With the rapid depletion of fossil fuels and the escalating issue of greenhouse gas emissions, the transition to renewable energy sources has become imperative. Solar energy, due to its abundance, cleanliness, and widespread availability, holds significant potential for large-scale adoption. Among solar technologies, concentrated solar power (CSP) systems are particularly promising for generating electricity by harnessing solar thermal energy. However, the intermittent nature of solar radiation—affected by diurnal cycles and weather conditions—poses a challenge to consistent energy supply. To address this, energy storage technologies are integral to CSP plants, enabling the storage of excess thermal energy during peak sunlight hours for use during periods of low or no sunlight. Thermal energy storage (TES) systems are categorized into sensible heat storage (SES), latent heat storage (LES), and thermochemical energy storage (TCES). While SES and LES have been widely implemented, TCES offers superior energy density, longer storage duration, and minimal thermal losses, making it a focal point for advanced solar power system development.
Thermochemical energy storage relies on reversible chemical reactions to store and release energy. For high-temperature applications in CSP systems, several reaction systems have been explored, including carbonates, hydroxides, metal hydrides, and metal oxides. The calcium looping (CaL) process, based on the reversible carbonation/calcination reaction of calcium carbonate (CaCO3) and calcium oxide (CaO), has emerged as a leading candidate due to its high energy storage density, wide operating temperature range, and the abundance and low cost of calcium-based materials. The CaL process integrates seamlessly with solar power systems, as it can utilize direct solar thermal energy for the endothermic calcination reaction, storing energy in chemical form. This review examines recent advancements in calcium-based thermochemical energy storage, focusing on material enhancements to improve cyclic stability, solar absorptivity, and mechanical durability for practical deployment in CSP plants.
The fundamental reaction governing the CaL process is the calcination/carbonation cycle: $$ \text{CaCO}_3 \leftrightarrow \text{CaO} + \text{CO}_2 \quad \Delta H^0 = \pm 178 \, \text{kJ/mol} $$ In this scheme, solar energy drives the endothermic calcination of CaCO3 at temperatures above 900°C, producing CaO and CO2. The products are stored separately, and when energy is required, the exothermic carbonation of CaO with CO2 occurs at temperatures between 600–700°C, releasing heat that can be converted to electricity via a power cycle. This process not only facilitates energy storage but also enables CO2 capture and utilization, aligning with carbon mitigation goals. Integration of CaL into solar power systems typically involves fluidized bed reactors for efficient heat and mass transfer, but challenges such as material sintering, poor solar absorptivity, and particle attrition hinder widespread adoption.
One of the primary issues with calcium-based materials in solar power systems is the degradation of cyclic stability. Sintering of CaO particles at high temperatures leads to reduced surface area and porosity, diminishing the carbonation conversion over multiple cycles. To combat this, researchers have developed various strategies to enhance the durability and performance of CaO-based sorbents. These include using alternative calcium precursors, incorporating inert support materials, and optimizing particle morphology through advanced fabrication techniques. Additionally, direct solar absorption by CaCO3/CaO particles is inefficient due to their white color, which reflects most solar radiation. Thus, enhancing optical properties through doping with dark-colored metals is crucial for improving the efficiency of solar thermal conversion in direct absorption systems.
In this article, we comprehensively review recent progress in calcium-based thermochemical energy storage for solar power systems. We discuss methods to improve energy storage performance, such as doping with inert supports and using organic precursors; approaches to enhance solar absorptivity via metal ion incorporation; and techniques for pelletization to enhance mechanical strength. Tables and equations are employed to summarize key findings, and future research directions are outlined to guide the development of commercially viable CaL-integrated solar power systems.
Enhancement of Energy Storage Performance in Calcium-Based Materials
The energy storage performance of CaO-based materials is typically evaluated using parameters such as adsorption capacity (Cn), effective conversion (Xef), and energy storage density (Qn). These are defined as: $$ C_n = \frac{m_{\text{car},N} – m_{\text{cal},N}}{m_0} $$ $$ X_{\text{ef}} = \frac{m_{\text{car},N} – m_{\text{cal},N}}{m_0} \times \frac{M_{\text{CaO}}}{M_{\text{CO}_2}} \times 100\% $$ $$ Q_n = \frac{m_{\text{car},N} – m_{\text{cal},N}}{m_0} \times 1000 \times \frac{\Delta H^0}{M_{\text{CO}_2}} $$ where mcar,N and mcal,N are the masses after carbonation and calcination in the Nth cycle, respectively, m0 is the initial mass, and MCaO and MCO2 are the molar masses of CaO and CO2, respectively. The enthalpy change ΔH0 is 178 kJ/mol for the reaction.
