Solar, storage-centric approaches to DC coupling

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From pv magazine USA

Anyone installing a distributed solar+storage system has to make a decision on how to couple the solar side with the energy storage side. Alencon has published a new white paper comparing the two main DC coupling approaches to combining solar and storage.

DC coupling describes a layout in which the solar array and battery share the same inverter, with configurations described by Alencon as “PV-centric” and “battery-centric.”

PV-centric coupling

PV-centric coupling is when a unidirectional DC:DC converter is installed between the PV panels and a DC bus that connects a battery energy storage system with an inverter. In this arrangement, the power flows from the PV panels in one direction through the DC:DC converter to directly charge a battery energy storage system, outlined in the schematic below.

In this layout, it is the job of the DC optimizer to optimally harvest solar energy by way of maximum power point tracking (MPPT).

Battery-centric coupling

Battery-centric coupling is where the DC:DC converter is installed between the battery racks and a DC bus connected to an inverter, while the inverter is connected directly to the PV system. In this case, power flows through one of Alencon’s optimizer products in a bidirectional manner to charge and discharge the battery while the inverter harvests power from the PV array.

While Alencon says a battery-centric approach appears to be the more valuable of the two at face value, PV-centric has some distinct advantages in specific scenarios.

One of these scenarios comes from projects using a bidirectional battery storage inverter, one of which can’t perform MPPT. In this case, the optimizer would perform that function, opening up the ability of the solar+storage system to benefit from more revenue streams, like grid services, through the use of the bidirectional inverter.

Additionally, this configuration can be useful for campus-wide installations, where arrays are highly distributed over a confined area. In this scenario, MPPTs distributed across the PV array can help to significantly increase PV yield.

Lastly, the PV-centric model shines in instances of maximizing DC overbuilds. As it becomes more economical to overbuild projects and add storage for the energy that would otherwise be clipped, the PV-centric approach, according to Alencon, “facilitates larger DC ratios than a typical PV inverter would allow by allowing more granular control and curtailment of PV production during edge-case scenarios when the battery is full and the PV is overproducing.”

The battery-centric approach is more about cost-effectiveness. When deploying DC-coupled solar+storage, the power rating of the PV array is typically much higher than the power rating of the battery, typically 3:1. When sizing power electronics like DC:DC converters, the number of devices needed is determined by the power rating of storage, meaning that if the power electronics are sized to the power rating of the battery – the battery-centric approach will be more cost effective.

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