1. Introduction

Energy is a notion that took a long time to be properly defined in physics (only in the 19^th century). Today, we have a clear idea of it and are able to understand how to describe it and how to use it.

The most important notion is the notion of conservation: energy is conserved in an isolated system: this is the first principle of thermodynamics.

The other important notion, which is more difficult to grasp, is the fact that most real energy transformations are irreversible and that it was necessary to introduce the notion of entropy to explain the experimental behaviour of energy transformations between systems. A consequence of this notion of entropy is that during an irreversible energy transformation (which is almost always the case) from one energy to another, entropy can only increase and does not allow access to all the useful energy available.

Humans have always used energy to enable societies to evolve and progress. First and foremost, we used the energy of our body or that of animals, but we very quickly realised that other, non-animal sources, could replace animal labour. As a result, today, humans use, on average accross the world, about ten thousand times more energy than we are able to produce with our bodies, because this energy is produced by industrial means.

2. What are these energies? Where do they come from?

Energy sources used by humans are most often classified into two categories: non-renewable and renewable. This term simply indicates that, in the first case, humans deplete a pre-existing stock on earth and, in the second case, he uses a source that renews itself over a sufficiently short time scale so that the resources are replenished as they are consumed.

Thus, fossil fuels (coal, oil, gas) are not renewable. They represent a "stock" that has taken hundreds of thousands of years to build up in the past and they have been consumed en masse for a hundred years. They will probably be depleted on this time scale.

Similarly, uranium, which is the basis of nuclear energy, is used by consuming the ore present in the earth's soil. It is not strictly speaking renewable, although some technologies (such as fast neutron generators or fusion) have production potential that would consume only a small part of the available stock.

On the other hand, photovoltaic energy and/or wind energy, which draws energy from the sun or from winds without affecting the source, are renewable energies (considering that the sun has an infinite life span, which is acceptable on the scale of human life).

Hydropower is also a perfect example of renewable energy. Winter rains fill the reservoirs at altitude, the stored water is then used to turn turbines by gravity. Biomass (wood, plants, etc.) can also be considered renewable, provided that the same amount of plant is replanted as is harvested (the example of the management of the Landes forest is a perfect example).

All these energies, in one way or another, make it possible to produce mechanical work and heat, which facilitates human life and has allowed the development of modern societies.

In fact, humans do not produce energy, we merely transform it from one form to another. For example, we convert heat into mechanical energy in heat engines and then possibly into electrical energy with turbines and alternators. This electricity is then used to produce heat, mechanical work or any other form of energy, such as light.

The sources of energy are therefore free and available to us: fossil fuels have been stored in the soil for thousands of years; the sun and the wind are also available to everyone. The cost of energy is therefore not the cost of "producing" energy since humans do not produce energy as such but the price of access to raw materials (rent) and the cost of transformation (often linked to industrial investments).

3. Trends in energy consumption

When monitoring global energy consumption compared to¹ population growth, we notice not only² that the curves have been growing almost exponentially since the beginning of the industrial era (19^th century), but also that their growth is relatively equivalent. Therefore, it can be said that in the first order, energy consumption follows population growth. When calculating global energy consumption per capita,³ however, we see, that in the second order, it also increases exponentially. More generally, there is a very strong correlation between Gross Domestic Product (GDP) and energy consumption.⁴ If we look country by country, it is interesting to see that if, on average, we find this correlation, there is however a certain dispersion in the curves. For the same GDP, some countries spend almost 10 times more energy than others. It can therefore be concluded that, unsurprisingly, energy consumption is linked to economic activity both by the size of the population and by the "production" of wealth. However, there are significant differences between countries that are roughly identical because cultural differences lead to large variations in the energy consumption of a country.

Let us now look at the evolution of the different energy sources over time.