Natural calcium-bearing minerals like limestone and dolomite are widely used as precursors due to their low cost and abundance. However, they suffer from rapid sintering. Dolomite (CaMg(CO3)2) outperforms limestone because MgO acts as an inert support, inhibiting sintering. For instance, after 20 cycles, dolomite retains a 28% higher conversion rate than limestone. Similarly, manganese calcite, which forms Ca2MnO4 upon calcination, shows improved stability with an energy density of 1932 kJ/kg after 20 cycles, nearly double that of limestone. Organic calcium precursors, such as calcium acetate (Ca(CH3COO)2) and calcium gluconate (Ca(C6H11O7)2), decompose to produce CaO with high surface area and porosity due to the release of CO2 and H2O during calcination. These materials exhibit superior cyclic performance, with adsorption capacities up to 0.52 g/g after 20 cycles.
Acid treatment with organic acids like acetic acid converts calcium sources to corresponding salts (e.g., calcium acetate), which upon calcination yield nano-structured CaO with enhanced porosity. For example, acetic acid-treated limestone and dolomite achieve effective conversions of ~70% after 30 cycles, compared to only 15–20% for untreated samples. High-calcium waste materials, such as steel slag and carbide slag, offer a cost-effective and environmentally friendly alternative. After acetic acid treatment and filtration, these wastes demonstrate stable energy storage performance, with effective conversions increasing by approximately 100% due to the removal of impurities.
Doping with inert supports is a common strategy to mitigate sintering. Materials like Al2O3, MgO, CaZrO3, Y2O3, CeO2, and Ca12Al14O33 act as structural stabilizers, preventing particle agglomeration and pore blockage. For instance, CaO doped with 30 wt% CaZrO3 maintains an energy density of 1390 kJ/kg after 20 cycles under pure CO2 atmosphere. Multi-element doping, such as Al-Zr/Ca composites, further enhances stability, with energy densities exceeding 2180 kJ/kg after 50 cycles. The table below summarizes the performance of various doped CaO composites under different cycling conditions.
| Heat Storage Material | Calcination Conditions | Carbonation Conditions | Energy Density (kJ/kg) |
|---|---|---|---|
| CaO/SiO2 = 70/30 (wt%) | 725°C, He, 5 min | 850°C, CO2, 5 min | 850 (20 cycles) |
| CaO/MgO = 75/25 (wt%) | 900°C, N2, 10 min | 650°C, 15% CO2, 30 min | 2345 (23 cycles) |
| CaO/Al2O3 = 83/5 (wt%) | 750°C, N2, 10 min | 850°C, CO2, 5 min | 1621 (30 cycles) |
| CaO/CaTiO3 = 72/28 (wt%) | 750°C, N2, 10 min | 600°C, 20% CO2, 10 min | 909.93 (10 cycles) |
| CaO/Ca12Al14O33 = 85/15 (wt%) | 900°C, 15% CO2, 5 min | 650°C, 15% CO2, 10 min | 1172.52 (15 cycles) |
| CaO/CaZrO3 = 70/30 (wt%) | 750°C, N2, 10 min | 850°C, CO2, 10 min | 1390 (20 cycles) |
| CaO/Y2O3 = 80/20 (wt%) | 950°C, CO2, 5 min | 650°C, 20% CO2, 30 min | 1940.73 (10 cycles) |
| CaO/Nd2O3 = 70/30 (wt%) | 1000°C, N2, 5 min | 650°C, 15% CO2, 30 min | 161.73 (100 cycles) |
| Ca:Mg = 8:1 (molar ratio) | 750°C, N2, 10 min | 850°C, CO2, 10 min | 2384 (20 cycles) |
The incorporation of inert supports not only improves cyclic stability but also influences the reaction kinetics. The rate of carbonation can be described by the shrinking core model: $$ \frac{dX}{dt} = k (1 – X)^{2/3} $$ where X is the conversion degree and k is the rate constant. Doping with nanosized supports like TiO2 and CeO2 enhances diffusion rates, leading to faster reaction kinetics. For example, CaO/CeO2 composites show a 20% increase in carbonation rate due to the formation of oxygen vacancies that facilitate CO2 diffusion.
