Mans energy history

The minimum energy requirement of man may be taken as the amount of "exchangeable" chemical energy that can be associated with the amount of food necessary to maintain life processes for someone performing a minimum of work and not losing weight. This minimum depends on the temperature of the surroundings, but for an adult man is generally considered to lie in the region of 60-90 W on average for extended periods, corresponding to (6 to 8)x106 J per day. The total life requirements are of course more than energy, comprising adequate supplies of water, nutrients, etc.

In order to perform any (muscle) work not purely vegetative, additional energy must be supplied in the form of food, or energy stored in the body will become depleted. The efficiency in converting stored energy into work typically ranges from 5% to 50%, the lower efficiencies being associated with activities involving large fractions of static conversion (e.g. carrying a weight, which requires the conversion of body energy even if the weight is not being moved). The percentage complementary to the efficiency is released as various forms of heat energy.

The maximum average rate of food energy intake that a human being can continue for extended periods is about 330 W, and the maximum average rate at which work can be delivered for extended periods is of the order of 100 W (Spitzer, 1954). During work periods, the "man-power" output level may be 300-400 W, and the maximum power which can be delivered by an adult male for a period of about a minute is roughly 2000 W.

Although it is not certain that the rates of energy conversion by the human body have remained constant during the evolution of man, it may be reasonable to assume that the average amount of "muscle power" used by the ear liest members of the genus Homo, which evidence suggests lived some 4x106 years ago in Africa (Leakey, 1975), was of the order of 25 W.

The total energy flux received by an individual man in a food gathering or hunting society is then the sum of the energy in the food, averaging say 125 W, and the absorbed flux of radiation and heat from the surroundings, which may reach considerably larger values, but is highly dependent on clothing, climate and the nature of the surroundings (cf. e.g. Budyko, 1974). The outgoing energy flux again consists of heat and radiation fluxes, turnover of organic material, plus the amount of energy converted into work. For growing individuals, the net flux is positive and the mass of biological material increases, but also for adult individuals with zero net energy flux, new biomass continues to be produced to replace "respiration losses".

Man has successively developed new activities, which have allowed him to gain access to larger amounts of energy. Solar energy may have been used for drying purposes, and as soon as fires became available a number of activities based on firewood energy may have started, including heating, food preparation and process heat for tool making. The earliest evidence for fires used in connection with dwellings is from Hungary, 350 000-400 000 years ago (H. Becker, 1977, personal communication),

A good fire in open air, using some 10-50 kg firewood per hour, may convert energy at a rate of 104-105 W, whereas indoor fires are likely to have been limited to about 103 W. Several persons would presumably share a fire, and it would probably not burn continuously at such a power level, but rather would be re-lit when required, e.g. from glowing embers. It is thus difficult to estimate the average fire energy per person, but it would hardly exceed 100 W in primitive societies. The efficiency of delivering energy for the desired task is quite low, in particular for open-air fires.

The next jump in energy utilisation is generally considered to have been associated with the taming of wild animals to form livestock, and the introduction of agriculture. These revolutions have been dated to about 104 years ago for the Near East region (cf. e.g. DuRy, 1969), but may have been developed in other regions at about the same time, e.g. in Thailand and Peru (Pringle, 1998). This time corresponds to the ending of the last ice age (see Fig. 2.91), which may have caused changes in the low-latitude climate, including altered precipitation rates. The introduction of livestock would have promoted the tendency to settle at a given place (or vice versa), increasing in turn the requirement for food beyond the capacity of a hunting society. Agriculture was based at first on wild varieties of wheat, for example, and it is believed that artificial irrigation was necessary at many of the sites where evidence of agriculture (various tools) has been found. The power for water transport and, later, pumping would then be derived from suitable draught animals in the livestock pool, as a substitute for man's own muscle power. The transition from a hunting to an agricultural society, often called the Neo lithic or "new stone age", occurred several thousand years later in the temperate zones of northern America and Europe.

The creation of cultures of growing size and level of sophistication, leading to the formation of large cities, for example at the rivers Euphrates, Tigris and the Nile, about 5000 years ago, witnesses a growing use of energy for ploughing, irrigation, grinding and transport (of food supplies and of materials, e.g. in connection with buildings and monuments), as well as the harvest of solar energy through agricultural crops. It is not known exactly how much of the physical work was performed by men and how much by animals, but it is likely that another 100-200 W was added to the average energy usage per capita in the most developed regions.

It is also important to bear in mind that there must have been large differences in energy use, both between different societies and between individuals within a given society. Throughout man's history (the meaning of "history" not being restricted to imply the presence of written records) there have been individuals whose access to energy was largely limited to that converted by their own bodies. Large regions in Asia and Africa today have an average energy spending per person, which is only a few hundred watts above the muscle power level (with firewood as an important source). This means that parts of the population today use no more energy than the average person during the Neolithic period.

