The scalability of solar photovoltaic and wind power is considered in
this post. Calculations performed by
David MacKay (author, Sustainable Energy Without the Hot Air) show that solar
and wind power would occupy extremely large physical areas to achieve
meaningful replacement of fossil fuels.
Replacing Fossil Fuels with Wind
Power and Solar Power
Atmospheric CO2 is rising due to fossil fuel emissions. If the science is correct (and I believe it
is), it will result in serious problems due to climate change.
The natural reaction to this knowledge is to try to replace fossil
fuels with renewable energy. Renewable
energy – power from the sun, the wind and from growing plants – is a growing
part of our energy supply. Renewables
are regarded as clean, of low impact to the environment, and most importantly,
emit no carbon dioxide. The use of
renewables, together with conservation, have reduced CO2 emissions in some
Western European nations, and slowed the growth of CO2 emissions in the United
States. Nevertheless, global CO2
emissions continue to rise due to expanding use of fossil fuels, relating to
population growth and economic development.
Environmental advocates propose large-scale investments in renewable
energy, and governments have implemented subsidies and policies to encourage
these investments. It is a worthy goal,
but before we make a wholesale commitment to building renewable energy
infrastructure, we must ask whether it is possible and reasonable to replace
fossil fuels with renewables.
Several criteria must be met in order for renewable energy technologies
to prevent climate change by replacing fossil fuels. Renewable energy technologies must be
efficient, scalable, timely, and reasonably certain to perform as expected.
This post and the next post will consider the scalability of wind and solar power; we will consider limits to the growth of solar
and wind electrical generation. Part one discusses the physical footprint
required for significant replacement of fossil fuels. Part two will discuss the limited
availability of key elements used in solar panels and wind turbines.
Growth Rates of Wind and Solar
Power
From 2005 to 2010, world electrical generation from wind power grew at
an average annual rate of 27%. Solar
power grew at a faster pace, at the rate of 53%. But these growth rates are on the basis of
small numbers. At the beginning of this
period, wind power provided only 0.6% of world electrical generation, and the
contribution of solar power was negligible.
By 2010, wind power provided 1.7% and solar power provided 0.2% of world
electricity. The current growth rates
are remarkable; if we assume a 20% growth rate for wind power, wind would meet
world demand by the year 2036. If these
growth rates could continue, renewables would soon eliminate CO2 emissions from
electrical generation. But there are
limits to the growth of renewable energy.
Sustainable Energy – Without the
Hot Air
David MacKay has written an extraordinary book, Sustainable Energy –
Without the Hot Air. The book is
available for free, in an electronic edition at this site:
MacKay puts hard numbers to the question of renewable energy in
Britain, as compared to energy demand.
MacKay is clearly an advocate of renewable energy. However, his analysis reveals the low
efficiencies and difficulty of replacing fossil fuels with renewables.
MacKay addresses the huge physical scale of generation facilities
required to replace fossil fuels, given the known efficiencies of various kinds
of sustainable energy.
MacKay considers paving 5% of Britain with solar panels, to produce
about 50 kWhr/day per capita. The cost
of power would be about 4-fold higher than today’s electrical rates. The number of photovoltaic panels would
require more than 100 times the total number of photovoltaic panels existing in
the world (2008). This installation
would produce only slightly more than the energy required for heating, cooling,
lighting and gadgets in the UK, or about 44% of the average per capita energy
consumption in the UK. The estimate
does not include transformation and transmission losses, or implicit energy
consumption in the form of imported goods, for which the energy of production
is expended in another place.
MacKay also suggests covering the windiest 10% of Britain with wind
turbines. This would provide 20
kWhrs/day per capita, and require about twice the number of wind turbines in
the entire world (2008). This number of
turbines would provide about one-half of the energy needed daily for automotive
transportation. MacKay gives further
consideration to offshore wind, while noting that some existing offshore
windfarms have had serious difficulties with mechanical lifetime due to
corrorsion. MacKay proposes covering
one-third of the shallow water (<30 m) offshore Britain with wind
turbines. It is an area equivalent to
putting a belt of wind turbines 4 kilometers wide around the entire coast of
Britain. These turbines would produce
16 kwh/day per capita, or less than half the energy required for automotive
transportation in Britain. Added to the
onshore wind assumption, it would involve nearly four times the number of wind
turbines in the entire world (2008).
Energy Density
Let’s use the term Energy Density to indicate the energy produced per
unit of surface area on the earth.
Let’s compare the energy density for wind, solar and petroleum. MacKay provides numbers for wind and solar
power. Wind power, in the windiest
parts of Britain, amounts to about 2 watts/m2. Solar power, after accounting for panel
efficiency, latitude, clouds, time of day and darkness, achieves about 5
watts/m2. By comparison, the
energy density of new “shale-play” onshore oil and gas developments in the United
States ranges from about 200 to 1100 watts/m2, assuming a 2.5 acre well pad and
a 15 year well life.1
The energy density of wind and solar power is quite low compared to
petroleum. Replacing even a part of the
energy provided by fossil fuels would require a physical footprint many times
larger than the land currently occupied by petroleum infrastructure.
Area Required to Replace Fossil
Fuels with Renewable Energy
The average Briton consumes about 125 kWhr/day of energy (MacKay). If supplied entirely by wind power, this
amount of energy would require a land area of 0.65 acres per capita, or 2.6
acres for a family of four. The average
American consumes about twice as much energy, 250 kWhr/day. To supply an American with energy from wind
power would require 1.3 acres, and more than 5 acres for a family of four. Solar energy is somewhat more efficient in
terms of energy density, but a Briton would still require a quarter acre of
solar PV panels, and his family would require an acre. An American family would require two acres of
solar panels.
Next, consider the area required to supply the full energy requirements
of large population centers with solar or wind power. The New York City metropolitan area contains
nearly 20 million people, and covers 13,300 square miles, or 24% of the state
of New York. If we provided the energy
requirements of the entire population with wind energy, it would require
covering 74% of the state with wind turbines.2 If we assume the same solar efficiencies as
Britain, we would cover 30% of New York State with solar panels, in order to
provide all of the energy needed by the people of New York City.
The dedication of such large
areas of land to energy production is clearly absurd. We need land for agriculture; we must
preserve lands for nature. Wind and
solar power can make some contribution to reducing global CO2 emissions,
particularly in places where they are most efficient. But sooner or later, the growth of solar and
wind energy will meet a limit in terms of the land area which can reasonably be
dedicated to energy production.
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1. The energy density of an
onshore oil or gas well assumes the performance of the shale-gas and shale-oil
wells currently under development in the United States. Wells in the Eagle Ford Shale are expected
to produce an average ultimate recovery of 200,000 barrels of oil (SPE). Wells in the Bakken shale are expected to
produce 500,000 to 900,000 barrels (EERC).
Wells in the Marcellus shale are
expected to produce an average recovery of 1 BCF gas, or 166,000 barrels equivalent
(USGS). The energy density calculation
assumes a 2.5 acre well pad per well, which is the area required during
drilling. After the well is on
production, much of the area of the drilling pad can be reclaimed for the
producing life of the well.
2. This is under the generous
assumption of the best wind power productivity of Britain, which would not
exist through most of New York State.
References:
David MacKay, 2008, Sustainable Energy Without the Hot Air, http://withouthotair.com/
USGS technically recoverable reserves, Marcellus Shale
USGS Marcellus Per Well Recovery
Bakken Oil EUR per well
Eagle Ford Oil EUR per well
Swindell, G. S., 2012, Eagle Ford Shale, an Early Look at Ultimate
Recovery, SPE.
