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Issue #211 February 2008
Intelligent Energy Solutions
Solar-Powering the Circuit Cellar
Part 3: Wiring & Electronics
by Steve Ciarcia
Start | Back to the Solar Panels | Wiring and Inverters | It's All About Architecture | Maximum Power Point Tracking | So, Does It Work? | Conservation | Soaking Up Some Photons | Sources & PDF
BACK TO THE SOLAR PANELS
I’ve been beating around the bush for two articles, but now we have the ingredients in place to connect the arrays to the inverters and generate some power. While some thought had to be given to sizing all the system components before starting the project, this is where the interrelationship among solar radiance, solar panel I-V, wiring losses, and inverter dynamics all come into play to greatly affect system costs and performance. The same wattage solar system can be cheaply configured on the hairy edge of acceptable tolerance as one that is significantly more expensive, and, “electrically speaking,” bulletproof. I’d like to think my system is the latter.
In order to illustrate what I mean, I have to go back and talk about the effects of shading and temperature on solar panels, how much power we lose in all that wire, the conversion efficiencies and input tolerances of the inverter, and maximum power point tracking (MPPT).
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| Figure 1—This is an I-V curve for the SunPower SPR-210 solar panel showing the maximum power point. |
Figure 1 is the 25°C I-V curve of the SPR-210 solar panel used in my system. (The SPR-205 is essentially the same but 5 W less.) There are four factors that determine solar panel output: photovoltaic cell efficiency, load resistance, intensity of the sun, and cell temperature.
The first two are very straightforward. Solar cell efficiency is determined by the manufacturing process, and for the SPR-210, it is quoted as 16.9%. The load resistance sets the operating point on the current and voltage (I-V) curve and is controlled by the inverter MPPT algorithm (discussed later).
Solar intensity is a bit more nebulous. The current generated by the solar panel is directly proportional to the intensity of the sun and is pretty much independent of temperature. After a five-minute Internet search, you’ll have more charts, maps, and diagrams of local area sun intensity than you’ll know what to do with. The one unquantifiable anomaly affecting irradiance is edge-of-cloud effect and reflection (typically snow). When the sun enters or exits a cloud, there is at times an increase in solar intensity caused by reflection from nearby clouds that causes a sudden increase in current output. In the short time my system has been running, I have personally seen 2,000-W power spikes as clouds pass (i.e., a jump from 7,500 to 9,500 W).
The final solar cell issue is temperature. As the sun’s brightness increases, the output voltage and power decrease as the temperature of the cell increases. The STC power spec is based on a PV cell temperature of 25°C and the SunPower A-300 cells used in the SPR-210 will decrease 0.38% per degree centigrade. Especially in the summer, cell temperatures can typically run anywhere from 30° to 35°C above ambient when the module is exposed to “full” sunlight. So, if the cell temp is 35°C above a 30°C (86°F) ambient, cell temperature will be 65°C (149°F). That’s 40°C above the 25°C spec, and the effect could be 40°C × –0.38%/°C = –15.2% loss in power.
Of course, the opposite should be true as well, especially in New England. It’s quite possible that we can have days when the panels are operating at –18°C (about 0°F) and we should see 43°C × 0.38%/°C = 16.3% power increase. In general, there isn’t a lot to be concerned about low temperature increases as long as it doesn’t exceed the inverter’s voltage input range (it doesn’t). Basically, take the benefits and run.
For typical operation, solar arrays should be mounted in the sunniest place and kept as cool as possible. (This is specifically why my roof panels are mounted on high standoffs to allow air circulation under them.) No part of a PV array can be shaded. Large solar panels consist of many PV cells in series, and therefore, all of the cells in the same string are conducting the same current. If any of the PV cells are shaded, they cannot produce current and will become reverse biased. This means the shaded cells will dissipate power as heat, and over a period of time, failure can occur. Because it is impossible to prevent occasional shading, solar panels incorporate bypass diodes to conduct current around shaded strings. (I read someplace that while the A-300 cells don’t necessarily need diodes for cell protection, diodes were included in the panels to provide comfort to customers used to diodes being present in virtually every other brand of panels.) ;-)
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