The social challenges, conflicts and legislative provisions concerning the supply of energy are as old as humanity itself. Alongside the supply of food, the energy problem has always been a central topic. Until the industrial revolution, humans lived primarily in equilibrium and harmony with nature. The earth supported only as many people as were capable of using regenerative energy sources. The taming and domestication of fire enabled them to use heat obtained from the combustion of regenerable materials, such as wood and straw, for cooking and heating, but also for the smelting and burning of mineral resources. Animal, wind and water energy was used to transport goods and people, and drove the first machines for the conveyance, preparation and processing of plants and minerals.
In those days, equilibrium was frequently lost as a result of the effects of the weather (periods of drought, floods, cold winters, etc.) and of over-exploitation of nature, with the result that many people suffered hardship, or even perished. Special methods of cultivation for the fields, pastures, woods and rivers, such as the so-called “Hauberg” system used in Germany’s Siegerland region, were introduced in order to counteract such effects. This forestry management system, adapted to unfavourable climatic conditions, for centuries (first mentioned in 1467) up to the early 20th century, supplied wood for heating and for the production of charcoal for iron smelters on an approximately 18-year felling cycle and, shortly before the felling season, oak tanbark as a tanning agent for the leather industry. Between felling times, the Hauberg was used as woodland grazing for cattle, and for planting of winter rye [1, 2, 3].
Around the world, humans thus developed the most diverse range of local economies as a function of their local climatic and geological conditions. Crises nonetheless occurred frequently, but were, equally, the incentive to develop more elaborate concepts for the use of the planet. The very idea of “sustainability” itself arose as a result of a scarcity of wood caused by the excessive use of timber as support in underground mining, for fire-setting in ore mining, and for the distillation of charcoal for the metallurgical industry in the Erzgebirge range. Hanns Carl von Carlowitz for this reason developed in the early 18th century a conceptual solution and coined the term “sustainability” in his “Sylvicultura oeconomica”  (Figure 1).
Human history was radically transformed at the start of the industrial revolution in the 18th century, which began when humans learned to use fossil energy sources, firstly coal, followed by oil and, later, gas and ultimately, from the mid-20th century onward, also nuclear energy. The various branches of industry, such as mechanical engineering and chemistry, and also scientific disciplines, and the natural and engineering sciences, in particular, have since developed, up to the present day, increasingly more dynamically. The global population grew as a consequence, accompanied by the growth of prosperity previously unachievable, and inconceivable, for large numbers of people. This trend was founded on the increasing use of energy, and of fossil sources of energy, in particular. Even today, the supply of the population with energy continues to be based primarily on the utilisation of fossil energy sources, which claim a share of above 80 %. Regenerative energy sources have up to now contributed not even one seventh of the global energy demand [5, 6, 7] (Figure 2).
For many people, and particularly those in the developed economies, but also increasingly in the threshold countries, it is nowadays a matter of course to use energy at any time whatsoever and in an adequate quantity and the desired manner, whether for lighting and mobility, or for cooking and heating. For this reason, energy consumption continues to rise unabated. In recent decades, however, it has again become apparent that there are limits on an increasing use of energy. Reserves of fossil and nuclear energy sources are finite, and the use of these energy sources not infrequently generates negative aspects, such as effects on the climate, the atmosphere, the earth’s water resources and the soil. Humanity no longer lives in equilibrium and harmony with nature. Technological progress has up to now concealed and, indeed, solved many of the negative consequences of rising use of energy (by means of filtration and flue-gas desulphurisation systems, for example), but a rising percentage of the population has nonetheless become aware that sustainable solutions to the energy problem must be sought and, it may be hoped, found. Many entertain the illusion that future energy supplies can be based solely on regenerative energy sources without negative effects on their surroundings and on the environment, and without any diminution of their accustomed standard of living. The following therefore firstly analyses the energy situation, and then outlines and evaluates possible solution scenarios.
