Environmental & Water Quality Operational Studies: Improvement of Hydropower Release Dissolved Oxygen with Turbine Venting

摘要

This report summarizes various in-lake, in-structure, and downstream techniques to enhance the dissolved oxygen concentration of hydropower releases. In-lake and in-structure techniques appear to be the most applicable for Corps of Engineers projects because of the large discharges of most hydropower projects. Of these, the in-structure techniques, particularly turbine venting, appear very attractive considering cost and degree of improvement. Tests were conducted at the Clarks Hill Dam powerhouse to evaluate various aspects of turbine venting and thereby provide a data base to develop predictive and design capabilities for turbine venting systems. Test results indicated that, at most, the oxygen deficit in the penstock could be reduced by about 30%, i.e., if the penstock oxygen deficit is 8.0 mg/(l), then, at most, about 2.4 mg/l of oxygen could be absorbed into the release flow. Two reaeration processes that contributed to the overall oxygen transfer were observed: (a) due to the turbulence in the tailrace area and (b) due to the air bubbles (vented through the turbine) as they traveled through the draft tube. A numerical computer model was developed that included the impact of the changes in thermodynamic (pressure) state as the bubbles passed through the draft tube. By developing the 'pressure time history', the increased potential for oxygen transfer due to the increase in hydrostatic pressure was included in model formulation. Good agreement was obtained in comparing model predictions and oxygen uptake data from previous tests at Clarks Hill.