The enormous reserves of natural fuels formed over the course of our planet's evolution, settled in the depths of the earth's crust and preserved there for millions of years, we are burning them all within a single century.

Oil, coal and natural gas today cover 80% of global energy demand; a further 6% or so is covered by fissile material (essentially uranium-235, derived from natural uranium) in nuclear power plants. This means that nine tenths of global energy demand are still met by dirty, outdated, exhaustible sources, and that only the remaining share already comes from renewable energy sources (hydroelectric roughly 6%, while biomass, geothermal and wind together account for around 5%). The Key World Energy Statistics published annually by the IEA – International Energy Association sets out the terms of an issue that demands to be understood with a certain urgency. Fossil sources are destined to run out. This will happen within a time frame that, on a historical scale, is relatively short, making way for renewable energy.

How long will fossil fuels last before making way for renewable energy

For example, at current rates coal could last another century. Perhaps natural gas for two. The peak of oil production is currently expected to arrive within a window of between 5 and 30 years — if it has not already been passed — after which its price will begin to rise until it becomes economically unsustainable. For lower-grade coal and bituminous residues, supply timescales are on the order of one or two centuries, but with greater problems of energy yield and atmospheric emissions that are already scandalously problematic today.

Globally, energy demand is growing continuously. At the same time, so too is the need to reduce CO2, combat the greenhouse effect and decrease atmospheric emissions of substances harmful to health and the environment that alter the climate. Two needs that seem difficult to reconcile.

One possible solution is the production of electricity from renewable sources.

Fossil energy sources vs. Renewable energy sources

Fossil energy sources (oil, coal, tar, gas)	Renewable energy sources
• Limited (the world's existing resources are on the verge of running out)	• Unlimited (the sun, water, wind and geothermal energy are renewable and unlimited)
• Cause air pollution and CO2 and greenhouse gas emissions	• Non-polluting, emit no carbon dioxide / greenhouse gases
• Concentrated in only a few countries (geopolitical tensions and conflicts)	• Found everywhere: the sun shines throughout the world
• Risks during transport (tankers, pipelines…)	• No transport risks
• They are the past	• They are the future

What is renewable energy and why does it matter?

"The age of fossil fuels is drawing to a close. An energy regime capable of channelling civilisation towards a radically new path is being born," says Jeremy Rifkin, President of the Foundation on Economic Trends in Washington and lecturer at the Wharton School.

Renewable energy is the name given to electricity produced from all sources that are alternatives to traditional fossil fuels (oil, fuel oil, coal).

The meaning of renewable sources

These sources are called renewable because they renew themselves — they have the inherent characteristic of renewing themselves, that is, of not being depleted as a result of our conversion of them into usable energy. On closer inspection, the term "alternative energies" is unfair, given that throughout history it has always been the energy of the wind that propelled ships, the energy of water that turned mills, the energy of the sun that set clouds, rain and winds in motion, and the Earth that made plants and trees grow for our nourishment and to build houses and shelters, to give us food and firewood.

Renewables are diffuse, scattered energy sources, inherent in the force of the elements, in the water of rivers and oceans, in the wind and the sun. The heat of the subsoil, agricultural crops, underwater currents and high-altitude winds, and ocean waves are also energy sources. They are inexhaustible, as well as requiring electricity-generation processes with a low environmental impact and producing minimal or zero CO2 emissions.

The development of renewable energy plants would make it possible to reduce dependence on oil-producing countries, to stop generating hazardous or hard-to-dispose-of waste, to leave behind oil spills, disasters at sea, and ruptures of platforms, pipelines and conduits. The widespread presence of renewable sources allows us to envisage a distributed generation network, which in turn makes it possible to reduce energy transport costs, to keep the wealth of local production in place, and to avoid exporting capital to distant countries, to the Middle East or, worse, to countries that host international terrorist movements.

According to IEA forecasts, renewables will go on to supply 13.7% of primary energy worldwide by 2030. If governments introduce incentive schemes, that figure will reach 16%.

Renewable energy in a global perspective

As we explore the potential and innovations of individual renewable energy sources, it is essential to also observe the broader picture. At a global level, we are witnessing unprecedented expansion in the field of renewable energy. One figure that stands out strongly is the substantial growth in electrical capacity generated from these sources.

By the end of 2023, annual additions of renewable electricity capacity are expected to exceed 440 gigawatts (GW), bringing the cumulative global total to around 4,500 GW. This remarkable progress underlines the growing global recognition of the importance of renewable energy and its crucial role in shaping a sustainable energy future.

