Hydropower plants are a form of renewable energy. They operate using stored water in a dam; the water falls by gravity through penstocks to water turbines located below the dam. There are various types of water turbines used to drive power generators, producing electricity for the National Grid.
Water power has been used for centuries as a means of operating water wheels for various purposes. Hydropower is one such use of water and this has been in existence since the 1890’s. Hydropower is considered as renewable energy, produced from natural resource of water with zero CO2 emissions during operation.
It produces electricity by using a stored supply of water from a reservoir, which runs down large bore pipes known as penstocks, into water turbines located below the reservoir. These turbines drive power generators supplying electricity to the national grid.
This is an article on my series on electrical power generation from renewable sources. We shall have a look at the construction of the dam, and examine the supply of water to turbines and pumped storage systems.
So we start therefore by the factors which influence a suitable location and construction of the dam.
There a two essential conditions to be met before the location of a reservoir is confirmed. The first one is to have an adequate supply of water, usually from a river of high volume flowing through a mountain valley. The second one is the difference in height between the dam outfall and the turbine inlet in valley below the reservoir.
An Environmental Impact Statement is a mandatory requirement; carried out to ensure the impact to the area takes the environment and the indigenous people into consideration.
Once the EIS has been passed and these conditions established the construction of the dam can commence.
This is the longest period in the hydro power plant construction phase and requires large amounts of stone filling and, steel reinforced concrete.
When constructed, it can be expected to supply water to the turbines for up to a hundred years, some hydro plants being in operation since 1900 here near my home in Scotland. As the dam wall progresses, apertures are left in the walls for the water outfalls to the penstocks. Large steel gratings are installed in front of these outfalls to catch major debris brought down by the river.
Near the top of the dam wall, at the maximum water level, spill-pipes are inserted to prevent overtopping of the reservoir. This can happen after a particularly wet season or during a snow thaw. The water from the overflow is run down through spill-pipes to join the tail water.
Categories and Types of Turbines
As with steam turbines there are two categories of water turbines being impulse and reaction turbines only instead of steam pressure defining the category, the head of water and rate of flow is used to select the relevant category of the water turbine.
Impulse Turbines
These are used where a high head of water, combined with a low flowrate is available. The water is injected onto the turbine buckets or blades through nozzles arranged around the impeller.
Turgo Turbine
Pelton Wheel
Crossflow Turbine
From the above types of impulse turbines shall examine the operation of a pelton wheel.
Pelton Wheel
A pelton wheel has spoon-shaped buckets arranged around a runner connecting to a central hub. The water supplied by the penstock enters nozzles located around the buckets. The nozzles convert the potential energy from the water entering from the penstock to kinetic energy striking the buckets causing an impulse reaction which rotates the hub. The hub is connected to the generator driving it to produce electricity.
.Reaction Turbines
These turbines operate under low head combined with high flow rate.
Propeller Turbine
Francis Turbine
Kaplan Turbine
From the above list of reaction turbines, we shall examine the operation of a propeller type.
Propeller Turbine
As the name suggests this is in the form of a ships propeller, having between three to six blades. The blades and hub are completely submerged in the water that rotates the propeller. The propeller drives the power generator, produce the electrical power.
Supply of Water to the Reservoir
The main supply of water is from the river, but during the summer the river flow can diminish. To avert a water shortfall to the dam, tunnels can be installeds through the mountains, connecting the main reservoir with lochs further up the mountains.
In the old days these tunnels were very labor intensive, being hewn by air driven hand tools and explosives. Today they are formed by a huge boring tool which drills through the rocks with amazing speed and accuracy; the overburden can be used in the construction of the dam wall.
The main problems associated with the water supply is the sediment carried down by the river and deposited in the reservoir; the silt building up over the years and impeding the water outflow to the turbines.
The penstocks are connected to the outflows in the dam wall and run from here, usually at an angle downwards to the turbine water inlets either within the dam wall or outside the wall. The difference in height between the outfall and the inlet to the turbine is one of the governing factors of the hydropower plants electrical output capacity. The quantity and flow rate of the water through the penstock is another limiting factor.
A surge tank can be fitted to the penstocks to stop water hammer. Water hammer occurs due to the pressure exerted in the penstock when the water supply to the turbine is shut off, the surge tank absorbs this pressure.
The penstocks can be protected against corrosion by application the application of coatings on the external of the pipes along with an internal lining.
Coal-tar products were widely for external and internal protection until the mid eighties when it was thought that the coal-tar lining was detrimental to health.
Epoxy resins can be used internally or a concrete lining used in steel penstocks. Regular inspections for corrosion and erosion of the external and internal surfaces are carried out where possible.
The Operation of a Hydropower Plant
The reservoir dam wall contains the outflows which allow the stored water to pass into the penstocks. From the penstocks the water falls by gravity down to the turbine house where it enters the turbine. Depending on the type of turbine employed the water expends its energy in the rotation of the turbine and power generator. After passing through the turbine the tail water is expelled either into the original river course or into a lower reservoir.
A sketch of a typical hydropower plant along with an image of the Gordon Dam in Tasmania, is shown below; please click on images to enlarge.
The water in the lower reservoir can be pumped back up to the supply reservoir at night when the power is cheap. This is known as pumped storage supplementing the top reservoirs contents. Pumped storage can be accomplished by using a separate high pressure electric driven pump or depending on the type of turbine, or the existing system can be employed. In this case the generator is supplied with electrical power acting as a motor and the turbine rotated in reverse acting as a pump.
