Green sorption materials for thermochemical energy storage

Introduction
Thermochemical energy storage (TES) utilizes sorption-desorption reactions between a sorbent and a sorbate to reversibly store and release energy. TES can store energy approximately 2-10 times higher over sensible or latent thermal energy storage via a more compact system and for a longer time. Heat is charged into the system to separate the sorbate (usually water) from the sorbent via endothermic desorption reactions. The recombination of these components causes a heat discharge via exothermic sorption reactions. Salt hydrates have high energy densities reaching up to 3.12 GJ.m-3 for water/space heating (<100 °C), based on reversible hydration-dehydration reactions as follows:
Salt.xH_2 O (s)+Heat□(↔) Salt+xH_2 O (eq. 1)
Salts´ high affinity towards water molecules, however, causes deliquescence, the formation of saturated solutions at high relative humilities. Deliquescence and consequent swelling and agglomeration of the salt particles cause severe mass transfer barriers and vapor penetration leading to low levels of energy charging and discharging and low cycling stability for long-term operation of TES. The impregnation of salts within the porous matrices can prevent salts´ dissolution and agglomeration issues. As such, herein, inexpensive CaCl2 has been impregnated in porous carbon for TES.
Materials & methods:
Porous carbon was produced via pyrolysis of wood under nitrogen gas condition. Wood was used as the source biomass because it is a common product of forestry and easily accessible worldwide. The composite was prepared via vacuum impregnation method. Three different ratios of salt to biocarbon as 70/30, 65/35, and 60/40 wt% were prepared and characterized by scanning electron microscopy (SEM) and simultaneous thermal analysis (STA).
Results & Discussion:
SEM confirmed a successful impregnation through pore filling mechanism. The surface area of the pure biocarbon was 600 m2/g that was reduced to around 60 m2/g after salt impregnation into the pores. The compositions showed no leakage or agglomeration of the salt while still providing high energy storage density. STA recorded the storage capacity of hydration-dehydration reactions, which reached up to 1 kJ/g of charging and discharging heat. These results show the potential of salt-impregnated biocarbon as a green energy storage material.