Hiwassee Dam was converted by engineers at the Tennessee Valley Authority into an enormous battery amid the picturesque mountain terrain of western North Carolina.
Built on the Hiwassee River in 1940, the power plant received a revolutionary pump turbine, Unit 2, in 1956.
When demand for electricity is low, the unit pumps water from the river to the upper reservoir; when demand is high, the reverse action is used to produce 85 MW of electricity.
Pioneering pumped-storage technology in western North Carolina
Hiwassee Dam was built by the Tennessee Valley Authority in 1940 as part of a hydroelectric power generation project and downstream flood control project along the Hiwassee River.
In the mid-1950s, the western North Carolina power station was chosen to experiment with a new energy storage technology.
In 1956, Unit 2 was erected at this site; this installation had the largest reversible pump turbine and motor-generator set for that time.
This revolutionary engineering solution enabled one hydropower unit to serve simultaneously as an electric pump and power generator.
With the help of reversible equipment, a conventional hydropower dam was turned into a multifunctional energy storage system.
Unit 2 increased the generating capacity of the entire power plant to 185 MW.
Operating reversible pump-turbines for grid balancing
In times when electricity usage is at its lowest levels, regional power plants can end up with extra electrical energy produced throughout the transmission system.
This extra electricity from off-peak hours gets used by the Hiwassee Dam to run the Unit 2 motor backwards.
Running the motor in reverse causes the turbine runner to rotate backward, pulling water out of the lower Apalachia Reservoir and forcing it to move uphill.
Pumped storage refills the upper Hiwassee Reservoir, storing vast amounts of gravitational potential energy behind the concrete barrier.
The water gets directed back down through the turbine when there is high consumer electricity demand on scorching summer afternoons or freezing winter mornings.
As a result, electricity production reaches a level of 85 megawatts of generating capacity.
Moving millions of gallons uphill during off-peak hours
Unit 2 functions like a hydraulic pump with a capacity to move large amounts of water against gravity.
At full pumping capacity, the unit can move up to 1.75 million gallons of river water per minute into the upper reservoir, uphill.
Filling the upper reservoir stores mechanical energy in the form of gravitational potential energy, eliminating the need for a chemical battery.
Storage of the water in the reservoir allows for quick production of electricity in cases of power outages and emergencies.
Water is pumped during the nighttime when prices of power are low throughout the Tennessee Valley region.
Pumped storage is a mechanical way of storing energy for decades to come.
Supporting World War II torpedo testing operations
Before installing reversible pump turbines in Hiwassee Reservoir, the location played an important national defense function during World War II.
Hiwassee Reservoir was chosen by the U.S. Navy as one of the best secret locations for testing experimental torpedoes.
High mountains around the lake helped in maintaining confidentiality because of natural physical camouflage, and clean water was useful for underwater observation of torpedoes’ work.
Unarmed test torpedoes were released in Hiwassee Reservoir, and their speed, depth, and performance were tested, as reported by the Tennessee Valley Authority.
Specific boats collected test torpedoes after tests on the surface of the lake and in its deep waters.
Torpedoes were tested in the reservoir because of its importance in naval weapons development.
Evaluating operational limits and long-term energy storage
The Hiwassee Dam is an example showing how mechanical pumped storage allows dependable long-term stability of the regional power grids.
Operation of a single reversible machine requires constant maintenance of water levels both at the upper and lower reservoirs.
Drought periods or high seasonal drawdown could also limit the operation of pumping within the two interconnected lakes.
Also, engineers should perform regular mechanical maintenance to protect large turbine blades from mechanical erosion and cavitation.
In spite of certain environmental restrictions, many years of stable operation allow us to conclude about the longevity of pumped storage facilities.
The example of the Hiwassee Dam shows how old hydroelectric plants are able to adapt to new energy requirements.
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