5
Critical Densities of Energy Self-Sufficiency and Carbon Neutrality
Finding the routes to self-sufficiency. The end of the two-century long fossil fuel interlude. Creating a new global convergence, a myriad network of local self-sufficiency. What is the limiting balance between the consumption and collection of renewable flows? What is the density, the level curve of an energy topography that identifies self-sufficient regions? What are the good governance scales for orchestrating energy self-sufficiency and carbon neutrality? 5.1. Introduction This chapter originates from a presentation made at the Ecole Polytechnique Fédérale de Lausanne entitled “Territories with 1 watt”1. Before this conference, in November 2015, Léo Benichou sent the following question to Fanny Lopez, Marc Barthelemy and me: “[...] Inspired by Marc Barthelemy’s work on networks, Raphaël Ménard’s work on energy catchment areas, and of course Fanny Lopez’s work on the disconnection dream, I would like to raise the following question: Given the challenges of mobilizing solar (and derived) energy resources at the service of human societies. Given the infrastructures’ energy costs (fixed and mobile) that make it possible to collect, transform, store, transport and deliver energy carriers and services. [...] How can an optimal scale for the expansion and pooling of our energy infrastructures be determined? When sizing our
Chapter written by Raphael MÉNARD. 1 In 2016 as part of the IDEAS doctoral seminars.
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networks what would be the relevant dimensions to reveal the thresholds and the balance between optimality regimes? Between dependence and self-sufficiency, “Nucleocrat Jacobinism” and the “ideal of disconnection”, how can physics help us give some meaning to warm water? [...]” Jotting down my first thoughts I answered the following: “[...] One watt per m² is for me the right balance, the order of magnitude of the primary flow’s threshold, the possible equilibrium between the density of an abundant and complex renewable supply (with a mix combining biomass and solar-wind energy, excluding local specificities of other flows: geothermal and hydraulic) and the combined consumption density. For a decrease of primary consumption per individual of about 4,000 W, the average density P+E2 of the area considered would be 250 people per km². In my opinion, outside metropolitan areas, the large cantons (or small departments) represent the convergence area between these two densities. [...] In southern countries this value would probably be smaller due to lower individual consumption and higher solar potential [...]” Before explaining these values, this permanence scale, I would like to share the following thoughts inspired by the works of David MacKay. This Cambridge physicist, who died prematurely in April 2016, wrote a major piece on energy issues. In his book Sustainable Energy, Without the Hot Air, MacKay examined the technical and spatial issues of an energy-carbon transition [MAC 09]. In this book, he explains the appropriate sizing that would allow the convergence between our energy consumption and an adapted mix of renewable products3. This method joins the work carried out by the members of the Négawatt Association and their proposals regarding the triptych-slogan “sobriety, efficiency and renewable”. Another researcher, the Canadian Vaclav Smil, is gathering information on the spatial densities of energy flows both for production and consumption [SMI 15].
2 P + E for the density adding population and jobs which is sometimes a more relevant indicator. 3 He specifically describes the capacity for decreasing energy consumption in Great Britain. He simultaneously analyzes the contextual implementation of a tangible cluster of renewable energies. He is thus depicting in this geographical perimeter the technically and socially conceivable paths with the aim of bringing together supply and demand.
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5.1.1. What can environmental measures be related to? I include myself in this “MacKayan” affiliation, a methodical construction linking energy consumption and production, and carbon emissions and sequestration. Amory Lovins, founder of the Rocky Mountain Institute, also explains the issues of self-sufficiency, although Smil may be critical about the way he includes the spatial challenges of renewable energy [LOV 11]. The strength of MacKay’s description lies in measuring energy at the individual’s level, thus involving our civic responsibilities. MacKay reminds us of the urgent cultural transition and emulates Buckminster Fuller’s aphorism: “There isn’t an energy crisis but simply an ignorance crisis”. MacKay and the 2,000-Watt Society’s quantitative framework coincide in placing energy uses at the core of the changes needed [MAC 09]. Regarding the size, this variable focusing on the individual4 differs from the concept currently used by energy system designers for whom energy advantages are traditionally brought back to the surface. In energy system design accounting, annual kWh and CO2 emissions are measured per square meter. Consumption is derived from the space available from the container. This quotient5 is inappropriate because a structure with no use does not generate any externality: it does not need to be heated, or to be well lit, etc., and so it would not consume any energy6. Aspects relating to density have often aroused my curiosity. In the article Dense Cities in 2050: The Energy Option? [MEN 11], I introduced an initial controversy to this line of thought: urban density would be the necessary condition for ecological qualities. Indeed, 20 years ago, Australian researchers, Newman and Kenworthy, revealed the correlation between urban density and energy consumption regarding the use of cars. Based on surveys carried out in major cities around the world, this correlation revealed that fuel consumption decreased with increasing density [NEW 99]. This curve shows the impact of density on a portion of our energy consumption. However, this asymptote, which has become famous with time, creates the illusion that individual consumption sharply decreases with a high urban density, as if the latter made it possible to offset the effect of population. Interpreted in a rush, it could even suggest that the surface density of energy consumption has become infinitesimal.
4 Or on use. 5 The same applies to transport efficiency, cars in particular. Most efforts seem to focus on reducing the vehicle’s fuel consumption and/or its emissions, whereas the real criterion is the service provided and thus the massive increase in occupation rate induced. 6 Common language misuse: energy is not consumed, the first law of thermodynamics states that it is conserved. It is the energy quality that is consumed.
