Modular assembly (E) for storing heat by phase-change material including a plurality of heat-storage modules (M1) attached to one another, the heat-storage assembly comprising a vessel , each module (M1) for storing heat by phase-change material being delimited by a plurality of external walls , and comprising: - at least one heat exchanger including first (4.1) and second (4.2) connection ends intended to be connected to a heat-transfer fluid system, said first (4.1) and second (4.2) connection ends entering and emerging from the vessel , - a structure intended to contain a phase-change material, said structure comprising a porous matrix (MA, MA') with communicating cells through which said heat exchanger passes and in contact with the external surface of said heat exchanger , said matrix (MA, MA') being obtained by moulding around said heat exchanger , at least one of the external walls of the module, referred to as a solid external wall , being formed by moulding directly on the porous material so as to form a part of said vessel of the heat-storage assembly, at least one of the other external walls of the module, referred to as a porous external wall , being formed by the porous matrix (MA, MA'), and, among the modules (M1) of the heat-storage assembly, at least two adjacent modules are disposed so that a porous external wall of one of the modules (M1) is arranged facing a porous external wall of the other one of the modules (M1), and so that a solid external wall of one of the modules (M1) is attached to a solid external wall of the other one of the modules (M1). each module (M1) for storing heat by phase-change material being delimited by a plurality of external walls , and comprising: - at least one heat exchanger including first (4.1) and second (4.2) connection ends intended to be connected to a heat-transfer fluid system, said first (4.1) and second (4.2) connection ends entering and emerging from the vessel , - a structure intended to contain a phase-change material, said structure comprising a porous matrix (MA, MA') with communicating cells through which said heat exchanger passes and in contact with the external surface of said heat exchanger , said matrix (MA, MA') being obtained by moulding around said heat exchanger , - at least one heat exchanger including first (4.1) and second (4.2) connection ends intended to be connected to a heat-transfer fluid system, said first (4.1) and second (4.2) connection ends entering and emerging from the vessel , - a structure intended to contain a phase-change material, said structure comprising a porous matrix (MA, MA') with communicating cells through which said heat exchanger passes and in contact with the external surface of said heat exchanger , said matrix (MA, MA') being obtained by moulding around said heat exchanger , at least one of the external walls of the module, referred to as a solid external wall , being formed by moulding directly on the porous material so as to form a part of said vessel of the heat-storage assembly, at least one of the other external walls of the module, referred to as a porous external wall , being formed by the porous matrix (MA, MA'), and, among the modules (M1) of the heat-storage assembly, at least two adjacent modules are disposed so that a porous external wall of one of the modules (M1) is arranged facing a porous external wall of the other one of the modules (M1), and so that a solid external wall of one of the modules (M1) is attached to a solid external wall of the other one of the modules (M1). <EOS>