A system to generate electricity from a fluid flowing in a pipeline, the system comprising: a mounting arrangement to mount the system in a portion of a pipeline; an elongate shaft; a turbine rotor which is mounted to the shaft, the turbine rotor being operable to rotate about the elongate axis of the shaft when fluid in the pipeline acts on the turbine rotor; an electrical generator arrangement comprising a first part incorporating at least one magnet and a second part comprising at least one winding, wherein one part of the generator arrangement is mounted to the turbine rotor and the other part of the generator arrangement is mounted to a stator element which is positioned adjacent the turbine rotor; and a housing which at least partly houses the turbine rotor, the shaft and the electrical generator arrangement, the housing comprising a fluid inlet and a fluid outlet, wherein one or both of the fluid inlet and the fluid outlet has a cross sectional area that is less than the cross sectional area of the portion of the pipeline; characterised in that the system further comprises: at least one inlet guide vane which is detachably attached to the housing adjacent to the fluid inlet, each inlet guide vane being held at an angle relative to the direction of the flow of the fluid in the pipeline such that each inlet guide vane changes the direction of flow of the fluid in the pipeline to be at least partly in line with a rotor vane on the turbine rotor; and at least one outlet stay vane which is detachably attached to the housing and positioned adjacent to the fluid outlet, each outlet stay vane being held substantially in line with the direction of flow of the fluid in the pipeline; and wherein the housing comprises two parts (24, 25) that are releasably attached to one another such that the two parts (24, 25) of the housing can be at least partly separated from one another to permit access to the turbine rotor and the electrical generator arrangement and to permit removal of each inlet guide vane and each outlet stay vane. a mounting arrangement to mount the system in a portion of a pipeline; an elongate shaft; a turbine rotor which is mounted to the shaft, the turbine rotor being operable to rotate about the elongate axis of the shaft when fluid in the pipeline acts on the turbine rotor; an electrical generator arrangement comprising a first part incorporating at least one magnet and a second part comprising at least one winding, wherein one part of the generator arrangement is mounted to the turbine rotor and the other part of the generator arrangement is mounted to a stator element which is positioned adjacent the turbine rotor; and a housing which at least partly houses the turbine rotor, the shaft and the electrical generator arrangement, the housing comprising a fluid inlet and a fluid outlet, wherein one or both of the fluid inlet and the fluid outlet has a cross sectional area that is less than the cross sectional area of the portion of the pipeline; characterised in that the system further comprises: at least one inlet guide vane which is detachably attached to the housing adjacent to the fluid inlet, each inlet guide vane being held at an angle relative to the direction of the flow of the fluid in the pipeline such that each inlet guide vane changes the direction of flow of the fluid in the pipeline to be at least partly in line with a rotor vane on the turbine rotor; and at least one outlet stay vane which is detachably attached to the housing and positioned adjacent to the fluid outlet, each outlet stay vane being held substantially in line with the direction of flow of the fluid in the pipeline; and wherein the housing comprises two parts (24, 25) that are releasably attached to one another such that the two parts (24, 25) of the housing can be at least partly separated from one another to permit access to the turbine rotor and the electrical generator arrangement and to permit removal of each inlet guide vane and each outlet stay vane. at least one inlet guide vane which is detachably attached to the housing adjacent to the fluid inlet, each inlet guide vane being held at an angle relative to the direction of the flow of the fluid in the pipeline such that each inlet guide vane changes the direction of flow of the fluid in the pipeline to be at least partly in line with a rotor vane on the turbine rotor; and at least one outlet stay vane which is detachably attached to the housing and positioned adjacent to the fluid outlet, each outlet stay vane being held substantially in line with the direction of flow of the fluid in the pipeline; and wherein the housing comprises two parts (24, 25) that are releasably attached to one another such that the two parts (24, 25) of the housing can be at least partly separated from one another to permit access to the turbine rotor and the electrical generator arrangement and to permit removal of each inlet guide vane and each outlet stay vane.