The energy sources emerged so far are direct solar radiation, environmental heat, animal biomass as well as primary (plant) biomass in the form of food and later as firewood, plus mechanical work from the muscle power of animals. In the Near East, oil was used for lighting, and bitumen had non-energy uses. Boat travel in the open sea (the Mediterranean) is believed to have started over 9000 years ago (Jacobsen, 1973), and there is evidence of wind energy utilisation by means of sails in Egypt about 4500 years ago (Digby, 1954). Per person, wind energy may not at this time have contributed a significant proportion of the total energy use in the Mediterranean region, but later, when trade became more developed (from about 4000 years ago), the total amount of energy spent on transportation on land and at sea constituted a less negligible share (maybe a few per cent) of the total amount of energy spent in the "developed regions" of the world at the time.

The building of houses in many cases implied the creation of a required indoor climate with utilisation of solar energy. In the low-latitude regions, structures of high heat capacities were employed in order to smooth out day-to-night temperature variations, and in many cases the houses were built partly underground, and the evaporation of soil moisture was utilised to create cool environments for living (during hot periods) and food storage (Bahadori, 1977). In regions with a colder climate, a number of insulating* building materials (e.g. roofs made of straw) were employed to reduce heat

* The term "insulating" is taken to include suppression of convective heat transfer.

losses, and heat production not involving fires was increased by keeping livestock within the living area of the houses, so as to benefit from their res-pirational heat release.

Water mills and windmills (e.g. the vertical axis panemone type probably derived from water wheels, or the sail-wing type presumably copied from sail-ships) also played a role from a certain stage in development. The earliest mention of windmills in actual use is from India about 2400 years ago (Wulff, 1966). Considering its low efficiency and overall size, it is unlikely that wind power has at any time accounted for a large proportion of the average energy use. On the other hand, windmills and water mills offered the only alternative to muscle power for high-quality (i.e. low-entropy) mechanical energy, until the invention of the steam engine.

The industrial revolution 200-300 years ago was connected with placing at the disposal of man amounts of power capable of producing work far beyond his own muscle power. However, at that time firewood was barely a renewable resource in the developed regions of the world, despite quite extensive programmes to plant new forests to compensate for usage. The increase in energy usage made possible by the growing industrialisation did not really accelerate, therefore, before large amounts of coal became available as fuel. In the 20th century, large growth in energy consumption has been made possible by the availability of inexpensive fossil fuels: coal, natural gas and oil.


Time relative to present ( I06 years)

Figure 1.15. Trends in average rate of energy conversion per capita, not including fluxes associated with the local thermal environment.

An outline of the possible development in energy usage up to the present is presented in Figs. 1.15 to 1.17. Only over the past century or two have reliable world-wide data on energy usage been recorded, and even for this period the data comprise mainly direct use of commercial fuels, supplemented with incomplete information on biomass and other renewables. One reason for this is that it is more difficult to specify the remaining energy use, because e.g. solar collectors are often not individually monitored, local biomass use is not quantified in energy units, environmental heat gains vary from day to day, and so on. In Figs. 1.15-1.17, which are anyway only indicative, fuels are included in terms of their gross energy value, independently of end-use efficiency. The use of renewable energy flows, on the other hand, is given as an estimated net energy at the primary conversion stage, i.e. the energy in food intake rather than the total amount of energy absorbed by the plants or the total biomass of plants and animals. The environmental energy contribution to maintaining man's body temperature as well as the regulation of indoor climate by the choice of materials and building systems ("passive energy systems") are excluded.

Time relative to year 2000 (years)

Figure 1.16. Trends in average rate (solid line) of energy conversion per capita, not including fluxes associated with the local thermal environment (same as Fig. 1.1, but on a logarithmic time scale). Dashed lines indicate the corresponding trends for the societies, which at a given time have the highest, and the lowest average energy usage. For the more recent period, data from Darmstadter et al. (1971) and European Commission (1997) have been used, in a smoothed form.

Figure 1.15 shows the trend in average rate of energy conversion per capita, on a linear time scale, and Fig. 1.16 shows the same trend on a logarithmic time scale, extending backwards from the year 2000. Figure 1.16 also indicates the estimated spread in energy usage, with the upper curve representing the societies with highest energy use, at a given time, and the lower curve the societies with the lowest energy use. These curves, which do not reflect any great degree of accuracy, do not represent rigorous limits, and values outside the interval may certainly be appropriate for individuals of a given society - the very rich or the very poor.

The energy conversion rate corresponding to food only has been taken as 125 W throughout the time interval. The increase in energy usage from about -105 y is associated with access to fire. The amount of energy derived from fires depends on whether fires were used only for cooking or also for heating. The choice of the average curve further rests on the assumption that between -7x104 and -104 y (i.e. during the latest ice age, cf. Fig. 2.91) about half of the world population used fires for heating purposes.

