Evaluating the thermal efficiency of microencapsulated phase change materials for thermal energy storage in cementitious composites

Microencapsulated phase change materials (MCPM) have been shown to be a promising material that can be used in cementitious composites for thermal energy storage. This paper evaluates the thermal efficiency of MPCM incorporated in the cementitious composite and quantitatively identifies the influential mechanical and chemical damages that can lead to reductions in thermal efficiency of MPCM with melamine formaldehyde based shell. Longitudinal Guarded Comparative Calorimetry (LGCC) and Low Temperature Differential Scanning Calorimetry (LT-DSC) were used to quantitatively evaluate the thermal response of cementitious composite containing MPCM, in relatively large (10(-2) m) and small (10(-3) m) scale samples, respectively. The results indicated an average reduction of similar to 32%-34% in the effective thermal efficiency (i.e., the average ratio of measured over expected heat release/absorption) of cementitious composite containing MPCM due to mechanical as well as chemical damages to MPCM. Mixing speeds of various rate was also studied and it was found that sample preparation at slow mixing speed (i.e., 55 rpm) could cause minimal damage to the MPCM (i.e., similar to 5.5 %-7.5% reduction in thermal efficiency), while using fast mixing speed (i.e., 165 rpm) can reduce the thermal efficiency of MPCM by up to 50%. Exposure of plain MPCM to separate constitutes of synthesized pore solution indicated that the main cause of MPCM thermal efficiency reduction is its chemical reaction with the pore solution of the cementitious composite. It was found that the main constitute of pore solution that could chemically interacts with MPCM was SO42-. Finally, it was observed that the incorporation of MPCM in the cementitious composite can reduce the compressive strength of mortars due to the soft nature of MPCM. To increase the thermal efficiency of MPCM, this work recommends potential solutions such as using a cement with low sulfate content or development of a shell material for MPCM which is unreactive when exposed to pore solution.