The Rise of the DC Coupled Microgrid

on December 21, 2020

The defining feature of any microgrid is the fact that it generates and consumes its power without having to rely on the support of the broader utility grid. According to Bloomberg New Energy Finance, solar is the least expensive form of new build power generation. The challenge with solar is that it is an intermittent resource, so relying on it solely will not be sufficient in building a viable microgrid. Of course, the solution to this deficiency is pairing solar with battery energy storage.

Thus far, most deployments of solar plus storage have utilized the AC coupled technique, whereby the solar and batteries are connected on the AC side of inverters connected to each resource. In the DC coupled approach, which, thus far, has been less frequently deployed because it is less understood, the solar and storage are connected on the DC side of the inverter by using DC:DC converters to marry the differential levels of voltage from the PV and battery, and they use a common DC bus to divert energy into either the battery or the ultimate load. 

When it comes to the goal of most microgrids, i.e., creating a reliable supply of energy to consistently feed loads, the DC coupled approach offers a number of benefits, which are explained below.

Benefit 1: Harvest more energy 

A typical technique when building a solar plant is to overbuild the DC capacity of the solar array relative to the inverter. Historically, the reason for doing this has been to maximize the utilization of the inverter’s AC nameplate rating over the course of an entire day, from sunrise to sunset. During midday hours, when a solar array is overproducing the nameplate rating of the inverter, the solar energy will be curtailed or “clipped” by the inverter to ensure the generation does not exceed the capacity of the inverter.  

While this technique makes sense for stand-alone, grid connected solar projects, it can be counterproductive for a microgrid that needs to capture every electron the solar array generates. With the DC coupled approach to combining solar and storage, excess generation coming from the solar may be diverted into a battery, instead of essentially being “thrown away’”when it is clipped by the inverter.

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Fractal Energy Storage ConsultantsThe Rise of the DC Coupled Microgrid