Performance evaluation of a solar thermal storage system proposed for concentrated solar power plants

Utilizing renewable energy sources, such as solar energy, has become essential due to the world’s increasing need for fossil fuel energy and the resulting greenhouse gas emissions. Concentrated Solar Power (CSP) plants are becoming the favored alternative since they can create electricity without contributing to localized increases in greenhouse gas emissions or other environmental pollutions [1]. Solar energy has been increasingly popular due to its many benefits; nevertheless, it is not without its share of problems, such as irregular availability. As a result, researchers have been attempting to develop a practical method of storing solar energy for later use in CSP plants. Thermochemical thermal energy storage method is favored over sensible and latent heat storage because of its higher energy density [2].

Consequently, thermochemical energy storage has the greatest heat storage capacity without incurring thermal losses during the storage period. The type pairs of metal hydrides (MHs) used in a thermochemical energy storage system have a substantial effect on the amount of heat storage capacity [3]. In the literature, there exist metals that can react with hydrogen and absorb a significant amount of the gas to form a MH [4]. When these metals absorb hydrogen, exothermic reactions occur and heat is released into the environment. In contrast, endothermic processes necessitate the presence of heat in order to desorb the hydrogen held in the MH, which may be supplied by another source. For each cycle, heat is lost, reducing the effectiveness of the MH bed [5]. To comprehend the complex coupled heat and mass transfer in MH tanks, numerous researchers conducted experimental and numerical studies [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25].

