Solar Pathfinder

I borrowed a Solar Pathfinder from GCC yesterday and used it to analyze the solar access and gain for the site (once the trees are cut).

How the Pathfinder works:

The device is quite simple. (pictures below) A semi reflective/semi transparent dome that sits above a sun path chart for our latitude. The dome reflects the entire horizon and sky and effectively superimposes it on the sun path chart. You can than see what trees and buildings obstruct the sun throughout the entire year. One can do out the calculations by hand, it is easier and quicker to take a digital image looking down on the Pathfinder and load it into the associated software.

Setting up the Pathfinder:

Orient the Pathfinder so that the it is level and pointing towards magnetic South using the bubble level and compass that are on it. Make sure there is no magnetic metals near the pathfinder including nails and belt buckles. The tricky part is correctly rotating the sunpath chart so that it is facing the solar south and not magnetic south. The amount and direction of rotation depends on where you are on the planet. For Western Massachusetts the difference between magnetic south and solar south (declination) is about 14.5 degrees West of North. It is somewhat confusing to explain without pictures which direction to rotate; suffice to say one rotates the chart clockwise. If one is in the western part of the country one would be rotating the chart a certain number of degrees counter clockwise. If you are in the correct location in Mississippi river valley, there is no declination, magnetic south and solar south align.

Links:

Declination Calculator http://www.ngdc.noaa.gov/geomagmodels/Declination.jsp

Declination World Map http://www.ngdc.noaa.gov/geomag/WMM/data/wmm-D05.pdf

Results:

As you can see (at least for the center of the lot) the solar gain is fantastic once the trees are gone. All of the trees will be cut except for on large silver maple on the west side of the lot. Incredibly, this silver maple is situated so that it won’t impinge on the winter sun and will block the summer afternoon sun from the west (which is typically the most problematic from the point of view of overheating). What luck!

I need take more readings with the Pathfinder from different locations so I can get a better idea of what the house will experience. Also, I need to recalculate the kWh/m^2/day with a 64 degree tilt. Sixty four degrees is the optimum angle to maximize winter gain.

Picture of me with the pathfinder. You can see the large silver maple in the back left.

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