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Issue #209 December 2007
INTELLIGENT ENERGY SOLUTIONS
Solar-Powering the Circuit Cellar
Part 1: Preparing the Site
by Steve Ciarcia
Start | Getting Started | So, How Do I Describe this Project? | Location, Location, Location | The Solar Panels Are the System | What's Next | Sources & PDF
LOCATION, LOCATION, LOCATION
Photo 1 shows the roof-mounted solar panels that produce approximately 40% of the total PV power. Figure 2 is a block diagram of my overall solar installation and the elements to be discussed (neglecting for the moment some of the electrical disconnects and fuses).
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| Figure 2—This is a block diagram of my overall solar installation (neglecting for the moment some of the electrical disconnects and fuses). |
I know it sounds like a joke that the first PV system consideration is walking around the house and looking for the sun, but you can’t generate much energy if your panels are always shaded. When you live in the middle of the woods, finding the sun is often easier said than done.
Array orientation determines how much energy you can produce. Solar panels are typically aimed due south at a specific tilt angle that optimizes the incidence angle of the sunlight striking the panel. Maximum energy is produced when this tilt angle is equal to the latitude of the location (reduced by a location correction factor). Typically, the optimal tilt angle during the summer is the latitude minus 15°, and the optimal angle for the winter is the latitude plus 15°. Hartford, CT, is located at 42° latitude and the optimum tilt angle (minus an 8° correction factor) ranges from 19° in the summer (34° – 15°) to 49° in the winter (34° + 15°). The Connecticut rebate program suggests that if a fixed tilt is used, it be set at 35°. Of course, these are computer-generated optimizations that don’t necessarily accommodate real-world conditions. While it requires some nontrivial computer calculations to show authenticity, it is my understanding that as long as the non-optimal differences in azimuth and tilt are less than 20°, the loss in maximum power production is typically only about 5%. It is exactly for that reason that the most cost-effective PV installation is typically a fixed-pitch roof-mounted array.
My system includes both variable and fixed-pitch arrays. The roof-mounted panels are located on the solarium roof and oriented at a fixed pitch of 17.5° facing SSW (see Photo 1). According to Sunlight Solar Energy’s calculations, efficiency is still about 92% of the desired maximum because the 17.5° roof angle actually allows higher efficiency during longer summer hours even though it isn’t the optimum tilt for winter.
Pole-mounted arrays are more efficient than a fixed-pitch roof array by design. My configuration is single-axis adjustable. The pole-mounted arrays are oriented due south and enable seasonal adjustment in the tilt angle to optimize the incidence angle of the sun. For everyone ready to e-mail me asking why I didn’t put in a tracking solar array since this is a pole mount, let me just say that you can also send me financial contributions for doing it via the magazine.
Seriously, even though I have a large property area, the house sits below a ridgeline of trees, and short of clear-cutting half of the county (and some of the neighbors’ trees too), the only place with a due south view is a small area at the solarium end of the house (the same reason I put the greenhouse there years ago). A 52-panel solar system is not small. To accommodate almost 700 square feet of collectors in that area, I had to put 20 panels on the solarium roof and 32 panels on two poles. Besides questioning the expensive electromechanics necessary for tracking that adds only a few percent more energy, to my knowledge, I haven’t seen residential pole-mounted tracking systems that accommodate more than eight to 10 panels. The four poles necessary to mount 32 panels in a tracking arrangement would have been too high and just plain wouldn’t fit in the available area. Certainly, there are some well-designed solar tracking installations among the Circuit Cellar readership, so I invite you to fill in the knowledge gaps with your own Circuit Cellar article. Who knows, I still have to do something with the annex later on, too.
When you find that you have shade over the PV panels, there is only one solution: a chainsaw. Typically left to the homeowner to solve in PV installation contracts, tree removal can add thousands of dollars to the total installation cost of a PV system. My location was certainly no exception. Four tree-removal people spent a week cutting, chopping, hauling, and chipping the area around my house in search of due south. As you can see in Photo 2, it was not an insignificant task getting at some of these trees and a lot of heavy equipment was involved.
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| Photo 2a—It was not an insignificant task getting the trees out of the way and lots of heavy equipment was involved. b—The easiest way to remove some of the smaller trees was to grind them into mulch. |
I had to remove a considerable number of trees, but they mainly affected solar panel view and not house shading. Because I pretty much keep the really big trees away from the house as a hurricane measure, the trees that were removed this time were farther away from the house and didn’t drastically increase my air conditioning load from reduced shading. In fact, the uniqueness of my solarium may have actually benefited from a little more spring and fall sunlight.
The present exercise is all about adding active solar electricity to my house, but the 800-square-foot solarium under the roof-mounted solar array in Photo 1 is already a passive heat collector. When I built it a number of years ago, I made the floor into a large energy heatsink that supplies heat to the rest of the house. The 10² thick, 800-square-foot ceramic tile floor is one gigantic slab of concrete insulated by 6² of Styrofoam. The 100,000 lb. of concrete act like a large heatsink absorbing energy during the day. The heat is redirected at night to the adjacent living room via thermostatically controlled vent fans in the wall. With the improved solar view, it will probably be a little bit more efficient in the spring and fall when the trees used to block the rising and setting sun. Finally, in case you thought that all those trees might go to waste, please note the stainless steel wood stove chimney on the peak of the roof above the solar panels. A wood stove in the solarium also contributes energy to the cause and one of the six outbuildings (the size of a single-car garage) is used solely for firewood.
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