Power density

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W/m2 As already noted, my primary measure of power density will not be the energy flux per unit of the working surface (vertical or horizontal) of a converter (as in Umov-Poynting vector) but per unit of the Earth’s surface, for comparability expressed always in W/m2. I have been advocating the use of this measure since the early 1980s (Smil 1984) and chose it as key analytical variable in my first synthesis of general energetics (Smil 1991) and again in its thoroughly revised and expanded sequel (Smil 2008). Since the late 1990s power density expressed as energy flux per time per unit of horizontal surface has been receiving more attention as a result of the growing interest in renewable energy resources and their commercial conversions to fuels and electricity. Power densities of harnessing these energy flows are appreciably lower than the power densities of fossil fuel-based systems, something that even often uncritical proponents of new renewables (as opposed to the traditional hydroenergy and fuelwood uses) cannot ignore.

Perhaps the greatest advantage of power density is its universal applicability: the rate can be used to evaluate and compare all energy fluxes in nature and in society. I will use it for a systematic assessment of all important energies, be they natural (renewable) flows or fossil fuels burned to produce heat for many direct uses or as the means of generating thermal electricity. Because fossil fuels are the dominant source of primary energy I will look first at power densities of producing processing and transporting coal, crude oil and natural gas and then at combustion processes in general and at thermal generation of electricity in particular. A significant share of the latter activity is also energized by uranium fission and I will quantify power densities of the entire nuclear fuel cycle.

I will also quantify, in a brief historical survey and in more detailed sectoral comparisons, the hierarchy of power densities of energy uses and the challenges of heat rejection arising from highly concentrated energy conversions. In the closing chapter I will assess the importance of power densities, their heuristic value and their practical implications. This will be done mainly through two sets of contrasts, those of renewable and fossil-based energy flows, and those of all commercially important energy production activities with the power densities of modern economic activities.

In the penultimate chapter I will summarize the book’s main findings in order to demonstrate why power densities matter and why their appreciation and assessments should be among the key ingredients of any attempts to understand the energy predicament of modern, high-energy civilization and to guide the development of its future energy supplies. Although there is no imminent global prospect for running out of fossil fuels (the very notion of running out as a complete physical absence of a resource is incorrect, as rising prices will end an extraction long before the last reserves are exhausted), their resources are finite and, eventually, our civilization will have to accomplish the transition to renewable energy supply. Its early stages have been unfolding during the past generation and I will explain some of the challenges involved in shifting from a high power density system of energy supply to low power density renewable energy flows and their commercial energy conversions to produce electricity as well substitutes for oil-based fuels.

But before starting a systematic review of specific power densities I should take a closer look at qualitative differences that are hidden by a simple quantitative expression. They affect the rate’s numerator: all energies can be converted to a common denominator but that measure cannot convey their qualities. But their range and their consequences are particularly important as far as the denominator is concerned: space attributable to energy production and use may be easily measured in common units but its qualitative attributes range across a very wide spectrum of land cover transformations.

Typology and caveats

Power density is an inherent property of all forms of energy production and energy use. These processes unfold on scales ranging from sub-atomic to universal but, obviously, we are interested above all in their varied and ubiquitous commercial manifestations that sustain our civilization. Power density, however, is not a primary design parameter. Energy systems of modern societies are designed for overall performance and reliability achieved at acceptable cost: annual output (and hence maximum available power), price per unit of product, and desired parameters of safety and environmental impacts govern their construction and performance, and a specific power density is simply a key consequence of their operation.

The measure is deceptively simple –- power (energy flow per unit of time) per unit of area, in standard form W/m2 –- but both the numerator and the denominator hide important differences and complexities. Neither of them can convey quality of the two measured variables: watts and square meters are measures that will accurately quantify power and area without telling us directly anything either about the quality of a specific energy flow (its environmental externalities, its ease of use, its reliability, its cost; its durability) or about the initial quality of land that was claimed by energy production and use and about the eventual extent of its destruction or alteration. A closer look at some of the most important qualitative attributes of the two variables that are used to calculate power density is important in order to appreciate inherent deficiencies of the measure, and I will pay a particularly close attention to the denominator.

