A modular thermal energy store configured for assembly with another like modular thermal energy store to form a stacked series array of modular thermal energy stores or back-to-back with another like modular thermal energy store to form a parallel array of modular thermal energy stores; said modular thermal energy store comprising: a housing having a first, second, third and fourth walls; said first wall being coupled at opposite extremities to said second and third walls, respectively; said fourth wall being coupled at opposite extremities to said second and third walls, respectively; said first, second, third and fourth walls co-operating to define: (i) an internal chamber, (ii) an inlet adjacent a first periphery of said first, second, third and fourth walls, and (iii) an outlet adjacent a second periphery of said first, second, third and fourth walls opposite said first periphery; a plurality of receptacles provided within the internal chamber and spaced from one another so that heat transfer fluid can flow over and around the receptacles as the fluid moves from the inlet to the outlet; each said receptacle defining an internal cavity for the storage of phase change material so that thermal energy can transfer between the stored PCM material and the heat transfer fluid as the fluid passes over and between the receptacles from said inlet to said outlet; first and second structural features extending outwardly from the first and second peripheries respectively of said second, third and fourth walls, said first and second structural features being configured so that said modular thermal energy store can be assembled in a stack with a like modular thermal energy store so that said outlet of said modular thermal energy store aligns with and is adjacent to said inlet of said like modular thermal energy store; and third and fourth structural features extending outwardly from opposite third and fourth peripheries, respectively, of said first wall; said third and fourth structural features being configured so that said modular thermal energy store can be assembled back-to-back with a like modular thermal energy store so that said first wall of said modular thermal energy store abuts said first wall of said like modular thermal energy store; wherein said fourth wall defines an opening for accessing openable ports by means of which PCM can be poured into said receptacles. a housing having a first, second, third and fourth walls; said first wall being coupled at opposite extremities to said second and third walls, respectively; said fourth wall being coupled at opposite extremities to said second and third walls, respectively; said first, second, third and fourth walls co-operating to define: (i) an internal chamber, (ii) an inlet adjacent a first periphery of said first, second, third and fourth walls, and (iii) an outlet adjacent a second periphery of said first, second, third and fourth walls opposite said first periphery; a plurality of receptacles provided within the internal chamber and spaced from one another so that heat transfer fluid can flow over and around the receptacles as the fluid moves from the inlet to the outlet; each said receptacle defining an internal cavity for the storage of phase change material so that thermal energy can transfer between the stored PCM material and the heat transfer fluid as the fluid passes over and between the receptacles from said inlet to said outlet; first and second structural features extending outwardly from the first and second peripheries respectively of said second, third and fourth walls, said first and second structural features being configured so that said modular thermal energy store can be assembled in a stack with a like modular thermal energy store so that said outlet of said modular thermal energy store aligns with and is adjacent to said inlet of said like modular thermal energy store; and third and fourth structural features extending outwardly from opposite third and fourth peripheries, respectively, of said first wall; said third and fourth structural features being configured so that said modular thermal energy store can be assembled back-to-back with a like modular thermal energy store so that said first wall of said modular thermal energy store abuts said first wall of said like modular thermal energy store; wherein said fourth wall defines an opening for accessing openable ports by means of which PCM can be poured into said receptacles.