Chapter 4

Introduction

Solar energy will play a major role in the energy transition, but it cannot solve every part of the problem on its own. Modern energy systems are far too diverse for a single technology to meet every need.

Homes require heating and electricity. Cars, trucks, ships and aircraft need energy for transport. Factories require high temperatures, chemical feedstocks and reliable power. Entire industries depend on processes that are difficult to run directly from solar panels or batteries.

The challenge is therefore not to find one perfect technology.

It is to build an energy system in which different technologies work together, each solving the problems it is best suited to.

Solar may provide enormous quantities of electricity during daylight hours. Wind can generate at different times and in different weather conditions. Hydropower can provide both renewable generation and storage. Batteries can shift electricity through time. Electrification can replace fossil fuels in transport and heating. Hydrogen, nuclear power and carbon management may help address parts of the system that are harder to decarbonise.

The path to net zero is therefore likely to be a combination of solutions rather than a single breakthrough.

There Is No Silver Bullet

Every energy technology has strengths. Every energy technology also has limitations.

Solar is modular, increasingly affordable and capable of being deployed almost anywhere, but it only generates electricity when sunlight is available. Wind can produce large quantities of low-carbon electricity, including at night, but its output varies with weather conditions. Batteries can respond almost instantly and shift electricity from one part of the day to another, but storing enormous amounts of energy for weeks or months remains more difficult.

Hydropower can provide reliable renewable generation and large-scale storage, but suitable geography is limited. Nuclear power can generate large quantities of low-carbon electricity continuously, but projects can be expensive, complex and slow to build. Hydrogen can store energy and provide a fuel for some difficult applications, but producing it requires significant amounts of energy.

The same principle applies across the entire transition. No technology performs every task equally well. This is not a weakness of the energy transition. It is simply the nature of engineering.

Complex systems are rarely solved by one perfect component. Instead, engineers combine technologies so that the strengths of one compensate for the limitations of another. A net-zero energy system is likely to work in exactly the same way.

Wind Power

Wind is one of solar's most important partners. Where solar captures energy from sunlight, wind turbines capture the kinetic energy of moving air. Both technologies generate electricity without burning fuel during normal operation, but they often produce power at different times.

Solar generation follows the daily cycle of the Sun and usually reaches its highest output around the middle of the day.

Wind behaves differently. Depending on geography and weather conditions, wind generation can remain strong overnight, during cloudy periods or at times of year when solar production is lower. This means that combining solar and wind can produce a more balanced renewable generation profile than relying heavily on either technology alone.

Onshore wind farms can provide large amounts of electricity at relatively low cost in locations with strong wind resources. Offshore wind expands the opportunity further by accessing stronger and more consistent winds at sea, although construction and maintenance are generally more complex. The two technologies therefore complement each other. Solar performs best under strong sunlight. Wind performs best under strong airflow. When one resource is weak, the other may still be available.

This does not eliminate variability, but it reduces the likelihood that an electricity system will depend on a single weather pattern. The wider and more diverse the renewable generation mix becomes, the easier it can be to balance changes in individual resources.

Hydroelectricity and Pumped Storage

Hydropower provides another important piece of the energy system. As we saw earlier, hydroelectric power stations convert the gravitational energy of water into electricity. Water flowing through turbines turns generators, producing electricity without the need to burn fossil fuels during operation.

One of hydro's greatest strengths is that some systems can be controlled. If water is stored behind a dam, operators can often increase or decrease the flow through the turbines depending on electricity demand. This makes reservoir hydropower very different from solar and wind. Instead of generating only when the resource happens to be available, stored water can often be released when electricity is most valuable.

A related technology, pumped hydro storage, takes this principle one step further. When electricity is plentiful, pumps move water from a lower reservoir to a higher one. This stores energy in the form of gravitational potential energy. When electricity is needed later, the water is released downhill through turbines, generating electricity as it returns to the lower reservoir. 

In this way, pumped hydro acts like an enormous rechargeable battery.

The electricity used to pump the water uphill is stored indirectly and recovered later when the water flows back down.