This information loss is a common problem when using single measures for variable whose quality matters at least as much, and sometimes even more, than their quantity. Food consumption is an excellent example of this information deficiency as every foodstuff can be quantified in terms of energy density (kcal/g, MJ/kg) and as average per capita food intake can be expressed in kcal/day or J/day –-but these commonly used quantities hide fundamental nutritional qualities: proportions of macronutrients (carbohydrates, lipids, proteins), their specific qualities (all animal proteins are nutritionally superior to plant proteins), presence of micronutrients. As a result, people can consume energetically adequate quantities of food but be malnourished; unfortunately that is not an uncommon situation for some population groups even in many affluent countries, particularly as far as some key micronutrients (above all iron and zinc) are concerned (Smil 2013b).

Energy qualities Quantification of the numerator is subject to frequent but minor inconsistencies arising from conversion of fuels to common energy equivalents. The most common cause are slightly different assumptions regarding the average, or typical, energy density rates (J/kg or J/m3) of fuels and the choice (often undefined) of using either higher or lower heating values. But by far the greatest challenge arises from converting various forms of electricity to primary energy equivalent. With electricity generated by fuel combustion it is simply the matter of adjusting for overall efficiency of thermal generation: an aging coal-fired station may have efficiency of just 33%, the latest variant may convert 42% of coal into electricity and for combined gas cycle generation the rate may be as high as 60%. Consequently, primary energy equivalents of the three processes will be, respectively, 10.9, 8.6 and 6.0 MJ/kWh.

But there is no universally valid (or accepted) way of converting nuclear, solar, wind and hydro electricity, and the two commonly used procedures differ by roughly a factor of 3: simple thermal equivalent is 3.6 MJ/kWh, conversions using the prevailing efficiency in large central fossil-fuelled thermal stations range from 8.6-11.0 MJ/kWh. This disparity explains most of the differences in primary energy totals offered by data aggregators and statistical services. For example, British Petroleum’s widely quoted Statistical Review of World Energy uses straight thermal equivalent (BP 2013), while the International Energy Agency uses that equivalent for all form renewable electricity generation but calculates the primary energy equivalent of nuclear electricity by assuming that 1 kWh equals 10.9 MJ.

These realities also mean that the no matter which conversion rates are chosen power densities of fuel combustion are always qualitatively different from power densities of primary electricity use. The key difference, and one with enormous environmental and hence socio-economic implications, is that fossil fuel combustion is always associated with CO2 emissions, the principal source of anthropogenic carbon releases and the leading cause of the human interference in the global biogeochemical carbon cycle. Combustion of coals and hydrocarbons also emits oxides of nitrogen (precursors of photochemical smog) and burning of coal and of many liquid fuels also releases sulfur oxides (key contributors to acid deposition). In contrast, primary electricity (nuclear fission ad renewable flows) is not a direct source of CO2 (carbon emissions are, of course, attributable to construction and maintenance of the requisite facilities and converters).

Other qualitative differences pertain to the convenience and flexibility of energy use. Electricity is always easier to use (with a flip or push of a switch) than fuels, although in household setting a modern high-efficiency natural gas furnace with electronic ignition connected to a programmable thermostat comes close in reliable convenience: except for an annual check-up its operation is entirely automatic. And the only two important commercial applications where electricity cannot compete with fuel are (as already explained) production of iron from its ores (where coke or charcoal remain indispensable) and commercial flying where kerosene remains the only practical choice for jetliners; capability of experimental unmanned solar-powered airplanes is orders of magnitude behind (Paur 2013): they can stay aloft for many hours and Solar Impulse crossed the US -- but it needed 12,000 photovoltaic (PV) cells to do so, it carried only its pilot, its speed averaged less than 50 km/h while its wingspan is almost the same as that of a wide-body Boeing 747-400 that carries more than 500 people at Mach 0.92.

