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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
THE SOLAR PANELS ARE THE SYSTEM
As I mentioned previously, I have a limited area where I can mount solar panels. As such, the efficiency of the panels is an important consideration. The more efficient a solar panel is, the less real estate it takes up to produce the same wattage.
One of the reasons I chose Sunlight Solar Energy as my PV contractor was that they are a SunPower panel distributor. I think the SunPower panels are among the best and most efficient available. There is a variety of technologies used to construct solar panels. These use monocrystalline silicon solar cells covered by tempered glass with 16.9% overall conversion efficiency. (SunPower quotes 21.5% for individual A-300 cells.)
I chose the SunPower panels with the A-300 solar cells because they seem to be the most efficient solar cells for residential use. They get their efficiency multiple ways. For example, traditional solar cells use soldered ribbon to connect the front of a cell to the back of the adjacent cell. The connection ribbons on the front reduce the effective solar collection area. In contrast, the A-300 cells have all of their connections on the back of the cell and don’t sacrifice front collection area. They also eliminate the front-to-back bend between cells, which is often the location of weather-related expansion/contraction interconnect failures in other panels. Finally, because the A-300 cell is formulated from a single crystal, it has a high solar conversion efficiency. It can also respond to a broad spectrum of light (330 to 1,170 nm).
Of course, there is a lot more to the physics of solar cells, but please forgive me for avoiding discussions about quantum mechanics, Planck’s constant, and bad gap energies instead of just digging holes and pouring concrete. Gladly, we have the benefit of using the A-300 and not having to invent them. All solar cells have specific current/voltage (I/V) and temperature-response characteristics that greatly affect downstream component choices such as wiring and inverters. We’ll discuss specifics later as necessary. For now, let me just say that the idea behind using off-the-shelf solar panels is that someone has already done the heavy lifting.
SunPower manufactures various finished panels for residential installations based on the A-300 cell. These panels have an aluminum support chassis with a substrate that holds a laminated sandwich of the solar cells between two layers of ethylene-vinyl-acetate (EVA), an optically clear material similar to hot glue. This sandwich is in turn laminated to a 4-mm thick sheet of glass covered with an antireflective coating with a total level of integrity that allows SunPower to claim a 25-year panel lifetime.
My installation uses two different panel models: SPR-205 and SPR-210 (see Figure 3). The SPR-205 is rated at 205 W, while the other is 210 W per panel. The panels have a 12 × 6 grid of solar cells wired in series. The output voltage is 0.56 V per cell and the 72 series-connected solar cells produce an output of 40 V at a rated current of 5.13 A (SPR-205) and 5.25 A (SPR-210), respectively. The reason for choosing different types was aesthetics not power. The SPR-210 has a lot of white color on the panel and, pardon me, but it looks like a solar panel. This was fine for a roof-mounted system I wasn’t planning on staring at all the time. However, since I get to look at 400 square feet of pole-mounted panels every time I sit on the solarium deck, I selected the black-coated SPR-205 panels for a more pleasing view.
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| Figure 3a—The SPR-210-WHT solar panels are used on the solarium roof. b—The SPR-205-BLK solar panels are used on the pole mounts. |
Figure 4 is the spec sheet for the SPR-210-WHT solar panel used on the solarium roof. Both models are essentially the same, but using 32 SPR-205 panels results in 160 fewer total system watts than all 210s.
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| Figure 4—These are the specifications for the SPR-210-WHT. | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
One of the technical details that many people don’t realize is that a grid-tied PV system is very high voltage compared to the 24 to 48 V typically employed when batteries are involved. The maximum capacity of parallel-connected battery-backed PV systems is often limited as a sheer consequence of the high cost of conducting the massive currents involved. The benefit of a grid-tied system is that it uses series-connected panels to create higher voltages at much lower currents. For example, if my 10-kW system were configured with the 48-VDC input inverters typical of battery-backed systems, the current would be 229 A! Instead, grid-tied inverters such as those chosen for my system have a 195 to 600 VDC input range. The solar panels are always wired in series or series-parallel combinations so that their combined output fits the “sweet spot” of the inverter efficiency and solar panel I-V curves.
In my particular case, the roof array is wired as two parallel sets of 10 panels in series (400 VDC at 10.5 A), and each pole mount is two parallel sets of eight panels in series (320 VDC at 10.26 A). When we are talking about PV wiring losses, it is Power = (I2)(R), where R is the wire resistance. If we double the voltage, we reduce the current by half and the power lost by four. For a very long wire run, this can add up to real savings very quickly!
The numbers are absurd, but if we compare a 10-kW system running at 320 VDC to the same 10-kW system at 48 VDC, there is a 44× difference in power losses due to wiring resistance. Now you understand why utility long transmission power lines are over a half million volts, and perhaps you can also see why it isn’t a smart idea to stick your fingers in the electrical box of a grid-tied PV system. That’s one of the reasons why there are so many disconnects and fuse points in the typical system.
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| Photo 3—Because the efficiency of solar panels also depends on temperature, rather than flush-mounting the roof panels, the UniRac mounting rails were mounted on raised-flange flat-top standoffs with no-caulk flashing. That added about 6² of ventilation space under the panels. |
Solar installers like roof-mounting panels because they are relatively quick and easy to install compared to a ground or pole-mounted system. It took only a couple of days for Sunlight Solar Energy to mount the 20 panels on the solarium roof. Since good engineering was more important to me than aesthetics, I chose not to flush-mount the panels to the roof like most people. Instead, because I am aware of the negative effects that heat has on solar panel efficiency, I wanted some ventilation space beneath the roof-mounted panels (see Photo 3). Sunlight Solar Energy’s solution was to put the entire UniRac mounting system on raised-flange flat-top standoffs with no-caulk flashing. Photos 4, 5, and 6 detail the installation process.
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| Photo 4—After the UniRac mounting rails are installed, it’s time for the panels. |
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| Photo 5—Moises Rivera and Alex Fox attach the first SunPower SPR-210 panel to the UniRac mounting rails on the roof. |
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| Photo 6—The Sunlight Solar Energy team moves into high gear installing panels on the roof. |
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