We're using a solar site evaluation tool to help with siting some solar thermal and pv arrays. If the "fuel" is the Sun, then it makes sense to find a location to mount your panels that has little or no shade for the hours of at least 9am to 3 pm. Shading solar t
hermal is not a huge issue, if you are partially shaded you will just get a proportional decrease in thermal output. Solar pv (ie. solar electric panels a.k.a. Photovoltaic) on the other hand has a bad habit of producing very little energy at all if they are even partially shaded.
So how do you measure how often a site is shaded? Setting up the lawn chair for a year-long vigil in a location under consideration is not practical. Luckily, if you recall astronomy 101, the Sun's path in the sky is absolutely predictable for any day of the year! Traditionally, a solar site evaluation would be done with a neat tool called the Solar Pathfinder. This simple instrument can show you exactly when objects on the horizon will cast shadows on your site for any month of the year. Neat. You start with some paper templates that are specific to your latitude. T
hese templates show the suns path for every month and every (daylight) hour. You insert the template under a plastic dome and make sure the assembly is level and pointed due south. Standing directly above the pathfinder dome, you look straight down and you'll see the reflection of objects around you in the dome. Using a pencil, you trace the reflections of the objects onto the template. Seeing where the objects you've drawn on the template intersect the solar path lines on the template tells you exactly when those objects will cast a shadow on your location.
We took this one step further with a really cool instrument that uses a digital camera and custom software to replace the plastic dome and paper template. It uses a special mounting bracket that holds a camera and attaches to your tripod. It has 15 degree detent
stops that allows you to take a series of pictures from east to west in 15 degree increments. You save these pictures onto your computer and you use special software to "stitch" these pictures back together in a panorama. After you enter your latitude, the software overlays the sun's path for the entire year over the panorama. Of course, the sun takes a different path across the sky depending on the season, so what you see is a series of arcing lines across the photo. The software tries to identify when an object intersects these arcs and thus would present a shadow on your array. A red line indicates when the software thinks shadowing will be an issue. This isn't an exact science - sometimes one 
photo of the sky might be a little darker than the others, and the computer interprets the contrast change as a sun-blocking object instead of the sky. A little touch up in photo shop or some other photo-editing program will blend the sky and help the software interprets the photo correctly. What can you tell from this analysis? Quite a bit. The simplest and most important thing is when you can expect your site to have a shading issue. Take a close look at the blowup of the right side of the solar panorama photo. You can double-click the photo to open it full
size in a new window. Notice the "horizontal" lines are labelled with months of the year. This is the path that the sun will take in that particular month. Notice also that the "vertical" lines are labelled by hour. You can see that the equipment garage will present a shadow on the location that the picture was taken, at around 1:30 pm in December all the way to around 4:30 pm in June. See - you don't have to camp out in your lawn chair all year long after all! Now this isn't all this product can do - it has a nifty .csv output that has all kinds of interesting and useful information like how much energy you can expect to capture given the shading at this site, how much more energy you could expect if you had a tracking mount versus a fixed mount, or how much more energy you could expect if you cut down your neighbours tree!!
hermal is not a huge issue, if you are partially shaded you will just get a proportional decrease in thermal output. Solar pv (ie. solar electric panels a.k.a. Photovoltaic) on the other hand has a bad habit of producing very little energy at all if they are even partially shaded.So how do you measure how often a site is shaded? Setting up the lawn chair for a year-long vigil in a location under consideration is not practical. Luckily, if you recall astronomy 101, the Sun's path in the sky is absolutely predictable for any day of the year! Traditionally, a solar site evaluation would be done with a neat tool called the Solar Pathfinder. This simple instrument can show you exactly when objects on the horizon will cast shadows on your site for any month of the year. Neat. You start with some paper templates that are specific to your latitude. T
hese templates show the suns path for every month and every (daylight) hour. You insert the template under a plastic dome and make sure the assembly is level and pointed due south. Standing directly above the pathfinder dome, you look straight down and you'll see the reflection of objects around you in the dome. Using a pencil, you trace the reflections of the objects onto the template. Seeing where the objects you've drawn on the template intersect the solar path lines on the template tells you exactly when those objects will cast a shadow on your location.We took this one step further with a really cool instrument that uses a digital camera and custom software to replace the plastic dome and paper template. It uses a special mounting bracket that holds a camera and attaches to your tripod. It has 15 degree detent
stops that allows you to take a series of pictures from east to west in 15 degree increments. You save these pictures onto your computer and you use special software to "stitch" these pictures back together in a panorama. After you enter your latitude, the software overlays the sun's path for the entire year over the panorama. Of course, the sun takes a different path across the sky depending on the season, so what you see is a series of arcing lines across the photo. The software tries to identify when an object intersects these arcs and thus would present a shadow on your array. A red line indicates when the software thinks shadowing will be an issue. This isn't an exact science - sometimes one 
photo of the sky might be a little darker than the others, and the computer interprets the contrast change as a sun-blocking object instead of the sky. A little touch up in photo shop or some other photo-editing program will blend the sky and help the software interprets the photo correctly. What can you tell from this analysis? Quite a bit. The simplest and most important thing is when you can expect your site to have a shading issue. Take a close look at the blowup of the right side of the solar panorama photo. You can double-click the photo to open it full
size in a new window. Notice the "horizontal" lines are labelled with months of the year. This is the path that the sun will take in that particular month. Notice also that the "vertical" lines are labelled by hour. You can see that the equipment garage will present a shadow on the location that the picture was taken, at around 1:30 pm in December all the way to around 4:30 pm in June. See - you don't have to camp out in your lawn chair all year long after all! Now this isn't all this product can do - it has a nifty .csv output that has all kinds of interesting and useful information like how much energy you can expect to capture given the shading at this site, how much more energy you could expect if you had a tracking mount versus a fixed mount, or how much more energy you could expect if you cut down your neighbours tree!!The software creates a .csv file with a lot of great information including the potential monthly energy production of the array. While I haven't yet been able to independently verify the proposed energy production (this survey was not a "real" location) I'm assuming that the differences between fixed, 1 axis tracking and 2 axis tracking are reasonable. 954 kwh /1159 kwh/1309kwh shows a fairly significant increase in energy production with the tracking arrays. Tracking arrays can be pricey however, and it may make more sense to make a 30% larger array on a cheaper fixed mount.
1 comment:
What kind of results would one expect if a tracking (vs fixed) solar panel system was used?
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