Three-dimensional hydrodynamic modelling in a bay of the Lake Mälaren to assess environmental impacts from a cooling and heating power plant production

The power production company Norrenergi owns a power plant in Solna, Stockholm county, Sweden, to produce heating or cooling energy by exploiting intake water’s temperature. In its current configuration, the power plant is connected to the network of the Bromma sewage treatment plant, located in the northern part of Stockholm and uses this water for production. The planned commissioning of a new sewage treatment plant in Henriksdal in the southern part of the city, thought to replace the Bromma sewage treatment plant, will lead the current sewage network used by Norrenergi’s power plant to be decommissioned. In order to maintain water supply for its power plant, Norrenergi is now seeking environmental permit to use water from the Bällstaviken bay in Solna where the old intake and outlet structures exist. This article presents the hydrodynamical modelling performed as part of the environmental permit application. The work has focused on describing the thermal dispersion conditions for an extreme energy demand in the summer (cooling production), where the released water temperature is increased between 5.5 and 10.0 °C during a daily cycle. Hydrodynamic modelling has been performed with the software TELEMAC-3D version 7.3. The first part of the modelling work has been to perform flow measurements in order to calibrate the model with respect to currents. Some preliminary dispersion tests showed that in their current configuration, the current intake and outlet structures, both located in the Bällstaviken bay, do not offer optimal production capacity due to a very high risk for a temperature shortcut. To mitigate this risk, a new location for the inlet has been investigated. A location in the adjacent Ulvsundasjön bay has been chosen which offers the possibility to take water from a greater depth with a cooler temperature especially during summer months, making it possible to reduce the relative temperature difference between the intake and outlet depths, hence lowering the environmental impacts. The thermal dispersion modelling has been performed for this intake configuration for several representative wind speeds and directions. The results show that the water temperature in the Bällstaviken bay is significantly increased due to its relatively small volume whereas impacts are, as expected, lower in the adjacent Ulvsundasjön bay which has a greater volume. The results also show a clear influence of wind direction on thermal dispersion conditions. The environmental risk assessment shows that the planned production is not expected to significantly alter the flora and fauna in the study area mainly due its current low ecological status