The extraction and disposal of heat from lakes and rivers is a large yet scarcely exploited source of thermal energy. In Switzerland, this renewable source of heating and cooling would be readily available, as many urban centers are located near lakes and rivers, and the technique is well-established. Its use would help to reduce consumption of fossil fuels and emissions of CO2, and promote local know-how and economy. The aquatic research institute Eawag is conducting a project funded by the Federal Office for the Environment on this theme.
Using waterbodies as heat sources and sinks implies the discharge of thermally-altered water into aquatic systems (thermal discharge). As part of this project, the possible physical, chemical and ecological impacts of thermal discharge in lakes and rivers have been reviewed. In most cases, it appears that the main impacts remain local (e.g., certain species avoid the discharge area). However, intense thermal discharge could potentially cause large-scale impacts such as disruption of winter mixing (in lakes) or of fish migration (in rivers). Warming in summer is particularly critical, as many ecosystems are already under stress due to climate warming.
Based on simple assumptions, the potentials for heating and cooling from the main lakes and rivers in Switzerland have been estimated (see the potential map). These potentials are remarkably large, often well above the regional demand. The use of this thermal energy is particularly adapted at the scale of a neighborhood, a large company or industrial parks. The sustainable use of this resource requires a well-thought-out energy strategy and good coordination between the different actors.
Currently, mostly small to medium-sized systems are in operation (see the map of installations). However, given judicious management and supportive policies, lakes and rivers could become a significant source of thermal energy in Switzerland in the near future. The main challenges of using this renewable, local and reliable energy on a larger scale will be installing the required infrastructure, ensuring profitability and preventing negative impacts on aquatic systems.