Shape-stabilization of organic phase change materials as mechanically stable silica boards with high latent heats synthesized via sol-gel route

Abstract

Monolithic shape-stabilized phase change materials (ss-PCM) are of great interest for energy-saving construction materials, solar-water heating systems and passive cooling systems of batteries and photovoltaic cells. However, the application range of ss-PCMs is currently limited by either low mechanical stabilities or low heat storage capacities. Therefore, we recently developed ss-PCMs based on silica and butyl stearate (BS) with high compressive strengths (1.2 MPa, 10 °C), but only moderate latent heats (∼100 J/g). Here, we focus on validating the applicability of our porogen-assisted sol-gel process to other organic PCMs apart from butyl stearate. To increase the latent heat of ss-PCMs, we create a mixture of 70% hexadecane (HD) or octadecane (OD) with 30% BS and combine this PCM mixture with our novel-sol-gel route to synthesize ss-PCMs with high compressive strengths (1.2 MPa, 5 °C) as well as high latent heats (160 J/g). Due to the well-interconnected silica matrix confining the paraffins, the ss-PCMs have 70% higher thermal conductivities (0.43 W/mK, 5 °C) than pure paraffins, high shape-stabilities (∼100%), high PCM loadings (83 wt%), and high chemical and long-term stabilities (>3000 thermal cycles). Moreover, they are thermally stable up to ∼170 °C with PCM melting points of 17 °C and 22 °C. We also synthesize ss-PCMs with pure polyethylene glycol 600 (PEG600) and pure acetamide (Ac) to analyze the effect of non-paraffinic PCMs on the physicochemical properties of ss-PCMs. These ss-PCMs have a ∼30% lower latent heat than expected because the PCM either reacts strongly by hydrogen bonding with the silica (PEG600) or was chemically degraded under the basic conditions during gelation (Ac). In contrast to the low compressive strength of shape-stabilized PEG600 (35 kPa, 5 °C), the ss-PCMs based on Ac have high compressive strengths (9.6 MPa, 5 °C). Moreover, they are translucent above the PCM melting point due to a nanoscopic silica matrix. The high supercooling effect (37 °C) and the moderate long-term stability (3000 thermal cycles, 20% lower latent heat) of shape-stabilized Ac restrict its application to special, glassy materials.