Enhancement of Solar Absorptivity in Calcium-Based Materials
Direct solar absorption by CaCO3/CaO particles is inefficient because of their high reflectivity. To address this, researchers have doped calcium-based materials with dark-colored metal ions such as Fe, Mn, Co, Cu, and Cr. These dopants improve the solar absorptivity, enabling more efficient direct solar thermal conversion. The average solar absorptivity (Asol) is calculated as: $$ A_{\text{sol}} = \frac{\int_{300}^{2400} A(\lambda) I(\lambda) d\lambda}{\int_{300}^{2400} I(\lambda) d\lambda} $$ where A(λ) is the spectral absorptivity and I(λ) is the solar irradiance under AM1.5 direct spectrum.
Doping with multiple metals, such as Ca-Mn-Fe or Ca-Cr-Mn, results in composites with absorptivities exceeding 80%. For instance, a composite with Ca:Mn:Fe:Al = 100:12:8:5 achieves an absorptivity of 84.27% and retains 93.03% of its initial energy density after 20 cycles. Similarly, Ca-Co-Mn-Mg composites show absorptivities of 79.3% and energy densities of 1295 kJ/kg after 30 cycles. The table below compares the solar absorptivity and energy storage performance of various metal-doped CaO composites.
| Energy Storage Material | Calcination Conditions | Carbonation Conditions | Energy Density (kJ/kg) | Solar Absorptivity (%) |
|---|---|---|---|---|
| Ca:MnFe2O4 = 100:6 | 700°C, N2 | 700°C, CO2 | 1330 (20 cycles) | 85.67 |
| Ca:Cu:Co = 100:5:5 | 700°C, N2 | 700°C, 50% CO2/50% N2 | 504 (20 cycles) | 71.80 |
| Ca:Zr:Mn:Fe = 100:6.7:6:12 | 750°C, N2 | 850°C, CO2 | 1090 (19 cycles) | 79.60 |
| Ca:Al:Fe = 100:28:8 | 700°C, N2 | 700°C, 50% CO2/50% N2 | 950 (50 cycles) | 45.60 |
| Ca:Mn:Fe = 85:3:12 | 700°C, N2 | 700°C, 50% CO2/50% N2 | 1310 (40 cycles) | 81.50 |
| Ca:Cu:Mn = 100:5:5 | 700°C, N2 | 700°C, 50% CO2/50% N2 | 1952 (20 cycles) | 60.28 |
| Ca:Ce:Co:Mn = 100:5:3:6 | 700°C, N2 | 700°C, 50% CO2/50% N2 | 1350 (20 cycles) | 83.26 |
The enhanced absorptivity is attributed to the formation of mixed metal oxides that exhibit broad-band absorption in the solar spectrum. For example, Mn-doped samples form Mn3O4 or MnFe2O4, which have high absorption coefficients. Additionally, these dopants often act as secondary stabilizers, reducing sintering and maintaining porosity. The economic cost of dopants is a critical factor; Fe and Mn are preferred due to their low cost (Fe at $0.069/kg and Mn at $2.06/kg) compared to Co ($68.0/kg) or Ni ($11.4/kg). Thus, Ca-Mn-Fe composites are promising for large-scale solar power system applications.
Pelletization of Calcium-Based Materials for Mechanical Strength
In practical solar power systems, powdered CaO-based materials are prone to attrition and elutriation in fluidized bed reactors, leading to material loss and reduced efficiency. Pelletization enhances mechanical strength and reduces dust formation. Common methods include extrusion, spheronization, rotary molding, and graphite bed casting. The mechanical performance of pellets is evaluated by compressive strength and attrition resistance, often measured using a friability tester.
