Solar Design

Site Survey

Upon arrival at the site, roof orientations were verified with a compass . Measurements and photographs were taken of the south, east and west facing roof areas and recorded.  The width of the roofing valleys was also measured to ensure that the PVL product would fit.  The height and location of structures and trees that might shade the roof were noted.

Design

Grid tie solar electric systems utilize high voltage series strings of solar panels.  This maximizes power harvest by improving DC to AC conversion efficiency and minimizing DC wiring voltage drop.  These series strings must be composed of solar panels with as close to identical voltage output as possible.  The system will operate at the voltage of the lowest solar panel in the string.  The analogy of the chain being only as strong as its weakest link is also accurate for strings of solar panels.  This also applies to multiple directional orientations.  If some panels in a string face south and others face another direction, there will be variations in solar exposure between the two.  This will cause differences in voltage resulting in total system performance degradation.  The degradation can be negated by using separate inverters for each orientation.  In addition to similar voltages, solar panel strings must also be sized to meet minimum and maximum inverter input voltages.  Xantrex GT series inverters were chosen due to their wide input voltage which allows for a wide range of solar panel strings sizes to be utilized.  SMA and Fronius inverters were also considered but couldn’t be used.  Uni-solar PVL solar panels are available in 68, 124 and 136 watt sizes and can be placed only in the valleys of the roofing pans.  This means that the number and size of solar panels is restricted by the length and number of the valleys in any section of roof.  Valleys with roof penetrations such as plumbing vents and skylights can be unusable and further reduce the number of available valleys.  South facing roof is optimal since it is exposed to sunlight from dawn to dusk year round.  In lieu of south facing roof, east and west facing roof space is used since each will receive excellent sun exposure for part of the day.  North facing roof surfaces are not usable at the 34o N latitude of the survey location.  Solar panels mounted to this surface will provide little or no power

due to the fact that the sun angle will too low for most of the year any but reflected solar exposure.  

The south facing roof area was addressed first since it is oriented optimally.  Only the PVL-68 panel  would fit the available roof dimensions.  Using the Xantrex online string sizing program, it was determined that the minimum string length was 14 and the maximum was 22 for the PVL-68 panel.  A maximum string of 22 panels was selected since at least 22 valleys of the proper length were available.  A GT-2.8-NA-DS inverter was chosen for power conversion for this solar array.

West facing roof area was addressed next since it had the fewest roof penetrations.  This roof area was found to be long enough to use the PVL-136 panel.  Again, using the Xantrex online string sizing program, it was determined that the minimum string length was 8 panels and the maximum was 10 panels.  There was not room for two 8 panel strings so a single maximum string of 10 panels was selected.  A GT-2.8-NA-DS inverter also was chosen for power conversion for this solar array.

East facing roof area was address last.  Having the same dimensions as the west facing roof, PVL-136 panels were again selected.  A maximum string of 10 panels was also used with a GT-2.8-NA-DS inverter. The inverter manufacturer recommends that the distance between the inverter and the utility connection be as short as possible to minimize AC frequency and voltage interference.  For this reason the inverters

will be located in the basement near the utility service panel.  Wiring from the solar arrays to the inverters will be contained in conduit and sized to minimize voltage drop.  The inverters will be connected to the building side of the utility meter so that solar production will reduce the need for utility power.

System Performance

The total amount of solar is 4,216 watts DC measured at Standard Test Conditions (See the PVL product data sheets for full STL information.)  A computer model has been generated using the RETscreen modeling software.  This software uses 30 years of local climate and solar insolation data to accurately model solar power system performance.  The various sizes and orientations of the solar arrays were

modeled.  Estimated annual output for the system without any shading is 6.243 MWh.  Since shading from adjacent trees will cause approximately a 20% reduction in total output, annual output is estimated to be 4.994 MWh.  This is an average of 416 kWh per month.  One unique characteristic of the amorphous silicon technology used in the PVL panels is that the output will begin approximately 15% higher than specified.  This will gradually drop over an 8 to 10 week period to within +/- 5% of the manufactures specifications.  If the owner wishes to monitor and record the system output, Xantrex

offers an optional GT Inverter Monitor that can be used with up to 5 inverters.  It connects to the inverters with standard Cat5 cable and can be remotely mounted up to 100 feet away.