2 Energy demand
Global utilisation of energy by humans is subject to extremely great local variations. Annual per capita consumption of primary energy may be less than 10 GJ in many developing countries, such as Ethiopia and the Congo, all depending on the level of prosperity, the climate and the economic structure, or it may exceed a level of 100 GJ in industrialised countries such as Germany and the USA. Although there is no precise correlation between the standard of living and the energy consumption of any society, it can be assumed, as a general observation, that the universally desired improved living conditions will increase the energy demand [8, 9] (Figure 3). Even the transition from an industrial to a service society has proven to be a macroeconomic aberration since, on the one hand, national energy balances need to be adjusted as a result of the indirect energy imports inevitably associated with the importation of goods (“energy footprint”) and, on the other hand, since their energy intensity (as a result of transportation of goods and of the use of computers, for example) has been totally underestimated.
In parallel to this, the growing global population also increases consumption, particularly in the developing countries. Forecasts predict a figure of nine billion inhabitants on the planet by the year 2050 [10, 11] (Figure 4). Increases in the efficiency of technological and macroeconomic processes and systems have the effect of reducing demand for energy. They necessitate significant investments, however, and also technological and social developments. For these reasons, forecasts of growth rates and of the use of the various forms of primary energy differ considerably (see e.g. [5, 6, 12, 13, 14, 15]). They range from zero growth resulting from ever more rational utilisation of energy, up to and including predictions which assume the doubling of energy demand to around 1000 EJ/a within the next fifty years (Figure 5).
The forecasts which predict zero growth are based on concepts which assume significant technological and social increases in the efficiency of energy conversion and of energy utilisation. Such concepts envisage, for example, that the industrialised countries will cut their per capita energy consumption to a greater degree than the developing countries increase their per capita consumption, with the ultimate aim of energy utilisation rates approaching a common value over time (e.g. the “Contraction and Convergence” model, the “2000 Watt per capita Society” model). Historical stages of development which the industrialised countries passed through are here to be avoided in the developing countries, and development leaps made possible by means of knowledge and capital transfers (the so-called “leapfrogging” strategy), i.e., energy-intensive technologies are not to be installed at all in the developing countries, which are to get immediately the latest and most efficient technologies. This process is to be accompanied by the decoupling of energy consumption and economic growth, or standard of living (the “decoupling strategy”) . It will not be possible, particularly in the industrialised countries, to achieve this aim without a new awareness, and without modifying our conceptions of quality of life and prosperity.
Macroeconomic experience up to now tends to indicate the contrary effects. Energy-savings and the more efficient use of energy frequently result not in a fall in overall energy consumption, but instead in the increased use of more efficient technologies and products, or in the opening of new markets, with the consequence that the energy-saving potentials are achieved only incompletely, or not at all (Jevons’ Paradox, the “Rebound Effect”) or are even cancelled out by still higher consumption (the “backfire effect”).
It is therefore necessary, to estimate a realistic scenario, to bear in mind that humanity’s total energy consumption Eges is multiplicatively linked to the three principal influencing factors:
Eges = EB ∙ ((eb ∙ ew)/ ee)
in which EB basic energy consumption
eb population factor
ew prosperity factor
ee efficiency factor
Assuming that global population rises from its present seven billion to nine billion by the middle of the 21st century, and that energy consumption remains constant for one third of humanity, but triples for the poorer two thirds, in particular, the efficiency factor must more than double in order to keep total energy consumption constant at its present level. This is unrealistic, however. It should be noted in this estimate, in particular, that all concepts concerning future use of energy assume a rising share of primary energy conversion for electrical energy. Electricity’s percentage share rises, for example, from its present around 16 % in the growth scenario and in the energy (r)evolution scenario to around 21 %  (Figure 6); the most easily usable and most versatile form of energy, generally involving the longest conversion-process chain (exception, for example: photovoltaics), and therefore frequently also the poorest overall efficiency, increases at above-average rates. It is, for these reasons, extremely probable that it will, for the foreseeable future, be necessary to provide more energy. The focal questions are therefore how growing energy demand can be met, and the extent to which the individual energy sources can make their contribution.
