Chapter 3

Introduction

For more than a century, electricity systems were designed around a simple idea. A relatively small number of large power stations generated electricity, transmission networks carried it across long distances and homes, businesses and industry consumed it. That structure is now changing.

Utility-scale solar farms, wind farms, rooftop solar systems, batteries and electric vehicles are becoming part of the electricity network. Some generate power, some consume it, some store it and some can do all three at different times of the day.

The electricity grid is therefore evolving from a one-way delivery system into something far more dynamic.

Building enough renewable generation is only part of the challenge. The electricity produced still has to reach the places where it is needed, remain balanced with demand and support a stable network every second of every day. That requires transmission infrastructure, energy storage, smarter controls and increasingly flexible electricity demand.

Together, these technologies are creating a new kind of electricity system.

From Centralised to Distributed Power

Traditional electricity networks were built around large centralised generators. Coal, gas, nuclear and hydroelectric power stations produced electricity at a relatively small number of locations before sending it through transmission and distribution networks to millions of customers.

Modern renewable electricity systems are becoming much more distributed.

A utility-scale solar farm may contain millions of solar cells connected together across a large site. A wind farm may contain dozens or hundreds of turbines spread across land or sea. At the same time, individual homes and businesses can generate electricity directly from their own rooftops.

A single rooftop solar system may be small compared with a conventional power station, but millions of systems operating together can represent a significant source of generation.

This also changes the role of the electricity customer.

Traditionally, households simply consumed electricity supplied by the grid. A home with rooftop solar and battery storage can now generate electricity during the day, store some of it, consume it later and export surplus electricity back into the network.

Such customers are sometimes described as prosumers because they are both producers and consumers of electricity. Electric vehicles add another layer. When connected to the grid, they represent large electrical loads that can potentially choose when to charge. In some future systems, vehicles may even be able to return stored electricity to buildings or the wider grid.

Instead, it is becoming a network connecting utility-scale generators, rooftop solar systems, batteries, electric vehicles and millions of different consumers.

The Challenge of Variable Generation

One of the main differences between renewable generation and conventional fossil-fuel power stations is that some renewable resources cannot be controlled in the same way.

A gas-fired power station can usually increase or decrease its fuel consumption to change its electrical output. Solar panels cannot simply be told to generate more electricity after sunset, and a wind turbine cannot generate at full output when the wind is not blowing.

This characteristic is often described as variable renewable generation. Solar output follows a relatively predictable daily pattern. Generation begins after sunrise, increases through the morning, often reaches its highest level around the middle of the day and then declines towards sunset.

Electricity demand does not necessarily follow the same pattern.

In many electricity systems, demand can be relatively modest during the middle of the day before increasing later in the afternoon or evening as people return home, cook meals, use appliances and switch on heating or cooling. Wind introduces another source of variability because generation changes with weather conditions. A region may experience periods of strong wind and high output followed by calmer periods when generation falls.

The challenge is not simply that renewable generation changes. The challenge is that electricity supply and electricity demand must remain closely balanced continuously.

If generation greatly exceeds demand, the system must find somewhere for that electricity to go, reduce generation or shift consumption into that period. If generation is too low, other resources must increase their output, stored electricity must be released or demand must be reduced.

Managing this balance across an electricity system containing large quantities of variable renewable generation is one of the central engineering challenges of the energy transition.

Connecting Renewable Energy

Renewable electricity can only be useful if it can reach the people who need it. The best locations for solar and wind generation are not always close to major centres of electricity demand. Large solar farms are often constructed in sunny rural areas where land is available, while wind farms are commonly built in exposed countryside, mountain regions or offshore where wind conditions are stronger.

Cities and large industrial facilities may be hundreds of kilometres away. This makes the transmission network an essential part of the energy transition.

High-voltage transmission lines allow large quantities of electricity to travel over long distances with relatively low losses. As renewable generation expands into new regions, new transmission lines and substations may be required to connect those projects to the existing grid.

