# Sustainable Energy Solutions: Utilising UGS for Hydrogen Production by Electrolysis
## 1 Introduction
The water content was determined using standard chemical methods. The hydrogen production rate is given for different values of solar radiation during the year. The study showed that a small plant with solar panels of 10 \(\times\) 10 m\({}^{2}\) could produce a total of 9700 m\({}^{3}\)/year of hydrogen. Furthermore, the efficient operation of electrolysers means not only the provision of renewable electricity but also the utilisation of waste energy. At a time when prices for primary energy are rising, the use of energy-efficient technologies in existing process plants is becoming profitable and relevant. Energy recovery from natural gas transport or distribution networks is a growing area of research. The process of reducing the pressure of natural gas, which is usually achieved by conventional check valves, can be upgraded implementing turbo expander technology, enabling energy recovery from the pressure drop. The use of expansion turbines in the natural gas transport system was analysed by more authors [32; 33; 34]. The exergy potential available in the LNG transport chain was analysed by [PERSON] et al. [35] and by [PERSON] [36] and [PERSON] and [PERSON] [37]. Some recent publications provide up-to-date insights into the latest advances, technologies, and methods related to the use of turboexpanders for energy recovery in gas pressure reduction stations (PRSs). Currently, only a few energy recovery systems are installed in PRSs, mostly consisting of radial turboexpanders coupled to combined heat and power combustion engines or gas-fired heaters that provide the necessary preheating [38]. [PERSON] et al. [39] present a novel plant configuration consisting of a two-stage expansion system whose energy performance is analysed using numerical dynamic simulations. By comparing the energy efficiency of PRS with high- and low-temperature configurations, it is shown that the two-stage expansion is more energy efficient and can be effectively integrated with low-enthalpy heat sources. [PERSON] et al. (2022) [40] investigate the potential of energy recovery in the gas extension pipeline, through which energy waste can be effectively utilised to supply green energy to the city of New Al-Alamein in Egypt. Static and dynamic simulations of energy recovery from natural gas pressure reduction stations were carried out by [PERSON] and [PERSON] [41] to investigate the possibilities of using the electricity generated to meet the needs of the reduction and measurement stations. Analyses were done regarding the possibility of reselling surplus electricity. An increase in the electricity purchase price and a reduction in the natural gas purchase price may help to make the investment worthwhile in the future [41]. The pressure-related potential that is considered exploitable can be described with the concept of \"exergy\", i.e., the maximum work that can be recovered from the system considering the environmental conditions. This paper focuses primarily on the utilisation of exergy in the conventional gas reduction station of the underground gas storage (UGS) to produce electricity and ultimately hydrogen through the electrolysis system and using the produced formation water. The use of the turboexpander (TE) to generate electricity and hydrogen via the electrolysis system was analysed in a simulation case study. A mathematical model was created in industrial process software, and the dependencies of the process parameters were simulated for the UGS gas reduction station. This research uses a mathematical model that integrates several commercial software packages, each specialising in different parts of the system, to predict the dynamic response of the entire gas storage system. This approach, which is commonly used in oil and gas production optimisation, enables the analysis of the impact of gas storage parameters on operating characteristics under different pressure conditions. The model was developed and calibrated based on actual field production and hydrodynamic data to ensure its applicability in different scenarios.