Contents lists available at ScienceDirect Journal of Energy Storage journal homepage: www.elsevier.com/locate/est Experimental investigation of thermal degradation of phase change materials for medium-temperature thermal energy storage and tightness during cycling inside metal spheres Nikolaos Stathopoulos a, ⁎ , George Belessiotis b , Petros Oikonomou b , Elias Papanicolaou a a Solar & Other Energy Systems Laboratory, NCSR “Demokritos”, Patr. Grigoriou & Neapoleos 27, Agia Paraskevi, 15341 Attiki, Greece b Institute for Nanoscience and Nanotechnology, NCSR “Demokritos”, Patr. Grigoriou & Neapoleos 27, Agia Paraskevi, 15341 Attiki, Greece ARTICLEINFO Keywords: Medium temperature PCMs Cycling in realistic conditions Encapsulation Packed bed D-Mannitol H160 ABSTRACT The ability of phase change materials to maintain their thermal properties after prolonged use is an important parameter, strongly infuencing the performance of a Thermal Energy Storage (TES) application. To investigate possible degradation, materials are usually cycled under laboratory conditions by testing small samples in a calorimeter. In this study, three materials are macroencapsulated in spheres specifcally developed for a medium temperature packed bed TES system. In contrast to previous studies, this allowed to thermally cycle and to quantify the degradation of the materials under realistic conditions, i.e. bigger sample mass, sphere en- capsulation, tightness and exposure to air. At the same time, diferent sealing methods, leading to confgurations where the materials are under small, minor and no exposure to ambient air, are proposed and compared re- garding thermal degradation. Results show that exposure to ambient air signifcantly infuences the behavior of the materials, almost doubling the decrease of phase change enthalpy and temperature values for the same number of cycles. No changes in the chemical structure of the tested materials were observed. Furthermore, it was found that the same material can present a diferent degree of degradation depending on its supplier, namely, 24,5% against 38,9% decrease to the melting enthalpy respectively under the same conditions. The material exhibiting the best performance presented a decrease of 24,5% and 16% of its melting and solidifcation enthalpy values respectively, after 60 cycles within spherical capsules and under no exposure to ambient air. Lastly, even though supercooling is observed, this phenomenon was found to be less pronounced when the material is encapsulated in spheres compared to small samples in calorimetry measurements. 1. Introduction As the share of renewables in the energy production increases, thermal energy storage (TES) becomes a necessary technology, ad- dressing intermittence issues and increasing efciency. One of the main types of TES is Latent Heat Thermal Energy Storage (LHTES) with Phase Change Materials (PCMs) [1,2]. PCM technology takes advantage of the phase change process (usually between solid and liquid state) which leads to high energy density within narrow and fxed temperature bracket [3]. A PCM candidate must fulfll several requirements related to its thermophysical and other properties, such as increased phase change enthalpy value, specifc melting temperature range, non-corrosive, nontoxic and low cost [4]. Another important factor is the degradation of the phase change temperature and enthalpy values over consecutive melting-solidifcation processes, i.e. cycling stability, and over exposure to elevated temperatures, i.e. thermal stability [5]. Rathod et al. [6] conducted a literature review of studies that investigate the thermal stability of organic, inorganic and eutectics PCMs, stating that a ma- terial is reliable if it is thermally, chemically and physically stable after a number of repeated and consistent thermal cycles. Stability of latent heat and melting temperature is noted as the most signifcant criterion, most researchers using Diferential Scanning Calorimetry (DSC) tech- nique to measure these properties. Ferret et al. [7] performed a similar survey, stating that DSC is mostly used for the thermal cycling proce- dure, followed by specifcally designed equipment such as thermal bath, beaker, glass or oven. DSC is the only reported method used for the thermal characterization of the materials. Dynamic (isothermal stages amid heating and cooling segments) and pyramid (no isothermal stages) are reported as the two main cycling methods, the latter being the most used one. A big variety of heating rates is observed, 5 °C/min, 7 °C/min and 10 °C/min being the main chosen ones. The authors https://doi.org/10.1016/j.est.2020.101618 Received 14 March 2020; Received in revised form 12 May 2020; Accepted 14 June 2020 ⁎ Corresponding author. E-mail address: nistatho@gmail.com (N. Stathopoulos). Journal of Energy Storage 31 (2020) 101618 2352-152X/ © 2020 Elsevier Ltd. All rights reserved. T