Successful PV Site Evaluation: Page 3 of 3

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Successful PV Site Evaluation
Successful PV Site Evaluation
Solmetric’s Roof Azimuth Tool Screenshot
Solmetric’s Roof Azimuth Tool can determine orientation from aerial photos. Google Earth has a similar tool that can determine approximate roof area.
An inclinometer
An inclinometer is an inexpensive tool for quickly determining the a surface’s slope. The needle is pointing to the angle in degrees.
Smartphone app
An inexpensive smartphone app can measure the roof angles in degrees, percentage, rise-to-run ratio, and even radians.
Solmetric's free online tool
Solmetric offers a free online tool for determining annual insolation based on various combinations of azimuth and tilt. This example roof is within 5% of the ideal tilt and orientation.
Solmetric’s SunEye 210
The SunEye 210 is Solmetric’s latest solar siting tool, which displays an annual sunpath view, showing annual and seasonal solar access values.
Solmetric Screenshot
The SunEye 210's accompanying software includes additional displays, such as the obstruction elevation.
The Solar Pathfinder
The Solar Pathfinder uses a highly reflective dome to reveal obstructions, which can be outlined on a paper sun-path chart. It has a built-in level and compass, and the base can be adjusted for the site’s magnetic declination.
Solar Pathfinder Screenshot
Digital photos taken of the Pathfinder reading can be used with the Assistant PV software to generate detailed summary reports.
Capture readings from all four corners of the intended array area.
To properly evaluate a proposed PV array location, capture readings from all four corners of the intended array area.
A solar siting tool can be used to determine a “shade-free” distance from roof obstacles
A solar siting tool can be used to determine a “shade-free” distance from roof obstacles, such as this plumbing vent. This should be recorded on the site evaluation form.
Check the roof’s condition
Check the roof’s condition—repairs or reroofing should happen before installing a PV system.
Assess the roof’s structural condition
Inspecting the attic is another way to assess the roof’s structural condition, check for leaks, and map rafters for future array mounting.
Plan for wire and conduit runs in advance
Plan for wire and conduit runs in advance. The path between array and balance-of-system components needs to meet the Code as well as logistical requirements. This junction box makes a transition between PVC conduit coming from the pole-mounted array to EMT conduit running through the building interior to the inverter.
Check for obstacles
Check for obstacles—a propane tank in the yard indicates underground pipe runs that should be avoided when trenching for wire runs.
AC service entrance
Generally, a new grid-tied system will connect to the existing electrical infrastructure at the AC service entrance. Besides checking equipment condition, look for service amperage rating, main breaker rating, busbar ratings, and adequate breaker space. Incompatibility with any of these things may dictate upgrading the panel.
Balance-of-system components
Balance-of-system components take up space, and have Code and practical access requirements. Map their placement in advance.
Successful PV Site Evaluation
Solmetric’s Roof Azimuth Tool Screenshot
An inclinometer
Smartphone app
Solmetric's free online tool
Solmetric’s SunEye 210
Solmetric Screenshot
The Solar Pathfinder
Solar Pathfinder Screenshot
Capture readings from all four corners of the intended array area.
A solar siting tool can be used to determine a “shade-free” distance from roof obstacles
Check the roof’s condition
Assess the roof’s structural condition
Plan for wire and conduit runs in advance
Check for obstacles
AC service entrance
Balance-of-system components

Performing a comprehensive solar site analysis is the first step toward ensuring a well-performing solar-electric system. New tools and technologies are available for evaluating a site’s solar potential. But while these tools can bring us powerful information at the touch of a button, nothing can take the place of conducting a thorough assessment of the proposed array location.

Orientation & Tilt

As a PV array’s orientation (azimuth, the direction it faces) or tilt varies from “optimal,” potential energy harvest decreases. The optimal combination of array tilt and azimuth will depend on a few variables, including geographical location, seasonal weather patterns and system application. Azimuth and tilt are both required data for system design and energy estimating programs such as the PV Watts calculator (see Access).

A PV array’s orientation is often referenced to true south. A compass doesn’t always point to true south, so magnetic declination, the angle variation between magnetic north/south and true north/south, must be taken into account, since it varies by location. There are various resources available to determine magnetic declination for most locations around the globe (see Access). With the use of a compass and local magnetic declination, a roof’s orientation is simple to determine (see “Methods” in this issue). Since most buildings are not constructed with solar in mind, roofs that face true south are uncommon. Fortunately, most PV arrays within the continental United States oriented within 30° of true south still offer excellent potential energy harvest, often within just a few percent of the maximum.

