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ABSTRACT
The structural optimization of a reactor in an open thermochemical energy storage (TCES) system utilizing composite salt hydrates is missing. This study concentrates on the structural optimization of a honeycomb reactor utilizing Wakkanai siliceous shale (WSS) combined with 9.6 wt.% LiCl by developing a three-dimensional numerical model considering coupled heat and mass transfer. Experimental validation of the model was performed for both thermal energy storage/release (TES/TER) processes. The length of the WSS + 9.6 wt.% LiCl unit inversely affects TES density. A thicker salt hydrate layer extends the duration of high outlet air temperature release but reduces the volumetric TES density due to elevated transfer resistances. A cross-sectional area ratio of the air channel at 0.45 demonstrates superiority in achieving high TES density. Furthermore, a hexagonal-shaped channel exhibits the highest TES density. From a fabrication perspective, adopting a TCES unit with a hexagonal shape, a 0.15 mm layer, and a cross-sectional area ratio of the air channel of 0.45 proves optimal for cyclic storage/release processes. The structured optimized WSS + 9.6 wt.% LiCl unit has shown a comparative TES density of 890 MJ·m−3, higher heat recovery efficiency of 92.6%, and lower regeneration temperature of 60°C compared to other TCES systems. Factors like cost, scalability, and integration with existing energy systems would be considered in future applications.
Nomenclature
Abbreviation
| DF | = | Degree of Freedom |
| RMSE | = | Root Mean Square Error |
| SS | = | Sum of Deviation Square |
| TCM | = | Thermochemical Material |
| TES | = | Thermal Energy Storage |
| MS | = | Mean Square |
| PCM | = | Phase Change Material |
| TCES | = | Thermochemical Energy Storage |
| TER | = | Thermal Energy Release |
| WSS | = | Wakkanai Siliceous Shale |
| Symbols | = | |
| a | = | The side length of each air channel in an element, m |
| b | = | The side length of the TCES unit, m |
| Cp | = | Specific heat capacity, kJ·kg−1·K−1 |
| D | = | Diffusion coefficient, m2·s−1 |
| e | = | The cross-sectional area ratio of the air channel, - |
| G | = | Volumetric flow rate of air, m3·s−1 |
| Hs | = | Sorption heat, kJ·kg−1 |
| H0 | = | Evaporation heat, kJ·kg−1 |
| L | = | Length, m |
| M | = | Molecular weight, g·mol−1 |
| P | = | Pressure, Pa |
| Patm | = | Pressure with a unit of atm, atm |
| qv | = | Volumetric thermal energy storage density, MJ·m−3 |
| qs | = | Specific thermal energy storage density, kJ·kg−1 |
| R | = | Gas constant, J·kg−1 K−1 |
| RH | = | Relative humidity, % |
| r | = | Mean radius, m |
| t | = | Time, s |
| T | = | Temperature, K |
| u | = | Velocity, m·s−1 |
| V | = | Volume, m3 |
| X | = | Water uptake, kg·kg−1 |
| x | = | Absolute humidity, kg·m−3 |
| δ | = | The thickness of the composite salt hydrates, mm |
| ∆F | = | Free sorption energy, kJ·kg−1 |
| ε | = | Porosity, - |
| η | = | Heat recovery efficiency, - |
| λ | = | Thermal conductivity, W·m−1·K−1 |
| μ | = | Viscosity, Pa·s |
| ρ | = | Density, kg·m−3 |
| τ | = | Tortuosity factor, - |
| Subscripts | = | |
| eff | = | Effective |
| ds | = | Dry porous composite salt hydrate |
| g | = | Gas |
| H2O | = | Water |
| kd | = | Knudsen diffusion |
| out | = | Outlet |
| s | = | Composite salt hydrate |
| sd | = | Surface diffusion |
| 0 | = | Initial state |
| eq | = | Equilibrium |
| f | = | Final state |
| gs | = | Gas in the solid matrix |
| in | = | Inlet |
| md | = | Molecular diffusion |
| p | = | Pore |
| sat | = | Saturated state |
| tr | = | True |
| 1, 2, 3 | = | 1 cm, 12 cm, and 19 cm from inlet |
Acknowledgements
Financial support from the National Natural Science Foundation of China (51906157) and the Shanghai Pujiang Program (No. 23PJ1409400) is greatly appreciated.
Disclosure statement
No potential conflict of interest was reported by the author(s).
Additional information
Funding
Notes on contributors
Hongzhi Liu
Hongzhi Liu is an assistant professor at the Faculty of Engineering at Hokkaido University. Her research mainly focuses on thermal energy storage technologies and heat pump systems.
Han Liu
Han Liu is a master’s student at the School of Environment and Architecture, University of Shanghai for Science and Technology. His research focuses on thermochemical energy storage technology.
Minglu Qu
Minglu Qu is an associate professor at the School of Environment and Architecture, University of Shanghai for Science and Technology. She is doing research on dual source heat pumps and energy storage technologies.
Katsunori Nagano
Katsunori Nagano is a professor at the Faculty of Engineering at Hokkaido University. One of the current research activities in his laboratory is energy storage technology.