Power Density

by Vaclav Smil

  Thermal electricity generation proceeds either by burning fossil fuels or by fissioning uranium, and accounting for its space requirements is a rather complex endeavor. Direct claims for plant sites are fairly small, but indirect claims for energy inputs (including their transport), the management of wastes, and electricity transmission reduce the overall power densities.

  Most of the world's electricity-almost exactly 80% in 2010-is produced by large thermal power plants burning fossil fuels or fissioning uranium. The development of thermal electricity generation began with the first Edisonian coal-fired stations of the early 1880s, which relied on large steam engines and dynamos, and the standard setup that prevails to this day-the combustion of coal in large boilers and the generation of electricity using steam turbogenerators invented by Charles A.Parsons (whose first patent was granted in 1884)-became the norm by the beginning of the twentieth century (Smil 2005). Subsequent progress was slower, and large gains in capacities and performances resumed only after World War II (Smil 2006). After 1950 an increasing share of thermal electricity was produced by burning hydrocarbons, beginning with oil and progressing to natural gas, with most of the latter fuel eventually destined for gas turbines rather than for large boilers.

  In the late 1950s came the commercialization of nuclear electricity generation based on highly exothermic fissioning of uranium to generate steam in a variety of reactors. Fuel is arrayed in rods, and water can serve both as a moderator (to slow down neutrons) and as a working medium, but some designs use gas for cooling. The pressurized water reactor (PWR) eventually emerged as the dominant design: in 2013 the world had 435 operating reactors, of which 274 were PWRs (IAEA 2013). Other relatively common choices have included British gas-cooled reactors and Canadian CANDU (CANadian Deuterium Uranium) reactors using natural rather than enriched uranium.

  The rapid expansion of nuclear capacities came during the 1960s and 1970s, followed by the virtual cessation of building in the United States and Canada and limited additions in Europe (mostly in France, the country with the highest reliance on nuclear generation). Since the year 2000 most new additions have been in Asia (China, Japan, South Korea, India), but the Fukushima plant disaster of March 2011 has affected those prospects. All thermal plants share key components: large halls with turbogenerators, cooling towers (in locations where there is no possibility for once-through cooling), and large transformer yards. But fossil-fueled stations require fuel storage, and all modern coal-fired plants have appropriate facilities to control air pollutants. Complete accounts of the land needed for thermal electricity generation must go beyond power plants and quantify the impacts of the requisite fuel production and its delivery. That is why my account of the power densities of thermal electricity generation is divided into the three categories that now dominate the industry: coal-fired plants, gas-fired installations, and nuclear power stations.

  Coal-Fired Power Plants

  Since the early twentieth century, increasing shares of global electricity have come from hydroelectric power, the burning of hydrocarbons, and, since 1956, also from nuclear fission, but coal has remained the single largest energy source for thermal electricity generation. The fuel has been particularly prominent in the world's two largest economies: in the United States it still accounted for 56% in 1990 and 51% in the year 2000, and only the recent rise of gas-fired electricity generation has brought its share to 43% in 2012 (USEIA 2014d). In China, coal-fired power plants generated about 80% of electricity in 1975, and the share was still 81% of the muchexpanded output in 2012 (World Coal Association 2013).

  The principal components and configurations of coal-fired power plants have not changed for decades, but the stations have larger boilers and turbogenerators (the two forming a generation unit), and large multi-unit stations have become the norm. The largest turbogenerators eventually reached ratings in excess of 1 GW, and the largest plants, made up of a series of units (boiler-turbogenerator assemblies), have capacities up to 5 GW. In light of the wide range of their designs and performances and the different origins and qualities of their fuel-the extremes would be a mine-mouth station located next to a large surface coal mine and burning poor-quality lignite, and a coastal station receiving high-quality coal shipped in bulk carriers-it is not surprising that land claims of coal-fired stations vary widely.

