Power Density - A Key to Understanding Energy Sources and Uses

298 Pages·2015·11.218 MB·English

by  Vaclav Smil

A Key to Understanding Energy Sources and Uses
Vaclav Smil
The MIT Press Cambridge, Massachusetts London, England
Preface
Acknowledgments
1 How Power Density Matters
2 Quantitative Keys to Understanding Energy
Power of Rates
Power Density: Sorting Out the Rates
Typology and Caveats
3 Renewable Energy Flows
Solar Radiation and Its Conversions to Heat and Electricity
Wind and Wind-Generated Electricity
Water Power and Hydro Generation
Phytomass for Traditional and Modern Uses
Geothermal Energy
4 Fossil Fuels
Coal Extraction, Preparation, and Transport
Crude Oils, Refining, and Long-Distance Deliveries
Natural Gas, Pipelines, and Liquefied Natural Gas
5 Thermal Electricity Generation
Coal-Fired Power Plants
Hydrocarbons in Electricity Generation
Nuclear Generation
Transmission
6 Energy Uses
A Brief Historical Perspective
Hierarchy of Modern Energy Uses
The Challenges of Heat Rejection
7 Making Sense of Power Densities
Recapitulations
Comparisons
Aggregate Land Claims
8 Energy Transitions
Biofuels
Wind and Solar Electricity
What It Would Take
Appendix
References
Index
Modern societies have many concerns about their energy supply. Above all, it
should be affordable, reliable, and convenient: affordable in order to drive
economic development and improvements in quality of life, reliable so as to be
available on demand in its various forms, most of all as uninterruptible electricity,
and convenient to give consumers virtually effortless access to preferred
household, industrial, and transport energies. During the closing decades of the
twentieth century two other concerns became prominent: energy supplies
should also be environmentally benign and, preferably, renewable. Since the
1970s-that is, once energy matters began to receive, belatedly, an
unprecedented amount of professional and public attention-all of these
concerns have been addressed and analyzed in hundreds of books and
thousands of papers, to say nothing of the instant expertise proffered by the
mass media.
Any modestly informed person knows that energy prices matter (we need only
think of OPEC's crude-oil price fixing), as do a reliable electricity supply (to avoid
blackouts) and the finiteness of fossil fuel resources (underscored by claims
about an imminent peak of oil extraction), and that the environmental impacts of
energy use are not only local but worldwide (global warming). But does power
density matter? Power is simply energy flow per unit of time (in scientific units,
joules per second, which equals watts, or J/s = W), spatial density is the quotient
of a variable and area, and hence power density is W/m2, that is, joules per
second per square meter. Why should we care about how much power is
produced, or used, per unit of the Earth's surface? Some say explicitly that we
should not bother at all.
Amory Lovins spent a lifetime making exceedingly optimistic forecasts about
the speed with which renewable energy conversions (and other energy-related
innovations) would be adopted by modern energy systems (for these claims, see
Lovins 1977 and 2011a; for their critique, see Smil 2010a). In 2011 he dismissed
any need to consider power densities, concluding that "land footprint seems an
odd criterion for choosing energy systems: the amounts of land at issue are not
large, because global renewable energy flows are so vast that only a tiny fraction
of them need be captured" (Lovins 2011b, 40). In contrast, nearly two
generations ago Wolfgang Hafele and Wolfgang Sassin, two leaders of energy
research at the International Institute for Systems Analysis, wrote that "the
density of energy operations is one of the most crucial parameters that
predetermine the structure of the energy systems" (Hafele and Sassin 1977, 18).
Even so, most of the modern energy literature has simply ignored the subject.
Energy economists have been worrying about prices, their elasticities,
oligopolies, taxation, and links between energy use and economic growth. As
interest in energy matters began to rise, Malcolm Slesser offered an explanation
of this omission:
Land has always had a firm place in classical economics, where attention focused
upon the agrarian economy, and land as a factor of production. Ricardo and
Malthus founded their ideas around the land factor, in marked contrast to more
recent economic thought in which land virtually dropped out of the scene, and
production was viewed essentially as a synergy of labour and capital. (Slesser
1978, 86)
Similarly, modern engineering analyses of energy systems look at fuel
qualities, mass throughputs, the rated power of energy converters, and annual
and peak production. Of course, the design of specific energy extraction or
conversion facilities must, inevitably, consider land requirements and land
qualities in requisite detail, but space is not a common analytical denominator
used to assess their performance. Still relatively uncommon interdisciplinary
inquiries into the nature and linkages of modern energy systems look at per
capita energy use, energy's role in economic performance, and the impacts of
energy conversions on environmental quality, but only very few recent
publications have looked at power densities.
