This post will help you to determine the best location for a photovoltaic (PV) system. After you have sized your PV system based upon the calculated the power requirements, you will have to select a location that has maximum sun exposure and limited shading throughout the year. PV arrays can be mounted on rooftops, ground, or another type of structure. Some of the considerations for the array location are:
• How can the solar array maximize the photons received?
• Are there times of the day where the solar panel will be shaded?
• How will the array be maintained?
• How far will other system components be?
• Are there wind load concerns that could affect the PV system?
• Is the surface area large enough to support the PV array?
• Is the support structure design strong enough to support the array?
• Are there any special installation, safety or maintenance concerns?
Answering some of these questions will aid in determining the best possible locations for installing the PV arrays. In this post, we will review several considerations for selecting the best locations based on site conditions and other factors.
As we have seen in a previous post, the surface area of the array is dependent upon the peak power requirements, PV panels, array spacing, and panel connection type (series versus parallel). PV arrays do not have to be installed flat on a surface – they can be installed on tilted racks, solar trackers, or other configurations to help maximize the angle to achieve the ideal sun position.
The surface area of a PV array will be specified on the manufacturer’s specification sheet. The power density of the PV array will also be specified and typically ranges from 6 to 20 Watts, depending upon the module efficiency and layout. To illustrate the relationship between power density and surface area, let’s assume that we have a 240 W crystalline silicon PV array, and the manufacturer’s specification sheet has a surface area of 20 square feet, then we can calculate the watts per square feet by:
If the initial system design calculations required 4 kW of power, the total module surface area required would be:
The total module surface area required would be 333.3 ft2.
When planning the design and installation of a PV system, an important consideration is the position of the sun and the angle of solar radiation with the latitude and longitude coordinates of the solar panels. Two angles are important:
• Solar azimuth: the sun’s horizontal projection relative to the placement of the solar panels
• Solar altitude: the elevation of the sun above the horizon
On a standard compass, north is 0° (or 360°), east is 90°, south is 180° and west is 270°. Some solar calculation programs use south instead of north because the calculations are simpler relative to the sun’s position. If the south is the zero reference, the solar azimuth angles west of south will have negative angles (due west is -90°), and east of south is represented as a positive angle (due east is +90°). The solar altitude angle is 0° when the sun is on the horizon and is 90° if the sun is directly overhead. Depending on the time of day and the season, the range of latitude is defined by the sun angle and position.
An easy way to look at the sun’s position at a specific latitude and day of the year is by using a sun position diagram, which is a graphical representation of the sun’s altitude and azimuth angles. The sun position diagram charts can be used to determine the sun’s position at any latitude during any time of the year. Sun path diagrams help evaluate the effects of shading on PV arrays and other types of solar collectors. These charts consider the sun paths for the solstices and at the equinoxes. On the equinoxes, the solar altitude is 90° at solar noon at every point along the equator. The solstices define the minimum and maximum solar altitude angles and sun path range over a year.
The solar window represents the range of sun paths between the winter and summer solstices at a specific altitude. PV arrays should be angled towards the solar window for maximum solar energy collection. The sun paths are longer in the summer and shorter in the winter. The maximum altitude of the sun paths will vary by 47° between the winter and summer solstices.
Before designing the PV system, information should be collected on the PV surface area, local weather conditions, and any other issues that could affect the installation. The level of detail depends on the size and type of the system to be installed. Larger PV projects will require more significant detail than small PV projects. The site assessment usually includes the following:
• How times of day affect the shading on the PV panels
• How seasons change the shading on the PV panels
• The optimal location of the array
• Position of the other components required for the PV system
• The interface with the existing electrical system
Shading analysis is a process of quantifying the impact of shading on solar output. Many obstructions can cause shading:
• Power lines
• Other array parts
Shading can also occur by accumulated debris on the surface, which can be more severe in certain regions of the world. The PV array should not be shaded for at least 6 hours during the middle of the day to produce the maximum amount of energy. It is preferable that there is no shading during the hours of 9 a.m. and 3 p.m. because most photons are absorbed during these hours. If this cannot be achieved, then the obstructions that are causing the shading need to be removed, or the PV system needs to be mounted elsewhere.
Sun path charts help to conduct shading evaluations. The altitude and azimuth angles of a shading object can be measured from the array location and plotted on a sun position chart for a specific latitude. The shading analysis can help you to estimate the reduction in solar radiation received during the shaded times of the day to help appropriately size the PV to provide enough power with the shading.
There are many software tools that have been developed to help simplify shading analysis. The automated tools are based upon sun path charts and the solar window at the location coordinates. Some useful shading evaluation tools and software include Solar Pathfinder™: www.solarpathfinder.com and Solmetric SunEye™: www.solmetric.com.
In some larger PV systems, a row of modules can shade another one if the rows are closely spaced. A shading for several inches can shut down another PV array row if enough spacing has not been built between rows. A rule that is often used is to allow a space three times the height of the top of the obstruction in front of the array. For example, if the array height is two feet, the minimum separation distance should be six feet since the height is two feet above the front of the next row.
In this post, we reviewed some aspects of how the best location can be selected for a PV system. We examined how to calculate the panel area based upon the power requirements and the manufacturer’s specifications. We also looked at sun position, solar window and the importance of conducting a shading analysis on a PV system.