Date: Tue, 22 Jul 2008 02:03:59 -0400 (EDT) From: Mitch Golden Subject: AC Well we took the plunge and today added an AC in the living room to our inventory of appliances. Next time you come to visit our son, you won't have to schvitz. I know you were thinking of buying a Prius because of your desire to lower your carbon footprint. Apropos of our discussion of air conditioning the other night, I thought you might want to see some relevant statistics that you might find interesting. Electricity generation, not transportation, is the single largest sector contributing to carbon emissions in the US, and HVAC (Heating Ventilation and Air Conditioning) is a big part of that. Here is an example from my own life. *) A current Prius gets about 46 mpg combined, according to the EPA. A more traditional car of the same sort, say a Chevy Malibu, gets about 27 mpg. (1) Let's make an estimate - assume you drove 12000 miles per year in your car - this is a liberal figure I suspect, given that it's 33 miles per day, high for someone who lives and works in NYC. If the car were a Prius, you would use 261 gallons of gasoline, while in a Malibu you'd use 445. Each gallon of gasoline when burned emits about 20 pounds of CO2 (2). So, from driving your Prius, you'd emit 5220 pounds of CO2, while in your Malibu, it'd be 8900. The savings is about 3680 pounds of CO2 per year. BTW, a point of controversy about the Prius is that *manufacturing* it emits more CO2 than for a normal car, due to the energy input needed to make the batteries, so the actual carbon comparison may not be as favorable to the Prius as above. This is discussed a bit on the Prius wikipedia page (3). *) For air conditioning - as an example the AC in the bedroom is rated at 13500 BTU/hr, which uses 1.45kW. Therefore, it uses 1.45 kW-hr every hour. Now how much CO2 is emitted from electricity use depends on the method of generation. Coal is the worst form, having just over 2 pounds of CO2 per kW-hr; Wind and nuclear have nearly zero; natural gas is somewhere in between. In the Mid-Atlantic states, according to the EPA (4) the mix of fuels is such that one kW-hr of electricity generation creates 1.07 pounds of CO2. Therefore, each hour that our AC runs is about 1.45*1.07 pounds of CO2. The next question is how much one uses the AC. One guess would be how many days in the year are hot, and how many hours a day the AC might be on. Of course that depends on whether you are in the apartment or not during the day. (Unfortunately, many people leave the AC on all day so their house will be cool when they get home.) So suppose that we use the AC for 100 days a year during the summer for 10 hours a day. I pick that number because most people leave the AC on overnight and for several hours while they're home, but OTOH, the AC probably doesn't run full-tilt all the time. I see that the EPA estimates that in NYC, the AC is on 1089 hours per year (5). (On the other hand, the tag that came with the new AC said that it costs $73 per year at 10.65 cents per kW-hr - a lower rate than we pay in NYC - meaning that they are using expecting the average use to be 685 hours per year. Presumably use in NYC is above the national average.) Therefore, to use just the one AC in the amount an average New Yorker does, we will get 1089*1.45 = 1579 kW-hr = 1690 pounds of CO2. We actually have 3 rooms in the apt, so if we used AC in them all we would exceed the difference between the Malibu and the Prius. And one should bear in mind that we have an apartment that is small by the standards of dwellings nationwide. In contrast, the fans we have use only about 10-30 watts or so on the low speeds we use, so they amount to a trivial percentage of the AC. (In other words, we could run 50 fans on the energy of the one AC.) *) Just for fun, consider this additional point: In practice, an airplane has about the same fuel efficiency as a car (6) if you include the different numbers of passengers they carry and the different miles per gallon they get. That means that for every mile you travel in a plane you emit the same CO2 more or less as a mile you drive. But you can fly 12,000 miles, your annual auto usage, in a single international round-trip flight. In fact, for many of us, myself included, the emissions from our plane flights dwarf the emissions from our terrestial travel. So if you gave up a couple of plane flights, you could go out and buy a Hummer, and still be ahead of the game. Now you know why I am such an eccentric about these matters: it's important to keep one's eye on the real source of emissions, and not just the sexy, fun things like hybrid cars. - Mitch (1) http://www.fueleconomy.gov/feg/FEG2008.pdf (2) http://www.fueleconomy.gov/Feg/co2.shtml (3) http://en.wikipedia.org/wiki/Toyota_Prius (4) http://tonto.eia.doe.gov/ftproot/environment/co2emiss00.pdf (5) I can't find the real refererce, but I see it referenced in the energy calculator spreadsheet on the energy star web site http://www.energystar.gov/index.cfm?c=roomac.pr_room_ac (6) See chapter 2 of the Transsportation Energy Data Book, table 2.12. Note that it is expressed in BTU, not gallons, and it is not directly convertable to CO2 emmissions, but the general point is valid. http://cta.ornl.gov/data/tedb27/Edition27_Chapter02.pdf