Tamahara Pumped Storage Power Station

Overview

The Tamahara Pumped Storage Power Station (玉原発電所) is a major hydroelectric facility located in Gunma Prefecture, Japan. It operates as a pumped-storage plant on the tributaries of the Tone River, utilizing the water resources of the Hotchi River and an adjacent tributary. The station plays a critical role in energy management by generating electricity during periods of high demand and pumping water back to the upper reservoir during times of low demand, such as at night. The facility is operational with a total installed capacity of 1,200 MW, commissioned in 1986.

The power station relies on two key dams. The upper reservoir is created by the Tanbara Dam, a rock-fill embankment dam located 14 km north of Numata. Construction of the dam began in 1973 and was completed in 1981. The Tanbara Dam is 116 m tall and withholds a reservoir with a total storage capacity of 14,800,000 m³, of which 13,000,000 m³ is active for power generation. The lower reservoir is formed by the Fujiwara Dam, located 4 km to the northwest on another Tone River tributary. The elevation difference between the upper Tamahara Reservoir at 1,177 m and the lower Fujiwara Reservoir at 651 m provides an effective hydraulic head of 518 m.

The station contains four reversible Francis turbine pump-generators, each with a capacity of 300 MW. These units serve to both pump water and generate electricity. When pumping, the system can move up to 210 m³/s of water, and when generating, they discharge up to 276 m³/s. This configuration allows for efficient energy storage and release, supporting the regional grid's variable demand patterns.

History and Construction

Construction of the Tanbara Dam and the associated Tamahara Pumped Storage Power Station began in 1973, marking the start of a significant infrastructure project in Gunma Prefecture, Japan. The primary structure, Tanbara Dam, is a rock-fill embankment dam designed to impound the headwaters of the Hotchi River, which is a tributary of the Tone River. The dam is located 14 kilometres north of Numata and serves as the upper reservoir for the power station.

The dam construction was completed in 1981, establishing a reservoir with a total storage capacity of 14,800,000 cubic metres. Of this total volume, 13,000,000 cubic metres is designated as active capacity for power generation. The dam stands 116 metres tall and creates the upper Tamahara Reservoir at an elevation of 1,177 metres. This upper reservoir works in conjunction with the lower Fujiwara Reservoir, which is created by the Fujiwara Dam located 4 kilometres to the northwest on another Tone River tributary. The lower reservoir sits at an elevation of 651 metres, providing an effective hydraulic head of 518 metres for the pumped-storage system.

The Tamahara Pumped Storage Power Station itself was commissioned in 1986, three years after the completion of the Tanbara Dam. The station has an installed capacity of 1,200 megawatts, generated by four reversible Francis turbine pump-generators, each with a capacity of 300 megawatts. These units are designed to both pump water and generate electricity, allowing the station to store energy during periods of low demand and release it during peak energy demand. When pumping, the system can move up to 210 cubic metres per second of water, while generating discharge can reach up to 276 cubic metres per second.

Reservoir Infrastructure and Geography

How does pumped storage hydroelectricity work?

The Tamahara Pumped Storage Power Station operates on a cyclical mechanism that converts gravitational potential energy into electrical energy, functioning as a large-scale battery for the regional grid. The system relies on two distinct reservoirs separated by a significant elevation difference. The upper reservoir, known as the Tamahara Reservoir, sits at an elevation of 1,177 metres (3,862 ft) and is formed by the Tanbara Dam. The lower reservoir, the Fujiwara Reservoir, is located at 651 metres (2,136 ft) and is created by the Fujiwara Dam, situated 4 km (2 mi) to the northwest. This arrangement provides an effective hydraulic head of 518 metres (1,699 ft), which is the vertical distance the water travels to drive the turbines.

Operational Cycle: Pumping and Generating

The station's operation is divided into two primary phases: pumping and generating. During periods of low energy demand, such as at night, electricity from the grid is used to drive four reversible Francis turbine pump-generators. These units move water from the lower Fujiwara Reservoir up to the upper Tamahara Reservoir. During the pumping phase, the system can move up to 210 cubic metres per second (7,400 cu ft/s) of water. This process stores energy in the form of water held at a higher elevation.

When energy demand peaks, the cycle reverses. Water is released from the upper Tamahara Reservoir, flowing down through the penstocks to spin the same Francis turbines, now acting as generators. This discharge rate can reach up to 276 cubic metres per second (9,700 cu ft/s). The potential energy of the falling water is converted into kinetic energy, which drives the generators to produce electricity. The station has a total installed capacity of 1,200 megawatts (1,600,000 hp), with each of the four units contributing 300 megawatts (400,000 hp).

Reservoir Capacity and Efficiency

The efficiency of this pumped storage system depends on the volume of water available for movement. The Tanbara Dam creates an upper reservoir with a total storage capacity of 14,800,000 m3 (11,999 acre⋅ft). Of this total volume, 13,000,000 cubic metres (11,000 acre⋅ft) is considered "active" capacity, meaning it is actively cycled between the upper and lower reservoirs to generate power. The dam itself is a rock-fill embankment structure, 116 metres (381 ft) tall, located 14 kilometres (8.7 mi) north of Numata in Gunma Prefecture. By shifting water between these two bodies, the Tamahara station helps balance the grid, absorbing excess power when supply exceeds demand and releasing stored energy when demand surges.

