Original article: Microgridknowledge.com
Author: Hanan Fishman, president of Alencon Systems LLC, Hatboro, Pennsylvania, United States
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.
For a microgrid, the implications of being able to capture clipped energy are significant. Specifically, it means that a solar array and battery that are much larger than the inverter can be used, and the solar and battery can be optimally sized to service the continuous load of the microgrid without necessarily having to choose overly large inverters.
Benefit 2: Costs less
The DC coupled microgrid approach to building microgrids can be much more cost-effective than the AC coupled approach. DC coupling only requires one inverter, whereas AC coupling requires two inverters and related switch gear. This downside of AC coupling is often particularly amplified with microgrids because the inverters need to be sized far larger to accommodate the continuous consumption needs of the microgrid. With the DC coupled approach, the inverter can be sized appropriately to the microgrid’s continuous loads and thus be much smaller and cost less.
Benefit 3: More efficient
On an intuitive level, DC coupling of solar and storage makes sense because solar is a DC source while battery energy storage is a DC load (and a DC source when discharging). With the DC coupled technique, the battery can be charged directly from the solar in a highly efficient manner with the assistance of a DC:DC converter. Only when the energy needs to be delivered to an AC load will it need to be converted. In AC coupling, the energy generated from solar makes a very long “”round trip” from the solar panels through the solar inverter and into a battery inverter which then rectifies that solar back to DC to charge the battery. When energy needs to be drawn from the battery, it is then converted back to AC through the battery connected inverter. Every DC:AC conversion typically comes with about a 2% loss of energy. Because it involves so many fewer conversions, the DC coupled technique is far more efficient, and it allows more energy to be captured.
Benefit 4: Smaller interconnect
Ultimately, while the idea of a microgrid is to be a self-reliant island of energy, in most cases, it will still be interconnected to the grid. This is particularly true of behind the meter microgrids for commercial and industrial facilities, one of the best and still highly underutilized applications of the microgrid concept. When building a power project, you need a grid interconnect. Grid interconnects are granted and priced by the interconnecting utility based on their size, i.e., the larger the requested interconnect is, the more expensive and more difficult it will be to obtain. DC coupling helps address this challenge in a couple of ways. First, by only requiring a single inverter, it reduces the size of the interconnect by upwards of 50% since the interconnecting utility generally views the size of the required interconnect as the sum of the AC capacity of the inverters at the site. Additionally, the DC coupled technique for pairing solar and storage, as explained above, also allows for the use of smaller inverters to service the microgrid’s load since inverters are rated by the instantaneous power they provide (kW), not the energy they can deliver (KWh). Ultimately, a microgrid is concerned with energy delivery, and project developers should size their interconnecting power electronics accordingly.