Wind and Solar Power

Learning objectives

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

Understand how wind turbines and solar photovoltaic (PV) systems generate electricity.
Explain the strengths and limitations of wind and solar generation.
Distinguish between variable and dispatchable generation.
Understand the concept of capacity factor.
Appreciate the role of wind and solar within a low-carbon electricity system.
Recognise why renewable technologies should be evaluated according to the services they provide, not simply their cost per megawatt-hour.

Introduction

Wind and solar power have transformed electricity systems around the world.

Over the past two decades they have become some of the cheapest sources of new electricity generation in many countries, while producing almost no greenhouse gas emissions during operation.

Unlike coal, gas or nuclear stations, however, wind and solar do not produce electricity whenever operators choose.

Instead, they generate electricity whenever the underlying natural resource is available.

This simple difference has profound implications for how electricity systems are designed and operated.

Understanding both the strengths and limitations of renewable generation is essential for understanding modern electricity markets.

Renewable energy

Wind and solar are examples of renewable energy technologies.

Renewable resources are naturally replenished over human timescales.

Unlike fossil fuels, they do not rely on consuming finite fuel reserves.

Their primary energy sources include:

Sunlight.
Wind.
Water.
Biomass.
Geothermal heat.

In this lesson we focus on the two technologies that have experienced the most rapid global growth: wind and solar.

Wind power

Wind turbines convert the kinetic energy of moving air into electricity.

The process is relatively straightforward.

Wind flows across the turbine blades.
Lift forces cause the blades to rotate.
The rotating shaft drives an electrical generator.
Electricity is supplied to the network.

Modern turbines are highly sophisticated machines.

They continuously adjust:

Blade angle (pitch).
Rotor speed.
Turbine orientation (yaw).

These adjustments maximise energy production while protecting the turbine during strong winds.

Onshore and offshore wind

Wind farms may be located either:

Onshore

Built on land.

Advantages include:

Lower construction costs.
Easier maintenance.
Faster deployment.

Offshore

Located in coastal waters.

Advantages include:

Stronger winds.
More consistent wind speeds.
Larger turbine sizes.
Higher energy production.

Offshore construction is significantly more expensive, but the higher energy output often compensates for these additional costs.

Solar photovoltaic generation

Solar photovoltaic (PV) systems convert sunlight directly into electricity.

Unlike thermal power stations, no turbines or steam are required.

Instead:

Sunlight strikes semiconductor materials.
Photons transfer energy to electrons.
Electric current is produced.
Power electronic converters transform this into alternating current for the grid.

Because no moving parts are required, solar PV systems are generally simple, reliable and relatively inexpensive to maintain.

Utility-scale and rooftop solar

Solar generation occurs at many different scales.

Large solar farms may occupy hundreds of hectares and supply electricity directly to the transmission network.

At the other extreme, individual households increasingly install rooftop solar panels.

Distributed rooftop generation has two important characteristics.

It:

Produces electricity close to where it is consumed.
Reduces demand on the wider electricity network during sunny periods.

However, widespread rooftop solar can also create new operational challenges, particularly for low-voltage distribution networks.

Variable generation

The defining characteristic of both wind and solar is that they are variable generators.

Their output depends upon weather conditions rather than operator instructions.

For example:

A gas turbine can usually increase output whenever instructed.

A wind farm cannot produce additional electricity if there is no wind.

Similarly, solar panels cannot generate electricity during the night regardless of electricity prices.

This distinction is fundamental.

Variable generation provides energy when nature permits.

Dispatchable generation provides energy when operators request it.

Capacity factor

A useful measure of generation performance is the capacity factor.

Capacity factor compares the actual energy produced over a period with the energy that would have been produced if the generator operated continuously at maximum output.

For example:

A 100 MW wind farm operating continuously for one year would theoretically produce:

100 MW × 8,760 hours

In practice, the wind does not blow continuously.

Its actual production may therefore correspond to a capacity factor of around 35–50%, depending on location.

Solar capacity factors are generally lower because generation occurs only during daylight hours.

Importantly, a lower capacity factor does not mean the technology is inefficient.

It simply reflects the availability of the natural resource.

Strengths of wind and solar

Wind and solar offer several important advantages.

Very low operational emissions

Electricity is generated without burning fuel.