Technical Specifications and Turbines

The Tamahara Pumped Storage Power Station is equipped with four reversible Francis turbine pump-generators, each rated at 300 MW, contributing to the plant's total installed capacity of 1200 MW. The hydraulic efficiency of the system is driven by the significant elevation difference between the two reservoirs.

Hydraulic Head and Elevation

The power station utilizes the Tanbara Dam, which creates the upper Tamahara Reservoir at an elevation of 1,177 metres. The lower reservoir, formed by the Fujiwara Dam located 4 km to the northwest, sits at an elevation of 651 metres. This configuration provides an effective hydraulic head of 518 metres for the pumped-storage operation. The Tanbara Dam itself is a rock-fill embankment structure standing 116 metres tall, with a total reservoir storage capacity of 14,800,000 cubic metres.

Turbine and Flow Data

During the pumping phase, the reversible Francis turbines move water from the lower Fujiwara Reservoir to the upper Tamahara Reservoir at a rate of up to 210 cubic metres per second. When switching to generation mode, the units discharge water at a flow rate of up to 276 cubic metres per second. The following table summarizes the key technical specifications of the turbines and hydraulic parameters.

Significance

The Tamahara Pumped Storage Power Station serves as a critical infrastructure asset for energy stability within Gunma Prefecture and the broader Tone River basin in Japan. With a total installed capacity of 1,200 megawatts, the facility provides substantial flexibility to the regional electrical grid, enabling efficient load balancing through its four 300-megawatt reversible Francis turbine pump-generators. This configuration allows the station to rapidly adjust output in response to fluctuating energy demands, a key function for maintaining grid frequency and voltage stability in an increasingly dynamic power market.

Grid Flexibility and Load Balancing

Pumped storage hydroelectricity is essential for smoothing out the variability of power generation and consumption. The Tamahara station operates by pumping water from the lower Fujiwara Reservoir to the upper Tamahara Reservoir during periods of low energy demand, typically at night. This process stores potential energy in the upper reservoir, which sits at an elevation of 1,177 metres. When energy demand peaks, the water is released back down to the lower reservoir, which is located at 651 metres, generating electricity through the hydraulic head of 518 metres. This cycle allows the station to act as a giant battery for the regional grid, absorbing excess power when supply exceeds demand and releasing it when the grid requires additional capacity.

The operational parameters of the station are optimized for high throughput. During pumping phases, the pump-generators can move up to 210 cubic metres of water per second. These high flow rates, combined with the significant elevation difference between the two reservoirs, enable the station to deliver substantial power output quickly. This rapid response capability is vital for covering peak demand periods, such as midday in summer or early evening in winter, thereby reducing the need for less efficient peaking thermal power plants.

Regional Energy Infrastructure

Located in the headwaters of the Hotchi River, a tributary of the Tone River, the Tamahara station leverages the natural topography of Gunma Prefecture to maximize energy storage efficiency. The upper reservoir, created by the Tanbara Dam, has a total storage capacity of 14,800,000 cubic metres, of which 13,000,000 cubic metres is actively used for power generation. The lower reservoir is formed by the Fujiwara Dam, situated approximately 4 kilometres northwest of the upper reservoir. This strategic placement within the Tone River basin integrates the station into the wider hydrological and energy network of the Kanto region, contributing to the reliability of power supply for millions of residents and industrial consumers.

Commissioned in 1986, following the completion of the Tanbara Dam in 1981, the station has been a cornerstone of regional energy planning for decades. Its continued operation underscores the enduring value of pumped storage technology in managing the interplay between renewable energy sources, such as solar and wind, and traditional thermal generation. By providing essential grid services, including frequency regulation and reserve capacity, the Tamahara Pumped Storage Power Station plays an indispensable role in ensuring the resilience and efficiency of Japan’s energy infrastructure in Gunma Prefecture.

What distinguishes Tamahara from other Japanese pumped storage plants?

The Tamahara Pumped Storage Power Station is distinguished by its significant effective hydraulic head of 518 metres (1,699 ft), a result of the substantial elevation difference between its upper and lower reservoirs. The upper Tamahara Reservoir sits at an elevation of 1,177 metres (3,862 ft), while the lower Fujiwara Reservoir is located at 651 metres (2,136 ft). This vertical separation allows the facility to generate 1,200 megawatts of capacity using four reversible Francis turbine pump-generators, each rated at 300 megawatts.

Reservoir Infrastructure and Location

The plant’s upper reservoir is formed by the Tanbara Dam, a rock-fill embankment structure that impounds the headwaters of the Hotchi River, a tributary of the Tone River in Gunma Prefecture.

This dual-dam configuration on separate tributaries of the same river system is a key characteristic of the Tamahara facility. The Tanbara Dam is situated 14 kilometres (8.7 mi) north of Numata.

Operational Characteristics

These flow rates, combined with the 518-metre head, enable the plant to deliver its 1,200-megawatt output efficiently.

See also

• Fukushima Daiichi nuclear disaster
• Fukushima nuclear power plant accident and comprehensive health risk management
• Waste-to-energy incineration plants as greenhouse gas reducers: a case study of seven Japanese metropolises
• Fukushima Daiichi nuclear accident
• Fukushima nuclear accident casualties and health impacts