Module 7 — The Changing Electricity System
Lesson 2 of 8
Variable renewable generation
Learning objectives
By the end of this lesson you should be able to:
- Understand what is meant by variable renewable generation.
- Explain why renewable generation differs from conventional generation.
- Distinguish between variability and uncertainty.
- Appreciate how renewable generation changes power system operation.
- Understand the growing importance of forecasting and flexibility.
- Recognise both the opportunities and challenges associated with renewable electricity generation.
Introduction
For most of the history of electricity systems, generation followed demand.
When consumers required more electricity, operators instructed additional power stations to increase their output.
When demand fell, generators reduced their output.
Modern electricity systems increasingly operate in a different way.
Much of today's new generating capacity comes from renewable energy sources such as wind and solar.
Unlike conventional power stations, these generators depend upon natural resources that cannot be controlled.
The wind blows when it blows.
The sun shines when it shines.
Rather than deciding how much electricity renewable generators should produce, operators must increasingly decide how best to operate the rest of the electricity system around the electricity that nature provides.
What is variable renewable generation?
Variable renewable generation refers to renewable energy sources whose output changes naturally over time.
The most important examples are:
- wind power,
- solar photovoltaic (PV) generation.
Unlike fossil-fuel or nuclear power stations, their output depends upon weather conditions rather than operator decisions.
Generation therefore varies throughout the day and across the seasons.
Variable does not mean unpredictable
Variable renewable generation is sometimes described as unpredictable.
This is not entirely accurate.
Weather cannot be controlled, but it can often be forecast with considerable accuracy.
For example:
- sunrise and sunset occur at predictable times,
- cloud cover can be forecast,
- wind speeds can be predicted hours or even days in advance.
Forecasts are never perfect, but they continue to improve through advances in meteorology, computing and artificial intelligence.
The challenge is therefore not complete unpredictability, but managing forecast uncertainty.
Variability versus uncertainty
It is helpful to distinguish between two related concepts.
Variability
Variability refers to the fact that generation changes over time.
For example:
- solar output increases after sunrise,
- peaks around midday,
- falls towards sunset.
These changes are expected.
Uncertainty
Uncertainty refers to differences between forecasts and reality.
For example:
A weather forecast may predict moderate winds tomorrow.
Actual wind speeds may be slightly higher or lower.
Electricity systems must therefore be capable of responding when conditions differ from expectations.
Renewable generation has low operating costs
One of the defining characteristics of wind and solar generation is their very low marginal cost.
Unlike gas or coal-fired power stations, they require no purchased fuel.
Once installed, producing one additional unit of electricity costs very little.
As we learned in the previous module, this means renewable generators typically appear near the beginning of the merit order and are often dispatched whenever they are available.
Weather becomes part of system operation
Historically, electricity system operators focused primarily on forecasting demand.
Today they must also forecast generation.
Weather has become a central part of electricity system operation.
Operators routinely monitor forecasts for:
- wind speed,
- solar irradiance,
- temperature,
- cloud cover,
- rainfall,
- storms.
Electricity systems have become increasingly dependent upon accurate weather forecasting.
Different renewable technologies behave differently
Not all renewable technologies vary in the same way.
Wind generation
Wind output may change significantly over minutes, hours and days.
Large weather systems can affect entire regions simultaneously.
Solar generation
Solar generation follows a clear daily pattern.
It increases after sunrise and falls after sunset.
Cloud cover can also produce short-term fluctuations.
Hydroelectric generation
Many hydroelectric plants can be controlled in much the same way as conventional generators.
Hydropower is therefore often used to help balance variable renewable generation.
Each renewable technology contributes differently to the operation of the electricity system.
Renewable generation changes net demand
One useful way to think about renewable generation is through the concept of net demand.
Total electricity demand still exists.
However, some of that demand is supplied directly by renewable generation.
The remaining demand must be supplied by other resources.
For example:
Suppose consumers require:
40 GW
Wind generation provides:
15 GW
The rest of the electricity system must supply only:
25 GW.
System operators therefore increasingly schedule conventional generation around renewable output.
Curtailment
Sometimes renewable generation produces more electricity than the network or the electricity system can accommodate.
When this occurs, some renewable generators may be instructed to reduce their output.
This is known as curtailment.
Curtailment can occur for several reasons, including:
- transmission congestion,
- insufficient demand,
- system stability requirements,
- operational security.
Although renewable energy is available, it cannot always be fully utilised.
Flexibility becomes increasingly important
As renewable generation increases, so too does the need for flexibility.
Flexibility allows the electricity system to adapt as renewable output changes.
Examples include:
- battery storage,
- pumped hydro,
- flexible demand,
- fast-ramping gas turbines,
- interconnectors,
- electric vehicles.
These resources help maintain the balance between supply and demand when renewable generation changes.
A changing operational philosophy
Traditional electricity systems largely followed a simple principle:
Adjust generation to match demand.
Increasingly, electricity systems must do something more sophisticated.
They must coordinate:
- renewable generation,
- conventional generation,
- storage,
- flexible demand,
- network constraints.
Rather than simply controlling generators, operators increasingly coordinate an entire ecosystem of resources.
Opportunities
Variable renewable generation offers many important benefits.
These include:
- lower greenhouse gas emissions,
- reduced dependence on fossil fuels,
- very low operating costs,
- improved energy security in many countries,
- reduced exposure to fuel price volatility.
Renewable generation has become one of the principal drivers of electricity system decarbonisation.
Challenges
At the same time, variable renewable generation introduces new operational challenges.
These include:
- forecast uncertainty,
- changing generation patterns,
- increased need for flexibility,
- transmission constraints,
- renewable curtailment,
- maintaining system balance.
These challenges do not make renewable generation undesirable.
Rather, they require electricity systems to evolve in how they are planned and operated.
Variable does not mean unreliable
It is important to distinguish between the variability of an individual generator and the reliability of the overall electricity system.
A single wind turbine may generate different amounts of electricity from one hour to the next.
However, a reliable electricity system can still be built using variable renewable resources if sufficient flexibility, storage, forecasting, network capacity and complementary generation are available.
Reliability is therefore a property of the entire electricity system, not of any individual technology.
A key insight
Variable renewable generation represents one of the most significant changes in modern electricity systems.
Unlike conventional generators, wind and solar output depends upon weather conditions rather than operator decisions.
This changes how electricity systems are operated, increasing the importance of forecasting, flexibility and coordination while enabling significant reductions in fuel consumption and greenhouse gas emissions.
Key takeaways
- Variable renewable generation includes technologies such as wind and solar whose output depends on natural conditions.
- Variability refers to changing output over time, while uncertainty refers to differences between forecasts and reality.
- Renewable generators typically have very low marginal operating costs.
- Weather forecasting has become an essential part of electricity system operation.
- Renewable generation changes the amount of electricity that must be supplied by other resources.
- Flexibility resources help maintain system balance as renewable output changes.
- Variable generation does not prevent reliable electricity systems but does require new approaches to planning and operation.
Looking ahead
The transition to renewable electricity is only one part of the transformation occurring within modern power systems.
At the same time, societies are increasingly replacing fossil fuels with electricity in sectors such as transport and heating.
In the next lesson, we examine the electrification of heat and transport and explore how this is reshaping patterns of electricity demand.