Passive Solar Basement Efficiency
The graphs below summarize the results for the 2005-2006 heating
season.
Click on the graph for a larger version.
The graph above shows the observed daily average temperatures for this
heating season. The grey squares (along with the y axis on the right
edge of the plot) show how many hours per day the backup propane furnace
ran. The lower graph shows the ratio of daily average surface solar
flux to the top of atmosphere solar flux. The surface solar flux on a
horizontal plane is measured by the weather station, and the top of
atmosphere solar flux is calculated for the date and location. Sunny
days have a solar transmission near 0.7, and cloudy days have lower
values.
In September and October there was a fairly steady rise in basement
temperatures as the sun moved lower in the sky and the solar energy was
absorbed in the basement. There was a period of three cold and cloudy
days in mid-October when the average house temperature dropped below
65oF because we were not at home and closed vents prevented
heat transfer from the basement. Most of November was sunny and the
basement temperature remained above 80oF. However, while we
were away over Thanksgiving (late November) the automatic basement
window insulation system malfunctioned, causing 1.5 days of sun to be
blocked from heating the basement. This malfunction, along with the
following cold weather, caused the basement temperatures to drop into
the 70s and required the first use of the propane backup furnace.
December, January, and February were rather sunny, and it was quite warm
at the end of February. Thus we used the backup furnace very little in
the winter. As the sun rose higher in the sky in spring, much less
solar energy was captured in the basement (due to the vertical windows),
and there was significant furnace use during cold or cloudy periods.
To improve the solar heating performance, the computer control
implements a ramping thermostat, with the desired temperature increasing
smoothly from 65oF at 8:00 am (usually) to 75oF at
4:00 pm MDT, with a decrease during the night. This makes the solar
heating much more effective because it transfers heat to the house when
the basement is warmest. The backup furnace thermostat is set to
65oF during the day and 60oF from 9:00 pm to 6:00
am. The computer operates the backup furnace within a ±1.0K
temperature range around the desired minimum temperature.
Click on the graph for a larger version.
The above graph shows the total heating degree days (HDD) accumulated
through the season and total BTUs provided by the propane furnace in our
house. These HDD are defined relative to the standard 65oF.
The furnace BTUs are calculated using the 70,000 BTU/hour furnace rating
and the time the furnace is on as measured by the computer. This season
the furnace was used for only 34 hours, consuming 26 gallons of propane.
The ratio of BTUs to heating degree days divided by the floor area of
the house (1508 square feet) is plotted in the lower panel. This ratio
is called the heating intensity index, and is a measure of how efficient
the house is.
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