In October, California regulators proposed allowing DC-coupled solar plus storage systems to be compensated via net metering for electricity stored in their batteries. This came after NEXTracker's NX Flow system was verified by UL as charging only from solar power.
On September 1, 2017, the California Solar & Storage Association (CALSSA) filed and served a petition for modification of D.14-05-033 (pdf) in order to allow net metering and interconnection of DC-coupled solar plus storage systems and in October, the California Public Utility Commission (CPUC) suggested moving forward. The impetus for this decision was inverter firmware that was tested and approved by Underwriters Laboratories (UL) as never importing electricity from the grid, but instead only charging from the locally installed solar power.
In A Technology and Policy Review: California DC-Coupled Solar-Plus-Storage Net Metering Ruling, NEXTracker and SepiSolar show off one line diagrams behind the NX Flow solar+flow battery hardware that the firmware was tested on, and some of the logic used in moving this decision forward.
The above image is of the hardware requirements needed to track batteries, solar, and import/export from the power grid in an AC- and a DC-coupled system. These designs are using only hardware to control and monitor flows of electricity, as these are the components trusted by the utility. Notice in the DC-coupled model there are three utility meters, and a relay. This hardware was required as these systems weren’t allowed to export any energy to the grid.
The firmware tested by UL laboratories is shown in layout below. In comparison to the above eight components in the AC/DC coupled systems above, the simpler five component designs allowed by a trusted firmware to get rid of inverters, meters, relays and much more balance of system electrical gear. Additionally, every time the above systems send electricity through a new component, just a little bit of efficiency is lost as heat.
The most interesting analysis from the document came in the suggestion that “super-sized” solar systems might come as a result of this rule change:
Because the DC-coupled solar-plus-storage system size is measured by the inverter’s AC nameplate rating, DC solar systems can now be “super-sized” to exceed the 1 MW limit on NEM. Any excess generation over 1 MW may be stored in the DC battery, while up to 1 MW of power can be exported into the grid at favorable or optimized NEM tariffs. Thus, a 1 MW standalone solar system can be increased to 2.8 MW with a complementary DC-coupled 1.8 MW storage system and a 1 MW AC inverter that has the new firmware.
This sort of design follows along behind NEXTracker’s philosophy that it is time to move solar installations up the food chain, to bona fide baseload solar power grid resources.
Fluence Energy has given guidance on optimizing solar module loading ratios, suggesting that a 1.9:1 DC:AC solar panel to inverter ratio, makes economic sense with integrated DC-coupled energy storage in some circumstances. The logic is based upon the cost to add extra solar panels and additional balance of system (racking+wiring) separate from the already sunk costs of the interconnection hardware and inverters. Fluence estimates 60¢ per watt for that extra DC-sided hardware.
SepiSolar and NEXTracker even go so far as to suggest a 2.9:1 DC:AC ratio:
With the new DC-coupled firmware, a storage system can capture the excess energy that would normally be lost due to clipping on the 2.2 MWdc PV system through the 750 KWac point of interconnection. The new DC-storage system configuration provides the farmer with the following benefits: Demand charge reduction; TOU arbitrage; A super-sized 2.2 MWdc PV system (by keeping the AC export at or below 1 MW); Elimination of utility infrastructure upgrade costs
Expect to see ever increasing DC:AC ratios, eventually moving toward 6:1 and 10:1 ratios. NextERA’s Babcock Solar Farm is currently the nation’s largest operational solar+storage power plant, with a 10MW/40MWh energy storage system complementing the 74.5 MW-AC/126MW-DC solar power system.