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Figure 5.1. Graph taken from the article “Dense Cities in 2050: The Energy Option?” showing the evolution of energy consumption linked to car use and the production of renewable energies according to urban density [MEN 11]
↓ Incoming flows
Outgoing flows ↑
Energy
Local production of renewable energy
Final energy demand or energy consumption
Carbon
Pumping of atmospheric CO2 and carbon sequestration
Emission of direct and/or indirect greenhouse gases
Figure 5.2. Incoming and outgoing flows for a given region
In that same article [MEN 11], I showed that this correlation would evolve in the coming years as a result of the improvement of the efficiency and use of the vehicle fleet and also as a result of the massive integration of renewable energies due to their technical maturity and economic competitiveness. I questioned this balance between consumption and production densities within a particular scope of energy accounting. Thus, as a continuation of said article, this paper will examine more globally the flow tendencies according to density. We will see how this parameter
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determines the maximum pressures of energy self-sufficiency and/or carbon neutrality. To analyze this, we will use fluid mechanics and its description models. There are two options to choose from: the so-called “Eulerian” point of view or the “Lagrangian” one. These are two different frames of reference, two different ways of describing nature. In the Lagrange description, the equations are written from the moving particle’s point of view; an account as seen by the protagonist of the scene, the particle of matter itself. To quantify consumption, we will therefore preferably follow the moving particle that represents the individual. In section 5.2, we will follow MacKay’s theory which diverts the consumption flows toward the person7 [MAC 09]. The same applies to carbon, discussed in section 5.5, by estimating the emissions as being equivalent to the individual8. For Euler, the narrative is omniscient: the world is described from a motionless position and matter is observed by a static observer. In section 5.3, we will evaluate, according to this framework, the renewable energy production capacities of a territory (or its carbon sequestration capacities in section 5.5) such as Smil does in Power Density [SMI 15]. We will also see how the energy consumption space density9 enables the transposition of the different points of view. 5.1.2. Critical densities and catchment areas With the use of some quantification tools with the aim of testing the hypothesis of energy self-sufficiency in 2050, section 5.4 will look at a specific case, the example of a small urban area in Lille a few hectares in size [MEN 15]. We will analyze the reasons that prevent it from ensuring its own energy self-sufficiency. We will show various types of energy transition paths according to different densities where the “critical point” is the famous watt per unit area, the “reference level curve of energy self-sufficiency”. We will describe the dual situation of carbon, the spatial issues between highly emitting regions and those that can act as atmospheric carbon pumps, trapping the carbon atom in biomass, in the countryside and forests in particular, i.e. low-density regions. The concept of exergy rate10 will appear in some of the diagrams herein. This value measures the energy quality. The closer this rate is to 1 (or to 100%), the higher the energy quality. On the other hand, low heat (for example the heating of a 7 For example in watts per person. 8 In tons of CO2 equivalent per person per year. 9 Or population space density. 10 As part of the Réforme project, we set the bases for “energy algebra”, a two-dimensional expression of energy to describe quantity and quality such as changing real numbers to complex numbers [MEN 14].
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home) will have a very low exergy rate of the order of a few percent. The prevailing unit of measure for this quantity is the watt (W). It measures a time flow of energy, i.e. power. By using this unit, we do not need to relate an energy quantity (such as kWh11) to a given time (e.g. a year). Therefore, a home at “50 kWh per m² per year” will be at “6 watts per m”12. Another example: Paris’ solar potential is equal13 to 130 watts per m²: this corresponds to the average solar flux, both in summer and winter, during day and night. According to this convention, time no longer intervenes in this equation; flux averaged over a certain period of time is measured14. “KWh per day or kWh per year” are nothing more than average power: an energy quantity divided by time15. Therefore, unless specified, the energy units16 will be expressed as primary energy17, upstream in the energy’s lifecycle. In some cases, the term “electrical” or “thermal” will refer to the energy quality of the flow being considered, and will therefore provide an indication of the “exergy rate” of the flow or the quantity in question. 5.2. Energy consumption density 5.2.1. Differences regarding the 2,000 watts Energy consumption is Lagrangian and here we return energy needs to the individual. The 2000-Watt Society vision developed in Switzerland18 established a framework for developing urban scenarios aimed at dividing individual energy consumption by at least 3. This method is global; it incorporates building-related consumption, mobility, infrastructure and food consumption. In Europe, primary energy consumption averages around 6,000 watts per person, more than triple the world average of just over 2,000 watts per person. Two hundred years ago, before the fossil fuel transitions, consumption was much lower: between 500 and 1,000 watts per person.
11 This unit is also composite since typographically it joins power (W or kW) with a quantity of time (1 h). 12 50kWh divided by 8,760 hours in a year. This value should of course not be confused with that of the sizing of heating which would be close to 50–100 watts per m² and which would correspond to a peak value. 13 Expressed in kWh, horizontal solar irradiation is about 1,100 kWh per m² and per year. 14 As a general rule this is a year unless otherwise specifically stated. 15 The unit of time will eventually appear when we evaluate energy reserves: for example to quantify the storage capacity of an electric battery (for example in the form of Wh or kWh). 16 Of flux (in watt) or storage capacity (as described above). 17 Primary energy comes from raw energy products as provided by nature: wood, coal, oil, natural gas, uranium (for non-renewable energy forms; solar, wind, biomass, geothermal, tidal and hydro power for renewable forms). 18 http://www.2000watt.ch/fr/.
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Figure 5.3. Individual consumption of primary energy (in W per person) and its correlation according to latitude (according to Réform [MEN 14]). For a color version of this figure, see www.iste.co.uk/lopez/local.zip
At present, the quantity and quality19 of our energy needs are strongly related to living standards20. In Réforme [MEN 14], we tested the relationship between consumption and latitude. In the regions contained within the wide ring centered on the equator and located between latitudes 30° south and 30° north, lives more than two-thirds of the world population (see diagram below); this “large society” is living at 1,000 W, a power close to that of westerners’ great-grandparents. Obviously, this is not a question of a new geographical determinism, or the discovery of a correlation between climates and energy consumption. This graph reveals the
19 In two centuries, average demand has increased from 500 to 2,000 W. In fact, the consumption of our ancestors was mainly based on heating needs (to keep warm, cook or transform matter). Since the postwar economic boom, the large increase in mobility, communication and the digital age created final energy needs that are more qualitative from a thermodynamic point of view. 20 Generally measured in annual GDP per capita.
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relationship between living standards and latitude, showing the existing division between northern and southern countries. There are also clear paths to be followed by the different regions of the world to move toward continuous energy production (100% renewable) and carbon neutrality. 5.2.2. 0.1 watts per square meter as average for mainland France The density of use makes it possible to transpose individual demand into spatial density according to the relation below. The density of a region is usually expressed in persons per km² or in persons per hectare. Before applying the product directly, we will perform the appropriate conversions. With the density of use, the Lagrangian approach is transformed into an Eulerian one. For mainland France, with a population of around 120 inhabitants per square kilometer and an average consumption per person of about 6,000 W, the energy consumption density is of around 0.6 W/m2. For Paris, with over 20,000 inhabitants per square kilometer, the consumption density reaches 120 W/m2, which is close to the value of the average solar flux striking the capital.
