Evaluating Forced Evaporation with Comparisons to EPRI and USGS Approaches (AL, USA)

Electricity production accounts for one of the largest water usages in the United States and worldwide. Water for thermoelectric power is used in generating electricity with steam-driven turbine generators. Surface water was the source for more than 99 percent of total thermoelectric-power withdrawals according to USGS surveys. Forced evaporation (FE) in a receiving water body due to added heat from once-through cooling for thermal electric generating plants has been studied by Electric Power Research Institute and the USGS. However, these approaches are for screening level analyses and/or analytical solutions which do not address site specific FE. DSI supported Alabama Power Company to develop a robust environmental tool to compute site specific FE using the Environmental Fluid Dynamic Code (EFDC). The site specific approach has been compared to the analytical solution approach to validate the tool. The evaporation computed by both approaches were in good agreement. The FE analysis using the EFDC+ tool was then extended to more realistic, though generic, FE analysis of a simplified Lower Mississippi River. The impacts on FE due to dams were evaluated by comparing the two cases, without dam to with dam river domain. Presence of the dam was found to significantly decrease the evaporation rate downstream of the dam compared to the river without the dam. It was also found that there is a significant range of FE estimates depending on the estimation method and parameter selection. The recommendation from the study was FE estimates should be based on a calibrated thermal model, thus providing the most defensible approach.

There are two major types of cooling systems in common use in thermoelectric power plants: recirculating cooling systems and once-through cooling systems. The processes involved in once-through cooling systems are shown below. Once-through cooling systems, initially the most popular and widespread in eastern USA, withdraws the ambient water from the intake canal / river and release warmer water into the discharge canal / river. Once-through cooling systems return the same amount of water to the environment as it is withdrawn. However, the discharged water at a higher temperature results in the evaporation from the receiving water body at a rate greater than background levels. Forced evaporation is the excess evaporation due to increased temperatures in the receiving waters from the discharge of the heated water. The use of the term forced evaporation (FE) generally applies to the consumptive use of once through cooling systems.

Diagram of Once Through Cooling Systems