperatures for advanced power cycles such as the Rankine or Brayton cycle. This results in higher thermal energy efficiencies, allowing for greater solar flux on the receiver. Pure liquid metals and their alloys exhibit high thermal conductivity, indicating the potential for efficient heat transfer. However, liquid metals also present compatibility issues, such as molten salts and steam. The raw material cost for liquid metals is higher compared to molten salts, and the cost of the fluid and storage volume for substances such as sodium can be significant. Safety is another major concern, particularly with liquid sodium due to its high reactivity with water.

3.2. Solid Materials

The problems of high-water vapor pressures and the limitations of other liquid substances at high temperatures can be avoided by using solid materials [24]. However, the lower heat storage capacity of solids compared to water necessitates larger storage volumes. Solid materials, such as rocks or stones, can be utilized for energy storage in insulated containers, even at high temperatures (up to 1000 °C). The key advantages of using rocks include their non-toxic nature, non-flammability, and low cost. Other refractory materials suitable for high-temperature storage include magnesium oxide, aluminum oxide, and silicon oxide.

To minimize costs, direct contact between the solid material and the transfer fluid is required. In the case of packed beds, depicted in Figure 13, a container is used to hold a layer of rocks [28]. These rocks can be arranged randomly or in an organized manner. Packed beds enable the absorption and release of heat while maintaining a solid–solid state throughout the storage process, without any phase transition [28].

During the heat storage and recovery processes, a heat transfer fluid is employed to flow through the hollow spaces within the porous medium, facilitating the addition and removal of thermal energy. In this arrangement, the cold and hot fluid regions coexist within the same volume without physical separation, forming a thermocline structure. This thermocline structure serves as the foundation for heat exchange between the solid material (the storage medium) and the fluid inside the storage tank, giving rise to a dual-media thermocline storage system. During the charging phase, heat is transferred to the rock bed, allowing for the storage of thermal energy. The hot fluid is injected into the top of the casing and flows downwards, exiting through the bottom at a lower temperature. In the discharge phase, the cold heat transfer fluid flows upwards, recovering the thermal energy stored in the solid at a higher temperature. Each charge and discharge operation is terminated when a predefined fixed-point temperature is reached.

3.2.1. Solid Particles

Solid particles for CSP technology can overcome the temperature and stability limitations of molten salts by utilizing solid particles as the material for the thermal energy storage (TES) system and as the heat transfer fluid (HTF). It is expected that solid particles in TES systems will exhibit high performance due to their high service temperature and relatively low material cost. The proposed solid-particle-based materials are chemically inert and stable beyond 1100 °C. These particles can store energy over a wider temperature range compared to other currently used media, thus increasing the energy storage density. Material costs are also predicted to be relatively low.

These systems offer high performance at higher temperatures compared to other storage materials, allowing for the use of the Brayton cycle for enhanced heat-to-energy conversion efficiency [29,30]. By employing solid particles, CSP systems can operate within a temperature range of 600 °C to over 1000 °C (

Table 4. Summary of solid particle thermal properties to be used for CSP plants [31].

Material	Melting Point [°C]	Temperature [°C]	Cp [J/kg°C]
Silica sand	1220–1400	400–600	742–1175
Hematite	1565–1597	-	650
Silicon carbide	2150–2250	1470–1450	663–677
Alumina	2050	977–1030	790–800
Zirconia	2550–2700	2150–2250	418–436
Titanium dioxide	1830–1850	1570–1640	683–697
Magnesium oxide	2810–2860	1980–2130	880–1030
Graphite	3530–3680	2580–2690	852–941
Basalt	1410–1490	500–850	840

), thanks to the availability of stable materials [31]. Research on agglomeration has found that alumina, silica, and zirconia exhibit the best behavior when used in the temperature range of 1000 °C to 1200 °C.

There is also a relationship with solar absorption, as low solar absorption indicates that the particle medium may perform better in terms of agglomeration. Therefore, further studies on material selection and research must be conducted to find a good balance for these contrasting conditions. The melting point of a solid is the phase transition from solid to liquid. For solid particles in CSP plants, it is crucial to avoid this transition during the materials’ lifespan. For ceramic materials, low melting points (even if above the service temperature) can indicate potential agglomeration and sintering issues. Sintering is promoted by increased pressure, temperature, and/or surface area. For ceramic materials, the sintering temperature is close to 70% of the melting temperature.

Other solid–solid phase changes must be considered and studied within the service temperature range, as they can cause density variations that can physically alter the particles, leading to breakage or increasing the specific surface area (thus increasing the potential for chemical reactivity). This technology is commonly used as both a heat transfer medium and a thermal storage medium: the heated particles are then transported to the TES system, where heat can be transferred to a secondary working fluid, or the solid particles themselves can serve as the working fluid, flowing through a fluidized bed heat exchanger. The original design proposes two containers for storing the particle medium: one for hot storage and one for cold storage. The hot storage container will hold particles coming from the high-temperature (800 °C) solar receiver and store them until electricity production is economically viable. The cold storage container will hold the solid particles after they have exited the heat exchanger, keeping them at a lower temperature (350 °C) before sending them back to the solar receiver for reheating. The cold tank will store the particle medium until solar resources are available again for solar heat collection.