Accordingly, scientists have developed a useful technique for preventing the waste of heat energy during exothermic processes by storing it in a Phase Change Material (PCM) and then using it to initiate the endothermic reactions. In this regard, Garrier et al. [6] developed a new concept of an MH-PCM tank in which the central tube carring the MgH₂ compacted disks is encircled by an exterior tube containing the PCM. Experimental results show a significant enhancement of the storage capacity of the tank in comparison to a basic tank without PCM. Another configuration of a MH bed was proposed by Marty et al. [7]. It consists of 71 disks filled with compacted Magnesium alloy with natural expanded graphite. A stainless steel wall separates the inner cylinder from the outer tube housing the PCM. They studied the dynamic behavior of the reactor according to various operating parameters and they concluded that the presence of a PCM around the hydride improves the storage capability. Ben Mâad et al. [8] investigated heat and mass transfer within a MH reactor equipped with the Lithium Nitrate Trihydrate (LiNO₃-3H₂O) as a PCM. The proposed geometrical configuration consists of two concentric tubes with LaNi₅ in the central cylinder and PCM in the annular space between the two cylinders. In their study, they suggested a new mathematical model based on the Heaviside step function to describe the liquid fraction in the PCM. As a result of incorporating the PCM, they discovered that the reactor could release up to 80% of its stored hydrogen. Later, they examined the same setup with the intention of determining the impact of various physical PCM properties on the absorption and melting processes [9]. Mellouli et al. [10] investigated the hydrogen storage phenomenon in cylindrical and spherical MH tanks including a PCM. The numerical results show that, compared to the cylindrical case, considering a spherical bed permits to improve the thermal performance of the system by 22%. In another research, Mellouli et al. [11] numerically investigated the effect of the tank design by considering four conceptions with integrated PCM: a tube placed in the center of the tank, hexagonal tubes, spherical shells and cylindrical tubes. Authors found that, in comparison to the basic configuration, incorporating cylindrical PCM tubes into the MH bed is the best solution and enhances the filling time by 58.1%. Another numerical study was proposed by Mellouli et al. [12] in order to evaluate the impact of using a heat transfer fluid pipe in an MH-PCM system. Three different configurations were studied and compared. It was concluded that the loading process can be enhanced by 94% when considering open heat transfer fluid pipes. Furthermore, Darzi et al. [13] conducted a numerical study to investigate the absorption and desorption processes of hydrogen by an LaNi₅ bed integrated with the Rubitherm PCM jacket and a metallic foam. Various supply and discharge pressures and bed porosities were analyzed. They noted that the performance of the MH-PCM system improves with a higher supply pressure for absorption, lower pressure for desorption, higher tank porosity and by inserting a metal forma in the MH bed. Ben Mâad et al. [14] proposed a new conception of a MH reactor filled with Mg₂Ni as a hydride metal and equipped with a PCM heat exchanger. They developed a numerical simulation to study the dynamic behavior of the system during the desorption process. They proved that a good selection of a PCM affects considerably the efficiency of the tank. Later, the same design was considered by El Mghari et al. [15]. They evaluated the effect of the PCM physical properties on the kinetics of the MH bed and the melting of the PCM. Authors concluded that an increase of the PCM thermal conductivity and the melting enthalpy leads to a significant improvement of the system behavior. Also, keeping the same conception and combining MH and PCM with metal foams, Tong et al. [16] were interested to determine the effect of some physical parameters on the metal hydrogen storage reservoir. Given the important effect of increasing the PCM thermal conductivity, various metal foams (copper and aluminum) were integrated into the PSMs to enhance the heat transfer between the MH bed and the PCM. It was shown that the composition of PCM and copper foam enhances the system performance by about 34% more than aluminum foam does. Moreover, El Mghari et al. [17] studied coupled heat and mass transfer in LaNi₅ beds equipped with five PCMs to determine its impact on the hydrogen storage process. After selecting the proper PCM, the authors studied the impact of integrating three different foams into the selected PCM. Besides, they were interested on evaluating the design effect of PCM jackets by considering cylindrical and spherical configurations. Results showed that the two tanks exhibit identical behavior. Alqahtani et al. [18] proposed a novel conception of a high temperature MH reactor. The MH bed was equipped with two cascaded PCM beds. The two PCMs with different physical properties were analyzed. Their results show that the proposed design is an effective solution to ameliorate heat transfer and reduce times required for absorption and desorption processes. Another MH-PCM unit design was developed by yang et al. [19]. The configuration is a cylindrical tank that contains a defined number of layers filled with the MH. There are compacted disks made of Mg/MgH₂ and expanded graphite. The PCM is sandwiched between two layers of the MH. Considering a sandwiched structure, it was discovered that the storage capacity is enhanced due to the larger heat transfer area. Also, in order to increase the heat transfer area, Alqahtani et al. [20] introduced a new design for the MH-PCM storage unit where the cylindrical MH bed was encircled by a sandwich bed filled with PCM. The authors prove that doubling the number of interfaces separating the MH part and the PCM rigorously improves the absorption and desorption processes. In fact, the time required to absorb or desorb the maximum amount of hydrogen was enhanced by 81.5% and 73%, respectively. Corgnale et al. [21] revealed a method for selecting high-temperature MH beds that are appropriate for use as thermal energy storage (TES) units in CSP plant applications. As part of the screening procedure, a technoeconomic analysis of the cost, energy storage capacity, and exergetic efficiency of these materials was conducted. The results showed that TiH₂, CaH₂, and NaMgH₃ have high volumetric energy densities (over 25 kWh/m³) and high operating temperatures (over 600 °C), making them ideal for CSP plants. To boost the production of a solar energy unit, dissipated heat is retained in PCM and reused during the evening and night. Vikrant P. et al. [22] provided a review study investigating the optimal PCM for solar still based on the current state of solar energy unit with PCM. The past few decades have seen a rise in interest in PCMs due to its potential use in energy storage and efficiency. F.S. Javadi et al. [23] provide a thorough overview of the basics of PCM, including PCM types, applications, and bottlenecks, with a focus on the incorporation of PCM in solar thermal applications. One practical approach to reaching the “dual carbon” goal is to replace fuel-fired boilers with heat pump facilities that recover heat from industrial waste. Through the use of a dual-temperature evaporation approach accomplished with an ejector for cascading heat absorption from the heat source, Baomin Dai et al. [24] developed three unique transcritical CO₂ high-temperature heat pump systems. A proof-of-concept thermal battery based on two MH beds was designed and tested by Fang et al. [25]. The results showed a COP of 0.384 and a total cooling energy of 13.6 kWh. A prototype of the MH-TES system was proposed for the CSP plant by Paskevicius et al. [26]. The results demonstrated that there was minimal hydrogen capacity loss during thermal cycling of the MH bed at 420 °C. A thridimontional mathematical model was presented by Malleswararao et al. [27] to predict the performance of an MH-TES unit. The paired beds were made with Mg2Ni/LaNi5 hydrides. The findings show that a density of 156 kWh/m³ and an energy storage efficiency of 89.4% were both achieved. Mellouli et al. [28] performed a computational investigation of an MH-TES unit based on paired beds of Mg₂FeH₆/Na₃AlH₆. The results demonstrated that the heat storage could yield an energy density of 90 kWh/m³ with an energy storage efficiency of 96%. Energy density is a measure of how much heat can be stored in a given volume of material, and thermochemical storage materials have a distinct advantage over latent and sensible heat storage materials. MHs have high enthalpies and are very stable when subjected to repeated thermal and mechanical stress. They can improve the adaptability and productivity of CSP plants and have uses in both short- and long-term heat storage. The US Department of Energy has set goals for CSP systems with heat storage at 600 °C or higher in order to make solar electricity competitive with other kinds of unsubsidized energy. Currently, molten salts are the most widely used storage medium, however they degrade at temperatures beyond 600 °C, hence an alternative must be found [28]. Proper research into hydrides could lead to a bright future for them in this field due to their many desirable properties, including very high energy density, reversibility, and low cost [29]. Heat quantities involved in hydriding/dehydriding reactions are utilized in a hydride-based (thermochemical) TES system. Hydrides such sodium magnesium, titanium, calcium, and lithium can all be used at temperatures between 600 °C and 1000 °C. As of now, the predicted costs of MH heat storage systems are comparable to those of molten salt systems; however, additional research is likely to find hydrides, especially high-temperature ones, that are cheaper than molten salts [28].

