Solar Builder

JUL-AUG 2018

Solar Builder focuses on the installation/construction of solar PV systems. We cover the latest PV technology (modules, mounting, inverters, storage, BOS) and equip installers/contractors with tips and tools to make informed purchasing decisions.

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SOLARBUILDERMAG.COM | 31 SOLAR + SCALE T he value offered by DC-coupling of solar + storage is clear, but there is a challenge in facilitating DC-coupling on a large scale. The answer might be in-line power electronics. Example: A 1,500-volt utility-scale DC-coupled solar + storage system with string level MPPT was brought online for the first time on the East Coast. The spe- cif ic solution deployed was Alencon's SPOT-ES, a string level DC-DC opti- mizer for voltages ranging from 600 to 1,500, in a DC-coupled solar + storage system. In an AC-coupled solar + storage sys- tem, the batteries are charged after the solar energy has passed through the PV inverter. As a consequence, such systems need two inverters, one for the PV and one for the battery. In a DC-coupled approach to solar + storage, the battery is placed on the DC side of the inverter, so the battery is charged with DC power from the panels before passing through the inverter and experiencing DC:AC conversion losses. This approach works well because just as PV panels are a DC-source of power, bat- teries are a DC load when being charged as well as a DC-source when being dis- charged. The DC-coupled approach to solar + storage has many advantages to AC-coupling including lower cost, higher efficiency and perhaps most importantly the ability to capture every kilowatt-hour of energy generated by PV plants with DC:AC ratios greater than one by eliminating inverter clipping. Alencon's approach to the DC-coupling of solar + storage places a galvanically isolated, string-level DC-DC optimizer between the PV array and the DC-bus connecting the batter y and the PV inverter. A rising and falling DC-bus voltage commanded by the battery ener- gy storage system's (BESS) system con- troller dictates the behavior of the sys- tem, i.e. directing the system to charge or discharge the battery. Alencon's SPOTs extract the maximum amount of power from the panels by performing string level maximum power point tracking (MPPT) while their voltage output is linked to the DC-bus voltage, allowing the output voltage to follow that of the rest of the BESS. By placing the MPPT function at the string level, the SPOT- ES har vests the absolute ma ximum amount of power possible from the PV array. By connecting the PV panels directly to a battery on the DC-side of the inverter, the SPOT-ES assures that the entire generating capacity of the plant is captured for eventual delivery to energy consumers. How this large-scale solar + storage site stayed DC-coupled

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