Happening on the Megawatt Design Discussion Board…

Expert Instructor Ryan Mayfield on three student questions regarding Megawatt Design equipment and software selection

1. String inverters & AFCI failures…

Student 1: I have seen many people staying with central inverters on big jobs mostly due to AFCI failures. I think the AFCI issues are being resolved, but it burned a lot of people trying string inverters on big jobs. What has everyone else seen?

Ryan Mayfield: I have been seeing an overall trend towards string inverters. We just went to permit on a 1.25MW where we used string inverters. From talking with others in the industry, it is becoming commonplace for sites up to 3+MW to use string inverters.

I haven’t had people cite AFCI issues to me but rather the ease of install and the granularity of the production as primary reasons. The costs for the string inverters have been driven down so much… and there is the reduction of 600-1,000Vdc combiners and re-combiners.

2. Utilizing software to define intra-row space…

Student 2: How wold you define the intra-row space for the following example:
-My roof has 4 slopes, each 13° tilt, azimuth South and North.
-I want to tilt my modules at 25°, so I’ll be adding 12° to my modules.
-I want all the modules to face South, on my North and South slopes.
I cannot do that on Helioscope, as the software thinks my area is flat. How can I simulate it? What software should I use? Thanks!

Ryan Mayfield: I haven’t tried to do something like that in HelioScope. For the energy production, you could trick the program and have your arrays set at the proper angles. But for layouts, it will be difficult to get HelioScope to work well.

I would suggest either drawing it in CAD or possibly in a 3D program such as SketchUp (or even a combination of the two). SketchUp would give you a good idea of shading lines and setback requirements while a CAD drawing would be more accurate in terms of array placement and layout.

What is your ultimate goal for the drawing, to determine the required spacing and racking heights? Roof layout?

Student 2: First, I want to quickly simulate the kwh with such design and use North slope vs. North/South flush mount modules. But this does not work, as my shade loss will not be correct with a slope of 13°. Moreover we are introducing bi-facial technology and want to maximise the reflection of white roofs. Those modules are costly and we need to maximise kWh/kWp. So an additionnal module tilt would be good.

I guess SketchUp would be the right way to set the intrarow space.

All industrial buildings have sandwich panels and a tilt of 6-15° in Turkey. There are not flat like in the US!

Ryan Mayfield:  You could use SketchUp to figure out your spacing requirements and then use HelioScope to help with the kWh estimate. You would have to either create keepouts in the HelioScope model or use multiple field segments to complete the design.

It isn’t the intended use of the software but a way you can get your needed result.

Burying wiring for ground mounted systems…

Student 3: I have read before that the wiring for ground mounted systems should be buried underground. Is there any truth to that? What are the advantages/disadvantages of buried cables compared to using cable trays?

Student 4: Exterior mounted conductors need to either be in a cable tray or conduit. A cable tray usually leaves the conductors exposed to the elements for 20+ years. Both a cable tray or conduit will add material costs and depending on the racking they could add install time. Direct burial is straightforward and PV wire (which is rated for direct burial) is used for 1000v. Direct burial will require trenching, though, but that’s not typically a huge cost.

Ryan Mayfield:

Well said, Student 4. From my experiences, with only a couple of exceptions, I have found it is unusual for ground mounted arrays to use cable trays for the outputs.

In a recent job we did, there was bedrock six inches below the top soil. The costs associates with trenching the conductors was outrageous so the final design had nearly all the conductors in cable tray. This took careful coordination with the contractor and inspectors to make sure everyone was happy with the design.

Like Student 4 said, you can utilize direct burial cables to avoid the cost associated with conduits. I know some contractors that don’t feel comfortable with direct burial and always run conduit.

Enroll in Megawatt Design and spend 10 weeks learning from industry professional and Solar PV Technical Editor at SolarPro, Ryan Mayfield! Now approved for: 30 NABCEP Advanced Credit Hours , 6 NABCEP Technical/JTA , 6 NABCEP Elective , 3 NABCEP NEC , 30 AIA LU/HSW Credits

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About Instructor Ryan Mayfield

Renewable Energy Associates

Ryan Mayfield has been working in the renewable energy field since 1999 and is the President of Renewable Energy Associates, a consulting firm providing design, support and educational services for electrical contractors, architectural and engineering firms, manufacturers and government agencies. Ryan serves as Photovoltaic Systems Technical Editor for SolarPro Magazine, regularly writing feature articles in SolarPro and Home Power magazines, and wrote PV Design and Installation for Dummies. Ryan was also a contributor and video team member for Mike Holt’s Understanding the NEC Requirements for Solar Photovoltaic Systems. Ryan teaches various PV courses across the nation for electricians, existing solar professionals, code officials, inspectors and individuals looking to join the solar industry. Class topics include National Electrical Code and PV systems, residential and commercial PV systems. Ryan holds a Limited Renewable Energy Technician (LRT) license in Oregon, is an Oregon Solar Energy Industries Association (OSEIA) board member and chairs the state’s LRT apprenticeship committee.

Ryan is teaching…