Energy

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As several municipalities considering or even implementing bans on disposable grocery bags, I have noticed some claims emerging that reusable bags are actually worse for the environment than disposable ones. The basic argument is that reusable bags not only take more energy to manufacture (they are heavier, so this is doubtless true), but moreover the need to wash reusable bags negates any energy benefit from re-using them. A similar case was made for using recycled paper napkins rather than cloth in restaurants, and the numbers do favor recycled paper. At least, they do for restaurants, which replace their napkins when they get the slightest stain; at home, using that author’s numbers, cloth is likely to be the more “green” choice.

However, I didn’t find any good analyses for reusable and disposable grocery bags (while I did find some contradictory claims), so I decided to see what I could work out myself. I did a bit of searching one weekend to try to figure out how much energy it took to manufacture canvas and plastic bags, estimated how much energy it took to wash the canvas bags, and made a quick calculation of how many plastic bags I avoid using in a year by carrying around my canvas ones. The details of the references, assumptions, calculations, and rough uncertainties are here, and I am posting a summary below.

For manufacture, the canvas bags are heavier, and therefore it takes a lot more energy to manufacture one canvas bag than it does one disposable plastic one. I weighed the bags that I’ve used, and found that my Trader Joe’s 2003-vintage canvas bags weigh about 180 g each, and the disposable plastic bags I get when I forget the reusable ones weigh on average 6 g each. My canvas bags should have taken about 25 MJ to make [PDF] (growing the cotton, weaving the fabric, and assembling the bag), whereas one plastic bag should take about 0.5 MJ to make.

The energy used in transportation per bag should be strictly proportional to the difference in weight. I assumed that it took 5 MJ to transport my 180 g canvas bag [PDF], and 0.2 MJ to transport a 6 g plastic bag.

Only the canvas bags need to be washed. The energy used will depend upon the size of the load, the temperature at which the water is washed, the amount of water used for the load, and whether or not the bags are run through the dryer (mine are). I tend to wash fairly large loads, about 4.5 kg at a time. I would use hot water for the wash, since I wash the bags with our napkins and dish towels, and cold for the rinse. I estimate that each wash takes between 0.6 MJ per bag for our front-loading machine (for a top-loading machine, this would be about 0.9 MJ per bag). Drying the bags takes another 1 MJ per bag (I should get a clothes line! Um, and a backyard. Oh, and a sunny, dry climate). I wash the bags about once a month at most, so the yearly energy budget for washing the bags is 19.2 MJ. Washing the canvas bags nearly doubles their energy footprint.

Summing the numbers, in the first year of purchasing a canvas bag, I estimate that I use 49.2 MJ of energy for that bag, versus 0.7 MJ per disposable plastic bag. The canvas bags carry more, and the plastic ones are always double-bagged, so I estimate that each canvas bag replaces three plastic ones on each trip. I go shopping once a week, so one canvas bag replaces 156 plastic ones in a year. Those 156 plastic bags would take 109 MJ of energy — more than twice the energy used by a new canvas bag. After the first year, I would only need to wash the canvas bag, taking 19.2 MJ of energy, so my reusable bags are 5.7 times more energy-efficient.

I might be off on my numbers by a factor of a couple, so perhaps in the first year the canvas bags are about equally energy-intensive as the plastic ones. However, in the long run, it appears that canvas bags are much more efficient than plastic ones. This would be especially true if I were to line-dry them, as someone truly eco-conscious would… But I mainly use them because I hate stuffing my closet with plastic bags. It just turns out that it’s also the more energy-efficient choice.

I wanted to get an idea of how much it would cost to undo global warming by engineering the atmosphere. A friend stated emphatically that it was cheap, but I wasn’t so sure. So, I decided to do a back-of-the-envelope calculation. You can find more reliable numbers elsewhere, but I’m posting this anyway, in case someone might find it fun to see what one can do with some numbers found with Google, some arithmetic, and an hour or two of spare time.

The simplest thing one could do to change the climate (aside from burning fossil fuels) is to eject a chemical into the atmosphere that reflects sunlight back into space. The obvious candidate chemical is sulfur dioxide. In 1991, Mount Pinatubo erupted, and put somewhere between 17 million and 20 million tons of sulfur dioxide into the stratosphere. As a result, the Earth cooled by about 0.5 degree Celsius for a few years. To temporarily alleviate global warming of a few degrees, we would need to put a comparable amount of sulfur dioxide (within factors of a few) into the atmosphere each year.

How much would it cost to inject this much stuff into the upper atmosphere? As a rough estimate of the cost, I assumed that it would cost about the same amount as getting things into the air by airplane. A 747 freighter uses about 10,000 kg of fuel to take off to a height of 10,000 feet, for a gross take-off weight of 360,000 tons, about 140 tons of which are cargo. Fuel costs $0.70 per kg ($2/gallon, density of 2.8 kg/gallon). So, I get a cost of $50 per ton to get things into the air, based on the fuel alone. That estimate doesn’t get the material into the stratosphere, but most of the air resistance is near the ground, so this shouldn’t be too far off (feel free to correct me if I’m wrong).

So, I estimate that getting 20 million tons of sulfur dioxide into the stratosphere will cost at least $1 billion dollars a year. My number should be accurate to within an order of magnitude. I don’t know whether it is expensive to make sulfur dioxide (probably not), and the distribution system might have different costs than a simple airliner (these don’t typically fly in the stratosphere, after all).

