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Vital Signs
Project: Siegel House Case Study
"Rock Tubes" Acting as Thermal Mass:
Hypothesis, Methods, Data, and Analysis
Hypothesis
As stated above on the hypothesis page, we believe the
rock storage is acting as thermal mass, though not in the manner intended by the original
design.
Methods
In order to determine if the rock wall was acting as thermal mass even though the
thermosiphon loop was not operating, we decided to use two different methods. First, we
looked for the "lag" associated with thermal mass in action by comparing the
peak ambient and rock tube temperatures collected from the temperature sensors. Second, we
modeled the Siegel House on the Energy 10 computer program with and without the
thermal mass, and examined the two results for differences. If the thermal mass was acting
in the house, there should be differences in the way the house used energy between the
with and without mass cases.
 A
section diagram of the sunspace and living room,
showing the location of the temperature sensors used
to test thermal mass performance
To look for "lag," we used temperature data. We placed sensors on the surface of
one tube at the top and at the bottom, about 16 feet and 2 feet from the floor, and we
placed an ambient sensor at a central point in the living room, about 6 feet off the floor
(see diagram at left). We expected the rock wall temperature to lag the ambient living
room temperature--it should take longer for the rock wall to reach its peak temperature
during the day than the ambient living room temperature. Also, it should take longer for
the thermal mass to cool off in the evenings. This result would occur due to the thermal
mass soaking up heat during the day, then releasing it at night when the house was cooler.
Secondly, we used the computer building-energy-use simulation model Energy 10 to
examine this hypothesis. We modeled the Siegel house with and without the thermal mass of
the rock wall in order to compare the differences in energy use between the two cases. We
expected an identical house without a rock wall to exhibit greater daily temperature
swings than a house with the thermal mass of the rock wall. (We could not examine only
temperatures in the living room specifically because Energy 10 models buildings
with only one inside temperature.) These larger temperature swings should show up in the
energy used by the house because of the need for more mechanical heating and cooling to
keep the house within the range set by the thermostat.
Data and Analysis
Here is a graph of three days worth of temperature data we obtained from the sensors in
the house. Click on the graph to see a graph of all the data we collected over the three
weeks.
This graph shows little evidence of the lag expected from the
thermal mass (17 k gif)
We were unable to discern the lag we expected as an indication that the thermal mass was
acting. The peak temperatures at the top of the tube, the bottom of the tube, and the
ambient temperature in the living room all correspond closely.
We also entered the Siegel House into the computer model Energy 10 , and ran
simulations of the building with and without the rock wall. Here are two output graphs
that resulted from the simulations. Click on either for a more detailed view.
 Output screen from the Energy-10
computer
program showing energy use with and without
the thermal mass (18 k gif)
 Output screen from the Energy-10
computer
program showing monthly hourly average HVAC
energy use with and without the thermal mass (29 k gif)
The first chart is a bar graph comparison of the energy use between the two cases. The
chart shows that the energy use in each case is virtually identical. The second chart
shows a 6 month period and the energy consumed for heating and cooling over that time as
well as indoor temperature profile. Again, the top chart (with the mass) and the bottom
chart (without the mass) are identical.
We did not find any differences between the simulations with and without the rock wall.
This finding indicated to us that the rock wall is not acting as effective thermal mass in
the Siegel House. If it was having an effect on the temperatures inside the house, some
difference in the temperature profiles or the energy use of the buildings would have been
evident.
One item worth noting is that we had a tough time modeling the 14 foot high metal tubes
filled with fist-sized rocks with the Energy 10 computer program. As you can
imagine, this was not an option in one of the drop-down boxes. After many tries, we
decided that the closest we could come to the rock tubes were square steel columns filled
with a concrete. We reduced the overall volume of the concrete as compared to the volume
of the rock tubes to compensate for the air-space volume in the rock tubes. This
approximation may not very accurately describe the real behavior of the rock tubes.
Another consideration to bear in mind about Energy 10 is that in another project
for this class, we had trouble with it. Often, when making drastic changes to the
configuration of a house, only very small differences in the outputs from the program
could be noted. This same problem may account for some of the similarity in this Siegel
House comparison.
Conclusions
None of the evidence supports our hypothesis that the rock tubes were acting effectively
as thermal mass in the Siegel House, even though they are not operating as originally
designed. We measured ambient temperature in the living room and in two locations on the
surface of the rock wall for several weeks. As described along with the temperature data
chart, the rock wall temperature does not lag the ambient temperature at all. We also used
the Energy 10 software program to simulate two houses identical except one had the
rock wall. The six-month simulation results show that the monthly average hourly HVAC use
are identical. We were unable to confirm our hypothesis that the rock wall is functioning
as an effective thermal mass in the Siegel house. |
