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Pumped storage power stations are a facility that produces green and renewable energy in a similar way to hydroelectric plants. The main difference between the two being that water just flows from a high point to a low point in a hydroelectric plant, but the water in a pump storage power station can be pumped back up to the top and used again. It is an alternative to hydroelectric plants that allows more electricity to be produced in times of need. In this article, we look at how they work, and the machinery used within them.
What Are Pumped Storage Power Stations?
A pumped powered water station is similar in nature to a conventional hydroelectric power station, with the main difference being that the same water can be pumped back round through the system and be used again and again. When there is a low electricity demand, water is pumped up to a higher reservoir to be stored for times of need– so when a high demand manifests, extra water can be passed through the turbine which generates more electricity.
Whilst there are many different parts and machinery to some aspects of the station, the general process of how they produce electricity stays the same. In a conventional pumped storage power station, the water will travel from an upper reservoir into an underground tunnel called an intake tunnel, where it will then pass through a powerhouse (which contains turbines connected to generators) and into the lower reservoir via a discharge tunnel. If the demand for electricity is low, the system goes into reverse and turbines spin backwards which pushes the water from the lower reservoir to the upper reservoir, ready to be used again.
The Machinery Used Within These Power Stations
As with any type of hydroelectric energy plant, there are certain components of the energy generation process that are ubiquitous, such as the turbine, the generator and transformer to ensure that electricity is produced and can be transported. However, given the reversible nature of pumped storage power stations, additional equipment is required that helps to facilitate the movement of water in two directions.
Variable Speed Motors
One of the most important pieces of machinery is the variable speed motor that rotates the turbine head. There are many different parameters that can influence the optimal operating efficiency of a turbine head, including flow speed, size of the head and speed of the head, so having a motor with a variable speed for different scenarios is a huge benefit.
By using a motor that can operate at variable speeds (and in forward and backwards directions), the operating speed can be varied across a range of optimal speeds to coincide with the turbine and generator units. So, when the head speed and flow speed of the water change, the motor can change its speed to keep the whole operation close to its optimal speed. This means that the turbine operating speed can be extended, and the pump capacity can be adjusted to just use the amount of energy available at a given moment in time. It also helps to stabilize the whole grid within a pump storage power station.
Variable speed motors vary in their operating principles to classical motors that only move the turbine head in one direction. In one-way systems, the motor usually uses many projected (salient) poles and a direct current (DC) to generate a magnetic field. By comparison, variable speed motors use a three-phase winding mechanism and a low-frequency alternating current (AC). A frequency convertor is also used within the circuit of the motor to control the frequency– where changes in the frequency change the speed of the motor.
The torque convertor is involved with the starting up and shutting down of a storage pump and has been found to be the most efficient way to perform these functions, as it enables the pump to be connected to, or separated from, the shaft system in a few seconds.
The torque convertor transmits torque and/or power from the motor/generator system to the pump shaft through the filling of process water. The storage pump starts up during the filling process, but as the pressure within the system rises from the larger volume of water, the torque from the torque convertor increases and accelerates the pump. The initial torque acceleration is very high to start the process, but this equilibrates over time and settles out to an optimal operating speed.
A ternary set is two separate hydraulic machines that consist of a motor, generator, pump set and a separate turbine. Because they are two separate machines, the rotational direction can be the same in both operational modes. They can also switch between pump and turbine modes by using a clutch, starting turbine and a torque convertor. These are a newer component of many pump storage power stations, but they have found use in systems which require a fast grid response and have started to become widely used in new, and technologically up-to-date, power stations.
Sources and Further Reading