Table of Contents
Photovoltaic Performance
Photovoltaic (PV) modules are essential components in solar energy systems, converting sunlight into electricity. Various factors influence their performance, impacting efficiency and energy output. In this post, we will explore the primary elements that affect PV module performance, contributing to optimized solar energy solutions.
1. Factors Affecting PV Module Performance
Factors Affecting PV Module Performance – A PV module’s performance is directly related to the amount of sunlight it receives. If a PV module is even partially shaded, its performance will be significantly reduced. Other factors affect the output of a solar power system.
These factors need to be understood so that the customer has realistic expectations of overall system output and economic benefits under variable weather conditions over time.
1.1 Environmental Factors
1.1.1 Location
When designing a PV system, location is the starting point. The amount of solar access received by the photovoltaic modules is crucial to the financial feasibility of any PV system. Latitude is a primary factor.
1.1.2 Solar Irradiance
It is a measure of how much sunlight intensity or power in Watts per square meter is falling on a flat surface, or what you are getting at your location. Because weather conditions are somewhat similar over the years, it is possible to predict the average monthly and annual energy production of a system using historic, standardized weather data.
There are maps available of solar resources showing how much energy reaches the surface of panels. The data is presented
in standardized maps that show how many standard sunshine hours can be exacted over a month or a year. It is expressed by the term “solar insolation”.
1.1.3 Solar Insolation
Solar insolation is a measure of the solar irradiance that reaches a PV surface at any given time. Solar energy available in a given location is expressed as kWh/m2/day. This is commonly referred to as Peak Sun Hours (PSH). For example, if solar radiation for a location is 5kWh/m2/day, then PSH for that location will be 5 hours. Now, if you install a 1kW solar panel on that location, it will produce 1kW x 5h = 5kWh of energy per day without considering any losses.
More intense sunlight will result in greater module output. Lower sunlight levels result in lower current output. Voltage is not changed appreciably by variations in sunlight intensity.
The map below shows the amount of solar energy in hours available each day on an optimally tilted surface during the worst months of the year to generate electricity (based on accumulated worldwide solar insolation data). This is very useful because it allows you to calculate the energy generation of your solar system.
Statistical estimations of average daily insolation levels for specific locations are commonly used in the PV design process and measured as kilowatt-hours per square meter per day (kWh/m2/day).
1.1.4 Electricity Generation V/s the Sun Hours Available per Day
Several factors influence how much sunlight your modules will be exposed to:
- When will you be using your system – summer, winter, or year-round?
- Typical local weather conditions
- Fixed mountings vs. trackers
- Location and angle of PV array
The Annexure-2 at the end of the course provides the sun hour ratings for several cities in North America for summer, winter, and year-round average. If you use your system primarily in the summer, use the summer value; if you are using your system year-round, especially for a critical application, use the winter value. If you are using the system most of the year (spring,
summer and fall) or the application is not critical, use the average value.
Note that it is more difficult to produce energy during the winter because of shorter days, increased cloudiness, and the sun’s lower position in the sky.
1.1.5 Air Mass
Air mass refers to the “thickness” and clarity of the air through which the sunlight passes to reach the modules (sun angle affects this value). The standard is 1.5.
1.1.6 Sun Angle and PV Orientation
The direction that a solar panel faces is referred to as its orientation. The orientation of the solar array is very important as it affects the amount of sunlight hitting the array and, hence, the amount of power the array will produce. The orientation generally includes the direction the solar module is facing (i.e., due south) and the tilt angle, which is the angle between the base of the solar panel and the horizontal. The amount of sunlight hitting the array also varies with the time of day because of the sun’s movement across the sky.
Solar modules should be installed so that as much radiation as possible is collected. Ideally, the PV installations North of the equator perform optimally when oriented to the South and tilted at an angle 15 degrees higher than the site latitude. If the PV array is mounted on a building where it is difficult for the panels to face the South, then it can be oriented to the East or West, but under no circumstances to the North, as its efficiency will then be very limited.
The highest efficiency of a PV module or peak power occurs when its surface is perpendicular to the sun’s rays. As the rays deviate from perpendicular, more and more of the energy is reflected rather than absorbed by the modules.
Most PV systems are mounted in a fixed position and cannot follow the sun throughout the day. It is possible to improve the output by installing PV modules on trackers to follow the sun from east to west during the day (single-axis trackers), and from north to south during seasonal changes (dual-axis trackers). This can be expensive, so it is not common practice for most PV system applications.
PVs should be tilted toward the sun’s average elevation, equal to the latitude of the array’s location, to capture most of the solar energy throughout the year. For example, a system used throughout the year at a latitude of 25° can have a tilt angle of 15° to 35° for the maximum amount of electricity over a year.
1.1.7 Shading
Shading may be one of the most important parameters for energy loss in a PV array. Even the partial shading of one cell of a 36-cell module can reduce the power output significantly. Potential shading sources can be trees and bushes, neighboring buildings, and self-shading by the multiple rows of modules themselves. Calculations need to be done to find the minimum distance between PV Array rows to avoid winter mid-day shading.
The general rule of thumb is to locate the array at a distance away from the object that is at least twice the height of the object. This will ensure that the object will not cast a shadow for 4 hours on either side of solar noon.
As a rule, with a lower tilt angle, there is less shading, and the area can be better exploited. However, in that case, the solar yield drops throughout the year. For this reason, a tilt angle of 15° is usually chosen.
Thin film PV modules are more tolerant to partial shading than crystalline silicon PV modules.
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References:
A. Bhatia, Course No: R08-002, https://www.cedengineering.com
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