What Exactly is a Microgrid?

What, exactly, is a microgrid?

The electricity grid is evolving, moving away from the traditional centralized grid and toward a more-distributed grid utilizing local sources of generation, resulting in more and more microgrids. At the same time, islands and remote communities around the world operate grids that are completely separate from any larger grid, and these are also often referred to as microgrids.

In general, microgrids are small power systems that use local generation resources to meet local electricity needs. However, since different people use the word “microgrid” to describe different things, it can be difficult to have a clear understanding of what is meant in specific situations. More important than agreeing on a standard definition of a microgrid is having a shared understanding of the specific microgrid in question in a particular situation.

1. Who owns it?
One way to learn more about a microgrid is to understand who owns it. Bloomberg’s research group divides microgrids into five ownership categories that include commercial or industrial, community or utility, campus or institutional, military, and off-grid or remote. For example, the Borrego Springs microgrid mentioned above serves that community and is owned and operated by the local utility, SDG&E. The U.S. military has been implementing microgrids, both at stationary bases in the U.S., and at forward operating bases overseas. Understanding the ownership type for a given microgrid can provide useful insight into the microgrid being discussed.

2. Are there multiple owners?
Along with the question of which type of entity owns the microgrid, it can also be useful to understand if there are multiple owners. For example, in military microgrids there is usually just one owner, the military. Other microgrids however may have different entities owning various pieces of the system, such as the generation, the storage, and the connecting infrastructure. At the Philadelphia Navy Yard, which is a 1,200-acre former naval shipyard that has become a modern business campus, several partners including Penn State and Alstom have come together to further advance microgrid technologies.

3. Does it interact with the larger grid?
Many microgrids are connected to the larger grid, with an option to disconnect and become an electrical island. Alternatively, some microgrids may not interact with the larger grid at all, but instead operate as completely separate electricity systems. These types of systems are operating today on many islands and in remote communities around the world. While these are really just smaller versions of the grid, they are often referred to as microgrids.

4. What is the size?
Another key defining aspect of each microgrid is size. Size can be expressed in terms of installed generation capacity, peak load, number of people served, or amount of land covered. Some might consider a smart home that includes distributed energy resources such as solar PV and battery storage to be a microgrid, while others might suggest that something as small as a home is really more of a nanogrid, or even a picogrid. Meanwhile, many island systems such as those in the Caribbean are considered microgrids, even those that have several MW of peak load. The lines between what size fits into the pico, nano, micro, and macrogrid categories are blurry, so asking about the size can bring clarity to the conversation quickly.

5. What is the main purpose?
As another distinction, microgrids may be built to serve varying purposes. In some cases, the main purpose may be to provide emergency backup power to critical loads. On the other hand, some microgrids (like those on islands and in remote communities) are built to operate normally at all times. Other microgrids might fall somewhere in between; for example at the University of California, San Diego, the campus microgrid provides power in parallel with the grid during normal operation, but can also meet most campus loads during a grid outage (such as the 15-hour outage that occurred in 2011).

Read the full story by Kaitlyn Bunker: RMI

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