How the Power Is Extracted
The head is critical to the design of the turbine/generator combination in any hydro-power station. The technology is very mature and design choices for getting the right turbine/generator combination to suit the conditions at a particular reservoir are well established . Rotational speeds in the range 100 rpm to 600 rpm are typical of water driven prime movers, and the choice of turbine speed is governed by the priority placed on turbulence-free interaction between the flowing water and the moving turbine blades. Speed synchronisation of the streaming water and the blades is essential for high efficiency. Turbine speed is held constant by incorporating a large heavy flywheel onto the drive shaft, together with some form of controllable water flow deflectors. At limited reservoir heads in the range 10-100 ft, when water pressure is low, the turbine tends to be of the propeller type (Kaplan design - Fig. 3.1). The turbine has a compact diameter (1-5 m) for efficient conversion of relatively slow axial water flow into high rotational speed and optimised torque at the output shaft, which is connected to the generator. When there is plenty of head (more than 100 ft) the turbine is more likely to be of the water-wheel genre (water scoops on the end of radial support arms - Pelton design as shown in Fig. 3.2). While the rotating Pelton wheel itself is not greatly different in size from other types (typically 3-4 m in diameter) the water feed arrangement
to achieve efficient transference of power from the fast flowing water is complex, so that the overall diameter of the turbine can be as much as three times the diameter of the rotator. Modern water turbines are actually quite efficient in converting water power to shaft power. The value generally varies between 70 and 90% depending on precise operating conditions.
The generally slower rotational speeds available from water turbines, when compared with steam turbines, dictates that synchronous generators in hydroelectric stations are much larger than those encountered in fossil fuel power stations. In saying this it is assumed that generator outputs in the range 40 MW to 400 MW at a frequency of 50 Hz (or 60 Hz in the USA) are desired. In power stations associated with very large dams, providing potentially vast quantities of water but with a moderate head, the size of the generator means that it must be installed with the armature rotating around a vertical axis to ease bearing problems. The four generators at the Cruachan power station in Scotland, for example, which is by no means big by hydro-electric standards, are tightly housed in a cavern, which is 50 m long and about 60 m in diameter, located within Ben Cruachan. The excavation of this cavern required the removal of 220,002 m3 of rock and soil.
Although much larger than the generators typical of fossil fuel power stations, the electrical and mechanical loss mechanisms inherent in hydro-electric generators are of a similar nature to their fossil fuel counterparts and lead to similar efficiency levels of the order of 90%. Transmission losses on the grid are likely to be relatively high for hydro-electric power owing to the longer than average distances to the users from remote stations. The generally quoted figure for grid loss, as
noted in Chap. 2, for all types of power station in the USA and Europe is 7%. More remote hydro-power stations will obviously incur slightly higher losses in transmitting power through the grid resulting in a loss figure of nearer 8%.
The Aswan High Dam (Fig. 3.3), which holds back the waters of Lake Nasser on the Nile, is 550 km in length, has a surface area of 5250 km2, and contains approximately 111 km3 of water . This volume of fresh water (density = 1000 kg/m3) has a mass of 111 x 109 x 1000 = 111 x 1012 kg. Consequently, assuming that the average height of the water in Lake Nasser above the dam outflow is 55 m, the energy stored in the dam is 111 x 1012 x 55 x 9.81 = 60 x 1015 J = 60,000 TJ. However, like the pendulum discussed in Chap. 2, this potential energy yields power only when it is converted to kinetic energy. Water can be discharged at a rate of 11,000 m3/s through the base of the Aswan dam. By performing a calculation of the kinetic energy per second (power) associated with a moving water column, a simple formula for estimating the power represented by the flow of water emerging from the dam can be constructed. The power is equal to the flow rate multiplied by the head multiplied by a conversion factor (9810 J/m4). For a dam of this type, assuming that the in-flow is much smaller than the out-flow, the head will decrease linearly when maximum power is being extracted. The average head will be about half the maximum head. Consequently, for Aswan this gives a potential power at the maximum flow rate of 2.85 GW. Water turbine efficiencies are typically of the order of 85% while the generators are unlikely to be much better than 90% efficient, therefore of the 2.85 GW contained in the rushing water, only about 2.2 GW is available to the grid. This is very close to the claimed capability of the twelve generator sets incorporated into the Aswan High dam complex . In hydro-generation systems located in mountainous terrain the head, as suggested above,
can be greatly enhanced by arranging for the turbines to be far below the base of the dam. Gravity means that high flow rates occur at the turbine. On the other hand high confined mountain valleys are unlikely to provide very large volumes of water. In some mountainous hydro-generation schemes several reservoirs are used in cascade to raise the potential energy.
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