by Vaclav Smil
The production of liquid biofuels from crops is even more spaceintensive. Yields of sugar cane and grain corn, the two most important feedstock crops, have been rising (as have the efficiencies of their fermentation), but the power density of producing ethanol from US corn yielding 10 t/ha is no higher than 0.25 W/m2. In Asia, where yields are much lower, the power density of corn ethanol production would be just above 0.1 W/m2. In contrast, the power density of ethanol made from high-yielding Brazilian sugar cane is about 0.4 W/m2, and cellulosic ethanol, as yet to be commercialized on a large scale, would have a similar power density.
The production of biodiesel from oil seeds is limited by the inherently low yields of oil crops. Even for the relatively high-yielding Dutch rapeseed, the power density is only 0.18 W/m2, and the EU's biodiesel mean is only 0.12 W/m2. Finally, the small-scale conversion of biomass wastes to produce biogas is rather inefficient: even the best commercial conversion, using corn silage as feedstock, translates to power densities of about 0.6 W/m2 for the gas and 0.2 W/m2 for using that gas to generate electricity. The inherent limits of a phytomass-based energy supply mean that even an unlikely early doubling of yields would keep the power densities of woodbased electricity generation and of ethanol, biodiesel, and biogas production mostly below 1 W/m2.
Large geothermal projects have power densities between 100 and 800 W;/m2 of directly affected land. These values go down an order of magnitude, mostly to 50-80 We/m2, when actual generation is prorated over all affected land of accurately assessed projects in the United States, Iceland, and New Zealand. The power densities of geothermal electricity generation are thus similar to those of alpine stations, an order of magnitude above typical PV performance in Atlantic Europe (5 We/m2) or in the more sunny United States (8-10 We/m2), and also an order of magnitude higher than the spacing densities for the best large wind farms. Supplies of geothermal heat for individual houses have similar densities, commonly between 40 and 100 W,/m2.
Fossil Fuels
Compared to the sprawling land claims of phytomass energies or hydroelectric power generation, extraction of the richest deposits of fossil fuels is almost punctiform; at the same time, the power densities of fossil fuel extraction show even wider ranges than do the conversions of renewable energies. The specific rates of coal mining depend on the depth, thickness, and quality of coal seams. The permanent structures of large underground mines (machinery, maintenance, storage, and office buildings, parking lots, coal processing and shipping facilities) occupy relatively small areas (on the order of 1 ha/1 Mt of annual output); much more land is taken by the on-site disposal of rocks separated from coal and by tailing ponds that receive small-particle waste. The largest underground mines producing high-quality coal from thick seams have power densities in excess of 10,000 W/m2, while the rates for smaller operations extracting lower-quality fuel from thinner seams (and hence generating more waste) are often above 2,000 W/m2 and usually not below 1,000 W/m2.
The ratio between variable and fixed land claims is much larger for surface mining because large volumes of overburden must be stripped and repositioned to access coal seams. The highest ratios of overburden to coal seam are now nearly 7, and the deepest surface mines go below 300 m. Many destructive and unsightly mountaintop removals in central Appalachia produce coal with power densities of just 200 W/m2, some well below 100 W/m2, while the operating densities of the largest surface mines that extract the thickest seams (102 m), be it in Wyoming's Powder River Basin or Australia's Latrobe Valley, are more than 10,000 W/m2 and even above 15,000 W/m2.
The lowest power densities of coal extraction are thus comparable to those of PV-based electricity generation, while the highest rates are as high as hydrocarbon production in some major oil and gas fields. Most surface mines do not belong to either of these extreme categories: their operating densities are typically 1,000-5,000 W/m2, and the figures do not change significantly even when dedicated railroad links (built solely to move coal from a large mine to a power plant) are included because land disturbed by surface mining over the lifetime of an operation greatly surpasses the rightsof-way of a typical rail line.
Early oil field development was often marked by drilling too many wells in close proximity in a predatory quest for maximized output; modern development optimizes production, with 5-30 wells/km2 and with as few as 2-3/km2 where multiple directional or horizontal wells can be drilled from a single pad. A small number of giant oil reservoirs produce a disproportionate amount of oil, but their lifelong power densities cannot be accurately calculated: the largest reservoirs are still producing, with no firm numbers on their ultimate aggregate output. North American oil fields show long-term cumulative power densities of about 2,500 W/m2 for more than 80 years in California and about 1,100 W/m2 for 50 years in Alberta, with both rates calculated for land occupied by wells.