Consumption density [W/m2] = Individual consumption [W/pers.] × Population density [pers./m2]
Framed text 5.1. Consumption density formula
If we take the product of individual consumption with the geographical latitude density function, we obtain the relation between surface consumption and latitude. Average continental consumption is 0.1 W/m2 and drops to 0.03 W/m2 if we include oceans and seas. Land lying between latitudes 25° and 55° north is, on average, the most energy-hungry: more than 0.2 W/m2. This, however, remains three times lower than the French consumption density for two reasons: France is on average denser than the regions within these continental bands and its population requires 6,000 W compared to the 4,000 W required on average per person in the regions between 44° north and 51° north. Smil makes a historical summary of the chronology of consumption densities for the most important eras for humanity [SMI 15]. In the era of hunter-gatherers, tens of thousands of years ago, human density was one person per 10 km² in arid environments and increased to one person per km² near the coast. The consumption
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density was about 0.1 mW/m², which is one thousandth of the current value21. With agriculture and the intensity of life during the Neolithic, consumption increased to 20 people per km² and consumption flow increased to 4 mW/m2, equal to the values of Sudan or Guyana at present. In Rome, inside the Aurelian walls, the population density reached 67,000 people per square kilometer and led to an intensity of 7 W/m² [SMI 15], a value not far from the fictional example we will later share for Lille in 2050 [MEN 15].
Figure 5.4. Energy consumption density according to latitude (according to Réforme [MEN 14]). For a color version of this figure, see www.iste.co.uk/lopez/local.zip
Table 5.1 summarizes in descending order current consumption density values. Singapore is in first position with more than 100 W/m², a combination of high individual demand and very high density; at the other end, Mauritania consumes 1 mW/m2, a hundred thousand times less than Singapore.
21 About 0.1 watts per m2 as average for mainland France.
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Table 5.1. Some energy consumption flow values per country. The population data are from 2012. The energy data are from 2009 (source: International Energy Statistics, 2012)
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5.3. Renewable energy production density 5.3.1. Renewable energy production is Eulerian Upstream of demand, the production of renewable energies is the result of regional planning. Production flows are the result of land-use choices: for agricultural or forestry production; for installed renewable capacities, such as a wind farm or a solar park or a hydraulic dam. Watts per unit area are produced without any direct relation to human density. Smil cites an impressive amount of data according to the type of renewable production: 5–10 electric watts per m² for a solar park22; 2–3 electric watts per m² for a wind farm; and, in general, about 0.1 W/m² for biomass23 [SMI 15]. MacKay reports similar ratios [MAC 09].
Table 5.2. Some production density values by MacKay serve as average values to quantify the result of energy harvesting plans (according to Réforme [MEN 14]). For a color version of this table, see www.iste.co.uk/lopez/local.zip
How can a mix of local productions be structured? What energy, material and knowledge investments are necessary to obtain space systems capable of converting flows crossing through a region? Primary flows are solar, wind, hydraulic, wave or geothermal. Biomass is an “intermediate energy carrier” resulting from photosynthesis and therefore from solar energy. In addition, the most homogeneous energy on the earth’s surface is solar flux. It is the densest of renewable energies: 169 W/m2 on average on the Earth’s surface. In France, solar flux is 130 W/m² on average, with many moments where it is zero (at night) and a few rare times when it reaches 1,000 W/m². In the south of France, the solar energy deposit generally exceeds 150 W/m². In Mauritania, it exceeds 22 Value not to be confused with the peak power of the park. 23 Thermal or metabolic use.
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200 W/m² whereas in Norway it reaches 80 W/m². This primary flow is therefore fairly evenly distributed over the planet and very available in those areas where the vast majority of the world population is located: most people receive more than 200 W/m². Each region receives at least 100 watts per unit area: this is more than most of the consumption densities we have listed above. It is also necessary to efficiently convert this flow, according to a “selection of renewable energies” in harmony with the region’s needs: food as “fuel” for the metabolism of living things, low heat to keep inhabitants warm, moderate heat for washing, heat at higher temperatures for our stoves24 and more qualitative forms of energy such as those allowing movement, light or electricity, an essential aspect of modernity that enables the processing, dissemination and storage of information. For production, the relationship below is the ruthless product of our regions, the yield being a weighted average of the different contributions25 of each energy harvesting26.
Offer density [W/m2] = Renewable energy deposit [W/pers.] × Yield [%]
Framed text 5.2. Production formula
5.3.2. Energy harvesting plans Energy harvesting plans are graphic instruments developed within the Réforme framework [MEN 14]. They make it possible to estimate the equivalent conversion efficiency of a region and visualize its mix of renewable energy productions. A specific typical yield is associated with each energy production facility27. Much like land use mapping (where maps show how land is being used), this new representation establishes an energetic land use pattern. If the region is considered as a solar system, as a “portfolio of renewable yields”, weighted by the relative areas and their yields, our regions can probably reach higher values than 1 Wm2, which is the target consumption value for regions with population densities between 100 and 1,000 persons per km². 24 And some other uses for material transformation. 25 Formula for estimating the equivalent density of renewable production of a section in space in horizontal projection. At the regional level, the yield will correspond to an average yield, the abundance of a variety of different yields: biomass, solar, wind, hydro, geothermal, etc. 26 Specific energies such as geothermal or hydropower will be evaluated separately. Here we are interested in land, whatever its size, and its solar collecting power. 27 Agriculture being one of them.
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Figure 5.5. An example of an energy harvesting plan carried out by RĂŠforme [MEN 14]. The example of a Paris district, a 100 Ă— 100 m2, summarized into a square showing a breakdown of energy productions (and the inefficient part in gray). For a color version of this figure, see www.iste.co.uk/lopez/local.zip
5.3.3. Quantification of the production flow of a region Harvesting plans evaluate each region regardless of its scale, indicating the inefficient areas and those that deliver a certain quantity and quality of production. In the future, according to each energy harvesting plan and its geographical location, we can imagine the creation of a temporal signal of its production flow, both in quantity and quality. This representation could also be used for carbon and to visualize the potential atmospheric pumping flows of a given region (section 5.5). In retrospect, 1,000 years ago, the Neolithic was already an energy revolution with the composition of the first land plans in order to maximize the yield of agricultural supply and its resistance. Our ancestors made harvesting plans specific to biomass. According to biomass polycultures, the thousandth of the typical yield of farming areas consisted of 0.1 W28 regions. Furthermore, the individual energy 28 Consumption ranging between 1,000 and 10,000 W per person.