Due to wind-loading and aesthetic concerns, most rooftop arrays are mounted parallel to the roof plane—so the roof’s pitch determines the array tilt. For grid-tied systems here in the United States, “optimal” roof angles—which maximize an array’s production—range between 25° and 35°. The roof pitch can be measured with an inexpensive inclinometer or a smartphone, which uses an app that takes advantage of the phone’s internal sensors to measure angles.

As roof pitch decreases, the roof’s orientation to true south becomes less influential on a grid-tied system’s performance. At a steeper tilt, the array will capture less energy during early mornings and late afternoons due to the sun’s oblique angle. As the array’s tilt approaches 0° (horizontal), the array is able to capture more of the early morning and late afternoon sun, which can increase overall energy production (see the “Orientation & Tilt Reference” table that shows the effects of decreasing the tilt angle with respect to azimuth). Since the goal for most grid-tied PV systems is to maximize the annual energy yield, winter production can often be “sacrificed” to maximize production in the summer, when the available peak sun-hours increase.

With your preferred solar design tool, such as Solmetric’s Annual Insolation Tool, you can create a chart for quickly evaluating potential energy production as it varies with array tilt and orientation (see “Free Tools for Estimating PV Output” in HP147). The results will reveal the ideal array orientation and tilt for an unshaded array in that particular location. This chart can be used to compare different mounting scenarios, such as multiple roof planes, as well as pole- or ground-mount options (for more information, see “Methods” in HP155).

To use Solmetric’s online tool, select your state from a drop-down menu and then select the location that best matches your local weather patterns. In our example, Grand Junction, Colorado, has very similar weather to the site’s location. Selecting the location generates an “annual insolation” graph that can be customized with specific orientation and tilt angles. To interpret the chart, find your tilt angle or roof pitch along the vertical axis and then match that up to the azimuth along the horizontal axis. This will lead to an intersecting point in the graph with a color designation that reveals a percentage value as compared to the ideal conditions, which are listed at the top of the legend area. The annual insolation value can be used to quantify the impacts of various tilt and azimuth angles. The scale along the right provides color-coordinated values expressed as a percentage of the ideal conditions and the associated insolation value in kilowatt-hours per square meter.

The Solar Window

The most common professional siting tools—the Solar Pathfinder and Solmetric SunEye—can help determine a particular site’s solar access or solar “window”—the average amount of sun that falls on a particular site, usually between 9 a.m. and 3 p.m. Maintaining shade-free access during this time is necessary to maximize system production while sunlight is at its peak. Solar siting tools allow quick comparisons between multiple locations on a property to determine the optimal location for system production, reliability, and overall aesthetics. Note that there are also some smartphone apps available, such as Solmetric’s IPV (for iPhone), Comoving Magnetics’ Solar Shading (for Android), and Onyx Solar Energy’s app (iPhone & Android).

Both of the pro tools, however, have optional software for analyzing the data and creating a detailed report. Standard software features include the ability to overlay multiple points of data, like a “four corners” evaluation to determine the overall solar access for the area. The data can generally be presented several ways, graphical or numerical. The software also allows users to digitally remove obstacles, such as trees or chimney vents, from the captured image and will recalculate the solar window and estimated energy production.

Evaluating a site’s solar access requires capturing data points for the four corners of the proposed PV array. You can either use a tripod or a beanbag to create a level surface for the siting tool. Obstacles and trees adjacent to the building may require additional readings along the perimeter of the proposed array location. Protrusions, such as chimneys and vents, on the roof can be quickly evaluated using the siting tool to determine a “shade-free” distance from the obstacle. Using a piece of roof chalk, the shade-affected area can be marked around the protrusion. The resulting shaded area can then be recorded on the roof layout part of the site survey. Mapping roof obstacles beforehand can minimize or eliminate having to make expensive redesign decisions during the system’s installation.

Assess the Roof

While on the roof, analyze the condition of the roof surface and structural integrity of the rack attachment area. A roof’s estimated age and thickness should be documented. Identifying signs of deterioration and damage varies depending on the roofing material. Having an experienced roofing contractor evaluate the roof can be money well-spent.