Annual statistics make it possible to trace a gradual decline in power densities (assuming, liberally, 2 ha/well). Between 1972 and 2012 they dropped from about 40,000 W/m2 to 23,000 W/m2 in Saudi Arabia, and from almost 25,000 W/m2 to less than 9,000 W/m2 in the Middle East. Very low US and global rates (100 and 650 W/m2) are biased by the inclusion of thousands of America's old marginal wells, kept in production because of recent high oil prices. The span is thus between 102 W/m2 for mature fields that have been producing for generations to 104 W/m2 for the world's most productive reservoirs, with modal values near the lower end of 103 W/m2.
The two most important ways of nonconventional oil production are horizontal drilling and hydraulic fracturing of oil-bearing shales in the United States, and extraction of oil from Alberta's sands. Oil shale wells have rapid, hyperbolic productivity declines. For North Dakota's Bakken shale, now the largest oil shale producing region, power densities decline from about 4,000 W/m2 in the first year to a mean of 1,600 W/m2 for the first five years and less than 1,000 W/m2 for ten years of extraction. The production of oil from Alberta's oil sands began with surface mining of sands; the power densities of these operations are similar to those of surface coal mining, ranging from roughly 2,000 to 4,000 W/m2. In contrast, in situ recovery, the dominant way of future production, has power densities of 7,000-16,000 W/m2 and averages about 10,000 W/m2. These high densities drop significantly with inclusion of the land required to produce natural gas use for bitumen extraction and steam generation: the power densities of surface mining fall to about 2,300 W/m2 and those of in situ extraction to less than 3,200 W/m2.
The transport and processing of crude oil have fairly high throughput power densities. Crude oil pipelines need construction corridors 15-30 m wide, and ROWs typically are 15 m for buried lines. The average US throughput power density is nearly 700 W/m2, with major trunk lines rating well above 1,000 W/m2. Pumping of crude oil into large tankers has very high throughput power densities of 105 W/m2. A disproportionate amount of liquid fuels comes from a relatively small number of large refineries whose processing power densities are mostly between 4,000 and 8,000 W/m2.
The power densities of natural gas and crude oil extraction are similar, with most operations rating between103 and 104 W/m2: 2,300 W/m2 for all of Alberta's extraction to nearly 50,000 W/m2 for Groningen, the Dutch supergiant field. The gas output from hydraulically fractured horizontal wells shows the same rapid decline as oil production from shales, from power densities of 103 W/m2 in the first year to a low of 102 W/m2 just a few years later. The processing of natural gas has high throughput densities of 104 W/m2, and the power densities of long-distance pipeline transportation range from 102 to 103 W/m2. The rising trade in liquefied natural gas (LNG) involves high-power-density operations: gas liquefaction proceeds at a high power density of 103 W/m2, while the power densities of regasification can be well into 104 W/m2.
Thermal Electricity Generation
The steam-driven generation of electricity remains the dominant way of producing the most flexible kind of energy, and the two most important ways to produce steam are by burning pulverized coal in boilers and by
fissioning uranium in nuclear reactors. Burning gas in boilers is much less common because gas turbines are much more efficient (as well as more flexible), and using crude oil or refined liquid fuels for electricity generations is rarer still (because liquids are too expensive). The core structures of thermal stations are quite compact, and plant sites are mostly occupied by essential infrastructures. Many plants also hold additional land in reserve for possible expansion or as a buffer zone, and nuclear plants require a safety belt that excludes permanent habitation. Some thermal electricity generation proceeds with a very high power density, but plants with large adjacent cooling lakes, with large areas reserved for on-site storage of captured fly ash and sulfates from flue-gas desulfurization, and with extensive (often forested) buffer zones have much lower ratings.
