The present invention relates to an energy system (100) operable to generate mechanical energy. The energy system (100) comprises a cluster of elongated energy cells (10). The energy cells (10) are arranged parallel to each other in their longitudinal direction in a cylinder block means (102). Each energy cell (10) is operable to generate mechanical energy when a phase change material (PCM) changes from solid phase to liquid phase. The energy system (100) also comprises a cylinder head means (104), and a cylinder bottom means (106), both connected to the cylinder block means (102). In FIG. 1, the energy system (100) is partly shown dismantled.