Passive Solar Basement Efficiency
We use a computer to monitor the temperature of the house and basement
and ingest data from a
weather station . The computer also controls the
solar-basement-to-house fan and the backup propane furnace. This
detailed monitoring allows us to measure the backup furnace use and
estimate the solar performance of the system. The graphs below
summarize the results for the 2008-2009 heating season. Results for the
2005-2006 season , for the 2006-2007 heating season , for the 2007-2008 heating season , for the 2008-2009 heating season are also
available.
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.
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 MST, 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 56 hours, consuming about 43 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 energy intensity index, and is a measure of how efficient
the house is.
Home Energy Magazine has a good introductory article explaining this
benchmark. A table in that article indicates that the median single
family home energy intensity is about 10 BTU/(HDD ft2).
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