# Special Issue \"Advanced Phase Change Materials for Thermal Storage\"
## 2 Advanced Phase Change Materials for Thermal Storage  
The key subjects included in this special issue were related not only to materials in terms of new PCM formulations and concepts, validation and assessment procedures, characterization and simulation, but also to PCM applications in terms of implementation and testing in storage prototypes, innovative approaches and the simulation of novel storage modules for latent heat. Despite COVID-19 crises and lockdowns in most countries, there were still six papers submitted to this special issue, and five of them were accepted, which proves the quality of the research and the strong interest in the field of latent heat storage. In the following paragraphs, a summary of these papers with their most relevant contributions is presented. The first paper included in this issue dealt with a procedure for selecting the appropriate PCM for two kinds of innovative compact energy storage systems implemented in residential buildings: the Mediterranean (MED) concept, intended for space cooling, and the continental (CON) concept, used for heating and domestic water [1]. The selection methodology consists of a qualitative decision matrix, which uses some common features of PCMs to assign an overall score to each material so that different options can be compared. The most important PCM features to be considered in the decision matrix for material selection are the melting enthalpy and temperature range, availability, cost and, in the case of the CON concept, the maximum working temperature range. Apart from the qualitative results, the authors made an experimental characterization of the best candidates, consisting of differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA), before making a final decision. This selection process led to various possible candidates, but two commercial PCMs were selected as the most promising ones: RT4 (T\({}_{\text{melt}}\) = 4 \({}^{\circ}\)C) for the MED cooling system and RT64 HC (T\({}_{\text{melt}}\) = 64 \({}^{\circ}\)C) for the CON system providing house heating and domestic hot water. The main contribution of this paper is that it provides a simple, quick tool for pre-screening a PCM before being implemented in any application and selecting at least the most promising candidates to be included in further validation tests. The second paper presented a methodology that allowed comparing latent heat energy storage (LHES) modules with different designs with respect to their compactness and heat transfer performance [2]. Nowadays, many novel and promising heat exchanger designs and concepts have emerged, aiming to enhance the heat transfer inside LHES devices. However, the wide range of experimental conditions that can be found in the literature for their characterization makes it difficult to compare their performance. In light of this, the methodology described in the paper established just two key performance indicators (KPIs) which were minimally influenced by the experimental conditions: the compactness degree (\(\Phi\)PCM) and a normalized heat transfer performance coefficient (NHTPC). In the paper, these KPIs were calculated for several LHES units already reported in the literature, allowing a leveled performance comparison with regard to operating conditions at different scales, while characteristics like geometry, structural materials and PCMs remained intrinsic. The robustness of the proposed KPIs was confirmed for units at different size scales by varying the heat transfer fluid mass flow rates and temperature levels. The evaluation procedure was applied to various LHES systems, and the most promising designs for different applications were identified and discussed. The authors clarified that the storage units analyzed were application-oriented and, in most cases, a high heat transfer rate was not a requirement, which led to low values for the KPIs. However optimized versions of the evaluated LHES systems are expected to deliver considerably higher performance indicators.