The first source of energy besides humans themselves was wood and more generally biomass. As long as the human density was low enough and the environmental exploitation was only marginal compared to the amount produced naturally, this resource was renewable. Unfortunately, the combination of population growth and the increasingly widespread use of heating for higher, more confortable temperature in housing has led to massive removals that have contributed, in some parts of the world, to deforestation. Other sources of heat had to be found to produce well-being and work. The discovery of fossil fuels was a revolution because of their abundance, ease of access and use. From the beginning of the 19^th century, coal has been an important source of energy and has greatly contributed to the industrialisation of Europe and the rest of the world. An examination of the consumption of different forms of fossil energy is interesting.⁵ For nearly 100 years, there has been an increase in the use of coal per capita, finally reaching a ceiling since the beginning of the 20^th century. Oil then took over towards the end of the 19^th century and has not yet stopped growing. More recently, at the beginning of the 20^th century, natural gas was introduced. Its use per capita is still increasing. Electricity, historically produced with fossil fuels, has seen a diversification of production sources. Hydroelectricity, first of all, with the construction of dams which took place in France from the 1930s to the 1970s. Then, just after the Second World War, a new form of electrical power generation linked to nuclear energy emerged. Civilian nuclear reactors have been developed. As early as the 1960s, France took the decision (a decision reinforced by the oil crises of the 1970s) to implement this technology, leading to full production power at the end of the 1990s, i.e. 30 to 40 years after the decision to launch the programme.

This example shows us that the implementation and evolution of the use of energy resources at a national scale take place over very long periods of time, several decades. It is therefore necessary to anticipate developments well in advance, otherwise reacting in a hurry leads to difficulties that can be major.

When assessing a country's energy consumption, the distinction between final and primary energy is important. Indeed, depending on whether one counts in final energy or in primary energy, serious differences appear. This is because while fossil fuels are considered primary energies, electrical energy is not considered: electricity comes from the transformation of primary energy into electrical energy and, as we have seen, part of the primary energy is "lost" (not usable) in this transformation…Electricity comes largely from the transformation of heat into mechanical and then electrical energy. The example of gas (or coal or oil) power plants proves useful: a fossil fuel is burnt to produce heat (thermal energy), with which pressurised steam is made, which turns turbines (mechanical energy) to produce electricity with alternators. The ratio between the thermal energy consumed and the electrical energy supplied is of the order of 3, i.e. an efficiency of about 30%. Thus, for 1 kWh of primary thermal energy, we obtain 0.3 kWh of final electrical energy. If this electricity is then used to heat a building, it is quite logical to apply a multiplication factor when counting in primary energy: thus 3 times more than the final energy used. One consequence is that, when counted in final energy, fossil fuel consumption (coal, oil, gas) in France is 70% of the total energy, whereas it is only 48% when counted in primary energy.

In the case of nuclear energy, things are more complex and it has been decided to apply, arbitrarily, a factor corresponding to the thermal efficiency of the turbines, without returning to the true “primary” source of nuclear energy, which would be the fission of atomic nuclei. Thus, in France, primary electrical energy corresponds on average to 2.58 times (by convention) the final energy. It must be realised that this figure is conventional (especially in the case of nuclear power) but it is important, when it comes to, for example, calculating the EPC (Energy performance certificate) of a building.

This explains the significant difference between the comparative weights of fossil energy according to whether one counts in final energy or primary energy. With this convention, electricity consumption in France corresponds to about 20% of total final energy, but about 38% of primary energy.

A few simple ideas can be drawn from the figures presented. Today, 84% of the world's final energy consumption is fossil energy (70% for France). Overall, therefore, we are still using energy that is mainly of fossil origin and therefore carbon-based (i.e. emitting excess CO₂). The characteristic times of changes in energy consumption are of the order of 30 to 40 years. We can therefore see that all the speeches explaining that we aim for carbon neutrality within 10–20 years through a profound change in our energy consumption are extremely optimistic or simply unrealistic.