Thermal electricity generation is clearly in a high-power-density category, but its surprisingly large range of values means that its outcomes can be orders of magnitude above any renewable alternatives (mine-mouth coal-fired stations burning high-quality fuel, gas-fired plants burning LNG)-or that it can have almost as low a power density as the best instances of large-scale PV-based generation (coal-fired stations burning low-quality fuel from surface mines with a high overburden to seam ratio). Here is the sequence of diminishing power densities for coal-fired electricity generation, from its burning core to complete plant-and-fuel claims.
Large boilers have power densities in the low 106 W,/m2 of their footprint; boilers and turbogenerator halls are in the mid- to high 105 We/m2 range; stations, including all on-site infrastructures, are in the mid-103 We/ m2 range; and stations including coal extraction and delivery infrastructure are in the low 103 We/m2 or high 102 We/m2 range (determined by coal quality and shipping distance). Actual generated power densities for coal-fired stations in the United States, Europe, and Asia confirm model calculations, as they range from a high 102 We/m2 to a mostly low 103 We/m2 range for plant sites only; depending on the mining methods, coal quality, and coal shipping; inclusion of the entire coal-to-electricity chain leaves the densities well above 1,000 We/m2 or lowers them to the mid-102 We/m2 range, even to just above 100 We/m2.
Stations burning heavy oil and crude oil have operating power densities typically well into the 103 We/m2 range. Natural gas-fired central electricitygenerating plants do not need any land for extensive on-site waste deposits, and continuing gas supply by pipelines or LNG tankers reduces their fuel storage requirements; thus they generate electricity with power densities mostly between 2,000 and 6,000 We/m2. Even after the claims of natural gas production are included, the power densities for the entire extractiongeneration sequence remain on the order of 103 We/m2. More natural gas is now burned in gas turbines than in the boilers of large central stations. Gas turbines have limited ratings (100 to mid-102 MW) but operate with very high power densities, commonly 104 We/m2. This makes it possible to site new gas turbines on land belonging to the existing central stations. The same is obviously possible for combined-cycle plants, whose operating power densities will also be on the order of 103 We/m2.
The power densities of nuclear reactors (per unit of their footprint) are 106 Wt/m2, but subsequent energy flows are the same as in fossil-fueled central stations, and hence the layout of nuclear plants and their overall land requirements are also very similar. Because these plants do not require any extensive fuel storage and do not generate any waste from controlling air pollutants, they can be quite compact, rating between 2,000 and 6,000 We/m2. Land claims for the entire uranium cycle lower the densities of the entire fuel generation-disposal sequence. The most productive underground mines have exceptionally high extraction densities (104 We/m2), typical surface operations produce ore with power densities ranging from the high 102 We/m2 range to the low 103 We/m2 range, and in situ leaching projects reach 400-600 We/m2.
The milling of ores to produce U308 concentrate proceeds with high power densities, and the production of fuel rods filled with slightly enriched UO2 is even less space-intensive, but the enrichment process has fairly low power densities once the claims for its electricity requirements are included. Differences in uranium mining, processing, fuel enrichment, and eventual radioactive waste disposal produce a range of overall power densities very similar to that of coal-fired plants, with rates as high as 103 We/m2 and as low as the low to mid-102 We/m2 range. Several recent studies offer all-inclusive rates of 230 to 960 We/m2 for US nuclear power generation, in good agreement with my detailed, stepwise quantification.
Early electricity-generating plants served their immediate neighborhoods, but modern stations are connected to load centers (cities, industries) by high-voltage AC transmission lines. Their ROWs add up to mostly between 4 and 6 ha/km, while high-voltage DC links (mostly from distant hydro stations) need at least 40% less land for handling the same capacity. Naturally, short, high-capacity links will have higher power densities-a 200-km, 1-GW, 765-kV line would rate about 80 W/m2 of its ROW-but the means for large utilities or for nationwide grids will typically be no more than 30-40 W/m2 of ROW.
Energy Uses
As huge as it is in absolute terms (17 TW in 2013), global energy use remains a tiny fraction of the Earth's natural radiation balance: it prorates to 0.03 W/m2 of the Earth's surface and to 0.125 W/m2 W/m2 of ice-free land, the latter rate being less than 0.07% of the mean global insolation and a small flux compared to the 2.3 W/m2 of aggregate radiative forcing due to greenhouse gases. Average power densities of energy use within national territories range over four orders of magnitude: desert countries with sparse rural population rate only 10-4 W/m2, while the Netherlands, Belgium, and South Korea reach about 3 W/m2 (fig. 7.2). Densely populated and industrialized urban areas use about 75% of the world's energy, and the global average of their power density reaches roughly 20 W/m2.
