Power density

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Actual power densities But measurements of many actual power plant sites, now relatively easy to do thanks to satellite images on the Google Earth, show that coal-fired power plants often occupy substantially larger areas and hence their operating power densities are appreciably lower even when the variable claims of annual coal extraction are excluded. This is not at all surprising because of three main reasons: the sites have to have sufficient land for at least three decades (minimum life expectancy of a modern station) of fly-ash and FGD sludge disposal; most sites also fence-in land that is held for future expansion; and many sites contain green areas (groves, grasses, wetlands) included to buffer the operation and to make its presence environmentally more acceptable. As a result, areas within a station’s perimeter fence are, particularly in the US, often two or three times as large as the land presently claimed by the plant’s structures and storages.

The world’s largest coal-fired power plant, with capacity of 5.824 GWe, is on about 300 ha of reclaimed land on the eastern shore of the Taiwan Strait near Taiwan’s city of Taichung (GIBSIN Engineers 2006). The station burns coal imported from several countries (mainly from Australia) and with annual generation of 42 TWh its power density (excluding coal extraction) is about 1,600 We/m2. Two of America’s three largest coal-fired electricity generating plants belong to the Southern Company headquartered in Atlanta: Robert W. Scherer (four units, 3.52 GW) located on the northern shore of Lake Juliette southeast of Atlanta, and Bowen (four units, 3.499 GW) in Bartow county of Georgia. Scherer is a sprawling installation with a large oval-shaped coal storage yard (36 ha), ash-settling pond of 120 ha and ash disposal pond of 300 ha (designed to last for the plant’s lifespan of some 50 years), and the plant’s total operating area covers about 14 km2 (Georgia Power 2013).

With an average load factor of 75% the plant generates 23.1 TWh/year and the power density of its operation is about 190 W/m2. But the entire property, including part of the Lake Juliette, is 34 km2. Bowen plant claims about 3.7 km2 (with main buildings, cooling towers, coal yard and transforming area accounting for only about 10% of the total area) and its annual generation of 22.6 TWh translates into overall power density of nearly 700 We/m2. The third largest US coal-fired plant is Duke Energy’s Gibson in Indiana (five units, 3.34 GW). The plant owns about 16 km2 of land of which 12 km2 is man-made lake used as the plant’s cooling pond; the plant itself and its associated coal-storage and pollution-control infrastructures takes over 2.5 km2 resulting in power density of close to 1,000 We/m2. TVA’s Bull Run station (with a single 900 MWe unit) uses once-through cooling and the plant itself, including a compact coal storage yard, occupies only about 65 ha; the plant generates 6 GWh/year, resulting in power density of about 1,050 W/m2.

Bełchatów, Poland’s and Europe’s largest coal-fired plant (5.298 GWe, 28 TWh annual generation), burns lignite from the adjacent surface mine and its rather large area results in generation power density of about 500 We/m2 and the entire mining-generation system rates less than 250 We/m2. Drax, England’s largest thermal station, has capacity of 3.96 GW and in 2012 it generated 27.1 TWh; that implies load factor of 78% or 3.09 GW (Drax Group 2013). The plant owns 750 ha but it structures and storages cover less than 200 (with the rest of the site under trees and meadows) resulting in actual generation power density (exclusive of coal mining) of about 1,500 We/m2. Jänschwalde, Germany’s largest lignite-fueled station near Peitz in Brandenburg (very close to the Polish border), has installed capacity of 3 GWe (6 x 500 MWe), it generates annually 22 TWh (Vatenfall 2013). The station occupies about 220 ha: the site’s power density is thus about 1,100 We/m2 in terms of actual electricity production.

The two best conclusions are that when the calculation is done by considering the area actually occupied by plant’s structures then most coal-fired power plants generate electricity with power densities in excess of 1,000 We/m2, and that the inclusion of coal extraction lowers those rate by widely differing margins. As a result, power densities of the entire coal-to-electricity sequence can stay well above 1,000 We/m2 but in some cases they can reduced by an order of magnitude to just around 100 We/m2. For comparison, Mielke’s (1977) average US rate (including coal extraction) for uncontrolled generation was nearly 1,000 We/m2, 2,700 We/m2 for emission-controlled Western coal-based plants and less than 700 W/m2 for plants burning Eastern coal with air pollution controls.