Current research focuses on two main approaches: the interaction and integration of the storage system with other components of the plant that are being studied; the cost reduction of storage containers based on heat transfer understanding; and exploring innovative designs, materials, and insulation techniques to improve thermal performance and reduce heat losses during the storage process. The study of possible thermal losses is considered significant and should be investigated for different charge/discharge cycles. Efficient thermal insulation systems for energy storage are crucial due to the high temperature of the stored solid particle medium.

3.2.2. Concrete

Concrete is commonly utilized as a storage material, and when subjected to heating, various processes occur. These processes, outlined in Table 3, can involve phase changes within the material. At certain operating temperatures, the internal structure of the concrete can undergo microscopic destruction, resulting in impaired properties and reduced material performance [32]. Mechanical characteristics such as thermal stability and durability within the designated temperature range are crucial factors to consider.

Ordinary concrete, when heated up to 400 °C, can experience violent explosions due to the pressure generated by superheated steam dissociating from the cementitious material (CaCO₃·11H₂O). To mitigate these issues, the addition of polypropylene fibers has been considered, allowing for temperatures of approximately 550 °C to be reached [32,33] (

Table 5. Processes that occur at different temperature [32,33].

Temperature [°C]	Processes	Comments
105–440	dehydration	C-S-H phase 1
440–580	dihydroxylation	Portlandite phase 2
580–1000	decarbonization	--

¹ C-S-H: Calcium silicate hydrates, primarily responsible for the cement-based materials. ² Portlandite: affects the carbonation and corrosion resistance of concrete.

). Cracks observed in concrete can hinder thermal performance by affecting effective thermal conductivity. These cracks may arise from differences in thermal expansion between the steel tube heat exchanger and the surrounding concrete, or from significant temperature gradients within the concrete.

Compression tests have demonstrated that the significant reduction in compressive strength after heating is attributed to local overpressures in the porous system, which can damage the internal microstructure of the matrix. Cement-based storage technology presents an intriguing alternative for storing sensible heat in concentrated solar power plants. The thermophysical and mechanical properties of cement, such as density, thermal capacity, thermal conductivity, thermal expansion, and durability, play a vital role in determining the performance and longevity of this type of storage system over many years of operation.

Recent studies have indicated that concrete can be utilized as a dual-phase thermal storage system, addressing concerns such as spalling. Promising approaches include the implementation of shell-and-tube concrete or thermocline concrete in combination with molten salt. Concrete bricks have demonstrated effectiveness as a storage medium when paired with molten salts operating at temperatures as high as 600 °C. Storage systems, such as these, eliminate the need for pressurized tanks, thanks to the excellent mechanical properties of concrete. Additionally, this system is non-toxic and non-flammable. However, further experimentation is required to explore various modifications to the concrete structure, aiming to mitigate the issue of spalling caused by high temperatures and repeated loading and unloading cycles.

3.2.3. Rocks

Typically, cubic or parallelepiped geometries are preferred in the configuration of packed bed thermal storage systems due to their high volume-to-surface ratio.

Table 6. Most-used rocks.

Rock	Specific Heat [kJ/kgK]	Hardness
Basalt	1.47	Medium high
Quartzite	0.8–0.9	Very high
Marble	0.7–1	Low high
Granite	0.6–1.2	High

presents the most used types of materials for such systems. Among these materials, gravel is often favored over crushed stone due to its lower resistance to airflow. However, one of the main limitations of packed bed systems is the requirement for stones of uniform size. The packed bed system utilizes a porous medium composed of solid particles to store thermal energy provided by the thermal system. This technology enables the storage of thermal energy in the form of sensible heat for air or other heat transfer fluids. The stored energy can be discharged when needed by the user [34].

During the charging phase, the heat transfer fluid from the solar system enters the storage tank from above and transfers heat to the solid particles or rocks. In the discharge phase, the flow direction is reversed, and the fluid is pumped towards the bottom of the tank to extract the previously accumulated heat. Quartzite and silica sand are commonly used rocks for this purpose. When selecting a material for thermal storage, it is important to consider its thermal capacity and optimal geometric configurations that allow for efficient use of materials in the storage tanks. This technology enables the utilization of readily available materials that can effectively store heat over a wide temperature range. It is necessary to study the chemical stability of the chosen material and ensure the rocks have suitable geometry to minimize pressure losses caused by irregular shapes [35].

3.3. Gaseous Substances

Air and other gases (CO₂, H₂, and He) have also been evaluated as HTFs for CSP plants. Compressed gases offer many advantages over traditional HTFs, such as availability, reduced safety and environmental concerns, easy operation and maintenance, and high operating temperatures. Additionally, a gaseous HTF could potentially be used as the working fluid in the power block turbines, eliminating the need for additional heat exchangers [28]. The direct expansion of solar-heated gases allows for the use of high-efficiency Brayton cycles, which could be combined with conventional Rankine cycles for a total efficiency above 50%. However, some drawbacks of gases are their poor heat transfer properties. Poor heat transfer presents challenges in receiver design. As a result, gases require high pressures to achieve adequate efficiencies, increasing the system installation costs with thick-walled structural elements and high pumping power needs.