Some countries are also developing dedicated renewable energy zones, where areas with particularly strong renewable resources are identified and transmission infrastructure is planned around clusters of future solar, wind and storage projects.

Interconnectors between regions and countries can provide another advantage. If one area has surplus renewable generation while another has higher electricity demand, transmission links allow electricity to move between them.

A larger connected electricity system can therefore make better use of diverse weather conditions and generation resources. Generating clean electricity is only half the challenge. It must also be transported from wherever nature provides the energy to wherever society needs it.

Keeping the Grid Stable

Balancing the total amount of electricity generated and consumed is only one requirement of a reliable electricity system. The grid must also remain electrically stable.

Traditional electricity networks were dominated by large synchronous generators. Inside coal, gas, hydroelectric and nuclear power stations, enormous turbines rotate generators at carefully controlled speeds. Because these machines contain large rotating masses, they store kinetic energy as they spin. This creates inertia.

If there is a sudden imbalance between electricity supply and demand, the rotating energy stored in these generators can briefly resist rapid changes in grid frequency, giving control systems time to respond.

Solar photovoltaic systems work differently. Solar panels generate direct current electricity, which is converted into alternating current using electronic inverters. Modern wind turbines also rely heavily on power electronics. These systems do not naturally contribute inertia in exactly the same way as large conventional generators.

As more inverter-based generation is connected to electricity networks, engineers therefore need new ways to maintain grid stability. Advanced inverters can respond extremely quickly to changes in voltage and frequency. Battery systems can inject or absorb power within fractions of a second, while sophisticated control systems continuously monitor conditions across the network.

Some modern technologies can even reproduce certain behaviours traditionally provided by synchronous generators through software and power electronics. The objective remains unchanged. Whatever technologies generate the electricity, the grid must remain stable enough to deliver reliable power whenever people need it.

Storing Electricity

One of the most powerful tools for managing variable renewable generation is energy storage. Storage does not create additional electricity. Instead, it moves electricity from one point in time to another.

When renewable generation is plentiful and electricity demand is relatively low, surplus energy can be stored. When demand increases or renewable generation falls, that stored energy can be released. Battery storage is particularly well suited to this role.

Large grid-scale batteries can be connected directly to electricity networks or installed alongside solar and wind farms. They can absorb electricity during periods of high generation and discharge it later when that electricity becomes more valuable.

Batteries can also respond extremely quickly. If a power station suddenly disconnects or electricity demand changes unexpectedly, a battery can adjust its output in fractions of a second. This makes battery systems useful not only for energy shifting but also for supporting grid frequency and stability.

The same principle applies at a much smaller scale in homes. A household with rooftop solar often generates more electricity during the middle of the day than it immediately consumes. Without a battery, that surplus electricity may be exported to the grid. A household battery can instead store part of that energy and release it later in the evening when solar generation has fallen. This increases self-consumption, reduces the amount of electricity imported from the grid and, depending on the system design, may also provide backup power during certain outages.

Not all energy storage relies on electrochemistry. Pumped hydro stores energy using water and gravity. When electricity is plentiful, pumps move water from a lower reservoir to a higher one. When electricity is required, the water is released downhill through turbines to generate power. The underlying purpose is the same.

Whether energy is stored inside a lithium-ion battery or as water behind a dam, storage helps shift electricity from periods when it is abundant to periods when it is needed most.

The Smart Grid

As electricity systems become more complicated, simply building additional hardware is not enough. The grid also needs better information.

Traditional electricity networks were designed mainly around predictable flows from large generators towards consumers. Modern networks may contain millions of devices capable of changing their behaviour throughout the day.  This requires a much more intelligent electricity system. A smart grid uses sensors, communication networks, forecasting tools and advanced software to monitor and manage electricity flows in real time.

