S265C  09 1120 

Lab IK20-30 Fixed Tilt vs. Solar Tracking PV Arrays
OEES 265 Alt. Energy Construction

Back to Main Page
 Get data from NREL's PVwatts calculator
  1. Download the fixed vs. tracking spreadsheet.
  2. Go to NREL's PVwatts calculator for Albuquerque.
    • (They don't have data for Grants.)
  3. Leave the DC rating at 4 kW.
  4. Specify fixed-tilt as the array type, use a tilt angle of 46 degrees, and click Calculate.
    • 46 degrees is an average tilt angle that will yield the most energy for the winter half of the year.
  5. In the fixed tilt column of the spreadsheet, type in the energy values (dollars) for October through March.
  6. Click the browser's Back button.
  7. Change the tilt angle to 24 degrees, and click Calculate.
    • 24 degrees is an average tilt angle that will yield the most energy for the summer half of the year.
  8. In the fixed tilt column of the spreadsheet, type in the energy values for April through September.
  9. Click the Back button.
  10. Change the array type to 1-axis, specify a tilt angle of 46 degrees, and click Calculate.
  11. In the 1-axis column of the spreadsheet, type in the energy values (dollars) for October through March.
  12. Click the Back button.
  13. Change the angle to 24 degrees, click Calculate, and record the energy values for April through September.
  14. Click the Back button.
  15. Change the array type to 2-axis and click Calculate.
    •  The tilt angle is irrelevant for a 2-axis tracker.
  16. In the 2-axis column of the spreadsheet, type in the energy values for all twelve months.

Determine which is more cost effective: fixed tilt, 1-axis, or 2-axis.
When comparing fixed vs. tracking PV systems, you'll want to be sure that they are all grid-tie or all stand-alone. Also, if batteries are needed, make sure that batteries are either included or not included for the all the systems you're comparing. Same goes for inverters. (You don't want to be comparing apples and oranges, as the saying goes.)
  1. Search the Internet to find a 1-axis photovoltaic system with a low cost per watt.
  2. Near the bottom of the fixed vs. tracking spreadsheet, type in the wattage and cost for this system. 
  3. If you wish, do the same for a 2-axis system, or do a 2-axis system instead of a 1-axis system.
  4. Find a low-cost-per-watt fixed tilt system, and type in it's data also.
    • You could either buy a system that includes a mounting rack, or you could plan on building your own mounting rack.
    • If you're going to build your own mounting rack, make a guess as to how much lumber and hardware is needed, and estimate its cost.

You'll want the fixed tilt system to have a higher wattage rating than the tracking system in order to end up with roughly the same annual amount of energy.

By trial and error, adjust the fixed-tilt DC wattage ratings (in the blue-shaded box near the bottom of the spreadsheet) to get an annual energy value roughly equal to that for a 1-axis or 2-axis system. Then, calculate the cost of the additional panels needed to supply the necessary wattage rating.

You can now tell which is more cost effective, a fixed-tilt system or a tracking system.

To be fair to tracking systems, they are able to supply more energy than fixed-tilt systems during the morning and afternoon hours. I would think that NREL simply used an average amount of sunlight for each day. If this is the case, a tracking system might perform better during July and August than what is indicated by NREL's PVwatts calculator. The afternoon thunderstorms we get during these months would greatly reduce energy production while they are occurring. A tracking system would be able to harvest more energy during morning hours than would a fixed-tilt system. Thus, a tracking system might produce more energy per day than what the PVwatts calculator predicts.

By trial and error, adjust the fixed tilt DC rating to give the same annual energy value as the 1-axis or 2-axis system.

Back to Main Page