I chose the measure as a key analytical variable in General Energetics (Smil
1991) and in its second, expanded edition, titled Energy in Nature and Society
(Smil 2008). David McKay's Sustainable Energy-Without the Hot Air (2008) also
uses the rate as an essential indicator that offers revealing insights into unfolding
energy transitions. And Juan Moreno Cruz and M.Scott Taylor offer a rare
example of economists who have explicitly incorporated energy density and
power density in their new analysis, concluding that "perhaps the most
important attribute of an energy source is its density: its ability to deliver
substantial power relative to its weight or physical dimensions" (Cruz and Taylor
2012, 2) and that "the very density of the energy resource we seek, fuels our
efforts to obtain more. Therefore differences in power density across energy
resources create large differences in energy supply" (Cruz and Taylor 2012, 48).
In this book I demonstrate that power density is a key determinant of the
nature and dynamics of energy systems. Its careful quantifications, their critical
appraisals, and their revealing comparisons bring a deeper understanding of
the ways with which we harness, convert, and use energies. A careful assessment
of power densities is particularly revealing when contrasting our dominant fossil
fuel-based energy system with renewable energy conversions. But before I start
laying the foundation for my systematic power density assessments (by
introducing the principal variables and reviewing different power density
concepts), I tell a story of early modern charcoal-based iron smelting and of its
replacement by coke-based production in blast furnaces. This historical
appraisal offers a compelling (yet curiously overlooked) example of why power
densities matter, why energy's rate of flow per unit of surface area is a key
determinant of the structure of energy systems, and how it constrains their
development.
This brief chapter starts with a 1548 King's Commission, proceeds to a great
English invention of the eighteenth century (not James Watt's steam engine!),
and ends with the consequences of trying to supply a rising pig iron output in
nineteenth-century blast furnaces by metallurgical charcoal.
In November 1548 the King's Commission, given to 20 men in Sussex, was to
examine "the hurts done by iron mills and furnaces made for the same" on the
Weald, the region of southeastern England between South and North Downs
that was the center of English iron-making during the sixteenth century. The
commission's most pressing questions to witnesses were these (Straker 1969,
115):
5. If the said iron mills and furnaces be suffered to continue, then whether
thereby there shall be great lack and scarcity of timber and wood in parts near
the mills?
6. What number of towns are like to decay if the iron mills and furnaces be
suffered to continue?
The cause of these hardships was obvious:
The iron mills and furnaces do spend yearly... above 500 loads of coals, allowing
to every load of coals at the least three loads of wood; that is every iron mill
spendeth at the least yearly 1,500 loads of great wood made into coals.
The petitioners went to great length to enumerate how wood scarcity would
make it impossible to build houses, water mills or windmills, bridges, sluices,
ships, boats, wheels, arrows, hogsheads, barrels, buckets, saddletrees, bowls,
dishes and to supply timber "for the King's Majesty's towns and pieces on the
other side of the sea" (Straker 1969, 118; Boulogne was an English possession
between September 1544 and March 1550). As long as the deforestation caused
by the rising demand for charcoal was limited to a few counties, local smelting
would simply decline or cease, and the production would move elsewhere. Data
assembled by King (2005) show that the production of pig iron from Weald's
charcoal furnaces (about 4,000 tonnes [t] at the time of the 1548 Sussex petition)
peaked by 1590 (at 14,040 t), and that by 1660 the region was making less iron
than in 1550, while the metal's output kept on increasing in the rest of England.
English Iron Smelting
By 1620 England was producing more than 26,000 t of pig iron annually, but
then production began to fall, and it was only about 18,000 t by the end of the
seventeenth century. We can reconstruct fairly reliably what this meant in terms
of energy demand and environmental impact. In the early decades of the
eighteenth century, charcoal-fueled blast furnace campaigns usually extended
from October to May, and during those eight months a typical furnace produced
300-340 t of pig iron (Hyde 1977). The efficiencies of wood conversion to
charcoal and the charcoal requirements for pig iron smelting and for
subsequent conversion of the metal to bar iron varied considerably (and about a
third of the metal was wasted in the conversion), but the best available data
(Hammersley 1973) indicate that at the beginning of the eighteenth century, at
least 32 t of wood were needed to produce a tonne of bar iron.
