Electricity Storage

Learning objectives

By the end of this lesson you should be able to:

Understand why electricity storage is becoming increasingly important.
Distinguish between different electricity storage technologies.
Explain the difference between energy capacity and power capacity.
Understand the strengths and limitations of battery energy storage.
Recognise the many system services storage can provide.
Appreciate why the value of storage depends on when and how it is used, not simply how much energy it stores.

Introduction

One of the defining characteristics of traditional electricity systems was that electricity had to be consumed almost immediately after it was generated.

Unlike coal, gas or oil, electricity itself was difficult to store economically at large scale.

As a result, power systems were designed around the principle of continuously balancing generation with demand.

Today, this is changing.

Advances in batteries and other storage technologies mean electricity can increasingly be stored and used later.

Storage is rapidly becoming one of the most important technologies in modern electricity systems because it increases flexibility, improves reliability and enables much greater use of renewable energy.

What is electricity storage?

Electricity storage refers to any technology that captures energy at one time and releases it later.

Importantly, storage does not create energy.

Instead, it moves energy through time.

For example:

Charge a battery during the afternoon.
Discharge it during the evening.

The amount of energy remains slightly smaller because every storage technology experiences losses.

The value comes from making electricity available when it is more useful.

Why storage matters

Imagine a windy night.

Electricity demand is low.

Wind farms are producing large amounts of electricity.

Without storage, some renewable generation may need to be curtailed.

Now imagine the following evening.

Demand rises sharply.

The wind has fallen.

Electricity becomes scarce.

Storage allows some of yesterday's surplus electricity to help satisfy today's demand.

In effect, storage reduces the mismatch between when electricity is generated and when consumers require it.

Power versus energy

Storage technologies have two important characteristics.

Energy capacity

Energy capacity describes how much electricity can be stored.

It is usually measured in:

kilowatt-hours (kWh)
megawatt-hours (MWh)
gigawatt-hours (GWh)

Power capacity

Power capacity describes how quickly stored energy can be delivered.

It is measured in:

kilowatts (kW)
megawatts (MW)
gigawatts (GW)

These two characteristics are independent.

For example:

A battery may have:

100 MWh of stored energy
Maximum output of 25 MW

This means it can supply:

25 MW for approximately four hours.

Understanding the difference between energy and power is essential when comparing storage technologies.

Battery Energy Storage Systems (BESS)

The fastest growing storage technology is the Battery Energy Storage System (BESS).

Most large-scale batteries today use lithium-ion chemistry similar to that found in electric vehicles and mobile phones, although many other battery chemistries are emerging.

A battery stores electricity chemically during charging.

When discharged, the chemical reactions reverse, producing electricity.

Power electronic converters connect the battery to the electricity network.

Strengths of batteries

Battery systems possess several important advantages.

Extremely fast response

Batteries can change output in milliseconds.

This makes them ideal for:

Frequency response.
Fast reserves.
Balancing sudden disturbances.

High efficiency

Many battery systems recover more than 90% of the electricity used during charging.

Modular design

Battery installations range from:

Residential systems.
Commercial installations.
Utility-scale battery farms.

Flexible location

Unlike hydroelectric reservoirs, batteries can be installed almost anywhere.

Zero operational emissions

Battery systems themselves produce no operational greenhouse gas emissions.

The overall environmental impact depends upon how the electricity used to charge them was generated.

Limitations of batteries

Battery storage also has important limitations.

Limited storage duration

Most commercial battery installations provide electricity for a few hours rather than several days.

Degradation

Battery performance gradually declines with repeated charging and discharging.

Eventually replacement becomes necessary.

Material requirements

Battery manufacturing requires materials such as lithium, nickel, cobalt and graphite, although alternative chemistries continue to develop.

Cost

Battery costs have fallen dramatically but remain significant for very long-duration storage applications.

Other storage technologies

Although batteries receive considerable attention, they represent only one form of electricity storage.

Other technologies include:

Pumped-storage hydro

Stores energy by pumping water uphill.

Suitable for very large amounts of long-duration storage.

Compressed air energy storage

Uses surplus electricity to compress air into underground caverns.

The compressed air later drives turbines to generate electricity.