Operators can use weather forecasts to estimate how much solar and wind generation may be available several hours or even days ahead. Smart meters can provide more detailed information about electricity consumption. Automated control systems can respond rapidly when network conditions change. Power flows can also increasingly move in both directions. A household may import electricity in the evening, export solar electricity during the afternoon and charge a battery when electricity is abundant.

Managing millions of these interactions manually would be impossible. Software allows the electricity network to continuously coordinate them. The physical grid of cables, substations and transformers therefore increasingly operates alongside a digital layer that monitors what is happening and helps decide how the system should respond.

In many ways, the grid is becoming not only larger and more interconnected, but also more aware of its own changing conditions.

When Demand Becomes Flexible

For most of electricity history, the basic assumption was that generation had to follow demand. If millions of people switched on appliances at the same time, power stations had to increase their output.

Renewable electricity creates another possibility. Instead of always changing generation to match demand, some electricity demand can be shifted to match generation. This is known as demand response or demand flexibility.

Many electrical loads do not have to operate at one exact moment. An electric vehicle may need to be fully charged by the following morning, but it does not necessarily matter whether most of that charging happens at 6 p.m., midnight or during a period of abundant solar generation. A hot-water system can heat water before it is required and store that heat for later. Commercial refrigeration systems can sometimes adjust their operation slightly while maintaining safe temperatures. Some industrial processes can also shift production towards periods when electricity is cheaper or more abundant.

Instead of reducing the useful service provided, demand response changes when electricity is consumed. This can make renewable energy easier to integrate because more demand can be moved towards periods when wind and solar generation are plentiful. The electricity system becomes more flexible from both directions. Generation can respond where possible, while consumption becomes increasingly capable of responding as well.

Thousands of Devices, One Power Station

A single rooftop solar system is small. A single household battery is small. A single electric vehicle is also small compared with a conventional power station. But thousands of these devices working together can become something much more significant.

This is the idea behind a Virtual Power Plant, or VPP.

A VPP uses software to connect and coordinate many distributed energy resources such as rooftop solar systems, household batteries, electric vehicles and controllable electrical loads. Instead of each device operating independently, the group can respond collectively to conditions on the electricity network. Thousands of batteries might charge when solar generation is abundant and electricity prices are low. Later, they could reduce charging or discharge some of their stored electricity when demand increases.

From the perspective of the wider grid, these thousands of individual devices can begin behaving like one coordinated energy resource. No single physical power station exists. The power station is created through communication and control. This represents one of the most significant changes taking place in modern electricity systems.

For most of the twentieth century, the grid connected a relatively small number of generators to millions of passive consumers.

The future grid may connect millions of active devices that constantly change between generating, consuming and storing electricity.

Bringing Everything Together

A renewable electricity system is far more than a collection of solar panels and wind turbines. Generation has to be connected through transmission infrastructure. Supply and demand have to remain balanced. Grid frequency and voltage must stay within acceptable limits. Surplus electricity needs to be stored or redirected, while consumption can increasingly be shifted towards periods when renewable energy is plentiful.

Each technology solves a different part of the problem. Transmission moves energy across distance. Batteries and pumped hydro move it through time. Smart grids provide information and control. Demand response changes when electricity is consumed. Virtual Power Plants coordinate thousands of smaller resources into something capable of supporting the wider system.

Together, these technologies allow electricity grids to accommodate levels of renewable generation that would once have seemed impossible. The electricity grid is therefore no longer simply transporting electricity from large power stations to passive consumers. Increasingly, it is coordinating millions of devices that can generate, consume and store energy.

Looking Ahead

Building a renewable electricity system solves only part of the wider energy challenge.

Electricity generation is enormously important, but modern civilisation also depends on transport, industrial heat, steel, chemicals, aviation and many other processes that cannot all be solved by solar panels and batteries alone.

In the next chapter, we'll look beyond solar and explore the wider collection of technologies that may be needed to build a net-zero energy system, from wind and electrification to hydrogen, nuclear power and carbon capture.