Calcium chloride hexahydrate/nano-SiO2 composites as form-stable phase change materials for building energy conversation: The influence of pore size of nano-SiO2

Development of phase change materials (PCMs) for thermal energy storage in building envelopes, is a promising method of energy utilization and retention. In the present study, three different kinds of nano-SiO2 (NS) were introduced to CaCl2•6H2O to obtain form-stable phase change materials (FSPCMs) with favorable thermal properties and non-flammability. The results of leakage tests show that the maximum CaCl2•6H2O mass fractions in FSPCMs containing NS1, NS2 and NS3, were 75%, 73% and 70%, respectively. There were no chemical reactions observed between CaCl2•6H2O and nano-SiO2, and CaCl2•6H2O was uniformly dispersed in the pores and on the surfaces of nano-SiO2. Differential scanning calorimeter (DSC) results show a lower melting temperature for FSPCMs with smaller diameter nano-SiO2 species, with latent heats equivalent to the calculated values based on the mass fraction of CaCl2•6H2O in the FSPCMs. Among the three types of FSPCMs, CaCl2•6H2O(75%)/NS1 FSPCM exhibited the highest latent heat of 148.2 J/g with a phase change temperature of 25.1°C. Thermogravimetric analysis and thermal cycling tests verified that FSPCMs possess good thermal stability and reliability. Smaller nano-SiO2 pore diameter caused a downward shift in the thermal conductivity of FSPCMs. The results of thermal performance tests using a laboratory-scale test chamber, show that FSPCMs, in particular CaCl2•6H2O(75%)/NS1 FSPCM, effectively decreased the indoor peak temperature during the heating process and extended the duration of heat preservation during the cooling process, further supporting their valuable potential as candidates for use in building energy efficiency applications.