Renewable energy explained: solar, wind, storage and the grid

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Renewable electricity has gone from a niche to a central part of energy systems in many countries. Understanding how the main technologies work, and the challenges of fitting them together, makes the news about energy much easier to follow. Figures in this field change quickly, so for current numbers check sources such as the International Energy Agency (IEA), national grid operators and government statistics.

Solar power

Photovoltaic (PV) panels turn sunlight directly into electricity using semiconductor cells, usually made of silicon. They range from rooftop systems on homes to large solar farms.
Strengths: panels have no moving parts, can be installed quickly and at almost any scale, and their cost has fallen dramatically over recent decades. The IEA's recent renewables reports identify solar PV as the largest source of new renewable capacity being added worldwide.
Limits: output follows daylight and weather, drops to zero at night and is lower in winter at high latitudes.

A different approach, concentrated solar power, uses mirrors to focus sunlight and generate heat, which can be stored. It is far less widespread than PV.

Wind power

Onshore wind turbines are among the cheapest sources of new electricity in many places, though planning and local objections can slow them.
Offshore wind benefits from stronger, steadier winds and room for very large turbines, but costs more to build and connect.
Limits: output varies with the weather, sometimes for days at a time, and turbines need maintenance in harsh conditions.

Other renewables

Hydropower is the longest-established renewable source and can often be turned up and down quickly, which makes it valuable for balancing.
Bioenergy burns or processes organic material; its climate benefit depends heavily on how the fuel is sourced.
Geothermal uses heat from underground and runs steadily, but suitable sites are limited.

The variability challenge

Electricity grids must match supply and demand second by second. Traditional power stations could be dispatched on demand. Wind and solar produce power when the weather allows, which creates two problems: times of surplus, when output may have to be curtailed, and times of shortfall, when other sources must fill the gap. The IEA has warned that where grid investment does not keep pace with new renewables, curtailment can become a growing problem.

How storage helps

Batteries, mostly lithium-ion, can respond almost instantly and are increasingly used to shift solar power from the afternoon into the evening and to stabilise the grid. They are typically used for hours rather than days.
Pumped hydro pumps water uphill when power is plentiful and releases it through turbines later. It is the largest form of grid storage worldwide but needs suitable geography.
Longer-duration options under development or early deployment include various battery chemistries, compressed air and producing hydrogen for later use. Storing energy across weeks or seasons remains difficult and expensive.

The grid is the hidden story

Many energy experts now see grids, not generation, as the main bottleneck.

Transmission lines must carry power from where it is windy or sunny to where people live, and they can take many years to plan and build.
Connection queues for new projects have grown long in several countries.
Flexibility includes interconnectors between countries, demand response, where large users shift consumption to cheaper times, and smart charging of electric vehicles.

Common questions

Can a grid run entirely on renewables? Some regions with abundant hydropower already come close. Others are working towards very high shares by combining wind, solar, storage, interconnection and flexible demand, while keeping some backup capacity.
What about nuclear? It is low-carbon but not renewable. Countries take very different views on its role.
Do renewables raise bills? It depends on the country, the market design, the cost of networks and the price of the fuels they replace. Be wary of simple answers either way.

What renewable projects are being built near you, and how are people reacting? Share your thoughts in the comments.