The use of air as an HTF in central receiver towers has been demonstrated since the 1980s. There are some pre-commercial scale CSP plants that use air as an HTF. Inert gases such as CO₂ are also being considered as alternative HTFs and are currently being reevaluated. Pressurized CO₂ offers advantages from an environmental and safety perspective. It is non-flammable, non-toxic, and widely available at a low cost. Helium has been evaluated as an HTF in CSP plants as well [29,30]. Its main advantages are its inertness and much higher specific heat capacity compared to air. However, similar to CO₂, it must be used in a closed cycle (meaning the fluid is recirculated from the solar receiver to the turbine and then condensed and sent back to the receiver), and leaks are a significant problem.

4. Thermochemical Heat Storage

Thermochemical systems commonly require higher temperatures to initiate energy storage but, conversely, provide higher temperatures during the release of that energy. The most relevant chemical processes for chemical energy storage in CSP plants are metal/metal oxide reactions and ammonia [7]. The thermochemical storage systems are based on the exploitation of reversible sorption processes or of chemical reactions:

$$AB+HEAT\rightleftarrows A+B$$

(3)

During the charging step, the heat is used to drive an endothermic process that leads to the dissociation of the reactant AB; then, A and B, the product of the charging step, can be stored separately. The inverse exothermic process, between the stored chemicals A and B, will have as a product the initial species AB and an amount of heat. The heat involved in the charging and discharging steps can be defined as

$$\dot{Q_s}=\alpha m\Delta \mathrm{h}$$

(4)

The reversible reactions that are suitable for a TES system can be characterized by reactant family, reaction enthalpy, and turning temperature.

Pardo et al., in their work [35], report that in the temperature range of 573–1273 K, the most promising chemical reactions because of the actual related development and cost are the following:

MgH₂ ↔ Mg + H₂

PbCO₃ ↔ PbO + CO₂

Ca(OH)₂ ↔ CaO + H₂O

NH₃ ↔ N₂ + H₂

The chemical loop, specifically the calcium looping, has the highest energy storage potential (4400 MJ/m³) and operating temperature (800–900 °C) and the lowest net efficiency penalty (5–8% points). The combination–decomposition chemical reactions of the carbonate-contained TCMs are as follows [36]:

K₂CO₃⋅1.5H₂O(s) ↔ K₂CO₃ (s) + 1.5H₂O (g)

SrCO₃(s) ↔ SrO (s) + CO₂ (g)

MgCO₃(s) ↔ MgO (s) + CO₂ (g)

CaCO₃(s) ↔ CaO (s) + CO₂ (g)

The process allows for increasing the net power output of the integrated system by over 50% and storing a permanent source of carbon dioxide, using excess electricity produced from renewable energy sources. The basis of the calcium carbonate is described as follows: solar heat is supplied to the calciner and used to heat and decompose CaCO₃ (calcite/calcium carbonate) into CaO and CO₂ (carbon dioxide) [37,38]. In the carbonator, CaO and CO₂ recombine to form CaCO₃ and, consequently, release the heat of carbonation. The heat is then stored in the form of CaO. Several configurations have been developed to integrate chemical looping into CSP systems.

Muñoz-Antón et al. [39] analyzed the integration of a regenerative Brayton cycle with near-critical CO₂ in a CSP plant without storage to achieve higher cycle efficiency. Cabeza et al. [40] explored a new CSP plant configuration based on thermochemical storage using the novel concept known as consecutive reactions.

Solar thermal technologies through thermochemical conversion pathways offer the prospect of systems with intrinsic energy storage for continuous (24 h) electricity generation. This issue will become increasingly significant as the world transitions to a truly renewable-energy-based economy.

5. Latent Heat Thermal Storage

This system enables the accumulation of a significant amount of heat by utilizing phase change processes in materials, with minimal variations in temperature. Latent heat storage can be categorized based on the type of phase change process involved, such as solid–solid, solid–liquid, solid-gas, and liquid-gas, as shown in

Table 7. Phase change transformation.

From/To	Solid	Liquid	Gas
Solid	Solid–solid transformation	Melting	Sublimation
Liquid	Solidification	-	Evaporation
Gas	Desublimation	Condensation	-

. However, the last two transformations are not commonly used due to the complexities and bulkiness associated with volume changes. Solid–solid transformations have low associated heat, so the most prevalent choice is solid–liquid transitions. This is due to their high heat accumulation density and the minimal volume changes required for effective storage.

