Apparatus is provided for use in power generation, including a fluid-flow-driven power generator for use in a fluid-containing pipe such as a drill pipe as used in oil and gas exploration and extraction. Parts of the generator (220,254) are adapted to be removable from the pipe - for example while a drill pipe is downhole within a drilling well - to leave a clear through bore for survey and fishing operations and to enable replacement of the removed parts. The removable part may be a flow control assembly (490), which includes an adjustable flow control nozzle (500) or which receives any one of a set of different size nozzles. The flow-driven generator comprises an impeller (220) connected to a magnet assembly (254) to rotate the magnet assembly when fluid flows past the impeller. This causes relative movement between the magnet assembly (254) and an adjacent electrical coil assembly (254), the relative movement and magnetic coupling generating an electrical current in the coil assembly. This generated electrical current is used to power electrical devices within the pipe.