DC to AC Ratio Not Properly Displayed

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kylenorthway
DC to AC Ratio Not Properly Displayed

I designed a DC to AC Ratio of 1.24 and then go to the time value graph series to show the clipping and degradation of DC over a 25 year period, along with the AC kW generation overplayed, and staying constant over that time. But this is not properly reflected on the summary screen for annual energy production of kWhs. The kWh are degraded over time. Please advise.

Paul Gilman

Dear Kyle,

The DC-to-AC ratio for the photovoltaic performance models is based on the nameplate capacities of the module and inverter, and therefor is a nominal value.

For the Detailed PV model, if you use the "specify desired array size" option and specify a value for the DC-to-AC ratio, the actual ratio may be different than the desired one if the nameplate capacities of the module and inverter do not allow it. As a simplified example, consider the following

Desired array size: 100 DC W
Desired DC-to-AC rato: 1.1
Module nameplate capacity: 95 DC W
Inverter nameplate capacity: 100 AC W

You could either use one module for an actual DC-to-AC ratio of 100 / 95 = 0.95, or two modules for a ratio of 200 / 95 = 1.9, but you cannot exactly achieve the desired 1.1 value.

If you want to model a system whose output is constant over the 25 year period, you can set the degradation rate on the Lifetime input page to zero.

Best regards,
Paul.

kylenorthway

Thank you for your response. Got it DC-STC and AC Rating of inverter is how SAM works. Your example did not make sense to me. In the end we all want to define the economics of a system. Among other reasons for stacking a high DC to AC ratio, the primary one is energy value over time. I need to substantiate that I can conservatively deliver the same kWhs of production from year 1 to year 25. Many inverters have a limited DC to AC stacking ability. This is one reason I use HiQ inverters as they can take a ratio up to 6.35. Not that anyone would do it, but I know I am safe with higher ratios that may be needed... by economic design, and depending on location temperatures. I usually factor modules degradation with PTC values and inverter AC-CEC ratings to be conservative. I like that SAM graphs the DC degradation mapped over the AC kW output... and demonstrates no AC kW degradation over the 25 year period. The net effect is no degradation of kWh over the 25 year period. I understand I can cheat SAM with inputting zero DC degradation of the modules, but this not a correct approach in my mind. Seems the summary would reflect the net affect of the design and show zero reduction of kWhs over the project life. The summary graph mis-represents a project using a high DC-AC Ratio.

Paul Gilman

For the kind of analysis you are doing, you can look at the more detailed simulation results on the Time Series tab and Data Tables tab of the Results page. The graphs on the Summary tab do not provide much detail as you suggest.

Best regards,
Paul.

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