A latent storage system consists of a substance capable of undergoing a phase transition in the considered temperature range, and the supplied heat is stored as latent heat. The thermal energy is used to break the molecular bonds and allow the change of state (fusion-vaporization) without temperature variation: it is an endothermic process that accumulates heat making it available later. The accumulated energy is a function of the mass and of the latent heat of fusion as illustrated in Equation (5):

$$E=m\lambda$$

(5)

It is usually difficult to operate isothermally at the phase change temperature. The system operates in a temperature range which includes the melting point. Therefore, the overall energy is given by the following Equation (6):

$$E=m\left[{\int }_{T_{in}}^{T_m}{C}_{ps}dT+\lambda +{\int }_{T_m}^{T_{out}}{C}_{pl}dT\right]$$

(6)

Different solutions have recently been tested which allow the PCMs to be placed in direct contact with the heat transfer fluids, favoring the exchange and reducing the costs of the exchange.

5.1. Phase Change Materials

PCMs are organic, inorganic, or eutectic substances (Figure 14) of natural or synthetic origin that are used to store and release thermal energy during the phase change from solid to liquid state and conversely, thus exploiting the sensible heat capacity, but also the latent heat capacity of melting [41]. PCMs help optimize daily temperature fluctuations by reducing internal heat peaks, resulting in energy and air conditioning savings in the environment.

The most-used PCMs are organic materials, including paraffins and fatty acids. Additionally, there are inorganic materials, such as hydrated salts, and a less common category known as eutectic materials. One of the significant advantages of using PCMs is their high thermal storage capacity. Compared to traditional materials, PCMs have an exceptional storage capacity that is 80–100 times higher at the same weight and within a temperature range close to the melting point [42]. However, their complexity in design and application time are also the main disadvantages as regards the need of special thermal conditions.

PCMs can be divided into three groups: organic, divided into paraffins and non-paraffins; inorganic, classified as salts, hydrates, and metals; and eutectic mixtures, divided into organic, inorganic, and organic–inorganic eutectics. The last group refers to solutions of two or more components, each of which melts and solidifies congruently at a lower temperature than that of the individual substances, forming a mixture of the component crystals.

5.1.1. Inorganic Type

Hydrated salts and metals are a part of the PCM category due to their ability to change the melting temperature of the salt by incorporating water molecules. They are highly desirable as PCM materials because of their high energy densities, appreciable thermal conductivity, low corrosivity, and compatibility with plastics. Mustafa et al. [43] conducted experiments to determine the optimal concentration for maximum solar storage using various PCM substances. The experiments involved a water bath placed on a magnetic stirred hot plate, a beaker container, temperature sensors, a stopwatch, and a data logger. The glass beaker contained 100 g of PCMs, including wax (B), pure Ca(NO₃)2.4H₂O (C), and a composite of Ca(NO₃)2.4H₂O:Mg(NO₃)2.6H₂O (D) at different ratios. The heating process gradually increased the temperature from the environmental temperature to 55 °C within one and a half hours, allowing the researchers to observe the substance’s behavior during the phase changes.

By analyzing the temperature data, they determined the heat storage capacity of each substance. The results showed that pure calcium nitrate tetrahydrate exhibited the best physical qualities, with a storage duration of 72 min for free cooling and 57 min for supercooling storage. It demonstrated high efficiency, a small phase change temperature gap, and low production cost, making it suitable for subtropical climates. In terms of stability, wax proved to be the most stable option for up to ten cycles, while calcium nitrate tetrahydrate was stable for up to six cycles. The addition of certain salts to water reduces the melting point of the water by weakening the hydrogen bonds between individual water molecules through the presence of salt ions [43,44].

By introducing a temperature regulator, the intermolecular strength can be enhanced. Common regulators include sodium chloride (NaCl), sodium peroxynitrate (NaNO₄), potassium chloride (KCl), potassium sulfate (K₂SO₄), and ammonium chloride (NH₄Cl). The study revealed that the addition of Na⁺ ions slows down the decrease in melting point [45,46]. The NH₄⁺ ion forms hydrogen bonds with water molecules, reducing the interaction between the inorganic salt and water, thereby altering the phase change temperature of the hydrated salt. According to a general classification, Na+ brings about the least variation, while NH₄⁺ causes the most significant variations, with K+ falling in between.

Zhu et al. [47] investigated the addition of NH₄Cl and KCl as regulators for the phase change of sodium sulfate decahydrate. The addition resulted in a reduction in temperature to 8.3 °C and an increase in latent heat. It is important to note that the added inorganic salts do not participate in the phase transition but only control the temperature to achieve the desired value. The addition of salt needs to be carefully calibrated to avoid negatively impacting the thermal accumulation capability of the PCM.

In recent studies conducted by Chen et al. [46], the regulation of the temperature of lithium nitrate trihydrate was achieved using potassium nitrate and sodium nitrate. The addition of inorganic salts in this case does not induce phase transition but solely serves to adjust the temperature to the desired level. It is essential to determine the appropriate quantity of salt to ensure it does not adversely affect the thermal accumulation capacity of the PCM.

