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Shadow on the solar panel – harmful effect

Intro

The effect of shadowing on the lowering of radiation and, consequently, production, is often overlooked in calculations of the output of solar power plants. Because shadowing can result in significant losses for a solar energy system, impact modeling is crucial for evaluating production.

 

Solar cells and panels connecting , local shade and bypass diodes

In order to obtain the highest possible output power, solar cells are connected in series, parallel, and in combination. Regularly connected cells increase the output voltage. The parallel connection of the cells increases the current’s
strength. It is very important that all connected cells in the panel have the same characteristics. Similarly, it is important that all connected solar panels be of the same characteristics, i.e., of the same manufacturer and type. A problem with solar cells (of the same characteristics) in a panel can arise if the radiation on one or more panels is less or if the solar cell is in the shade. Then, due to the very nature of the non-linear characteristics of the current-voltage (IV diagram) in the conditions of the output short circuit ending, more current will flow through the shaded solar cell than it can generate. Hence, it becomes inversely polarized and no longer works in current-generation mode but instead behaves as a consumer. The entire energy generated by the illuminated cell is spent on the shaded one. Due to the dissipation of energy on it, it
heats up and becomes a hot spot, which leads to its damage.

This problem is solved by installing bypass diodes in series on each parallel-connected cell.

The Bypass diodes  are inversely polarized in the normal operating mode of the cell. When a solar cell enters the shadow or its characteristics weaken , the bypass diode becomes conductive due to the appearance of an inverse voltage at its ends, and the current of the series connection flows through it. This avoided the occurrence of power dissipation in the shaded solar cell  that generates other solar cells, and minimal power loss was achieved.

 

Hot spot and bypass diodes

Harmful influence od shadow from surrounding objects

IIn order to consider the influence of shadows created by surrounding objects, it is necessary to define a reference
point in relation to which the azimuth angle will be determined, as well as the angle at which the object’s shadow falls. At the
reference point, we set the coordinate origin. Then you need to measure the shortest distance from the obstacle, for example, a tree, to the nearest part (corner) of the solar panel. This distance is marked in the picture with a “d.” There are two possibilities. 

a) The solar panel is on the roof of the house (building), so the height of the wall (h1) seems geometrically favorable for avoiding the shadow.

 b) The solar panel is fixed to the ground, so the probability of touching the shadow is higher than in the case of (a). Impediments Obstacles are usually trees and similar vegetation, as well as surrounding buildings. The lucky thing about
trees is that they do not block radiation completely; part of the direct solar radiation can pass through the trees. Therefore, the transparency of the surrounding buildings should also be taken into account. The transmittance marked with the Greek letter describes what percentage of direct solar radiation passes through the object—trees. By examining the permeability of deciduous trees, it was found that the value of this coefficient in the case when the leaves have fallen is about 65%, while in the case when the tree is completely in leaf, it is.22%. In the case of irregularly shaped obstacles, it is possible to make a geometric approximation to the closest possible shape. So, for example, a coniferous tree can be approximated to a cylinder, a smaller building to a cuboid, etc..

 

Shadow length by hours
Shadow length is changing by day
Shadow size change

Self Shading effect

As I have mentioned several times, photo voltaic systems are often installed on the ground or on roofs (flat or at an
angle). Brackets are used for mounting photo voltaic energy systems on the ground. Solar panels are placed at a certain angle in order to obtain the greatest possible radiation throughout the year.
Installation of solar modules horizontally due to poor use of solar energy and problems caused by the deposition of dirt on the surface of the panels, for example, by the deposition of pollutants in the air, bird droppings, or in some other way. Rain or snow can clean an inclined surface more easily than a horizontal surface. Yes, the rule is that the smaller the angle of inclination, the less effect rain and snow have on cleaning. Let me remind you that the smaller angles of the panel installation are in the warm climate parts of the Northern Hemisphere; for example, when installing the panel in the deserts  of North America or North Africa, problems of sand contamination may occur. In central European, Asian, and American climates, average fouling losses in the range of 2–10% can be expected for surfaces inclined at an angle of 30°, in case they are never cleaned by hand. We can roughly say that the tilt of the panel is equal to the latitude of the place. 

 

Shadow equation

The main disadvantage of a PV system installed on the ground compared to one installed on the roof of the house is that the former has a self-shading effect. This is a situation where the panels cast a shadow on each other. By optimizing the distance between the rows of panels, losses caused by the mentioned effect can be reduced. The optimal distance and angle of inclination can be determined as follows: 

Solar panel on the roof and shadows
Self shading representation and mathematical model

On the  roof

S – shadow length

β– Altitude angle – depends on of date, time and Longitude

h – The house wall height

h1 – Tree height

 

On the ground

S – shadow length

Σ -– PV module tilt

β– Altitude angle – depends on of date, time and Longitude

H – PV module length

h – PV modeule height

h1 – Tree height

 

Conclusion

Surfaces that are not exposed to the shadows of surrounding objects are hard to find in reality. Shading can cause
large losses in the production of a photo voltaic system. Apart from losses in production, the negative effect of shading is the
generation of relatively high power dissipation on the shaded cell, which creates hot spots and degrades the system. It can be concluded that consideration of these influences is important when evaluating the electricity production of a photo voltaic system because losses in radiation, and therefore in production, are significant. For approximate estimates, you can use the solar panel angle calculator to calculate the harmful effect of the shadow.

 

Miroslav Mitić

Mechanical engineer and programmer for 25 years. Solar energy expert

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