# Sustainable Energy Solutions: Utilising UGS for Hydrogen Production by Electrolysis
## 5 Conclusions  
The energy transition is a crucial process that society must undergo in order to reduce its dependence on fossil fuels and move towards clean, sustainable, and renewable sources of energy. At the centre of this transition is the \"hydrogen paradigm\"--the increasing importance of hydrogen as a key element in achieving this goal. Through a detailed analysis in this study, the potential application of underground gas storage technology for the sustainable production of clean energy and hydrogen was demonstrated. Our research showed that components of underground gas storage technology could be utilised for carbon-free energy production and, through the synergy of water production at the gas storage facility during the withdrawal cycle, for sustainable and closed-loop hydrogen production. The use of a turboexpander-generator-electrolyser system for hydrogen production was investigated as a method of recovering the pressurised gas energy lost during the gas withdrawal process from underground storage facilities. By applying an integrated mathematical model for UGSs, insights were gained into the potential of recovering lost pressurised gas energy to generate electricity and hydrogen during a single gas withdrawal cycle. This approach not only enables additional energy savings for gas storage facilities but also has the potential to improve the efficiency of energy resource utilisation. The exclusive use of commercial, industry-standard software (IPM 13.5 PETEX package, Schlumberger Petrel RE 2022.1, Eclipse 100, etc.) in our methodology does impose certain limitations on the direct reproducibility of our model results, but despite these limitations, the procedures outlined should enable sufficiently qualified experts to reproduce conceptually similar models for any other gas storage facility. The results of the simulation model indicated an extraordinary potential for the integration of turboexpanders into existing underground gas storage processes for the production of electrical energy and hydrogen. Based on the given analyses, approximately 70 tonnes of hydrogen per UGS withdrawal cycle could be produced in parallel to 3.82 TWh of electricity generated by the expander (at a constant pressure differential of 2 MPa across the turboexpander) and the utilisation of produced field water. Furthermore, the analysis of the results showed that hydrogen production was sensitive to changes in gas flow through the pressure reduction station, highlighting the importance of process optimisation to maximise hydrogen production. Additionally, a linear relationship was found between the gas flow rate and the available power of the turboexpander, further emphasising the importance of precise process control. The electrical energy generated by this pressure differential can be categorised as \"clean\" or \"green\" energy. Consequently, the hydrogen produced under these conditions should also be categorised as green hydrogen. Future regulatory frameworks should support the transportation of this green hydrogen through the gas transmission systems to end users, in order to meet environmental sustainability goals and promote the adoption of cleaner energy solutions. Also noteworthy was the persistent correlation between the total average water produced from the reservoir during a single gas withdrawal cycle and the total electrical energy generated by the exergy process, which suggested that all water production during a gas withdrawal cycle could be utilised or converted into hydrogen. Our current energy and climate path requires a reduction in greenhouse gas emissions. However, we cannot predict in advance what innovations will be available when we need solutions. Our research emphasises the crucial role of integrating innovative technologies into existing energy processes to achieve a transition towards a more sustainable and cleaner energy system. In this context, it is essential to advance techno-economic assessments.