###### Abstract  
In Europe, renewable energy sources such as photovoltaic panels and wind power plants are developing dynamically. The growth of renewable energy is driven by rising energy prices, greenhouse gas emission restrictions, the European Union's Green Deal policy, and decarbonization efforts. Photovoltaic farms generate energy intermittently, depending on weather conditions. Given the increasing number of new installations, ensuring the power balance and transmission capacity of the electrical grid has become a major challenge. To address this issue, the authors propose a technical solution that allows the energy generated by photovoltaic systems to be stored in the form of heat. Thermal energy from solar power and wind energy offers significant potential for energy storage. It can be accumulated during summer in specially designed sand-based heat storage systems and then used for heating purposes in winter. This approach not only reduces heating costs but also decreases greenhouse gas emissions and helps balance the power grid during sunny periods. Post-industrial areas, often located near city centers, are suitable locations for large-scale heat storage facilities supplying, among others, public utility buildings. Therefore, this article presents a concept for utilizing high-temperature sand-based heat storage systems built in decommissioned underground mining excavations. m +
Footnote †: journal: Applied sciences  
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capacity of PV systems, which reached 12.42 GW at the end of 2022, 17.08 GW at the end of 2023, 17.73 GW at the end of the first quarter of 2024, and 21.2 GW by the end of December 2024 [7; 8; 9]. The rapidly growing number of photovoltaic (PV) installations faces a significant challenge in balancing energy surpluses. This issue is linked to the temporary restriction of electricity production during periods of energy surplus. Initially, such situations occurred only on weekends and holidays, but they are now increasingly happening on weekdays as well. For example, in May 2024, the electricity transmission system operator, PSE, ordered photovoltaic farms to reduce production for 14 days to balance the national power system. Another issue related to the inability to balance PV-generated energy is the high percentage of refusals for connecting new PV installations to the grid. In 2023 alone, connection requests for up to 83.6 GW of PV capacity were denied [10; 11]. Another major issue in Poland is poor air quality. Air pollution, particularly the frequent exceedance--sometimes by several hundred percent--of the permissible levels of PM10 and PM2.5 particulate matter, is most common during the winter (heating) season. One of the main causes of smog in many areas is so-called low emissions, which result from the burning of solid fuels (often low-quality) by private individuals for heating purposes. This phenomenon negatively affects both the quality of life and the natural environment. In search of a solution to both issues--the challenge of balancing electricity generation and air pollution--the authors propose an innovative system that utilizes photovoltaic farms and other renewable energy sources, such as wind turbines, to produce heat, which can be stored in inactive mining excavations. This approach helps mitigate the problem of surplus electricity, particularly on sunny summer days, while also reducing air pollution and the emission of harmful gases, including greenhouse gases, during winter when the stored heat is used [6]. Waste heat from industrial plants can also be used for heat storage. The presented idea concerns seasonal thermal energy storage. There are also short-term thermal energy storage facilities. They are used to optimize and equalize the daily load, e.g., in thermal power plants [12; 13; 14; 15].