Pumped up power plans for Morocco4 August 2002
At the Afourir project in Morocco, an existing hydroelectric plant and storage reservoir will become part of a larger complex providing power, irrigation and grid management services. Janet Wood reports
MOROCCO'S Afourir project has been developed to satisfy a portion of the peak electric energy demand on the Moroccan grid by using stored low-cost energy generated from a hydroelectric power plant. This will be accomplished through the use of a pumped-storage system, and, according to the Arab Fund, which is helping to fund the project, it will preclude the need for installing several gas turbines, with a total power rating of 450MW, to satisfy peak demand.
The project will be located in the Azilal region, around 220km south of the capital city of Rabat. It includes the construction of upper and lower water reservoirs, connecting ducts and pipes, and two dual pumping and generating stations. The project has the following main components:
• Electromechanical and electric works which include the supply and installation of the following main components: upper reversible station including two vertical reversible pump/turbine units rated at 173MW each, two generators (which could operate as motors) and power transformer; lower reversible station including two vertical reversible pump/turbine units rated at 62MW each, two generators (which could operate as motors) and power transformers; two substations which include 225kV line cells, transformer cells and bus couplers; and network expansion and connection to the grid including overhead lines, underground cables and switch gear cells.
• Armouring works including the supply and installation of around 4000t of armouring material to be used to reinforce the high pressure pipes.
• Civil works including supply of building materials, the construction of water reservoirs, the substations, the pumping stations, the pipe works and the ducts.
• Technical services including consulting services and project supervision.
• Institutional support including design, acquisition and upgrade of the existing systems and computer hardware and software.
The loan from the Arab Fund has provided around 40% of the total project cost to Mexico's Office National d'Electricite (ONE). The loan is made at an interest rate of 4.5%, with repayment over 22 years after a five-year grace period.
alstom, as leader of a consortium comprising Moroccan civil works contractor La Société Générale des Travaux du Maroc (SGTM), has been selected by ONE as turnkey supplier of the plant, in a project worth over US$129M.
Alstom will supply, erect and commission the electrical, mechanical and hydromechanical equipment for the new plant including pump-turbines, valves, penstocks, hydrogenerators, the power plant control system and balance of plant.
SGTM will be responsible for civil works. Electricite de France (edf) has been responsible for the conceptual design of the plant.
The pumped storage power plant will be connected to an existing production grid and irrigation system. It will comprise two units, each with two pump turbines of respectively 60MW and 170MW capacity, connected to a 220kV switchyard.
The two units will function either in series or in differential pumping mode, while ensuring sufficient water flow for irrigation purposes. The use of a variety of operating modes is a novel application for pumped storage plants. Alstom has already installed approximately 800MW of hydro power capacity in Morocco, notably at Al Wahda, Matmata, Afourer, Al Massira and Idriss, as well as 1500MW of conventional power plant.
Armand Pineda, Alstom's project manager, described the project in detail. 'The upper reservoir is 1.3M m3 in volume, and its level of operation is 1284m asl. The intake underground gallery is 100m in length and is 5.5m in diameter. It connects to a low-pressure penstock 450m long and 4.9m in diameter,' he said. 'The penstock is of a relatively new design, similar to that of Motz, a similar plant in France built under a project run by EDF. The design mixes steel and concrete structures to reduce the quantity of concrete and the underground works required.'
The new design has a standard penstock at its centre, made of 6mm thick steel. This is surrounded by steel and concrete structures that are welded onto the tube and keep it rigid. Whereas a standard concrete penstock in this situation, with no steel tubing, would be 60-80cm thick, the combination of the steel, the concrete and the connections allow the thickness of the concrete to be reduced to around 30cm.
The low pressure penstock is connected, with a surge shaft attached, to a high pressure penstock some 1420m long and 3.7m in diameter. The penstock runs overground and the steel varies up to 35mm thick as the pressure increases.
Power plant one comprises two 173MW pump-turbines. The horizontal inlet to the plant is common to both turbines and it bifurcates into two pipes each 100m long and 2.6m in diameter.
