Potential Hydraulic and Power Effects of Hydrokinetic Turbines Deployed Within Hydropower Tailrace Channels

Abstract:

Riverine hydrokinetic energy is a source of significant undeveloped energy potential. Recently, interest has been given to deploying hydrokinetic energy devices, also known as current energy converters (CECs), within tailrace channels downstream of conventional hydropower plants. Deploying CECs within a hydropower tailrace has multiple advantages: higher flows than most river reaches, scheduled and quantifiable releases, proximity to existing structural and electrical infrastructures, and low risk of new environmental impacts. However, beyond the economic and technical challenges facing such deployment, the presence of operating hydrokinetic turbines within the tailrace may introduce unfavorable effects to the upstream hydropower plant production. While extracting energy from flowing water, CECs create significant flow resistance which induces a small but measurable water level increase upstream of their location (in subcritical flows). Depending on the distance between the devices and the upstream hydropower facility, this water level increase can diminish the available hydraulic head and, consequently, hydropower production, thereby offsetting a portion of the benefits of CEC deployment and production. This study, using simplified assumptions, presents a one-dimensional momentum balance approach to analyze the backwater effect and system net power production (hydropower loss vs. hydrokinetic gain) as a function of the tailrace hydraulic conditions and device characteristics. Based on the study presented herein, CEC power extraction applied too close to the upstream dam would likely be outweighed by the corresponding hydropower losses, resulting in a negative net power production. An optimal distance at which revenue from CEC production outweighs lost hydropower production and CEC capital expenditures (i.e., a “sweet spot”) could thus be modeled. In addition, if CEC tailrace applications prove a feasible option, existing hydropower dams could qualify as viable candidates for CEC full-scale testing sites