Thermochemical heat storage uses a reversible chemical reaction to store thermal energy. This thermal energy storage method has high energy density and allows long-term thermal energy storage. MgO/Mg(OH) 2 is a promising reversible reaction for thermochemical heat storage systems. The performance of this reversible reaction inside a reactor plays a key role in its practical applications. The direct type heat transfer reactor is a novel design compared with an indirect powder bed reactor. It allows the gaseous heat transfer fluid to flow through the powder in order to break the heat transfer limitation from the heat exchanger between the MgO/Mg(OH) 2 powder and the heat transfer fluid. In this study, we evaluated the performance of a direct type MgO/Mg(OH) 2 reactor under various operating conditions. Parametric analyses indicated that the heat output is limited by the reaction rate rather than the heat transfer efficiency. Results also showed the best bed thickness in order to achieve the highest temperature output. Material characterization indicated that the MgO/Mg(OH) 2 particles did not agglomerate with each other after 15 heat charging and discharging cycles.
The application of MgO/Mg(OH) 2 in heat storage technology has been limited due to slow hydration rate and easy agglomeration. In this paper, nanoporous carbon particle (NCP) as a carrier was used to prepare NCP/MgO chemical heat storage composite materials by impregnation and calcination methods, respectively. The microstructure characterization, hydration reaction, and simultaneous thermogravimetric analysis experiments were carried out on the prepared composite materials. The results showed that the internal distribution of MgO in the composite material prepared by calcination method was more uniform, and the particle size of MgO was smaller. Under the conditions of 110 ℃ hydration temperature and 57.8 kPa water vapor partial pressure, the hydration conversion ratio of the composite material prepared by calcination method after 120 min of hydration conversion ratio reached 93 %, which was 17.7 % higher than that of the impregnated composite material and 50 % higher than that of pure MgO. Under the comprehensive influence of specific surface area, pore structure, and active component particle size, compared with pure materials and impregnated composite materials, the dehydration rate of the composite material prepared by calcination method at 300 ℃ was twice that of impregnated method and 6 times that of pure MgO, and the heat storage density reached 1039kJ/kg. After 10 cycles, the heat storage density still maintained an initial state of 95.8 %. Compared with pure materials and impregnated composite materials, NCP/MgO composite materials prepared by calcination method had better heat storage performance and higher cyclic stability.
Thermochemical energy storage (TCES) is a promising energy storage method because of its high energy storage density and long-term storage ability. The reactor is the most critical part of the TCES system, and the fluidized bed reactor (FBR) is a novel reactor, which is not restricted by the low effective thermal conductivity of energy storage materials. However, there is little theoretical guidance about the design and operation of the FBR for a TCES system. In this study, we investigated the performance of an FBR using MgO/Mg(OH) 2 as reaction materials by establishing a numerical model followed by experimental validation. The influence of operating parameters on the bed temperature was discussed based on numerical results. According to the sensitivity analysis, the reactor inner diameter, material mass, and inlet gas velocity were the main factors affecting the bed temperature. The optimal boundary surface temperature was analyzed, which influences the proportion of TCES in the total heat storage.
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