Megawatt Systems: 3 Design Considerations

On the Megawatt Design discussion board, Ryan Mayfield answers student questions about designing systems. He discusses surge protection, tilted arrays, and “levelized cost of energy.”

Student 1: Do any of you specify surge or lightning-protective devices for your systems? Either on the DC or AC side of the inverter? Have any of you had cases where array equipment were damaged by a power surge or lightning?

Ryan Mayfield: We specify it about 50% of the time. Typically at the request of the contractor. We work with some companies in the Caribbean and they require it on 100% of their projects on both sides of the inverter.

I have seen arrays that have been hit by lightning. One took a direct hit on a module, and put a hole in it. But, none of the equipment beyond the combiner (where the surge suppressor was located) was badly damaged. The module, combiner and suppressor were replaced and we didn’t experience any problems afterwards. I have also seen the opposite– situations where all of the electronics were fried after a hit.

I think it is best to install the surge suppressor (depending on where you’re located geographically) and include language in your contract that states explicitly that you have taken precautions, but can’t be held liable for the effects of lightning strikes on or near the array.

Student 2: One of the course articles states “…For more steeply tilted arrays that produce more energy and power on a consistent basis, a more conservative ratio makes sense.” What does it mean when it said that tilted arrays produce more energy and power on a consistent basis? Does the power curve for the day change for more steeply tilted arrays?

Ryan Mayfield: I believe the author was referring to arrays tilted greater than 15 degrees. These arrays have the ability (and frequency) to see the 1,000W/Sq. meters much more than the arrays tilted at a lower angle. Therefore, there are more hours in each year where the array will be producing more power in the middle hours of the day.

Given that, you may want to consider a smaller dc to ac ratio in order to maximize the ability of the inverter to convert that power.

With the design tools that are available now (such as SAM), we can track the number of hours per year where the array is producing more power than the inverter can process, (referred to a power limiting) and we can quickly simulate different dc to ac ratios to figure out the best solution for our installation.

Student 3: When someone refers to the “levelized cost of energy” (the average total cost of installing and running a solar system throughout it’s lifespan, divided by the energy output it produces throughout it’s lifespan), does it take in to account all the costs of the system in its lifetime including replacement of inverters after its lifetime, etc.?

Ryan Mayfield: Exactly. You need to look at all the costs associated with generating the energy in order to determine the actual cost per kWh.

Join Ryan Mayfield in Megawatt Design for a 10-week deep dive into large-scale solar design. Learn how to get your projects permitted and installed faster and cheaper. This training covers many types of large solar PV systems, with a heavy emphasis on commercial rooftop systems.

Check out Ryan’s free Megawatt System Design Guide and learn about flat roof mounting solutions, megawatt system grounding, PV circuit sizing and more. 

About instructor Ryan Mayfield – President, 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.