Conducting a PV Array Site Assessment
A photovoltaic (PV) installer needs to know how to determine whether a proposed site design for a PV installation will be adequate for proper operation of the PV array system.
A PV site assessment involves
A site assessment involves determining whether the location of the PV array system will be shaded, especially between the hours of 9 a.m. and 3 p.m. solar time.
This is important, as the output of PV modules may be significantly impaired by even a small amount of shading on the array.
Crystalline silicon module outputs are generally more susceptible to shading than thin-film module outputs, because the thin-film cell structure traverses the full length of the module requiring more shading for the same effect.
For maximizing benefits to utility companies in some utility-interactive PV installations it may be desirable for the array to face either southwest or even west, provided that the array tilt is below 45º.
Westerly orientations tend to maximize the PV array output in the afternoon during utility peak usage hours, but do not necessarily maximize the benefit to the customer.
Some net metering programs offer time-differentiate rate structures to encourage the production of energy during utility peak hours.
A careful assessment using an hourly computer simulation program is necessary to determine the benefits of westerly orientations. A minimum of six hours of unshaded operation is still important for best PV array system performance.
There are formulas that enable the PV array system designer to calculate the position of the sun at any time of the day, any day of the year, at any place on the planet.
While an installer is not expected to know how to use these formulas, the PV installer should know that devices are available for observing the path of the sun at a location for each month of the year.
These devices can be used to determine whether any obstructions in the vicinity of the PV array will shade the array during critical sun times at any time during the year.
The three products most commonly used today to assess the impacts of shading are (1) the Solar Pathfinder ( www.solarpathfinder.com ); (2) the Sun Eye ( www.solmetric.com ); and the ASSET ( www.we-llc.com ).
These devices, and similar devices that may developed in the future can be used effectively to estimate the impact of shading at any location.
The location of the sun in terms of North, East, West, or South is determined by the azimuth angle, usually symbolized by the Greek letter Psi (ψ).
When a conventional compass is used, North is 0° or 360°, East is 90°, South is 180° and West is 270°. But for calculating sun paths, due South is often used as the zero degree reference, as the sun is due South at solar noon.
Note that solar noon is not necessarily the same as 12 p.m. clock time due to various offsets including daylight savings time. A simple method to determine solar noon is to find the sunrise and sunset times in a local paper and calculate the midpoint between the two.
In summary, for solar calculations, 0° typically represents true South, 90° represents East, 180° represents North, and 270° or -90° represents West. Azimuth angles west of south are typically represented as negative angles and azimuth angles east of south are conversely represented as positive angles.
The location of the sun in terms of the angle between the sun and the horizon, where horizon is defined at the plane tangent to the Earth’s surface at the point of measurement, is determined by the altitude angle, and symbolized by the Greek letter alpha (α).
At sunrise and at sunset, when the sun is on the horizon, the altitude is 0°. If the sun is directly overhead, then the altitude is 90°.
The sun will be directly overhead only in the tropics located between the Tropic of Cancer and Tropic of Capricorn (23.45° north and south of the equator, respectively).
When the sun is up, it is at an altitude angle between 0° and 90°. It is interesting to note that everywhere on the Earth, on the first day of spring and on the first day of fall (equinoxes), the sun rises directly in the east and sets directly in the west and is above the horizon for exactly 12 hours.
Knowledge of the azimuth and altitude of the sun at any particular date and time will enable the determination of whether the array will be shaded, but it is very difficult to quantify the effects of shading due to the variables involved (degree of shading, placement of shading, series/parallel connection of cells and modules, etc.).
Devices can be purchased that allow the user to view potential shading problems and quantify the impact of the shading and provide a reasonable energy loss estimate.
Although understanding the sun’s position using azimuth and altitude is important, calculating the impact of shading is even more important.
Inter-row shading is when one row of modules shades an adjacent row of modules. A six-inch shadow from an adjacent module is capable of shutting down a whole section of modules and can even shut down the entire PV array system down.
A simple rule for minimum spacing between rows is to allow a space equal to three times the height of the top of the adjacent module.