Space qualities The denominator of power densities -- a unit of horizontal surface –- is seemingly a much more simple measure, but a brief reflection makes it obvious that some very different spatial and functional qualities are subsumed under that ordinary quantity (usually m2). For all photosynthetic production in fields, grasslands and forests it is simply its annual yield, albeit expressed in W/m2 rather than (as in agricultural statistics) in t/ha (or, in non-metric units, in bushels/acre). For fossil fuels it is the unit of surface disturbed by their extraction (by mines, oil and gas wells and drilling pads) or otherwise claimed by infrastructures needed for their production (roads, rights-of-way for pipelines). For thermal electricity-generating plants it is a combination of permanent structures (boiler and generator halls, other buildings, cooling towers) and necessary infrastructures on-site (in coal-fired plants mainly coal storage, switchyard, fly ash depository) and off-site (rights-of-way for high-voltage transmission lines) and for nuclear stations it includes also the exclusion (safety) zone surrounding a plant.

For renewable energy conversions it gets somewhat more complicated. For direct combustion of biofuels it is obviously just the phytomass harvest per unit of area, but for liquid biofuels it is the energy of ethanol or biodiesel produced in a nearby or a distant processing plant from the feedstock harvested from a unit of cultivated land. For hydroelectricity it is the entire surface of a reservoir created by a dam but there are several possibilities to consider: maximum design level (which may not be attained), average level determined as the mean of seasonal fluctuations, or the modal level that prevails for most of the year (for all large projects land claimed by the dam and associated infrastructure amounts to only a small fraction of the are inundated and exposed by the extreme reservoir levels).

In order to be comparable with other conversions, power densities of wind-generated electricity require a transposition from vertical to horizontal planes. Working surfaces of wind turbines are vertical but power density of wind generation is calculated as a ratio of electricity produced per unit of horizontal (although in reality it can be also sloping) land surface. Power densities for solar PV electricity generation (and also for solar water heating) should be expressed, for conformity’s sake, per unit of horizontal surface area but they are usually given per unit area of actual working surface that is fixed at an angle in order to optimize radiation capture or (a much more expensive solution) that uses automated tracking to maximize the exposure.

As a result, power densities are calculated by using several obviously distinct types of land covers or land uses, and similarities (often identities) and differences embodied in these rates are cutting across production modes, infrastructural arrangements and diverse energy uses. My attempt at typology of space that becomes denominator in calculating power densities offers a few fairly simply classifications based on obvious structural and functional attributes: I will consider the degree of transformation, project longevity, likelihood that the land claimed by energy facilities will regain its former function (or at least some of it), and the possibility of concurrent uses of land (or water) used to produce or convert energies.

Space claims by all modern energy conversions are hierarchical. On the most intensive end of the spectrum is the obliteration extreme, whereby the original appearance of natural surfaces has been entirely erased and replaced by structures that are completely and exclusively devoted to a site’s new extraction, transportation or conversion function. Then comes an extended continuum of impacts of diminishing intensity that eventually joins the other extreme, land that is claimed, owned or managed by energy industries but whose surfaces have retained their previous (and often entirely undisturbed) soil and plant cover.

The most diverse group in the first category are the structures required by modern energy extraction, transportation and processing and for electricity generation. Every industry has many structures and assemblies of this kind, some highly standardized, other of specific design: buildings housing hoisting machinery, sorting and washing facilities and storage silos in underground coal mines; wellheads, gas processing, liquefaction and regasification plants, compressor stations, refineries and loading docks of oil and gas industry; boiler and turbogenerator halls, electrostatic precipitators, desulfurization units and cooling towers of thermal power plants. Given the high power densities of fossil fuel combustion, gas processing, crude oil refining or nuclear fission it is not surprising that most of the structures housing these activities account for only a small fraction of land claimed by entire facilities or plants.

In addition to buildings that house machinery and processing facilities there are also office buildings and many energy extraction and conversion sites must have infrastructural components whose construction and operation result in complete destruction of original soils and plants: these include often extensive areas of impervious surfaces (roads and walkways, large parking lots, storage sheds or open lots with stacked components and parts, railroad yards, switchyards) and frequently even more extensive fuel and waste depositories (large coal yards, oil and gas tanks, ponds storing captured fly ash and sulfates produced by flue-gas desulfurization).