5.1.2. Eutectic Type

Eutectic mixtures are formed by combining a hydrated salt with a basic hydrated salt to alter the melting point. This change occurs due to a modification in the hydrogen bonds, resulting in the formation of a stable structure [47]. In the case of eutectic mixtures, a substance is created with a melting point lower than that of the base salt. This ensures that the two substances do not undergo a chemical reaction but instead remain as stable compounds. By varying the proportions between the salts, multiple independent phase changes can be formed.

The point at which the phases coexist is referred to as the eutectic point (Figure 15) [48]. In a study aimed at overcoming the limitations of low thermal conductivity in fatty acids and fatty alcohols, the performance of a PCM eutectic mixture of lauric acid and hexadecanol (HD) was evaluated. The composite material was modified using two different approaches: seashell powder with varying mesh sizes and TiO₂ powder. The test results demonstrated that the use of seashell powder was more effective in improving thermal conductivity compared to the first method, thereby enhancing thermal energy storage. Eutectic compounds offer the possibility of modifying the thermophysical properties of the constituent salts, reducing their melting point to mitigate freezing issues. The potential for obtaining eutectic mixtures is vast, as each addition expands the temperature range of the phase change compounds (PCM).

Table 8. Most famous PCMs’ melting points [49,50,51,52,53].

Organic PCM	Tm [°C]	Inorganic PCM	Tm [°C]	Eutectic PCM	Tm [°C]
Naphthalene	80	COCl2	740	NaCl-KCl	360
Acetanilide	118	KOH	375	MgCl2-NaCl	430
Paraffin wax	64	Na2CO3	850	Al-Mg-Zn	440
Glycerine	18	Mg(NO3)26H2O	88	LiF-CaF2	770
Stibene	124	Al	660	LiCl-LiOH	270

provides an overview of the most-used eutectic types along with their corresponding eutectic points [49,50,51,52,53].

The most promising method that has been considered for latent heat storage is the utilization of molten salt mixtures as phase-change materials [54]: the study focused on the binary salt mixture of lithium chloride–lithium hydroxide (LiCl-LiOH) as a potential phase-change material for thermal energy storage. The thermal analysis revealed that a composition of 32% mol LiCl and 68% mol LiOH exhibited a melting range between 269 °C and 292 °C, with a heat of fusion of 379 J/g. Repeated heating and cooling cycles demonstrated minimal variations in the melting temperature and heat of fusion, indicating good thermal repeatability. Furthermore, thermal decomposition analysis showed negligible weight loss up to 500 °C. Based on these findings, it was concluded that the binary salt mixture has the potential to be used as a thermal energy storage material in applications up to 500 °C.

5.1.3. Organic Type

Yuxuan et al. [51] aimed to investigate the thermal storage capacity of various organic phase-change materials (PCM). Experiments were conducted on a structure filled with PCM material and air, with solar energy applied to induce solid–liquid phase transitions and evaluate thermal storage (Figure 16). The symmetrical dimensions of the structure simplified the problem. The materials compared for their capacities were fatty acid (phase-change temperature, tm = 70 °C), barium hydroxide octahydrate (tm = 78 °C, highest density of 1660 kg/m³), naphthalene (tm = 80 °C), magnesium chloride hexahydrate (tm = 117 °C), and erythritol (highest tm = 118 °C). In addition to symmetrical dimensions, ideal assumptions were made, considering the PCM as isentropic, applying a constant heat flux to the structure’s surface, and assuming the air above the structure as an ideal gas governed by the perfect gas law. The results of the experiment involved comparing the materials based on their behavior at different volume fraction values (φ = 40%, 50%, 60%, 70%, 80%, 90%).

Volume fraction represents the ratio of the solid phase volume to the total cavity volume containing the PCM material. Figure 17 and Figure 18 depict the variation of the phase change for different values of β (0.5 or 1.0 for half or fully melted), with a color legend provided on the side. The results revealed that higher values of φ corresponded to lower volume expansion of the PCM and longer duration in the melting phase. The most favorable thermal accumulation was observed at an 80% ratio due to the rapid development of the Rayleigh convention.

In the study in [52], the experiment aimed to evaluate the long-term performance and thermal stress of a phase-change material composed of 80% paraffin and 20% high-density polyethylene. The material was subjected to thermal loading with three different thermal velocities, measured in degrees Celsius over minutes, simulating 10,000 effective cycles of thermal storage. The test results showed that by incorporating polyethylene into the paraffin base, the thermal stress of the material could be significantly reduced. This modification allowed the material to withstand longer cycles without experiencing excessive thermal stress.

The use of paraffinic PCM is highly developed in thermal storage applications due to its wide melting temperature range and high heat capacity. These properties make it suitable for storing and releasing thermal energy efficiently [50]. This type of PCM is widely used for hot water production with materials that have a melting temperature in the range of 40 to 80 °C, ensuring high outlet temperatures regardless of mass flow [53]. Positioned according to the type of requirement, integrated or non-integrated into the solar water system, they allow a constant outlet temperature to be maintained and a higher system efficiency. The study by Eleni et al. [53] especially shows that the low thermal conductivity values and the consequent lower losses to the environment are emerging properties of paraffin- and myristic-acid-type PCMs. Therefore, there is great efficiency in the night period. Instead, the use of PCMs encapsulated in spherical cells at the top of the cylinder allows hot water to be available for a longer period [54].

