Balancing Supply and Demand in Electricity Grids

Introduction

Throughout the day, electricity demand fluctuates greatly, depending on the time of day and consumption, as well as the weather and the season. Similarly, electricity supply can fluctuate as more and more intermittent renewable energy sources are added to the grid. However, supply and demand need to be balanced to keep electricity grids reliable and stable, and to avoid shortages or blackouts.

Measuring and Managing Imbalances

In Europe, the equilibrium point for the European grid, which operates on alternating current, is at a frequency of 50 Hertz. In the USA, the reference frequency is 60 Hertz. Grid operators ensure that this frequency remains stable 24 /7, with a tolerance threshold of plus or minus 0.050 Hertz.

Changes in grid frequency immediately show if there is an imbalance and also signal its degree of severity.

An imbalance in the grid could be either:

• High electricity generation and low electricity demand
• Low electricity generation and high electricity demand

If too much electricity is fed into the grid in relation to consumption, the electrical frequency increases. This can be managed by reducing generation through taking power plants off the grid. For example, wind farms are disconnected when the wind is strong and demand for electricity is low.

When power generation is too low to meet demand, frequency drops. This can be managed by increasing inputs such as steam, fuel or water to increase power output. Grid-connected power stations usually have their own reserve capacity to increase electricity production. Gas plants can be dispatched at times of peak demand. It can also be managed by reducing demand, e.g. from industrial users who regulate their production for a period of time and receive compensation. Finally, load shedding is also an option to restore balance and avoid blackouts.

Storage Options

In order to cope with both high and low load situations, the storage of electricity is of great importance. However, the large-scale storage of electricity in the grid is still a major challenge and subject to research and development. 

The following technologies and approaches can, or are hoped to, provide a solution to balancing supply and demand in modern electricity grids.

Pump storage hydropower

A pumped storage hydroelectricity system (PSH) consists of two water reservoirs at different elevations. Low-cost surplus electric power is typically used to pump water into the upper reservoir. During periods of high electricity demand, the stored water is released through turbines to generate electricity.

Further Information

Grid Portal on energypedia