Research conducted in the framework of MIRACLE Project (Photonic Metaconcrete with Infrared RAdiative Cooling capacity for Large Energy savings, GA 964450), coordinated by Dr. Jorge Sánchez Dolado, from Centro de Física de Materiales (CFM). A cement-based device that can meet, partially or completely, the heating loads of a building by absorbing the solar radiation and converting it into thermal energy can be defined as a Massive Solar-Thermal Collector. The absorbing material for the incoming radiation is made of a cementitious composite, generally concrete, and flowing water inside tubes acts as a heat transfer medium. For an optimized performance, during periods of solar radiation, the device has to efficiently conduct the heat flow from the absorbing surface of the collector and transfer this heat energy to the water. Then, when the radiation is reduced or became null, the device should retain as much as possible the heat energy, reducing the heat that is escaping the collector and consequently the losses to the surrounding environment. In this work, by performing a parametric analysis, different absorbing materials are tested with the objective of finding the best configuration that maximizes the energy efficiency of the collector. Cementitious materials, in combination with Phase Change Materials with distinct melting (and solidification) temperatures, are selected as candidate absorbing materials. The weather variables of an entire year and for two different locations are considered to evaluate the behavior of these devices in opposite climates. After numerical simulations, in where an enthalpy-based finite element formulation is used to solve the physical problem, the obtained results allow to conclude that the inclusion of Phase Change Materials within the absorber material of the collectors, if it is done in a correct way, can improve the energy performance of these devices. In this study, 34 °C and 53 °C are chosen as the most appropriated melting temperatures, which conduct to considerable improvements in the achieved performances, and in both warm and cold climates. The authors gratefully acknowledge the financial support from: - The National Scientific and Technical Research Council (CONICET) of Argentina. - The CONICET through the project “Computational design of functional thermal metamaterials in transient regime taking advantage of phase changes” (PIP 11220200101018CO). - The National Agency for the Promotion of Research, Technological Development and Innovation (AGENCIA) of Argentina through the project “Computational design of metamaterials applied to the development of thermal diodes for building envelopes” (PICT 2020 SERIE A 03765). - The National Technological University (UTN) of Argentina, for Grant PID MAUTNFE0007745. - The Technical University of Darmstadt, for the “Future Talent (Guest Stay)” and “Career Bridging” Grants, both given to the first author of this work. - The NRG-STORAGE project (870114, 2020-2024,https://nrg-storage.eu/), financed by the European Union H2020 Framework under the LC-EEB-01-2019 call, IA type. - The MIRACLE project (964450, 2021-2025,https://miracle-concrete.eu/), financed by the European Union H2020 Framework under the FETOPEN-01-2018-2019-2020 call, RIA type. - The support to networking activities provided by the PoroPCM Project (part of the EIG CONCERT - Japan funding,http:concert-japan.eu/) is also gratefully acknowledged. Peer reviewed