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footprint was likely to be at least greater than 5,000 m² (or even 1 hectare) while the critical population density at that time was probably around 100 to 200 persons29 per km². 5.4. Self-sufficiency, convergence: 1-W regions 5.4.1. The 7 hectares, surface area per person in the world garden Over the past two centuries, the world’s population has experienced an exceptional growth with a sevenfold30 increase during this period. This demographic acceleration has induced the inverse effect with a drastic decrease in the average surface area of earth per person. If we evaluate the planetary area per inhabitant, the result is generally surprising: we tend to think of at least kilometric scales. Quite the opposite, our space is very limited and the spaceship earth metaphor has never been so relevant: we only have seven hectares per person and two-thirds are water31. When equally shared, each person cultivates a plot of land to fulfill all their needs32, which is a little less than 150 m per side. This fraction of the globe is representative of the diversity of landscapes: deserts, mountains, wastelands, etc., a sample, a geographical chimera33. On this piece of land, each person also offsets his or her greenhouse gas emissions to ensure carbon neutrality by 2050: this will be dealt with in section 5.5. Thus, this contemporary spatial rarity conditions the dual constraint of a reduction in consumption density (section 5.2), and a massive increase in renewable production density (section 5.3). In this third section, we will explore different situations that make the convergence of energy self-sufficiency tangible or not.
29 In Europe, the solar deposit is of the order of 100 W and the typical biomass yield is of the order of 0.1%, with local production density of the order of 0.1 W/m2. 30 In the preface to the second edition of Something New Under the Sun [MCN 10], John R. MacNeill considers that fossil fuels are at the root of most of the biosphere, lithosphere, atmosphere and the aquasphere’s transformations. This “energy tsunami” has at the same time generated an exponential growth of global population, one of the many “crooked curves”, typical of the Anthropocene. 31 Total surface area: 510 Mkm2; oceans’ surface area: 360 Mkm2 (71%); land masses surface area: 149 Mkm2 (29%). 32 And especially their energy consumption. 33 For educational purposes, we could consider making up a place to illustrate this, a thematic park dedicated to this concept, to this “physical experience of the maximum dimension”. This would undoubtedly seduce Olivier Rey, the philosopher who documents the “out of scale” and “out of proportion” in modernity in Une question de taille [REY 14].
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Table 5.3. Evolution of land surface area per person for the last 350 years. It shows the decrease from 65 hectares per person in 1750 to less than 7 hectares in 2050, making the assumption of 9 billion people on earth. In 300 years, the potential area to capture energy and matter has been reduced by a factor of ten, while individual energy and material requirements have grown by one order of magnitude. In 2050, the continental surface per human will correspond to a square with sides which are less than 140 m in length
Due to the enormous increase in energy requirements and the intensity of CO2 emissions, these nearly 8 billion gardens will have to be up to two orders of magnitude more productive than two centuries ago, given that the total energy consumption has been multiplied by nearly 70. However, if we can convert only 0.03% of the solar flux hitting each of these 7 hectares, we will obtain the famous 2,000 W required per person. This amount seems small enough, easily attainable and amounts to the composition of a planetary energy harvesting plan, which is finally perfectly feasible. 5.4.2. The story of urban transition in cities The following example is a prospective one, which we use here to illustrate this quest for self-sufficiency on a smaller scale. It takes place between today and 2050 in an area of a few hectares in the center of Lille [MEN 15]. At the end of 2015, the “La Fabrique de la Renaissance� project34 was awarded the EDF Architecture Bas 34 By the 169-Architecture, Obras and Elioth team.
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Carbone prize in a competition, which proposed to understand the required transformations to create an energetic landscape in 2050 at the scale of an existing Lille district.
Figure 5.6. General view of the Fabrique de la Renaissance (169-architecture, Obras and Elioth, drawing by Diane Berg). For a color version of this figure, see www.iste.co.uk/lopez/local.zip
The main idea of our project was to reintegrate workshops and factories into the heart of the city and develop a manifesto describing the trajectory of the interwoven mutations of energy, matter and culture [MEN 15]. The Third Industrial Revolution is not an “uberization� of the economy. This transformation is above all a cultural one, a political utopia where all the inhabitants participate in the transformation of the built fabric and the means of mobility with the enthusiasm of a joyful frugality. In 2050, this district is completely self-sufficient regarding all of the built environment’s energy needs because of the Duc, a low-tech solar infrastructure. 5.4.2.1. Is energy self-sufficiency accessible to 50 inhabitants per hectare? On a larger scale, with a surface area of 15 hectares, on which the project operates, we have described the strategy of decreasing energy consumption, a sort of local Negawatt scenario (section 5.2). We have then developed a scenario for increasing the production of renewable energies. At the end of this method, we will see how density guides energy self-sufficiency. When all efforts have been made to limit consumption and the urban form has offered its maximum solarization possibilities, only density allows the control of the energy self-sufficiency of the region. Energy consumption is evaluated over a wide area: the built environment, individual mobility35, added to food and to constructive depreciation, the embodied energy compared to its obsolescence. We followed the guidelines of the 2000-Watt 35 By exploiting the Newman and Kenworthy curves linking density and the distance traveled per person.
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Society: to include everything as consumption36. We analyze consumption using technical parameters and uses, including the density37 and building footprint variables38. In [MEN 15], this last parameter quantifies the decrease in the propersona residential area seen in the Euro-region from 2040 onwards: spatial happiness does not necessarily mean more space. 5.4.2.2. Initial state, a reminder of the situation in 2015: 22 W/m² At the beginning of our imaginary story, the site’s urban density39 is 50 people per hectare. The car fleet is almost exclusively thermal. The built fabric, although very close, has poor insulation. The typical annual consumption is 250 kWh/ m2. For a building footprint40 of more than 60 m², the consumption flow is of the order of 4,000 W per inhabitant. With a mix containing 85% hydrocarbons, annual emissions equal about 9 tons of CO2 per inhabitant41. By looking at the diagram, it appears that with standard equipment parameters, the consumption density can reach more than 70 W/m2 of urban surface. Hence, this consumption density becomes comparable to Lille’s horizontal solar potential42. With this consumption intensity, hoping for selfsufficiency is unreal since it would require an average conversion yield of over 50%. 5.4.2.3. Intermediate stage, the project’s first results in 2025: 15 W/m² In 2025, the effects of the Fabrique are felt by its neighbors. As a result of the district’s community of knowledge and practice, the passion for transformation has begun to positively affect some of the surrounding fabric. The district’s residents, helped by students and with the guidance of engineering students, transform their homes: draftproofing windows, double curtains, insulating from the outside with old books. Thus, the average consumption of buildings in the district has decreased from 250 to 150 kWh/m² per year. The thermal car fleet has reduced its average consumption; the share of electric vehicles is over 20% for all the distances traveled. Carpooling has increased sharply while some eco-rickshaws start to appear in the district. 36 For example, energy consumption related to leisure mobility is not included. It should be noted that a round trip Paris–New York is equivalent to about 6,000 kWh for a second-class user. To “energetically pay” an annual round trip over a similar distance is equal to 700 W, i.e. one-third of the 2,000 W. 37 Variable “d”, in persons per hectare. 38 Variable “f”, in built square meters per person. 39 i.e. “P+E” over the 15 ha. 40 Adding the residential footprint and that of buildings for professional uses. 41 This value is the one used as the emissions starting point for a Parisian resident in our Paris changes era study, which is around ten tons. 42 120 W per horizontal square meter.
