The Coo-Trois-Ponts Hydroelectric Power Station is a pumped-storage hydroelectric power station in eastern Belgium, located near Coo in the valley of the Amblève, on the territory of Stavelot and Trois-Ponts in the Province of Liège. Operated by Electrabel, it is Belgium's principal large-scale electricity-storage facility and one of the largest pumped-storage installations in Europe.
The station consists of two upper reservoirs on Mont Saint-Victor, a lower reservoir occupying a former meander of the Amblève, two large hydraulic circuits, and an underground powerhouse equipped with six reversible Francis pump-turbines. During periods of low electricity demand or surplus generation, water is pumped from the lower reservoir to the upper reservoirs. During periods of high demand, the water is released back through the turbines to generate electricity.
Coo was developed in two main phases: the first three units, known as Coo I, were commissioned in 1971–1972, and the three additional Coo II units were commissioned in 1979. Its original function was closely connected with the development of nuclear power in Belgium. The plant was designed to help balance the output of large, relatively inflexible nuclear power stations at Doel and Tihange, absorbing electricity during off-peak periods and returning it to the grid during peak demand.
Since the liberalisation of electricity markets and the growth of wind and solar generation in the Belgian energy mix, Coo's role has broadened. It remains a load-shifting plant but it is also used for short-term flexibility, reserve power, voltage regulation, black-start capability, and portfolio balancing in day-ahead, intraday, and ancillary-services markets.
Background and site
Pumped storage does not create energy. It stores electricity by using it to pump water uphill, thereby converting electrical energy into gravitational potential energy. When electricity is needed, the water is released downhill through turbines. The value of the system lies in its ability to shift energy from periods of low demand or surplus production to periods when dispatchable power is needed.
The site at Coo was chosen because the Amblève valley provides one of the few locations in Belgium with a local height difference of more than , making it suitable for high-head pumped storage. The upper reservoirs are located on Mont Saint-Victor, near Brume, while the lower reservoir lies in the valley below, beside the village of Coo.
The lower reservoir occupies an old meander of the Amblève known as the Tour de Coo. In the eighteenth century, monks from Stavelot Abbey modified the course of the river to protect the village of Petit-Coo from flooding and to supply a local mill. They created the Coo waterfall, leaving the former meander with only a limited natural flow. This topography later allowed engineers to convert the meander into the lower basin of a closed-cycle pumped-storage installation.
Construction and development
Belgian electricity producers began looking for a site for a pumped-storage plant in the 1960s. The selected Coo site combined a natural lower-basin form with nearby high ground suitable for artificial upper reservoirs. Construction was carried out in two phases. The first phase began in 1967 and created Coo I, composed of three reversible pump-turbine units. The first electricity from a Coo unit was supplied to the Belgian grid in October 1971, and the first three units were commissioned between 1971 and 1972. The second phase began in 1975 and added Coo II, with three further units commissioned in 1979.
The lower reservoir was formed by building two dams at the ends of the former Amblève meander. A diversion gallery was excavated to allow flood flows in the river to bypass the basin, and a drainage gallery was provided so that the lower reservoir could be emptied for maintenance or emergency purposes.
The two upper reservoirs were excavated on Mont Saint-Victor. Rock and soil removed from the interior of the basins were reused to form peripheral levees, limiting the need for imported material. The underground cavern was excavated in one phase so that it could eventually receive all six generating groups, including those installed during the later Coo II phase.
Design
General layout
Coo-Trois-Ponts is divided into two separate hydraulic and electromechanical sections. Each section has one upper basin, one penstock and three reversible pump-turbine groups. Therefore, the plant counts as two energy storage systems: Coo I and Coo II. The connection between the turbine groups and the lower reservoir is common to both sections.
The plant operates in a closed-water cycle. Rainfall and additional inflow are drained from the lower reservoir to the river, but the working volume used for energy storage is repeatedly cycled between the lower and upper reservoirs.
Reservoirs
The two upper reservoirs are artificial basins built on Mont Saint-Victor. The first upper basin, serving Coo I, has a useful capacity of about , with useful operating levels between and above sea level. The second upper basin, serving Coo II, has a useful capacity of about , with useful operating levels between and .
The lower reservoir has a useful capacity of about , corresponding to the combined active capacity of the two upper basins. Its useful operating levels range between and above sea level.
Both upper basins were sealed with an impermeable bituminous-concrete lining. The lining includes drainage layers and individually drained panels, allowing leakage to be monitored from control rooms outside the reservoirs. Rehabilitation of the lining of the first upper basin became an important element of later maintenance work.
Penstocks and hydraulic head
The maximum elevation head is approximately , though the effective head varies according to the levels in the upper and lower reservoirs. For Coo I, the head varies from about to . For Coo II, it varies from about to .
