A downhole power generation system includes a tubular power generation device configured to be disposed in an annular space around a portion of a production tubing, wherein the power generation device is switchable between a power generation mode and a bypass mode. The system also includes a power storage device electrically coupled to the tubular power generation device and configured to store power generated by the power generation device. The power generation device comprises at least one power generation path and at least one bypass path. The at least one power generation path comprises at least one power generation mechanism which generates power when traversed by fluid. The at least one power generation path is open in the power generation mode and closed in the bypass mode.