A penile prosthesis has an energy assembly coupled to a proximal end of a tubular body. The energy assembly has a housing enclosing a piston, a heating element, and a liquid phase change material (PCM) sealed between an interior surface of the housing and the piston. When the heating element heats the liquid PCM to a gaseous state, the piston moves in a distal direction to increase pressure in the tubular body and provide the prosthesis with erection.