Because reservoirs can be extremely large, pumped hydro can store far more energy than many electrochemical battery systems and can provide electricity for many hours or even longer.

Its main limitation is geography. Suitable sites require elevation differences, access to water and significant infrastructure.

Where those conditions exist, however, pumped hydro can provide an extremely valuable source of large-scale energy storage.

Electrifying Everything We Can

Replacing fossil-fuel power stations with renewable generation is only one part of reaching net zero. A large amount of fossil fuel is burned directly outside the electricity sector. Petrol and diesel power vehicles. Natural gas heats homes. Coal and gas provide high-temperature heat in factories. Oil products are used across transport and industry.

One of the most powerful strategies for reducing these emissions is electrification. The idea is simple. Wherever practical, replace a technology that burns fossil fuel directly with one that uses electricity instead. A petrol car can be replaced by an electric vehicle. A gas boiler can be replaced by a heat pump. Some industrial equipment can replace fossil-fuel burners with electric heating or other electrically powered processes.

This matters because electricity can become progressively cleaner as the generation mix changes. A petrol car is permanently tied to burning petrol during use. An electric vehicle, by contrast, uses whatever electricity is available from the grid. As more solar, wind, hydro and other low-carbon generation is added, the emissions associated with operating that vehicle can fall without replacing the vehicle itself. 

The same principle applies to heating and industry. Electrification also often improves energy efficiency. Electric motors are generally much more efficient than internal combustion engines, while heat pumps can move several units of heat for every unit of electrical energy they consume. 

The result is one of the most important ideas in the energy transition. Total electricity demand may increase significantly even while total greenhouse gas emissions fall. That is not a contradiction. It is a consequence of replacing direct fossil-fuel use with increasingly low-carbon electricity.

Hydrogen for What We Can't Easily Electrify

Direct electrification is usually the simplest route where it is technically practical. Using electricity directly avoids the additional energy losses that occur when electricity is first converted into another fuel and then converted again into useful energy.

But not every activity is easy to electrify directly. Some industrial processes require extremely high temperatures or chemical reactions that electricity alone may not easily provide. Long-distance transport can require very high energy densities. Certain industries also need molecules as raw materials rather than simply energy.

This is where hydrogen may become important.

Hydrogen is not a primary energy source like sunlight or wind. It is an energy carrier. Electricity can be used to split water into hydrogen and oxygen through a process called electrolysis. If that electricity comes from renewable sources, the resulting fuel is often described as green hydrogen. The hydrogen can then be stored, transported or used in another process.

One potential application is steel production, where hydrogen can help replace coal in certain methods of reducing iron ore. Hydrogen is also already an important industrial feedstock in chemicals and fertiliser production, where low-carbon production could replace hydrogen currently made from fossil fuels. It may also contribute to synthetic fuels for shipping, aviation or other applications where batteries are difficult to use. In some electricity systems, hydrogen could potentially provide long-duration energy storage. Surplus renewable electricity could produce hydrogen during periods of abundant generation, with the stored fuel used later.

The drawback is efficiency. Every conversion step loses some energy. Electricity must first produce hydrogen, the hydrogen must then be compressed, stored or transported, and it may later be converted back into electricity or another useful form of energy. For this reason, hydrogen is unlikely to replace direct electrification where electricity can be used efficiently.A useful hierarchy is therefore: direct electrification where practical, hydrogen where direct electrification is difficult.

Nuclear Power

Another technology frequently discussed in the energy transition is nuclear power. Nuclear power stations generate electricity using heat released through nuclear fission rather than combustion. Because they do not burn fossil fuels during normal operation, their direct operational carbon dioxide emissions are very low. Nuclear plants can also generate large amounts of electricity continuously, regardless of sunlight or wind conditions. This provides what is often described as firm low-carbon power.

Supporters argue that this reliability can complement variable renewable generation and reduce the amount of storage, backup generation or transmission required in some electricity systems. Nuclear power also has an extremely high energy density. A relatively small amount of nuclear fuel can produce enormous quantities of energy compared with fossil fuels.

However, nuclear projects bring significant challenges of their own.

