A system, comprising: an electronic system comprising electronic components and a chassis, the chassis in thermal contact with components; a first modular thermal energy storage card and a second modular thermal energy storage card inserted into the chassis, the first modular thermal energy storage card stacked on top of or placed adjacent to the second modular thermal energy storage card such that the first modular thermal energy storage card and the second modular thermal energy storage card comprise one of a horizontal or a vertical stack, wherein the stack provides cooling to the electronic system, characterized in that the first modular thermal energy storage card and the second modular thermal energy storage card each comprise: a thermally conductive enclosure (110, 210, 310) bounding an interior cavity; a cell wall structure comprising a plurality of cells disposed within the interior cavity and in thermal communication with the thermally conductive enclosure; a phase change material having a melting point, the phase change material disposed within the plurality of cells and in thermal communication with cell walls of the plurality of cells; a thermally conductive interface disposed between the thermally conductive enclosure (110, 210, 310) and the chassis, the thermally conductive interface extending from the interior cavity a distance beyond the interior cavity of the enclosure and in contact with the chassis. an electronic system comprising electronic components and a chassis, the chassis in thermal contact with components; a first modular thermal energy storage card and a second modular thermal energy storage card inserted into the chassis, the first modular thermal energy storage card stacked on top of or placed adjacent to the second modular thermal energy storage card such that the first modular thermal energy storage card and the second modular thermal energy storage card comprise one of a horizontal or a vertical stack, wherein the stack provides cooling to the electronic system, characterized in that the first modular thermal energy storage card and the second modular thermal energy storage card each comprise: a thermally conductive enclosure (110, 210, 310) bounding an interior cavity; a cell wall structure comprising a plurality of cells disposed within the interior cavity and in thermal communication with the thermally conductive enclosure; a phase change material having a melting point, the phase change material disposed within the plurality of cells and in thermal communication with cell walls of the plurality of cells; a thermally conductive interface disposed between the thermally conductive enclosure (110, 210, 310) and the chassis, the thermally conductive interface extending from the interior cavity a distance beyond the interior cavity of the enclosure and in contact with the chassis. a thermally conductive enclosure (110, 210, 310) bounding an interior cavity; a cell wall structure comprising a plurality of cells disposed within the interior cavity and in thermal communication with the thermally conductive enclosure; a phase change material having a melting point, the phase change material disposed within the plurality of cells and in thermal communication with cell walls of the plurality of cells; a thermally conductive interface disposed between the thermally conductive enclosure (110, 210, 310) and the chassis, the thermally conductive interface extending from the interior cavity a distance beyond the interior cavity of the enclosure and in contact with the chassis.