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In 2025, and with the same uses, individual consumption is now less than 3,000 W per person. With a mix containing 70% hydrocarbons, greenhouse gas emissions are now around 5 tons per year per person. At the district level, energy consumption density remains high: nearly 15 W/m², which is five times higher than the current average consumption density in the Netherlands (see table in section 5.2). 5.4.2.4. The project in 2050, a district at 2,000 W and a density of 8 W/m² In 2050, while the building footprint density has decreased43, the urban density has slightly increased44. The product of “f.e”45 has slightly decreased: some buildings have been deconstructed and their components have made it possible to increase the efficiency and the spatial quality of standing buildings. A sort of “donation of construction organs”, the tangible implementation of a local circular economy.
Table 5.4. Summary and comparison of energy consumption between 2015 and 2050. “Density-footprint” graphs that allow the estimation of individual and spatial energy consumption. In 2050, keeping a population density of 50 people per hectare, the energy consumption remains higher than 8 W per m² even though great efforts have been made to reduce all energy consumption. For a color version of this table, see www.iste.co.uk/lopez/local.zip.
43 “f”, the residential surface area per person has decreased compared to 2025. 44 Holiday rates have improved as a result of the district’s attractiveness. 45 Which corresponds to the district’s built density.
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Thus, the morphological density has decreased by about 10%: there are more voids, more interstitial spaces, meaning it is also possible to give that space back to living species, biodiversity and the pleasure of gardening. In the end, the population density remains the same, notwithstanding the “qualitative downsizing” of those living in the existing transformed spaces. Individual consumption becomes less than 1,500 W, or 8 W/m², which is equal to 6% of Lille’s solar potential. Thanks to the Fabrique and its inhabitants, by setting up an ingenious material loop, energy consumption per capita is now less than the equivalent of 1 ton of oil: one-third of the value in 2015, a decrease by a factor of three in less than 30 years. 5.4.2.5. A first assessment of the convergence between consumption and production If the residents’ consumption is reduced while maintaining the same density of use, the energy consumption density is now limited to 8 W/m². Given the complexity of this urban fabric and given the confusing urban geometry, will we succeed in producing that much? Now, with experience in energy harvesting plans (seen in section 5.3), and given the solar potential of 120 W/m², will we achieve a renewable yield of nearly 7%46? If this urban land consisted of half of the current buildings, and one-third of the roofs had solar panels with an average yield of 30%, even then the equivalent yield would only be 5%. In 2050, as a result of photovoltaics being highly competitive and due to a drastic increase in conversion efficiency, all individuals and condominiums have mostly opted for this very economical solution. The improved knowledge of construction works and the joys of eco-home improvements that prevail in the district have also largely favored this colonization of roofs and exposed windows, as much as it has favored harvesting microregions for renewable energy production. Solar electricity production is thus rapidly reaching 60 times the level it had in (albeit tiny) 2015, from 0.05 to 3 W/m²: enough to supply the district’s needs most of the time and to export large amounts of energy to the public grid. In addition to this informal solarization, our project developed a new type of urban infrastructure, a “low exergy solar system” known as the “Duc”. Its creation is extremely simple, a nod to the American counter-culture of the 1960s and 1970s. During that time, architect Steve Baer designed what was probably the most efficient solar system to produce low heat with a high efficiency for one of his
46 ~8 W/m² divided by a solar power of 120 W/m².
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zomes. The Duc uses this same principle while extruding it, linearizing it and spreading it over most of its roads. It is a low-tech smart grid that everyone can use: it proves that it is easy to “hack” the sun, to convert energy in order for us to become “urban energy peasants”. It stores in its water-filled tubes the solar energy for the day. At night, or when it is cloudy, flaps fall back to store the accumulated heat. It helps to significantly increase the district’s production by generating 2 W of heat per meter square of land of the entire district. Almost half of the thermal energy consumption47 is now produced and stored locally in energy and water tanks scattered around the district. It is also the supply umbilical cord of the entire urban system. Six meters above the ground, it ensures the transfer of all materials and objects, from the port to the workshops.
Figure 5.7. Some scenes near the Duc. Fabrique de la Renaissance Project (169architecture, Obras and Elioth, drawing by Diane Berg). For a color version of this figure, see www.iste.co.uk/lopez/local.zip
5.4.2.6. Energy self-sufficiency at the building scale At the end of this transition, “an” energy self-sufficiency by 2050 is emerging. Self-sufficiency is then assessed according to two distinct consumption perimeters. The first one is restricted, adding the energy consumption of the building in use. In this case, the district’s 15 hectares are mostly positive energy: production exceeds consumption by 35%. The second perimeter adopts a global point of view by also taking into account the energy necessary for individual mobility and the supply and depreciation of embodied energy. According to this broader scope, self-sufficiency is reduced as consumption increases while the supply remains the same. However, it still reaches nearly 60%, which is an achievement for a dense district.
47 Heating, cooling, domestic hot water.
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Figure 5.8. The non-convergence between demand and local production in 2050 in Lille
5.4.3. The fundamental equality of self-sufficiency 5.4.3.1. Energy densities of conventional utilities This urban prospective illustrates the difficulty of self-sufficiency when the consumption density is much higher than the watt per square meter. Yet, “a watt” compared to “a region” seems out of proportion. For conventional infrastructures, we usually think of capacities of about 10 MW for a heat network and up to several gigawatts of electricity for a nuclear power station or a large hydroelectric dam. These infrastructures are also different at the regional scale: they constitute large densities of energy transformation. However, as soon as we spatially average out these values over their total footprint, they become much weaker. In Power Density [SMI 15], Smil analyzes a wide variety of utilities: gas or coal power plants, nuclear power plants, etc. If we compare their production flow with the total amount of space they claim48, the average equivalent flow is generally around 100 W/m², values similar to those of solar energy. 5.4.3.2. Renewable energy densities As we saw in section 5.3, due to conversion efficiencies, densities are generally more modest for renewable energies. In Lille, the renewable energy production density was 5 W/m². Applying the formula below, and if our aim is 100% self-sufficiency, it would be necessary to reduce population density. However, this 48 The sum of the effect of stock, waste, intermediate processes, mines, etc.
