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Sustainable Design Cases

This page contains a few short case studies about sustainable design.

Zero Energy House

The Zero Energy House is a single-family dwelling designed and built in New Zealand that literally uses no net energy over a year. In particular, it's a combination of Passive House and solar technologies, with a number of site-specific custom features - like solar panels that also function as roof shingles to keep the underlying structure dry. The interior is spacious and beautiful - which shows that sustainability doesn't have to be ugly or uncomfortable.

Hummer versus Prius

A story published 19 May 2008 in Wired magazine emphasizes how hard it is to understand whether one product is more sustainable than another.

In 2007, CNW Marketing Research published a report1) suggesting that the total life-cycle energy cost of a Humvee (13 miles per gallon city, 16 highway) was lower than that of a Toyota Prius (48 city, 45 highway). The key argument involved the impact of the manufacture and disposal of the batteries used in the Prius, which when combined with other life-cycle costs, overwhelmed the impact of the conventional, albeit large, Hummer. This report made news all over the world.

A lesser known report, by the Pacific Institute (summarized here and here) cast significant doubts on the CNW report. The key finding of the Pacific Institute report was that the comparison was one of “apples and oranges” in that, for instance, the Hummer was assumed to travel much farther over a longer period of time than the Prius. While it is reasonable to think one might use the actual (average) total mileage of each vehicle, the costs associated with manufacture and disposal are fixed and thus can be made to seem lower when distributed over a longer period of time or total distance.

Furthermore, the 30 pounds of nickel that go into a Prius battery system means that making a Prius actually causes more carbon emissions than making a Hummer. Of course, the Prius is so much more efficient in operation that overall the Prius still wins handily.

The key issue here is not which vehicle impacts the environment more, but rather the uncertainty of the facts behind any such assessment.


It seems that it takes 113 million BTUs of energy to make a Prius, and a (US) gallon of gas has about 113,000 BTUs in it. So right off the assembly line, a Prius has already consumed 1,000 US gallons of gasoline.

The Wired article then suggests that this makes a case for used cars. The article's author, Matt Power, argues that the carbon footprint of a car's manufacture is “paid off” by the first owner. When you buy it as a used car, it has no footprint left.

This is based on the notion that the “unit of measurement” is the duration of the car's ownership by a single person, but this unit is completely arbitrary. Indeed, it's not clear exactly how the footprint of the car's manufacture is “paid off.” Certainly, this isn't literal: there are no such taxes or charges assumed by someone buying a car, new or used.

The only reasonable interpretation in this context is that you've bought a fuel-efficient used car that is quite old. Indeed, the example given is that of a 10 year old Toyota Tercel. Since such a car consumes less gas than, say, a Hummer, then the gas saved by not driving the Hummer offsets the footprint of the car's manufacture - assuming that we know what the footprint of making Tercel 10 years ago is known, and that it's not higher than that of making a modern fuel-efficient car.

That's not a good argument, because it assumes the average gas consumption is high and will always be high, and the suggested “savings” are only relative to that average.

The point is: measuring sustainability of products is hard and always relative, so someone somewhere is very likely to find an error in your calculations, or the calculations will become obsolete over time.

Exercise for the Reader: What does this imply with respect to achieving true savings of carbon footprints?

On-demand streetlights

Time Magazine reported in 2009 on a project in Dörentrup, Germany, using technology by Dial4Light, wherein streetlights are turned on at night via one's cell phone.

The idea is that the lights are not always needed, and that selectively turning them off would save a lot of energy (and, therefore, money). The lights are there only for pedestrians. (Cars have headlights.) So how can one arrange to turn on the streetlights only when there are pedestrians nearby? With the ubiquitous cell phone!

While on the face of it, it's a lovely idea, it isn't quite as good as one might think. You can read this blog post to find out why.

The Problem of LCA

There are many different methods for performing a life-cycle assessment. Which one is best? No one knows.

However, there is one systemic problem with all these methods, and that involves calculating the impact of a particular element of a product. One must calculate the flows to and from nature for the product. However, not all flows into (or out of) a product are to/from nature. For instance, when considering fuel consumption by automobiles, the exhaust of the automobile is a direct flow to nature, but the corresponding input (the gasoline) is not a flow from nature. Thus one must track back the gasoline put into the automobile by examining how the gasoline was produced, per unit volume. This becomes very difficult to do very quickly.

Consider the total life-cycle cost of purchasing a screwdriver at a hardware store. One must trace the metal used in the screwdriver back till it was mined (an input from nature), but a fractional amount of various machines resources were used to form those metal parts, so one must include some fractional amount of the impact of those machines too. A similar argument can be made for the plastic or wood handle of the screwdriver. Once you have calculated all that, then one must include a fraction of the impact of the store in which you buy the screwdriver, including lighting, heating, electricity to run the cash register, paper used for the receipt, ink/toner used to print on the receipt, etc.

Obviously, this quickly becomes an intractable calculation, so various assumptions are made. But each assumption can build an error into the calculation.

Other Products

See Also

The URL given in Wired is this one, but I can't find the report posted there. Fil Salustri (2013.08.06 10:57)
design/sustainable_design_cases.txt · Last modified: 2020.03.12 13:30 (external edit)