Conceptual Design and Performance Analysis of an Expander for the Recovery of Potential Energy Stored in Compressed Hydrogen

Abstract

In the scope of global decarbonisation, hydrogen has been identified as an essential energy carrier. Not only as an energy storage solution for further electrification in vehicles but also as a primary fuel source in processes that rely on chemical energy which cannot be easily electrified. However, at ambient conditions, hydrogen is also a very sparse gas with a low energy density. This requires it to be either liquified or compressed as a gas for most storage and transport solutions, both of which are rather energy-intensive processes. In the case of compressed hydrogen this leads to a significant amount of potential energy stored in the storage tanks. In current applications, the high pressure hydrogen is then expanded through a valve to reach the system’s desired working pressure. A process in which most of the potential energy stored in the compressed is lost to thermal effects.

The research in this thesis is focused on identifying a suitable application in which an expander could be applied to recover part of the potential energy stored in compressed hydrogen and to provide a conceptual design and a thermodynamical analysis of this expander to assess its efficiency and feasibility and has been caried out in collaboration with Ayed Engineering GmbH.
The literature study provided an overview of the current hydrogen landscape and the potential applications in which an expander could be applied. These have been evaluated and led to the selection of a hydrogen refuelling station as a suitable application for this purpose.

A system model has been developed to replicate a refuelling process in a hydrogen refuelling station and to serve as an operating environment for the to-be-designed expander. Based on the operating parameters of the hydrogen refuelling station, a number of expander designs have been developed, each optimized for the operating conditions in the station at a certain timestep. Empirical loss models have been applied to assess the performance of each of these designs across the entire operating range. From this, it was possible to find which potential design would provide the best overall performance. This selected design and the operating conditions at its optimal performance point have then been used in a 3D CFD validation case using Ansys CFX.

The results indicate that employing a radial inlet turbine for the recovery of potential energy stored in compressed hydrogen in a hydrogen refuelling station can indeed lead to notable operational cost savings but also that there are several practical challenges which remain to be solved. Specifically, the extremely high rotational speeds that are required in operation and the miniature design features of the turbine design. To address these, a sensitivity analysis of several system parameters and design assumptions has been included to provide guidance for future work.