To achieve effective long-term CO2 storage in saline aquifers, it is essential to understand and monitor CO2 distribution and trapping mechanisms, which are significantly influenced by groundwater flow. This study investigates the impact of background flow velocity and direction on CO2 plume behavior and different trapping mechanisms (residual and solubility) using numerical analysis. The results of simulation show that in the flat (0° dip) model, increasing background flow velocity significantly extends the plume migration distance, enhancing both solubility and residual trapping through a larger CO2-water contact area and increased pore space occupation. The analysis is further extended to a dipping aquifer scenario to assess the role of groundwater flow direction. In the co-current flow case, where water and CO2 move in the same direction, the plume attains its maximum lateral extension, resulting in the highest storage efficiency. Conversely, in the counter-current flow scenario, where CO2 and water move in opposite directions, lower CO2 trapping is observed because, particularly at high velocity, the drag force exerted by water overcomes buoyancy force and limits further plume extension.

This study uses the concept of exergy-return on exergy-investment (ERoEI) to evaluate the life-cycle exergetic efficiency and CO₂ intensity (grams CO₂ per MJ of electricity) of (diabatic and adiabatic) compressed air energy storage (CAES) systems. Several CAES configurations are assessed under defined system boundaries, including diabatic systems powered by methane (CH₄) or hydrogen (H2), and adiabatic system with a thermal energy storage (TES) facility.

The results show that conventional (diabatic) CAES system powered by natural gas has the lower exergetic efficiency and higher CO2 intensity compared to adiabatic CAES due to the heat dissipation during compression stage and additional fuel requirements for reheating the air during expansion. Integrating carbon capture and storage (CCS) plant with conventional diabatic CAES can nearly halve the CO₂ intensity for electricity generation although the additional exergy investment for the CCS process reduces the exergetic efficiency of the system. Transitioning to green H2 (produced from low-carbon electricity) as the primary turbine fuel in the diabatic CAES results in a 65–76 % reduction in CO₂ intensity. However, the average exergetic efficiency of system decreases by around 10 %, mainly due to the substantial exergy investment associated with hydrogen production. It is also found that the adiabatic CAES system integrated with TES demonstrates the highest thermodynamic and environmental performance. When 100 % of compression heat is captured and reused during discharge phase, the system reaches ERoEI values up to 61 % with CO2 intensity of 12–26 g CO₂ per MJe.

Disclaimer: The results and performance metrics presented in this study are based on modelled scenarios and literature-derived parameters under defined system boundaries. Actual performance of CAES systems may vary depending on site-specific conditions, technology maturity, and operational configurations. All efficiency values, CO₂ intensity estimates, and comparative assessments should be interpreted within the context of the assumptions and limitations described herein. This study does not constitute a commercial endorsement or performance guarantee. The authors have made every effort to ensure accuracy but accept no liability for decisions made based on this analysis.

Hydrogen plays a crucial role in the transition to low-carbon energy systems, especially when integrated into energy storage applications. In this study, the concept of exergy-return on exergy-investment (ERoEI) is applied to investigate the exergetic efficiency (defined as the ratio of useful exergy output to invested exergy input) and CO2 equivalent intensity of the hydrogen supply chain, with a specific focus on the underground hydrogen storage process. Our findings reveal that the overall exergetic efficiency of the electricity-to-hydrogen-to-electricity conversion process can reach up to 25 %. Among the hydrogen production methods, green hydrogen, produced via electrolysis powered by renewable energy, exhibits the lowest CO2 equivalent intensity. Blue hydrogen, produced from natural gas with carbon capture, can significantly reduce the carbon footprint of electricity generation, though this advantage comes at the expense of decreased exergetic efficiency. Analysis further indicates that the exergetic efficiency of underground storage components ranges from 72 % to 92 %. A substantial fraction of the exergy is lost during compression and injection of the stored hydrogen. Nevertheless, the subsurface operations contribute a minimal CO2 emission, between 1.46–4.56 grams of equivalent CO2 per megajoule (gr-CO2eq/MJ) when powered by low-carbon energy sources. Furthermore, it is found that hydrogen loss in the reservoir, along with methane and hydrogen leak during surface operations, notably affects the overall efficiency of the storage process.