Hydrogen

Electricity can produce hydrogen through electrolysis.

The hydrogen may later generate electricity using fuel cells or gas turbines.

Hydrogen offers the possibility of storing energy for weeks or even months.

Thermal storage

Some systems store energy as heat using:

Hot water.
Molten salts.
Phase-change materials.

The stored heat may later generate electricity or provide heating directly.

Storage is not one technology

Different storage technologies occupy different parts of the electricity system.

Some are designed for:

Millisecond response.

Others for:

Hourly balancing.

Others for:

Seasonal energy storage.

No single technology is optimal for every application.

Instead, electricity systems increasingly require a portfolio of storage technologies operating across multiple timescales.

Storage as a system resource

Many people think storage exists simply to buy cheap electricity and sell it later.

While this is one important application, storage provides many additional services.

These include:

Energy arbitrage.
Frequency response.
Operating reserves.
Voltage support.
Congestion management.
Black start capability.
Renewable integration.
Peak demand reduction.

The value of storage therefore extends well beyond shifting energy between different times of day.

Energy arbitrage

Perhaps the simplest application of storage is energy arbitrage.

The storage system:

Charges when electricity prices are low.
Discharges when prices are high.

This smooths price differences throughout the day.

It also allows surplus renewable electricity to be used later rather than curtailed.

However, arbitrage alone rarely captures the full value of storage.

Many battery systems earn significant revenue from providing system services in addition to energy trading.

Supporting renewable generation

Storage plays a crucial role in integrating renewable energy.

For example, batteries can:

Absorb sudden increases in solar generation.
Smooth fluctuations in wind output.
Delay expensive network upgrades.
Reduce renewable curtailment.

Storage therefore increases the amount of renewable generation that electricity systems can accommodate while maintaining reliability.

Why duration matters

Not all shortages are the same.

Some last:

Seconds.

Others last:

Minutes.

Some continue for:

Several hours.

Occasionally, low renewable output may persist for several days.

A battery capable of supplying electricity for one hour cannot solve a week-long shortage.

Similarly, a seasonal storage technology may respond too slowly to provide millisecond frequency control.

Choosing the appropriate storage technology therefore depends upon the problem being solved.

Storage and flexibility

Perhaps the greatest contribution of storage is flexibility.

Storage allows electricity systems to become less dependent upon matching generation and demand at every instant.

Instead, energy can increasingly be shifted through time.

As electricity systems become more decentralised and renewable generation increases, this flexibility becomes increasingly valuable.

Why £/MWh is not enough

Suppose two battery systems have identical storage costs.

One provides:

Four hours of storage.
Millisecond response.
Frequency support.
Voltage support.
Black start capability.

The other provides:

Four hours of storage only.

Clearly the first battery contributes much more to the electricity system.

Similarly, two batteries with identical energy capacity may have completely different power ratings.

One might discharge fully within thirty minutes.

Another might operate continuously for eight hours.

Comparing them using only the cost of stored energy ignores these important differences.

Storage should therefore be evaluated according to:

Power capability.
Energy duration.
Response speed.
Availability.
Location.
System services.

A key insight

Electricity storage does not produce energy.

Instead, it creates flexibility.

It allows electricity generated at one moment to be used at another.

In doing so, storage supports renewable integration, improves reliability and provides many of the fast-response services required by modern electricity systems.

As electricity systems continue to decarbonise, storage is becoming less of a specialist technology and more of a fundamental component of everyday system operation.

Key takeaways

Electricity storage shifts energy through time rather than creating additional energy.
Energy capacity and power capacity describe different characteristics of storage systems.
Batteries provide rapid response, high efficiency and flexible deployment.
Other storage technologies include pumped hydro, compressed air, hydrogen and thermal storage.
Storage supports renewable integration, balancing, reserves and many other system services.
Different storage technologies are suited to different timescales and applications.
The value of storage depends on how and when it is used, not simply how much energy it stores.
Modern electricity systems increasingly rely on storage as a source of flexibility rather than merely as a source of energy.

Looking ahead

Storage helps balance electricity across time.

The next lesson examines interconnectors, which balance electricity across geography by allowing neighbouring countries and regions to exchange power, improving reliability and making more efficient use of generation resources.