4. The role of renewable energy

As far as renewables are concerned, there is no reason why the increase in the production of these new energies should be any faster than what we have seen in the past for the increase in the production of various fossil fuels or nuclear power. Indeed, there are several technological problems to be solved to quantitatively replace the current system, which is largely dominated by fossil fuels, with electricity from renewable sources. The three main items of energy consumption are: heating of buildings, transport and industrial production. The massive replacement of fossil fuel or nuclear power by renewable energies will require fundamental adaptations, both in the means of production and also in the means of distribution of these energies. This is not unthinkable, but will necessarily take a long time and certainly not in one or two decades.

Let's look at some of the challenges: first of all, transport. They consume about 1/3 of primary energy and use fossil fuels very heavily (close to 100%); in this case, replacing oil with electricity, even if the beginnings of solution have appeared with the electric car, is far from simple (cost, autonomy, evolution of the fleet...). For trucks, planes or ships, there are currently no viable solutions with a timeframe compatible with carbon neutrality in 20 years. For these heavy vehicles, hydrogen could be a solution, provided that it is produced in a decarbonised manner, at an acceptable cost, and safety issues (particularly for planes) are not be an insurmountable obstacle. There remains the production of liquid fuel, identical to oil, but with a renewable carbon source (biomass). Here again, if we take into account the quantities to be produced as well as the cost, we are very far from having an acceptable solution.

For industry, electricity is already an important source of energy, but its production needs to be decarbonised (nuclear or renewable). Moreover, highly energy-intensive industries (cement plants, blast furnaces, etc.) that emit large quantities of CO₂ do not have viable alternatives on a production-wide scale. The possibility of capturing and storing CO₂ when it is emitted by factories is conceivable, but this requires the use of oxygen rather than air as an oxidant and proven geological storage solutions. We are still only in the pilot tests with a significant additional cost.

5. Buildings, perhaps a prime target for energy conservation

Indeed, the use of geothermal energy, biomass or decarbonised electricity can be considered provided that massive investments are made to change to heating systems.

In the short term, it is likely that the safest way to reduce our CO₂ emissions is to reduce energy consumption, particularly in buildings, as this is the area where we can expect the most energy savings.

Let's now take a closer look at the energy expenditure situation in buildings.

With a world population of around 7 billion, 56% of whom live in cities and the rest in rural areas, the world is facing major challenges in terms of housing. Demographers' projections for 2050 predict no less than 10 billion inhabitants, 66% of whom will live in urban areas. A quick calculation shows that between 2010 and 2050, as many buildings will have to be constructed in the city as exist today. This will require a doubling of the global capacity of cities around the world. To understand the crucial role of buildings in the energy and environmental challenges that lie ahead, it is important to remember that, to date, in Western countries, buildings are the largest consumer of energy and one of the largest emitters of CO₂. With 42% of Europe's energy consumption (26% for transport and 32% for industry), this corresponds to almost half of the energy we consume, as this energy is used to heat or cool the buildings in which we live and work.

To fully understand the issue, it is important to bear in mind that the average building consumes around 320 kWhEP/m²/year (EP indicates that the calculation is made in primary energy, as required by the regulations). By comparison, a new residential building in France, built according to the thermal regulations in force (RT2012) must consume around 40 kWhEP/m²/year. The average consumption of existing buildings is therefore almost ten times greater than that of new buildings.

There is another way of looking at the situation. If we were to reduce the average to 100 kWhEP/m²/year, this would save practically the same amount of energy as we consume in transport.

All Western governments are aware of this situation and have enacted two kinds of measures: thermal regulations for new buildings and renovation plans for old buildings. It must be taken into account that, because of the very low rate of renewal of old buildings (around 1% per year), we cannot rely solely on the thermal regulations for new buildings to improve the situation.