5.1.4. Biobased PCMs

The development of biological PCM [55] is turning towards the evaluation of the potential of animal-fat-based materials as tools for thermal storage. These, consisting of percentages of monounsaturated and polyunsaturated fatty acids, categorized as inedible fatty parts of pork and chicken, identify themselves as a low-cost biocompatible PCM. The potential use of expired palm oil from the food industry has been exploited as a phase-change material with potential for use not only in buildings but also as an upcoming development as a thermal storage system. The real innovation brought about by the study of these materials lies in being able to guarantee energy storage with the use of natural or processed products.

The study in [56] focuses on the results of the tests on inedible or waste oils and fats or appropriately modified oils. The best feature of using these materials is their chemical stability, non-toxicity, and lower flammability compared to paraffins. Furthermore, they are biodegradable and have minimal environmental impact. As for the melting point value, this depends on the length of the carbon chain of the fatty acids. However, on the other hand, for all these types, there is the problem of low thermal conductivity, a problem that is being worked on to achieve better results. In the case of bio PCMs, it must be considered that the cell housing the material must also be of a similar nature to the PCM to achieve optimum compatibility.

5.1.5. Nano-Potential PCM

The studies conducted by Banumathi et al. [57] have explored the enhancement of phase-change materials (PCMs) through the addition of nanoparticles for solar thermal applications. The addition of nanoparticles, such as carbon-based materials (carbon, soot, graphene, graphite, carbonized kapok fiber), as well as metals such as copper and indium, can improve the thermal conductivity of the PCM [58]. The choice of nanoparticles is crucial to ensure that they do not negatively affect the performance of the PCM. The density of the nanoparticles should be low to avoid excessive weight and minimize the impact on the fusion of the PCM. High density can also reduce the porosity of the PCM, limiting its ability to absorb and release thermal energy effectively. Therefore, it is important to select nanoparticles with low density for solar thermal storage applications.

Table 9. Low-temperature nano-enhanced PCMs.

Nano-Potential PCM	Application	Tm [°C]	Comments
Octanoic acid-Lauric acid (k = 0.336 W/m K)	Medical refrigeration and air conditioning	--	The best thermal conductivity; low melting point
Erythritol (0.73 W/m K)	--	118	It suffers from supercooling
Ba(OH)2⋅8H2O (k = 1.163 W/m K)	Industrial applications	--	Good thermal conductivity and heat recovery applications
Polyethylene glycol (k = 0.254 W/m K)	Thermal management	--	Good stability and has the best latent heat
Palmitic acid (k = 0.28 W/m K) with porous mullite	Solar energy storage and solar heating	60	Good stability
1-Octadecanol (0.415 W/m K)	Solar energy storage	58.54	It suffers from supercooling

and

Table 10. High-temperature nano-enhanced PCMs.

Nano-Potential PCM	Application	Tm [°C]	Comments
Li2CO3–Na2CO3–K2CO3	Thermal energy storage	407.13	Good latent heat but is corrosive
NaNO3/KNO3 (k = 0.806 W/m K)	Thermal energy storage	223.4	It suffers from supercooling
Pentaerythritol (PE)	Thermal energy storage	255–259	Good thermal conductivity but suffers from supercooling
50 wt% LiNO3–45 wt% NaNO3–5 wt% KCl (Lithium sodium and potassium ternary salt)	Thermal energy storage	--	Good thermal conductivity but suffers from supercooling

provide insights into high-temperature PCMs and how the thermal conductivity can be enhanced by incorporating different amounts of carbon, metal, and ceramic additives [24,42]. The study also explores the potential benefits of adding copper and indium nanoparticles to the PCM. These additives aim to improve the overall performance of the PCM in terms of thermal conductivity and storage capacity [9,59].

6. General Analysis of Different Energy Storage Systems

Cristina et al. [13] have proposed a qualitative comparison between a concentrated solar system, which uses a thermal accumulation made of two tanks filled with molten salt that act indirectly on the operation of the system (traditional technology) and the use of PCMs, arranged in small tanks, distributed in four groups for the type of salt used (Figure 19 and Figure 20). PCMs allow energy to accumulate through the phase change of the substance that takes place at an approximately constant temperature; the arrangement in the array allowed for highlighting the different proportions of the constituent salts. In the first group there is NaNO₃; NaCl (33%)–KCl (24%)–LiCl (43%) in the second one; NaOH (80%)–NaCl (20%) in the third one; and MgCl₂ (60%)–KCl (20.4%)–NaCl (19.6%) in the last one.

The test carried out under non-extreme environmental conditions and normal solar radiation conditions led to the conclusion that the possibility of using phase-change materials should be evaluated depending on the type of energy utilization and the geographic location of the solar field, as the thermal losses occurring in PCMs may be greater than the expected accumulation. In a net view over the period of one year, the two technologies are similar, at the expense of a more difficult construction technique for the traditional indirect system.

