Provided herein are multiple variations, applications, and variations for producing electrical power from a flowing fluid such as a gas or liquid under pressure, for example natural gas flowing through a pipeline, by means of one or more positive displacement devices that drive one or more electrical generators. The electrical generators may be immersed in the flow stream together with the positive displacement devices as disclosed, or alternately may be isolated from the flow stream, such as by magnetic coupling, in order to promote longevity and to decrease the risk of accidental discharge or explosion of the fluid in the flow stream. To further decrease such risks, the positive displacement devices may isolate the drive fluid from the environment without the use of dynamic seals.