Pumped Storage Power Station (Francis Turbine)

Introduction

Pumped storage power plants are a type of hydroelectric power plant; they are classified as a form of renewable (green) power generation.

Pumped storage plants convert potential energy to electrical energy, or, electrical energy to potential energy. They achieve this by allowing water to flow from a high elevation to a lower elevation, or, by pumping water from a low elevation to a higher elevation. When water flows to a lower elevation, the power plant generates electricity. When water is pumped to a higher elevation, the power plant creates a store of potential energy. Pumped storage plants use Francis turbines because they can act as both a hydraulic pump and hydraulic turbine.

Francis Turbine

Francis Turbine

Pumped storage power plants are used to balance the frequency, voltage and power demands within the electrical grid; they are often utilized to add additional megawatt capacity to the grid during periods of high power demand. For this reason, pumped storage plants are referred to as ‘peaking’ plants.

Electrical Grid Power Demand Graph

Electrical Grid Power Demand Graph

Because pumped storage plants can provide electrical grid operators with power ‘on-demand’, they have a high level of dispatchability (the ability to provide power to the grid quickly when needed).

Power Plant Design

Irrespective geographical location, all pumped storage plants require an upper reservoir and lower reservoir. The difference in elevation between the upper and lower reservoirs is referred to as the ‘head’ (head of pressure) and it must be significant in order for the plant to operate efficiently.

A penstock connects the upper reservoir to a Francis turbine located in the power house. A draft tube and tailrace connects the Francis turbine to the lower reservoir.

Pumped Storage Power Plant Layout

Pumped Storage Power Plant Layout

How Pumped Storage Plants Work

 

The below video is an extract from our Introduction to Hydroelectric Power Plants Online Video Course.

 

How Pumped Storage Plants Generate Power (Electricity)

Water flows from the upper reservoir, through the penstock, and to the Francis turbine. As the water passes over the Francis runner blades, a pressure differential is created that causes torque (rotary force) to be applied to the runner. The runner begins to rotate.

The turbine runner is connected on a common shaft to an electrical generator. As the runner rotates, so too does the generator rotor. As the rotor rotates through the electromagnetic field within the generator, it induces current in the stator windings and electrical current begins to flow. The electrical current is usually then dispatched to end consumers via a switchyard and electrical transformer.

Electrical Grid

Electrical Grid

Water discharged from the turbine runner enters into a draft tube where some of the kinetic energy is recovered and converted to potential energy; the water then enters the tailrace and is discharged to the lower reservoir.

In this example, the potential energy of water was converted by the turbine runner into mechanical energy. The mechanical energy was transferred on a common shaft to a generator, which converted the mechanical energy to electrical energy. The entire process can be continuous until the upper reservoir is empty.

How Pumped Storage Plants Store Potential Energy

Water is pumped from the lower reservoir to the upper reservoir by the Francis turbine runner. The flow path is the same as when generating electricity, except the flow direction is reversed because the Francis runner is used as a pump instead of a turbine.

Pumped Storage Plant Economics

Pumped storage plants rely upon the varying price of electricity to make a profit. Many thermal power plants (coal fired, gas fired etc.) cannot increase or reduce their MW output quickly because this would place large thermal stresses on the power plant components (water tube boiler, piping etc.). For this reason, thermal power plants produce almost as much power at night, as they do during the day.

During the day, power is in demand and electricity prices are high. At night, power is not in demand and electricity prices are low. Pumped storage power plants purchase power at night to pump water up to the upper reservoir, they then generate power and sell it back to the grid during the day, when the demand -and price- is higher.

Example 1

Power is purchased from the grid at 1ct/kWh to pump water from the lower to upper reservoir.

Power is sold to the grid at 2ct/kWh by allowing water to flow from the upper to lower reservoir.

The pumped storage plant has generated 1ct/kWh of profit during this process because:

2ct/kWh (sale) - 1ct/kWh (purchase) = 1ctkWh (profit).

Pumped storage plants have operated in this manner for many years, but the recent increase in renewable energy sources is changing the power industry dynamic.

Example 2

It is a sunny and windy day, many solar and wind power plants are online and generating power. Because of this increase in power availability, there is a surplus in the grid and the price of electrical power reduces proportionally.

The price of power during the day reduces so much that pumped storage plants can come online and purchase power to pump water up to the upper reservoir.

In this scenario, pumped storage plants are used both day and night to compensate for the surplus of power in the grid, and to provide power during peak power demand periods. This scenario is relatively new and has led to pumped storage plants becoming a more integrated part of the electrical grid.

3D Model Components

This 3D model shows all major components associated with a typical pump storage power station, these include:

  • Upper and Lower Reservoir
  • Penstock
  • Trash/Rubbish Grate
  • Isolating Valves
  • Switchgear
  • Turbine Runner (Francis)
  • Spiral/Volute Casing
  • Draft Tube
  • Generator
  • Electrical Transformer
  • Open Air Switchyard
Additional Resources

http://www.wvic.com/content/how_hydropower_works.cfm

https://science.howstuffworks.com/environmental/energy/hydropower-plant.htm

Encyclopedia - saVRee