Passive Solar Basement Efficiency

The graphs below summarize the results for the 2009-2010 heating season.

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The graph above shows the observed daily average temperatures for the 2009-2010 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.

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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 41 hours, consuming about 31 gallons of propane. In January 2010 we installed solar reflecting panels in front of the basement windows to increase the solar energy entering the basement during the spring when the solar elevation angle is increasing rapidly. The solar reflectors with adjustable tilt angle worked well, keeping the April average basement temperature 8 to 10 degrees warmer than previous years. As a result, the backup furnace was almost unused after March 1. Around May 22 we lowered the solar reflectors tilt angle from 28o to below 0o, which cooled the basement rapidly for the end of the heating season.

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.