The example in Figure 1 suggests that a separation distance (d) should be nine feet since the height of the adjacent row is three feet above the front of the next row.
In the southern half of the United States, a closer spacing may be possible. However, even in the lowest latitudes the spacing should not be less than two times the height of the top of the adjacent module.
In most cases it is better to take the orientation penalty of using a lesser tilt angle in order to prevent inter-row shading than it is to take the penalty of the shading loss.
If a chart of sun altitude vs. sun azimuth, such as the one shown in Figure 2, is to be used to determine whether the array will be shaded, then the worst-case values for the altitude and azimuth must be measured for the site.
When the worst-case altitude and azimuth angles corresponding to a shading problem have been measured, they are then compared with the position of the sun on a sun position chart for the latitude of the installation.
The chart in Figure 2 is for latitude 30°N. The chart indicates that on the first day of winter (December 21), the sun rises at about 7 a.m. sun time and sets at about 5 p.m.
Solar noon is when the sun is directly south and highest in the sky for that day. Notice that on December 21, the highest sun altitude is about 37° at noon.
On March 21 and September 21, the first days of spring and fall, the sun rises at 6 a.m. at an azimuth of 90° and the highest sun altitude is 60° at noon. On June 21, the first day of summer, the sun rises at about 5 a.m., reaches a maximum altitude of about 83° and sets at about 7 p.m. sun time. At 9 a.m. on June 21, the azimuth is approximately 95° (slightly north of east) and the altitude is approximately 49° (about half way between the horizon and directly overhead).
If an inclinometer or transit or other measuring device is not available, then a ruler, a straight stick, a level and trigonometry can be used to determine the angles.
Photovoltaic Array Orientation
Next to shading, orientation of the PV array is one of the more important aspects of the site assessment.
Fully understanding what the orientation will be at construction must be understood very early in the project.
Often the roof tilt is used as the orientation of a residential rooftop system due to the improved aesthetics of a parallel standoff roof mounted array rather than an array that is tilted to an angle greater than the roof tilt.
Most roof orientations are not the most ideal for the array orientation so the impact of a less than optimal orientation must be understood prior to solidifying the system orientation.
Much erroneous information has been circulated with respect to the impact of systems at non-optimal orientations, and it is difficult to find sources with accurate information on the subject.
Over recent years, the number of Building Integrated Photovoltaic (BIPV) array system installations for home systems have been increasing in the world as well as Malaysia.
For the optimum design of building integrated photovoltaic array systems installations, it is important to determine their performance at the site installation.
Since the amount of power produced by a PV array panels depends upon the amount of region’s solar irradiation and temperature.
Kuala Lumpur is located at North longitude 3.7 and East longitude 101.33, lies entirely in the equation region and the city is at an elevation of about 50 m.
The average monthly of solar irradiation and temperature is 131 kWh/m2 and 25°C respectively. Heavy rainfall, constantly high temperature and relative humidity characterize the Kuala Lumpur as well as Malaysian climate.
Generally, the rain falling most in the afternoon or early evening with the monthly average daily sunshine ranging from 4 hr to 8 hr.
The optimum values of tilt angles and direction of a PV array panels in Kuala Lumpur were determined using simulation method.
Where the optimum PV array module tilt angle (inclination) in Malaysia is between 0° and 15° and any orientation of the system facing either North, South, East or West.
Photovoltaic Array Location
PV arrays can be mounted on roofs, racks, and poles. The installer needs to determine or verify which method is best for the location of the installation.
Roofs are popular locations for PV array installations. Roof-mounted PV arrays provide protection for the modules from many forms of physical damage.
Additionally, rooftops usually provide better sun exposure, and installations do not occupy space on the ground that might be needed for other purposes.
Several disadvantages of roof installations are that they require lifting all of the modules, mounting materials, and wiring materials to the roof; they present a falling hazard; they are susceptible to leakage at the attachment points; and they need to be removed and replaced occasionally when the roof needs to be repaired or replaced.
Photovoltaic Array Sizing
The size of the photovoltaic array is determined by considering the available solar insulation, the tilt and orientation of the array and the characteristics of the photovoltaic modules being considered. The photovoltaic array is sized to meet the average daily load requirements for the month or season of the year with the lowest ratio daily insulation to the daily load.
