How to choose a Solar Module?
Solar Modules vary in size, design and looks based on the cell technology, module technology, number of bus bars, cell size, and other components such as the back-sheet used in the solar module. How will you select a Solar Module for your requirement? In this blog, we will discuss key performance indicators of Solar Modules and how we can compare different module technologies based on the same.
Note: The above data points are indicative for a sample set of technologies as on December 2021 and is not a comprehensive list.
Key Performance Indicators of Solar Module
A. Product Warranty
Solar module manufacturers provide product warranties from 5 years to 25 years. Product warranty is a measure of the reliability of the solar module. Hence, it is better to choose manufacturers who provide longer product warranty. A solar module performance warranty doesn't cover a degradation in performance due to product defects.
B. Performance Warranty
The power production of a solar module degrades with time. To cover the risk of customers against faster degradation or failure before the lifetime of 25 to 30 years, solar manufacturers provide a performance warranty. The duration of the performance warranty varies from 15 years to 30 years depending on the module manufacturer and the technology used. There are two common types of performance warranties in the industry today:
(a) Tiered performance Warranty
The manufacturer guarantees a minimum power production at different points of time. E.g. a typical tiered performance guarantee reads as below:
“Performance Warranty of 90% up to 10 years and 80% up to 20 years”
This implies that the panel manufacturer guarantees a minimum power production of
90% of rated power production from 1st year to 10th year
80% of power production from 11th year to 20th year
(b) Linear performance Warranty
Most of the manufacturers today offer linear performance warranties for Solar Modules. A linear performance warranty will define a drop in the first year due to cell stabilisation and then a degradation factor for the period of performance warranty. Typically in crystalline modules, due to cell stabilisation, there will be a decrease of 1.5% to 3.5% in power production during the first year. The degradation factor defines the rate at which the modules power production comes down. A degradation factor of 1% implies that the power production will decrease by 1% every year after the first year.
A typical linear performance guarantee will be similar to the statement below. The module data sheet will have a degradation factor defined as well.
“Company guarantees that the actual output of the product will amount to at least 97% of the output during the first year after purchase of the product and from second year onwards the effective output will decline annually by no more than .5% for 24 years”
It is always advantageous to have a linear performance warranty as compared to a tiered performance warranty, as it reduces the overall performance risk to the customer. Similarly, a lower degradation factor will result in a substantial increase in power production over the lifetime.
The efficiency of a solar module decides the area occupied by the solar PV module for a given Wp rating. For a solar module, the module surface area decides the input solar energy it receives. A high-efficiency solar module will occupy less area as compared to a low-efficiency one. Many times, customers confuse Wp rating with efficiency. Please note that the Wp of the module doesn't have a direct correlation with efficiency or performance. For E.g. a solar module of dimension 2mx1m with an efficiency of 10% will have a rating of 200 Wp, while an efficiency of 20% will increase the rating of the solar module to 400 Wp.
The Efficiency of the module also decides its performance during low light conditions. Most of the solar inverters require a minimum voltage to start production. Highly efficient solar modules attain this minimum voltage under low light conditions such as rainy or cloudy days or early mornings or late evenings leading to slightly higher energy yield.
D. Temperature Coefficient
The module efficiency and operating voltage decrease with an increase in cell temperature beyond the standard operating temperature of 25⁰C. Heat has the same effect on the cell as the effective resistance has on the flow of electrons. The modules should be mounted to promote airflow on all sides, to reduce the heating of the cell.
Every module has a specific temperature coefficient. Typically the temperature coefficient will be around -0.40%. This implies that for every degree rise in temperature from 25 ºC (Standard Test Conditions) the module efficiency will drop by 0.40% and for every degree fall in temperature from 25 ºC, the module voltage will increase by 0.40%. So a lower temperature coefficient will result in better temperature performance of modules.
E. NOCT (NMOT)
A Solar PV module is rated at Standard Test Conditions ( 25 °C under 1 kW/m2). However, at the site conditions, they typically operate at higher temperatures and at lower insolation conditions. To determine the power output of the solar cell under operational conditions, it is important to determine the expected operating temperature of the PV module. Nominal Operating Cell Temperature (NOCT) of a solar module is the temperature attained by the open-circuited PV cells in a module under the below-listed conditions:
1). Irradiance= 800 W/m2
2). Ambient Temperature= 200C
3). Wind Velocity= 10m/s
4). Mounting – Open
NOCT is an indicator of the heat sink of the module and decides at what temperature the module will operate. A Lower NOCT is better. Typically solar modules have NOCT around 45 deg.
You may use the formulae [ (NOCT - 25) x Temperature Co-efficient ] as a thumb rule to determine the effective temperature performance of a solar module.
F. Additional Factors
(a) Module design and Quality testing
Module design and the extent of quality testing decides the life of solar modules. For larger projects, it might make sense to request the manufacturer for the BOM of the solar module and evaluate the same to ensure reliability. It is equally important that all modules are installed as per the installation manual so that they can withstand the wind loads specified in the data sheet.
(b) Evaluation of Manufacturer
The Product and Performance Warranty of the Solar Module is a good indicator of the Module Reliability. Since Solar Modules design life is more than 25 years, we have to evaluate the credit rating of the manufacturer to ensure that the Manufacturer will last 25 years. In the Solar Industry Bankruptcy of Solar Manufacturers is frequent. So evaluation of the manufacturer credentials or third party insurance is equally important to ensure that a manufacturer can service the Warranty.
At Cares Renewables, we are focused on delivering quality solar solutions to our customers and partners. Please leave a comment, if you have any queries on this blog. Write to us at firstname.lastname@example.org, for your solar requirement.