There are a number of means available to increase solar panel output and efficiency — some of which may be utilized by the serious experimenter. These are listed as follows:
Solar Cell Technology
There are a number of technologies being researched and there are continual advancements. Experimental technologies and highest efficiencies include:
- Multi-cell gallium arsenide – 44%
- Single cell gallium arsenide – 29%
- Crystalline silicon - 25%
- Thin film copper-indium-gallium-selenide – 20%
- Emerging PV technologies (dye-sensitive cells etc.) – 11% (low efficiency, but very inexpensive)
http://upload.wikimedia.org/wikipedia/commons/7/71/PVeff%28rev121211%29.jpg
Unfortunately, we live in the real world and the highest efficiency technologies are either unaffordable or have not been put into production. As a result, the experimenter is generally stuck with crystalline silicon technology with efficiencies ranging from 15 to 21.5% — this is what I refer to as “practical efficiency.”
Fill factor
Fill factor is simply a fancy term for utilization of available surface area. Full utilization of fill area is required to obtain highest output for a given surface area. The fill factor ranges from about 70 to 90%. You have seen solar panels that utilize round or moon shaped PV cells — well, these have a lower fill factor than square cells. This is not really that important — all it means is that panels delivering a specific power may vary in dimensions somewhat. On the other hand, if attempting to maximize the amount of solar power out of a specific area, then fill factor is an issue.
Grade A, B, C, D
When purchasing name brand solar panels, you will be getting perfect grade A cells. If purchasing garage shop solar panels, the quality of its cells is unknown. If purchasing DIY solar cells, all grades are available, but beware — it is easy to get cheated on quality. If purchasing on eBay, check feedback ratings.
- Grade A: No imperfections – output = 100% – (name brand panels)
- Grade B: Cosmetic imperfections – output > 90% – (good for DIY panels)
- Grade C: Contains chips and/or micro-cracks – output = 75 to 90% – (serious experimentation)
- Grade D: Fallout – output = 25% to 75% – (just for messing around)
http://www.instructables.com/id/Make-a-high-powered-solar-panel-from-broken-solar
Note that micro-cracks effectively reduce the fill factor so that it takes a larger surface area (more cracked cells) to obtain the same power output. Micro-cracks can also cause localized heating and roof fires — beware!
For more info, check out http://reviews.ebay.com/Solar-Cell-Grading-A-B-C-D?ugid=10000000017991249
Solar Cells Glazing
For long life, solar cells must be protected from the elements (rain, snow, hail, bird dropping etc). Polycarbonate or low-iron glass is generally recommended due to high optical transmissivity — perhaps 90%. Surface coating treatments reduce reflections for even higher transmissivity. Ordinary window glass reduces the output by about 40% — not recommended. Note that my knowledge is weak in this area.
Solar Panel Orientation
For highest output, solar panels must be perpendicular to the sun’s rays. However, it is generally practical and common for roof-top installations to follow the roof pitch and orientation. For other types of fixed installations, the azimuth is oriented to the south and tilt adjusted for the winter sun. Note that solar power is minimized in the winter mostly due to the reduced daylight period; therefore that is the default for fixed orientation. While this is clearly not optimum in the summer, the longer daytime period more than compensates for the compromised tilt angle.
Check out this solar calculator http://energyworksus.com/solar_installation_position.html
Solar tracker
Solar tracking is a great way of increasing the output power. It rotates the panel or array of panels so that they always directly face the sun. However, the larger the array, the more difficult will be the mechanics of this task. Some trackers are simply driven by a “clock” motor like a telescope so that it follows the sun (or wherever it is supposed to be in the cloudy sky). Others have active circuitry that adjusts the orientation for maximum power output. Others may be controlled by a shadow feedback signal technique. Check out this DIY Solar Tracker
The optimum tilt angle changes slowly as the earth rotates on its axis, therefore it is not generally required to track this change automatically. The easiest way to handle this seasonable variable, is to go out and manually adjust the angle every month or so — not a difficult task.
Concentrators
Solar panel output power may be increased via a light concentrator such as a Fresnel lens or mirror. Note that such a lens must be substantially larger than the panel. Also, concentrators may not be practical for a large array, and orientation of the mirror creates an additional tracking problem. Output may be increased by perhaps 50%. Care must be taken to prevent overheating the panel.
Check out this brief video
Solar Charge Controls
Since the solar panel does not put out the correct voltage to charge a battery, it must be controlled via a solar charge controller to prevent battery overcharge. The series voltage regulator control wastes the excess power either by turning off the solar panel current or by dissipating the excess power in heat — that is the function of the heatsink in such controls. electroschematics.com has a number of these controls.
MPPT Controller
MPPT stands for Maximum Power Point Tracking. The MPPT control is different in that it does not turn the excess power into heat — it turns it into additional charge current so that if the solar panel is putting out 10A, the battery may actually be charging at a higher current (perhaps 12A). The control senses both input voltage and current, and then does some math with its microcontroller and makes adjustments accordingly in order to maximize power transfer. It uses switch mode technology.
On the other hand, when the battery is fully charged, it still turns off the solar panel. I was toying with the idea of what to do with this unused power, but have not come up with a really practical use other than perhaps to heat water in a hot water tank.
Here is a popular commercial MPPT control with a great description http://www.windsun.com/ChargeControls/MPPT.htm
Glossary of undocumented words and idioms (for our ESL friends)
transmissivity – noun – ability of a medium to transmit or pass electromagnetic energy — light in this case
garage shop – noun – small scale “bare bones” manufacturing operation (literally in a garage)
bare bones – adjective – simple, limited — literally all that is left after an animal carcass decays…
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