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self-sufficiency would not be complete: food self-sufficiency requires at least about 1,000 m² per person and this is in the case of an essentially vegetarian diet. The pressure caused by food generates a critical density (a maximum of 1,000 people per km²) about which the roofs of contemporary architecture cannot do much. Renewable energy source [W/m2] × Yield [%] Self-sufficiency [%] = Individual consumption [W/pers] × Density [pers/m2]
Framed text 5.3. Self-sufficiency formula
5.4.4. Some self-sufficiency paths according to density 5.4.4.1. 1 W/m2, the harvesting objective at the regional scale Equality establishes the different terms of the scale. The energy consumption’s global density is about 0.1 watts per continent square meter. Then why claim a harvesting objective which is 10 times larger? Over the next few decades, the population is likely to increase while the average consumption is likely to change as well. It would therefore be prudent to anticipate an increase in consumption density. From a spatial point of view, a large part of the territory must remain as a natural space, protected from an energy drain and dedicated to carbon sequestration as we will see in section 5.4.5.2. It would, therefore, make sense to gather harvesting near human densities as well as within already artificialized spaces. These spaces must produce more and the flow rate of 1 W/m2 is the right order of magnitude. Energy harvesting types can also be spatially cumulative: the agrivoltaic is an example of the interdependency between the greenhouse and photovoltaic cultures; agriculture and wind turbines are a good example. According to this method, the following sections illustrate the diversity of possible self-sufficiencies according to the territory’s density. 5.4.4.2. Path for a region with 100 persons per hectare According to this first fictional scenario, individual consumption drops sharply from 6,000 to 2,000 W. The spatial consumption density decreases from 60 to 20 W/m². At the same time, renewable energy production has been developed to a great extent: the production density reaches 5 W/m² in 2050. In the end, dependence is greatly reduced but complete self-sufficiency is not possible; it is about 25% and convergence does not seem physically possible at this intensity of use (10,000 people per km²). This solution is comparable to the imaginary case described in the Fabrique de la Renaissance.
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5.4.4.3. Path for a region with a moderate population density (10 persons per hectare) Another scenario: in a region which is 10 times less populated49, the individual consumption of 6,000 W decreases to 4,000 W. Hence, the consumption density is reduced from 6 to 4 W/m². At the same time, the energy planning of the area has led to a significant increase in local energy production. In 2050, the density of production will reach 3 W/m². Self-sufficiency now seems feasible and would equal 75%. This moderately populated region can therefore aim to reach full self-sufficiency for the supply of its energy needs in the long term. 5.4.4.4. Path for a region with a low population density (one person per hectare) The last scenario is as follows: a region with a population density much lower than the previous ones50, it posits an increase in energy consumption as a result, for example, of an improvement in living standards. This example could be that of a region in a developing country where the average consumption per citizen would increase from 1,000 to over 2,500 W. In this case, the consumption density would increase from 0.1 to 0.3 W/m². However, the renewable energy production capacity is able to make up for this consumption density (1 W/m²) and the region becomes a net energy producer (+0.7 W/m²). This region with a low-population density is not only self-sufficient (300%), it can even export energy.
Figure 5.9. Left: possible path for a densely populated region (100 pers/ha), the convergence between energy consumption and renewable energy production. Center: moderate population density (10 pers/ha), the region can tend toward full self-sufficiency. Some cantons in Germany have already attained this situation. Right: low population density (1 pers/ha)
5.4.4.5. Intermediate conclusion: energy catchment areas This last scenario illustrates the ability of some regions to become the exporters of renewable products. They would be the extension of the “energy catchment areas” of densely populated regions. This concept, a diversion of the vocabulary used in
49 Which roughly corresponds to the average density of the Ile-de-France region. 50 That is 100 people per km², or a population density similar to that of mainland France.
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hydrography, was created in 2008 alongside the Grand Pari de l’Agglomération Parisienne consultation [MEN 09], where the energy issue was the center of the concerns of our team’s response51: “Reducing the ecological footprint of a city’s future. What is the city’s ecological footprint? Through its metabolism, the city absorbs and swallows resources on at least a regional scale (food, waste), if not a national (electricity, water) or even a continental scale (fossil fuels, etc.). The impact of its activity can be seen today at a global level. As one of the symptoms of this state of affairs, the Greater Paris area makes up 2% of the national territory but represents 10% of greenhouse gas emissions. The challenge for cities in the 21st century is to reduce this footprint: to decrease the consumption and use of energy and to install self-production strategies in cities [...]”.
Figure 5.10. The before and after of Greater Paris. How to reduce cities’ energy dependence? How to bring energy catchment areas closer to the cities? Drawing by the author [MEN 09]. For a color version of this figure, see www.iste.co.uk/lopez/local.zip
Later, in 2013, as part of a conference52, my conclusion was unequivocal: with a density nine times greater than the national average, energy equity between the regions required the Greater Paris area to be enlarged. 5.5. Emission density and carbon neutrality 5.5.1. Post-COP21 and carbon neutrality The previous section described the possible paths to be followed to get rid of our addiction to fossil fuels as a supply of energy. In this section, the dual problem of 51 Elioth’s contributions to the AJN-AREP-MCD team in 2008. 52 Quel Grand Paris? Et avec quelles énergies ?, speech delivered in a conference at l’Ecole Spéciale d’Architecture.
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carbon accentuates even more the pressure that exists on densities. In fact, in order to respect the Paris Agreement on climate change, and in order to keep the chances of global warming limited to 2 °C by 210053, it would be necessary for humanity to be carbon neutral by around the 2050s54. Emissions of greenhouse gases of human origin would need to be balanced out by carbon storage to then become “net negative”. In the second half of this century, humanity would store more carbon than it emits.
