From photosynthesis to human digestion, from the combustion engine to the electric battery, energy is a fundamental conce

pt that underpins almost every aspect of our daily lives. However, it is difficult to define energy simply, as the topic and its applications are vast and complex.

In this effort of synthesis an

d popularization, we will explore what energy is, both in its definition and in its magnitudes. Then, we will examine its societal repercussions and, finally, explain the central role of energy in the climate crisis.

What is Energy?

There are many definitions of energy, from its Greek root « force in motion » to its physical definition: « the property of a material system capable of doing work. » It is characterized as a physical quantity with the ability to modify the environment. It takes various forms: radiation, chemical energy, nuclear, thermal, or mechanical energy. Energy can modify temperature, speed, shape, chemical composition, atomic composition, or even position in a field, whether magnetic, gravitational, or electric. Not only do we use it daily, but it is also an integral part of our bodies, which can be likened to a machine converting nutrients (from the food we eat) into heat and motion.

Its primary characteristic is its ability to be easily converted into another form of energy through machines or organisms. We can distinguish between primary energy, the energy entering the system, and final energy, the energy used. Let’s take a daily example: the combustion engine of a car. The primary energy is gasoline, a form of chemical energy, which is converted into heat energy through combustion, then into mechanical energy by the crankshaft system, and finally into motion. Here, we see that energy takes on four successive forms, with each transformation resulting in some loss. This is called energy efficiency, the ratio between final energy and primary energy, and energy vectors, which refer to the intermediate steps between the initial and final forms of energy. Energy is governed by two physical laws: the law of conservation (it can neither be created nor destroyed, only transformed; the energy entering equals the energy exiting) and the law of increasing entropy (energy always tends to disperse). In other words, as Lavoisier’s famous saying goes, « nothing is lost, nothing is created, everything is transformed. »

Another example is nuclear electricity. The primary energy comes from the fission of an atom, which is converted into heat in the reactor. This heat is collected by a fluid (often pressurized water) in motion (mechanical energy), which drives a turbine that converts mechanical energy into electricity. Electricity, in particular, is never the final energy but is the most universal and transportable vector discovered to date. However, it is very difficult to store, especially through batteries, which convert electricity into chemical energy with lower efficiency, as it will be reconverted into electricity for final use.

We also distinguish between the notion of energy and power. These concepts are related and complementary: power characterizes the capacity to transform energy in a given time. This is, however, the crux of human energy use. Machines allow us to multiply human power. Jean-Marc Jancovici aptly describes it as follows: the mastery of energy transformations has turned us into « Iron Man, » and we now use it daily without even realizing it. To illustrate, please refer to the below video:

 The energy required to toast a slice of bread is the same, whether the toaster is powered by the electric grid or a stationary bicycle. However, the effort required by a human to toast a slice of bread is considerable. The same goes for moving a cubic meter of earth: a bulldozer is much more efficient than a shovel and a pair of hands.

This leads us to the next concept: the origin of primary energy and energy density.

Primary Energy and Energy Density

Primary energy refers to the energy available as it exists in natural resources within the environment. These are unprocessed and unexploited energy products. Globally, the profile of primary energy consumption and origin is as follows:

We see here that 80% of primary energy consumed is from fossil sources (coal, oil, and natural gas). These originate from the fossilization of biomass, a process that takes millions of years. These energy sources have the advantage of being very dense, available in large quantities, easy to extract, transport, store, and thus cheap, and usable on demand.

To give an idea of the energy density of these sources, a 20-liter jerry can of gasoline represents the equivalent of 200 kWh of energy. It would take two months of operation of a 1 kW wind turbine to produce the same amount of energy (and the wind isn’t always blowing).

These energy sources come with major disadvantages. Firstly, they are non-renewable (or at least not quickly renewable, as it takes millions of years to fossilize biomass). They can therefore be considered stock resources, available in limited quantities and only in certain parts of the globe. Furthermore, their combustion (the way we transform their chemical energy into heat energy) emits large amounts of greenhouse gases, notably CO₂, which is responsible for global warming.

The rest of the primary energy consumed (approximately 20%) comes from renewable sources. They originate from resources that nature continually renews. They have the advantage of being available in unlimited quantities in the environment and, for the most part, are low in greenhouse gas emissions. Their major drawback is that they can be difficult to collect, are hard to store (except for biomass and hydropower), and some are intermittent, meaning they do not produce energy continuously or on demand.

So, what is the energy density between the two major families of energy sources? In 2019, humanity consumed 173,000 TWh of energy. One might wonder: is that a lot? Knowing that a standard solar panel produces 500 kWh of energy per year and the global population was 7.7 billion people, each person consumes an average of 22,367.5 kWh/year and would need 45 solar panels. This represents a surface area of 76.5 m², which is far from being available to each individual in some high-density regions. Furthermore, it is unlikely that the necessary materials would be available for such an area, making it a relatively expensive solution.

