Documenting my attempt to cut my energy usage in half.

Tuesday, February 14, 2012

Solar Site Survey And Collector Output Comparison


I recently conducted a solar site survey for the area by my shop where my solar air heaters are located.  This should normally be done before you build and mount your collector, but in my case, I had never heard of a solar site survey before I built my first collector.

The purpose of the solar site survey is to determine where obstacles are that will block the sun from reaching your collector and to determine if those obstacles will interfere with your collector all year or just certain times of the year.

To do a solar site survey you will need a few simple things…

  • You’ll need to know which direction solar south is
  • A sun chart
  • An elevation and azimuth gauge


Determining Which Way South Is

Solar south (or true south) differs from magnetic south depending on where on the planet you are.  For our collectors, we don’t care where magnetic north/south are, we care about how the sun tracks across the sky and where true south is in relation to the sun.

BuildItSolar has a list of several methods for determining solar south.  One that is not listed is the “I’m in the middle of nowhere but I have a stick and 2 rocks” method.  To determine true south take a mostly straight stick/rod that is about 3 feet long and stick it in the ground vertically.

Determining East-West First Mark

Stick with first marker placed.

Where the stick’s shadow stops, place a marker such as a rock, quarter, or large washer on the ground.  Then sit back, enjoy the sun and wait… 20 to 30 minutes should be fine.  After some period of time has elapsed, place a second marker on the ground where the stick’s shadow now is.

Determining East-West Second Mark
Stick with second marker placed.

With the two markers now on the ground, draw a line between the markers.  This line will run east – west.  A line drawn perpendicular to the east/west line will give you north/south.

Determining East-West Both Marks
Direction of true south determined.


Tools For The Survey

As mention above, you’ll need to obtain a sun chart and an elevation/azimuth gauge.

A sun chart shows the position of the sun (elevation and azimuth) for every minute of the day.  Sun charts are available from the University of Oregon here.  You just need to enter your zip code (or your latitude/longitude), download the pdf file, and print it out. 

The elevation/azimuth gauge can be downloaded from here courtesy of BuildItSolar.  You’ll need to print out 2 copies.  One will be used to make the azimuth gauge, the other will make the elevation gauge.

Once they are printed out, make the azimuth gauge first.  To do this, attach the print out to a piece of cardboard (I used some adhesive spray), and then attach a small wooden pointer to it with a nail through the center point of the reference circle.  Then take the gauge and place it on a flat surface and align it so that it is pointing south.  When you’re done it should look something like this:

Azimuth Gauge
Azimuth Gauge

The elevation gauge will also need to be attached to a piece of cardboard.  Once that is done, trim the paper and cardboard along the “site line” that is marked on the paper.  Next you’ll attach a piece of string with a small weight attached to the end (like a washer) to a nail that is pushed through the center of the reference circle.  When you’re done it should look something like this:

Elevation Gauge


Conducting The Survey

To start the survey, make sure the south marking on the azimuth gauge is inline with solar south and then move the pointer on the azimuth gauge so that it is pointing east.  As you move the pointer from east around to the west, note the position of any obstacles such as trees, buildings, mountains, etc.  When you find an object, take the elevation gauge and site along the “site line” to the top of the object.  Note the angle that the string is at and place a pencil mark on your sun chart for the corresponding azimuth and elevation angles. After you’ve worked your way around to the west, you’ll need to draw a horizon line on your sun chart by connecting the dots.  When you’re done it will look something like this:

Shop South Wall Solar Site Survey
Site Survey Sun Chart Showing Obstructions

Comparing The Survey to Collector Data

From the survey sun chart you can see that I have a large obstruction due south (at the 180 degree mark) of my collector location.  I have another tree 40 degrees west of that, and by the time the sun is 60 degrees west of due south, it’s behind the tree line and done for the day as far as generating useable heat.  If we look at the line for Jan 21 and where it intersects with the horizon line, I should start getting sun on the collectors from 9am – Noon, from 1pm – 3pm, and a little between 3pm – 4pm.  When we compare this to the graph of the temperatures from the collector we see that the data matches up pretty close.  The collector starts warming up a little after 9am and runs until roughly noon.  It then comes on again about 1:30pm and stays on until 3pm. 

