Abstract
Two-tank metal hydride pairs have gained tremendous interest in thermal energy storage systems for concentrating solar power plants or industrial waste heat recovery. Generally, the system’s performance depends on selecting and matching the metal hydride pairs and the thermal management adopted. In this study, the 2D mathematical modeling used to investigate the heat storage system’s performance under different thermal management techniques, including active and passive heat transfer techniques, is analyzed and discussed in detail. The change in the energy storage density, the specific power output, and the energy storage efficiency is studied under different heat transfer measures applied to the two tanks. The results showed that there is a trade-off between the energy storage density and the energy storage efficiency. The adoption of active heat transfer enhancement (convective heat transfer enhancement) leads to a high energy storage density of 670 MJ m−3 (close to the maximum theoretical value of 755.3 MJ m−3). In contrast, the energy storage efficiency decreases dramatically due to the increase in the pumping power. On the other hand, passive heat transfer techniques using the bed’s thermal conductivity enhancers provide a balance between the energy storage density (578 MJ m−3) and the energy efficiency (74%). The utilization of phase change material as an internal heat recovery medium leads to a further reduction in the heat storage performance indicators (142 MJ m−3 and 49%). Nevertheless, such a system combining thermochemical and latent heat storage, if properly optimized, can be promising for thermal energy storage applications.
Highlights
The hydrogen absorption into and desorption from hydride materials are accompanied by a high heat release and consumption, respectively
Mg-based hydrides have stimulated worldwide interest in their utilization in hydrogen/heat storage and high-temperature fuel cell technologies. This is true due to their relatively high hydrogen storage capacity (3.6–7.6 wt% and 110–150 kgH2/m3) and the high heat of the reaction (60–80 kJ/mol-H2) [3,4]. Their use in concentrated solar power (CSP) plants can improve performance in terms of energy storage density compared to the state-of-the-art two-tanks-based molten salts [5,6], one tankthermocline [7,8], and latent heat phase change materials [9,10], the economic analysis shows that metal hydrides are more expensive [11,12]
The results showed that TiH2, CaH2, and NaMgH3 showed a desirability for CSP plants, since their operating temperature is above 600 ◦C and their volumetric energy density exceeds 25 kWh/m3
Summary
The hydrogen absorption into and desorption from hydride materials are accompanied by a high heat release and consumption, respectively. The present study aims to numerically investigate the effect of the adoption of various thermal management techniques on the energy storage efficiency of the thermal energy storage system These techniques include passive (bed thermal conductivity augmentation, natural convection, phase change materials-based heat recovery/input) and active (forced convection) heat-transfer enhancements. It is found that the heat stored in the phase change material can be reutilized for H2 desorption in the case of heat discharging, which can significantly improve the energy efficiency [34,35,36,37] of the system, as will be discussed hereafter In these different configuration designs, we can see a mismatch between the heat exchangers’ heat transfer coefficient of the beds.
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