Extrusion involves mixing raw materials with water to form a paste, which is then extruded into cylindrical pellets. For example, pellets made from Ca(OH)2 and cement exhibit compressive strengths up to 4.61 MPa and attrition losses below 4.58% after 2000 rotations. Spheronization, combined with spray drying, produces spherical pellets with improved flow properties. Pellets with Ca12Al14O33 as a binder and urea as a pore-forming agent show CO2 adsorption capacities of 0.48 g/g after 25 cycles and attrition losses below 0.8% after 3000 rotations. Graphite bed casting is a novel method where Ca(OH)2 slurry is dropped into graphite molds, forming spherical pellets upon drying and calcination. These pellets demonstrate a 48% higher CO2 adsorption capacity than untreated pellets and compressive strengths of 3.2 MPa.
The table below compares the energy storage performance and mechanical properties of pellets produced by different methods.
| Energy Storage Material | Pelletization Method | Pellet Parameters | Energy Storage Performance | Mechanical Properties |
|---|---|---|---|---|
| Calcium nitrate/Zirconium nitrate | Graphite bed casting | 1 mm diameter | Energy density: 1150 kJ/kg (20 cycles) | Compressive strength: 3.2 MPa |
| Ca(OH)2/Manganese acetate/SiC powder | Extrusion-spheronization | 0.6–0.9 mm diameter | Energy density: 1310 kJ/kg (30 cycles) | Compressive strength: 15.8 MPa |
| Limestone/CA-14 binder | Rotary method | 0.425–1.000 mm diameter | Adsorption capacity: 0.2–0.3 g/g (20 cycles) | Good wear resistance |
| Organic calcium precursor/Calcium aluminate cement | Extrusion | 1.8 mm base diameter | Adsorption capacity: 0.19–0.38 g/g (20 cycles) | Compressive strength: 1.67–4.61 MPa |
| Ca(OH)2/Titanium dioxide | Extrusion-spheronization | 0.6–0.9 mm diameter | Energy density: 930 kJ/kg (20 cycles) | Compressive strength: 17.7 MPa, wear rate 9.2% (2000 rotations) |
| Sodium alginate/Calcium nitrate | Mold casting | 0.7–0.8 mm diameter | Adsorption capacity: 0.48 g/g (25 cycles) | Compressive strength: 3.48 MPa |
| Dolomite/Manganese nitrate/Iron nitrate | Extrusion-spheronization | 0.90–1.25 mm diameter | Energy density: 840 kJ/kg (30 cycles) | — |
The choice of pelletization method depends on the desired balance between mechanical strength and energy storage performance. Extrusion-spheronization yields high-strength pellets but may reduce porosity, whereas graphite casting preserves microstructure but offers moderate strength. Additives like pore-forming agents (e.g., urea) or binders (e.g., diatomite) can optimize both properties, making pellets suitable for long-term use in solar power system reactors.
Conclusion and Future Perspectives
Calcium-based thermochemical energy storage presents a viable solution for addressing the intermittency of solar power systems. The CaL process, with its high energy density and compatibility with CSP temperatures, offers significant advantages over sensible and latent heat storage. However, challenges such as cyclic stability, solar absorptivity, and mechanical durability must be overcome for commercial deployment. Through material modifications—including doping with inert supports, enhancing optical properties with dark metals, and pelletization—researchers have made substantial progress in improving the performance of CaO-based sorbents.
Future research should focus on several key areas. First, in-depth studies of material microstructure are needed to understand degradation mechanisms and guide the design of more robust composites. Second, reactor design must advance beyond laboratory-scale thermogravimetric analyzers to pilot-scale fluidized bed systems that mimic real-world solar power system conditions. Testing under pure CO2 atmospheres is essential for assessing CO2 separation efficiency. Third, the economic and environmental aspects of material synthesis should be considered, including the use of low-cost precursors and scalable fabrication techniques. For instance, the integration of waste-derived calcium sources and eco-friendly binders could reduce costs and enhance sustainability.
In summary, the development of high-performance calcium-based materials is crucial for the next generation of solar power systems. By optimizing cyclic stability, solar absorptivity, and mechanical strength, CaL technology can achieve efficient, long-term energy storage, contributing to the reliability and scalability of concentrated solar power. Continued innovation in material science and reactor engineering will pave the way for widespread adoption, ultimately supporting the global transition to renewable energy.