There are several technologies that can be used to reduce the energy requirement of a building, in particular thermal insulation. Air immobilised in a lightweight porous material is the most accessible technique for insulating a building. The purpose of a porous material is to prevent convection which, by allowing the air to move, would greatly increase the heat exchanges that we want to avoid. It is thus possible to use entangled lightweight fibres (glass wool, rock wool, wood wool, etc.) or synthetic foams (polyester, polyurethane, etc.). Due to the ever increasing requirements related to improving building performance, the thickness of the insulation in the walls is increasing, up to 30 to 50 cm depending on the climate. While these thicknesses are acceptable for new buildings, they are increasingly problematic for renovation, in particular for internal insulation, as living space may be lost. It is therefore necessary to find more efficient insulating materials which, for equivalent thermal resistances, can achieve significantly lower thicknesses. Two technologies can be used: vacuum insulation and aerogels.

The first is to use vacuum insulation. They are made by covering panels of fumed silica (very fine silica) with an aluminium foil package. Beforehand, the air was removed from the panels and they were sealed. In this way, the performance can be increased by a factor of 5 to 7 compared to conventional insulation.

The second technology involves aerogels: they are made from precipitated silica by forming very small pores. If the size of the pores becomes of the order of magnitude of the mean free path of air molecules (of the order of a hundred nanometres), the thermal conductivity of the air is greatly reduced. This effect was discovered by Martin Knudsen [Knudsen 1934] in 1934 at the Technical University of Denmark. This makes it possible, without using vacuum, to have highly insulating materials improving performance by a factor of approximately 2 compared to immobilised air.

While improving wall insulation is necessary, it is not sufficient. It is also necessary to improve the insulation of glass walls. To this end, the appearance of double glazing with the insertion of a gas (Argon, Xenon, etc.) between the walls has made it possible to make significant progress. This double glazing has now become the norm. Other, more recent advances have further improved performance. The introduction of a coating, i.e. a series of very thin layers (a few nanometers) of conductive materials (most often silver) and dielectric materials, makes it possible to transform the pane into a true interferometer, allowing visible radiation to pass freely but reflecting infra-reds (which carry a large part of the thermal radiation energy) towards the interior. This "low-emissive" glass also saves energy in winter and summer. Very recently, windows whose transparency can be controlled by a potential difference between two conductive glass plates have been developed. These electrochromic windows are batteries containing metal ions which, depending on their degree of oxidation, absorb light or not.⁶ They make it possible to regulate the entry of light radiation into the building, thus reducing heat inputs and allowing continuous adaptation of the brightness.

Apart from the improved performance of the walls, it is necessary to measure the effect of the introduction of these technologies on the final building. While the physics of buildings allows to calculate fairly precisely what can be expected from the theoretical performance of a well-constructed building, it is surprising to see that few comprehensive measurement techniques have been developed so far. The performance of each of the materials used in a building is of course measured and controlled, on the other hand, in comparison, few performance checks are carried out on the final building. One of the reasons for this is the relative difficulty of developing fast and light measurement techniques. In recent years, such measures have emerged. Their principle is simple, but the implementation requires a few tricks. This involves considering a building as an electrical circuit (composed of at least one resistor and one capacitor) and using perturbation methods to measure the thermal resistance and heat capacity of the building [Mangematin et al. 2012; Boisson and Bouchié 2014].

6. Conclusions

The assessment of the links between energy and climate impact clearly shows that CO₂ emissions come mainly from energy production, it is necessary to play on both fronts: reducing our consumption and decarbonising energy production. From this point of view, we can be pleased that, at world level, since 1990, the ratio of energy production to GDP has fallen by around 30%, as have CO₂ emissions per capita. Unfortunately, global GDP and population are growing faster than these relative declines, so that, far from decreasing, global energy consumption and CO₂ emissions are increasing in absolute terms. For France, the objectives of the low-carbon strategy are very ambitious. They aim to make the country carbon neutral by 2050 by reducing emissions by a factor of 6 and offsetting the rest with carbon sinks. These objectives are all the more ambitious because the recent past has shown us that our country, like many others, has previously set less ambitious objectives and has not been able to meet them.⁷ We can therefore legitimately ask ourselves the question of the realism of our country's ambitions as well as those of the rest of the world with regard to the climate challenge.