Noticeable waviness or spongy spots in the roof warrant further evaluation of the structure. Installing PV arrays on older roofs can later result in expensive removal and reinstallation of the array. Failure to recognize potential issues, such as compromised structural elements, can pose significant hazards if attachment points are stressed during large windstorms and other weather-related events. When attic access is available, inspecting and mapping the rafters is recommended.

Local weather considerations can also influence design choices and should be recorded during the evaluation process. In heavy snow areas, knowing the expected snow loads on the roof is necessary to select an appropriate racking product rated for the conditions, as well as determining the rail span distance between attachment points. Snow loads can also dictate module selection, since modules have specific static ratings. Wind-loading also must be considered for equipment placement and selection for many of the same reasons. Most rack and module products have wind load ratings that range between 90 mph and 120 mph. The perimeters of most residential roofs are exposed to greater wind forces than interior portions of the roof and may not be usable in many locations with high wind loads. This can limit array placement and size, as can required roof setbacks for firefighters (see “PV Array Siting & Mounting Considerations” in HP155). The local building department is a good place to glean information on expected snow and wind loads.

The Rest of the System

After completing the solar window analysis, you still need to plan for routing conduit, grounding conductors, and placing the balance-of-system components (inverters, disconnects, and combiner boxes). A thorough inspection of the existing electrical panel includes documenting the enclosure make and model, as well as verifying additional breaker space, and checking the main breaker size and bus bar ratings. The service equipment ratings will be used in system design calculations to determine allowable PV backfeed capacity and the associated breakers needed for interconnection. Ensuring adequate breaker space avoids expensive surprises later, like needing to upgrade the service panel.

Planning for Code-compliant conduit routing requires considering equipment placement, the mechanical integrity of equipment mounting and conductor runs, aesthetics, local regulations, and climate. Part IV: Wiring Methods, Article 690.31 of the 2011 National Electrical Code (NEC) dictates the type, placement, routing, and marking of conduit. Because of the higher temperatures commonly encountered on roofs, the details of conduit routing can affect system design due to required derating of conductor ampacity.

Identifying grounding components of the existing AC electrical system is another important step of the site survey. The location of the existing grounding electrode conductor (GEC) and grounding electrode (often a ground rod) will inform decisions for routing the PV system grounding conductor(s) to interconnect the two systems. In some cases, the inverter will have a GEC that must be connected to the building’s existing grounding electrode or GEC. Depending on the system, the PV array may also have a GEC that must be connected to the grounding system. Planning these routes will help ensure Code-compliant installation.

Inverter and disconnect placement involves satisfying  Code regulations that apply to the working space around the equipment, weather considerations, and aesthetics. Disconnect locations must be accessible and installed within height limitations. In cold climates, some inverter models need to be installed indoors to keep them within their operating temperature range. Inverters placed outdoors should avoid south or west orientations as the intense sun may cause the inverter to derate its power output to protect internal components from excessive heat damage. Direct solar exposure can also burn out the LCD screen, which includes state of operation information, energy production values, and potential error messages.

Additional Considerations

Analyzing the building’s loads—and researching measures for improving the overall efficiency—is another important part of the site evaluation. Shrinking a building’s loads can often result in a smaller PV system, resulting in PV system savings that are greater than the efficiency investments. Lighting and refrigeration are two of the easier places to start with efficiency upgrades—incandescent bulbs and old refrigerators are prime candidates for replacement. Insulation and other weatherization measures can also make a significant impact on electricity usage in homes with electricity-based space heating.

Access

Jeff Tobe is a PV curriculum developer and instructor at Solar Energy International (SEI). He has been instrumental in the development of SEI’s state-of-the-art PV lab training facility in Paonia, Colorado, and spearheads renewable projects for indigenous communities in the United States. He is an ISPQ-certified PV Instructor, has a NABCEP certification in Technical Sales, and is a NABCEP-certified PV Installation Professional.

Magnetic declination finder • bit.ly/CalcDeclination

PV-ready home checklist • bit.ly/PVHomeChecklist

PV system production estimator (PVWatts) • bit.ly/PVWattsV1

Solmetric roof azimuth tool • bit.ly/SolmTools

Solar Site Evaluation Training & Certification:

Solmetric • bit.ly/SolmTraining

MREA • midwestrenew.org

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