The sun: photovoltaic, concentrating solar power, solar thermal

The sun is a G2 star — slightly larger than an average star. In its core, every second, 700 million tonnes of hydrogen are converted into helium through nuclear fusion reactions, releasing energy equivalent to 386 billion billion megawatts.

In 2023, photovoltaic energy took on an even more central role in the renewable energy landscape. Two thirds of the overall growth in renewable energy during this year is attributed to photovoltaic technology. This impressive development is the result of a combination of factors: the reduction in the cost of photovoltaic modules, which has made the technology more accessible, and the increased spread of distributed systems, which have expanded the use of photovoltaics across different geographical contexts. These developments have been supported by policies and incentives in key markets, which have strengthened the attractiveness and viability of photovoltaics as a key energy solution. According to the International Energy Agency, this trend is evident in the significant increase in photovoltaic installations worldwide.

Could the sun fully meet humanity's energy needs? "Certainly," replied Hermann Scheer, the architect of German energy policy, speaking to LifeGate Radio. "Every day the sun projects onto the earth 15,000 times the amount of energy we produce from nuclear power and from other non-renewable sources."

Earth's surface receives 170,000 TW (1 TeraWatt = 1 million million Watts) of solar energy.

Of these 170,000 TW:

– 50,000 TW are reflected by the upper layers of the atmosphere;

– 30,000 TW are absorbed by the atmosphere;

– 90,000 TW reach the Earth's surface. Of these 90,000 TW, most is reflected by the surface or absorbed and re-emitted. A portion, however, is transformed:

– 400 TW lift ocean water up to the clouds,

– 370 TW set the wind in motion,

– 80 TW are transformed by plant photosynthesis.

In short, an immense flow of energy envelops the Earth, shifting and transforming itself. Harnessing this renewable energy source is the challenge: learning to intercept at least part of this golden cascade.

From solar radiation to electrical energy

Today there are at least four systems for doing so.

• Photovoltaic, in which solar radiation is converted directly into electrical energy by means of "silicon solar cells".
• Solar thermal, for heating a liquid either directly (the water-heating panels widespread in Mediterranean countries) or with a heat pump;
• Concentrated solar power, thermodynamic-electric, using mirrors that focus heat onto boilers which vaporise water and send it to a turbogenerator;
• Passive solar, in architecture, through the rationalisation of building shapes, structures, materials and orientations in order to capture and retain the greatest possible amount of sunlight and heat, or to regulate warmth and cold.

New concentrated solar power plants are on their way, along with more efficient, versatile and colourful photovoltaic cells, and transparent photovoltaic solar modules to be incorporated into windows and building glazing.

The potential of photovoltaics for global energy production

According to the Trans-Mediterranean Renewable Energy Cooperation, covering 4% of the Sahara's surface with photovoltaic panels would meet the energy needs of the entire globe.

Wind: wind energy and offshore wind

There are 5.2 thousand trillion tonnes of it above and all around each of us. It is air. An unimaginable quantity of air in constant motion, circulating, warming and cooling in a perpetual dynamic interchange with the oceans and the landmasses.

Wind is precisely this: the movement of atmospheric masses that swirl incessantly at a planetary level between pressure differentials (the "gradient"), from high-pressure fields towards low-pressure ones.

The instrument for converting the "power in the wind" (whose factors are air density, the area intercepted by the wind, and the instantaneous wind speed) into electrical energy is the wind turbine. These range from small ones, half a metre in diameter (capable of generating 20W, like the Marlec 500), to large ones, up to gigantic dimensions (a Vestas model producing 1,650 kW has a rotor diameter of no less than 120 metres, as does a model by the German firm Repower).

Advances in wind turbine design over the past twenty years have enabled them to operate at lower wind speeds as well, harnessing a greater quantity of energy and collecting it at greater heights, increasing the amount of exploitable wind energy and reducing rotation speed, noise and visual impact on the landscape. A great many territories could be used to generate wind energy in sparsely populated areas — windy regions such as the great plains of North America, north-west China, Eastern Siberia and the Argentine regions of Patagonia — in addition to the enormous potential of offshore installations. One of the world's foremost environmental scientists, Lester Brown, is convinced that wind represents the next energy future: "wind is abundant, cheap, inexhaustible, widely distributed, does not harm the climate and is clean".

