Sustainable Energy without the Hot Air

Jul 2, 2011 - David J.C. MacKay

Visualizing Sustainable Energy for the USA

David J.C. MacKay

Nuclear heat:
22.5 kWh/d

Let’s express energy consumption and energy production using simple
personal units. The units I’ll use are kilowatt-hours. One kilowatt-hour
(kWh) is the energy used by leaving a 40-watt bulb on for 24 hours. The
chemical energy in the food you eat to stay alive amounts to about 3 kWh
per day. Taking one hot bath uses about 5 kWh of heat. Driving an ordi-
nary car (delivering 25 miles per US-gallon) 100 km uses 80 kWh of fuel.
Americans use a total of 250 kWh per day per person for transport,
heating, manufacturing, electricity, and so forth. That’s equivalent to ev-
ery person having 250 lightbulbs switched on all the time. And, as ﬁg-
ure 1 shows, most of this energy today comes from fossil fuels. Roughly
81 kWh per day (kWh/d) per person of fossil fuels and 22.5 kWh/d of
nuclear heat go into making electricity. When wind, solar, and hydroelec-
tricity are added, the total electricity delivered is 37 kWh/d. We use a fur-
ther 44 kWh/d of natural gas and 105 kWh/d of oil for heat, for chemical
processes, and for transport. Geothermal sources and biomass contribute
smaller amounts.
What are our post-fossil-fuel options? We can match supply to demand
in two ways: by energy-saving and by increasing non-fossil-fuel sources of
energy-supply.
Among the energy-saving options, two promising technology switches
are (a) electrifying transport (electric vehicles can be about four times
as energy-efﬁcient as standard fossil-fuel vehicles) and (b) using electric-
powered heat pumps to deliver winter heating and hot water. (Heat pumps
can be three or four times as energy-efﬁcient as standard heaters.) We can
also save energy by making vehicles lighter; by insulating buildings bet-
ter; by improving the engineering of appliances such as refrigerators and
air-conditioners; and by enhancing consumers’ understanding of their con-
sumption through engaging energy-meter-displays.
Among all the energy-supply technologies, the four with the biggest
potential today are solar power, wind power, bio-energy, and nuclear power.
Figure 2 visualizes, for illustration, the sizes of solar, wind, bio-energy,
and nuclear facilities that would each supply 42 kWh per day per per-
son. (Remember, the total power consumption today is six times as big –
250 kWh per day per person. I picked 42 kWh per day per person from
each source on the grounds that, if some energy-saving measures are in-
troduced, roughly three or four times 42 kWh per day per person might be
enough to maintain today’s lifestyle. I am not recommending this particu-
lar energy mix; I picked equal amounts from each source so as to make it
easy to see the exchange rates, and easy to construct alternative mixes.)

• To supply 42 kWh per day per person from solar power (for everyone
in the USA) requires concentrating solar power stations with total
area equal to one eighth of Arizona. This area is shown on the map
by 21 hexagons, each with an area of 1650 square kilometres (twice

Coal
for electricity:
61 kWh/d

Wind:
0.5 kWh/d
Solar electricity:
0.008 kWh/d
Hydro: 2.3 kWh/d

Nuclear:
7.4 kWh/d

Electricity
from fossils:
27 kWh/d

delivered
electricity
(37 kWh/d total)

Oil for
electricity:
1.3 kWh/d

Natural gas
for electricity:
19 kWh/d
Geothermal: 0.9 kWh/d

Biomass:
10 kWh/d

Oil for
transport,
industry,
heat, etc:
105 kWh/d

Natural gas
for heat, etc:
44 kWh/d

Figure 1. Current consumption per
person in the USA is about 250 kWh
per day. The ﬁrst column shows the
chemical and thermal sources of
power, some of which are converted
to electricity; the second column
shows the breakdown of the
electricity, a little of which comes
from mechanical and solar sources.

the area of San Diego) and an average output of 25 GW. [This visu-
alization assumes that concentrating solar power stations can deliver
an average power per unit area of about 15 W/m2 .]

• To deliver 42 kWh per day per person from wind would require wind
farms with a total area roughly equal to the area of California (ten
New Jerseys) – a one-hundred-fold increase in US wind power over
2008 levels. [Windfarms deliver roughly 2.5 W/m2 .]

• To get 42 kWh per day per person from bio-energy would take roughly
10% of US land area (ﬁfty New Jerseys), assuming energy crops have
a power per unit area of 0.5 W/m2 . This chemical energy could re-
place some of today’s oil and natural-gas consumption.

• To get 42 kWh per day per person from nuclear power would re-
quire 525 one-gigawatt nuclear power stations – a roughly ﬁve-fold
increase over today’s levels.