Coo I and Coo II have different penstock designs. The Coo I penstock includes a vertical intake, a shaft, a long sub-horizontal section, an inclined section and a distributor leading to the spherical rotary valves. Its length is about , with diameter reducing from about to .
The Coo II penstock follows a more linear route, with a lateral intake, inclined sections and a distributor leading to the spherical rotary valves. It is about long, with diameter varying from about to .
Underground power station
The underground power station is located in a cavern excavated in the hill between the reservoirs. The cavern is about long, wide and high. The excavation of the cavern and associated access galleries removed about of rock.
The cavern contains six independent unit structures and a central area for control and common equipment. It is connected to multiple galleries, including the main access gallery, the visitor access gallery, galleries used for excavation, and galleries carrying high-voltage conductors from the alternators to the transformers outside the cavern.
Mechanical equipment
The six generating groups are vertical-shaft reversible Francis pump-turbines designed to permit removal of the runner from above. In generating mode, water flows from an upper reservoir through the penstock, spiral case, wicket gates and turbine runner, then discharges through the draft tube and tailrace toward the lower reservoir. In pumping mode, the same hydraulic machine reverses function and lifts water from the lower reservoir to the upper basin.
Each group is equipped with a spherical rotary valve upstream of the spiral case. This valve isolates the unit from the upper reservoir and is opened or closed during starting, stopping and maintenance operations. The Coo I valves are operated by pressurised water from the penstock, while the Coo II valves use oil pressure for opening and penstock water pressure for closing.
The runners are located at the lowest point of each group, beneath the alternator. The Coo I runners have a diameter of about and weigh about ; the Coo II runners have a diameter of about and weigh about .
Electrical equipment
In each group, the motor used for pumping and the generator used for turbine operation are the same synchronous machine. The Coo I machines have an AC power of about 200 MVA and the Coo II machines about 230 MVA. Coo I rotates at 300 rpm, while Coo II rotates at 272 rpm. The original manufacturer of the electrical motors was ACEC. Each generator is connected to a 20 kV switchgear. Power is carried out of the cavern to 20 kV/400 kV transformers, one per unit, and then connected to the Belgian transmission grid. The nearby Brume 380 kV substation links the installation to the high-voltage transmission grid operated by Elia, including high-voltage connections toward the Province of Luxembourg and the Tihange area.
Operation
Coo-Trois-Ponts can operate in both generating and pumping modes. In generating mode, the plant can reach rated power in about 90 seconds. In pump mode, starting takes about five minutes. The plant is capable of starting in less than two minutes and delivering power comparable to a 1-GW nuclear reactor for up to 6 hours. The change between turbine and pump mode is achieved by reversing the direction of rotation, with phase reversal carried out behind the 20 kV switchgear. Pump-mode starting is more complex than turbine-mode starting. The runner is first dewatered by compressed air, then a 6 kV auxiliary power unit accelerates the unit toward synchronous speed. After synchronisation, the main generator-motor is connected to the grid and the runner is refilled so that pumping can begin.
At maximum head, the three Coo I units are rated at about 3 × 158 MW in turbine mode, while the three Coo II units are rated at about 3 × 220 MW. At minimum head, the corresponding turbine-mode ratings fall to about 3 × 115 MW for Coo I and 3 × 187 MW for Coo II. Pumping-mode ratings are lower than turbine-mode ratings. The maximum turbine-mode flow is about 3 × for Coo I and 3 × for Coo II.
The station has about of storage capacity in Coo I and in Coo II, for a total of about in the original technical configuration. It is characterized by a roundtrip efficiency of 75%. Recent modernization works increased its storage capacity by about and power output by about . At full capacity, Coo I can sustain generation for approximately five hours and Coo II for approximately six hours before repumping is required.
Relationship with the development of the Belgian nuclear power program
From the late 1970s, Belgian electricity policy increasingly relied on the development of nuclear energy and the construction of large pressurised water reactors at Doel and Tihange. The first commercial nuclear reactors, Doel 1, Doel 2 and Tihange 1, entered service in 1975, followed by Doel 3 in 1982, Tihange 2 in 1983, and Doel 4 and Tihange 3 in 1985.
Coo-Trois-Ponts was built during the same period as Belgium's major nuclear-power program. The purpose of the project was to balance the electricity demand with the increasing supply from the non-flexible power generation of Doel and Tihange. These Belgian nuclear reactors were designed to provide large quantities of baseload electricity, but were not designed to operate in a load-following mode. They are less suited than hydroelectric or gas-fired plants to frequent short-term changes in demand. Pumped storage therefore complemented the nuclear fleet by using off-peak electricity to pump water uphill and then returning electricity to the grid during peak periods.