In the time interval -104 to -103 y, human settlements developed into a variety of societies, some of which had a very high degree of organisation and urbanisation. The increase in energy usage was mainly associated with more systematic heating and cooking practices, with tool production (e.g. weapons) and with transportation (e.g. by riding or by draught animals). With increasing population density, materials which previously had been available in the immediate natural surroundings had to be transported from far away, or substitutes had to be manufactured; either way, additional energy had to be spent. In several of the societies in question, mechanical work was performed not only by animals but also by human slaves, so that the average per capita energy usage was less affected. The trends of the curves also reflect the differences in development characterising different geographical regions. Simultaneously with the culmination of the civilisations in Mesopotamia and Egypt, northern Europe and northern America entered the Neolithic, with warm climatic conditions quite different from those of the preceding several thousand years.

During the last 1000 years, the increasing energy usage is in part due to the shift in population distribution towards higher latitudes, and to overall increased requirements for space heating in such regions (the "little ice age", cf. Fig. 2.91). It should also be mentioned that the efficiency of converting the energy of firewood (supplemented by animal dung and later by peat) into useful heat for cooking, craft work, hot water and space heating was quite low, for example in sixteenth-century Europe, but gradually improved as the twentieth century approached (Bjernholm, 1976). During the period 15001900, the curves are a result of this feature (in particular the early high maximum value attained for the most affluent societies) combined with increased energy demand (e.g. larger proportions of the population acquiring energy-demanding habits or lifestyles, such as taking hot baths, drinking hot beverages, washing clothes in hot water, etc.). The development in the last century is dominated by the energy consumption of the industrialised countries (industrial process heat, transportation, increased room temperature, refrigeration, lighting, etc.). During this period, the top curve in Fig. 1.16 represents the extravagant energy use of an average American, while the lowest curve represents the average energy use in the poor regions of Africa or India, in-

eluding non-commercial fuels such as cow dung and stray wood (which used to be absent from official statistics, as first noted by Makhijani, 1977).



Time relative to year 2000 (years)

Figure 1.17. Trends in the distribution on different types of energy resources of the average rate of energy use. The most recent period is based on smoothed data from Darmstadter et al. (1971) and European Commission (1997), and the basis for the estimates pertaining to earlier periods is explained in the text. Needless to say, such estimates should be regarded as very tentative, and the definition of average use is itself uncertain, in particular for the early periods (e.g. the 20% contribution from fires 50 000 years ago depends sensitively on the fraction of the world population living in regions where space heating was desirable).

Time relative to year 2000 (years)

Figure 1.17. Trends in the distribution on different types of energy resources of the average rate of energy use. The most recent period is based on smoothed data from Darmstadter et al. (1971) and European Commission (1997), and the basis for the estimates pertaining to earlier periods is explained in the text. Needless to say, such estimates should be regarded as very tentative, and the definition of average use is itself uncertain, in particular for the early periods (e.g. the 20% contribution from fires 50 000 years ago depends sensitively on the fraction of the world population living in regions where space heating was desirable).

In Fig. 1.17, a sketch of the distribution of the energy consumption on different sources of energy is attempted. Again, only for the past century or two have actual data been used. The shape of the curve describing the diminishing share of food energy starting about 105 years ago is again dependent on the picture of emerging cultures and geographical distribution of the population, outlined above. It is clear, however, that the energy basis for human societies has been renewable energy sources until quite recently. Whether all the wood usage should be counted as renewable is debatable. Early agricultural practice (e.g. in northern Europe) involved burning forest areas for farming purposes, and repeating the process in a new area after a few years, as the crop yield diminished owing to nutrient deficiency of the soil. Most forests not being converted into permanent agricultural land survived this exploitation, owing to the low population density and the stability of the soils originating from glacier deposits. Similar over-use, or over-grazing by livestock, would be (and was in fact) disastrous in low-latitude regions with a very shallow soil layer, which would simply be eroded away if the vegetation cover was removed (cf. section 2.4.2). Re-plantation of forests has been common in northern Europe during the last few centuries, but the strongly increasing demand for wood over the last century (not only for fuel purposes), as well as construction work associated with urbanisation, has led to an actual decrease in forest area in most parts of the world.

From the middle of the 19th century, the non-renewable fossil fuels have rapidly increased their share of the total energy usage, to the present 80-90 %. In the beginning, fossil fuels replaced wood, but they soon became the basis for exponential growth in energy use, associated with a number of novel energy-demanding activities. During the same period, usage of hydro-power has increased, and recently nuclear fission power passed the 1 % level. Growth has been interrupted by wars and periods of economic recession. The high dependence on non-renewable energy sources has developed over a very short period of time. The briefness of this era compared with the history of man on Earth stands out clearly on the linear scale used in Fig. 1.15.

Renewable Energy Eco Friendly

Renewable Energy Eco Friendly

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable.

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