  Principal Structures

  Dominant structures that are shared by all coal-fired power plants-tall boiler buildings, machine (turbogenerator) halls, electrostatic precipitators, tall stacks, maintenance and office buildings, a switchyard, and coal storage areas-occupy a relatively small amount of space. Coal combustion in large boilers and the expansion of highly compressed steam in turbines are two energy conversions that proceed at very high power densities. Modern plants burn mixtures of air and pulverized coal (with most particles as fine as baking flour, or less than 75 µm in diameter) in a swirling vortex, with flames reaching temperatures as high as 1800°C.

  Modern boilers (steam generators) are tall prismatic structures with walls covered in stainless steel tubing that circulates water to be converted to pressurized steam (Teir 2002). The boiler supplying steam to a large 1.3-GW turbogenerator is about 52 m tall, and its footprint is nearly 520 m2 (33.8 m x 15.5 m), which means that the combustion power density will be between 6.5 and 7 MW/m2; smaller boilers in less efficient plants have power densities of 2-4 MW/m2. Large modern steam turbines are also very compact: for example, the 250-MWe Siemens design is 19 m long and 8 m wide, the Skoda 660-MW unit is about 32 by 13 m, and the Siemens 1,350-MW turbine is about 70 m long and 30 m wide (Fiala 2010; Siemens 2013a). The rectangular footprint of these turbines implies approximate operational power densities of 0.6-1.5 MWe/m2. The core structure of a coal-fired power plant-boilers and turbogenerator halls-thus claims only a small fraction of a station's total area: for a 1.5-GW (with three 500-MW turbines) station it can be just 1.2-1.6 ha, while a larger (2-3 GW) station powerhouse can claim 2-3 ha, resulting in specific power densities of 105 We/m2.

  The undesirable consequences of high-efficiency coal combustion include the voluminous generation of fly ash and (where high-sulfur coal is burned) of SO2. That is why all modern coal-fired stations must invest into expensive techniques to limit the emissions of these air pollutants. Structures housing these air pollution control facilities are sited immediately adjacent to boilers and tall chimneys. A typical linear configuration would proceed from a coal bunker feeding a ball mill to pulverize coal, a boiler, an air heater, an electrostatic precipitator, a flue-gas desulfurization (FGD) unit, and a tall stack. In plants with capacities of 1.5-3 GW, electrostatic precipitators used to capture fly ash will occupy 1-3 ha, and FGD units will also occupy 1-3 ha. Adjacent walkways and separation spaces will raise these totals by 50%.

  Captured fly ash can be used to make cement, an appealing choice because it lowers the energy cost of the material and avoids landfilling. In China, two-thirds of all captured fly ash (a total of nearly 400 Mt/year) has recently been used by the cement industry (Lei 2011). In the United States, Virginia's Ceratech blends 95% fly ash and 5% liquid ingredients to make a stronger concrete (Amato 2013). FGD removes SO2 by reacting with lime or limestone to produce Ca504, which can be used in wallboard manufacture. Despite these efforts, large volumes of fly ash and FGD sludge are still deposited within many plant sites, claiming considerable amounts of land during the 35-40 years of typical power plant operation: for 2-3 GW plants these claims can add commonly to between 120 and 160 ha, that is, a disposal power density of 1,300-1,900 We/m2. For coal with a high ash content the areas will be obviously larger, 200 ha for an Indian 1-GWe plant burning domestic raw coal with 40% ash, and 480 ha for a 4-GWe station burning domestic sorted coal with 34% ash (CEA 2007).

  All thermal stations must condense steam discharged by the turbines and cool the condensate. This can be done by once-through cooling, which withdraws water from streams, lakes, or from the ocean with little additional space requirement; spray ponds require about
400 m2/MWe (2,500 We/m2), and ordinary cooling ponds need up to 5,000 m2/MWe (200 W/m2). Cooling towers allow water's continuing reuse. Thus, a complete water system for a 1-GWe plant (including water treatment and cooling) could occupy up to 20 ha for natural draft towers and only 10 ha for induced draft cooling (CEA 2007). Natural draft towers have inevitable evaporation losses but require no power to operate; their power densities (per square meter of tower base) are between 20,000 and 40,000 We/m2. Mechanical draft towers (counterflow or cross-flow designs) use fans to move the air, and in dry cooling towers there is no evaporation as water circulates in closed pipe circuits; the power densities of their heat rejection are high, on the order of 105 We/m2.