Hydrocarbons in electricity generation

Consumption of hydrocarbons in electricity generation has seen a two-fold shift: away from liquid fuels (that is mostly fuel oil in large power plants and diesel fuel in smaller generators) to natural gas, and from gas combustion in large boilers to widespread deployment of gas turbines. The shifts are easily explained by the post-1973 oil price increases and by superior efficiency, reliability and flexibility of gas turbines (Smil 2010a). As the world oil prices rose during the two consecutive OPEC-driven energy ‘’crises’’ (1973-1974 and 1979-1981) liquid fuels became simply too expensive to be used for electricity generation and most of the countries either shut down their fuel oil-based (and in some cases crude oil-based) capacities or converted them to burning either coal or natural gas.

US statistics show the rapidity of this shift: in 1980 the country’s utilities still generated 11% of electricity from petroleum fuels but a decade later the share was down to 4% and in 2012 it was only about 0.5% (USEIA 1993; 2014a). Rising use of natural gas made up the difference, and, as already noted, it has also replaced most of the declining coal-fired generation. In 1990 the US utilities produced only 9% of electricity from natural gas (compared to 24% in 1970, and to 21% from nuclear fission) but a decade later the share had nearly doubled to 17% and in 2012 it reached 30%. Not surprisingly, the world’s largest operating station burning fuel oil is in Saudi Arabia, but Japan’s Kashima (4.4GWe burning heavy oil and crude oil) surpasses Russia’s Surgut-1 plant in West-Central Siberia (3.268 GWe).

Shoaiba (installed capacity of 5.6 GWe), on the Red Sea about 100 km south of Jeddah, is also the kingdom’s largest plant and it now operates 14 generating units and combines electricity generation with desalination (Alstom 2013). The plant itself occupies about 250 ha, implying power density of about 2,200 We/m2 in terms of installed capacity. Kashima, located on about 60 ha of reclaimed land on the Pacific coast northeast of Tōkyō, rates more than 7,000 We/m2 and Surgut-1, also claiming about 60 ha, comes at about 5,400 We/m2, also in terms of installed capacity. One of the most unusual oil-fired stations was the Chavalon plant at Vouvry (Chavalon 2013): it had installed capacity of 300 MWe, it operated between 1965 and 1999, and its compact design -– occupying just 3.6 ha on small plateau on a steep slope above the Rhone valley -– resulted in power density of more than 8,000 We/m2 (gas-fired combined-cycle station of 400 MWe was approved for the site in 2010).

Natural gas in electricity generation Typical large central power plant fueled by natural gas and supplied by a pipeline claims much less land than a comparably sized coal-fired, and even oil-fired, operations: it may have only minimal emergency fuel storage, there is no fly ash to be stored and no SO2 to be captured. At a time when new construction of such stations was fairly common in the US Mielke’s (1977) data put their average power density at about 1,800 We/m2, and six years later the USDOE (1983) estimated that an 800 MWe plant with 55% load factor will occupy about 36 ha, implying power density (adjusted for partial load) of about 1,200 We/m2. Interestingly, the largest group of large central natural gas-powered stations (each with installed capacity more than 2 GWe) is in Japan, a country that imports 100% of its natural gas consumption, a choice driven by the quest for high air quality in Japan’s densely populated urban regions.

Japan has nearly 20 LNG-based high-capacity power plants, usually located on reclaimed coastal or close to an LNG terminal, with boilers and generator halls clustered around two or more tall chimneys and with adjacent gas storage tanks and security land buffers. The largest one of these gas-fired plants is TEPCO’s Futtsu in Chiba Prefecture rated at 5.04 GWe with adjacent LNG terminal and 10 storage tanks on the eastern shore of the Tōkyō Bay. Japan’s second largest natural gas-fired station is Kawagoe (4.8 GWe, about 30 TWh/year, with 6 large gas storage tanks) in Mie prefecture (Chubu Electric Power 2013).