E. González-Roubaud et al. [24] compared steam accumulator and molten salt sensible storage systems in commercial plant configurations (Figure 21). The indirect molten salt thermal energy storage system is the most widespread in concentrating solar power plants. One of the main advantages is the ability to discharge at constant conditions, maintaining high cycle efficiency. There are no concerns about the corrosion or degradation of salts as they operate at low pressures and do not require pressurized tanks, and this allows cheaper tanks to be used because of the minimum thickness (Figure 22, Equation (7)). However, their use is limited by the degradation temperature of the material. Large indirect molten salt storage systems will require large amounts of salt and a high number of heat exchangers. Additionally, a significant amount of time is needed to transition from charging to discharging conditions, which prevents the system from acting as a buffer storage or protecting the turbine from transients.

Indicating with ${\sigma }_1$ ≥ ${\sigma }_2$ the two principal stresses, we have

$${\sigma }_1={\sigma }_c=\frac{pr}{t}; {\sigma }_2={\sigma }_a=\frac{pr}{t}$$

(7)

$${\tau }_{max}=\frac{{\sigma }_1}{2}=\frac{pr}{2t}$$

Direct molten salt storage systems used in a solar tower plant offer the same advantages as the indirect system but with an increased temperature of around 565 °C (

Table 11. Estimation of molten salt and water/steam conditions for the steam generator in a molten salt tower [24].

Fluid	Temperature [°C]	Pressure [bar]
Hot salt	565	12
Cold salt	288	2.3
Superheat steam	550	130
Reheat steam	548	27

), thereby increasing the cycle efficiency. Both solutions require continuous heating to prevent material solidification.

For molten salt technology, the tower technology with a dual-tank system is utilized. In Figure 23, it is evident that the energy exchange between steam and molten salts will be penalized during the evaporation phase. In this case, heat exchange occurs from a lower temperature heat source to a constant temperature sink (steam), resulting in a loss of heat transfer area. The results of the economic analysis indicate that the steam accumulator system has the lowest thermal cost for storage capacities below 3 h, followed by the direct molten salt TES system and the indirect system, respectively. However, the trend reverses as the storage capacities increase, with the direct molten salt TES system being the best option. Burcu et al. [10] demonstrated that the energy density of storage materials can be defined as the amount of energy released per unit volume. More energy can be stored in materials with higher energy densities.

In Figure 24a [60], it is possible to compare the different storage capacities for various systems: thermochemical storage systems can store much more energy in a smaller volume, but it is the least developed technology among the three. Sensible heat storage materials require a larger volume to store the desired heat, and heat losses from the system increase as the storage volume increases. On average, latent heat storage systems can reduce the volume of water-assisted thermal systems by 50%, while thermochemical heat storage systems have the potential to reduce the volume of chemical storage tanks by a factor of 34 using chemical reactions. In addition, the thermal storage capacities of phase-change materials are higher compared to sensible heat storage materials (Figure 24b) [61], but the price of PCMs that can operate above 150 °C is very high.

Furthermore, Romani et al. [62], when comparing the storage capacities of different TES materials, observed from Figure 25a that water, as a sensible thermal energy storage material, has a lower storage capacity compared to PCM and thermochemical materials [63,64]. In terms of energy density [65], Figure 25b indicates that PCMs can store heat up to 1 GJ/m³, the sorption process can have high energy density values up to 6 GJ/m³, and chemical reactions can offer an energy density of up to 10 GJ/m³. To meet the heat demand using the sorption process of a TCM, half the volume of a thermal storage tank filled with a PCM is needed, and one-third of the volume of a water tank is needed to satisfy the same amount of heat [24].

7. Global Trends

Indeed, solar storage systems are gaining popularity globally due to their ability to reduce carbon dioxide emissions and combat climate change. The continuous improvement of these systems through research and testing has led to promising results in terms of global energy efficiency. To ensure optimal performance, it is important to conduct tests in various climatic conditions and combine the results obtained from different geographical areas. By doing so, it becomes possible to develop solutions that are optimized for maximum energy accumulation efficiency.

By studying the behavior of solar storage systems in different climates and locations, researchers can identify the most effective strategies and technologies to achieve efficient energy storage. This approach allows for the development of customized solutions that are tailored to specific environmental conditions and energy requirements. The goal of these efforts is to enhance the overall performance and reliability of solar storage systems, ultimately contributing to a more sustainable and environmentally friendly energy landscape.

It is correct that the industrial sector is a significant consumer of energy, and a large portion of industrial processes require high temperatures exceeding 400 °C. In contrast, about 30% of energy demand in industries falls within a range of 150 °C. As economic growth is often associated with increased energy consumption, it becomes crucial to harness solar energy to its fullest potential. To achieve this, it is necessary to simulate the operation of solar plants under climatic conditions representative of energy-intensive countries. Spain has emerged as a leader in solar energy, meeting 10% of its national energy demand through solar installations. However, worldwide solar energy systems currently account for only 0.8% of total energy production. To effectively address global carbon emissions, it is imperative to promote the use of renewable energy sources, including solar power, for both energy management and construction purposes.