Wind: from renewable source to electrical energy

The very different types of wind installations correspond to the need to adapt to the local terrain:

• onshore wind farms, with rows of turbines on flat ground and gently elevated ridges
• offshore wind farms, which have attracted increasing attention because wind at sea is stronger and less turbulent than on land, with beneficial effects both on the energy produced — which is the most important aspect — and on turbine lifespan… Future prospects point to installations in coming years resting on seabeds at depths of up to several hundred metres (today depths of a few tens of metres are reached), and subsequently wind farms on platforms anchored to the seabed
• mini-turbines, architecturally integrated into buildings or port structures (projects to which major architects from Dubai to London, from Philippe Starck to Renzo Piano, have committed themselves).

The potential of wind for global energy production

According to experts at the World Watch Institute, onshore winds alone, if systematically and strategically harnessed, could already supply energy equal to four times global energy demand.

That is to say, wind alone can give the world all the energy we consume today, with three times as much left over. And that is without counting offshore winds, which are even stronger, more stable and more promising.

In a study to quantify global wind energy resources called Wind Force 12, the European Wind Energy Association concludes that the global potential of energy generatable from wind would be fully double the world electricity demand projected for 2020.

Water: hydroelectric and mini-hydro

Water covers 71% of the Earth's surface and occupies an enormous volume, impossible even to imagine. One and a half billion cubic kilometres.

All the planet's waters — in the oceans, rivers, underground and in the atmosphere — are interconnected. Everywhere, water circulates and renews itself over time. The average timescales of this flow vary greatly. A single water molecule remains in the deepest underground aquifers for millennia on average; in the oceans the residence time extends to hundreds of years; in the atmosphere it does not exceed 4 days.

The complex dynamics connecting all 1,400 billion cubic kilometres of the planet's waters is known as the hydrogeological cycle.

The hydrological cycle, by contrast, refers to the dynamics of only those waters circulating in the atmosphere — approximately 577,000 km³, no more than 0.000041% of the total hydrosphere. Yet sufficient to blanket the entire globe, to determine meteorological conditions, the climate and life on Earth.

Atmospheric waters renew themselves completely as many as 40 times every year.

Viewed from a macroscopic perspective, it is this enveloping, restless, powerful force — triggered by solar radiation, which prevents water from remaining stagnant — channelled into rivers, streams, rainfall and evaporation, that must be harnessed for conversion into energy. It is the perpetual movement of water transforming itself.

A rough indication of its global potential can be drawn from the surface water map, that is, from the bodies of water in circulation and in motion through annual precipitation.

– North America: 18,300 km³

– South America: 28,400 km³

– Europe: 8,290 km³

– Africa: 22,300 km³

– Asia: 32,200 km³

– Australia: 7,080 km³

Every continent has thousands of cubic kilometres of water in motion, in the form of rainfall alone. Of this, the run-off (meaning the water that remains and does not re-enter circulation through evaporation) varies from 45% in North America and Asia to 20% in Africa (where the evaporation rate is the highest).

Water: from renewable source to hydroelectric energy

A hydroelectric plant is defined as the engineering and construction apparatus comprising the complex of hydraulic works, machinery and equipment used to convert the potential energy of water into electrical energy.

Hydroelectric power stations can be of two types: run-of-river, when they directly exploit the flow of a watercourse; or storage-based, when they make use of a reservoir upstream of the plant that allows the desired water flow to be channelled.

A dossier presented by the Istituto Ambiente Italia shows that there are 50,000 large dams in the world (those more than 15 metres high), with 67% concentrated in China, Turkey, Iran and Japan. As a result of these dams, between 40 and 80 million people have been forced into displacement, over 35 million in India alone. A nation larger than Italy has effectively been evacuated and destroyed — indeed, submerged. The solution for obtaining hydroelectric energy is not large dams, but the construction of "mini-hydro" plants: small turbines placed at the edges of rivers, which remain free to flow.

The potential of water for global energy production

Dams and river diversions are highly problematic and sometimes unacceptable operations today, from both a social and an environmental standpoint. But water can also mean energy from waves, tides and underwater currents. Various systems are under study — suspension bridges on the shore that oscillate, metal columns planted in the sea like gigantic barometers, pumps, buoys, basins, parabolic chutes to amplify jets of water — to harness wave motion which, being in constant and restless movement, is believed to be capable of generating usable energy. There are also enormous masses of water moving at const