Operational carbon emissions are therefore extremely low.

No fuel costs

Unlike coal or gas stations, renewable generators require no continuous fuel purchases.

Once constructed, operating costs are generally low.

Rapid deployment

Wind farms and solar installations can usually be built much more quickly than nuclear or large thermal power stations.

Scalability

Renewable technologies can be deployed across a wide range of sizes.

Examples include:

Domestic rooftop solar.
Community wind projects.
Utility-scale renewable farms.

Energy security

Because wind and sunlight are local resources, countries become less dependent on imported fossil fuels.

Limitations

Wind and solar also present important engineering challenges.

Weather dependence

Generation depends upon natural conditions rather than electricity demand.

Variability

Output changes throughout the day and across seasons.

Forecasting has improved considerably but uncertainty always remains.

Limited dispatchability

Operators cannot instruct wind or solar farms to produce more electricity when the resource is unavailable.

Network impacts

Renewable resources are often located far from major demand centres.

This can require substantial investment in transmission infrastructure.

Large quantities of rooftop solar may also create reverse power flows within distribution networks.

Reduced synchronous generation

Wind and solar connected through power electronic converters contribute less natural inertia than conventional synchronous generators.

As renewable penetration increases, additional technologies may be required to maintain system stability.

Curtailment

Sometimes renewable generators could produce electricity, but are instructed not to.

This is known as curtailment.

Curtailment may occur because:

Network congestion prevents electricity reaching consumers.
Electricity demand is already fully satisfied.
Maintaining system stability requires reduced renewable output.

Although curtailment appears wasteful, it often reflects physical constraints elsewhere in the electricity system rather than shortcomings of the renewable generators themselves.

Complementarity

Wind and solar often complement one another.

Solar generation peaks during sunny daytime periods.

Wind generation is often stronger overnight or during winter months in many regions.

Neither technology alone can provide continuous electricity, but together they reduce overall variability.

Combining multiple renewable resources across different geographical locations further improves reliability.

Wind and solar within the wider system

Wind and solar rarely operate in isolation.

Instead, they are combined with technologies such as:

Batteries.
Hydroelectric storage.
Flexible demand.
Interconnectors.
Gas turbines.
Nuclear generation.

Together these technologies create a system capable of delivering reliable electricity while reducing greenhouse gas emissions.

The challenge therefore shifts from evaluating individual technologies to designing an efficient portfolio of complementary resources.

Why £/MWh is not enough

Suppose two generators both produce electricity for £45/MWh.

One generates whenever requested.

The other generates only when weather conditions permit.

The two technologies clearly provide different value to the electricity system.

Similarly, two wind farms with identical costs may have very different value if:

One is located close to demand.
One experiences more consistent wind conditions.
One reduces network congestion.
One generates during periods of high electricity demand.

This illustrates an important principle.

The value of electricity depends not only on how cheaply it is produced, but also on when, where and how reliably it is available.

A key insight

Wind and solar have fundamentally changed electricity systems.

They provide:

Low-cost energy.
Low-carbon generation.
Excellent long-term sustainability.

However, they do not replace every service historically supplied by conventional generators.

Instead, they shift the engineering challenge towards coordinating a wider portfolio of complementary technologies.

The question is therefore no longer:

"Should we use wind or conventional generation?"

Instead, it becomes:

"What combination of technologies provides reliable, affordable and low-carbon electricity?"

Key takeaways

Wind turbines convert the kinetic energy of moving air into electricity.
Solar photovoltaic systems convert sunlight directly into electrical energy.
Wind and solar are variable generators whose output depends on weather conditions.
Capacity factor measures how much energy a generator produces relative to its maximum possible output.
Renewable technologies provide very low operational carbon emissions and require no fuel.
Their variability creates new challenges for balancing and operating electricity systems.
Wind and solar work best alongside complementary technologies such as storage, flexible demand and dispatchable generation.
Renewable technologies should be evaluated according to the timing, location and reliability of the services they provide, not simply their cost per megawatt-hour.

Looking ahead

Wind and solar depend on naturally variable energy resources.

The next lesson examines hydroelectricity, a renewable technology that not only generates low-carbon electricity but can also provide some of the fastest and most flexible responses available in modern power systems.