These are highly compact facilities: Kawagoe occupies only about 75 ha, Futtsu about 125 ha: when using actual generation in calculating their power densities are 2,800-4,500 We/m2. Other countries with high-capacity natural gas-fired stations include Australia, Malaysia and Russia (all burning abundant domestic gas) and China, Taiwan and South Korea (relying on imported LNG). In 2013 Russia’s Surgut-2, in Khanty-Mansyisk region of West-Central Siberia was the world’s largest natural gas-fired station (installed capacity of 5.597 GWe, annual generation of roughly 40 TWh) and with an area of about 80 ha its power density (including a large switchyard) was approximately 6,000 We/m2. And Ravenswood in Long Island City (Queens, New York) burns natural gas, fuel oil and kerosene to power units with total capacity of 2.48 GW situated on a small (just 12 ha) rectangular lot just south of the Roosevelt Island Bridge on the East River, has installed power density of about 20,000 We/m2.

But the most important destinations of natural gas in electricity generation are not central power plant burning the fuel in large boilers but gas turbines, efficient and flexible energy converter that now come in capacities ranging from 1 MW to 375 MW, the record rating by 2013, for Siemens SGT5-8000H model installed in Irsching near Ingolstadt (Siemens 2013). Swiss Brown Boveri Corporation pioneered the use of gas turbines for electricity generation with a low-efficiency (17.4%), small installation (effective output of just 4 MW) in Neuchâtel just before WW II; but widespread adoption of these machines came only during the 1960s, especially after the great November blackout in the northeastern US in November 1965 showed the need for swiftly deployable generators in case of emergency (Smil 2010a).

Continued expansion of turbine-driven generation was also helped by more common use of the combined cycle: exhaust gases leaving the turbine have enough energy to produce steam for an attached turbine and the set-up can now reach overall conversion efficiency as high as 60%, the best performance of all thermal electricity generation techniques. Another innovation has seen jet engines adapted for stationary uses: these aeroderivative machines, made by GE. Roll Royce and Pratt & Whitney (P&W), have efficiency of about 40% and some of them are available as fully assembled units on trailers, able to generate electricity in a matter of hours after arriving at a chosen site.

Modified Pratt & Whitney FT8 jet engine with capacity of 25 MWe fits on two trailers and it can generate eight hours after arrival (PW Powersystems 2013).The turbine itself occupies no more than 140 m2 and even with its control trailer, access roads, fuel and electricity connections, and a safety perimeter buffer it claims only 600 m2 (a 40 m x 15 m rectangle), implying power density of nearly 42,000 We/m2. A larger unit by the same company, 60 MW SWIFTPAC that is placed on concrete foundations, claims less than 700 m2 (power density of some 85,000 We/m2) and it could be ready to generate in just three weeks.

Compact size of powerful gas turbines means that multi-unit installations can be easily accommodated on small lots in industrial areas. Delta Energy Center in Pittsburg, California is a perfect example of this flexibility (Calpine 2013a). Calpine Corporation put it on an undeveloped 8 ha lot at the Dow Chemical Company facility, and with 835 MWe combined-cycle capacity its rated power density is nearly 10,500 We/m2. Usually there is also no problem to locate the turbines within the existing sites of established electricity-generating stations, a choice that eases contentious application and approval processes for new plant sites. Large (1.36 GWe) English Didcot-B plant in Oxfordshire is a perfect example of this option, as it was built between 1994 and 1997 within a larger pre-existing site of Didcot-A, a 2-GWe coal-fired station whose construction (it was completed in 1968) had met with a great deal of opposition, but a gas-fired plant of more than two-thirds of the original coal plant’s capacity occupies less than 10% of the entire site.

Because of their unequalled performance combined-cycle plants (converting fuel to electricity with 60% efficiency, compared to 40-43% for the best thermal stations) will also have a relatively high power density for the entire extraction-generation sequence. As an example, a conservative combination would be 2,000 W/m2 for extraction and 5,000 W/m2 for generation. A 500 MWe combined-cycle plant with capacity factor of 50% and 60% efficiency would require annually about 21 ha for extraction (500 MWe x 1.66 (60% efficiency) = 830 MWt x 0.5 = 415 MWt/2,000 W = 20.8 ha) and about 5 ha for the plant.

