A way oversized DC:AC ratio seen in the wild!


Historically, we’d been taught that solar modules were the most precious of goods – to be held closely, to be loved. There’s that famous graph showing the price per watt falling from $100 in the 1970s, to the less than 30¢/watt these days. Now, we still care greatly about our solar modules – however – we use them differently as the facade of ideology falls in the face of modern knowledge.

Solar FlexRack has announced that Swinerton Renewables has contracted its GX-3 fixed racking system for a 26.4 MWac solar power system located in Simsbury, Connecticut. The Tobacco Valley Solar Project was developed by Deepwater Wind, who signed a long term power purchase agreement (PPA) with Eversource of Massachusetts. The PPA suggested that the solar power plant would receive payments for both the electricity and value of capacity (like all major utility scale solar plants are paid these days).

The most interesting part of the design in the DC:AC ratio though – a very rough public filing at the end of May (see page 19 pdf) – suggests over 48.6 MWdc of solar power, versus the publicly announced 26.4 MWac.

The blurry public document shows a portion of the project will be greater than 84,000 400-watt solar modules, plus 38,000 395-watt modules manufactured by Jinko, though the model number couldn’t be found. These numbers total 33.6 MWdc of 400-watt, and 15 MWdc of 395-watt modules – totalling 48.6 MWdc. No mention of energy storage could be found in any designs.

The system is projected to generate enough electricity to power 6,800 homes in the United States over the course of a year. The average home uses 10,000 to 12,000 kWh/year – suggesting 68 to 81 GWh/year. This would compute to an AC capacity factor of 29% to 35% on a non-tracking solar power system in Connecticut – far to the north of the range where even tracking systems are dominant. The DC capacity factor is a much more sensible 16-19% value. A 1/10 sized rough production analysis in Helioscope (pdf) shows 11% of the electricity would be clipped, and total generation delivered to the grid would be 60 GWh – which suggests Swinerton Renewables (one of the nation’s leading builders of solar power) is able to optimize greatly their designs.

The last time the EIA reported on this (looking at data through the end of 2016), they showed that utility scale systems have slowly increased their DC:AC ratios to around 1.25:1.

That 2016 data also included a few plants built by NextEra/FPL that had very high DC:AC ratios. The Babcock Solar Farm, noted as the the nation’s current largest operational solar + storage power plant – with a 10MW/40MWh energy storage system – has a 126MWdc/74.5MWac solar power system. This is a 1.7:1 DC to AC ratio, but this plant has energy storage.

The Babcock plant was built in 2016, and it was probably designed a couple of years earlier. So that could mean that NextEra has deployed this model elsewhere, maybe several times more.

Consider that 8minute Solar Energy has modeled solar + storage with extreme DC:AC ratios like 3-4-5:1 that are coupled with massive amounts of storage, allowing for 24:7 solar power plants that they hope to deploy within five years. This is the company with the nation’s lowest priced power contract right now, and this author suggests that 8minute is making use of this technique to get that price.

Of course, not all solar power systems want a high DC:AC ratio – some want it flat. Ideal Energy of Iowa built a single axis tracker, with flow batteries, designed for peak shaving. The DC:AC ratio, approaching a 1:1 value, is because the facility is not allowed to export excess electricity per the interconnection agreement. The developer noted that getting rid of the export means the interconnection application was faster and cheaper.

Scream out loud “use case” from the rooftop.

Last year, Fluence Energy gave guidance on optimizing solar module loading ratios, suggesting that a 1.9:1 DC:AC solar panel to inverter ratio, in defined circumstances, makes economic sense with integrated DC-coupled energy storage. Fluence estimates 60¢ per watt for that extra hardware.

Research in Minnesota has shown that oversizing the DC:AC ratio for wintertime production will offset the need for long term storage (in sunny Minnesota). NREL and FirstSolar developed models showing how oversizing solar allows for predictable use of the excess power produced for ancillary services. In another use case, FirstSolar and the Tampa Electric Company, showed that oversizing and designing flexibility with that oversizing allows for more solar power to be installed, and greater amounts of solar electricity to be delivered to the grid.

The purpose of this article is to illustrate that it is time for us to let loose of the standard values we usually consider for DC:AC ratio when designing our solar power systems. Sometimes it makes sense to build on north facing roofs, sometimes the panels are flat, sometimes you have energy storage, or – like in Connecticut – you have an opportunity to make revenue from other streams than simply electricity generation, and extra solar modules are now a financially viable means to do that.

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