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...


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[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)

New Building Occupant Behavior Opportunities

An interesting bit of news on the Boston behavior change front: the Boston Redevelopment Authority is interested in pursuing behavior change technologies in two new college dormitories being built in Boston. Berklee College of Music and Boston College are both building new dorms to accommodate their burgeoning student bodies, and Gerald Autler, urban planner and senior project manager at the Boston Redevelopment Authority, wants to design the dormitories with occupant feedback systems in mind from the onset.


I have to applaud his approach. At MIT we were limited in the sense that we had to work with the building infrastructure we were stuck with, but if you can work in occupant feedback systems from the onset, you can make a significantly greater impact. My primary idea was to submeter the electrical systems keeping in mind the way students would socialize, so that any energy monitoring hardware added afterward would reflect the energy usage of a given social unit, rather than an arbitrary group of students. Typically electrical submetering is done just as a matter of convenience: a 3-story building with 2 wings may be submetered at the wings, grouping all three floors together. This isn't ideal, as it's more likely that students would socialize with the other students who share their hallway, rather than those directly above or below them. Submetering to the level of social aggregation makes conservation messaging a lot more effective. 


If I can pull through on the MIT-NSTAR Efficiency Forward campaign's behavior change program, it may be interesting to do some intercollegiate behavior change competitions between MIT, BU, and MIT.  

A Brief History Lesson

In 2006, a friend and I started what would become known as the Dorm Electricity Competition. The idea was simple; encourage students to conserve electricity by hosting a competition between dorms where the winner would receive $10,000 in energy efficient dorm improvements. The electricity savings resulting from the competition would be more than enough to pay for the dorm improvements, so MIT could benefit from lower electricity bills from both the competition AND more efficient dorms, while students in the winning dorm would benefit from infrastructure improvements. Everyone's a winner!

We held the first competition Spring of 2007 and, to our great surprise, saved 228 MWh of electricity. As with most first attempts, we learned a great deal from the experience:

1. MIT LOVED the idea of spending $10,000 for $20,000 in electricity savings. After the first year, Karen Nilsson of MIT's Division of Student Life added the funding for our project as a line item in their annual budget. 
2. Shockingly enough, MIT students don't think dorm infrastructure improvements are an alluring enough prize to substantially change their electricity-consumption habits. They DO, however, like the idea of a friendly competition between dorms.
3. At the time, the technology simply wasn't there to collect electricity consumption information on a daily, let alone continuous basis. We knew that in order for real progress to be made, students would have to have better information than the week-to-week reports about building-level energy consumption.
4. Finally, the measurement and verification ("M&V" as they say in the industry) is a tough nut to crack. We spent many late nights poring over spreadsheets of historical energy usage just to establish a fair baseline for each dorm. We would later use much more sophisticated methods to quantify savings precisely, like regressing per capita energy usage to population in order to isolate electrical loads under user control from building loads. 

So that brings us to where we are today: a lot of lessons learned, but thanks to our new involvement with the MIT-NSTAR Efficiency Forward campaign, we have more or less a blank slate to work with. 

I'll update a little later with more or less what I presented to the MIT Walk the Talk Task Force by way of schedule for 2011-2012. 

Welcome!

Short version: Welcome. I started this blog to chronicle my work for MIT starting an electricity conservation competition based on social network-based games. I’ll also post other relevant information as appropriate. Enjoy!


Full version:

Hi there! Welcome to The Energy Nerd blog. I’m Tim Grejtak, a self-proclaimed energy nerd and the author of this blog. I started this blog rather tangentially:

I’m a recent graduate of MIT and during my time there, I stared a project with another student called the Dorm Electricity Competition. It succeeded in the sense that it actually saved MIT enough electricity to power 100 US homes for a year.[1] It failed in the sense that it didn’t rigorously test the hypotheses behind the project: that behavior change is one of the fastest, most cost-effective routes to lower carbon emissions, and that game-based incentives and competition are the surest way to achieve sustainable behavior change.

I was enlisted by MIT this year to help them institute a formal behavior change policy as part of their NSTAR-MIT Efficiency Forward Campaign (details on the NSTAR-MIT Efficiency Forward Campaign here). My newly-started job as a development associate at the local energy start-up Sun Catalytix (shameless plug here) precluded me from being paid by two different companies for two different jobs. I also knew the project was important, but I wasn’t an MIT student any longer. So the incentives expert in me reasoned that if bribery and guilt didn’t work, peer pressure surely should!

Hence this blog. Announcing to the world my intentions to set up a behavior change project should compel me to actually set up a behavior change project. Right? So, dear reader, my integrity is on the line; either I must follow through with my intentions publically on the internets, or face ignobility and shame FOREVER!!!

In between updates and sure-to-be-brilliant discoveries (such as “people like to win” or “working with bureaucracy is annoying”), I’ll be posting relevant news articles, references to interesting papers, and maybe a few personal interest stories about the minutia of my oh-so-interesting life (though I promise to keep those to a minimum). By and large, new posts should come every weekend.

So welcome and thanks for reading. Hopefully you haven’t grown tired of my slightly-sarcastic writing style (and occasional interjections) and I’ll see you again.

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[1] 700 MWh total electricity saved. An average US home consumes 30 kWh electricity per day (source: EIA) [2]

[2] BTW, one of my tenets is, “You’re only as good as your references,” so expect lots of footnotes (and yes, I did just use nested footnotes, but don't expect much more of those)