Potential and Evaluation of Heat Sources

Ambient air is an inexhaustible and universally available heat source, often used for smaller and decentralized heat pumps. It offers the advantage of comparatively low investment costs and is easily accessible. However, this heat source is subject to strong seasonal temperature fluctuations, which can pose challenges, especially in the cold season. To ensure efficient operation during these phases, adapted operating modes are often required, such as return flow elevation. For large heat pumps, larger air volumes must be moved, which imposes special requirements on fans and noise protection measures. Another aspect is the possible icing of heat exchangers, which occasionally requires defrosting processes.

Near-surface geothermal energy up to a depth of 400 meters has the potential to cover up to 75% of the heat demand for space heating and hot water in Germany. The technology utilizes the year-round stable temperatures of the ground, requiring mining rights and water law permits depending on the type of use. Investment costs vary depending on the development: shallow earth collectors are more cost-effective, while deeper drilling for closed or open systems incurs higher costs but operates more efficiently.

Intermediate and deep geothermal energy, from drilling depths of about 600–800 meters, offers the potential for year-round high source temperatures and significant heat quantities. The temperature increases with depth by about 3 Kelvin per 100 meters, and regions like the South German Molasse Basin or the Upper Rhine Graben have high potentials. However, developing these geothermal sources requires large drilling equipment, leading to higher investment costs. The use is also subject to mining and water law and carries a high discovery risk, requiring elaborate hydraulic and seismic analyses. Due to these challenges, developing deep geothermal energy is costly and complex, with state support or private insurance models becoming increasingly relevant for risk mitigation. In Germany, deep geothermal energy is still little researched, so current studies aim to improve data availability and develop funding instruments.

Mine water represents a valuable heat source but is only locally available. In the Ruhr area, where large areas of the underground are traversed by former mines, coal mine water is permanently pumped to prevent ground subsidence and flooding. This permanent water retention offers great waste heat potential, with an average source temperature of about 35 °C.

Waterbody thermal uses environmental heat from surface waters such as lakes, rivers, and seas, requiring water law permits. Each type of water body offers different potentials: Lakes often have stronger seasonal temperature fluctuations. Rivers, on the other hand, are characterized by their flow and constant water movements, making them less prone to overheating. Seas offer smaller temperature fluctuations. However, extended requirements for components such as heat exchangers apply. Technologically, the biology of the waters and the use of safe refrigerants also pose challenges.

Wastewater and sewage treatment plants offer a potentially nationwide available heat source with stable temperatures of 17–20 °C in summer and 10–12 °C in winter. As a result, they are more efficient as a heat source, especially in winter, than air or water bodies. Theoretically, around 31 TWh of environmental heat could be gained annually in Germany with a temperature withdrawal of 3 Kelvin, although actual use is lower due to restrictions on technical accessibility. A reduction of sewage or wastewater by 4 Kelvin does not impair the function of sewage treatment plants according to studies. Challenges exist in fluctuating water flows and biological deposits, as well as strict clean water protection requirements that can be addressed through technical solutions such as double-walled heat exchangers.

Industrial waste heat can occur in various forms, such as cooling water, waste heat from compressed air generation, or flue gas waste heat during combustion processes. Depending on the process, the temperature level varies, often above district heating level, allowing for direct heat recovery via heat exchanger systems. With the use of large or high-temperature heat pumps, there is considerable potential for the technical use of industrial waste heat. Especially at higher temperatures, this waste heat should primarily be fed back into industrial processes to maximize energy efficiency. Utilization for process steam preparation at temperatures above 200 °C is also possible. Only if recovery is not feasible should the remaining waste heat be fed into district heating networks. Technical development requires special systems for storage and transmission to reduce losses, avoid possible contamination, and balance temporal fluctuations between source and sink. Integration into a district heating network requires close coordination with the industrial company and district heating network operators. Industrial companies often have shorter investment horizons than district heating network operators, complicating planning. Changes in the industry could also lead to an early reduction in the amount of waste heat, making collaboration between the parties even more important.

Data centers are gaining importance as a waste heat source due to ongoing digitalization. Despite efficiency improvements in power demand, an increase in this sector's energy consumption is expected in Germany. Since the electrical energy used is converted into heat, there is considerable potential for use by large heat pumps. The construction of data centers close to the heat sink is a prerequisite. Under optimal conditions, up to 70% of the waste heat could be utilized. The temperature of the waste heat depends on the cooling concept: in air-cooled data centers, it is about 30 °C, while liquid-cooled systems generate temperatures up to 60 °C.