A scenario-based, generalised model of the thermal impacts of cooling water discharge from thermoelectric power plants: thermopeaking and spatio-temporal interactions.

Thermoelectric power plants are the backbone of global power production, providing a consistent and reliable electricity supply while offering great flexibility to meet rapidly fluctuating demands. However, the discharge of cooling water from these plants, particularly those employing once-through cooling systems, raises the temperature of receiving rivers, acting as a potentially concerning additional stressor in the context of existing climate change impacts on river temperature. This study employed synthetic boundary conditions, using a UK-typical river as a template, to create a generalised model representing a large, low-gradient, single-threaded river channel to examine the whole-river-scale impacts of cooling water discharge from thermoelectric power plants. A conceptual, process-based model of power plant effluent impacts on stream temperature was developed to simulate stream temperatures at hourly time scales across a series of scenarios with different combinations of effluent discharges, temperatures, and distances between power plants. Results showed that thermal energy from power plants can propagate considerable distances downstream, even if the energy input is low, and cause cumulative thermal impacts when combined with inputs from other downstream plants. Comparative analyses indicate that to minimise the impact of multiple power plants along the river, it is crucial to fully utilise the river's cooling capacity by placing the power plant with the largest thermal input downstream and as far from the upstream plant as possible. This study also found that effluent discharge contributed to greater increases in river temperature than effluent temperature, indicating its potentially greater importance in the management and regulation of power plant discharge. Power plant operation mode and climate change were also considered by adjusting operation schedules and climate variables. Thermal effluent from single-day operations of a peaking plant caused additional short-term temperature fluctuations. With sufficiently large heat inputs, these effluents interacted across space and time, leading to cumulative impacts that extended both the duration and length of the river affected by temperature increases. Climate change, as a gradual and long-term process, caused only minor increases in river temperature in comparison to power plant effluent, but exacerbated the impact of this effluent. These findings emphasise the importance of strategic placement and discharge management of power plants to mitigate thermal pollution and safeguard riverine ecosystems.