This means that a typical furnace and an associated bar forge (sometimes
located closer to the market) would have required at least 9,600-10,900 t of
wood-and in the early seventeenth century, with less efficient conversions, the
total could easily have been twice as much. The preferred wood supply came
from coppiced plantings of beech or oak cut in 20-year rotation, which would
yield an annual increment of about 5 m3/ha; with an average wood density of
0.75 g/cm3, that would be 3.75 t/ha, and operating a furnace and a forge would
have required harvesting about 2,700 ha/year. In 1700 British furnaces produced
about 12,000 t of bar iron, and hence they consumed on the order of 400,000 t
of charcoaling wood. With an average productivity of 4 t/ha, this amount of
wood would have required at least 100,000 ha of coppiced growth, a square
with sides of nearly 32 km.
Obviously, the availability of suitable wood was a limiting factor in the further
expansion of the English iron industry. Hammersley (1973) estimated that the
maximum countrywide harvest would have been on the order of 1 Mt/year. Not
surprisingly, during the eighteenth century England became highly dependent
on iron imports: Swedish exports rose rapidly after 1650, Russian exports began
to dominate after 1750, and by the 1770s England was covering two-thirds of its
iron demand with foreign metal (King 2005). An end to the charcoal ceiling on
English pig iron production, the elimination of imports, and a massive expansion
of iron output (from only about 25,000 t in 1750 to 100 times that much, 2.5 Mt,
100 years later) were possible because of the switch from charcoal to coke. Coke
is a lightweight (apparent density of just 0.8-1 g/cm3) but energydense fuel (29
MJ/kg) that is produced from coal by pyrolysis, or heating in the absence of
oxygen.
In England, the first use of coke was for drying malt in the 1640s, Shadrach Fox
was the first industrialist to use it on a small scale, in a blast furnace, during the
1690s, and, starting in 1709, Abraham Darby became the fuel's best-known
promoter (Harris 1988). Coke's initially high production costs limited its rapid
adoption, and the two fuels coexisted in England for most of the eighteenth
century, and in the United States well into the second half of the nineteenth
century. This shift from metallurgical charcoal to coke was part of a much larger
energy transition from traditional biomass fuels (woody phytomass, charcoal,
and also crop residues, mainly cereal straws) to fossil fuels, first to coals and then
to hydrocarbons (Smil 2010b).
This transition introduced fuels that had a generally higher energy content: a
kilogram of high-quality coal has nearly twice as much energy (29 MJ) as a
kilogram of air-dried wood (17 MJ); a kilogram of liquid fuels refined from crude
oil had three times as much energy (42 MJ) as straw used for cooking (14 MJ).
The higher energy density also made fossil fuels cheaper to transport and easier
to store. But the replacement of charcoal by coke did not bring in a fuel with
superior energy content, as both charcoal and coke are essentially pure carbon
with a virtually identical energy content of about 30 MJ/kg, but the power
densities of their production are orders of magnitude apart.
From Charcoal to Coke
Harvesting coppiced beech or oak would yield, at 5 m3 or 3.75 t/ha and an
energy density (of dry mass) of 19 GJ/t, an annual phytomass harvest of about
0.22 W/m2. In contrast, a typical late eighteenth-century deep mine-based on
detailed data for the Toft Moor Colliery of the 1770s (Hausman 1980)-would
have produced about 15,000 t/year, and all of that coal would have been hauled
through a single narrow shaft. With the mine's surface structures occupying no
more than a hectare (10,000 m2) of land, that extraction would have produced
fuel with a power density of nearly 1,200 W/m2, more than 5,000 times higher
than the power density of charcoal production.
But more land was needed near the pithead because the fuel was usually
sorted at the site to remove associated rocks and to improve the coal's quality. If
we assume that the incombustible material (amounting to 10% of the total mass,
with a density of 2.5 t/m3) was deposited nearby in a conical heap just 20 m tall,
the area claimed after 50 years of a mining operation would have been no larger
than 4,500 m2, and the overall power density of coal production would have
been no less than 800 W/m2, or 4,000 times higher than the power density
associated with harvesting wood.
And we have to make adjustments for conversion to, respectively, charcoal
and coke (both calculations disregard the relatively small areas needed for