Microencapsulated Phase Change Materials in Solar-Thermal Conversion Systems: Understanding Geometry-Dependent Heating Efficiency and System Reliability

Abstract

The performance of solar-thermal conversion systems can be improved by incorporation of nanocarbon-stabilized microencapsulated phase change materials (MPCMs). The geometry of MPCMs in the microcapsules plays an important role for improving their heating efficiency andreliability. Yet few efforts have been made to critically examine the formation mechanism of different geometries and their effect on MPCMs-shell interaction. Herein, through changing the cooling rate of original emulsions, we acquire MPCMs within the nanocarbon microcapsules with a hollow structure of MPCMs (h-MPCMs) or solid PCM core particles (s-MPCMs). X-ray photoelectron spectroscopy and atomic force microscopy reveals that the capsule shell of the hMPCMs are enriched with nanocarbons and have a greater MPCMs-shell interaction compared to s-MPCMs. This results in the h-MPCMs being more stable and having greater heat diffusivity within and above the phase transition range than the s-MPCMs do. The geometry-dependent heating efficiency and system stability may have important and general implications for the fundamental understanding of microencapsulation and wider breadth of heating generating systems.3 Solar-thermal conversion, where solar irradiation is harvested and converted to heat for beneficial usage, has gained renewed interest in the past decade and made it a special asset in energy conversions due to its operational simplicity and high energy conversion efficiency. [1][2][3][4] Microencapsulated phase change materials (MPCMs, 1-100 µm diameter), often considered unique micrometer-scaled composites with a superior performance of latent heat thermal storage as compared with bulk PCMs, are currently emerging as positive additives/dopants to the solarthermal conversion systems. Nanocarbon-stabilized MPCMs are of particular interest as they combine the advantages of nanocarbons for their outstanding energy conversion/transfer performance, [5][6][7] MPCMs with an accelerated heat storage/release due to a relatively high surfacearea-to-volume ratio [8][9][10][11][12][13] and the PCM-nanocarbon interactions which often fosters an enhanced enthalpy and better crystallinity. 14,15 A new avenue is therefore opening to enhance the heatgenerating efficiency at a output temperature within and even higher than the solid-liquid phasetransition range (PTR). [16][17][18] By constantly storing and retracting latent heat, 19 the MPCMs are expected to maintain the dynamic equilibrium of output temperatures when the surrounding temperature is around the PTR. More attractively, since the liquid PCMs above PTR store a higher accumulative energy (latent heat + sensible heat) but exhibit a much lower specific heat capacity than the PCMs within PTR, 20,21 the temperatures of PCMs and heat-generating structures would increase synchronously. [22][23][24][25][26][27][28] Consequently, a higher energy storage capacity will be achieved; 17 meanwhile, more heat will be emitted from the MPCMs above PTR to eliminate the convective heat dissipation in the heat-gen...

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