## 4 Discussion  
The solution presented in this article, consisting of storing heat in high-temperature sand storage tanks located in underground mining excavations, has many advantages and can help reduce problems related to power balancing in power grids and also improve air quality and reduce greenhouse gas emissions. Large post-mining areas located in the vicinity of the centers of Silesian cities, or in close proximity to residential estates after the mine is closed down, are an excellent area for implementing such an investment. On the one hand, post-industrial areas will be used effectively, on the other hand, we can improve the quality of life of residents. This improvement can occur in several aspects. The first is the already mentioned improvement in air quality resulting from the reduction in the use of solid fuels for heating apartments and public buildings. The second aspect is the economic aspect; by storing thermal energy from renewable energy sources, residents can have long-term access to clean and relatively cheap energy for heating their apartments and houses in  
Figure 14: Mode of discharging the heat storage [own source].  
Figure 15: Diagram of the system based on Variant 2 of the heat storage facility [own source].  
winter heating periods. Another benefit, this time for the natural environment, is reduced CO\({}_{2}\) emissions resulting from the reduction in or elimination of solid fuel combustion, but also related to the cessation of the mine operation process and coal extraction. Another aspect is the possibility of obtaining waste heat from nearby industrial plants, which may have a positive impact on their economic results. However, it should be noted that the use of the presented technology is also associated with certain limitations and its implementation may encounter certain problems. The basic limitation that may be encountered in the case of the desire to implement the above solution is the issue of land-ownership and the development plan. When planning such an investment, it is advisable for the land to belong to local government authorities who intend to introduce an ecological heating system using the presented technology. Development plans for the selected post-mining area must also be submitted to the appropriate institutions developing the plan for the liquidation and reclamation of mining plants in advance. Otherwise, changing the plan and obtaining the necessary consents may be associated with a long and tedious formal procedure that does not guarantee success. Another aspect is the need to maintain underground workings (or at least a part of them) and the shaft. In the case of mining plants, where for safety reasons, e.g., pumping water, it is planned to maintain a certain number of underground corridors, the matter is relatively easier and the costs of maintaining the infrastructure are planned. The matter is much more difficult in the case of the lack of plans to maintain underground workings and the possibility of reaching these corridors. It is estimated that the costs of maintaining the workings will constitute the second, after the investment costs, component of the general costs. Of course, the investment costs will not be small, however, in this aspect the investor can apply for various types of targeted subsidies. To sum up, the technology of storing heat in high-temperature underground heat storage tanks proposed by the authors is undoubtedly a new and unprecedented way of using former mine workings. It can be an alternative to currently used heating methods. However, its implementation and implementation still requires further work related to adapting the mine and municipal infrastructure to use the new heating method.