Waste Heat Dissipation and Effluent Water Temperatures from Black Dog Lake

Northern States Power Company's Black Dog Power Generating Plant discharges its condenser cooling water into a pond called Black Dog Lake, which releases it through two outlets into the Minnesota River. In this study it was found that an average 63 percent of the heat load rejected by the plant was dissipated to the atmosphere by the lake during a period from March 1974 through January 1975. Daily heat dissipation varied from 15 to 128 percent because of temporary heat storage in the lake (lag) under transient weather conditions. Daily lake effluent temperature fluctuations were determined and compared to Minnesota River temperature fluctuations. The lake very effectively reduces the impact of thermal discharges on the Minnesota River. Its effectiveness was found to vary with season, with a maximum in midsummer and a minimum in midwinter, presumably due to sinking plumes. A mathematical model of the unsteady well-mixed type predicts summer effluent water temperatures well, but is not applicable for winter conditions.