What factors degrade the performance of the module?

Solar panels are the most important and robust component in your solar PV system and one must collect all the relevant information before buying solar panels for home. But as everything, the performance of the solar panels also degrades with time. The solar panels convert sun energy into electrical energy; the amount of sun energy converted into electrical energy defines the efficiency of the module.

It reduces with time and this phenomenon is called degradation of the solar modules. The degradation in the performance of the modules is caused by the multiple factors, which I am going to discuss with you in this post.

The degradation of the modules can be broadly classified into main categories:

• Reversible degradation
• Irreversible degradation

The reversible degradation can be rectified by you. The main reasons for this type of degradation are as follows:

• The shading of the solar panels caused by tree or the shadow of any other object
• The panel surface is covered with dust, snow or debris results in lowering of power output
• There is a loose connection between panels causing drop in the power output.

The above mentioned problems can be resolved by you and your solar module will again start giving optimum output.
The irreversible degradation is the reduction in the power output of the solar panels with time which cannot be rectified or reversed. The main reasons behind this are the changes in the temperature leading to thermal stress, humidity, wind, quality of the solar cell, mechanical stress and other manufacturing defects.
Let us discuss their effects on the performance of the solar module:

(1) Corrosion & breaking of the connection between solar cells

It is the degradation of the material due to chemical reaction with the surrounding atmosphere. The life of the module is majorly decided by its ability to resist corrosion. In a hot and relatively humid climate, the moisture penetrates through the module and reaches the solar cells which are joined or soldered together through wire. The moisture weakens the adhesive bond that joins the solar cells with the module frame and corrodes the soldering joints connecting the solar cells.  This whole phenomenon increases the resistance, resulting in reduced power output from the solar modules.

(2) High temperature acts as a catalyst in degrading the performance

The temperature variations affect the performance of the module. A high temperature put the thermal stress on the solar module that causes solar cells to break or crack down with time. The broken solar cell becomes an open circuit with high resistance, resulting in lowering down of the output power of the module. The temperature also acts as catalyst for the water vapor to penetrate and reach to the solar cells, accelerating the process of corrosion, thus  affecting output power.

(3) The short circuiting of the solar cells

The silicon solar cells which generate electricity are closely held in the casing, increasing the chances of the short circuiting at the interconnections of the cells. The short circuiting can damage the solar cell and lowers down the performance of the solar module.

(4) Aging of connecting wires and the joints

The interconnection resistance increases due to the aging of the wires and the soldered joints. With time, they crack down and separate, resulting in increase of inter cell resistance that lowers the overall module performance.

(5) The breaking of covering glass

The encapsulating glass may break due to thermal stress over time or due to wind storm, lowering down the performance of the solar module.

(6) The failure of the bypass diodes

As the name suggests, this diode by passes the current from non performing solar module to the performing solar module. The bypass diode plays an important role in proper functioning of the modules but it sometimes fails due to overheating and become ineffective in bypassing the current, resulting in increased inter solar cell resistance that affects the performance of the module.

(7) Hot spots

When there is high amount of power dissipation in the small area, it results in overheating which leads to breaking of cell, melting of solder and cracking of glass that degrades the performance.

(8) Depletion of encapsulating layer

The encapsulating layer, Ethyl Vinyl Acetate (EVA), acts as an adhesive between the solar cells, back surface and the top surface of the solar panel. Due to ultra violet radiations and temperature variations, this encapsulating layer starts degrading with time, affecting the overall performance of the solar module.

(9) The effect of metal ion migration

When the electric current flows through the solar cells, the ions from the metal also start flowing in the direction of the current, resulting in the increase of the overall current density.  The increase in the current density leads to overheating that may result in disconnection between solar cells.

The degradation is irreversible, although a good quality solar module will degrade at the lesser rate than a low quality module.

Most of the manufacturers give 25 years warranty about the performance of the solar panels. The degradation rate of silicon crystalline technology based solar modules is < 1% per year or an average of  0.8% per year. One can easily calculate the expected output from the solar module based on this information. Expect an around 3% decrease in the output of the solar panel in the first year and then  0.80 % per year afterwards. Let us analyze the degradation rate of 100 watts silicon crystalline technology based solar module with the help of the following table and the chart.

Year	Degradation rate	Power output in watts
1	3.00%	100.00
2	0.80%	97.00
3	0.80%	96.22
4	0.80%	95.45
5	0.80%	94.69
6	0.80%	93.93
7	0.80%	93.18
8	0.80%	92.44
9	0.80%	91.70
10	0.80%	90.96
11	0.80%	90.24
12	0.80%	89.51
13	0.80%	88.80
14	0.80%	88.09
15	0.80%	87.38
16	0.80%	86.68
17	0.80%	85.99
18	0.80%	85.30
19	0.80%	84.62
20	0.80%	83.94
21	0.80%	83.27
22	0.80%	82.60
23	0.80%	81.94
24	0.80%	81.29
25	0.80%	80.64

 

You can see in the table and the chart above that the power output in the 25th year is 80.64 watts. I have taken the average degradation rate of silicon crystalline solar module in order to explain the process of reduction in output power with time. The actual degradation rate may vary, depending on the quality of the solar module.

Conclusion

The degradation rate is very important from financial point of view. A high degradation rate means that the future cash flows will be on the lower side which can affect the feasibility of your solar roof. A good quality solar module is more effective in resisting the process of corrosion and can withstand the thermal stress for the longer duration. Also, the quality of the solar cells and their encapsulation will help in maintaining the right amount of power output for long. All these qualities will lead to lower degradation rate and can increase your future cash flows. As the technology is further improving, it will further reduce the degradation rate of the module and will increase the efficiency of your solar roof.

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