Editor’s Note: This article was originally published on HydroWorld.com sister site GenerationHub.com.
Loxbridge Partners LLC applied April 23 to the Federal Energy Regulatory Commission for a preliminary permit, which would allow it three years of exclusivity to study the feasibility of the roughly 700-MW McNary Second Powerhouse project in Oregon.
This application is made in order that the applicant may secure and maintain priority of application for a license for the project while obtaining the data and performing the acts required to determine the feasibility of the project and to support an application for a license.
The project will be located at a U.S. Army Corps of Engineers facility at the McNary Dam located at river mile 292 of the Columbia River in Umatilla County, Oregon. The existing dam is 7,365 feet long, rising about 183 feet above the streambed. It consists of a concrete structure with an earthfill embankment at the Oregon (south) abutment.
The existing powerhouse at the dam has fourteen 70,000-kW hydroelectric generator units – a 980-MW total powerhouse capacity. All 14 turbines in the powerhouse were operating by February 1957. The hydraulic capacity of McNary Dam, the amount of water that passes through the existing powerhouse turbines, is substantially less than that of any other project on the lower Columbia River. Congress authorized a second powerhouse in 1986, but it was deauthorized later due to a lack of economic feasibility in 1991. McNary remains a hydraulic constraint on the river.
The proposed project will be about 1,140 feet long and from upstream to downstream it will be about 256 feet wide. Excavation for the draft tubes will extend down to elevation 168 mean sea level (MSL). About nine million cubic yards of excavation will be required downstream of the powerplant to create the tailrace area. The originally planned second powerhouse envisioned ten turbine units that will each pass 25,000 cubic feet per second (cfs) of water will drive generators with continuous over-load ratings of 100 MW each.
This proposed project will be sized according to studies to be carried out in the future but will likely be fewer than 10 turbines and likely with a total capacity of less than 1,000 MW. An estimated 700 MW capacity is used for this preliminary permit even though the capacity may be any amount up to 1,000 MW.
Much of the supporting mechanical equipment will be independent of the existing powerhouse. It is proposed that the existing station service generators be permanently removed from service to facilitate cofferdam construction. The alignment of the powerhouse is to be determined after future study. It could be “in-line”, that is, the new generators could be installed directly in line with the existing 14-unit powerhouse. The second powerhouse will be placed in the area now occupied by the south abutment, which is a rock-fill, impervious core embankment adjacent to the existing powerhouse. It could also be possible that the new powerhouse be placed perpendicular to the dam.
No new dams or spillways are proposed for this project but the earthfill embankment at the Oregon (south) abutment will be removed/modified/relocated to make room for the additional powerhouse.
Electrical interconnection options will be studied. The construction of one to two transmission lines of about 1,300 yards from the project site may be required to connect to the McNary switchyard.