Relatively large shares of areas that are claimed by energy projects have retained their soils and vegetation but have been fragmented to such an extent that they lost capability to provide many of their former ecosystemic services. Easy access to satellite imagery illustrates a wide range of these energy-related fragmentations. Development of oil and gas fields on grasslands or in forests requires construction of access roads and drilling of many, often fairly closely spaced, wells; drilling pads and wellheads create pock-marked landscapes (often in regular, grid-like manner) that are dissected by roads, and further disturbances are created by installing pumps, compressors and (above-ground or buried) gathering pipelines. Measurements will show that all above-ground structures claim only a small share of the affected area but while most of the land is undisturbed land its fragmentation excludes any commercial use and degrades its value as habitat for plants and animals. Similar pock-mark fragmentation is also typical (albeit usually on a smaller scale) of in situ leaching of uranium.

Large-scale photovoltaic electricity generation with massed rows of panels fastened to elevated steel support entails much greater interventions. Support columns, arrayed in regular formations, disturb the ground, panels shade the ground and space must left for access needed for maintenance and regular cleaning is repeatedly frequently by foot and vehicle traffic. In contrast, large wind farms can be seen as perhaps the least disturbing example of this fragmentation. As I will explain in some detail in the next chapter, turbine siting requires minimum spacing between adjacent towers and this distance increases with the machine’s capacity. Consequently, large modern wind turbines stand hundreds of meters apart and even when the area of all access roads and transformer stations is added to space occupied by their concrete foundations at least 90% of a wind farm’s area is left undisturbed (albeit slightly fragmented) and, if located in a natural setting, the facility should not have an adverse effect on terrestrial fauna (of course, it is deadly to birds) or on such previous commercial land uses as grazing or cropping.

Many energy projects also include new, deliberately created green belts or buffers designed to provide at least partial visual, noise and air pollution screens separating them from their surroundings. Such projects are usually site-specific, some well planned, others are added as afterthoughts, others yet are a part of standard construction requirements. For example, India’s Ministry of Environment and Forests stipulates that the total green area of the country’s numerous new coal-fired power plants will equal one-third of the total plant area, or about 60 ha landscaped and planted for 1 GW station and almost 120 ha for a large 4 GW plant (CEA 2007).

Finally, many energy production facilities contain land that is entirely undisturbed by their construction; this includes land that was acquired before the project’s initiation and that is held in reserve for a possible expansion; land that was deliberately acquired in order to put some distance between a project and the nearest inhabited areas; and, in the case of nuclear power plants, land whose previous uses (unexploited, forestry, cropping, seashore) can continue as long as they do not include any permanent habitable structures. Some inland water bodies (lakes or reservoirs) associated with thermal power plants also belong to this category: because they are used for water cooling their area should be counted as a part of the project spatial claim while most of their former uses are largely unaffected.

As far as the overall impacts of energy infrastructures on land use and land cover are concerned it should be obvious that they cannot be expressed simply by adding up the affected space. Detailed, realistic appraisals require qualitative assessments because energy developments affect spaces ranging from unproductive, barren, hilly surfaces to extensive areas of highly fertile alluvial soils; similarly, many energy infrastructures have negligible impacts on flora and fauna but others destroy parts of highly biodiverse environments. More often, rights of way and access roads needed to bring fuels and electricity to distant markets are a major reason for habitat fragmentation, a change that can contribute to loss of biodiversity.

Project longevities As for the longevity of specific energy-related land use, it is obvious that only a few structures of modern fossil fuel industries and electricity generation can be called permanent even if that adjective refers to the span of just a single century. Mining facilities, oil wellheads, refineries, tanker terminals, fuel storages and thermal electricity-generating plants are designed for amortization spans of between 20-40 years, but many of these enterprises may be around, after upgrading and partial reconstruction, for 50-80 years. What is much more common is that the structures are completely replaced and modernized but the extraction and generation sites remain.