Nuclear generation

I called the worldwide achievement in post-1956 nuclear electricity generation a successful failure (Smil 2003). Successful because the fission of uranium gradually rose to supply about 13% of the world’s electricity by 2012, that is more than was contributed by hydroelectricity after 130 years of developing water turbines and building large dams. Failure because in the UK and, even more notably, in the US (the two countries that pioneered its introduction) the technique fell far short of the early expectations that envisaged it as the dominant (if not the only) way of electricity generation before the year 2000 (Smil 2010). Will this great pause -– never entirely global, because the development continued in Japan until the Fukushima disaster, and major expansion plans remain in place for China, India and Russia -– be followed by a long-awaited nuclear renaissance?

Whatever the outcome, high power density of nuclear fission has been one of its major appeals. Conversion of nuclear energy to heat in reactor cores proceeds at power densities (as used throughout this book as power per unit area, not as used by nuclear engineers as power per unit of reactor volume) ranging from 50-300 MWt/m2, that is up to an order of magnitude higher than the power densities encountered in boilers burning fossil fuels. In addition, nuclear stations do not require any extensive fuel-receiving and fuel-storage facilities, they have no need for air pollution controls and for land set aside for storage of captured waste products, and radioactive wastes stored temporarily at the site occupy only small areas.

Instead of large boilers nuclear plants produce heat required to generate steam in reactors placed in steel vessels and contained in massive reactor buildings designed to withstand not only such natural disasters as earthquake or hurricanes but also aircraft impacts and accidents resulting from internal system failures, and prevent any release of radioactive material into the environment. Attention to well-known breaches of these containments (Chornobyl in 1985, Fukushima in 2011) should not obscure the fact that the operation record of nuclear stations in the two countries with the largest number of commercial reactors (US and France) has been very good.

In common with all other thermal plants, nuclear stations have machines halls (housing turbogenerators, steam condensers, waste heat rejection systems and requisite pumps) and auxiliary buildings (containing water and waste treatment, fuel and maintenance stores, and administration), and road and rail access. Similarly, some nuclear plants use once-through cooling but many have tall cooling towers, and all of them must have appropriately designed switchyards needed to step up the voltage before transmitting generated electricity to a national grid. And, also in common with large fossil-fueled plants, nuclear stations vary greatly in their overall land claims.

In 2012 almost 60% of America’s total nuclear capacity (118 reactors in 74 plants) was located at sites of 200-800 ha, with the modal size of 200-400 ha of which 20-40 ha were actually disturbed during plant’s construction (USNRC 2012). But the extreme land claims range over two orders of magnitude, with the largest land claims due to cooling reservoirs, artificial lakes or extended buffer areas: California’s San Onofre (2.586 GW in three reactors on the Pacific coast in Oceanside, cooled by the ocean water) occupies just 34 ha, and William B. McGuire station near Charlotte in North Carolina (2.36 GW) is situated on a site of 12,000 ha.

Power densities of nuclear generation Nuclear power plant sites can be thus quite compact: San Onofre’s power density prorates to about 7,600 We/m2 in terms of installed capacity, and densities between 2,000-4,000 We/m2 are common. On the other end of the size spectrum, America’s nuclear stations with the largest land claims have power densities of less than 100 We/m2: McGuire rates at just 20 We/m2, Wolf Creek in Kansas (1.17 GW, 3,937 ha, most of it a large cooling lake) 30 We/m2. Given the fact that most of the world’s reactors belong to just two dominant types (the most common pressurized water reactor, PWR, and boiling water reactor, BWR, the latter one generating steam directly with water circulating through reactor core, the former using heat exchanger) and that their installed capacities range mostly between 400-1,200 MWe, it is hardly surprising that power densities of European and Asian nuclear electricity stations are very similar.