Available Roof Area
If a roof is selected for the array location, then it is necessary to determine whether the roof is large enough for the proposed number of PV modules.
Sometimes when a roof has non-rectangular shapes, it is a challenge to determine the amount of useful roof area. When laying out a plan for mounting modules on a roof, access to the modules must be provided in case system maintenance is needed.
For easiest maintenance access, a walkway should be provided between each row of modules. However, this consumes valuable roof area, so a balance needs to be made between the area for the array and access to the array.
Often, only 50% to 80% of the roof area that has a suitable orientation can be used for mounting modules when room for maintenance, wiring paths, and aesthetic considerations are taken into account.
To determine the size of the PV array (ultimately the power rating of the system) that can be installed, the usable roof area must be determined.
The physical size of the modules to be used is important, depending upon the shape of the roof area to be used. When a module is selected, it is necessary to check the total array dimensions against the roof dimensions to be sure the array will fit the roof.
As a rule of thumb, crystalline silicon modules with 10% efficiency will generate about 10 watts per square foot (100 watts per square meter) of illuminated module area (typically, today’s thin-film modules require more area for the same rated output).
Hence, by multiplying the usable and available roof area in square feet by 10, the size of the PV array (in Watts) that can be installed can be estimated.
For example, a roof with dimensions of 14’ by 25’ (350 ft2) has a usable area of 250 ft2 (71% of total). This roof area would be sufficient for a 2.5 kW (250 ft2 x 10 W/ft2= 2500 W) crystalline silicon array or an 8% efficient thin film array of 2 kW.
Roof Structure and Condition
If the roof appears to be bowed, or if it will not readily support the weight of the installer, it may not be strong enough to support the array.
A structural engineer should be consulted if the roof structure appears to be inadequate to support the PV array.
Generally, houses built since the early 1970’s have been through more rigorous inspection and tend to have more standard roof structures than those built prior to that period.
If the attic is accessible, a quick inspection of the type of roof construction is often worthwhile.
Also, the uplift force during windy conditions of a PV array must also be considered, as the total uplift force in a strong wind may reach up to 50 pounds per square foot (psf) or greater.
This is particularly important with standoff roof mounts. A 10-square-foot module could impose an uplift load of 500 pounds when the PV array system is attached to the roof.
A panel of four of these modules may impose a load of 2,000 pounds on the mounting structure.
If the panel is supported by four roof-mounts, and if forces are distributed equally, there would be a 500-pound force attempting to lift each mount from the roof, and the roof mount attachment method must be capable of resisting this maximum uplift force.
Several manufacturers of roof mounting systems provide engineering analysis for their mounting system hardware. Without this documentation, local inspectors may require that a custom mounting system have a structural analysis for approval.
The need for engineering documentation easily justifies the additional costs of purchasing mounting hardware from a mounting system manufacturer.
The final consideration for roof mounting is the age and condition of the roof. If the roof is due for replacement within the next 10 years, it typically makes sense to re-roof the building before installing the PV system, as the array would need to be removed when the roof is replaced.
Commercial Roof Mounting
Some roof mounting systems developed for the commercial PV array market use ballast instead of lag screws into the structure to hold the array in place.
These systems are engineered for specific wind speeds and for specific roof structures and have very specific stipulations on how to install the array.
Before recommending any roof mounting system, provide detailed information to the mounting system manufacturer to confirm whether or not your specific application is acceptable for the PV array mounting system design. (PDF 3.62MB)
Far too often, installers make assumptions about the applicability of mounting systems without consulting the manufacturer.
Also, many installers often copy manufactured mounting systems and make the assumption that the self designed system is equivalent to the manufactured product.
Unfortunately, many installers lack the background and experience to properly design and fabricate a mounting system and these structures often deteriorate or fail prematurely.
These self designed examples are why local jurisdictions often require some engineering documentation to qualify that the mounting system, as designed by the manufacturer, will keep the PV array installation on the roof and not pose a hazard to the system owner or those in the immediate vicinity of the array.