Solar Path Finder

   Trees are just a fact of life here in New England.  If you don't keep mowing your lawn, the trees will completely reclaim the area in about 10 years.  After a while, the trees will be 40 or 50 feet tall and casting wonderfully large shadows to keep you cool in the summer sun.

   Wonderful, that is, as long as you are not interested in collecting solar power.  Putting solar PV (electric) panels on my house necessitated turning this beautiful tree...

 

into this less than beautiful trunk.

    This time around, I was trying not to lose any more trees.  On the other hand, I do need good "solar coverage."

     Solar coverage is the percentage of the time that the site of interest has unobstructed exposure to the sun.  To measure this, there is a clever little device called the "Solar Path Finder" which can be rented or purchased.   The device costs about $300, but I rented my from Real Goods for $50.  They don't seem to advertise the rental, but if you call them they seem happy to rent.  Later on, I found one on ebay for $89, so I bought it.  If you are in the Boston area, perhaps I can help you out with a loan or site visit.

     The Solar Pathfinder comes with set of (optional) legs to support it on flat ground.  The device must be leveled, using the integrated bubble level, then it must be turned to face due north with the integrated compass.  There is also an adjustment for magnetic declination to adjust for the difference between true north and magnetic north.

      So if you are like most people, you are probably thinking that you put the solar pathfinder out in the sun for a day and it record the shadows.  But that is not how it works at all.  In fact, given that the angle of the sun changes month by month, you would have to record data for a full year using such a method.  The solar pathfinder does not have to see the sun to work.

      In fact, it is best to use the pathfinder on a cloudy day!  Yes that is right, you don't actually want the sun for this measurement.  The top of the solar pathfinder is covered with a partially reflective clear plastic dome.  If you look down at this dome you can see a hemispherical image showing all of the sky in one image.  This shows if any trees or structures are shading the area.

  So that all by itself is pretty clever, but there is one more piece of cleverness that makes this all come together.  Underneath the plastic dome is a piece of paper with two series of lines.  One series of lines represents the time in hours, as can be seen in the closeup image below showing 10 am, 11 am etc..  The other series of lines represents the months, which can also be seen in the image marked by the months June, July, May, etc.

    
   If a photograph is taken from directly above the solar pathfinder, it will show a reflection of the shading trees overlaid on the template showing the months and the hours as shown in the enhanced picture below for my preferred site.

 

The image clearly shows the location is far from ideal.  On the left side of the image, you can see that for most months, the sun starts to reach the site at about 9:00AM, which is great.  On the right side, you can see that sun continues until about 3:00 or 3:30 pm until shaded by an maple tree on the west side of my property.   At the top of the image, the real problem can be seen which is the winter months.  For these months there is almost no solar coverage.  

     In the closeup of the solar pathfinder chart, there are little numbers like 5,6,7 which are the percentage of time allocated for that 1/2 hour period.  For each month line, these number add up to 100%.  The 1/2 hour segments in the morning and evening are worth a lower percentage because the sun intensity is lower.  These number can be used to calculate the solar coverage for each month of the year.  My numbers are below.

January	22%
February	49%
March	71%
April	84%
May	91%
June	87%
July	91%
August	87%
September	80%
October	55%
November	36%
December	0%

The good news is that from April to August, the site will get more than 80% of the available sunlight.  The bad news in that in December, the site will not get any sunlight.  However, sunlight is a greatly diminished resource in the winter anyway.  Even with 100% solar coverage in the winter, the amount of energy is significantly less than in the summer due to the shortened day.

Despite the shortcomings, the chosen site on the ground will be used.

Next time, the location will be prepared to being the installation process.