By collecting and analyzing global results, researchers can gain valuable insights and develop strategies to optimize the performance of solar energy systems worldwide. Figure 26 provides an overview of the primary energy demand from both developed and developing countries, measured in exajoules. These boundary conditions help inform testing procedures and ensure that the test results are applicable and relevant on a broader scale. By leveraging solar energy and expanding its utilization across various sectors and countries, we can make significant progress in reducing carbon emissions and promoting a more sustainable and greener future [62,63].

Indeed, paraffinic phase-change materials (PCMs) are commonly used for medium- or low-temperature applications, typically up to temperatures of 180 °C. This is because these PCMs are easier to construct, more affordable, and have a latent heat of fusion in the range of 180–280 kJ/kg. In contrast, high-temperature systems require PCMs with a much higher latent heat of fusion, around 900 kJ/kg. Multiple tank configurations with PCMs in cascade and storage units in packed beds, as described in [64,65], are commonly used for high-temperature thermal energy storage. Investment in research and development is crucial for the advancement of solar thermal systems, especially considering the high initial economic cost associated with large-scale energy generation.

The intermittent nature of solar radiation poses a challenge that needs to be addressed for effective utilization of solar energy. Industrialized countries, despite accounting for only 15% of the global population, are the largest consumers of energy, contributing to most environmental issues. Among these countries, the United States ranks first in per capita energy consumption. With an annual consumption of 2297.8 million tons of oil equivalent (MTEP), a US citizen consumes nearly 8 tons of oil per year, approximately 800% higher than the world average. Italy, on the other hand, has a lower per capita energy consumption, but global energy consumption remains high. In contrast, African countries, with a population exceeding one billion, have significantly lower energy consumption, accounting for only 3% of the energy made available worldwide. This highlights the disparity in energy consumption between industrialized nations and other regions of the world. Overall, addressing the energy consumption patterns of industrialized countries and promoting sustainable energy solutions, such as solar thermal systems, can contribute to reducing environmental impact and achieving a more balanced global energy landscape [59,66].

8. Conclusions

This article analyzes the information available in the open literature regarding high- and low-temperature thermal energy storage (TES) for energy storage, focusing on the classification of storage system concepts and the description of materials used. TES systems can be integrated with solar thermal collectors for industrial applications to produce heat during periods of weak solar radiation, proportional to the operating temperatures required by the users. The requirements for a thermal energy storage system include high energy density in the storage material (also known as storage capacity); good heat transfer between the heat transfer fluid (HTF) and the storage medium; mechanical and chemical stability of the storage material (a primary safety requirement for a plant); good compatibility between the HTF, heat exchanger, and/or storage medium; complete reversibility of a certain number of charge/discharge cycles; low thermal losses; and ease of control.

Thermal energy storage systems are classified as sensible heat storage, latent heat storage, and chemical heat storage, while based on the storage concept, systems can be classified as active and passive. Active systems involve forced convection heat transfer in the storage material, either directly with the storage media (direct systems) or indirectly, with the heat transfer fluid and the storage medium being different substances. In passive systems, the thermal storage medium itself does not circulate. Most of the energy storage concepts used in solar power plants are active systems. Solid sensible heat storage (SHS) materials, such as water, have been used since ancient times as the primary developed TES systems, while latent heat storage (LHS) methods were discovered in the 1950s. Currently, all storage materials used in solar power plants are based on liquid sensible heat storage.

The two most commonly used molten salts are the so-called “solar salt,” a binary salt composed of 60% NaNO₃ and 40% KNO₃, and the commercially known HitecXL, a ternary salt composed of 48% Ca(NO₃)₂, 7% NaNO₃, and 45% KNO₃. Research and development are underway to overcome the current challenges related to high freezing points by exploring new salt mixtures. Several studies have been published on the effect of corrosion of molten salts on steel and stainless-steel tanks. It is concluded that the impurities typically present in commercial alkaline nitrate salts have relatively small effects on the corrosion of stainless and carbon steels in molten salts prepared from these constituents.

The use of solid particle materials has great potential for the development of a new generation of solar power towers. This technology can enhance CSP plants to reduce electricity production costs and add flexibility to the power grid, enabling greater implementation of renewable energies. This technology will be particularly advantageous for highly irradiated regions, which are expected to be the most developed regions in the coming years, exceeding their energy needs beyond the global average. Studies and tests have been conducted on solid sensible heat storage, including concrete and ceramics, as the most promising candidates. The low cost of solid materials needs to be balanced with the increased cost of the storage project.

Latent heat storage is a promising technology as it offers higher storage density and a nearly constant temperature. Organic PCMs have a low melting heat and are flammable, while salt hydrates have overheating and phase separation issues leading to a reduction in the melting heat in subsequent cycles. Nanoparticles can enhance the thermophysical properties of TES materials by increasing thermal conductivity, wettability, and improving intermolecular characteristics. Chemical heat storage technology is also promising but is less developed compared to latent heat storage for concentrated solar energy heat storage.