A turbine assembly for downhole use is being developed for use in drill pipe and annular locations in the well that require small power generation devices. Such devices are required in high speed data transmission drilling strings currently being introduced in the industry. Discrete array of sensor banks mounted on the wired drill string require a local power source that directs electric energy to tools located in a particular sensored section of the well being drilled. The thermodynamics of the compact generators is typically cyrocooling with reverse Brayton cycle expansion, liquefying in bulk gases such as helium. The high efficiency compared to membrane separation makes this method of helium distillation suitable for diverse uses such as NMR equipment and superconducting conductors. This paper details the thermodynamics of a high-efficiency cycle power generation system for such drill strings the development of components in the system used to arrive at the target efficiency levels defined by the generation cycle. The lessons learnt from the cryogenic compact turboexpanders, which include mixed radial inlets and axial discharge configurations are illustrated in the development of this turbine generator for downhole drilling use. Additionally, supercomputer simulations on structural and fluid mechanics conditions downhole are used are used to understand, in detail, the system and component level thermodynamics of the turbo-generator. The conclusions of this thermodynamic cycle development work is that a small profile, side-mounted power source can be development for use in drilling applications, particularly where the center bore of the drill string is required to be a clear pathway. The small profile of this solution enables completion strings used in intelligent fields to be powered, in addition, where the side-mounted power source is readily integrated into small cross-sections.