Temperature Graph From Solar Collector

One nice thing about the site survey is that you can look at the sun chart and determine how long certain objects will affect the collector performance.  The sun chart shows that starting in mid-Feb, the sun will clear the top of the large tree to the south of the collectors and that the collectors will get full sun from 8am - 2:30pm and from 3pm – 4pm. 

Monday, February 6, 2012

Solar Air Heater #2

I recently completed the build of a screen based solar air heater that is now attached to my shop next to the soda can based solar air heater that I built last year and detailed here.  It is based on designs from BuildItSolar and the Yahoo Simply Solar group. 


Screen and Soda Can Heaters



  • Frame:
    • Constructed from ¾" x 4 ⅝" oak. This size was selected to match the finished depth of the can based collector that it is mounted next to.
    • Outside dimensions of the box are 96" x 49"
    • The back of the box is a piece of ¼" plywood.
    • The outside of the box was painted with exterior latex paint that matches the paint on the building.
    • There is a cap on top of the collector box that is sloped to shed water.
    • There are trim strips on the sides to close off the gaps between the box and the siding so that the local wasp population can't build nests behind the collector.
  • Insulation
    • The back and sides are insulated with ½" R3 polyisocyanurate. ½" is the thickest any of the local home centers in this area carry.
  • Screen:
    • Two layers of 18 x 16 x 0.011 mesh aluminum insect screen spaced 1 ¼" apart and painted black with high temp grill paint.
    • The screen is mounted on a ¾" wide by 1 ¼" deep frame that fits just inside the insulated sides of the box.
    • The wood for the screen frame was not perfectly straight and required that a horizontal spreader be placed across the screen frame at the mid-point to keep the frame from bowing in.
    • The spreader is ¾" x ¾" and will block some of the vertical flow of air that is between the 2 layers of screen. This might possibly force some of the air back through the screen. Is this good or bad? After completion I realized I could have used a much smaller spreader, maybe 3/8 dowel.
    • The screen is mounted parallel with the glazing. It is not tipped outwards from the bottom to the top as some collectors I have seen.
    • The screen frame is held up off of the back insulation with 4 small wood blocks that are sized to center the screen in the box between the back insulation and the glazing.
  • Glazing
    • The glazing is 2 pieces of Suntuf from Home Depot.
    • There is a vertical support bar in the center of the box where the 2 pieces of glazing overlap. At first I tried just sealing the 2 pieces together with silicone but this failed to hold up while moving of the glazing panels prior to installing them and I realized it wouldn't hold up during expansion/contraction of the glazing panels either. The glazing is now screwed to the support bar to hold the 2 pieces together. There is a bead of silicone run between the 2 pieces to help with sealing.
    • The glazing is currently not sealed to the collector frame. It is screwed to the frame at every valley location on the wiggle strips. It is also screwed to the stiles of the collector box about every 15 inches.  This was done so that the glazing could easily be removed if I decide to add a third layer of screen.
  • Fan and Ducting:
    • The air inlet and outlet ports are 4" round duct. They are centered in the box horizontally and are offset from the top and bottom by 2".
    • The fan is a 125 cfm fan. General guidelines from the Yahoo Simply Solar email group suggests 3 cfm per sqft of collector.
    • This is the same model fan used in the soda can based collector.
    • The fan is mounted at the lower duct location and pushes air through the collector instead of being mounted at the top and pulling air through the collector.
    • The temperature control is provided by a 90/110 snap switch. It is mounted to the back right next to the outlet port.


Thoughts After Construction

  • Twin-wall polycarbonate looks much better.  However, 
    • The 2 pieces of Suntuf cost $42.
    • The closest place I can get twin-wall is 100 miles round trip and is $90 for a 4' x 12' sheet. 15 mpg in the truck and $3.15 for gas adds another $21 to the twin-wall price.
  • I was surprised that with 2 layers of screen how easy it is to see the insulation on the back of the collector.  Based on Gary Reysa’s tests on BuildItSolar it appears that 2 layers of screen will absorb approximately 52% of the incoming light.  This collector has a reflective back so the screens will get a chance to absorb some light on its way back out for a total of about 80% absorption.  While the collector seems to be performing well, I’m not pleased with having possibly 20% of the light reflected back out of the collector.  I need to investigate adding a third layer of screen or painting the back of the collector black.