Many European coalfields were in production (albeit, for generations, on a small scale) for more than a century, and some English ones for more than three centuries, before their production became uneconomical during the closing decades of the 20th century, not because they ran out of coal (Smil 2010a). California’s late 19th-century San Joaquin Valley oilfields are still in production more than a century later: Midway-Sunset was discovered in 1894, Kern River in 1899 (SJVG 2012). Even more impressively, Baku’s oilfields where modern oil production began in 1846 (a decade before the US drilling in Pennsylvania) are still in production (Mir-Babayev 2002). And several of the world’s largest oil producers are now older than 60 years: Saudi al-Ghawār (by far the world’s largest oilfield) began producing in 1951, Kuwaiti al-Burqān has been producing oil and gas since 1946 (for more details see chapter 4).

Many coal-fired electricity-generating stations have occupied the same sites for generations. In 2008 the US had 10 coal-fired powered units that were built during the 1920s, including the Sixth Street Generating Station in Cedar Rapids, Iowa that began to generate in 1921 and was shut down only in 2010 (SourceWatch 2014). There were additional 110 units that began to work before 1950, and plants of similar age, or even older locations with refurbished generating equipment can be found in Europe. And most reservoirs created by dams built during the closing decades of the 19th century are still in operation, and most of those built since WW II will be around for more than 100 years.

Rheinfelden on the German and Swiss border was the first large hydropower plant in European when it was completed in 1898, and after rebuilding between the years 2006 and 2011 its four new Voith turbines began producing up to 600 GWh/year (Voith 2011). Other well-known large hydro projects older than 70 years include America’s Grand Coulee on the Columbia (1942), Hoover Dam on the Colorado (1936), and Ukraine’s Dnieper station (1932), and the eventual life span of some of the largest reservoirs will be measured in centuries. That is true even for such impoundments affected by high rate of silting as the High Aswan Dam. Negm et al. (2010) modelled the reservoir’s silting and scouring processes and concluded that the steady value of life span of the dead zone (below the lowest water intake level for the turbines) is 254 years and that of the life zone is 985 years.

On the other hand, many small hydro stations built in China’s countryside since the late 1950s as a part of Maoist quest for inexpensive, mass labor-based solutions to the country’s energy shortages had silted very rapidly and were abandoned or dismantled after only a few years of unreliable generation (Smil 2004a). Similarly, large number of surface or shallow underground mines opened by Chinese peasants (legally and illegally) since the 1950s had operated for just a few years before more economic (and less dangerously produced) fuels became available. And even the latest techniques may result in relatively brief lifespans: average life expectancy of wells drilled for hydraulic fracturing of shales to produce crude oil and natural gas will be no more than 15-20 years (see chapter 4).

Taking these different longevities into account when calculating power densities of specific processes is not easy. The only possible way is to make serial assumptions regarding their ultimate lifespans –- and such assumptions may have large errors. Re-drilling of old reservoirs and secondary recovery of previously unobtainable oil has extended lifetimes of many oilfields to three, four or even five generations, while the initial expectations may have been for just 20-30 years of production. Horizontal drilling and hydraulic fracturing created commercial reserves out of resources that two decades earlier were thought uneconomical. And in many instances periodical refurbishing of old fossil-fueled power plants has maintained electricity-generating capacities on the same sites for 50 or 60 years for similar periods of time.

In contrast, once cheaper hydrocarbons became available many coalfields (in Europe, Japan, Taiwan) were abandoned long before they reached their expected lifespans, and in the Dutch case that unexpected shift away from coal to Groningen gas took place in just a few years (Smil 2010a). And because we do not have enough accumulated experience with new conversion techniques, assumptions have to be made about the longevity of different forms of solar energy capture and the durability of wind turbines. The standard assumptions for 30 years of service, but the first designs of large commercial wind turbines that was introduced during the late 1980s, and even many better models of the 1990s, were retired after less than a decade of operation. Consequently, calculating all power densities on the basis of life-cycle analyses may lead to some major errors and in general it may offer little improvement compared to the rates that are calculated simply on annual basis.

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