EET

Squeezing more power from shaded and soiled solar panels

Picture an array of brand new solar panels not shaded by trees or clouds. All of them are at the same temperature and unbesmirched by accumulated dust.

A solar array in this happy state will generate electricity at its optimum efficiency. But spots of shade on the array, a layer of dirt, and even different temperatures on different parts of the arry will cause problems that affect the array output. The difficulty stems from the practice of feeding the output of all panels into one central inverter. Shade or dirt on a panel can lower its output, as can a temperature difference. But the central inverter cannot compensate for one or more panels with low output. Modern inverters with their algorithms for tracking maximum power points cannot optimize an individual panel's output.

To solve this problem, engineers at National Semiconductor Inc., Santa Clara, Calif., (www.national.com) developed SolarMagic, which is essentially an intelligent inverter that is installed on each panel in the array.. With this addition, arrays can recoup up to 57% of the energy lost to shade and other panel mismatches, including panels with different shapes. National engineers are a bit cagey about exactly how SolarMagic accomplishes its “Magic.” But in essence, the device implements power point tracking for each panel to deliver a maximum output.

ORDINARY SOLAR CELL ARRAY

The left image shows the schematic of a typical solar array unaffected by shade. Under uniform conditions, each panel operates at its optimal current and voltage and the panel generates 200 W.

The right image shows what happens when shade covers one panel in the array. In this scenario, the partially shaded panel receives 50% of full sunlight. This causes the output of the shaded panel to drop to 25 W, and the output of unshaded panels drops to 135 W. This phenomenon, where shaded panels reduce the output power of unshaded panels within the same array, is the primary reason why shading losses are disproportional to the amount of shading. In this case, output power drops by 44%.

ARRAY WITH SOLARMAGIC INSTALLED

The diagram Array with SolarMagic Installed shows what happens when shade covers a panel in an array equipped with SolarMagic Power Optimizers. The affected panel now turns out 100 W instead of 25 W, a net gain of 75 W. The panel generates more power because SolarMagic changes its output voltage and current to a new max power point of 27 V at 3.7 A for shaded conditions. The panel gets only half the sunlight it previously received and puts out only half the power. Meanwhile, unshaded panels still produce their maximum power of 200 W. The distributed max-power-point tracking of SolarMagic Power Optimizers continues to keep the unshaded panels operating at their peak power point.

In this case, the array voltage remains at 150 V, and SolarMagic Power units adjust their output voltage and current to maximize the power of each panel in the string. In effect, it boosts up output voltages for unshaded panels and bucks down those of the shaded panels.

NATIONAL SEMICONDUCTOR CORP.

2900 Semiconductor Drive

POB 58090

Santa Clara, CA 95052-8090

(408) 721-5000

www.national.com

FIELD TRIAL RESULTS: SOLARMAGIC RECOUPES UP TO 66%OF THE POWER LOST DUE TO SHADE ON SOLAR CELLS

Time % of array in shade Power loss due to shade % of lost power recouped by SolarMagic
9:30 am 13% 44% 50%
10:30 am 11% 47% 58%
11:30 am 9% 54% 66%
12:30 am 6.5% 44% 65%
2:20 pm 3% 25% 40%
Resources: National Semiconductors Inc., www.national.com
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