Assessment of resources and environmental conditions for the implementation of low-temperature (low-enthalpy) geothermal energy as a source of carbon-free energy is one of the tasks carried out by the Polish Geological Institute - National Research Institute (the Polish Geological Survey). These activities are being implemented to assess the temperature regimes in the context of the local geology, tectonics, hydrogeology and anthropogenic influence, providing reference data for the development of the low-temperature geothermal potential maps and supporting decarbonization of the energy sector. In 2021 five boreholes were drilled at diverse locations across the country to take into account different natural conditions. Each borehole has a heat exchanger with a single U-pipe installed. In 2021, the test temperature measurements and TRT tests were conducted, while in 2022-2023 regular measurements were carried out at quarterly intervals for the individual year seasons. Results for the most shallow part of the subsurface, up to about 2-5 meters deep, indicate a strong relationship between its temperature and the climatic and weather conditions. Further on, this subsurface temperature zone is referred as to a zone of daily and seasonal temperature changes. Below subsurface temperature variability gradually decreases with depth and is less dependent on external factors. At the depth of usually 15-25 meters, depending on location, temperature stabilizes and is close to the value of average ambient air temperature at the given location. This subsurface temperature zone, called the zone of neutral or transient temperatures, can persist down to about 50-60 meters of depth or even more. Deeper down the subsurface temperature starts to increase according to the value of the geothermal gradient. Subsurface temperature values derived from research presented in this paper depend also to some extent on various geo- and anthropogenic factors, such as thermal properties of the rocks, e.g. thermal conductivity, presence of aquifers, climatic anomalies and presence of subsurface infrastructure.

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