09/24/2010 07:00:00

Sustainable Energy—Without the Hot Air

The statistics thrown around in the sustainable-energy debate are often chosen to impress rather than inform. "Los Angeles residents drive 142 million miles—the distance from Earth to Mars—every single day," for example. Sometimes there are no numbers at all, just adjectives: "We have a huge amount of wind energy just waiting to be tapped." But how does that hugeness compare to the hugeness of our energy consumption?

David MacKay (PhD '92), the chief scientific advisor to the United Kingdom's Department of Energy and Climate Change, has done the math. In fact, he wrote the book about how to do the math and what it tells us.

In Sustainable Energy—Without the Hot Air, MacKay converts all the statistics into one set of units—rather than the usual baffling mix of barrels of oil, terawatt-hours, petajoules, and the like—and reports the results on a per-person basis, reducing the problem to sets of easily comparable numbers. With such numbers in hand for supply and demand, he shows us how to begin to figure out an energy budget that makes sense.

It turns out that our everyday personal choices translate into small numbers of kilowatt-hours. For example, if you burn one 40-watt lightbulb for 24 hours, you use one kilowatt-hour of electricity. The chemical energy in the food you eat amounts to about three kilowatt-hours a day. If you take a hot bath, that's five kilowatt-hours of energy to heat the water. If you fly from London to Los Angeles and back, you use 10,000 kilowatt-hours, but if you only do it once a year, the average power consumption comes out to 26 kilowatt-hours per day. If you have a typical three-bedroom American house, you might be using 80 kilowatt-hours per day all told to run it. Share this among a family of three, and you're back in the ballpark of 25 kilowatt-hours per day. 

Viewed this way, the United States consumes an average of 250 kilowatt-hours per day per person. Europe uses half that amount, or 125 kilowatt-hours per day per person, and Australia and Canada use about 300 each. One kilowatt-hour per day is also the approximate power output of a human servant, so it's as if, in the modern age, every American has 250 mechanical servants on staff. This energy primarily goes into three categories of use: transportation, heating and air conditioning, and electricity. MacKay offers simple suggestions that use existing technology to reduce demand in each of these areas, a prerequisite to living sustainably without radical changes in our lifestyle.

On the supply side, harvesting most forms of renewable energy involves repurposing huge tracts of land, so it makes sense to think in terms of potential power output per unit area. Wind power, for example, can produce 2.5 watts per square meter at good sites. Multiplying Britain's per-person consumption by its population density, MacKay finds that Britain consumes about 1.25 watts per square meter. "So," he writes, "if you ask, ‘Can Britain power itself completely on wind?' the answer is, ‘Yes, if half the area of the U.K. is occupied by wind farms.'"

Doing the same sorts of calculations for other renewable energy sources allows planners to explore the trade-offs involved. Writes MacKay, "If someone says, ‘I don't want solar power from someone else's desert, thank you,' no problem—for each 60 square kilometers of solar power you cut, you need another one-gigawatt nuclear power station. Or if you say, ‘No no no, I don't like nuclear power,' no problem—each nuclear power station can be replaced (on average) by 2,000 wind turbines, occupying an area of about 400 square kilometers. Then you just need to decide where you're going to put all those extra wind farms."

MacKay hopes that his book will serve as a blueprint for informed discussions on a regional or national level by allowing all the options to be considered at once, rather than taking up the approval of this or that project piecemeal. "It seems to me to be a solvable problem," he writes. "It's not going to be easy to make an energy plan that adds up; but it is possible. We need to make some choices and get building."

Read the full story at E&S online.

Written by David MacKay