At power plant one, the level of the turbines is 642m asl and the head is more than 600m. 'This is not a record,' said Pineda, 'there is a double stage turbine in Korea with a head of 800m. But it is the highest head that has been used for this kind of pump-turbine.'
The equipment at this plant is held in caverns 50m deep and 19m in diameter. Pineda points out that this is a relatively small area: 'We have had to balance the location of the equipment in the cavern and the upper part of the plant at 687m and in the switchyard.' Pineda said the turbine is also highly developed.'We have a very high turbine speed, one of the fastest of its kind,' he claimed, 'the head and flow - which is 69.9m3/sec in turbine mode - delivers a speed of 750rpm.'
Pineda regards this high speed as a unique part of the plant design: 'The weight of the generator is around 200t, so as it turns so fast there is millions of tonnes of impulse to be managed. Imagine the technology that has to be set up to manage that.'
What is more, he said, the generator has been sized for a 1150rpm maximum overspeed. In pump mode the flow is around 50m3/sec.
From power plant one, the system is connected via another high-pressure steel penstock to the gallery that already exists connected to the power plant that has been in operation since the 1950s. 'This connection is very important,' explained Pineda. 'It's in between power plants one and two and it is the only way to get water from the dam into the existing system.'
Connecting the system
Connecting the new parts of the system to the existing galleries is one of the most challenging parts of the project: not because it is technically demanding, but because the surrounding farmers rely on the project for irrigation. 'We will have to cut the existing gallery and extend it to the high pressure penstock connection,' Pineda said. 'The difficulty is the timescale: we will have 45 days for this part of the job including the draining of the existing gallery. The farmers who are already customers of the project need the water for irrigation and the window for them is only 30 days.'
Between power plants one and two there are two surge shafts. 'We will use the surge shafts to maintain the hydraulic balance between the two power plants,' added Pineda. 'We will use the existing irrigation reservoir as a compensating reservoir, but we have no intermediate reservoir between the power stations. So in case of transient shutdowns the surge shafts have to be sized so they will not overflow.'
As a result, the surge shaft at power plant two is 30m deep and 7m wide, and the intermediate surge shaft is 30m deep and 8m in diameter, and an additional expansion chamber around 8m in diameter and 100m long is also available.
The low pressure penstock that transfers the water to power plant two is similar in design to the upper low pressure penstock and is 960m long. The high pressure penstock will run beside the existing connection and will have two pipelines 40m long and 2.5m in diameter.
At power plant two the turbines are at 445m asl. The available head at this point is around 200m and it produces 120MW of power from two 60MW pump turbines.
At the outlet of power plant two is another surge shaft - the fourth in the system - which balances the connection between this power plant and the lower reservoir. A connection to the irrigation system at this point will allow the system to deliver water for irrigation at up to 48m3/sec. Power plant two is connected to the lower reservoir via an underground gallery 1100m long and 5.5m in diameter. The lower reservoir, with a capacity of 1.3Mm3, is at 490m asl.
Alstom is planning to complete the project under a very tight schedule, according to project manager Armand Pineda. 'The order was placed in April 2001,' he said, 'and the first synchronisation is planned for April 2004. All four pump-turbines are due to be synchronised by October 2004, so that gives us 42 months.'
Part of the reason for that is a new 1320MW coal-fired station, called Jorf Lasfar, being built under a build, own, operate agreement by ABB and CMS Generation. ONE has agreed to a take or pay contract with this plant, so having the Afourir pumped storage plant available to absorb excess power from that plant will be economically very important. Morocco is also investing in 200MW or more of wind plant, which will add intermittency to the system.
But Pineda said the most technically challenging part of the project is already completed. That was modelling the transients and management of the two pumped storage plants. 'Using two pump-turbine sets in series without an intermediate reservoir was something new,' Pineda said. 'In fact the surge shafts, the low pressure penstock and the existing gallery take its place, but the transient analysis was very tough. We had to model all the plants.'
In operation, each unit will have its own control room and the staff will be trained by Alstom. The staff 'are already participating in the design and construction of the plant,' Pineda said, 'and they will know the plant well when it starts up.'