Figure 5.11. Decreasing trend of global annual CO2 emissions, compatible with limiting the temperature increase to + 2 °C (blue strand) or + 1.5 °C (red strand) (source: Joeri Rogelj et al.). For a color version of this figure, see www.iste.co.uk/lopez/local.zip
The climatic emergency calls for a global and extremely rapid reduction of current emissions, which at present are above 40 billion tons of CO2 equivalent. Thus, this first assessment of the “critical amount of carbon” is starting to take shape given that one person emits an average of 6 tons of CO2 per year, while 1 hectare of properly exploited forest pumps about 3.6 tons of CO2 per year. Hence, it would be necessary to allocate a significant portion of our small 2 hectares described at the beginning of section 5.4 exclusively to carbon neutrality, nearly 1.6 hectares, a plot 53 Compared to the pre-industrial era with reference date 1880. 54 In its 2014 report, the IPCC developed “carbon budgets” associated with goals to limit the average temperature increase: between 2011 and 2100 humanity could still emit a maximum of 550 GtCO2 (i.e. 550 billion tons of CO2 equivalent) to guarantee a limited warming of + 1.5 °C, or a maximum of 1,000 GtCO2 stocked to hold back an average increase of + 2 °C. Beyond this value, and therefore reaching a concentration considered critical by experts, the terrestrial “climate machine” could go out of control: scientists admit to being unable to predict the probable runaway global warming. At the current rate of about 40 GtCO2 equivalent being emitted annually, and without rapid action to decrease our emissions, the first estimate will be surpassed in about 2025 (+ 1.5 °C) and the second one (+ 2 °C) in about 2035.
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of 130 m per side. In order for limit the area destined for the sole purpose of sequestration, it is therefore necessary to massively reduce our greenhouse gas emissions and thus make neutrality tangible. 5.5.2. Equivalent emission densities To illustrate this pressure, if we consider that the annual CO2 emissions of a Parisian are more than 10 tons, then the density of annual emissions for Paris is greater than 2,000 tons of CO2 per hectare. On the other hand, in Mauritania, with an average population density which is slightly higher than 4 persons per km² and very low individual emissions55, the CO2 annual emission density is of around 0.03 tons per hectare. This equivalence arises from the transposition of the formula found in section 5.2 for energy consumption. Emission density [tCO2eq/ha/year] = Individual emissions [tCO2eq/pers./year] × Population density [pers./ha]
Framed text 5.4. Density of greenhouse gas emissions
5.5.3. Carbon sequestration density Sequestration through afforestation is undoubtedly one of the most effective56 carbon sinks. At the national level, forests in mainland France occupy roughly 30% of the territory, i.e. 16 million hectares. The French Environment and Energy Management Agency (ADEME) recalls that “forests contribute to the mitigation of climate change through two levers: a sequestration effect and a substitution effect”. Regarding sequestration, French forests constitute a “net sink” of 59 Mt of CO257 per year, or about 3.7 tons of CO2 per hectare per year. Afforestation may lead to a change in the use of certain soils and, if dietary patterns tend toward less meat consumption, the conversion of certain cereal fields currently used to feed livestock into forests could have a greater carbon impact than the unique sequestration generated by afforestation. In the future, we will probably draw plans for atmospheric carbon harvesting similar to the energy harvesting plans described in the second section of this work.
55 Roughly 0.7 annual tons per capita. 56 Ben Caldecott, Guy Lomax and Mark Workman, Stranded Carbon Assets and Negative Emissions Technologies – February 2015, SSEE, University of Oxford, p. 15. http://bit.ly/ 1ESZYzT cited by http://adrastia.org/technologies-emissions-negatives-racicot/. 57 French national inventory report under the United Nations Framework Convention on Climate Change and the Kyoto Protocol, CITEPA, 2014.
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Figure 5.12. The balance between emission and sequestration densities
5.5.4. The fundamental equation of carbon neutrality In 2014 in Paris, emissions amounted to 25.6 million tons of CO2 equivalent. Under the assumption of compensating with forests, the carbon neutrality of Paris would have required a forest area of approximately 50,000 km² exclusively for emission sequestration, i.e. 500 times Paris’ surface area. In the study carried out by the Elioth group, Paris change d’ère (Paris changes era) [MEN 16], the carbon neutrality strategy implied the allocation of nearly 9,000 km² to the sequestration of persistent emissions: a hundred times the cadastral surface area. Considering the scale of Greater Paris and its seven million inhabitants, and assuming that they would follow a similar reduction path, the surface area for this amount of sequestration would need to be as a first approximation of about 30,000 km², a circle about 200 km in diameter which is 5% of mainland France. The formula below is the same as that governing energy self-sufficiency but transposed to carbon and allows us to analyze the carbon neutrality ratio of a given territory. Surface sequestration [tCo2eq/pers./year] × Fraction of land [%] Neutrality [%] = Individual emissions [tCo2eq/pers./year] × Density [pers./ha]
Framed text 5.5. Carbon neutrality capacity
5.6. Conclusion 5.6.1. Continent–sea balance In sections 5.4 and 5.5, we described the spatial tensions induced by energy and carbon constraints. The Earth’s surface is taken into account and so growth limits
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appear in these critical energy and carbon flux densities. On the global scale, there is no doubt that energy self-sufficiency and carbon neutrality will necessarily require the support of maritime surface area. Undoubtedly, when faced with continental restraints and the many dilemmas posed by the carbon issue, marine territories will be precious allies providing a more flexible definition of our energy catchment areas: harvesting wind and sea currents, floating solar technology, development of aquatic biomass crops, etc. Will we eventually reach polarization: use the sea for energy and food, land for matter and carbon? 5.6.2. The city–countryside dichotomy At the regional level, the energy and carbon catchment areas question the evolution of the links between the city and rural areas, but they do so also regarding the regions’ other metabolic flows [BAR 04]: materials, supply and drainage. John McNeill recalls that the firewood used in Delhi is produced 700 km away from the Indian capital; he also points out that the harvesting area required to heat Vancouver is 20 times greater than that of continental France [MCN 10]. In order to understand this intensity of metabolic consumption, we can point out that the densest megacities in Asia, with nearly 50,000 inhabitants per km², have a human mass per unit area of more than 2 kg/m2, a density three orders of magnitude greater than that of large seasonal gatherings of herbivores in green regions of Africa. Are these hyperdensities sustainable? 5.6.3. The city, an energy-carbon monster Two centuries ago, before the thermoindustrial era, the city–countryside dichotomy was clear: rural areas were for agricultural production, exploitation of forestry, breeding and hunting. The countryside was the space for the conversion of solar energy, organized by the production and transformation of biomass. The countryside was the offer. On the other hand, the city was where people concentrated and therefore the concentration of energy consumption and carbon reemissions through the combustion of biomass. Demand was shaped by food and heating needs. Here and there, some buildings and structures ensured a production function: the water and wind mills. Eunhye Kim quoted a possible definition of the city: “a space where inhabitants do not produce their own food”. The city remains the consumption territory and thus maintains a singular link with its hinterland [KIM 12]. The city (and all the more so the metropolis) has become a region of very high consumption and very high emission of greenhouse gases. Conversely, the rural production of the beginning of the 19th Century was not able to follow the qualities and ease of extraction of most hydrocarbons. This situation worsened with the
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exploitation of fossil fuel resources during the 20th and 21st Century [KIM 12]. The countryside has basically turned into a territory mainly devoted to the production of our food needs58 and whose productivity has been boosted by fossil inputs: thermoindustrial mechanization of agriculture, extensive use of fertilizers, intensive use of phytosanitary products, etc. This catchment area aspect also conveys the opposite concept as recalled by Bonneuil and Fressoz [BON 13]: “The 19th century was marked by very strong concerns about the metabolic break between urban and rural areas: urbanization, i.e. the concentration of humans and their feces prevented the mineral substances’ return to the soil, which are essential for fertility�.
