Maintaining hydrothermal compliance at TVA's nuclear and fossil plants

The Tennessee Valley Authority (TVA) currently operates three nuclear plants and eleven fossil plants, all of which rely on nearby rivers to dissipate waste heat. To avoid permit violations of the National Pollutant Discharge Elimination System (NPDES), TVA is required to curtail generation at a given facility if the temperature of the plant discharge exceeds allowable limits. To minimize the threat of NPDES violations and, consequently, reduce generation losses, TVA has developed a number of innovative techniques to monitor, forecast, optimize, and control instream water temperature. This paper highlights those for the Widows Creek Fossil Plant. The Widows Creek Fossil Plant (WCF) is located on the northern shore of the Tennessee River, 58.9 miles upstream of Guntersville Dam and 17.2 miles downstream of Nickajack Dam. Hydroplants located at the upstream and downstream dams are operated in the summer to provide generation solely for peaking. This operation, however, creates reverse flow in the river causing heated water from the WCF discharge to migrate upstream and enter the plant intake. This recirculation occurs within several hours after cessation of operations at the upstream and downstream hydroplants. To avoid NPDES temperature violations, generation curtailment occasionally is necessary at WCF during these events. For example, in August 1995, excess water temperatures caused a loss of about 230 unit-hours of generation, valued at over $250,000. To minimize reverse flow at WCG, alternative generation schedules were simulated for the upstream and downstream hydroplants. This was accomplished using a water quality model. The results show that the hydro releases could be effectively altered to reduce the peak water temperature, and consequently, the amount of generation curtailment at WCF. In all cases, however, modifying the hydroplant schedule reduces the available capacity to provide peaking