3 Energy sources
Various energy sources are available to meet energy demand. These are, primarily, the sun, the earth and the moon (Figure 7). The surface of the earth has been and is supplied constantly with Energy E˙i from Initial Energy Resources E∞ in the form of radiation, thermal conduction and fluctuating gravitation. These forms of energy can be further converted by means of Natural Conversion Cn using the various media, such as water, air, rocks or biomass. These forms of energy, also referred to as regenerative energy sources, are tied either directly or indirectly to Energy Flows E˙js.
Nuclear fusion occupies a special ranking, since the fuel for this reaction is de facto infinitely available . Fusion-based power-generating plants will, at best, go into operation only in several decades’ time, if they prove to be feasible on a commercial scale at all. For this reason, they will not be examined in any further detail in this paper.
Fossil and nuclear energy sources make use of Energy ET, which is stored in the earth’s crust. This storage occurred during the genesis of the earth in the form, for example, of inclusions of nuclides in rocks, and during the history of the planet, as a result of the deposition and conversion of biomass. The earth’s crust continues even today to take in Energy E˙js3, by way, for example, of the sedimentation of biomass and of thermal conduction from radioactive decay.
Part of regenerative Energy E˙js2 and stored Energy ET is converted by means of Technical Processes Ct into the energy flows most frequently used by humans: Electricity E˙E, Heat E˙H, Power E˙P and Fuel E˙Ch.
Energy-policy discussions now frequently assume the differing availability of the various energy sources. Fossil and nuclear energy sources are based on geological stocks ET, and thus have a finite period of use. Regenerative energy sources are chronologically infinite, it is true, in relation to future human history, but their utilisation is nonetheless limited. Due to their being tied to an Energy Flow E˙js, they are dependent on the area on the earth and on the mass of the media and their chemical and physical parameters. The utilisation of regenerative energy sources can therefore also not be increased at will, since these factors are all finite.
We may summarise by stating that future energy supplies for the global population will be finite, although for differing reasons. For this reason, geological stocks ET of fossil and nuclear energy sources are estimated below, and the Energy Flows E˙js2 of regenerative energy outlined.
4 Reserves and resources
The deposits of individual raw materials are dependent on geological conditions and their origin. These materials are, for this reason, unevenly distributed on and in the earth. The total quantity of any raw material is referred to as the “total initial resources”. Where these initial resources are geologically proven, and also technically and economically recoverable, they are referred as “reserves”. “Resources”, on the other hand, are initial resources which are geologically known but not yet technically and economically recoverable, or technically/economically recoverable and geologically conceivable, but not yet proven. The remaining, largely unknown future reserves and resources above and beyond the aforementioned are referred to as “geopotential”  (Figure 8). The total initial resources of energetic raw materials actually present in the portion of the earth accessible currently and in the future are unknown.
The quantities of the individual types of initial resources result, and change, as a function of price, geological exploration and the technical/technological status of development. Technical progress therefore does not necessarily signify growth in reserves at the cost of resources, for example. Whereas, fifty years ago, the mining of hard-coal seams of 30 cm thickness was economically possible in Germany, the present economically rational seam thickness is now 60 cm or more, as a result of the further development of mining technologies, i.e. reserves have fallen as a consequence of technical progress .
The delineation of these types of initial resources, and determination of quantities, have not yet been unequivocally defined internationally, and vary from country to country. In many countries, and in those which exploit a large number of sub-surface deposits by means of open-pit mining, in particular, only those deposits located at a depth of not greater than 300 m to 700 m (down to 350 m in South Africa, and 671 m in the USA) are included in assessments of hard-coal reserves and resources, for example, whereas coal located at depths down to 1800 m is included in the initial resources calculation in countries with pronounced deep-level mining (down to 1200 m in India, 1500 m in Germany and 1800 m in Ukraine) . Coal is also found in much deeper seams, as is the case below the North German Plain. The same caveat applies to seam thickness for calculation of reserves and resources (minimum seam thickness included in calculation: 1 m in South Africa, 0.25 m in the USA, 1 m in India, 0.6 m in Germany and 0.55 m in Ukraine).
