Field device for process instrumentation

The device (1) e.g. sensors, has a generating unit generating electrical energy for operation of the device. A terminal supplies auxiliary energy through supply lines. The generating unit is formed as reaction cells of a flow battery and connected to supply and return lines (11a, 11b, 12a, 12b) to supply and return positive and negative electrolytes. A loading unit (10) generates the electrolytes, and tanks (6, 7) store the electrolytes. The electrolyte pumps (8, 9) generate flow of the electrolytes from the tanks over the supply lines through the reaction cells, respectively.