That, too, is roughly the US urban mean, including all treed and grassed area within cities; after their exclusion the power density per unit of impervious US urban surfaces is about 35 W/m2. Worldwide, annual urban means range between 10 and 100 W/m2; downtowns average in excess of 100 W/m2 and their hourly extremes often approach and can substantially surpass 1,000 W/m2, and that much can be the annual mean for the densest city blocks in Manhattan, a flux equaling or exceeding noontime insolation. The variability of rates among individual buildings in the same climate is highly influenced by their function and construction.
The power densities of detached houses and apartments now differ less than just a generation or two ago because the ownership of many household energy converters (refrigerators, electric or gas stoves, washing machines, TVs, computers) has approached or reached saturation. Moreover, higher efficiencies have cut the household rate in North America and Europe by half or more since the 1960s. The most efficient commercial buildings need less than 10 W/m2 of floor area, a tenth of the 1970s level. But in temperate climates, with winter heating and summer cooling, power densities are around 20-30 W/m2 of floor area in single - and multifamily housing. Schools have similar rates. Parking garages and warehouses have the lowest power densities, hospitals the highest (about 70 W/m2 of floor area). Adjustments from floor area to building footprints yield some very high power densities, up to roughly 1,000 W/m2 for the nearly 40-story housing estate towers in Hong Kong, 2,000 W/m2 for a 50-story luxury hotel in a hot climate, and more than 6,000 W/m2 for Burj Khalifa, the world's tallest building.
Figure 7.2
Power densities of large-scale energy use. Carl De Torres Graphic Design. Notes: Population density (persons/km2) is displayed on the x-axis, energy consumption (W/capita) on the y-axis. Power density (W/m2) is arrayed on the diagonals.
Energy converted by road vehicles is the second largest component of urban fuel use. The free-flowing traffic of cars and trucks will have a shortterm power density of about 500 W/m2 of a paved lane, and this figure may nearly double when vehicles have to idle. When the vehicular energy use is prorated over the entire ROW (lanes, shoulders, and medians), the densities are roughly halved (and c
ut even more in rural settings), and a further major reduction comes when parking lot space is included. For entire urban areas, with traffic averaged on an annual basis, the rates are usually less than 5 W/m2.
Comparisons
Comparisons of power densities inform us in two important ways: they reveal the hierarchies of supply and use, and the contrasts of supply and usage rates allow us to quantify relative claims of particular energy requirements and to assess the future demands for land devoted to energy industries. Of course, we should keep in mind that only a few power densities have very narrow ranges, and that in many specific cases the ratings will be far from the quoted typical performances or well outside the usual ranges. Perhaps the most consequential reality is that the extraction and conversion of fossil fuels and uranium produce useful heat and electricity with power densities that are usually at least two and up to five orders of magnitude higher than the exploitation of renewable flows. Only in exceptional cases do some renewable energy conversions (most notably those involving alpine hydro stations with high heads and small reservoirs) have higher production power densities than the extraction of fossil fuels and the generation of thermal electricity (fig. 7.3).
The production of phytomass in general, and of liquid biofuels in particular, has the lowest power densities of all commercially exploited resources. This is a fundamentally unalterable outcome of inherently low photosynthetic efficiency and environmental constraints on phytomass yield. Crops can be produced with a higher power density than woody biomass: the latter has a higher energy density, but the former has a higher rate of growth. Although yields have increased for both field crops and plantation trees, and although further increases are coming, there is no realistic prospect for biofuels to be produced with a power density significantly surpassing 0.5 W/m2 in temperate environments and 1 W/m2 in tropical environments, where the current rates are less than half these thresholds. Moreover, if the inputs needed to produce biofuels (machinery, fuel, fertilizer) were to be energized solely by renewable energies rather than (as is the case) by fossil fuels and nuclear-derived electricity, then the power densities of such completely renewable operations would drop to less than 0.1 W/m2.