Converting 100% of the energy consumed into renewables presents a considerable technical challenge, as the technological maturity of these solutions lags behind fossil energy. This explains why a shift to 100% renewables, as desirable as it may be, is not feasible in the short term and represents the major challenge of the 21st century for humanity. Since the Industrial Revolution, humanity has significantly improved living conditions. Now, in the 21st century, the challenge is to sustain this system while making it accessible to an increasingly large population. This is what is known as the energy transition.

Societal Transformations Due to Energy

Before addressing the notion of the energy transition, let’s go back to the Industrial Revolution. Prior to this revolution, all primary energies used were renewable. Agriculture was done by manual labor or animal traction, mills powered by wind or water were used for forges and refining foodstuffs, particularly grains, and transportation was powered by animal traction or sailboats. Naturally, everything took more time and effort. At the time, it was difficult to generate mechanical energy from heat.

It all began with the invention of the steam engine, which allowed steam to be transformed into motion. The depletion of forest resources in England led the British to use coal as fuel. Coal, with a much higher calorific value than wood, allowed for much more powerful machines to emerge in the early 19th century. These machines enabled the rise of more efficient industrial sectors in all areas: textiles, metallurgy, construction, transportation, and agriculture, reducing the manual labor burden and industrial dependency on forceful tasks, shortening transit times, and facilitating trade. This liberation of labor, previously used mainly in agriculture, allowed for a tertiarization of activities and urban densification. More people were assigned to administrative tasks, whose demand exploded proportionally with the intensification of trade, as well as research and development. This period also saw an intensification of our knowledge of the world and improvements in techniques in all fields.

The discovery of oil, initially for the creation of high-performance lubricants and later as a source of energy through the combustion engine (in the second half of the 19th century), greatly facilitated transportation. Easily transportable, refined oil in the form of gasoline, diesel, or kerosene became dominant in mobility, enabling vehicles to achieve greater autonomy. Ford’s application of Taylorism allowed for the emergence of personal automobiles, within the broader spectrum of the consumer society in which we now live. Finally, the widespread use of electricity as an energy vector allows everyone to easily use controllable energy in a wide range of manufactured devices that we can no longer do without.

In a sense, an energy transition has already occurred in the past: from renewable energies to fossil energies. It enabled unprecedented development, resulting in the emergence of our modern, tertiary, urbanized society, much wealthier, with a much longer and more comfortable life expectancy. The rise of capitalism, Western democracies, and the emancipation of women are consequences of this energy transition.

Did you know that, in pre-industrial France, the indoor comfort temperature was only 13 degrees? That 98% of the population was born, lived, and died within a 10 km radius?

All these advances are the result of the discovery of these dense, abundant, and cheap energy sources. To illustrate my point, here is the evolution of global energy consumption. You will see that new energy sources discovered and exploited never completely replace those already in use but are added to them. And even if it seems that renewables are gaining ground, remember this: we have never extracted as much coal globally as in 2023.

The Central Role of Energy in the Climate Crisis

As mentioned earlier, 81% of the primary energy consumed worldwide comes from fossil fuels. If we now look at the sources of greenhouse gas emissions, we find that 75% come from energy production and use.

If we break down these emissions by sector, we get the following graph:

By itself, energy production accounts for only 4% of total global greenhouse gas emissions. However, its use across other sectors represents 75% of total emissions. Reducing energy-related emissions, therefore, has repercussions across all sectors.

This graph highlights why public energy transition initiatives particularly target emission reductions in key sectors:

• Transport: Regulations like Euro standards in Europe aim to reduce emissions related to mobility, which is mainly driven by the combustion of petroleum derivatives. The goal is both to reduce fuel consumption and to promote less carbon-intensive propulsion methods, such as electric vehicles. This has led to ambitious measures, such as the ban on the sale of internal combustion engine vehicles by 2035.
• Buildings:
  • Renovating and insulating existing buildings is encouraged to improve energy efficiency.
  • The widespread use of energy performance certificates (EPCs) for property sales in Europe aims to raise awareness and encourage energy improvements.
• Green energy production: Subsidies are provided to support renewable energy production projects for self-consumption (wind, solar, geothermal, biomethane). These incentives benefit both individuals and businesses, positively impacting sectors such as construction, agriculture, transport, and manufacturing.

These initiatives aim to promote a transition to more sustainable solutions and reduce the carbon footprint of the economy.

Conclusion

In conclusion, energy is both the driving force of our modern civilization and one of the main challenges humanity faces. While we still rely heavily on fossil fuels—sources of high energy density but responsible for the majority of greenhouse gas emissions—the need to transition to renewable energy sources is becoming urgent. This transformation, though essential, is complex and involves unprecedented technical, economic, and social challenges.

History shows that mastering new forms of energy has always been a catalyst for societal progress and transformation. However, the current energy transition differs from past ones. This time, it is not just about improving our living conditions but about ensuring the sustainability of our planet in the face of climate disruptions caused by the intensive use of fossil fuels.

The challenge lies in balancing the growing demand for energy, reducing carbon emissions, and developing greener, more efficient technologies. If managed well, this transition could not only help mitigate the effects of climate change but also pave the way for a new era of prosperity based on clean and renewable energy sources.