The modification with dark metallic oxide is identified as the crucial strategy to enhance optical absorptions of calcium-based materials for the direct solar-driven thermochemical energy storage. The effect of modification on the heat release behavior in carbonation of calcium-based material has been widely investigated, but its effect on the heat storage behavior in calcination is lacking of sufficient research, typically for low-cost calcium resource such as carbide slag. The Fe-modified and Mn-modified carbide slags for CaCO₃/CaO heat storage were synthesized and their optimum decomposition temperatures, effective heat storage conversions, heat flows and heat storage rates in endothermic stage were investigated. Although the Fe modification exacerbates the CaO sintering due to the formation of Ca₂Fe₂O₅, that is still effective in reducing the regeneration temperature of CaO in CaCO₃/CaO cycles. The Mn modification enhances significantly sintering resistance by forming the CaMnO₃ and its transformation into Ca₂MnO₄. The effective heat storage conversion of Mn-modified carbide slag after 30 cycles is 3.2 times as high as that of untreated carbide slag. Mn-modified carbide slag exhibits the lowest regeneration temperature and the highest heat storage rate after cycles. The loose and stable porous structure of Mn-modified carbide slag contributes to its superior endothermic performance. Therefore, Mn-modified carbide slag seems to be the potential candidate for calcium looping thermochemical heat storage.

Thermal storage is a key technology in concentrating solar thermal power (CSP) system, which can provide continuous and stable high quality electricity, improve the efficiency of the power system and extend the system life. Molten salt is an important material for heat storage and heat transfer in solar thermal power generation, the addition of nanoparticles can synergistically and effectively enhance both specific heat capacity and thermal conductivity. In this study, a base salt with mass percentage of 31.5% Na₂CO₃-31.5% Li₂CO₃-37% K₂CO₃ was employed. SiO₂ nanoparticles with varying particle sizes, different concentrations of SiO₂ and Al₂O₃, as well as composite nanoparticles, were dispersed in a salt solution to create ternary carbonate nanofluids using a two-step solution method. The melting point, specific heat capacity, crystal structure, and surface microstructure of nanofluids were measured using a differential scanning calorimeter, X-ray diffractometer and scanning electron microscope, respectively. The results show that among the selected nanoparticles, SiO₂ nanoparticles are the most effective at enhancing the specific heat capacity and thermal conductivity of the ternary carbonates. The mass addition of 1.0% of 30 nm SiO₂ results in 83.5% increase in specific heat capacity in the solid phase and 159.4% increase in the liquid phase compared to pure ternary carbonates, and the thermal conductivity increases by 20.8%. Meanwhile, scanning electron microscopy has revealed the formation of rod-like nanostructures after adding nanoparticles to ternary carbonates. XRD results confirm that there are no chemical reactions between ternary carbonates and the added nanoparticles. After exposure to a constant high temperature of 600°C for 100 h and undergoing 100 cycles of large temperature differences (ranging from room temperature to 600°C), the thermophysical properties of this composite material remain relatively stable, demonstrating good long-term and heating-cooling cycle thermal stability.

Air source heat pump has insufficient heating performance under the low ambient temperature conditions;  meanwhile, the thermal storage device in heat pump system has a wide range of application. This study proposes a  thermal storage air source heat pump heating system (HSASHP) with a novel structure, and has established both the  mathematical models and simulation models of each component of the single-stage and the thermal storage air source heat  pump heating systems in MATLAB/Simulink respectively, with three operation modes proposed for the latter (i.e., the thermal storage air source heat pump heating system); by using the outdoor ambient temperature during the heating period in Baotou, China, the heating capacity of the two heat pump systems are simulated and the economy of both systems’ operation are investigated. The results show that within a 7-day heating period, the total heat production of the thermal storage heat pump unit and the single-stage heat pump unit is 442.58 kW·h and 355.68 kW·h, respectively, with HSASHP 24% higher; the average heating Coefficient of Performance (COP) of the two heat pump units is 2.11 and 1.51, respectively, with HSASHP 39.74% higher; the power consumption of the two heat pump units is 202.74 kW·h and 239.74 kW·h, respectively, with HSASHP 15.44% lower. These all illustrate the effectiveness of the new structure in improving the performance of heat pump units. However, the total power consumption and operational economy of both air source heat pump heating systems do not differ significantly.

Latent heat storage technology plays a critical role in storing and utilizing geothermal energy. By combining cascaded phase change materials (PCM) with mine filling technologies, mine geothermal energy can be stored thermally more effectively. Therefore, this paper designed a physical model of double casing cascaded latent heat storage (CLHS) system in mine. Paraffin RT28 and RT35 were encapsulated in annular gap 1 and annular gap 2, respectively, and this backfill mode was defined as Case 1. The scheme whose backfill sequences of the two PCM were exchanged is defined as Case 2. The heat transfer process of backfill body and PCM was simulated and analyzed by using FLUENT software, and compared with the single stage latent heat storage process. The temperature, liquid fraction (LF), heat transfer capacity, and heat transfer rate were used to evaluate the thermal properties of the CLHS process. It was necessary to study the effect of the filling sequence of PCMs on the heat storage and release process of the backfill body using these results as a starting point. The results show that the main factor affecting latent heat storage in cascaded system is the heat transfer of surrounding rock. Compared with the single-stage heat storage process, the heat storage time of cascaded heat storage process is reduced by 73 min, which is significantly decreased by 20.9%. Moreover, the whole liquid phase fraction (β) of the single-stage has little change during the heat release, while the PCM of the cascaded heat release process can fully release the latent heat. In terms of layout order of PCM, compared with Case 1, the latent heat storage time of Case 2 is increased by about 40 min, and the heat release rate (ε_s) is significantly lower than that of Case 1. In the initial heat release stage, the heat release rate of Case 2 reaches 95.6 W, which is 30.6% lower than that of Case 1. In comparison, the heat storage and release effect of Case 1 is better than that of Case 2. This paper provides a reference for the improvement of heat storage and release rate of the backfill coupled cascaded latent heat storage system (BCCLHS).

The improvement of composite phase change materials in energy storage density and thermal conductivity is significant for the efficient use of energy. This study proposed novel composite materials based on forsterite with high specific heat as matrix materials, chloride salts (NaCl-KCl) with high latent heat as phase change materials and SiO₂ nanoparticles as fillers. The results indicated that forsterite and chloride salts exhibited excellent chemical compatibility, and the composite materials containing 40% (in weight) chloride salts achieved an energy storage density of 882.5 J/g within the range of 100°C to 800°C, a latent heat of 108.1 J/g, and a thermal conductivity of 0.68–0.81 W/(m·K) at 300°C–500°C. Furthermore, the addition of SiO₂ enhanced the thermal conductivity and energy storage density of composite materials due to the formation of unique nanostructures. More importantly, the removal of structural water during heat-treatment process resulted in the formation of micropores and increased specific surface area of forsterite particles, which facilitated the adsorption and stabilization of molten chloride salts. Combined with the stabilization effect of forsterite and synergistic effect of SiO₂ nanoparticles, the obtained composite materials with 2.0% (in weight) SiO₂ nanoparticles exhibited good thermal stability with 1.80% weight loss and 2.34% reduction in latent heat after 150 cycles, indicating a promising application in high-temperature thermal energy storage.