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Posted on 12-02-2016

SHADING ON SOLAR PANELS

by Shen Ge

Introduction

Is your house suited for solar? Previously, we have talked about the effects of orientation of the roofs(east, west, south, or north) and the roof tilt on the amount of solar energy your home can generate but we have yet to touch on the important issue of shading on solar energy production. Unlike solar thermal panels used in hot water heating that can tolerate some shading, PV solar panels are very sensitive to shading. Partial shading of even one cell on a 36-cell solar panel, will reduce its power output. Because solar cells are connected in series (electrical term for a line like knots on a string), the weakest cell will bring the others down to its reduced power level. Therefore, whether 1/2 of one cell is shaded, or 1/2 a row of cells is shaded, the power decrease will be the same and proportional to the percentage of area shaded.

The video below demonstrates the strong dependence of solar on shading.

Soft Shading

Shading obstructions can be from “soft” or “hard” sources.  If a tree branch, roof vent, chimney or other item is shading from a distance, the shadow is diffuse or dispersed. These soft sources significantly reduce the amount of light reaching a solar panel’s cells. However, regardless of the irradiance (light) amount, as long as there is sufficient light (~50W/m2) the voltage remained the same. The voltage of a PV cell depends more on temperature and the electron band-gap in the materials than on the light itself.

Hard Shading

Hard sources are defined as those that stop light from reaching solar cells, such as a blanket, tree branch, bird dropping, etc. sitting directly on top of the glass. If even one full cell is hard shaded, the voltage of that module will drop to half of its unshaded value in order to protect itself. If enough cells are hard shaded, the module will not convert any energy and will, in fact, become a tiny drain of energy on the entire system.

More technically, if there’s not a completely illuminated path between electrodes or the entire cell is shaded, then no current will flow through the cell, and its voltage will drop to zero.  Most modern solar cells have silver wires imprinted across them to allow charge to be carried more easily across the silicon, similar to a highway which allows more rapid travel through a densely packed city. As long as these silvered cells have any light exposed to them, they will generate power. One of the only companies that do not add silver to their cells is Sunpower, since their solar cells are made with a different process to collect more light. The trade-off is more wattage per module in exchange for letting even tiny amounts of shade devastate production.

How We Design

We want the most efficient design for you. None of our designs would ever have panels under shading for your home roof. This is why we use sophisticated satellite imagery to determine where not to put the panels. A site survey conducted either by you via our platform or via an in-person team also helps us greatly in the design.

Interested in more of Shen Ge’s posts on solar energy and other energy-related posts? Follow the author on Twitter @shenge86.

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