Monday, May 9, 2011

The Fascinating World of Air Conditioner Efficiency

I was inspired to write about the oh-so-sexy topic of air conditioner efficiency and consumer behavior patters after reading an innocuous article in Slate today (article here). The author, Brian Palmer, did a thorough job of summarizing pros and cons, however (and here's where I think he opened up his article to vehement commentary) concluded that it's difficult to say which method of cooling is more energy efficient.  In fact, window ACs use less energy than central ACs despite lower SEER ratings, but rather than narrowing our focus on trying to criticize a single article, let's open up the discussion a bit, because, surprisingly, we use a great deal of energy just to keep ourselves comfortable.


According to the DOE's Energy Information Administration (probably my favorite reference), Americans used 24.5% of their electricity to keep themselves cool at home [1]. Looking at the US, this accounts for 7% of ALL electricity usage [2]. And the major contributor to AC energy use is central AC systems at 3745 kWh/year vs. 1259 kWh/year for window AC systems. Now, these are just household-level numbers, which don't take into account any confounding variables. One study, by a David Rapson of UC Berkley's Department of Economics, points out that households with central ACs are larger and are in climates with more cooling degree days (see Table 2) [3]. Even factoring that in, window ACs use less energy on a per cooling degree day and per square footage basis by a factor of 1.75. So there you have it: window ACs are better at cooling a space efficiently than central ACs.


But why? Why should window ACs, which have a lower SEER rating, use less energy than central ACs? Murphy's law provides a bit of insight here. Central AC systems are more than just the air handler and compressor that comprises the window AC unit. There's dozens of linear feet of air ducting, an automatic control system, and the whole system has to be engineered and installed by some contractor.  A quick survey of central AC units in Austin, TX by a few University of Texas, Austin engineering students revealed blowers to circulate cooled air were operating too high, unsealed ducts leaking cooled air to the environment, control systems extending operating times, and systems being oversized from specification [4]. The result is a high efficiency compressor but low efficiency ducting and control systems. 


But that only covers energy efficiency. What about consumer behavior? After all, this is a blog about behavior change. Well, the brilliant minds at the EIA have that covered; a section of their RECS questionnaire was about thermostats and usage. Of the 64 million central AC systems, 40 million consumers reported running them all summer, while 11 of the 27 million households that use window AC units report using them "only a few times when needed." And while fewer window AC units had programmable thermostats (only 15%), users simply turned them off or on when necessary. Conversely, 39% of central AC units had programmable thermostats, but 64% of users actually use the programmable thermostats as indicated. 


The bottom line seems to be that households with window air conditioners use them more consciously (and conscientiously) to cool smaller areas within the home, while households with central AC allow their units to run continuously and cool the entire home, whether it's necessary or not. Hence the factor 1.75 lower energy consumption per household for homes that use window AC units vs. central AC units. 


ACs represent an interesting home appliance where improvements in consumer usage behavior can be just as important as improvements in unit energy efficiency. Now it's just a matter of encouraging the right behavior...


--------------------------------------------------------------------------------------------------------
[1] http://www.eia.gov/emeu/recs/recs2005/c&e/airconditioning/pdf/alltables1-11.pdf
[2] http://www.eia.doe.gov/cneaf/electricity/epm/table5_1.html
[3] http://www.econ.ucdavis.edu/faculty/dsrapson/Rapson_LR_electricity.pdf
[4] http://bit.ly/msvo1j (unfortunately, I don't have the full article on this one, just the abstract)

7 comments:

  1. This blog is the best find ever.

    -Phil Narodick

    Ps typo in 'about me'

    ReplyDelete
  2. I live in a NYC co-op that is more than 50 years old. We have through-the-wall AC units in each room. They are very expensive to replace [around $3K-$5K, depending on the size and maker], and I think very inefficient. I'm trying to convince the building's board to allow window ACs, which currently aren't allowed. Do you have any thoughts or data on the efficiency of window AC units vs through-the-wall units, such as Simon-Aire? Many thanks. Bob Kozma

    ReplyDelete

  3. TemperaturePro provides the best in both in-home and business comfort and air quality. Heating, cooling, indoor air quality, and maintenance are just a few of TemperaturePro’s specialty areas.

    Furnaces, AC units, air cleaners, and any other kind of home comfort products are serious investments that families depend on to keep them comfortable throughout the year. Serious investments must be protected, and at TemperaturePro, we know exactly how to do that. Our qualified, experienced HVAC technicians will serve any and all of your heating and cooling needs and keep your units running smoothly.

    hvac repair bee caves
    lakeway ac repair services


    ReplyDelete
  4. Yeh that's true window ac consumes much electricity than the central ac. Thanks for such an amazing article.

    Heating and Cooling Markham

    ReplyDelete

  5. Really very useful tips are provided here.thank you so much.Keep up the good works.
    Heating and Cooling Toronto

    ReplyDelete

  6. Hi,
    I have visited your post,, you have added many important information of your post,its really good ,I hope you will be adding more.here is another link if you want you can visit it air condition

    ReplyDelete
  7. great info you shared here thanks for this useful info.
    Heating and Cooling Mississauga

    ReplyDelete