  Mine-mouth stations can be supplied directly from adjacent surface or underground extraction, but stations receiving deliveries by trains, barges, or ships need on-site storage for 90-120 days of normal operation. The energy storage density of coal yard will depend on the coal quality and the thickness of a coal pile (commonly 10-12 m). The area needed for storing 90 days' worth of coal for a 3-GW station will be 15-20 ha. The total area of a 3-GW station with adequate on-site coal storage and requisite ash and FGD sludge disposal will thus be between 350 and 500 ha, that is, a power density of up to 850 We/m2 for installed capacity and (with a 75% load factor) up to 650 We/m2 for actual generation.

  Variable space requirements surpass the fixed land claims, which remain constant or change only in minor ways if there is no major reconstruction or expansion. Coal mining claims, the single largest space requirements, can be accurately quantified only for plants supplied by adjacent mines or for the stations that receive their fuel from a single distant place of origin. In reality, many plants receive fuel from different source, while others change their suppliers during the decades of their operation (in Europe the switch has commonly been from original domestic coal to cheaper fuel imported from Australia, Indonesia, South Africa, or the United States). This makes it difficult, if not impossible, to impute any definite power densities to individual plants. Generalizations are also impossible for fly ash storage (ash content can range from less than 10% to more than 30%) and for FGD wastes. Finally, there are major differences in land claims imposed to connect the stations to existing high-voltage grids. Some plants will require only short new links, others will necessitate upgrading of older lines or the construction of new connectors.

  Two Realistic Examples

  To indicate the actual range, I will present two realistic examples with substantially different power densities. The first case is a high-density setup: a plant with an installed capacity of 1 GWe and operating with a high conversion efficiency of 40% will require 2.5 GW, of coal. Its mine-mouth location means that coal can be supplied either by high-capacity conveyors or by short-haul trucking directly from the mine, obviating a large storage yard. The plant burns good-quality bituminous coal with an energy density of 24 GJ/t, an ash content of 4%, and a sulfur content below 0.5%. It has access to a nearby source of cooling water and hence can do without any massive cooling towers. Finally, it operates with a high capacity factor of 80%. That station would generate annually about 7 TWh (about 25 PJ), and with 40% efficiency it would require about 63 PJ of coal.

  I also assume that the plant's bituminous coal (energy density of 25 GJ/t, specific density of 1.4 t/m3) comes from a large surface mine whose main seam is 6 m thick and whose recovery rate is 95%; this means that under every square meter of the mine's surface are 8 t of recoverable coal containing 200 GJ of energy. To operate the plant would require annually coal extraction from an area of 31.5 ha (315,000 m2). Adding 10% for access roads, buildings, and parking lots would raise the total annual mine claim to almost 350,000 m2 and would mean that the mining would proceed with a power density of about 2,300 W/m2.

  In this optimal case I assume that, except for the initial cut to expose the seam, the overburden would not claim additional land but would be redeposited into the mined section as the extraction front moved forward as an advancing indentation in the landscape; eventual recultivation would leave behind a flat landscape approximately 6 m lower than its surroundings. In the absence of large coal storage and with once-through cooling, the largest areas occupied by the plant itself will be its generating halls, maintenance and office buildings, parking lots, switchyard, and the pond used to deposit the captured fly ash.

  Box 5.1

  Coal for a 1-GW plant

  Box 5.2

  Power density of coal mining

  A generous allowance for plant structures would be on the order of 10,000 m2, the switchyard would take no more than 50,000 m2, and because all captured fly ash would be used in nearby cement production, there would be no need for any on-site storage. Even after adding another 40,000 m2 for roads, walkways, parking lots, and a green buffer zone, the site's total would be 150,000 m2, and it would prorate to about 5,300 W/m2. After adding the coal mining area, the annual electricity generation at a rate of 800 MW would thus require about 500,000 m2, and the power density of the entire extraction-generation sequence would be about 1,600 W/m2 (800 MW/500,000 m2 = 1,600 W/m2).

