# Sustainable Energy Solutions: Utilising UGS for Hydrogen Production by Electrolysis
## 1 Introduction  
Recognising climate change and environmental degradation as major threats to global society, the European Commission has introduced the EU Green Deal [1] as a comprehensive plan intended to make the EU's economy sustainable. It encompasses various policies and initiatives to reduce carbon dioxide (CO\({}_{2}\)) emissions by at least 55% compared with 1990 levels and aims to achieve climate neutrality by 2050, promote clean energy, and preserve biodiversity. The circular economy is one of the key components of the EU Green Deal. In a circular economy approach, there is a focus on maximising resource efficiency and minimising material inputs, with waste-to-energy (WE) projects as examples of best practices [2, 3, 4]. The need to create a sustainable society through clean energy and a circular economy is recognised in the Sustainable Development Goals (SDGs), which were adopted by all member states of the United Nations in 2015. These goals aim to eradicate poverty, reduce inequality, and promote peaceful and prosperous societies by 2030. Goal 7 (SDG 7) in particular provides for universal access to clean, affordable, reliable, and modern energy for all. At the same time, SDG 12 prescribes sustainable consumption and production patterns that are an integral part of the principles of a circular economy [5]. As one of the main objectives of integrated climate-energy policy is to increase the production of electricity from renewable energy sources (RES) [6], the share of renewables in the total electricity supply has increased significantly over the last ten years thanks to various incentives. Nevertheless, the REpowerEU plan envisages that the share of electricity generation from renewable energy sources should increase to 72% by 2030, compared with 44% in 2023 [7]. However, in some countries, the overproduction of electricity from renewable energy sources has meant that the electricity generated cannot be fed into the grid due to the limitations of the grid and the impossibility of storing the energy generated. One of the options being considered to solve this problem is the so-called power-to-gas (P2G) technology. In principle, P2G involves the conversion of electrical energy into gases, hydrogen, or methane [8; 9; 10]. The advantage of this technology is that large amounts of energy can be transported more flexibly and easily, and there are fewer limiting factors (such as the problem of energy storage). This technology is favoured by the fact that the capacity of gas pipelines and gas storage facilities is much higher than the capacity of electricity transmission lines, whose lack of capacity limits the generation of electricity from renewable energy sources [11; 12; 13; 14]. Serving as a feedstock, fuel, or energy carrier and storage medium, it is believed that hydrogen (H\({}_{2}\)) will play an important role in future integrated energy systems. Currently, it participates only with a small fraction of the global and EU-wide energy mix, primarily derived from fossil fuels, leading to the release of 70 to 100 million tonnes of CO\({}_{2}\) annually in the EU. However, to achieve climate neutrality, energy production must be decarbonised so that the share of hydrogen in the energy mix is 13-14% by 2050. This objective is feasible if at least 6 GW of renewable hydrogen electrolysers are installed by 2024 and 40 GW by 2030 [15]. Several strategic documents contribute strongly to the objectives of the European Green Deal. However, the Hydrogen Strategy for a Climate Neutral Europe (COM/2020/301) [14] and the Strategy for the Energy Systems Integration (COM/2020/299) [15] are crucial for the hydrogen economy. The integration of energy systems is about the connection between different energy sources and the end-use sector, which makes it possible to optimise the entire energy system. While today's energy systems involve linear flows with a lot of wasted energy, such future systems would allow flows between users and producers, resulting in less wasted energy.