Nonetheless, my estimate isn’t too different from one reported in ArsTechnica (quoting an
article in Geophysical Research Letters Alan Robock and collaborators): they estimate $4 billion dollars if one uses F-15Cs, and $375 million dollars if one uses KC-135 Stratotankers. However, from the description at ArsTechnica (I’d have to use a computer at work to get the actual paper), the authors of the proper study seem to think that we only need to add enough sulfur dioxide to counter the warming trend, so that Mount Pinatubo only needs to be reproduced once every four to eight years. This sounds to me like an underestimate, but either way, the numbers are within my order-of-magnitude tolerance.

My friend suggested that a single developing country might undertake geoengineering on its own. Setting aside the political problems, I also wanted to know, is this amount of money reasonable? I think so. For comparison, the budget of India’s space agency is about $750 million dollars a year (assuming an exchange rate of $1 to 46.7 Rupees, where a Crores Rupees is 10 million Rupees). Space agencies are a bit of a luxury, so $1 billion does not seem out of reach.

This scheme is also inexpensive compared to the cap-and-trade legislation that is heading through congress, which may cost on order $150 billion per year, and that’s for the U.S. alone.

So, in terms of cost, my friend is probably right: putting sulfur in the stratosphere is relatively cheap and accessible, especially when compared to changing our economy to use less carbon.

However, I still don’t think that anyone will do it, because fears of the potential side effects will probably dissuade us. The sulfur dioxide from Mount Pinatubo exacerbated the hole in the ozone layer, for instance. However, who am I to divine the heart of man? If you want politics, FiveThirtyEight probably has a better discussion of those issues.

According to a press release from GM, estimates based on preliminary EPA standards have the Volt getting 230 miles per gallon in city driving. I’m not sure what these “preliminary standards” are, but after running some numbers, I have a hunch (and I’m not the only one): I think the EPA is only counting gasoline burned, and not the electricity the car uses for short city trips. Here’s why.

The LA Times article states that the Volt uses as little as 25 kWh per 100 miles of city driving. This number really impresses me — that’s probably significantly less than my 2003 Prius uses (see below).

However, I have to ask, How much gasoline would it take to produce that 25 kWh of electricity? The energy density of gasoline is about 37 kWh per gallon. So, if the Volt only uses electricity, and if electricity could be produced from gasoline with 100% efficiency, the Volt would get a mileage of at most about 148 miles per gallon.

Of course, it is not possible to convert gasoline to electricity with 100% efficiency. I found this electrical plant that converts fossil fuel (natural gas) to electricity with an efficiency of 58%. A a typical efficiency is closer to 40%. Electrical transmission losses dissipate another 5-10% of the power. So, if, optimistically, my electricity is made by fossil fuel at about 45% efficiency, the actual mileage would be closer to 67 miles per gallon.

I do think that if we must use cars, electric cars are the way to go. Hopefully, in the future, more of our electricity will be made from wind and solar farms, nuclear power, and maybe even fission reactors (I can dream). In that case, the electricity comes with much less pollution, and the Volt is a winner.

Unfortunately, in the near term, when our electricity is made from coal and natural gas, the preliminary EPA mileage estimates for the Volt have little meaning. The 230 miles per gallon number is laughable. The Volt will get better mileage than a hybrid or diesel sedan, but it is not yet a big gain.

Instead, if you really want to make a difference, think in terms of person-miles per gallon (or gallons per mile per person), and carpool!

I believe strongly that energy conservation is crucial toward securing our energy future, so you might think that I would be happy with last week’s announcement by the Obama administration that they will be implementing standards to make light bulbs more efficient. Instead, though, the way the announcement was made has bothered me, because its impact is actually pretty tiny.

The problem is, as Obama stated in his speech, that lighting only consumes about 7% of U.S. energy use. The new standards will not eliminate the energy used for lighting, it will only make it more efficient. How much more efficient? We can cut the press conference numbers Obama used down to size.

First, the speech stated that in the most optimistic analysis, the savings over 30 years are equivalent to powering all American homes for 10 months. That means we will be changing our residential energy use by (10/12)/30 = 2.8%. Moreover, residential uses account for only 21% of total U.S. energy use (according to the Energy Information Administration), so this plan should cut total U.S. energy use by only 0.6%.

Similarly, the speech states that over 30 years, it will be equivalent to taking 166 million cars off the road for one year. Why not phrase this as taking 5 million cars off the road for 30 years, or roughly 2% of the 250 million cars on the road? Well, with this number, the savings seem even smaller. Passenger vehicles account for 17% of U.S. energy use, so the savings may well be only 0.3%.

I’ve been reading the book, Sustainable Energy – Without the Hot Air by David MacKay, and he wittily explains the logical flaw in arguments like the one made in Obama’s speech. The problem is, Obama’s writers took paltry numbers and multiplied them by big ones, to make the impact seem bigger. The real effect has been put better by Prof. MacKay:

If everyone does a little, we’ll achieve only a little.

The cap-and-trade legislation that Obama has been helping through Capitol Hill will be much more effective (if Congress doesn’t get in the way too much). Unfortunately, reducing energy use at the consumers’ end will take serious broad-based efforts that are much bigger than changing light bulbs, and that I am only beginning to appreciate. . .

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