The special strategic and technological concerns of individual states and companies should also not to be underestimated. The Lübtheen coal deposits, despite having been explored, were not found in any official geological publication during GDR times, since the necessary processing technologies had not yet been developed. Various criteria for the delineation of initial resource types have also become established for the individual energetic raw material sources. The cost-efficiency of mining of hard coals is incorporated indirectly in the calculation via the depth range of the deposits and seam thickness, whereas getting costs are used directly in the case of the nuclear energy sources. In the case of uranium, similarly to that of metal ores which frequently occur finely distributed in large but low-grade deposits (disseminated ores), getting costs are taken at 40, 80 and 130 US$/kgU for classification of reserves and resources .
Classification and quantification of initial resources therefore differ very greatly internationally, and are highly dependent on country and raw material specific circumstances. Against this background, numerous international, national and industry-specific classification systems have been evolved, some of which are still in use (e.g. [20, 21, 22, 23]). For these reasons, a UN commission has developed a classification system which is intended to specify and determine the individual initial resources world-wide on standardised criteria. This international United Nations Framework Classification (UNFC) for Solid Fossil Fuels and Mineral Resources is based on the following three criteria:
• Economy E, i.e., the degree of economic/commercial viability
• Feasibility F, i.e., field project status and feasibility
• Geology G, i.e., the level of geological knowledge
The degree of exploration, i.e. the status of knowledge, is specified and quantified for these criteria (Figure 9). Initial resources classes then result via the combination of these criteria in terms of their degrees of exploration. They are thus coded and terminologically defined by means of a three-digit numerical sequence. This concept therefore supplies a three-dimensional classification of the initial resources (Figure 10). Similar concepts have also been pursued and developed for corresponding initial resources classes for liquid, gaseous and nuclear fuels [24, 25, 26, 27].
These initial resources classes could now be used as the basis for the estimation of the supply and production potential of fossil and nuclear energy sources, presupposing that this concept was globally accepted and applied. It must be noted, however, that significant deviations exist not only in the determination of the quantities in the individual initial resources classes (as a function of technical development, geological exploration and recoverability), but also in the special strategic concerns of individual states. The classification of initial resources is therefore a dynamic concept, and is frequently time- and location-dependent (Figure 11).
The dependence on time of the development of reserves can be illustrated extremely clearly. The ratio of reserves and annual production, also referred to as static lifetime R, is characteristic for any raw material, and varies within a certain bandwidth, despite generally rapidly increasing production . In the case of oil, this index has for decades varied between 30 ± 10 years, and between 50 ± 10 years in the case of gas  (Figure 12). A greater range would, for macroeconomic reasons alone, also be uneconomical, and would allocate too much capital unnecessarily. This index is, therefore, merely a “long-exposure image” of the dynamic system of initial resources, and cannot provide any information on the actual range of any raw material.
The problems discussed illustrate the limits of informational accuracy and the determination of initial resources data. It is nonetheless a vital necessity to estimate the initial resources of individual energetic raw material sources, in order to obtain a basis for political, macro- and microeconomic, and also technical, decisions. In view of the difficulties involved, only a pragmatic approach, which must, in addition, be modified at certain intervals and in case of new developments, can be considered [19, 28]. In the context of this examination, reserves and resources are therefore stated on the basis of available compilations of data concerning the individual energetic raw material sources .