The electrical efficiency of a conventional steam-electric power plant is typically 33–48% [29]. However, the CSP plants employing thermochemical storage TES systems have in general lower storage efficiency (11.2%) compared to plants with molten salt two-tank systems (17.8%) [30]. Thermodynamic features of some materials, including as CaH₂, MgH₂, and Mg2FeH6, may allow for CSP system operation at temperatures higher than those reached by molten salts (i.e., > 565 °C) at lower prices, larger heat storage capacities, and higher efficiencies. However, the cost of the hydrides systems is substantially lower, with Mg₂F₂H₆ at 2.25 $/kWh_th, CaH₂ at 2.56 $/kWh_th and MgH₂ at 2.89 $/kWh_th compared to the 5.25 $/kWh_th that molten salts (0.6NaNO₃/0.4KNO₃) cost [29].

It is typical practice for CSP plants to employ a two-tank heat storage system with two MHs that run at slightly different temperatures. In contrast to the high-temperature metal hydride (HTMH) bed, which maintains a constant active heat management connection to the high-temperature heat source (i.e., the solar field), the low-temperature metal hydride (LTMH) bed can take any of two approaches to heat removal/addition during the charging/discharging processes.(i.e., (i) the LTMH bed is heated by the losses heat of the condenser of the power block and (ii) include PCM-based heat recovery/input). Thus, there is diversity in TES system configuration, which can have a major impact on overall system’s performance. However, no research has been conducted to compare the two methods and their impact on system’s performance. This study’s goal is to numerically explore how these methods (three configurations) influence the thermal energy storage system’s overall efficiency.

Three designs are evaluated in this study. One base design which uses only thermochemical heat storage mode and two designs of the HSTS system with two distinct PCM heat exchanger designs (i.e., shell-and-tube and cylinder with truncated hollow cones). For the three designs, a pair of Mg₂FeH₆/Na₃AlH₆ metal hydrides is used. Key performance indicators used to evaluate the performance of the three storage systems include volumetric storage capacity, specific power, state of hydrogen charge, and energy storage efficiency.