materials Review Phase-Change Materials in Hydronic Heating and Cooling Systems: A Literature Review Rok Koželj 1, * , Eneja Osterman 1 , Fabrizio Leonforte 2 , Claudio Del Pero 2 , Alessandro Miglioli 2 , Eva Zavrl 1 , Rok Stropnik 1 , Niccolò Aste 2 and Uroš Stritih 1 1 Faculty of Mechanical Engineering, University of Ljubljana, Aškerˇ ceva 6, 1000 Ljubljana, Slovenia; eneja.osterman@fs.uni-lj.si (E.O.); eva.zavrl@fs.uni-lj.si (E.Z.); rok.stropnik@fs.uni-lj.si (R.S.); uros.stritih@fs.uni-lj.si (U.S.) 2 Department of Architecture, Built Environment and Construction Engineering, Politecnico di Milano, Via Edoardo Bonardi, 9, 20133 Milano, Italy; fabrizio.leonforte@polimi.it (F.L.); claudio.delpero@polimi.it (C.D.P.); alessandro.miglioli@polimi.it (A.M.); niccolo.aste@polimi.it (N.A.) * Correspondence: rok.kozelj@fs.uni-lj.si; Tel.: +386-1-4771-102 Received: 15 May 2020; Accepted: 30 June 2020; Published: 3 July 2020   Abstract: When considering the deployment of renewable energy sources in systems, the challenge of their utilization comes from their time instability when a mismatch between production and demand occurs. With the integration of thermal storages into systems that utilize renewable energy sources, such mismatch can be evened out. The use of phase-change materials (PCMs) as thermal storage has a theoretical advantage over the sensible one because of their high latent heat that is released or accumulated during the phase-change process. Therefore, the present paper is a review of latent thermal storages in hydronic systems for heating, cooling and domestic hot water in buildings. The work aims to offer an overview on applications of latent thermal storages coupled with heat pumps and solar collectors. The review shows that phase-change materials improve the release of heat from thermal storage and can supply heat or cold at a desired temperature level for longer time periods. The PCM review ends with the results from one of the Horizon2020 research projects, where indirect electrical storage in the form of thermal storage is considered. The review is a technological outline of the current state-of-the-art technology that could serve as a knowledge base for the practical implementation of latent thermal storages. The paper ends with an overview of energy storage maturity and the objectives from different roadmaps of European bodies. Keywords: PCM thermal storage; latent thermal storage; latent heat storage; PCMs in hydronic systems; PCMs for heating; PCMs for cooling; PCM heat storage; PCM cold storage; PCM maturity; TRL of PCM 1. Introduction—The Use of Energy Storage in Buildings Buildings’ CO 2 operational emissions account for 30% of the total energy-related carbon emissions [1]. Indeed, the decarbonization of the building sector plays a central role in the action against climate change. The reduction in building carbon emissions goes through the improvement of energy efficiency, the electrification of the building final energy consumptions and the spread of renewable energy sources (RES) [2]. A higher RES penetration and the increase in building-related electricity consumption require a smarter energy management at the building level, as outlined by the revised Energy Performance of Buildings Directive (EPBD 2018/844/EU) [3]. In fact, the integration of RES introduces several problems in the management of electric systems, since renewables that are more easily integrable in buildings have unpredictable energy production profiles and high variable rates (e.g., solar energy) [4]. Thus, as RES integration increases in the building sector, the need to properly Materials 2020, 13, 2971; doi:10.3390/ma13132971 www.mdpi.com/journal/materials