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Tacoma Power Rebuilds 1968 Machines Inside Washington’s Tallest Dam

By Dian McClurg
The Chronicle

MOSSYROCK — What weighs 62½ tons, is 16 feet in diameter and will soon help generate 1.9 billion kilowatt-hours of energy each year? If you said a new turbine at the Mossyrock Dam, you’re correct.

One such re-machined turbine was delivered to the dam on U.S. Highway 12 early Saturday morning.

“It’s pretty impressive,” said Randy Stearnes, community relations officer for Tacoma Public Utilities. “This is something you only do every 40 years or so, so it’s something I probably won’t see again in my professional lifetime.”

This is the first upgrade to the power generators at the Mossyrock Dam that Tacoma Power has done since construction of the hydroelectric dam in 1968.

Saturday’s delivery marks the beginning of work on the second of two generators. The first generator was rebuilt in 2008.

Dean McLeod, project manager, estimates that the second generator will be operational in October. They learned last time that the process takes six to nine months from start to finish which means that between now and this fall, the dam will run on just its one rebuilt generator.

The project is costing Tacoma Power $50 million. All the major power-producing parts inside the dam’s power house at 439 Onion Rock Lane are being updated and rebuilt.

Mossyrock Dam, which forms the 23.5 mile long Riffe Lake, is one of the two dams owned by Tacoma Power on the Cowlitz River. West of Mossyrock is Mayfield Dam, completed in 1963, which forms Mayfield Lake.

Together, these two dams create enough hydroelectric power to serve more than 136,000 Northwest homes, according to Tacoma Power.

The generators, running typically twice per day during peak use hours in the morning and evening, consist of three major components including magnetic rotors spun by the turbines, which are turned by the water rushing through them.

These spinning magnets create an electromagnet field that charges the wires inside a stator, and it’s this energy that is converted to power and transmitted to various transfer stations all over the Northwest.

“We’re simply spinning magnets around wires, and with this we can produce energy that is over 90 percent efficient,” Stearnes said.

Mossyrock Dam has been closed to visitors since 2008, when the rebuild project began. Tacoma Power plans to give its annual dam tours again beginning in the spring of 2011, according to project manager McLeod.

During one Saturday each year an average of 1,000 to 1,200 people make the trip to Mossyrock Dam for this annual tour of Washington’s tallest dam, which beats the Seattle Space Needle, at 605 feet, by just one foot in height.

The Chronicle
Centralia, WA
April 19, 2010

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