  The second case aggregates the factors that maximize the land claim, which results in a much lower overall power density. Again, it is a 1-GWe plant, but an older one operating with only 35% efficiency and with a lower load factor (70%). It is located far from a coal mine and is supplied by a unit train peddling between the mine and the plant. It burns low-quality subbituminous coal (18 GJ/t) extracted mainly from a 6-m-thick seam that contains 10% ash and about 2% sulfur, and it requires large cooling towers. The plant burns an almost identical amount of coal energy, but the mining of that fuel claims a much larger area and proceeds with a power density of only about 1,600 We/m2. Adding, once again, 10% to account for land claims accompanying coal extraction lowers that rate to about 1,500 W/m2.

  Box 5.3

  Power density of a 1-GW coal-fired plant

  The second plant would also need off-loading facilities and coal storage large enough to supply the plant for up to 60 days, a larger area for fly ash disposal, and a pond for storing slurry from FGD, and land on which to site the cooling towers. Coal for 60 days of generation would amount to 575,000 t (about 410,000 m3) and, when stored in a yard 10 meters deep, would occupy 41,000 m2 and up to 50,000 m2, including approaches and off-loading ramps. Captured fly ash (with a density of 1.8 t/m3 after compaction) deposited in a 5-m-thick layer in a settling lagoon would annually claim about 38,500 m2 (nearly 4 ha), and FGD working with 85% efficiency would remove annually nearly 60,000 t of sulfur; when captured as CaSO4 to be deposited in a 5-m-thick layer in a pond near the plant, its storage would add annually nearly 22,000 m2.

  With, again, some 150,000 m2 for all plant structures and a switchyard, the land required for the plant's operation would add up to about 260,000 m2, implying a power density of about 2,700 W/m2. The plant (260,000 m2) and coal extraction (480,000 m2) would thus require a grand total of 740,000 m2, and the overall power density of producing 700 MW of electricity would be about 950 W/m2, a claim more than one-third more extensive than in the first case. These two realistic constructs set a low 103 We/m2 as the right order of magnitude for power densities of large coal plants: roughly 2,500-5,000 We/m2 for the plant itself (including all structures and storages) and roughly 1,000-1,500 We/m2 for a compact plant and coal mining of good-quality fuel.

  Box 5.4

  Power densities of fly ash and sulfate disposal

  Land claims are greater for plants burning low-quality coal and requiring extensive coal - and fly ash-handling arrangements. A report on land requirements of India's new coal-fired plants (burning domestic fuel containing 15 GJ/t and up to 40% ash) illustrates these needs for a typical 1-GWe (two 500 MWe units) station (CEA 2007). The plant buildings would occupy only 12 ha, but the total area needed would be 240 ha (with coal handling and a surrounding green belt accounting for about 60% of that total); fly ash storage (18 m high, sufficient for 25 years) would alone claim 200 ha, raising t
he overall claim (excluding coal mining) to more than 500 ha and resulting in a power density of about 150 We/m2 for generation at 75% of installed capacity. A 4-GWe Indian station would claim about 1,000 ha and would have a generation power density of about 300 We/m2,

  Actual Power Densities

  Coal-fired power plants often occupy substantially larger areas than those outlined in the preceding calculations or in typical recommendations. This comes about for two main reasons: many sites keep fenced-in land in reserve for possible future expansion, including additional land that might be needed for fly ash and FGD sludge disposal, and most sites contain green areas (groves, grasses, wetlands) to buffer the plant operation and make its presence environmentally more acceptable. As a result, areas within a station's perimeter fence are, particularly in the land-rich United States, often two or three times as large as the land presently claimed by the plant's structures and storages.