Thermal flow simulation for spatial energy planning with seawater

Lakes and other surface waters represent an enormous natural energy reserve that can be utilized both ecologically and economically efficiently. By using lake water heat pumps, this stored thermal energy can be harnessed for the heating and cooling supply of buildings, thereby contributing significantly to the decarbonization of municipal heat supply. Thermal flow simulations form an indispensable basis for spatial energy planning and enable a well-founded assessment of the framework conditions and requirements for sustainable thermal utilization of lake water.

Flow simulation as a planning tool for authorities, planners, and energy suppliers

A significant advantage of thermal flow simulations lies in their communicative function: they make the expected thermal and fluid mechanical conditions in bodies of water clearly and understandably visible to all stakeholders – authorities, planners, and energy suppliers. This considerably simplifies water law approval processes and creates a shared planning basis on which well-founded decisions can be made regarding locations, performance classes, and operating concepts for the use of lake water.

3D modeling through depth map digitization

The basis of any thermal flow simulation is a precise digital terrain model of the water body. For this purpose, existing depth maps are digitized and transferred into a three-dimensional computational model that accurately depicts the real basin geometry with its depths and shorelines. The more accurately this 3D model reflects the actual bathymetric conditions, the more reliable and meaningful the results of the CFD simulation will be.

Simulation of temperature stratification in heating and cooling operation

The flow simulation realistically depicts the temperature stratification in the body of water, considering both intake and discharge pipelines for heating and cooling operations. In natural waters, solar radiation, wind mixing, and seasonal influences create a characteristic thermal stratification – the so-called thermocline – which significantly influences temperature availability and flow behavior. The simulation quantifies these complex interactions between heat input/output and natural stratification, thus providing reliable planning data for the design of seawater heat pump systems.

Temporal resolution of seasonal temperature fluctuations

The time-resolved observation of temperature fluctuations in the body of water allows for a realistic assessment of the seasonal performance availability of the seawater system over a complete annual cycle. Temperature profiles and their dependence on weather influences, usage intensity, and operating regimes can thus be analyzed in detail and used for system design.

Optimization of the supply and return geometry to avoid short-circuit flows

By systematically varying the supply and return geometries—that is, the number, arrangement, and orientation of the intake and outfall structures—the respective thermal influence zones in the water body are determined. In this way, hydraulic and thermal short-circuit flows can be reliably avoided, the efficiency of seawater utilization can be maximized, and potential ecological impacts on the water body can be reduced to a minimum.

As part of an innovative energy concept, a harbor basin is being used as a natural heat source and sink for the cooling and heating operations of surrounding office buildings. Surface waters such as harbor basins, lakes, or rivers are ideally suited as the basis for water-based heat pump systems due to their thermal storage capacity and can achieve significant energy savings compared to conventional air conditioning systems.

Thermal simulations were carried out to assess the thermal impacts on the water body, quantifying the temperature influence of heat withdrawal and heat input on the harbor basin. Such simulations are essential to ensure that the water temperature remains within ecologically and legally permissible limits and that no undesired thermal stratification occurs.

Particular attention was paid to the hydraulic design of the supply and discharge lines at the quay wall. Thermal or hydraulic short circuits—that is, the direct return of already tempered water to the intake point—would significantly reduce the efficiency of the system. Through careful positioning and flow engineering of the inlet and outlet structures, this effect can be reliably prevented.

To avoid excessively high flow velocities in the harbor basin, the number of inlets and outlets was varied and optimized. High local flow velocities can cause sediment resuspension, negatively impact aquatic life, and lead to increased wear on technical equipment. Distributing the volumetric flow rate across multiple discharge points reduces these risks and ensures more uniform flow through the basin.

The intake boxes were specifically sized to prevent the entry of fish and other aquatic organisms into the pipeline system. In practice, fine-mesh grates, screens, or special protective grids are used, with flow velocities kept so low that fish are not sucked in. Corresponding limit values for inflow velocity are enshrined in water law regulations and environmental requirements.

Finally, the biological compatibility of the entire system was investigated. Possible impacts on the harbor basin ecosystem were assessed, particularly with regard to temperature changes, altered oxygen levels, and the introduction of non-native organisms. Environmentally compatible planning ensures that the operation of the facility is in accordance with water law approval requirements and the objectives of the European Water Framework Directive.