Krakow has an extensive district heating network, which is approximately 900 km long. It is the second largest city in terms of the number of inhabitants in Poland, resulting in a high demand for energy—for both heating and cooling. The district heating of the city is based on coal. The paper presents the conception of using the available renewable sources to integrate them into the city’s heating system, increasing the flexibility of the system and its decentralization. An innovative solution of the use of hybrid, modular heat pumps with power dependent on the needs of customers in a given location and combining them with geothermal waters and photovoltaics is presented. The potential of deep geothermal waters is based on two reservoirs built of carbonate rocks, namely Devonian and Upper Jurassic, which mainly consist of dolomite and limestone. The theoretical potential of water intake equal to the nominal heating capacity of a geothermal installation is estimated at 3.3 and 2.0 MW, respectively. Shallow geothermal energy potential varies within the city, reflecting the complex geological structure of the city. Apart from typical borehole heat exchangers (BHEs), the shallower water levels may represent a significant potential source for both heating and cooling by means of water heat pumps. For the heating network, it has been proposed to use modular heat pumps with hybrid sources, which will allow for the flexible development of the network in places previously unavailable or unprofitable. In the case of balancing production and demand, a photovoltaic installation can be an effective and sufficient source of electricity that will cover the annual electricity demand generated by the heat pump installation, when it is used for both heating and cooling. The alternating demand of facilities for heating and cooling energy, caused by changes in the seasons, suggests potential for using seasonal cold and heat storage.

Ten geothermal bathing and recreation centres have been established in Poland from 2006 to 2013. The largest number of geothermal bathing and recreation centres is located in the Podhale region and in the Polish Lowlands. These structural units have the best geothermal conditions. The maximum reservoir’s temperatures in the main aquifer of the Podhale Geothermal System can reach ca. 120–128 °C in the deeper parts of the system. The most prospective and exploited aquifers occur within the Middle Triassic formations and in overlaying Middle Eocene formations at the depth of 1,000–3,700 m. At present, five operating centres are located in the Podhale region. They use geothermal waters with the temperature of 27–82 °C (the lowest and the highest wellhead temperature of water from wells which are connected for recreational purposes) and total dissolved solids (TDS) of 0.4–3 g/dm³. In the Polish Lowlands, the reservoir’s temperatures vary from 20 to 130 °C at depths of 1–4 km. The most prospective geothermal aquifers of the Polish Lowlands occur within the Lower Cretaceous and the Lower Jurassic formations. Five geothermal recreation centres are currently operating. The centres use waters with the temperature of 38–72 °C and a wide range TDS from 0.4 to 79 g/dm³. Geothermal waters of the Polish Lowlands are generally high in Na and Cl concentrations. The new centres will use the waters of Lower Jurassic aquifers, the outflow temperatures of which vary from 27 to 82 °C and a TDS from 6 to 140 g/dm³.

Model simulation allows to present the time-varying temperature distribution of the ground source for heat pumps. A system of 25 double U-shape borehole heat exchangers (BHEs) in long-term operation and three scenarios were created. In these scenarios, the difference between balanced and non-balanced energy load was considered as well as the influence of the hydrogeological factors on the temperature of the ground source. The aim of the study was to compare different thermal regimes of BHEs operation and examine the influence of small-scale and short-time thermal energy storage on ground source thermal balance. To present the performance of the system according to geological and hydrogeological factors, a Feflow^® software (MIKE Powered by DHI Software) was used. The temperature for the scenarios was visualized after 10 and 30 years of the system’s operation. In this paper, a case is presented in which waste thermal energy from space cooling applications during summer months was used to upgrade thermal performance of the ground (geothermal) source of a heat pump. The study shows differences in the temperature in the ground around different Borehole Heat Exchangers. The cold plume from the not-balanced energy scenario is the most developed and might influence the future installations in the vicinity. Moreover, seasonal storage can partially overcome the negative influence of the travel of a cold plume. The most exposed to freezing were BHEs located in the core of the cold plumes. Moreover, the influence of the groundwater flow on the thermal recovery of the several BHEs is visible. The proper energy load of the geothermal source heat pump installation is crucial and it can benefit from small-scale storage. After 30 years of operation, the minimum average temperature at 50 m depth in the system with waste heat from space cooling was 2.1 °C higher than in the system without storage and 1.6 °C higher than in the layered model in which storage was not applied.

The geothermal energy sector continues to develop dynamically in the world. Within this work, preliminary feasibilities of different geothermal applications were presented for the Lower Cretaceous formations for wider geothermal uses’ development in the Mogilno–Lodz trough, Central Poland. The Lower Cretaceous formations are one of the most prospective geothermal reservoirs in Central Poland. Geothermal waters that occur within them are mostly of Na–Cl type and contain specific elements like I, Br or Fe. These are low-temperature geothermal resources together with highly variable mineralization, from 0.4 to even more than 90 g/L. A simple estimation procedure for hydrogeological information, such as water and thermal conditions of the reservoir obtained from an archival chemical analysis and borehole temperature loggings was developed. The potential for geothermal applications was estimated for three zones of the Mogilno–Lodz trough. Possibilities of conjunctive geothermal applications, such as space heating, balneotherapy and balneorecreation, mineral extraction, electricity production in binary systems and aquifer thermal energy storage were examined for the Lower Cretaceous formations.