David MacKay’s book (described here) makes an excellent point about the asymmetry between energy supply and demand, in terms of the difficulty or ease of increasing either:

It’s so simple for me to consume an extra 30 [kilowatt-hours] (kWh) per day. But squeezing an extra 30 kWh per day per person from renewables requires an industrialization of the environment so large it is hard to imagine.

For instance, buying a car and traveling 50 km per day in it means adding 40 kilowatt-hours per day (kWh/d) to your energy consumption. By contrast, surrounding all of the United Kingdom with wind turbines – with 15 per km of coastline, extending 4 km out to sea – would produce 16 kWh/d for every UK resident, if the wind was blowing all the time, and probably about 1/3 of that in actuality.

Statistics like that deepen my suspicion that a world without fossil fuel consumption will be one where there is much less energy consumption going on, overall. While increased efficiency can offset part of that, it also seems extremely likely that some very energy intensive activities will need to cease.

## Author: Milan

In the spring of 2005, I graduated from the University of British Columbia with a degree in International Relations and a general focus in the area of environmental politics. In the fall of 2005, I began reading for an M.Phil in IR at Wadham College, Oxford.
Outside school, I am very interested in photography, writing, and the outdoors. I am writing this blog to keep in touch with friends and family around the world, provide a more personal view of graduate student life in Oxford, and pass on some lessons I've learned here.
View all posts by Milan

Hmm, this guy sure likes figures. Too bad they seem to be wrong every time. 40kw/hrs to drive 50km eh? If we look at some actual numbers, say, for electric cars (which seems fair, if we’re talking about generation energy from wind stations after all), that doesn’t seem to work out. The Tesla, according to the vast numerical calculations on the wikipedia site, has an efficiency of 135 watt hours per kilometer. In other words, 0.135kwh/km. That’s a bit better than what is here assumed – almost 1kmh/km.

If we run this 50km driving per day scenario again with electric cars (which have been being built by enthusiasts, probably with similar efficiency numbers, since the 70s), we get adding 50km per day means adding 6.75kwh/day. A bit less than the 40 estimated here. So, to add 40kwh/day with a Tesla (which isn’t exactly optimized for efficiency – its optimized to be as fast as a Ferrari – it lapped the Top Gear test track as fast as a Porsche 911 GT3), you’d need to drive an extra 300km per day. I think I’d notice that, if I started driving 300km per day more than usual.

p.127

“At the other end of the spectrum is the Tesla Roadster. The Tesla Roadster 2008 has a range of 220 miles (354 km); its lithium-ion battery pack stores 53 kWh and weighs 450 kg (120 Wh/kg). The vehicle weighs 1220 kg and its motor’s maximum power is 185 kW. What is the energy-consumption of this muscle car? Remarkably, it’s better than the G-Wiz: 15 kWh per 100 km.”

“I’ve looked up the performance figures for lots of electric vehicles – they’re listed in this chapter’s end-notes – and they seem to be consistent with this summary: electric vehicles can deliver transport at an energy cost of roughly 15 kWh per 100 km. That’s five times better than our baseline fossil-car, and significantly better than any hybrid cars. Hurray! To achieve economical transport, we don’t have to huddle together in public transport – we can still hurtle around, enjoying all the pleasures and freedoms of solo travel, thanks to electric vehicles.

…

OK, the race is over, and I’ve announced two winners – public transport, and electric vehicles. But are there any other options crossing the finishing line? We have yet to hear about the compressed-air-powered car and the hydrogen car. If either of these turns out to be better than electric car, it won’t affect the long-term picture very much: whichever of these three technologies we went for, the vehicles would be charged up using energy generated from a “green” source.”

The key point raised above stands: it is easier to add to consumption per person than supply per person. Even if everyone in the UK got an electric car, it would require thousands of kilometres of offshore wind to power them all, if you took that route to getting the electricity.

You may want to read MacKay’s motivations and methodology.

In Part I, he compares current energy usage (i.e. 40 kWh per day conventional cars) with all possible forms of renewable generation in the UK, deployed to an extreme extent.

In Part II, he looks at options for reducing energy consumption, including more efficient vehicles, homes, etc.

Also, the man is a professor of physics at Cambridge. It seems sensible to be a bit less dismissive of his mathematical skills. One final element of his methodology to draw attention to:

“What I’m aiming to do in this book is to make these numbers simple and memorable; to show you how you can figure out the numbers for yourself; and to make the situation so clear that any thinking reader will be able to draw striking conclusions. I don’t want to feed you my own conclusions. Convictions are stronger if they are self-generated, rather than taught. Understanding is a creative process. When you’ve read this book I hope you’ll have reinforced the confidence that you can figure anything out.

I’d like to emphasize that the calculations we will do are deliberately imprecise. Simplification is a key to understanding. First, by rounding the numbers, we can make them easier to remember. Second, rounded numbers allow quick calculations. For example, in this book, the population of the United Kingdom is 60 million, and the population of the world is 6 billion. I’m perfectly capable of looking up more accurate figures, but accuracy would get in the way of fluent thought. For example, if we learn that the world’s greenhouse gas emissions in 2000 were 34 billion tons of CO2-equivalent per year, then we can instantly note, without a calculator, that the average emissions per person are 5 or 6 tons of CO2-equivalent per person per year. This rough answer is not exact, but it’s accurate enough to inform interesting conversations. For instance, if you learn that a round- trip intercontinental flight emits nearly two tons of CO2 per passenger, then knowing the average emissions yardstick (5-and-a-bit tons per year per person) helps you realize that just one such plane-trip per year corresponds to over a third of the average person’s carbon emissions.

I like to base my calculations on everyday knowledge rather than on trawling through impersonal national statistics. For example, if I want to estimate the typical wind speeds in Cambridge, I ask “is my cycling speed usually faster than the wind?” The answer is yes. So I can deduce that the wind speed in Cambridge is only rarely faster than my typical cycling speed of 20km/h. I back up these everyday estimates with other peoples’ calculations and with official statistics. (Please look for these in each chapter’s end-notes.) This book isn’t intended to be a definitive store of super-accurate numbers. Rather, it’s intended to illustrate how to use approximate numbers as a part of constructive consensual conversations.

In the calculations, I’ll mainly use the United Kingdom and occasionally Europe, America, or the whole world, but you should find it easy to redo the calculations for whatever country or region you are interested in.”

Deliberately imprecise is fine. Being off by a factor of six is not fine, one could say, its irresponsible.

And, for reasons I’ve already pointed out, the 1kw electric furnace is irresponsibly misleading.

His estimate: 15 kWh per 100 km.

Your estimate: 0.135kwh/km = 13.5 kWh per 100 km

They are basically the same.

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