system design - pv
PV Design Choices
Tilt and Azimuth
Getting the most energy production (maximizing capacity factor) from a photovoltaic system within a set geographic area has to do with maximizing the exposure to direct sunlight. As mentioned earlier, one key factor for this is avoiding shade. Another factor is exposing the panels to the most direct sun for the greatest amount of time. This is accomplished by installing a PV system at the most appropriate tilt and azimuth possible. (Tilt and azimuth are also factors for solar thermal collectors, but to a lesser degree of importance than for PV systems). The tilt of the array is the angle of inclination from horizontal (0° = horizontal, 90° = vertical). Very generally, installers aim for a tilt equal to the geographic latitude minus 15 degrees in order to achieve yearly maximum output of power. An increased tilt will favor power output in the winter months, which is often desired for solar water heating, and a decreased tilt will favor power output in summer months. Research performed by the University of Oregon Solar Center shows that Oregon solar sites west of the Cascades have an optimum tilt of 30 degrees and east of the Cascades, 36 degrees. This has to do with the distribution of solar radiation on an annual basis. Washington sites will be slightly different, but generally follow the same pattern.
The azimuth is the angle clockwise from true north of the direction that the PV array faces (0 or 360 = North, 180 = South). Solar installations in the Northwest should generally be designed with an azimuth within 45 degrees of true south (180) to maximize energy production. Increasing the azimuth angle favors afternoon energy production, while decreasing the azimuth angle favors morning energy production.
Note: All modules wired to one inverter (or all modules sharing a string in the case of a multi-string inverter) should be mounted at the same tilt and azimuth. This is to maintain consistent voltage production throughout the array (or string). If voltage differences occur, energy production from the entire array may be compromised.
Fixed vs. Tracking
While it is not difficult to determine optimal tilt and azimuth for a system design, the type of mounting selected will impact the options. A fixed system means that the PV panels are installed at a set tilt and azimuth and will not move. When installing panels flush to a pitched roof, the tilt and azimuth of the array will be that of the roof. For new construction, the roof can be designed with solar access in mind. For an existing roof, there are limitations. Panels can be fixed in any type of rack or mounting system. A tracking system is one that moves to track the sun. There are two different axes that can be tracked- the tilt which would change over the course of a year, and the azimuth, which would change over the course of a day. Tracking with either a one or two axis system allows the PV production to stay closer to maximum capacity for many additional hours. A manual tracker may be as simple as a rack with different slots for adjustment two or more times per year (similar to a chaise lounge that has an adjustable backrest). Even this modest tracking to the sun’s seasonal migration can boost production up to 10%. Motorized trackers can be single or dual axis trackers that turn arrays to face the sun throughout the day and year. Motorized trackers are typically used in large, field-mounted arrays, as they add cost to a system and consume a small amount of the power produced. Additionally, as PV systems are often touted for being low-maintenance with “no moving parts,” the added maintenance is a trade-off with a motorized tracker.
Other mounting considerations
Roofs: PV systems installed to provide power to a local building load are often rooftop mounted. Various mounting technologies are available to incorporate solar panels onto flat roofs (typically gravel or rubber-membrane) or onto pitched, asphalt shingle roofs. While solar panels should be expected to last upward of thirty years or more, a roof’s remaining lifetime might be much shorter than this. To prevent future cost and effort to take down and reinstall the system, solar panels should be installed only on roofs in good condition, with a remaining life of at least 10 years.
Building Integrated Photovoltaics
When incorporating a solar energy system into construction, there are a myriad of possibilities in design. BIPV (Building Integrated Photovoltaics) refers to design and integration of PV technology into the building envelope, and replacing conventional building materials. This integration may be in vertical facades, replacing view glass, or other material; semitransparent skylight systems; roofing systems, replacing traditional shingles; shade awnings over windows; or other building envelope systems. The benefits of BIPV can include added insulation or shading, as well as the cost savings through avoided materials and of course, the production of energy.
Pole and Ground-Mounted
The small scale and modularity of solar panels makes them a great fit for buildings in urban environments, particularly where other renewable energy technologies may not be appropriate. However, buildings and roofs are not required for mounting. PV systems are increasingly being installed in many different ways. While small-scale systems are still most commonly found on rooftops (allowing for easy wiring into a building’s electrical service), pole and ground-mounted systems are catching on. The advantage to a pole-mounted system for a small solar array is that it can be sited to optimize exposure to the sun (where existing rooftop space may face shadows from trees, buildings, or other rooftop structures). As the scale of a solar energy system increases in power output, the size or footprint of the solar array will also increase. Large industrial rooftops can be a great place for mid-sized solar arrays. For utility-scale power, or for an application where roof space is not available, ground-mounted systems offer the flexibility in space and sizing needed. One drawback to a pole or ground-mounted system is the added need for a foundation. When using a rooftop to house a solar array, the use of the existing structure eases design and cost of building the system. However, pole and ground-mounted systems have more opportunities for incorporating tracking to maximize production than do roof-mounted systems.