Repurposing gas transmission pipelines for medium-duration compressed air energy storage

As demand for natural gas declines in the pursuit of cleaner energy grids, large-scale infrastructure such as gas transmission pipelines may be rendered obsolete. While some pipelines will likely transport hydrogen, many could be adapted for Compressed Air Energy Storage (CAES), particularly for medium-duration energy storage. There has been a previous study on using transmission pipelines for CAES, however, it assumed the pipeline compressed air storage behaved in the same way as a cavern with the same volume, neglecting friction loss along the pipeline. This study focuses on the estimation of the potential of gas transmission pipelines for compressed air storage considering the different dynamics between a cavern and a pipeline through simulation using a Finite Volume Method. The potential storage was estimated based on a pipeline of length 100 km and 0.9 m diameter operating between 4 and 7 MPa, compatible with transmission pipelines found in the UK, Europe and the US, and the results compared to storage using a cavern with the same volume. During both pipeline charging and discharging, the process stopped before the average pressure reached the target pressure. This happens because pressure is controlled based on the inlet/outlet node, reflective of real operation, and the target pressure is reached first in those nodes. Therefore, the average pressure will be lower than the maximum pressure at the end of charging and higher than the minimum pressure at the end of the discharging process. In contrast, caverns maintain uniform properties, therefore the pressure in the entire volume will be at the target pressure at the end of both processes. This resulted in a reduction in total exergy capacity and exergy density of 3.5 % compared to the operation using caverns, considering only the exergy from the operation between 4 and 7 MPa. Using the gas transmission infrastructure as compressed air storage presents extra losses and reduced exergy density compared to traditional caverns for compressed air storage. On the other hand, these shortcomings are compensated by the reduction in costs and construction time needed to adapt salt caverns or other suitable geological locations while already having connections to densely populated city centres and also avoiding expenses with transmission pipeline decommissioning. Assuming storage between 7 and 4 MPa, there is a potential of up to 31 GWh of energy storage in the UK alone and might offer an opportunity for rapid expansion of medium duration energy storage capacity.