The fossil energy sources nowadays meet by far the largest portion of global energy demand. These energy sources, however, are effectively not renewable, if one sets aside natural production of biomass, which is constantly being fed into the geological process (see Figure 7). These energy sources are thus finite, and their range limited. Coal, oil and natural gas will nonetheless retain their significance for the supply of the world population with energy for the foreseeable future. The question of lifetime is also a justified one, but one which cannot be answered even approximately correctly, when data obtainment (see Section 4), evaluation methods and the special interests of individual states and companies are taken into account.
Despite these difficulties, global initial resources data are based on surveys performed by individual organisations and institutions, such as the World Energy Council (WEC), the United States Geological Survey (USGS) and Germany’s Federal Institute for Geosciences and Natural Resources (BGR). The initial resources are surveyed separately for “reserves” and “resources”.
In the compilations used here [28, 29], coals are differentiated into soft brown coals and hard coals (hard brown lignite, bituminous coal and anthracite), energy content being taken as the differentiation criterion (soft brown coals: less than 16500 kJ/kg, hard coals: more than 16500 kJ/kg). Classification of coals differs extremely greatly in the various states, with the result that classification is not universally possible without difficulties (in the case, for example, of sub-bituminous coals).
In the field of oil and natural gas, additional differentiation is made between conventional and unconventional deposits. Conventional oil is liquid, and possesses a density of lower than 1 g/cm3, and thus includes low-viscosity oil, heavy fuel oil and liquid hydrocarbons yielded in the production of natural gas, such as gas condensate and liquefied gas (natural gas liquids, or “NGL”). Unconventional oil includes extra-heavy crude oil and oil immobilised in oil shales and oil sands, with a density above 1 g/cm3 and a high viscosity.
Mobile natural gas which flows to the production well without any further technical provisions is referred to as “conventional gas”. This classification includes gas from natural gas and gas-condensate deposits, and also so-called associated petroleum gas (APG). Unconventional natural gas is defined as gas from dense rock (tight or shale gas), coal-bed methane (CBM), aquifer gas and gas hydrate.
On the basis of current consumption, initial resources of fossil fuels will be adequately, if differingly, available for the next few centuries [28, 29] (Figure 13). Peak annual production, followed by reduced production, will probably be reached first in the case of oil . This peak is anticipated around the year 2023 in the case of conventional oil , and may be an indication of an impending scarcity of oil. The forecasts for gas and coal permit the anticipation of a significantly longer range, particularly in the case of natural gas. Only natural gas fixed to gas hydrates is estimated at 1 to 120 ∙ 1015 m3 , corresponding to an energy content of approx. 38 to 4500 ∙ 103 EJ.
Uranium and thorium are the principal nuclear energy sources destined, on the basis of present-day assessments, to contribute to future global energy supplies. Both of these elements are widely distributed in locally differing concentrations in the Earth’s crust, with the result that assignment to the individual initial resources classes is frequently performed on the basis of recovery costs; reserves and resources thus also rise as a consequence of rising prices when demand increases. In addition, uranium resources can be significantly further expanded if the reprocessing of nuclear fuel rods and nuclear weapons, and recovery from seawater, are included. The use of nuclear energy sources can, furthermore, be considerably further increased by the use of breeder reactors .
So-called “reasonably assured resources” (RAR), i.e., uranium located in explored deposits of a known metal content and with recovery costs of less than 40 US$/kgU, are classified as reserves. Reasonably assured resources with higher recovery costs, “inferred resources” (IR), the recovery costs of which are lower than 130 US$/kgU, and undiscovered resources, the existence of which can be surmised from geological circumstances, are designated “resources” [24, 28]. Initial resources of nuclear energy sources are undeniably finite, but will, thanks to the available technical potentials and the price situation, remain adequate for many centuries [28, 30] (Figure 14).
This article is dedicated to His Magnificence Prof. Dr. Dr. e.h. mult. Gennadiy Grigoryevich Pivnyak, Rector of the National Mining University of the Ukraine in Dnipropetrovsk, on the occasion of his 70th birthday.