Figure 5.13. A diagram showing the energy consumption densities for French territory with steeper vector lines imitating hydrographic flowlines [MEN 14]
5.6.4. The mathematics of density, relocating according to the right proportions The sustainable limits of the thermoindustrial era have been exceeded. How can the future, energy, carbon, food and material catchment areas be reconstructed? How can fairer divisions be made using the population density map? How can different regions be outlined to heterogeneously and equally divide up the map? This method is also relevant and could be applied to the energy consumption density map in order to determine where the homogenous consumption territorial blocks and islands are. 58 Direct or indirect, in particular biomass production for cattle food.
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Figure 5.14. Some examples of regional energy catchment area consolidation. The diagram shows the regional redistribution according to the energy density analysis performed by the Réforme team [MEN 14]. For a color version of this figure, see www.iste.co.uk/lopez/local.zip
5.6.5. The scales in question Sustain the balance between regions by converging them toward similar consumption densities: the effort required to build sustainable catchment areas should be the same for all parties involved. Self-sufficiency for each region would then evolve homogeneously. According to this principle, the sum of the parts, that is the country as a whole, would be stronger and more resilient. The philosopher and mathematician Olivier Rey reminded us in Une question de taille of the advantage of benevolence at the small scale [REY 14]. “[...] Kohr was convinced throughout his life that the appropriate unit of distance to organize a healthy society was of the order of that separating his native village from the country’s capital, Salzburg, twenty-two kilometers away. Are small societies inevitably closed in on themselves, marked by provincialism and parochialism? The Athens of Antiquity or Florence during the Renaissance (which had 40,000 inhabitants at the start of the 15th century, which was the time of its greatest splendor), to mention just two spectacular examples, prove otherwise. In fact, it is the political unification of vast territories which makes all other smaller cities sterile by pumping all the energies towards a few enormous centers [...]”59. 59 p. 88.
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In light of the different examples provided in this chapter, the value of watts per unit area or the space requirements for carbon neutrality make us think about the scales and proportions of territorial development (and the updating of political and administrative governance in line with ecological priorities). We have seen how the intensity of cities requires more space to decide on the different self-sufficiency and neutrality paths to follow. Said area would probably be of the order of hundreds of kilometers, i.e. a large region would be the right scale. Outside these metropolitan areas, the cantonal scale of about ten kilometers would be more appropriate to bring together human energies and contextualize harvesting. Introduced between blocks of buildings in the cities, these smaller scales would increase fluidity and an important adaptation reactivity. 5.7. References [BAR 04] BARLES S., Mesurer la performance écologique des villes et des territoires : Le métabolisme de Paris et de l’Île-de-France, Final research report on behalf of the City of Paris, 2004. [BIH 14] BIHOUIX P., L'âge des low tech, Seuil, Paris, 2014. [BON 13] BONNEUIL C., FRESSOZ J.-B., L'Événement anthropocène: La Terre, l'histoire et nous, Seuil, Paris, 2013. [HOP 10] HOPKINS R., Manuel de Transition De la dépendance au pétrole à la résilience locale, Guides Pratiques, Montréal, 2010. [KIM 12] KIM E., BARLES S., “The energy consumption of Paris and its supply areas from 18th century to present”, Regional Environmental Change, 12(2), 2012. [LOP 14] LOPEZ F., Le Rêve d'une déconnexion. De la maison à la cité auto-énergétique, La Villette, Paris, 2014. [LOV 11] LOVINS A.B., Reinventing Fire, Chelsea Green Publishing, Chelsea, 2011. [MAC 09] MACKAY J.C., Sustainable Energy – Without the hot air, UIT Cambridge, Cambridge, 2009. [MAN 14] MANIAQUE C., Go West – Des architectes au pays de la contre-culture, Parenthèses, Marseille, 2014. [MCN 10] MCNEILL J.R., Du nouveau sous le soleil. France, Editions Champ Vallon, Paris, 2010. [MEA 12] MEADOWS D.H., RANDERS J., MEADOWS D., Les limites à la croissance (dans un monde fini), Rue de l’échiquier, Paris, 2012. [MEN 09] MENARD R., “Introduction au développement durable” in NOUVEL J., (ed.), Naissances et renaissances de mille et un bonheur parisien, du Mont Boron, Paris, 2009.
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[MEN 11] MENARD R., “Dense Cities in 2050: The Energy Option?”, Summer Study Proceedings, ECEEE, 2011. [MEN 14] MENARD R. et al., Reforme, Final research report, Programme Ignis Mutat Res, 2014. [MEN 15] MENARD R., La Renaissance des Fabriques. Un monde possible en 2050, October 2015. [MEN 16] MENARD R. et al., Paris, change d’ère. Stratégie de neutralité carbone de Paris en 2050, Report, Elioth, 2016. [NEW 99] NEWMAN P., KENWORTHY J., Sustainability and Cities: Overcoming Automobile Dependence, Island Press, Washington, 1999. [PER 13] PERLIN J., Let it Shine: The 6,000-Year Story of Solar Energy, New World Library, Novato, Paris, 2013. [REY 14] REY O., Une question de taille, Stock, Paris, 2014. [SMI 15] SMIL V., Power Density: A Key to Understanding Energy Sources and Uses, MIT Press, Cambridge 2015.