Large reactors can require substantial upfront investment and long construction periods. Financing can be difficult because investors may need to commit large sums of money many years before the plant begins generating electricity.

Nuclear waste must also be carefully managed over very long periods, while safety and public acceptance remain important considerations.

Different countries have therefore taken very different approaches. Some continue expanding nuclear generation as part of their low-carbon energy strategies. Others have reduced or eliminated it, while some are investigating newer reactor designs such as small modular reactors.

There is no single answer that fits every country. The role of nuclear power depends on factors such as existing infrastructure, political priorities, electricity demand, financing, geography and public acceptance. Like every other technology in the transition, it involves trade-offs.

Carbon Capture and Carbon Removal

Even with widespread renewable energy, electrification and cleaner fuels, some sources of greenhouse gas emissions may remain extremely difficult to eliminate. Certain industrial processes produce carbon dioxide as part of the chemistry itself. Cement production is an important example. Some emissions may also remain from aviation, agriculture and other activities where completely eliminating greenhouse gas release could be technically or economically difficult.

This is where carbon capture and carbon removal become relevant. Carbon capture and storage, commonly known as CCS, aims to capture carbon dioxide from large industrial facilities before it enters the atmosphere. The captured CO₂ can then be compressed, transported and injected deep underground into suitable geological formations for long-term storage.

This could allow industries such as cement, chemicals or certain forms of heavy manufacturing to reduce emissions that are difficult to avoid through electrification alone.

Carbon removal addresses a slightly different problem. Instead of capturing carbon dioxide before it reaches the atmosphere, removal technologies take CO₂ that is already present in the air and store it elsewhere. Forests and soils provide natural forms of carbon removal because plants absorb carbon dioxide through photosynthesis. Engineered approaches are also being developed, including direct air capture, where machines chemically extract CO₂ directly from the atmosphere.

These technologies are unlikely to provide an excuse for continuing unlimited emissions. Capturing or removing carbon is generally more complicated than avoiding the emission in the first place. Their most valuable role may therefore be dealing with the final fraction of emissions that cannot realistically be eliminated. This links directly back to the idea of net zero. If some residual emissions remain, equivalent removals may be required to balance them.

Putting the Pieces Together

The future energy system is unlikely to have a single winner. Different countries have different climates, geography, industries, resources and existing infrastructure. A sunny country with large areas of available land may rely heavily on solar. A windy island nation may develop enormous offshore wind resources. Mountainous regions may have access to hydroelectricity and pumped storage. Countries with established nuclear industries may continue using nuclear power, while others may choose different combinations of renewables, storage and interconnection. Heavy industrial economies may require large quantities of hydrogen, carbon capture or other technologies that smaller service-based economies need far less of. The exact mixture will therefore vary.

But the underlying system could contain many of the same building blocks: solar + wind + hydro + batteries + transmission + electrification + hydrogen + nuclear + carbon management + smarter demand. Each solves a different part of the problem. Solar and wind generate low-carbon electricity. Hydro and batteries provide flexibility and storage. Transmission connects regions and resources. Electrification allows clean electricity to replace fossil fuels in transport, heating and industry. Hydrogen provides an alternative where direct electrification is difficult. Nuclear can provide firm low-carbon generation. Carbon capture and removal can help address emissions that remain particularly difficult to eliminate. Smart grids and flexible demand help coordinate everything together.

Seen this way, the energy transition is not a competition to identify the single best technology. It is a systems-engineering challenge. Engineers must determine what combination of technologies can provide the required energy at the required time, in the required location, while balancing cost, reliability, security and environmental impact. 

Solar is one of the most important pieces of that system. But its greatest value may ultimately come from how effectively it works alongside everything else.

Looking Ahead

The technologies available today are already capable of transforming large parts of the energy system. But engineering never stands still.

Solar cells continue becoming more efficient, new photovoltaic materials are emerging and engineers are finding new places to generate electricity from sunlight, from reservoirs and agricultural land to buildings themselves.

In the next chapter, we'll look at the future of solar technology and explore how the panels of tomorrow could become more efficient, more versatile and more deeply integrated into the world around us.