(418y) Lithium Chloride and Expanded Graphite-Mediated Hydration Performance Enhancement of a Novel Chemical Heat Storage Material for Lioh/Lioh•H2O Reaction

Low temperature exhaust heat below 373K is enormously unused and discharged from various fields in the world due to the difficulty in utilization caused by its low exergy; e.g. it accounts for about 65% of waste heat discharged from industrial sector over the world. Besides, combined heat and power is recently expected to play an important role in improving total energy efficiency. The key point of this system is to store and utilize heat below 373 K with high efficiency generated from power generator, such as gas engine and fuel cell.

Reversible chemical reactions are appropriate for thermal energy storage because of their high energy densities compared to conventional sensible or latent heat storage systems. Chemical heat storage for such low temperature thermal energy is expected to play a key role to establish a highly energy utilization system. Therefore, in this study, we have first tried to find the promising compounds for low-temperature heat storage by measuring endothermic heat and temperature with a differential scanning calorimetry. As a result, it was found that lithium hydroxide monohydrate dehydrated at 337 K with endothermic heat of 1,440 kJ/kg. However, a serious problem needing to be settled is that hydration of lithium hydroxide proceeds very slowly. The conversion of LiOH hydration was only 21% at 2h, which severely reduce the cycle efficiency of heat storage in the reaction system.

The hydration performance enhancement for LiOH/LiOH·H₂O reversible chemical heat storage reaction was discussed. A novel composite, denoted as ELL, was developed by mixing pure lithium hydroxide with lithium chloride and expanded graphite, which were employed as hygroscopic material and heat transfer enhancers, respectively.  A high-yield impregnation method was used to prepare the composite as follows: a specified amount of LiCl and expanded graphite were mixed with 100 mL deionized water, followed by addition of pure LiOH powder into the solution. Next, the mixture was sonicated for 40 min to generate a sufficient dispersion. Subsequently, the excess water in the solution was removed by centrifugation, and the remaining wet product was dried at 110℃ in an oven overnight.

Fundamental study of dehydration behavior of ELL was also performed with a thermo gravimetric analyzer. With respect to pure lithium hydroxide powder, ELL showed much higher reaction rate for hydration transformation without a great loss of energy storage density, the heat output capacity after 30min hydration was nearly 1,200 kJ/kg. The dehydration kinetics of the ELL composite was well fitted with a first-order rate equation in the temperature range between 336K and 373K, addition of LiCl and expanded graphite was found to decrease the estimated activation energy in the dehydration. In conclusion, it was established that the newly developed ELL composite is a promising candidate for novel chemical heat storage materials for chemical heat pumps at working temperatures below 373K.