Solar Panel System Guidelines
Renewable and Sustainable Energy with Solar Panel System
10
Chapters
All
Skill Level
20 Min
Duration
English
Language
Chapter 6
Table of Contents
Solar Panel System Guidelines
Solar Panels String Sizing
Designing a solar panel system by yourself is a difficult task.
While most people prefer buying the systems directly, a few like being more hands-on with their devices. They modify them according to their specifications.
If you prefer being hand-on, string sizing is for you. We will first discuss what string sizing is and then give you a detailed guide to figure out the optimal string size for your panels.
What Is String Sizing?
A string refers to a group of panels plugged into a single input through a solar inverter.
String sizing are the calculations you need to make to ensure that your inverter receives the exact amount of power required for optimal performance. Since an inverter has a specific range of voltage which it can handle, your string sizing calculations must be correct.
The purpose of string sizing is to ensure that the voltage falls within a narrower margin called the maximum power point range. At this range, your inverter will operate at its optimum efficiency.
How To Calculate Solar Panel System String Size
Calculating your string size is a slightly complex process that requires analysis of many factors. For simplicity, we will proceed step-by-step. Let’s start with the voltage.
Every solar panel has an output voltage, as mentioned earlier, which goes straight to the inverter. With regards to the output voltage, you should know 2 things:
1. Open-circuit voltage (VOC): This condition occurs when there is no electricity passing through your electrical system, i.e., the circuit is open. The inverter is turned on during this time.
2. Maximum-power voltage (VMP): This is a measure of the voltage that flows in your circuit after you turn on the circuit.
The next value you should know is the maximum power point range. It was explained earlier as the sweet spot where your inverter has the highest performance.
One thing you should always know is the maximum direct current input voltage. Remember that your inverter’s circuitry will be overheated and damaged if it exceeds a specified value.
Case Study
Let’s apply the formula to see how it works. We will take a 9-kilowatt grid-tied system as an example.
We have all of the information we need. Let’s begin with a step-by-step calculation.
1. Find the Minimum String Size
You will identify the number of panels needed in a string by finding the minimum string size.
You divide the least amount of the MPP range by the VMP of the panel to obtain this value.
The result we obtain is 6.87. Round this to the next whole number. You can’t have a partial solar panel. Right?
2. Find the Highest String Size Below the Operating Voltage
We find the highest string size which won’t exceed the operating voltage to ensure that we don’t overburden our inverter.
3. Check Whether the String Size Falls within the MPP
The answer we get is 471.36 volts.
This value falls within our MPP range, as we can see. What are you supposed to do if it is more than your MPP? Decrease the number of panels by one and do the calculation again to see if the new value falls within the desired range.
4. Temperature
Finding out the optimal number of panels isn’t enough. The temperature of the area of solar panel system installation play a significant role in determining the MPP conditions.
Then, open your solar panels’ specification sheet once more and find out what the temperature coefficient of VOC is. This number tells us the amount of change in the voltage away from the normal operating cell temperature for (NOTC) every unit change in degree.
The temperature coefficient we will take is 0.28%
Recall that the temperature coefficient was 0.28. This means that for every single drift from the NOTC, our solar panel system will produce 0.28% greater voltage.
The answer we get is 0.134624.
If you’ve been able to follow so far, you know that this is the voltage our panel will produce above its VOC voltage on the coldest day ever recorded. After that, add the two values to get the complete voltage the solar panel system will produce.
701.83 volts is the answer.
What About High Temperatures?
The information presented above relates to temperatures that are extremely cold. But what about the other way round? What should you to do when the temperatures are quite high?
This is not a cause for worry. The NOCT falls within a range of 44-47 degrees Celsius during measurement, approximately 112-115 degrees Fahrenheit. Since most places don’t go beyond this range, no need for concern.
High temperatures affect string size, though, so you should recheck to make sure that your string size is still within the optimal MPP range required for the highest efficiency. The calculations needed to determine that are the same ones we did before, just with different values.
- In place of VOC, use the VMP value
- Instead of the Temperature Coefficient of VOC, use the value of the Temperature Coefficient of Pmax
- Don’t add the voltage compensation to the panel VMP to find out the true voltage; subtract it instead.
1. (VMP)*(Temp Coefficient of PMax) = Voltage Change per unit Celsius
39.28*0.00377= 0.148
2. The total difference in Voltage due to the 12.7 degrees Celsius above the NOCT
0.148*12.7= 1.8796
3. This value subtracted from the VMP gives you the true panel voltage
39.28- 1.8796= 37.4004
4. Lastly, multiply this value with your minimum string size to obtain the total voltage that you can use as input.
Recall that in step 1, we mentioned the lower end of the MPP as 270 volts. Since our obtained value of 261.8 falls below that, the ideal solution would be to increase the number of string panels to 8.
Start String Sizing Your Solar Panel System
Before we part, we’d like to ease your worries by telling you that solar panel systems are built while keeping these complex calculations in mind, so you don’t need to do any of the work yourself.
If you still need assistance in optimizing your solar energy system to match your needs, you can request for a free consultation with our experienced team.
