Monday, November 2, 2009

Analyzing Usage and Weather

I located some logged online Cooling Degree Day data nicely packaged by these folks.
To review, one Cooling Degree Day (CDD) recorded at a base of 65 degrees, for example, is a day where the temperature averaged over a day was 66 degrees. If the temperature for that day averaged 85 degrees, the CDD would be 20 for that day. Negative CDDs are ignored - actually, they would be counted as Heating Degree Days, which I am currently not using in my analysis (I may at some point in the future when I look at my natural gas usage, which heats my house).
The first question was which weather station to use; there are many in my area, and their numbers are all different. Not _too_ different, but somewhat. I decided to use the airport data since that data has the fewest gaps, even though the airport is a good half-hour drive away, and is well outside the urban heat island that I live on. One of the issues in dealing with long term archived data is dealing with the gaps. In my case, only a few months were missing, and I filled those in with data from another source.
I have the vague idea that my energy usage is mostly driven by air conditioner usage, supported by the fact well known to many Texans that the electric bills are largest in July and August, and are still not fun in June and September. Of course, I'm also using energy for other things such as lighting, computers, televisions, refrigerator, etc. but those loads should not vary as much seasonally, although, in yet another complication, lighting usage varies seasonally as the days get shorter.
So we have some known issues with our analysis, which we can hope will not matter too much:
  1. Location difference between airport (out of town) and house (in town, downwind from downtown in the "heat island")
  2. Small gap in the CDD data filled by data from a different source
  3. Many things sum to make energy usage; air conditioning is only one, albeit a big one
  4. CDD's measurements themselves can be taken different ways. Measuring the temperature every hour, and summing those results over a day, yields a different number than just looking at the (highest - lowest)/2 value that some data providers might use.
  5. CDD's at base 65 might not be the wrong "baseline" for my house. Perhaps my air conditioner does not kick on until the daily average is over 70, for example.
  6. CDD's do not consider sunlight, which delivers far more heat than convected air, particularly on cooler days. It would not surprise me to see some air conditioners running on a day with 0 CDD's but is sunny, and not run on a day with a few CDD's but is cloudy. However, installing a radiant barrier should have reduced this problem for me; I have far less sunny heat gain than before.
  7. CDD's do not consider the non-air environment, except as it affects air temperature. What do I mean by this? Well, a lot of us who live here have seen 100 degree days in May. Although unwelcome, those days never seem so bad as 100 degree days in August. Why? For one thing, there is still moisture in the soil in May. Grass is green and growing, plants are moist and lush, and the ground hasn't been baked for months on end to a nice shade of brown. All of these things will reduce the heat radiating around the area and hitting me and my house, even in high air temperature. In August, on the other hand, the grass is dormant and not evaporating water, cooling the ground. The streets, sidewalks, and bricks in the houses are storing a lot of heat built up over the summer that they didn't have in May. All of that heat is radiated and hits me and my house in August, causing more cooling load, even though very little of it affects the air temperature (particularly the air temperature at the airport, which is out of town).

One way that I hope to discover whether some of these factors matter, and perhaps how much they matter, is to simply graphically look at the data. Do the data make sense? If I graph my monthly consumption in kWh vs. the CDD's for that month, a relationship should emerge if there is one. In a nice, pretty world, it would be a linear relationship, with a slope showing how much energy I need to expend to handle one CDD, but we'll see if that is the case.

Here is a look at the Cooling Degree Days at the airport since 2005. You can clearly see the seasonality reflected, and the amazingly cool summer of 2007 right in the middle of the graph. You can also see how our January's have been getting warmer every year fairly consistently, even while the summers fluctuate, and you can see that the summer of 2009 was all-record-breaking in terms of heat. The graph has been 5-point smoothed (each point has been averaged with the 2 before it and the 2 after it) to make it look nicer.

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