Wind:
42 kWh/d

Nuclear:
42 kWh/d

Arizona

Texas

Roa

Biomass:
42 kWh/d

1000 km

Solar in deserts:
42 kWh/d

Figure 2. Visualizing sustainable energy options. Grey-green squares: wind farms.
Purple dots: nuclear power stations (not to scale). Light-green squares:
bio-energy plantations. (Some of these could occupy the same land as
the wind farms.) Yellow hexagons in the southwest: concentrating
solar power facilities in deserts, to scale. For comparison, the blue-
grey squares in the Atlantic show the total area devoted to roads in the
USA.

Let’s re-visualize these national numbers in personal terms.
would individuals or communities need to do?

• To obtain 42 kWh per day from solar power, each person requires
either roughly 80 square metres of solar photovoltaic panels, or a
share of a concentrating solar power station. One person’s share
would be 30 mirrors, each one square metre in size, and a one-four-
hundredth share of a solar collector tower.

• We can get 42 kWh per day per person from wind (on average) if
every 300 people have one 2-MW turbine.

• To get 42 kWh per day from bio-energy, each person needs the output
of 1 acre (4000 square metres) of land – that’s half a football ﬁeld.

• To get 42 kWh per day per person from nuclear power, each city the
size of Denver, Boston, Las Vegas, and Portland needs its own one-
gigawatt nuclear power station, occupying about one square kilome-
tre. Bigger cities would have proportionally more – 7 for Los Angeles,
5 nukes for Chicago, and 4 nukes for Houston, for example.

I hope these numbers convey the scale of action required to put in place
a sustainable energy solution.
Of course there are other technologies I haven’t mentioned in this short
note, which can also contribute to a plan that adds up. Home-mounted so-
lar hot water panels, for example, can easily deliver at least half of the hot-
water demand of a typical family in almost all climates; seasonal heat stor-
age systems might allow excess heat to be harvested in the summer and
stored until the winter; proponents of enhanced geothermal systems have
estimated that, with investment, geothermal resources in the US could de-
liver at least 100 GW of electricity – in personal units, that’s 8 kWh per day
per person; and clean coal and clean gas with carbon capture and storage
are crucial technologies to reduce the risks associated with fossil-fuel burn-
ing – roughly 22 kWh per day per person of low-carbon electricity could
be delivered if all US fossil-fuel-burning power stations were converted to
perform carbon capture and storage. And ﬁnally, we may wish to keep on
the table the option of lifestyle changes that reduce energy consumption,
for example switching from car-driving to public transport, cycling, and
walking; ﬂying less; and buying less stuff.
It’s not going to be easy to make a energy plan that adds up; but it is
possible. We need to make some choices and get building.

What

Photos by eSolar.com

Draft 2.1 – April 22, 2010

Sustainable Energy – without the hot air

David J.C. MacKay

This remarkable book sets out, with enormous clarity and objectivity, the vari-
ous alternative low-carbon pathways that are open to us.
Sir David King FRS
Chief Scientiﬁc Adviser to the UK Government, 2000–08

For anyone with inﬂuence on energy policy, whether in government, business
or a campaign group, this book should be compulsory reading.
Tony Juniper
Former Executive Director, Friends of the Earth

MacKay’s book shows how, when it comes to energy, you too can do the simple
arithmetic and learn the simple scientiﬁc facts needed to work out what energy
you need and where it might come from.
Prof David Mumford
Professor of Applied Mathematics, Brown University
Member of the US National Academy of Sciences

Common sense, technology literacy, and a little calculation go a long way
in helping the reader sort sense from nonsense in the challenges of developing
alternatives to fossil fuels. MacKay has provided a high priority book on a high
priority problem.
Professor William W. Hogan
Raymond Plank Professor of Global Energy Policy
John F. Kennedy School of Government, Harvard University

This is a complete resource for assessing the many options for choosing between
different energy options and for using energy more efﬁciently. Teachers, students,
and any intelligent citizen will ﬁnd here all the tools needed to think intelligently
about sustainability. This is the most important book about applying science to
public problems that I have read this year.
Prof Jerry Gollub
Professor of Physics, Haverford College and University of Pennsylvania
Member of the US National Academy of Sciences

MacKay’s book is the most practical, solidly analytical, and enjoyable book on
energy that I have seen. This heroic work gets the energy story straight, assessing
the constraints imposed by physical reality that we must work within.
Prof Tom Murphy
Associate Professor of Physics, UC San Diego

This book is a tour de force . . . As a work of popular science it is exemplary.
The Economist

“Sustainable Energy – without the hot air”
was published in hardback and paperback
by UIT Cambridge on 2nd December 2008
in the UK, and on 1st May 2009 in North
America. The book is also available for free
online at www.withouthotair.com.

David MacKay FRS is Professor of Natural
Philosophy in the Department of Physics at
the University of Cambridge. In October
2009 he was appointed the Chief Scientiﬁc
Advisor to the UK Department of Energy
and Climate Change.