Wednesday, October 03, 2007

Geothermal Analysis: How's It Working (Part 1)?

Now that it has been about a year since we have converted to geothermal for our cooling and our first-stage heating needs, it is time to check the results. It hasn't been easy to do this analysis. A lot of time was spent digging out old bills and check registers to find out how much heating oil we actually used, how much we are now using, and whether or not the conversion was worthwhile. The short answer is yes, it has been worthwhile, but there are a few side-effects and mis-steps we made along the way.

Locating our old heating oil bills wasn't easy. With having to move our for our renovations in 2005, we boxed everything, and of course, the process wasn't as efficient as one would have hoped. We are packrats to a large extent, so most of the bills were located, and the information was cross-checked with the checking account records and verified by the weather data. I think the analysis is as complete as it can get here, and the results pass some simple consistency checks.

The first figure below shows the amount of heating oil we have used on a daily basis since late 2000. The gap starting in the Spring, 2005 is because of the renovations, and the start of the geothermal period starts in late-2006. We had hot water starting May, 2006, but I've left that period out due to complications of how much heating was used in the renovation period.

The amount of heating oil used are given by the blue bars and are in daily amounts. The use of daily amounts is because the deliveries are not on a periodic schedule and vary depending upon how cold it has been. For instance, the very first bar represents 386 gallons used over 65 days. The very next bar is 278.3 gallons but is was used in only 24 days, and so on. Conversion to daily rates makes the analysis easier.

There are many parts to this first graph that deserve comment. One issue is shown by the red dashed line, and it represents the baseline heating oil use. The line is drawn at 2 gallons per day, and interestingly enough, it is the same for all years--both before and after the renovations. I'm assuming that this 2 gal/day is predominantly the amount we use for water heating. Here is one area where we blew it. The geothermal units can come with a water heater attachment (the desuperheater) that effectively gives you free hot water during periods of high geothermal heating and cooling, and we didn't get it. I think that was a big mistake.

The most interesting item is the major reduction in the amount of heating oil used since the installation of the geothermal. The total rate for last season was about 4.2 gallons/day, and about half of that was for heating water! So, yes, the geothermal heating has significantly cut into our heating oil use! However, it has come at a cost, namely more electricity, and that discussion is taken up in part 2.

Most remarkable about the reduction is that the renovated house is 60% larger than the older house. We are now at 4200 square feet instead of 2600 square feet. Countering that, though, is the fact that the present house is insulated and thermally wrapped whereas before, there was very little insulation, and the house was drafty. Nonetheless, the data suggest we went from roughly 2000 gallons of heating oil per heating season to about 700--a reduction of 65%. This reduction estimate is a little too high, as we will see below.

There is another complication, and that is that each year is a little different in terms of how cold it gets, and that factor needs to be taken into account. The way this is done is with something called the Heating Degree Days (HDD). A heating degree-day is a measure of how cold it is, and the amount you need to heat increases as the number of HDDs increases. The exact definition is that the number of HDDs is the total number of degrees the daily average temperature (in fahrenheit) is below 65 degrees. For instance, if the high and low temperatures for the day are 78 and 50, the number of HDDs for the day is 1 degree day [65-(78+50)/2]. For average temperatures over 65, the number of HDDs is zero. For a cold day where the average is 30 degrees, the number of HDDs would be then be 35. The next figure shows the number of HDDs for the same periods shown in the first figure.

From looking at the HDDs, the proper year to compare the new data to is the 2001-2002 season, because it was a similar winter in terms of overall heating. Taking out the water heating, we went from 6 gallons/day for heating to about 2.3 gal/day. The reduction is still 60% in our heating oil use.

The next figure shows the relationship between the amount of heating oil used and the number of heating degree days.

In the figure, I've noted 2 blue data points that are below the trend in the old house, and that is because we were on vacation during these two periods, so our demand was indeed down, but not for weather-related reasons. The red points are from the renovated house with geothermal heating. The red line shows a simple linear model for the new period, but it is not correct. The way the heating works is that the first stage is always geothermal. If the geothermal cannot keep up with the needed heating, the second stage heating (heating oil) comes on-line. The non-linear dashed line is probably closer to the current set-up in how our heating needs use heating oil.

Our biggest goal is to reduce our foreign oil dependence, and the switch to geothermal heating as first-stage heating has surely done that. In this manner, the new units have been successful. But at what cost? To answer that, the electricity data need to be analyzed, and that will be a subject of a later post.


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