The world’s largest nuclear station, Japan’s Kashiwazaki-Kariwa (as all of the country’s plants, closed in 2011 after the Fukushima disaster, but some of its reactors are to be restarted) had originally installed capacity of 7.965 GW in seven reactors located within a 420 ha area along the coast of the Sea of Japan in Niigata prefecture (Sakai, Suehiro and Tani 2009). That implied installed power density of about 1,900 We/m2 but a large part of that area is the surrounding wooded buffer zone and there is also a broad green strip between two groups of reactors (Google Earth 2014). Plant structures and infrastructures take up less than 200 ha, and the plant’s highest output of 60 TWh in 1999 (before it began to experience repeated operation problems) implied actual generating power density of at least 3,500 We/m2.

In contrast, the now infamous Fukushima Dai-ichi –- hit by the March 2011 tsunami that began a sequence of events that ended with the crippling of four of the six reactors and immediate release of radioactivity into the atmosphere and later also into the Pacific Ocean -– was located on a much larger piece of costal property; with overall area of about 350 ha and installed capacity of 4.698 GW its pre-disaster power density was 1,300 Wi/m2. The plant’s twin, Fukushima Dai-ni, 12 km south of the crippled station, has capacity of 4.4 GW in four reactors on a much smaller site of 147 ha, that is installed power density of 2,900 Wi/m2.

Gravelines, the largest of 21 French stations and the world’s fifth largest nuclear plant (with 5.7 GW in six 900 MW reactors, producing about 38.5 TWh/year) occupies only about 90 ha just west of Dunkerque on the Pas de Calais coast and its installed power density is thus about 6,300 We/m2. Cruas on the Rhône in southern France (3.6 GW in four 900 MW units) claims 148 ha for installed power density of some 2,400 We/m2. My last example is Swiss Beznau, in 2013 the world’s oldest operating PWR plant with two 365 MW reactors: the plant and its switchyard occupy only 20 ha on an island in the Aar river, resulting in installed power density of about 3,600 We/m2.

The evidence is clear: land occupied by structures and infrastructures of most of the operating nuclear stations translates into installed power densities of 2,000-4,000 We/m2, and the inclusion of immediately adjacent buffer zones (that do not prevent agriculture or forestry but exclude any permanent habitation or industries) reduces that rate by 40-60%. And, unlike other thermal stations -– some of which, like the already mentioned New York’s large (2.48 GW) Ravenswood in Queens, are not only within cities but relatively close to city centers -– nuclear station are preferentially and deliberately located in areas of lower population density. In the US more than half of all nuclear plant sites have 80-km population densities of less than 80 people/km2, and more than 80% have 80-km density lower than 200 people/km2 (USNRC 2012).

As in the case of other thermal power plants we now have to proceed beyond the plant sites: as must be expected, land claims for the entire uranium cycle will significantly lower the densities of the entire fuel-generation-disposal sequence. Commercially extracted uranium ores contain as little as 0.15% and as much 20% of the heaviest of all metals (WNA 2010). Their mining is followed by milling (on site or in a nearby facility serving several mines) to produce U3O8 concentrate (commonly known as yellowcake) that contains more than 80% U and that is sold in drums: about 200 t of the concentrate are needed to produce fissionable fuel required to run a 1 GWe for a year. U3O8 is refined to UO2 (this unenriched oxide fuels Canada’s CANDU reactors) and then converted to UF6. This gas undergoes isotopic separation (centrifugal or by gaseous diffusion) to produce enriched UO2 and this oxide is formed into ceramic pellets and (mostly) encased in zirconium alloy fuel rods.

Typical progression (nuclear fuel chain material balance) from mining uranium ore to the enriched fuel in reactor rods results in mass reduction of two orders of magnitude. For electricity generation in a 1 GWe plant operating at its full capacity and consuming fuel derived from ore with 0.2% U by weight the sequence is as follows (IAEA 2009): about 108,000 t of uranium ore (containing 217 t U) is processed in a uranium mill to yield 245 t of U3O8 (containing nearly 208 t U) and that compound gets converted to about 306 t of natural UF6; enrichment produces about 38 t of enriched UF6 and the hexafluoride is then converted to almost 29 t of UO2 (containing 25.4 t U) that is fabricated into fuel rods; this reactor fuel will generate 8.765 TWh/year (averaging about 2.9 t U/TWh) and produce 28.8 t of spent fuel.

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